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[
"Why does a string with a knot on one end seem easier to swing in a circle than one without a knot?"
] |
[
false
] |
If I have a uniform piece of string, and try to hold it by one and and swing it around in a circle repeatedly, it seems quite difficult. But, if I tie a couple of knots on the end of the same string, it seems a lot easier to swing around. What's the major effect at play here? Is it not actually any "easier" and just my senses playing tricks on me?
|
[
"You're not crazy! If more of the string's weight is on the end of that string, the force you apply to the string is translated there (because where mass is is where the force is). It's way easier to generate centripetal force (i.e. the reason the string spins) when the mass is concentrated in one place."
] |
[
"Why is it easier to generate centripetal force when the mass is concentrated?"
] |
[
"[Not a professional in mechanics]",
"I think there may be multiple related issues in play here. Two of them are rigidity and density relative to the air. Consider whether you would be having the same problem if you were spinning a thin metal rod, like a coat hanger that you straightened out and then bent at one end to give yourself a little handle. It would be pretty easy, even though there is no more weight on the end than at any other point along the length of it. ",
"I would suggest that this is because of two things, among others. (a) The metal holds its straight shape, so pulling at the center also efficiently pulls the tip, whereas with a loose string, you're more likely to be pulling on the part of the string near the center and having the rest of the string just dragging along behind it. Also (b), the metal is very dense relative to the air, and so when you impart the force needed to pull it through the air, the air obligingly gets out of the way. Even though the tip is moving much faster than the center, there isn't enough air resistance when you're doing this by hand to either deform the metal rod or to challenge your muscles. Consider trying to do the same thing but with a giant plastic drinking straw that you hold by the ridged, bent end. You can imagine that it would be quite a bit harder than with the rod. The straw is a very light, hollow tube that would induce a lot of drag relative to its mass, and the differential in drag between the center and the tip would be considerable. So as you try to spin it, you'll have trouble getting the tip up to speed, particularly while avoiding buckling the straw near your hand. ",
"Now return to the string. Depending on what type it is and how frayed, it's going to be fairly light and very flimsy. Because it's light, the tip needs quite a bit of push to get it cutting through the air, and because it's flimsy, providing the force at the center is liable to just deform the string rather than accelerate the tip, so you end up with a string wrapped around your hand.",
"Now consider what you've done by adding a concentration of mass at the tip of the string. For one thing, you've made the tip more dense, so it's going to have an easier time punching through the air and so not falling behind the portion of the string at the center that is moving slower and experiencing less drag. And you have at the same time provided a way to make the middle portion of the string more taut. ",
"Say you have unlimited strength, and imagine pulling on a tug-of-war rope with either the Stanford chess team or the Samoan rugby team on the other end, pulling as hard as they can, and you balancing the force. Now someone comes along and sits down on the middle of the rope. It's easier for him or her to deform it in the first case. When you have a stronger counterforce on the other end, the rope is more rigid.",
"And while it may not always feel like it, when you are swinging a string in a circle, you are always pulling toward the center. The centripetal force you apply is balanced by the \"opposite reaction\", the tension in the rope that is trying to just continue on its path even as you keep pulling it in a new direction. When you add or concentrate mass at the tip, you are augmenting the momentum of the fastest-moving part of the system. Because Force = Mass x Acceleration , and you are accelerating the tip the most by deflecting it around and around in a circle at high speed, putting density into the tip means both that there will be more more power to fight the air at the highest speeds and that there will be more tension in the string to keep it rigid against the drag differential that is trying to bend it. "
] |
[
"Why do we feel a pumping or throbbing sensation when we have an injury such as a cut?"
] |
[
false
] | null |
[
"In the immediate aftermath of the cut your body reacts with localized inflammation and swelling as an immune response and to lessen blood loss. This combined with an increase in blood flow to the area puts a great deal of pressure on your blood vessels. Because of this increased pressure, you can feel your pulse there not unlike the pulse points across your entire body. ",
"If you read my previous attempt at answering this, ignore it. It was so very regrettably wrong."
] |
[
"This is a great question, I've answered it briefly in some of my other posts. What you are experiencing when you have that \"throbbing\" sensation is actual part of the definition of inflammation. It was first described by Celsus (a Roman physician from the 1st C. AD) and later expanded upon by Galen. Inflammation is defined as having five cardinal signs: rubor, calor, dolor, tumor, and functio laesa (redness, warmth, pain, swelling, and loss of function). You can see this exactly in your example! A splinter in the finger will injure and subsequently cause some keratinocytes, basal cells, maybe mealnocytes, and immune cells within the dermis of the skin to upregulate genes that start creating specific proteins. One of the most famous and well known genes encode for an enzyme called cyclooxygenase. This takes arachadonic acid (a special lipid) and turns it into another class of lipids called prostaglandins. These are very important in the body as they are a potent mediator to activate the inflammatory process along with a cousin of these lipids called leukotrienes. These biologically active lipids first cause the capillaries near the wound to become \"leaky\" so that blood components literally leak into the extracellular matrix (the \"space\" cells take up within the body's tissue) from inside the blood vessel. This will cause the redness you see at the wound site along with the swelling or what we call \"edema\" as well. When the swelling is forming, you can get local compression of nerve fibers (specifically, C-fibers -- I learned them as C-delta fibers in medical school but it appears they are not called that anymore? -- which are small caliber, non-myelinated, afferent or \"sensing\" neurons for pain and temperature) which follow pathways up to the brain and register as a noxious feeling at the site of the injury. So, we have rubor, dolor, and tumor covered -- what about the other two? Calor or heat is produced because, again, of the leakiness of the capillary beds near the injury and the influx of blood into the extracellular matrix. Lastly, if enough inflammation is occurring, you can get loss of function. I think the easiest example of this is if your cut happens to be right at the knuckle of your hand. Your hand can swell a fair amount at the location of the injury and if swollen enough, you may find it difficult to bend the digit. That is one of the ways we define loss of function. All of these processes together is what causes what we see at the injured site and local c fiber neurons allow is to feel the pain. And what about the \"pumping\" sensation -- there is a long-standing myth believed that this matches your heart rate and this sensation is due to the increased blood vessel permeability and intravascular contents being \"leaked\" into and around the cells of the injured site. This is not true! A neat study was published in 2012 that showed there was no relation what-so-ever regarding the rate to which a study participant sensed these throbs and their recorded heart rate. ",
"The current answer is: we don't know. Pain can take on many forms. In acute appendicits, during the early phase, the pain is typically recounted as \"dull\" and is sensed \"near the belly button.\" Later, this pain shifts to the right lower quadrant of the belly and becomes much more sharp in nature. This is accounted for in two very interesting pathways. When the appendix becomes infected, local inflammation causes the 5 cardinal signs of inflammation! So, the appendix will get red, warm, swollen, and due to the swelling of the appendix -- you'll have compression on VISCERAL afferent nerves which are sensed by the body in a \"dull\" sensation, kind of like a gnawing pain. Later, when the appendix swells enough, it'll actually rub against the peritoneum (depending on the location and orientation of the appendix) which is the thin lining of the abdominal cavity and cause inflammation of the peritoneum . This is called peritonitis and it is not visceral afferent nerve fibers that allow us to sense it but rather somatic c fibers which is why we perceive that pain as very sharp and we can localize it to a specific area.",
"I hope this was informative for you and answered your question! I specifically left out details that get a little too muddy and that I think are beyond the scope of the question but if you have any questions or would like more detail please do not hesitate to ask! I'm just a lowly surgeon and it's been quite a while since I took pathophysiology so if there are glaring inaccuracies I would like my colleagues to correct me -- though I believe the basic premise of what I've been describing is correct."
] |
[
"Hi there! It’s not actually due to blood flow into the area but is a common misconception. There have been a couple studies that debunked this. I left an overly detailed answer in this thread as well, if you’d like to read it! I hope if you have any questions or things to discuss you’ll reach out!"
] |
[
"Publications from North Korea..."
] |
[
false
] | null |
[
"It wouldn't really matter much for me, but I can't really recall seeing a North Korean paper in any context, really. I did once hear a second or third hand account about a North Korean guest researcher they had at my university back in the 80's, who was accompanied by an appointed chaperone (I'd suppose, to make sure he didn't defect or do anything 'subversive'). People talked about how weird it was that he'd be working in the lab with this guy just sitting there, watching. It was only for some months though.",
"I don't really know anything about science in NK (I mean, it doesn't seem like ",
" knows too much about anything going on inside the country) I'm sure they read foreign journals, but I'd guess they mostly publish (to the extent that they do) in their one ones, in their own language. Their scientists are probably very isolated; I doubt many get permission to leave the country to go to conferences and such (even the Soviet were restrictive with that back in the day, and yet they weren't nearly as shut off from the rest of the world as NK is) The ones who don't do 'important' military-related research probably have very little resources as well. ",
"My feeling is that in these extreme societies, it's not so much about the politicians telling the scientists what to do, but rather bad scientists who cunningly exploited their totalitarian systems to eliminate their scientific rivals and further their own careers. That happened in the Soviet Union, mainly during the Stalin era, ",
"Lysenkoism",
" being the most infamous example. (they also executed Hans Hellmann, who wrote the first textbook on quantum chemistry. Although probably mainly because he was German) In Nazi Germany there was ",
"\"Deutsche Physik\"",
", and part of Hellman's tragedy was that he'd fled Nazi Germany because he couldn't get a professorship there due to his wife being a Russian Jew. ",
"But I don't think scientists or journals would ever put a blanket ban on a particular country. (Although in this case, they don't really participate much in the first place) We're capable of evaluating the research on its own merits, so there'd be little reason to. These things haven't been consistent either; even while Lysenkoism was a huge blow to Soviet biology, there was still good work being done in physics (e.g. Lev Landau), mathematics (Kolmogorov) and chemistry (Frumkin) among other things. "
] |
[
"When it comes to Lysenkoism in USSR, similar things were starting to happen in other fields as well. However, that was also the time when Soviet Union was developing their first nuclear weapons, and scientists working on those managed to persuade the government against supporting followers of Lysenko in other fields. There still was some damage in physics, but relatively small. "
] |
[
"True. In chemistry their society of chemists denounced resonance theory as \"bourgeois science\" in a letter to Stalin. I'm doubtful how far that ever went in practice though, or their organic chemistry would've been in just as bad a situation as their biology. ",
"Things lightened up once Stalinism and Lysenkoism with it were later denounced. My perception is that in the later Soviet Union, the perhaps most damaging bit of dogma (to the natural sciences) was the omnipresent priority to people from the \"right\" background with the right connections; working class family, member of the ",
" from a young age, etc. In conjunction with the ",
"unofficial-yet-institutionalized discrimination",
" against Jews and other \"undesirables\". ",
"(in fairness, ",
"several democracies",
" were not much better in that regard, pre-WWII. There's also the MacCarthy-era blackballing of Oppenheimer and other scientists. One shouldn't get too self-congratulatory; What Teller did to Oppie was eerily similar to what Lysenko did to his competitors. Teller was a better scientist than Lysenko, but they were both lousy as human beings) "
] |
[
"Does a full fridge cool more efficiently than an empty one?"
] |
[
false
] |
[deleted]
|
[
"The full one - it usually even states this in your manual. But, when you open the door, you let that cold air escape, and the overall temperature inside the fridge goes up. Hopefully your beers don't escape when you open the door, but assuming they stay put, you won't add as much heat in every time you open the door."
] |
[
"No, just always have a supply on hand to replace them, right?"
] |
[
"So if a guy wants to save on his electricity bills, the key is to not drink the beers. Oh life, why are you so full of difficult choices?"
] |
[
"How does a spiral galaxy STAY a spiral galaxy?"
] |
[
false
] |
Dark matter, no dark matter, MOND, giant space whale shenanigans, you name it, but orbital periods still increase as you go out from the galactic core, and they range in the hundreds of millions of years, yet spiral galaxies keep distinct spiral arms for of years. How do?
|
[
"The spiral arms of the galaxy aren't a 'winding' phenomenon. i.e. They didn't start as straight lines that have been wound around the center as the galaxy rotates. ",
"All of the stars/gas etc in a galaxy have elliptical orbits - just like all of the planets in the solar system. The Spiral arms are a result of the axes of these elliptical orbits not lining up , so rater than concentric rings of ellipses you get a spiral pattern where the parts of each orbit come closest together like in ",
"this",
" diagram. ",
"Thus as stars/gas/dust continue along their own individual orbits within the galaxy, there will always be these spiral regions where you have a higher than average density of stars clustered, making the spiral pattern visible to external observation. ",
"If you google 'spiral galaxy density wave theory' you can find a lot more resources about this."
] |
[
"For a visualization, take a look at ",
"this youtube video",
". The left side describes the spirals as you think about them, while the right side are how they actually behave. The video description has more details."
] |
[
"Great video thank you for sharing :)"
] |
[
"If there was a body of water that was as deep as the Marianas Trench but perfectly clear and straight down, would you be able to see all the way to the bottom?"
] |
[
false
] | null |
[
"Even pure water absorbs and scatters light so you could not see the bottom of the \nMarianas Trench (10,994 m depth). The amount of sunlight in the ocean decreases exponentially from the surface. The ",
" varies with colors (wavelength) with the blues penetrating the deepest to a few hundred meters. So even if you had the brightest floodlights in the Marianas Trench and perfectly ",
" pure water (the open ocean is pretty close except for the salinity), you can't see anything more than a few hundred meters away."
] |
[
"Obviously we can see that far...I see the moon all the time and it's ",
" 7 miles away..maybe 8."
] |
[
"But those reflect light. But then so does anything that you can see.\nI'm not sure what anyone is talking about."
] |
[
"What would happen if you just took a commercial plane and drive it straight up into the atmosphere trying to reach space?"
] |
[
false
] | null |
[
"If you actually tried to fly it straight up, it would stall.",
"If you tried to fly at the maximum rate of ascent, eventually the air would get too ",
" thin for the engines to work, or for the angle of attack on the wings to have much of an effect, so you'd be effectively limited to a certain altitude (what that is depends on the aircraft)"
] |
[
"Well it wouldn't ever be able to ever reach outer space but say we just plopped a plane into orbit there would still be a problem. Designing a body to maintain it's internal pressure with the near vacuum of space requires a more robust design than modern commercial aircrafts employ."
] |
[
"Disagree on what grounds? A commercial aircraft at 30000 feet experiences external pressure of 4.3 psi, while maintaining an internal pressure of about 8 to 11 psi - a difference of 4 to 7 psi that comports with articles I found indicating that commercial airframes can withstand a maximum pressure differential of 8.6 psI.",
"And that's in addition to the fact that aircraft pressurization is accomplished by bleeding air from the engine compressors. An airframe in outer space would have no way to maintain its pressurization even if the cabin could withstand the pressure differential."
] |
[
"In a reaction between enzymes and substrates, does increasing the number of enzymes affect the rate of reaction whatsoever?"
] |
[
false
] |
I've been getting mixed answers here and there when I tried to look it up. Some say only acidity, temperature, and substrate concentration affects. Some say enzyme concentration also affects. Others say enzyme concentration only affects the maximum rate of reaction and nothing else. So what's the answer?
|
[
"Enzyme concentration affects reaction rate as long as there are more substrates than enzymes, which we typically assume for most cases. The typical representation of enzyme action looks like this:",
"rate=k[E][S]/K+[S] where k and K are reaction constants (meaning they don't change based on concentrations of species, but these are the things that will change with pH and temperature), and [X] is the concentration of species X, E being enzyme and S substrate. So as expected, the rate of the reaction is proportional to the amount of enzyme.",
"For more information, you can start with the ",
"Michaelis-Menten",
" treatment, an introductory treatment of enzyme mechanics that is appropriate for a large variety of enzymes, although there are entire books devoted to how the assumptions can be relaxed and which treatment is appropriate to different classes of enzymes."
] |
[
"Yes increasing the amount of enzymes will affect the rate of the reaction. But temperature and pH will also affect the rate of the reaction as there are enzymes that work more efficiently in specific temperatures and pH. Several factors affect the rate of a reaction. "
] |
[
"Let's say you have a reaction A --> B catalyzed by enzyme X. ",
"You accumulate product B at 1 mM / min",
"Now dilute your enzyme 10,000-fold and try again",
"You will not accumulate B as quickly. ",
"On the flip side, if all of your substrate is bound by enzyme, and you have free enzymes floating around, bored, then adding more enzyme won't do anything for you. "
] |
[
"How do my cat's hairs have multiple colors on a single strand?"
] |
[
false
] |
I have a grey/black male tabby, and his hairs have bands of grey, black, and blonde on individual hair strands. He's an indoor cat, so it's probably not just faded from the sun. How does the pigment change on a single strand, and how is this different than the way human hair is pigmented?
|
[
"Cells responsible for pigmentation",
" can change pigments, rapidly, mid-hair, in some species.",
"\nMy cat Max is a brown ",
"Mackerel Tabby",
" (mutt cat, just describing the color accurately) and he has an Agouti (changing colors through the shaft of one hair) coat. ",
"More information on animal hair pigmentation ",
"can be found here",
" ",
"Human hair grows without changing pigments mid-hair, although some people's hair is lighter on the ends due to bleaching (via the sun, or by chemicals) ",
": people have been purposefully bleaching their hair through antiquity, often with strong lye soap. Others have used urine. ",
"Probably good at keeping down lice and fleas, as well as altering the appearance of hair color. "
] |
[
"Human hair grows without changing pigments mid-hair",
"I regularly pull hairs from my chin/beard that are combinations of red/brown/black/blonde.",
"Sometimes the tip of the hair is black and the part closer to the skin is blonde, sometimes the other way around, but that's the most common one I see.",
"Were you over-simplifying or am I abnormal?"
] |
[
"Thanks for the specific answer. ",
"Agouti",
" perfectly describes his hair. Coincidentally, he is also a tabby named Max. Are you me?"
] |
[
"When you burn yourself pretty bad, say on the hand, the pain seems to stay on the surface of your skin. When you subject your hand to extreme cold, it feels bone deep. Why?"
] |
[
false
] |
This is from my experience. Does a severe burn also feel bone deep? I thought of this question today at where I work, it's a frozen yogurt place and I was holding some in my hand. After a while, my bone started to hurt, as it always does, all the way up to my elbow. I have never experienced a burn that felt bone deep. It's igniting the same nerves, no?
|
[
"Because a burn ",
" damage to the surface of your skin. If your hand was burned all the way through, your nerves would be dead from the heat. The low temperatures you're referring to don't actually do damage to your hand, which is why you can feel them so acutely.",
"If you put your hand in really hot water that wasn't hot enough to scald you, you'd probably notice your hand feeling uncomfortably hot around where your bone is too."
] |
[
"Painful stimuli travels to the brain via two different pathways. One, the Neospinothalamic tract, transmits pain much faster and is associated with more acute stinging type pain which is generally more localized (i.e., to a burn on the surface of your skin). ",
"The other, the Paleospinothalamic tract transmits pain more slowly and is associated more with dull, aching pain that is more difficult to localize. The thermal nociceptors that transmit cold sensations send signals to the brain via the second, slower system, so the result is a more vauge sense of pain. ",
"So, in answer to your final question: no, these are not the same nerves. While heat and cold are both interpreted by thermal nocireceptors, there are different variations of these receptors for heat and cold stimuli."
] |
[
"Thank you!"
] |
[
"Ask Anything Wednesday - Biology, Chemistry, Neuroscience, Medicine, Psychology"
] |
[
false
] |
Welcome to our weekly feature, Ask Anything Wednesday - this week we are focusing on Do you have a question within these topics you weren't sure was worth submitting? Is something a bit too speculative for a typical post? No question is too big or small for AAW. In this thread you can ask any science-related question! Things like: "What would happen if...", "How will the future...", "If all the rules for 'X' were different...", "Why does my...". Please post your question as a top-level response to this, and our team of panellists will be here to answer and discuss your questions. The other topic areas will appear in future Ask Anything Wednesdays, so if you have other questions not covered by this weeks theme please either hold on to it until those topics come around, or go and post over in our sister subreddit , where every day is Ask Anything Wednesday! Off-theme questions in this post will be removed to try and keep the thread a manageable size for both our readers and panellists. Please only answer a posted question if you are an expert in the field. . In short, this is a moderated subreddit, and responses which do not meet our quality guidelines will be removed. Remember, peer reviewed sources are always appreciated, and anecdotes are absolutely not appropriate. In general if your answer begins with 'I think', or 'I've heard', then it's not suitable for . If you would like to become a member of the AskScience panel, . Past AskAnythingWednesday posts . Ask away!
|
[
"[Psychology] Posting here because my post didn't show up on the \"new\" page after hours. ",
"Some years ago I have watched a youtube video about persuasion techniques used in the media to indoctrinate people. The video spoke about a technique whereby media will show many photos of something to give you the impression that a phenomenon is widely common when in reality it is not. In the same way watching a video of planes crashing can give you the idea that it is unsafe to ride a plane. or watching videos of shark attacks can make you think sharks are deadly (I think 7 people on average die each year because of shark attacks while half a million die because of mosquito bites).",
"What is this technique called ? But more importantly, can you share some ressources about persuasion techniques in the media in general ? The only one I know for now is use of subliminal messages. I'd like to have a comprehensive list of all the techniques that are used."
] |
[
"The technique (or really phenomenon) that you’re describing is called the “availability heuristic.” There are a million articles on it, but kahneman and tversky are highly cited co-authors who have written extensively on the topic (as well as other common heuristics that humans rely on) if you want a more in depth understanding of it. ",
"As for persuasion (in general, not exclusive to media, but certainly used by the media and in advertising) you can look at the Elaboration Likelihood Model by Petty & Cacioppo which examines how people can be persuaded in different situations. There have been a lot of responses and rebuttals to this theory (I have my own concerns with it) but it is still regarded as somewhat of a “pillar” in human persuasion research."
] |
[
"[Biology, taxonomy] If there were aliens and they were examined, where would they be classified in the current systems of classification? would they be put in one of the existing Domain/Kingdom, or have a completely new one if they were:",
"1) Not based on carbon;",
"2) completely different to any form of life we know;",
"3) humanoids with similar atributes to Homo Sapiens, yet with completely different genetic code;",
"4) whatever assumptions you need to make that matters in the process of classification (would be nice if you stated them, as I'd learn something more about this).",
"I think I have to put disclaimer that I don't care about probabilities in this instance - this question is a product of thinking in the shower, purely hypothetical.",
"If it doesn't fit this sub, please, redirect me."
] |
[
"during the annealing stage in PCR, why don't the recently split complimentary DNA strands attach back to each other instead of the primer?"
] |
[
false
] |
is it because they are so much bigger? or do they attach to each other like 1/2 the time, and the other 1/2 the time the primer attaches first?
|
[
"Yes to your first supposition; it is because the primers are much shorter, and there is a much higher concentration of primers than template. This is also why annealing is not at body temperature. During initialization and denaturing, the strands not only break their hydrogen bonds but also mix around the tube and each other due to convection currents. When the temperature is lowered, the primers anneal much faster, and are more likely to anneal to the right place than the other strand because there are so many more primers in the mix. This is only a concern for the first few cycles, before there is exponentially more replicated DNA than template DNA. The concentration of primers is so influential that lower the annealing temperature is likely to result in nonspecificity of the amplicons (which might look like a smear on a gel) instead of stopping the reaction. I looked for sources, and I found that ",
"this site",
" had a more clear explanation than wikipedia.",
"From personal experience, I had a lot of trouble getting DNA from some high quality samples, and I figured out it was because the templates were too concentrated. The only reason I could imagine why too much DNA would stop a PCR is because the concentration of template and primers are closer to equal, so the strands anneal back to one another.",
"edit: ninja'ed by the ron, who is also more concise and quantitative. I'll leave mine up in case someone finds it informative."
] |
[
"They can, but it is unlikely. ",
"At the beginning of the reaction you probably have 10",
" more primers than target. Even by the end of the reaction, you will have about 100 times more primers than target. "
] |
[
"ah crap. now i feel retarded. i wasn't even thinking about the concentrations. thanks guys."
] |
[
"Can an element's properties be predicted from the structure of its atom?"
] |
[
false
] |
I.e., imagine there was no gold on Earth and humans had never encountered the stuff before. Would we be able to guess that "Unknown Element 79" would be yellow in color, very dense, and melt at 1947.52 °F based on the fact that it had 79 electrons, 79 protons, etc?
|
[
"To my knowledge color is beyond our ability to predict right now, but phase, density, and melting/boiling points are things that are calculated, although to nowhere near that level of precision. You can see that today with synthetic elements, like ",
"Copernicium",
", which is predicted to be (possibly) the only metal that is a gas at room temperature, and to be denser than any other element as a solid.",
"In addition, from periodic trends and quantum mechanical simulations, the kinds of things we can predict are on the atomic or molecular level: ionization energies, electron configuration, oxidation states, atomic radii, and the existence and structure of some simple chemical compounds like oxides and halides."
] |
[
"Actually, the chemical properties of a substance are much more dependent on the electron structure than the nuclear structure. For example, the way we would know about the metallic nature of any atom is by looking at its valence electrons and seeing if it is holding on to them very strongly. Generally, metals tend to have low ionization energies and therefore give off valence electrons quite easily to form positive ions. The melting point can also be predicted from the strength of inter-molecular bonding, and generally metals tend to form \"seas of electrons\" between eachother's atoms that keeps them held together quite strongly which is why, on average, metals have higher melting points than nonmetals. So yeah, the fact that some atom just has 79 electrons wouldn't be as helpful as the knowledge of the specific arrangement of these electrons for determining such properties as you described. Now, the nucleus doesn't tell you much chemically, but knowing the nuclear structure as well as the ratios of neutrons to protons could help you to predict if the atom would be radioactive (unstable) or not. Hope this helps!"
] |
[
"Nuclear structure affects electronic structure way too much to divorce the two concepts like that.",
"Not really. The Z of the nucleus determines the number of electrons in an atom. Then there's things like isotopic shift and hyperfine structure, which are very small effects."
] |
[
"Do we have any systems in which other planets rotate around a giant planet instead of a star?"
] |
[
false
] |
Like something with star-level mass but not out of material that can react in a fusion reaction under that much gravity. If no, then why not?
|
[
"Well, I think anything with high enough mass would become a star. I guess if you would collect heavier elements than helium in a large enough quantity, that wouldn't start fusion and won't be a star. But given that 98% of material is hydrogen or helium, that just doesn't happen naturally."
] |
[
"Hydrogen and helium are by far the most common element in the universe. It's just not plausible to form an object of stellar mass that isn't a hydrogen-helium composition.",
"There are planets orbiting white dwarfs and neutron stars, but those used to be stars."
] |
[
"The Planetary Solar System Model dictates that in order to be classified as a Solar System you would not find a planet at the center only a Star. We do have systems and that would be classified as a planet with moons. It is part of the classification verbiage. So the answer is yes but it is not an actual Solar System the Latin name of our sun is Sol. Most stars have at least one planet in orbit and some stars (brown dwarfs become planets). Hope this helps!"
] |
[
"How do we measure the caloric content of food?"
] |
[
false
] |
How, for instance, do we know that there are 100 calories in a serving of whatever (oreos, for instance)?
|
[
"Realistically, you just determine the protein, carbohydrate and fat content (or know that content for all the ingredients and calculate from there) and, from the known energy contents of each of those, calculate the total energy content.",
"As for how those values are initially obtained, you literally do that by burning it (in a device called a calorimeter), and measuring how much heat is put out (by measuring how much is heats up a known amount of water)."
] |
[
"I remember using a calorimeter to measure kilocalories in a chemistry lab in school. Burn a sample and measure the generated heat. I don't think that is how calories are calculated for food labels. The following link that describes the Atwater system. The components (protein, carbs, and fat) are measured and calories calculated (4,4,and 9 respectively).",
"http://www.scientificamerican.com/article/how-do-food-manufacturers/",
"That article has a link to the usda (which doesn't work). I'm curious too, i'll keep looking. The next question I have is how the macronutrients are measured. ",
"This seems to be the authoritative source ",
"https://ndb.nal.usda.gov"
] |
[
"Awesome, thanks!"
] |
[
"What would happen if you remove the electrons from a sand grain and put them inside a second sand grain and release them at some distance from each other?"
] |
[
false
] | null |
[
"The charge must be considerably more than 1.3 x 10",
" C, as even one electron is 1.6 x 10",
" C. The molecular mass of silicon dioxide is 60g/mol, so here we have 8.3 x 10",
" moles, which is still 5.0x10",
" molecules.",
"I think the charge should then be 5.0x10",
" x 1.6x10",
" C = 8.0x10",
" C.",
"Now the force is actually very large. At 1cm, it's about 6x10",
" N. For a grain of mass 10",
" kg, this translates to an enormous acceleration: a = 6x10",
" ms",
" , or about 1 trillion times the acceleration due to gravity.",
"EDIT: This was referring to the case where each molecule loses one electron. It wasn't clear from the question how many electrons are being transferred from one grain to the other."
] |
[
"Ah I see what happened if you type \"electron charge per mole\" into google it gives you nonsense.",
"This will give you the correct charge: \n",
"https://www.google.com/search?q=electron+charge&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-beta#hl=en&client=firefox-beta&tbo=d&rls=org.mozilla:en-US%3Aofficial&sclient=psy-ab&q=electron+charge+*+6.02*10^23+*+%28500+*+10^-9+g%29+%2F+60g&oq=electron+charge+*+6.02*10^23+*+%28500+*+10^-9+g%29+%2F+60g&gs_l=serp.3...58602.58602.10.58841.1.1.0.0.0.0.54.54.1.1.0.les%3B..0.0...1c.1.X_LJLiSUFxU&pbx=1&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&bvm=bv.1355325884,d.cGE&fp=b51e90b0395185a7&bpcl=39967673&biw=1680&bih=960",
"And this is the correct force: \n",
"https://www.google.com/search?q=electron+charge&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-beta#hl=en&sugexp=les%3B&gs_nf=3&gs_rn=1&gs_ri=serp&gs_mss=%28electron%20charge%20*%206.02*10^23%20*%20%28500%20*%2010^-9%20g%29%20%2F%2060g%29^&tok=zFGtKdNUHIBkS2TPuqz4gw&pq=%28electron%20charge%20*%206.02*10^23%20*%20%28500%20*%2010^-9%20g%29%20%2F%2060g%29^2%20%2F%20%281%20cm%29^2&cp=91&gs_id=nl4&xhr=t&q=%28electron%20charge%20*%206.02*10^23%20*%20%28500%20*%2010^-9%20g%29%20%2F%2060g%29^2%20%2F%20%281%20cm%29^2%20*%208.98*10^9%20N%20m^2%20%2F%20C^2&pf=p&client=firefox-beta&tbo=d&rls=org.mozilla:en-US%3Aofficial&sclient=psy-ab&oq=%28electron+charge+*+6.02*10^23+*+%28500+*+10^-9+g%29+/+60g%29^2+/+%281+cm%29^2+*+8.98*10^9+N+m^2+/+C^2&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&bvm=bv.1355325884,d.cGE&fp=b51e90b0395185a7&bpcl=39967673&biw=1680&bih=960"
] |
[
"For amusement I throw in the gravity between those two grains of sand and it's 1.7 × 10",
" newtons"
] |
[
"Did NASA really just say they performed an apparently successful test of a reactionless drive?"
] |
[
false
] |
and its comments suggest that NASA engineers tested some kind of reactionless thrust device with positive results. From the comments I was led to , but I can't penetrate the jargon well enough to get a clear idea of what it's saying, nor find the full paper. Commenters and I found say that the NASA device is kind of like something called the EmDrive, the description and for which come across like so much snake oil to me. Can someone with a better understanding explain if the thing NASA's talking about is really a reactionless drive? And are they really saying they got a test result indicating such a thing might work? Or is this just another case of misinterpretation and overstatement?
|
[
"Effectively, yes. Everyone is still relatively skeptical, as is natural, but basically, three independent agencies, one of which is NASA, have tested one of two variants on this drive. The one China and Argentina (I believe) is the EmDrive, and NASA tested a version called the Cannae Drive. The one NASA tested seems to have generated less thrust than the EmDrive due to its design, but they say their preliminary testing has indeed detected thrust.",
"\nBasically, it amounts to this: NASA has done basic testing and determined that the results show there is something there worth investigating further, that it's not snake oil yet. A lot more testing has to be done, and you won't be seeing any satellites or probes with this drive anytime soon. "
] |
[
"You're right they were careful, but the control they used suggests some kind of mistake is reasonably likely. They used a disabled version of the drive to ensure that they would measure 0 thrust, but they actually still got thrust. Generally speaking, if your control experiment gives the same result as your test experiment, the experiment is considered a failure until you can figure that out. In this case, maybe a lack of understanding of the physics behind the device means that they didn't actually disable it, but its hard to say for now. Really, I don't know why they would go public with results until they can get a null result from their control."
] |
[
"You could not be more wrong. The lack of propellant used is such a vast advantage, such an incredible prospect that it would utterly revolutionise the space industry.",
"You totally underestimate the limits placed on spacecraft by having to carry fuel. "
] |
[
"[Physics][Astronomy] Is there any property of space which enables it to \"remember\" its previous configuration?"
] |
[
false
] |
This is my thinking process: Consider a room. It has objects; tables, chairs and so. Since these objects have mass, they cause a very small dent in space-time. Now, if these objects are removed from the room, the space-time will come back to its original state; that is, it will no longer be curved/distroted. Does this happen immediately? Or does it have some sort of latency. Is it possible to theoretically observe the configuration of space-time just after the objects were removed & conclude that "an object of this shape was positioned here a while ago". Thanks!
|
[
"Gravity moves at the speed of light, so space-time will restore itself pretty much instantly. Gravitational radiation ",
"http://en.m.wikipedia.org/wiki/Gravitational_wave",
" might allow us to capture the potential impact that an object had on space-time though. "
] |
[
"Gravity propogates at the speed of light. When we refer to the speed of gravity, we are essentially talking about gravity waves that travel at the speed of light.",
"Source"
] |
[
"C isn't technically the \"speed of light\", it's actually the speed of massless particles.",
"\"Thus, the speed of \"light\" is also the speed of gravitational waves and any other massless particle.\" ",
"http://en.wikipedia.org/wiki/Speed_of_gravity"
] |
[
"Can polar and nonpolar molecules ever mix?"
] |
[
false
] |
What if polar and nonpolar fluids had the exact same specific densities? Is there anything keeping them from mixing?
|
[
"When two liquid phases, A and B, are in equilibrium (even if they are immiscible) there will some A in the B phase and some B in the A phase. A common measure of this phenomenon is the partition coefficient ",
"http://en.wikipedia.org/wiki/Partition_coefficient"
] |
[
"There is one product which you may have seen if you've ever looked at the ingredients in Mountain Dew. Brominated vegetable oil is used to tune the density of citrus flavors so they are the same density as the water. They form an emulsion and prevent an oily layer from forming.",
"http://en.wikipedia.org/wiki/Brominated_vegetable_oil",
"Although if you have a pretty large concentration that substances can't dissolve, then what will probably happen is you'll have a drop floating in the middle of the other liquid.",
"https://www.youtube.com/watch?v=MUKFaxy-z7s"
] |
[
"Yes, but for all purposes the non-polar tails of soaps don't interact with the water. The soap molecules form micelles (an aggregate of soap molecules forming a spherical shell) with the tails facing into the shell and the polar heads forming the surface of the shell and interacting with the polar water. ",
"That's how soap gets grease and stuff off. The grease is pulled into the micelle to interact with the non-polar tails and washes away with the water."
] |
[
"Would the sun's core be pitch black as it is too hot for the visible spectrum?"
] |
[
false
] | null |
[
"No. The distribution of the radiation wavelengths will vary according to temperature (with all wavelengths increasing in intensity as a black body becomes hotter), and any part of the sun undergoing fusion would emit radiation in the visible spectrum. ",
"http://en.wikipedia.org/wiki/Black-body_radiation#Spectrum"
] |
[
"Absolutely. Nuclear weapons achieve, for the briefest of instants, an exponentially tiny fraction of what is going on and has been going on every second since the Sun began nuclear fusion. The Sun transforms 600 million tons of hydrogen into 596 million tons of helium every second. The remaining 4 million tons are being converted into energy. Every. Second. A small nuclear weapon such as the one dropped on Hiroshima transforms something like less than half a Coke can's worth of mass into energy. And where is all this energy that the Sun is producing going? Along random paths out from the core and through the radiation zone. Thus, if you were to be able to perceive a random point in the radiation zone (called such not because there is radiation but because the energy is radiating outward from the core), it would, and I quote the book, \"be bathed in X rays trillions of times more intense than the visible light at the surface.\" I hope that answers your question. The book I am quoting is The Cosmic Perspective, Sixth Edition by Bennett, Donahue, Schneider and Voit.",
"edit: added a millions, whoops"
] |
[
"An even stronger statement can be made: A blackbody will emit ",
" light at every wavelength than a cooler blackbody.",
"The center of the sun would be much much ",
" in visible light than the surface."
] |
[
"How is energy harnessed from a fusion reactor? Is it the same principle as fission reactors or is it something entirely different, given the vast difference in temperature involved?"
] |
[
false
] | null |
[
"When you think about it, aside from wind, solar and hydro, most of our electric power plants are basically steam engines. The only thing that varies is what makes the heat to boil water."
] |
[
"When you think about it, aside from wind, solar and hydro, most of our electric power plants are basically steam engines. The only thing that varies is what makes the heat to boil water."
] |
[
"having steam turbines is in fact very beneficial for running a grid because they give builtin stabilisation measures allowing you to react fast enough when the frequency of the power grid drifts, the inertia of the rotating masses softens sudden frequency drops. photovoltaics for instance don't have these mechanisms which makes them unsuitable for feeding the a grid. "
] |
[
"Is there a cryptographic system that does not depend on a specific problem being hard to solve?"
] |
[
false
] |
[deleted]
|
[
"Yes. The one-time pad. It is a substitution cipher that hashes a message with a specific key, previously distributed between both parties, and as the name implies, that key is discarded after use. That key can be as simple or as elaborate as needed, derived from a purely mathematical arrangement or a book or anything else that can used generate a hash. All it has to be is identical on both ends."
] |
[
"It's very simple and completely unbreakable (excepting things like rubber hose cryptanalysis), but you have to distribute the keys between the communicating parties without them being intercepted, and each key can only be used once."
] |
[
"It's very simple and completely unbreakable (excepting things like rubber hose cryptanalysis), but you have to distribute the keys between the communicating parties without them being intercepted, and each key can only be used once."
] |
[
"Biologists: Is there an animal \"Y\" that can breed with \"X\" and \"Z\" but \"X\" and \"Z\" can not interbreed?"
] |
[
false
] |
This would be in relation to Evolution and Speciation. I am curious how common the blurring of the line is. I know some dogs can not interbreed because the birth is normally fatal for the mother (if the mother is the smaller breed). But is the function common on a genetic level? Like, if "Fox" could breed with "dog". "Wolf" could breed with "dog" but "Wolf" and "Fox" could not breed. (this is an illustration of what I'm asking, not a specific example I want to know about)
|
[
"Ring species",
" is what you're looking for."
] |
[
"Ring species, as bperki8 says. \nBut interbreeding is more complex: ",
"Absolutely can't interbreed (no mating, no viable offspring)",
"Won't interbreed: Don't ever get to meet, nocturnal vs diurnal, don't approve of each other's mating behaviour.",
"Can hardly interbreed (hardly ever fertile offspring (donkey/horse/zebra, risks to parent (Great Dane/chihuahua)).",
"Will hardly interbreed: rarely meet, don't appeal all that much to each other (Grizzly/polar bear)",
"Interbreed in the wild vs in a zoo...",
"Bad effects like interspecies mating killing the individual animals involved (there was a story regarding some seal that mated with females of a different species, killing them in the process)"
] |
[
"Richard Dawkins observes that ring species \"are only showing us in the spatial dimension something that must always happen in the time dimension.\"",
"That's a pleasing way to think of it."
] |
[
"Are EM waves a ripple in a universal field, or do EM waves propagate into areas where there is no EM field?"
] |
[
false
] |
Sorry if this is a dumb question, but I was reading another thread in which someone asked about whether radio waves are considered light, and one of the responses mentioned something related to the question I've asked. I'm wondering if there's an EM field that's everywhere, sort of like a "gel" that's at rest, so to speak. If that's the case, I'm imagining that an EM wave is just a ripple that makes its way through this "gel." (I use gel instead of a blanket metaphor just because I'm thinking of three-dimensional space instead of two.) Or, does EM follow a particle model in this case, in which portions of space don't have any EM "potential" in this way, and instead photons are just travelers passing through and once they leave the EM field is gone?
|
[
"This question is by no means dumb, in fact our conceptual understanding of this problem lies at the root of modern quantum field theory. The best analogy I have heard for describing fields is to visualize a field as a giant mattress with springs throughout, like ",
"this",
". In the simplest case, let us imagine that all the springs are ideal (they have no internal resistance and the force constant is linear) and independent of each other. Let's say you hit one of the springs. In the absence of any resistance, the spring would continue to bounce up and down in a periodic motion forever. In the language of field theory, we would say that at that given point, we have populated one of its higher energy modes. ",
"With this model in mind, you can think of a wave as a ripple propagating through this mattress. In fact an excitation of a spring at any given point would be interpreted as the probability of a particle being located at that point in space, which in the case of the electromagnetic field would be a photon. Note that this picture applies also to all types of particles, we can similarly construct electron fields, neutrino fields, etc, etc. "
] |
[
"Just to tack on to this, A. Zee's ",
" does an amazing job tackling the mattress/spring analogy in an understandable way for those who are interested. Very easy to find on Google, and the underpinnings of field theory are outlined very neatly in the first chapter."
] |
[
"Firstly, \"particle physics\" is a field which suffers from one of the worst cases of poor naming in science. Particle Physics (i.e. the standard model) is not a theory about particles, it's a theory about fields. We talk about electron particles and photon particles but the mathematical language is actually that of a special type of field called a quantum field. Everything is \"ripples\" in fields.",
"In some sense your question may seem akin to: If a tree falls in the forest and there is no one there to hear it does it make a sound? An electron is an excitation, a \"pulse\" or \"bump\" in the electron field which, if you like, \"permeates the universe\". Now the question is if I look at the electron field at a place where it has no bumps or pulses, where no energy has been put into it, is it still there? ",
"It turns out this isn't a philosophical question because it is actually possible for one of these field to actually not be zero even when it has no energy in it. I.e. even at \"vacuum\" it still isn't zero. The most famous example of this is the Higgs field. The Higgs field is very much like the the electron field, if I drop a certain amount of energy into it I get an excitation: The Higgs Boson or an electron respectively. The big difference is when I don't put energy in the electron field's \"natural state\" is to simply go to zero and one can ask if it is even there at all, but the HIGGS FIELD's natural state is to have some finite value. It's the interaction of particles with this universal non-zero-valued-in-its-lowest-energy-state/vacuum field that creates the effect/illusion of mass in the excitations of the other fields. The Higgs field is definitely there regardless of whether you've put energy into it. ",
"Thus I think it's \"philosophically sound\" to say that the photon field is also there even if it is currently not carrying energy in my vicinity."
] |
[
"When the James Webb telescope is complete what are we expecting to see?"
] |
[
false
] |
I know we'll be able to see farther but will we be able to see closer things with more detail? Like the surfaces of planets we think might have life.
|
[
"The main limit on resolution (the ability to differentiate two objects separated by a certain distance) is the size of the primary mirror; diffraction becomes a problem if you go too small. The James Webb telescope will have a 6.5 meter mirror compared with Hubble's 2.4 m, so ",
"this figure",
" shows that the angular resolution will be about 0.03 arcseconds to Hubble's 0.1. Three times better is a significant improvement, but it's not the many orders of magnitude we'd need to zoom in on extrasolar planets.",
"Edit: Explanation of the figure copied from my reply to a deleted comment below:",
"It takes a minute to figure out what's going on, but it can answer most questions about telescopes, microscopes, cameras, eyes, etc. Start with the aperture size on the left, follow it across until you get to the wavelength you're interested in, then look down to see the resolution.",
"Angular resolution is dependent on wavelength (which we interpret as color for visual light), and on the size of the aperture... Larger aperture gives better resolution, as does shorter wavelength. The same applies to sonars."
] |
[
"Assuming an earth-radius planet at a distance of 10 parsecs, about 30 kilometers. Not to fear, we can synthesize a telescope with an aperture of 30 kilometers using interferometry - in fact that's sort of what the TPF was supposed to do... before it got cancelled."
] |
[
"Nope. You can thank the politicians (and NASA's tiny budget - they can barely afford JWST!) for that one."
] |
[
"On Apollo 13, it mattered what order they turned systems on in - why?"
] |
[
false
] |
On Apollo 13, some time was given to making sure that spacecraft systems were turned on in the correct order. Specifically, I was reading which discusses how if systems had been turned on in the wrong order, there would have been insufficient power. Can somebody explain this to me? Why does the power drawn depend on the order systems were switched on in?
|
[
"Check out ",
"this article",
", part of the series of articles cited in the Wikipedia article you provided above.",
"But even with fully charged batteries, the Odyssey risked running out of electricity before it splashed down. Batteries are rated using a term called ampere-hours. If you start with a 40 amp-hour re-entry battery, and then turn on a piece of equipment that uses 1 amp-hour, and it takes 8 hours to finish the re-entry and splashdown, you have only 32 amp-hours left to power everything else. But if you can delay turning on that piece of equipment until 2 hours before splashdown, now you have 38 amp-hours to go around. ",
" says Aaron. Once a system had been turned on in the Odyssey, it had to stay on, so \"the only variable was how few systems could we turn on and how late could we wait?\" he explains.",
"Emphasis added. It's not that power drawn is different, but that you're turning it on later so it draws the power for a shorter duration."
] |
[
"Sorry I realize this might not be your area of expertise but do you know why \"once a system had been turned on in the Odyssey, it had to stay on\" ? Why not turn it on for a quick minute then off until you need it again?"
] |
[
"It Was Now over three days since the explosion in oxygen tank two. It was time to get ready for re-entry. The first step was to recharge the batteries in the command module, which had been significantly depleted before the lunar module came on line.",
"Remember how, while figuring out lunar module lifeboat procedures after the Apollo 10 simulation, Legler had worked out a way to run power from the lunar module to the command module back along the electrical umbilicals that connected the spacecraft? That was about to come in handy now, because that power could be used to recharge the Odyssey's batteries.",
"\"The biggest problem was that initially the lunar module guys didn't know how much power they were going to need\" for the Aquarius to serve its role as a lifeboat, remembers Aaron. For the first 30 hours, Aaron's power-up team didn't think the lunar module guys were going to have any power to spare for the Odyssey: about twelve hours after the explosion, \"we talked to them about getting some power,\" says Aaron. \"They threw us out of the room.\"",
"But the PC+2 burn had shortened Apollo 13's return flight sufficiently that the Aquarius would be able to supply the power needed to charge the batteries. Working with North American Aviation and Grumman, through lunar module gurus Hannigan and Mel Brooks in the SPAN room, to refine the procedure, Legler and Bill Peters wrote up the needed instructions. The charging process was \"only 20 to 25 percent efficient,\" remembers Legler, but it was enough.",
"But even with fully charged batteries, the Odyssey risked running out of electricity before it splashed down. Batteries are rated using a term called ampere-hours. If you start with a 40 amp-hour re-entry battery, and then turn on a piece of equipment that uses 1 amp-hour, and it takes 8 hours to finish the re-entry and splashdown, you have only 32 amp-hours left to power everything else. But if you can delay turning on that piece of equipment until 2 hours before splashdown, now you have 38 amp-hours to go around. \"It's not only a matter of how large a load is, but how long that load is on for,\" says Aaron. Once a system had been turned on in the Odyssey, it had to stay on, so \"the only variable was how few systems could we turn on and how late could we wait?\" he explains.",
"Aaron had an inspiration. Normally in a spaceship power-up sequence, one of the first things turned on is the instrumentation system so everyone can be sure that the rest of the sequence is progressing normally. But for Apollo 13, the instrumentation would be turned on last for a final check of the Odyssey just before re-entry began.",
"Source",
"Figured I'd take a stab at it since it's been 8 hours without a response. My best guess was that there had to be some kind of charging or power transfer done first, and that the movie was embellished during this scene. ",
"Simulating the startup: In the movie, Mattingly spends hours in a simulator putting together the procedures for starting up the cold, dead command module in time to bring the astronauts safely back to Earth. While that is a good way of conveying the mission’s aim to the public, the simulation runs (done by other astronauts, Mattingly said) were more of a verification of already written procedures.",
"We said, “Let’s get somebody cold to go run the procedures.” So I think it was [Thomas P.] Stafford, [Joe H.] Engle — I don’t know who was the third person, might have been [Stuart A.] Roosa. But anyhow, they went to the simulator there at JSC [Johnson Space Center], and we handed them these big written procedures and said, “Here. We’re going to call these out to you, and we want you to go through, just like Jack will. We’ll read it up to you. See if there are nomenclatures that we have made confusing or whatever. Just wring it out. See if there’s anything in the process that doesn’t work.”",
"Source",
"At this point the question is how possible is this in real life (starting various systems in a specific order with the purpose of changing total power usage), so I talked to a buddy of mine who is an electrician. Upon startup, many systems require a large amount of power, which then levels off as the system begins running. Think of an electric engine; it takes more power to get it spinning than it does to keep it spinning. You may see an initial draw if 10amps as it starts, then it drops to 5 amps continuous to keep it going. if you have two electrical motors running in series (not saying this was abourd the spacecraft, just using it as an example) than powering up the first motor (12amp initial) can help spin the second, so then you apply power to the second motor (5amp continuous) and you've saved power overall starting both. (note: starting the first motor takes 12amps because of the additional mass of spinning the second as well, alone it would have taken 10amps+10 amps to start both) ",
"There is also a hurdle in the fact that some systems may be wired in parallel vs in series. Think of christmas lights, which have a plug on each end for more lights to be plugged in. The first strand has a plug at the beginning (B) and also a plug at the end (E). If you plug in a second strand of lights to (B), they are wired in parallel, where if you plug them into (E), then they are wired in series. Since the power has to travel through the first strand to get to the second strand when running in series (E), power has to be applied to the first system (main strand) before power can get to the second strand in series. If that circuit is broken, power fails to make it to the rest of the lights. On board Apollo 13, I'm sure some systems were wired in series so certain systems had to be powered on before others so that the second system in line could get power. ",
"What it all seems to boil down to however is the fact that they had to keep everything shut down for as long as possible to avoid draining the batteries. They went through a fairly well known startup procedure, but only starting a system when it was absolutely necessary. "
] |
[
"Does Quantum Mechanics really say there's a universe where Elvis didn't die?"
] |
[
false
] |
This has bothered me for a long time. I thought that whole interpretation was about different states of atoms, not macroscopic events but I see people say a variation of that variation all of the time
|
[
" ",
"I'll try to give you a fair introduction to the Copenhagen interpretation and Many Worlds - others are free to disagree with me though. I'm sorry for how long this wall of text became.",
"The idea that this is hinting at is a thing called quantum immortality, which is a sort of modified Schrodinger's cat problem. In the Schrodinger's cat paradox, there's a cat in a closed box with the radioactive sample and a poison, and if the radioactive sample decays it sets off a detector which breaks open the poison, killing the cat. The idea is that the radioactive sample is in a superposition of decayed and not decayed, and until it's observed you don't know which state it's in (or in the Copenhagen interpretation, the observation ",
" it to a definite state). Since the box is closed you can't observe the system, so maybe the cat is entangled with the radioactive sample, so the cat is in a superposition of dead or alive until you observe it. ",
"Anyway, it's not meant to seriously argue that the cat is in a superposition - that's nuts. Schrodinger's cat is meant to ",
" the notion of the Copenhagen interpretation (which allows a quantum system to be in multiple states, which are collapsed into definite states by observation). ",
"So if you don't like Copenhagen, what's the alternative? Well, the first one that comes to mind is Many Worlds. The Many Worlds Interpretation argues that every time an observation would occur, rather than collapsing the state (like in Copenhagen) multiple new universes are spawned, one for each state available. If your particle is in a mixture of states A, B, and C, and if your measurement observes B then you are now in the universe where B happened, and there are two others where A and C happened. ",
" \"Observer\" and \"observation\" don't have anything to do with consciousness, and the people who claim that are making shit up. An 'observation' is better described as an 'interaction' in quantum mechanics, like bouncing a photon off of an electron. The photon is the 'observer' - it's not magic, it's just a weird naming convention that stuck.",
"'Quantum immortality' implies that for every quantum mechanical event like this a universe is spun off, so in the case of Schrodinger's cat it would imply that there will always exist a universe where the cat doesn't die (if you repeat the experiment many times), even if the probability of survival in the Copenhagen interpretation approaches zero. ",
"So just replace \"Schrodinger's cat\" with \"Elvis\" and it should be clear where people get the idea that \"Quantum mechanics says there's a universe where Elvis is alive!.\" In fact, it's crazier than that - MWI could imply that there are ",
" universes where Elvis is still alive, among tons of other alternate histories. Just imagine every little event that ever conspired to kill Elvis happening exactly in the opposite way (at some kind of atomic level?) and perhaps you could justify the argument that Many Worlds would allow him to still be alive.",
"That said, I don't think there's a problem with the Copenhagen interpretation in the Schrodinger's cat problem. Clearly the detector in the box is the thing making the observation, and it's collapsing the radioactive particle's state. The cat is never in a superposition. Many-Worlds is just chocked full of problems like... how the fuck do you confirm it empirically? But I'll spare you a rant about why I dislike MWI. ",
"In general, people use MWI as the basis of a lot of made up stuff, like \"Every possible universe exists!\" without qualifying the word 'possible.' Even if there are an infinite number of universes spawned by MWI, that doesn't mean literally anything you make up has it's own universe. I once heard the example \"There are an infinite amount of numbers between 0 and 1, but 2 is not one of them.\" Kinda like that. ",
"Anyway, quantum immortality is just meant as a thought experiment. No physicists are doing serious calculations about alternate reality Elvisses (Elvi? Elvodes?), or experimeting on cats in boxes for that matter. Personally, I find quantum immortality absurd and I think it illustrates exactly what's so bad about MWI in the same way that Schrodinger's cat was meant to illustrate what's wrong with Copenhagen, but that's just my interpretation. ",
"In fairness, MWI is second in popularity only to Copenhagen, but I think MWI is far more speculative and fails Occam's razor (don't make unnecessary assumptions!), and if MWI is speculative, you can be sure that quantum immortality is a double dose. ",
"If you followed none of that, then here's a take-away. As a general rule if someone says something that sounds like utter magic about something other than a subatomic particle and then tries to justify it with ",
" they are usually just talking out of their ass."
] |
[
"You can't confirm Copenhagen empirically either, as far as we know. That's why we call it and MWI \"interpretations\". "
] |
[
"It just isn't true that \"timeline-splitting\" in MWI is an \"ad hoc\" method of solving the problem of measurement. That's kind of the whole point: MWI is just unitary QM. If you think the MWI posits that worlds magically start \"splitting\" in order to solve the measurement problem, then you don't understand the MWI. The MWI is just the observation that we don't ",
" any ad-hoc explanation for wave function collapse, because it is ",
" with unitary evolution of a universal wave function. The point is that any additional \"collapse postulate\" is at best redundant and at worst logically inconsistent. Now, that doesn't mean the MWI is true or that it doesn't have problems, even some of which may be rightly claimed to be ad hoc -- and there are other perfectly respectable interpretations -- but you have to understand what the MWI is and is not, and the aspects of it that you have described are simply not ad hoc ",
"."
] |
[
"When I'm walking or running, how is it that what I see isn't all shaky like when I try the same thing with a video camera?"
] |
[
false
] |
I just bought a steadicam for my DSLR and it got me thinking. How is it that the body acts like a natural steadicam for my eyes? ELI5 if necessary.
|
[
"Would this effect be compromised in zero gravity?"
] |
[
"Would this effect be compromised in zero gravity?"
] |
[
"Cameras with \"",
"image stabilization",
"\" often do something that's analogous to this. (Compensating for movement via an accelerometer, not saccades.)"
] |
[
"Do train tracks build up an electrical charge when a train approaches?"
] |
[
false
] | null |
[
"You can achieve the same affect by putting a metal bar across both rails",
"Could you do the same thing with a simple insulated wire? It seems like that would leave every railroad crossing vulnerable to pranksters. It's a good thing I didn't hear about this when I was a teenager!"
] |
[
"You can achieve the same affect by putting a metal bar across both rails",
"Could you do the same thing with a simple insulated wire? It seems like that would leave every railroad crossing vulnerable to pranksters. It's a good thing I didn't hear about this when I was a teenager!"
] |
[
"If it is powered by overhead electric lines, there would be some voltage on the rails in between grounding points, but it wouldn't be between the rails. It would be from rails to ground. I don't know if that would be enough to shock you. "
] |
[
"For animals that live closely with humans, do we know of any specific adaptions they have undergone in response to eating human's food."
] |
[
false
] |
Also, can we infer probable changes in our own physiology by looking at the adaption of animals like rats who presumably would adapt faster owing to their quicker reproduction and shorter lives.
|
[
"Coincidentally, ",
"this research",
" article was just published last month about genetic variation between wolves and dogs. Dogs had many more copies of the gene for amylase (30 to a wolf's 2), which means they have adapted to a more starch-rich diet."
] |
[
"Do you know of a primary source for this? I heard the buzz about it after it was presented at one of the big evolution conferences in 2011, but even now all I can find is buzz."
] |
[
"Do you know of a primary source for this? I heard the buzz about it after it was presented at one of the big evolution conferences in 2011, but even now all I can find is buzz."
] |
[
"Does the pavement compound of a road depend on the climate of its area? e.g. Would there be a different compound in a Michigan road vs. one in Florida?"
] |
[
false
] |
Thanks!
|
[
"Hot mix Asphaltic concrete (HMAC) or asphalt consists of aggregate and a binder. Binders are petroleum based and sort of glue your rocks together much like cement in concrete. The grade of petroleum binder is determined in part by the climate. If you pick an asphalt binder that is too soft for a warm climate, you will get rutting, shoving and other failures. Similarly, if you pick a binder that is too thick for a cooler climate, you will have a brittle mix that can crack and pothole. Other considerations are the traffic loading, road speed, and subgrade support. Further reading from txdot: ",
"http://onlinemanuals.txdot.gov/txdotmanuals/pdm/performance_graded_binders_pg_binders.htm",
"On the other hand, if you are referring to concrete paving, typically air is entrained to increase flexibility of concrete to protect from freeze-thaw cycling in colder climates. Many times down here in Texas, entrained air isn't much of a requirement due to the warmer climate. (Interesting side story, the Romans would put animal blood into their concrete to help entrain air to increase workability and flexibility.)",
"Source: civil (geotechnical) engineer EIT in Texas."
] |
[
"Yes. After working with Highway Engineers, as a non engineer (In CA) they practically wax poetic about 'road mix'. ",
"In CA you have to design for desert, mountain snow, real extremes in temp. So the mix of rock, oil, etc. varies by temperature. Some mixes are so temp sensitive, that they can only be 'laid' if the temperature is at a certain range. ",
"Many times, paving ops would be called off: too hot or too cold."
] |
[
"I don't suppose there's any type of asphalt that can withstand years of freezing and heat? I've wondered like OP and many folk in MI why they have only two driving seasons: winter and construction..."
] |
[
"What makes certain liquids flammable and some not?"
] |
[
false
] |
I know nothing about this question, honestly.
|
[
"Volatility is a part of it, but another is whether the vapour can combine with oxygen present in the air to form a more stable compound (such as carbon dioxide and water), releasing heat in the process. Low molecular-weight hydrocarbons (such as butane and propane) vaporize easily (have high ",
" at ambient temperatures) and can combine chemically with atmospheric oxygen, releasing heat. The heat released in the vicinity of the liquid drives further vaporization (a function of its ",
"), which supplies fuel to the flame."
] |
[
"Even if the vapor pressure of water were equal of that to the atmosphere (think: boiling water), it would still not be flammable. You can't light steam on fire (with the exception of superheated steam, but even then its flammability is not due to its volatility).",
"If the substance is capable of undergoing ",
"combustion",
" (as atchemey linked to), and if that substance is able to be subject to an environment with enough oxidant around, then it will be flammable. Given that water is the product of combustion, it will generally force the reaction towards extinction."
] |
[
"Flames come about (from most fluids you would think of) because of [combustion](Wikipedia.org/wiki/ combustion).",
"More generally, flames can come about when energy is released by a reaction (that is: it is an exothermic reaction). Take cryogenic rocket fuels, for example. Liquid oxygen and liquid hydrogen are mixed and react to form water, releasing large amounts of energy, including some in the form of flames. Or you can talk about unsymmetrical dimethylhydrazine and nitrogen tetroxide, which react on contact with each other (are \"hyperbolic\"). ",
"Tl;Dr: energy is given off, sometimes flames are a part.",
"Apologies if errors. Late night, lots of calc studying, not much chemistry today:/"
] |
[
"Is gravitational bending of light wavelength-dependent?"
] |
[
false
] |
Obviously it is dependent on the mass of the object bending the straight path of light. But does the bending depend on the wavelength of the light? (Ie. is there something like chromatic aberration in optical lenses when looking at gravitational lensing?). Also, does gravitational lensing itself cause an additional redshift effect? (Which I would expext as the length of the path of light is lengthened/stretched). (I wasn't quite sure whether this eas better flaired Physics or Astronomy; mods having experience, please feel free to reassign). Thanks for your insightful answers!
|
[
"No, because gravity is curvature of space time, and anything moving along the same trajectory at the same velocity will take the same path, as long as we can assume that the object's own contribution to curvature is negligible."
] |
[
"Thank you for your answrr, howerver I consider this tautological, as your argument cones from a model (which is necessarily based on assumptions).",
"I rather asked about observational evidence.",
"Wrt my second question, I seem to have found the answer of Shapiro Delay, ergo a minuscule redshift.",
"However, doesn't gravity (in heneral relativity too) effect relativistic mass, rather than rest mass, and such shouldn't different wavelength light rays be bent to a different amount by gravity?"
] |
[
"It is a local statement, there is no difference between the two in vacuum.",
"In matter you get a phase velocity, a group velocity and a signal propagation velocity, in general they can all be different and the first two can even be faster than the speed of light in vacuum, only the last one is limited by the speed of light in vacuum."
] |
[
"Do plants ever get \"full\" of sunlight/water? If not, how do they store their surplus sunlight/water?"
] |
[
false
] |
AFAIK most plants don't have fat reserves like animals, so how do they store excess energy? And as for excess water, do plants have some process analogous to urination?
|
[
"Plants store energy as sugars and starches."
] |
[
"Plants constantly lose water through their leaves via transpiration (evaporation through pores). I believe they just don't absorb more water than they transpire, so they generally don't wind up with an excess. "
] |
[
"I believe they just don't absorb more water than they transpire",
"I am not a biologist, but they should take in more water than they transpire. The naive reaction going on inside the leaves is",
"H2O + CO2 + Light -> CXH + O2",
"Some water should go in building the sugar/starch and the expelled O2",
"/edit: The actual reaction is more complex (else, why would they need the clorophyl) and has 2 stages from what I remember."
] |
[
"How accurate is the due date of a child?"
] |
[
false
] |
[deleted]
|
[
"There are a few ways an OB/Gyn will estimate due date and date of conception. The first way is by dates. On an OB floor are things called ",
"pregnancy wheels",
". You ask the mother the first day of her last menstrual period and then line up the wheel, but the wheel follows ",
"Naegele's rule",
". This is called gestational age and due date is at 40 weeks.",
"A second method is by using a ",
"obstetric ultrasound",
" and can be used to measure a ",
"crown-rump length",
" and compare that to the gestational age by dates. The crown-rump length can also be compared to a standard. This is done in the first trimester. Also in the first trimester you can measure the gestational sac, the yolk sac, or assess cardiac activity.",
"In the second and third trimester you can measure biparietal diameter, cephalic index, head circumference, abdominal circumference, or femur length on ultrasound.",
"Not only can the OB/Gyn estimate date of delivery with these measurements, but an outlier measurement is followed up and investigated. Perhaps there is a fetal abnormality that can be treated, etc.",
"Probably more information than you expected, but do you have any questions about one of the particulars?"
] |
[
"Nope, no real questions. This is my first child (the reason I was asking this question in the first place), so my curiosity has been nagging me and I haven't bothered to ask the OB/GYN any of this. "
] |
[
"It's just a estimate.",
"http://transitiontoparenthood.com/ttp/parented/pregnancy/duedate.htm"
] |
[
"Could there be bacteria in landfills right now evolving to digest plastics?"
] |
[
false
] |
We've all heard the facts about plastic water bottles, that they take many thousands of years to degrade, etc. But given that the ability to metabolize ubiquitous plastic products would confer a distinct evolutionary advantage to any organism that could do so, is it possible that such a bacterium could evolve to do so within a shorter amount of time than the physical degradation rate of plastics? Similarly, could scientists direct the evolution of these organisms by, for example, placing a bacteria culture in a dish in the presence of polymers with enough natural media to get a sufficiently large level of variation, so that once the natural nutrients are expended the bacteria will have to evolve to digest the polymers to survive? EDIT: Is this process akin to what happened when plants first started producing cellulose? And if this evolutionary feature does not happen, will the plastics that make their way into the earth's crust turn back into oil?
|
[
"There are already bacteria which can eat some plastics. A nylon eating bacteria was discovered in 1975 and is one of the best examples of evolution in action\nEdit: It actually digests one of the byproducts of nylon manufacture",
"https://en.wikipedia.org/wiki/Nylon-eating_bacteria",
"And there is good evidence of polyethylene digestion by bacteria too.",
"http://pubs.acs.org/doi/abs/10.1021/es504038a",
"So it's quite probable that more microbes will evolve plastic digesting abilities for a wider range of plastics. Possibly some of these already exist at the bottom of landfills.",
"Personally I don't doubt that we could direct the evolution of microbes to digest further plastics. The difficulty comes identifying which plastic will be amenable to enzymatic digestion and which microbes already have enzymes which with a little evolution will be able to accomplish such a task."
] |
[
"But can it break down polymers in order to digest them? Oil would be in the form of long carbon chains, but probably not on the same scale of length as a polymerized plastic, right?"
] |
[
"It has been a few years since my course, so the specifics of its functions have left me. Granted, synthesized plastic is much more complex than crude oil, but we discussed how the microbe could still break it down. I do remember that my prof stated that the reason landfills are the size that they are is because it is an aerobic microbe, and that there isn't enough Oxygen for it in regards to the buried garbage. This means the buried garbage doesn't decompose. "
] |
[
"When a photon is blocked by a filter, say, a polarized filter, what happens to those blocked? Are they reflected or?"
] |
[
false
] |
Thought of this when thinking about the whole Bell's theorem thingy.
|
[
"Photons that don't pass through a filter are typically absorbed by the filter material. This causes the filter to heat up (slightly)."
] |
[
"One exception is a dichroic filter. They pass some wavelengths and reflect others. They use thin film interference."
] |
[
"If it's 'blocked', there are just 2 options: the photon is absorbed or the photon is scattered.",
"'Absorbed' means that the photon seizes to exist and its energy transferred to another particle.",
"'Scattered' means that it will be reflected to another direction and it might, or not, lose part of its energy to the surface it hit (inelastic vs. elastic scattering)",
"Each time the photon loses energy (absorption or inelastic scattering), it will do so by increasing the energy of the other particle. In practice, for most daily applications, you can think that the particles will be electrons (although we could talk about atom nuclei) and the increase of the energy means an increase in temperature, or simply raising the electron into another energy level from which it will go back to the lower energy level by emitting another photon, or a combination of photon + increase of temperature. You might read emission of 'phonons', which are pseudo particles that basically hold the vibrational energy. There is aso the possibility that you ionize atoms if the absorbed photon is highly energetic (x-ray and up), which basically means that the electron is ripped out from the atom."
] |
[
"Where does matter sit on the energy spectrum?"
] |
[
false
] | null |
[
"Hi ComeWatchTVSummer thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s):",
"This question is based on fundamentally flawed premises. Please conduct some background research and revise your question if you wish to resubmit.",
"Matter is not the same thing as light. You can't put them both in the same graph. This is a bit like asking where a car fits on a chart of apples.",
"If you disagree with this decision, please send a ",
"message to the moderators."
] |
[
"thanks for nothing"
] |
[
"Is there any area of science that a committed amateur with a few hundreds worth of equipment might make a worthwhile contribution?"
] |
[
false
] | null |
[
"Over the years many amateur astronomers have discovered things which have remained unnoticed by \"professionals\"."
] |
[
"In Computer Science, you can make discoveries/advancements for the cost of a used laptop/desktop and several years of study.",
"I might suggest the specific fields of computer vision or artificial intelligence.",
"Source: Undergraduate in Computer Science"
] |
[
"Mathematics. You need a pencil and a piece of paper."
] |
[
"Why does muscular dystrophy only happen in certain muscle groups?"
] |
[
true
] |
[deleted]
|
[
"There are two actual reasons.",
"The first is that certain muscles need to be very bulky and strong for them to be useful. These tend to be things like your hip extensors. If you lost 50% of the strength in your hip extensors, you probably could not stand from a seated position. You would have to use your arms to assist (this is called Gower's sign). Things like muscles in the hands or face can be weaker and still work decently well.",
"The other thing is that there is differential expression of genes in the body. Muscles in your face/mouth express some proteins differently than muscles in your legs, since those muscles have different properties and different purposes. Most forms of genetic muscular dystrophy are defects in structural proteins in muscle cells, which are present to varying degrees in different muscle groups."
] |
[
"It's that genes are variable expressed. Certain muscles may express dystrophin more than other muscles. This is likely influenced by signalling during early development and by strain placed on the muscles after birth. Sorry that I don't have any specifics."
] |
[
"It's that genes are variable expressed. Certain muscles may express dystrophin more than other muscles. This is likely influenced by signalling during early development and by strain placed on the muscles after birth. Sorry that I don't have any specifics."
] |
[
"What is the explanation for this flat earther claim?"
] |
[
false
] |
[deleted]
|
[
"The airplane is spinning around with the Earth. The 500 mph is with respect to the air.",
"Consider what is happening from the perspective of an inertial observer. The plane is already rotating at 1000 mph when it's on the ground. Then when it takes off, it has that rotational momentum. So the inertial observer sees it rotating at 1500 mph (well, assuming it travels with the rotation). This is the same reason why, when you jump, the Earth doesn't just fly past underneath you at 1000 mph. "
] |
[
"If there were absolutely no wind, then east-west and west-east flight times would be the same. That's because the speed of the airplane with respect to the air (which, with no wind, is also the speed with respect to the ground) is the same no matter what direction the airplane travels.",
"The difference in travel times is caused by headwinds (traveling against wind) and tailwinds (traveling with the wind)."
] |
[
"Shouldn't flying west be more energy efficient? Or is it? "
] |
[
"On what day(s) of the year is the ray from the Sun to the Earth pointing in the same direction as the Sun's (instantaneous) velocity around the galactic center?"
] |
[
false
] |
I'm trying to get a better sense of our solar neighborhood, and finding out which "direction" the sun is moving in as it orbits the galactic center would be helpful. Perhaps the ecliptic is aligned so that the Earth is never directly ahead of the Sun as the Sun orbits the galactic center--if so, then what time of year is the Earth "ahead of" the Sun?
|
[
"(Not remotely my area.)",
"I would guess the answer is probably \"it never is\", because there's an angle of ",
"about 60 degrees",
" between the plane of the ecliptic (in which the Earth orbits) and the galactic plane (in which the Sun orbits, approximately). Perhaps ",
"this picture",
" will help?"
] |
[
"Thank you. I didn't realize the angle between the ecliptic and the galactic plane is so big--I guess the angle of the milky way in the sky should have tipped me off."
] |
[
"I just did some googling and a couple of quick calculations, and here's what I found:",
"One of the points at which the ecliptic and the Galactic plane intersect is only 12 degrees (roughly) from the Galactic center. The other point is correspondingly about 12 degrees from the Galactic anti-center.\nAs was suggested by luchack, the ecliptic and the Sun's direction of motion don't line up.",
"I couldn't (quickly) find any concrete numbers for the average motion of the stars in the Sun's environment (the so-called Local Standard of Rest). If I assume a circular orbit about the Galactic center, the direction of that motion is in the constellation Vela, in the southern hemisphere. (Assuming I worked that out right.)\nAlso, according to Wikipedia, the motion of the Sun ",
" to the nearby stars/LSR is the direction of Vega, in the constellation Hercules.",
"I didn't know that before. Thank you for making me think about that."
] |
[
"If the solar system were to survive Andromeda–Milky Way collision and flung into deep space, what effect would it have on the solar system if it were not part of a galaxy?"
] |
[
false
] |
Assuming the Solar system is intact when the collision happens and comes out unscathed. Would the solar sytem existing out of a galaxy have any major changes?
|
[
"There would be no difference except for the sky at night, which would be dark."
] |
[
"dark"
] |
[
"This event has a high probability (from 12% to 50%) but it will have no impact on earth or the solar system.",
"Sources : ",
"http://www.newscientist.com/article/dn11852",
"http://www.universetoday.com/1604/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/",
"http://fr.arxiv.org/abs/0705.1170"
] |
[
"With chirality, why are the properties of R and S forms so different?"
] |
[
false
] |
I'm trying to wrap my head around the idea of chirality. I am getting there in terms of the understanding of identifying chirality through the whole left hand-right hand and mirror image explanations. But in terms of the R/S forms, why do they have such drastic difference in properties - such as taste and smell? Are these forms always so drastically different? And is there a general preference for R/S?
|
[
"Well you understand the left hand/right hand metaphor which will help. \nTry to put your right hand in a left-handed glove or vice versa and it’ll make sense. The gloves, like your hands, are chiral. The left hand won’t fit properly in a right-handed glove. \nI looked it up and taste buds are chiral. So if a chiral molecule gets on your tongue and it’s metaphorically a “left hand” it won’t fit with a “right-handed” taste bud and thus won’t trigger that taste bud to signal your brain."
] |
[
"As others have pointed out, they are actually EXTREMELY similar chemically speaking. I'll shed a little more light on it as someone who studies asymmetric catalysis.",
"Take two enantiomers R and S. Their NMR spectra are exactly the same. Their IR spectra are exactly the same. Their UV-vis spectra are exactly the same. They will have the same melting points, boiling points, and densities. They would have exactly the same Rf by any achiral chromatography method (eg TLC). Obviously, mass spec would also be exactly the same.",
"It's kinda hard to tell enantiomers apart with chemical analytical methods. Chiral chromatography (use a chiral stationary phase- now the two enantiomers have different Rfs) is one method. Optical methods like optical rotatory dispersion or circular dichroism (CD) are others. You can also use chiral shift reagent (a la Whitesides) to resolve the NMR spectra. Even then, once you know you have two different enantiomers, it's even harder to tell which is R and which is S. One thing you can do is use quantum chemical models to simulate the CD spectrum and compare to the empirical results. R and S will have different spectra, so if the empirical spectrum matches the simulated R spectrum there's a good chance it's R. The gold standard is to do single crystal x ray diffraction, and this is what 99% of researchers do to prove configuration when publishing.",
"As you point out, smell and taste are different. This is because the body itself is a chiral environment! Your body is like that chiral HPLC column. It discriminates R and S because they bind differently to protein receptors. All your proteins are pure L amino acids (the S configuration) and are thus chiral. Sometimes I like to imagine the \"mirror universe\" where your body is composed of D amino acids. In this universe the properties of the enantiomers are switched. Natural cocaine is inactive, as is LSD. Enantiopure thalidomide is now highly toxic, and would never make it to market. Achiral compounds like fentanyl would remain at their same activity because they do not interact differently in the mirror universe.",
"Chirality is bonkers!"
] |
[
"Some simple analogies that are helping me understand more, thank you."
] |
[
"The closest star to earth is 4.3 light years away. Is this par for the course in the Milky Way?"
] |
[
false
] |
Basically, is our cosmic neighborhood crowded, isolated, or about average
|
[
"But this doesn't take into account the fact that most star systems are binary, and some are trinary or more. Does anyone know the average distance between systems, rather than the average distance between stars? I think that would be much more informative, and more in line with the intent of OP's question."
] |
[
"But this doesn't take into account the fact that most star systems are binary, and some are trinary or more. Does anyone know the average distance between systems, rather than the average distance between stars? I think that would be much more informative, and more in line with the intent of OP's question."
] |
[
"Because it's like someone saying \"the sky\" in response to the question \"what's up?\". Everyone knows full well what the intended question is, because conventional methods for asking that question are being used, but the person replying is intentionally breaking with convention and giving a \"more literal\" answer solely for the purpose of self-interest (either in an attempt to be funny or witty, or in an attempt to make themselves look smart, or whatever). This is rather anti-social behavior and is looked down upon by basically every human culture.",
" is why doomgoblin is being downvoted. It's also why ",
" being downvoted, because you know all of this and yet you too act similarly uncooperatively in defense of such uncooperative behavior.",
"tl;dr: It's an assholish response."
] |
[
"Is constant light exposure beneficial to plants, or do they also require periods of low light?"
] |
[
false
] | null |
[
"The concept you're referring to is ",
"Photoperiodism",
" ",
"Depends on the plant, and the phase of growth that plant is in. Some plants do just fine with 24/7 light and can benefit from no darkness. ",
"Other plants, particularly those that flower in the fall, are dependent on changing day/light periods to trigger blooming. ",
"Many of those plants will still be fine under 24/7 light but will stay in a vegetative state, growing, but not producing fruit or flowers."
] |
[
"Tomatoes need at least 4 hours of darkness. It has something to do with reallocation of sugars produced. I have seen the ill effects off too much light in our lit greenhouses - in spring especially it is not hard to go over the edge with artificial light + natural light (as the day length rapidly increases)."
] |
[
"An additional point for clarification: many of you may remember hearing about the light and dark reactions of photosynthesis, but these are now commonly referred to as light dependent and light independent reactions. These two crucial sets of reactions can both operate during exposure to light, so at least in this sense, plants are fine. Just wanted to share just in case that's in the back of the mind of anybody who hasn't had a biology class in a while"
] |
[
"Why do cows have four stomachs when others grass eating mammals don't require that many?"
] |
[
false
] | null |
[
"Actually, quite a few mammals have stomachs with multiple compartments. People tend to say four \"stomachs\" but it's technically one stomach with four compartments. These animals are called \"ruminants.\" Some examples other than cows are sheep, deer, camels, llamas, and goats. There are quite a few others, as well. ",
"When a ruminant eats, the grass goes to the first chamber, called the rumen, where it mixes with saliva. The next chamber, the reticulum, separates solids and liquid. The solids are regurgitated as cud, and the ruminant chews it up some more. It is then swallowed again and passed on to the third chamber, the omasum. Here, water and inorganics are absorbed into the animal's bloodstream. Finally, what's left moves to the fourth chamber, the abomasum. The abomasum is basically the same as our stomachs and it digests everything that's left. ",
"Questions like this are why I never threw away my old 4-H books. I remembered the basic process from my cattle projects, but I had to pull out the book to recall the names of each part of the stomach. "
] |
[
"Vertebrates need microbes in their guts to break down cellulose in plant matter. There are different ways to go about this, but it basically involves lengthening the digestive tract to give the microbes more time to do their thing.",
"Broadly speaking, there are foregut fermenters, like cows, and hindgut fermenters, like horses. The foregut refers to the digestive tract up to the middle of the duodenum, which connects the stomach to the small intestine. Foregut fermenters typically elongate the stomach. In ruminants, the stomach is even divided up into ",
"distinct chambers",
". This is what people are talking about when they say cow have four stomachs. ",
"The hindgut technically starts at the last third of the large intestine, but I hear animals that have expanded the cecum, which is a pouch where the small and large intestine join, referred to as hindgut fermenters. Lagomorphs like rabbits have an expanded cecum to break down plant matter. However, at that point they're pretty close to the end of their digestive tract, so they can't absorb a lot of those nutrients. That's why rabbits ",
"eat their poop",
". In animals like horses, the large intestine is elongated.",
"These have each evolved multiple times in multiple groups, so it's more a discussion of general strategy than a single adaptation. For example, ",
"marsupials",
" and ruminants are both foregut fermenters, as are ",
"some monkeys",
"."
] |
[
"Similarly, some animals like horses don't have multiple stomach compartments and are hindgut fermenters, meaning instead of using their foregut for microbial fermentation of plant matter, they do it instead in an enlarged cecum in the hindgut.",
"Other animals, like kangaroos, have a single enlarged, curved stomach which is where most of there plant digestion takes place, both via the stomach secretions and microbial fermentation."
] |
[
"Could someone describe what was occurring in the early Universe? From the time of the Big Bang to the creation of protons and neutrons, where/when/why did quarks pop up in this primordial soup?"
] |
[
false
] | null |
[
"Well, the current hypothesis goes something along the lines of ",
"this",
":",
" after the Big Bang (about one Planck time unit) - \"Planck epoch\" - Basically we have no idea what happened because all of our models fail in the extreme conditions around this time. It is hypothesized that around this time there is a single unified force from which gravity becomes distinct. But really we have no clue.",
" - \"Grand unification epoch\" - We think with gravity separated from the other forces, there is probably some mechanism during this time that is able to produce an abundance of baryonic matter over baryonic antimatter (called \"baryogenesis\"), resulting in an abundance of what is likely to be some phase of quark-gluon plasma. There may also be a mechanism to either produce more leptonic matter than leptonic antimatter, or convert some of the baryon asymmetry into a lepton asymmetry as well (\"leptogenesis\"), but we don't know yet. Also inflation should occur roughly during this time period, where the universe very rapidly expands in volume by a large factor about ",
".",
" - \"Electroweak epoch\" - After inflation has ended and the universe continues expanding more slowly, the temperature falls enough for a phase transition where the electroweak force becomes distinct from the strong force. We think.",
"All of the above stuff is basically hypothesis. We think it happens but we don't know. The rest of the stuff below is more grounded in observation and factual science.",
" - \"Quark epoch\" - Now, things spread apart further, allowing the temperature to drop enough to permit the electromagnetic interaction to become distinct from the weak interaction, and all fundamental particles acquire a mass through the Higgs mechanism.",
" - \"Hadron epoch\" - Things continue to expand and cool, and the strong interaction dominates, pulling quarks together into stable hadrons (protons and neutrons) once the temperature drops enough to make this energetically favorable.",
" - \"Lepton epoch\" - More expanding, and annihilation between hadrons and antihadrons, and toward the end of this period, annihilation between leptons and antileptons, resulting in a small excess of both over their antimatter conterparts.",
" - \"Photon epoch\" - The only matter that's around during this time is very highly charged (electrically) and includes protons, neutrons, and electrons, but things are still hot enough and close enough that the protons can't capture electrons to form stable hydrogen atoms. Eventually, things expand and cool off enough though, and they combine to form neutral hydrogen. At the end of this period, the universe is filled with neutral particles (hydrogen and helium) and it becomes dark, as electromagnetic radiation (photons) becomes free to propagate, and becomes the cosmic microwave background.",
"And that's about it. That's pretty much the first half a billion years. Not long after, the first stars and galaxies begin to form under the collapse of gravity.",
"So, TL;DR: We have a fairly good idea of ",
" quarks came about (in the first second after the Big Bang) but we really don't know why or how. The \"where\" is \"everywhere\" because inflation drives everything apart, making the universe homogeneous on the large scale."
] |
[
"http://en.wikipedia.org/wiki/Big_bang#Timeline_of_the_Big_Bang",
"Read this and specify what you want to know! ;)",
"Quark-gluon plasma appeared after the inflation period!"
] |
[
"Thanks! I'll definitely take a look."
] |
[
"What would happen if a coronal mass ejection similar to the size of the one in 1859 happened today?"
] |
[
false
] |
[deleted]
|
[
"Nothing good. We're not ready to deal with it.",
"Mostly electric grids would experience a high voltage rise due to magnetic induction. This could burn the transformers that lower the voltage for domestic use. Other devices, such as solar panels, would be affected to a lesser extent due to the smaller loop area, though the sources I have are not conclusive on whether they'd be fine.",
"This article in NASA's science magazine",
" focuses precisely on your question if you want more info. Quoting from it:",
"widespread power blackouts, disabling everything that plugs into a wall socket."
] |
[
"Cellular networks would likely go down quickly because satellites aren't as protected. Electrical grids would blow quickly. There are some good documentaries on this, I would google solar flare documentaries. I know I've seen a good one before but I'm blanking on the name unfortunately."
] |
[
"Yes, the astronauts would be safe. This kind of events are usually preceded by a ",
"solar particle event",
" that can give a lethal dose of radiation to unprotected astronauts within a few hours. But the walls and equipment of the ISS offer an equivalent aluminium thickness of 20 g/cm",
", which is enough to protect them. The sleeping compartments are shielded a bit more heavily so they can use them as a safe place during these events."
] |
[
"Standing on the other side of the world, would I have felt the Chicxulub impact?"
] |
[
false
] |
I know that puts me in the Indian Ocean today and that the landmass was . I only mean hypothetically. If Chicxulub's 4.2×10 joule impact was too weak to feel, then how about the impacts? What other immediate effects might I notice on the other side of the world? To what extent did any of these alter the world's spin, axis, or orbit around the sun? BONUS: .
|
[
"I did a quick ",
"calculation",
" for the Chicxulub impact with the ",
"Earth Impact Effect Program",
": Energy and crater size match pretty well. On the other side of the planet, you would not feel much of the impact. Very slight shaking after an hour and bad weather after 16."
] |
[
"The calculation is inaccurate. It simply models the effect as a function of distance from the impact site, without treating any point on the earth’s surface as “special”. But the antipode (the point on the earth’s surface exactly opposite the impact) is indeed a special point, because of the way the spherical earth focuses the seismic waves.",
"These scientists created a computer model",
" of the Chicxulub impact and specifically tested the situation at the antipode. ",
"This figure",
" shows how the displacement induced by seismic waves is spread across the earth and focused at the antipode. As you can see, for both a 90-degree and 45-degree angle of impact, there is significant focusing on the antipode.",
"So it’s clear that you can’t simply model the seismic effects as a function of distance from the impact site, you also need to account for the focusing of seismic waves at the antipode because of the earth’s shape. The exact magnitude and effects of this focusing are less clear.",
"The paper splits up the effects into four categories:",
"Triggering of seismicity: ",
"Triggering of volcanism: ",
"Fracturing of rock: ",
"Melting of rock: ",
"So while some of these calculations are ambiguous about whether earthquakes or volcanism was triggered at the antipode due to the impact, the very fact that they are talking about secondary effects strong enough to trigger these things strongly implies that the shaking from the primary event must have been pretty darn strong, and was almost certainly felt by any critters standing there.",
"More generally, the effects of impact events are complex, and modeling the effect as a function of distance from the impact site as your link does is only an approximation. The stresses travel via “channels” so the magnitude of the effect is much higher in these channels than in surrounding areas. ",
"This figure from the paper",
" shows the magnitude of the stresses at a depth of 60 km. The shaking was much stronger along the channels, and may well have triggered secondary earthquakes along them."
] |
[
"It certainly appears so from this article: ",
"http://www.geologyinmotion.com/2011/10/chicxulub-impact-what-happened-on.html",
"I'm not able to make out whether the 4 meter displacement is as huge as it sounds, as the discussion of earthquakes seems fairly mild."
] |
[
"Can I preemptively get lots of sleep to help stay awake for a long time?"
] |
[
false
] |
Basically, if I sleep ~6 hrs a night, can I sleep 10 hours tonight and have an easier time pulling an all-nighter tomorrow? Please do not lecture me on how all-nighters are a bad idea.
|
[
"If ALL you want to to do is decrease your sleep drive for one night, then technically the answer is yes it ",
", but it doesn't mean it ",
". The amount of benefit you'll get really depends on how well rested you were to begin with, and the quality of your sleep over the several weeks preceding the \"all-nighter\". If a person averages 6 hours a night they may be somewhat under-rested and one night of 10 hours is unlikely to make a huge difference. Obviously there are a whole host of reasons why not to do this, but it seems that isn't relevant to the current question. "
] |
[
"You're sleep deprived enough that you're seeing effects of sleep inertia once you do start to sleep.",
"Also, the \"the longer I'm up, the less tired I get\" is due to the circadian influence on sleepiness. It has a natural up-down that goes through the time of the day, and once you pass the lowest point of it, you can still continue to accumulate sleep debt but feel less sleepy."
] |
[
"Awwww, I wanted to overclock my cells!"
] |
[
"If electrons repel each other due to their negative charge, how do any bonds form to make molecules?"
] |
[
false
] |
I thought I understood this but I hit me recently. How does anything exist? Electrons are negative so they should repel each other, yet it is somehow a lower energy state to share electrons and become one molecule. I probably don't understand it all or have all the information. Does it have something to do with quantum mechanics?
|
[
"The electrons in one atom are attracted to another atom's protons. The protons in one atom are attracted to another atom's electrons. This also works for ions - for ionic bonding. To form a stable molecular bond (covalent or ionic) the octet rule should be observed, since all molecular bonds work with valence electrons. The octet rule dictates, that to have a stable bond between at least two different atoms, the total valence electron count should be eight. This works, in smaller molecules, with a central atom having most of the valence electrons (the default electrons) and the surrounding atoms (of a different kind) having the rest shared proportionally between them to form an electron sharing system of sorts. The octet rule derives from a somewhat complicated matter of allocating electron orbitals and the spins all around for stability.",
"Say we have sodium. And then someone introduces chlorine to sodium. Between the both of them, they have eight valence electrons - sodium has one valence electron, and chlorine has seven. Then they bond, since they share eight electrons perfectly. The eight electrons separately are attracted to all the protons. The octet rule is observed. So now, you have NaCl. "
] |
[
"The electrons are not bound to other electrons, they are bound to the positively-charged nucleus (or nucl",
" for molecules)."
] |
[
"The negative charge of multiple electrons is countered by the positive charge of the multiple protons in the nucleus. This applies to both atoms and molecules. The orbitals of the electrons in the molecule can be described by quantum mechanics."
] |
[
"Why do optical illusions make us hallucinate?"
] |
[
false
] | null |
[
"What's an example of this?"
] |
[
"Generally if you stare at the center of a rotating vortex for a certain period of time and then look away you would see things moving around"
] |
[
"This is called the motion aftereffect.",
"https://en.m.wikipedia.org/wiki/Motion_aftereffect",
"https://michaelbach.de/ot/mot-adapt/index.html"
] |
[
"What is it that I can get from fresh fruit and vegetables that I can't get from a multivitamin pill?"
] |
[
false
] | null |
[
"Quick preface: this is a vastly oversimplified response, but it hits the high notes. Links provided throughout as a quick springboard into related literature in the field. ",
"Nutrition isn't only about the intake of 'essential' nutrients, i/e/ the compounds that our bodies can't synthesize (or can't synthesize enough to satisfy demand). In addition, we don't know everything about how diet influences our health--far from it. We're still learning. Nevertheless, there's a central assumption involved in research that examines our eating habits with respect to the effect these habits have on our biological lives: until the very recent past, the principal sources of food and nutrition for our species, and the species that predate our own to which we are genetically related, have been sourced from the ecosystem. That is, the \"whole foods\" that you hear about so much--plants and the occasional animal that have not been modified beyond cooking or fermentation before eating--are what we've had at hand, essentially forever.",
"And so we get to the issue of how a multivitamin differs from these food substances. Most apparently, these pills are not foodstuffs. They're sometimes derivatives or concentrates of select fractions of foodstuffs, but they're just a often synthetically manufactured from a combination of inorganic and organic starting materials. In focusing on the production of these specific fractions of interest, multivitamins reflect our incomplete understanding of diet, nutrition and health.",
"We know that multivitamin supplementation has its uses. Prenatal and maternal care, for instance, are areas in which supplementation can dramatically reduce or prevent certain kinds of morbidity (especially regarding specific issues with ",
"neural tube development",
", and possibly even some types of ",
"pediatric cancers",
").",
"That said, we know that marketing can be deceptive about the usefulness, or even the safety, of supplements. Vitamins and minerals that are essential have a very pro-health association, and this belies awareness of safe dose ranges. Vitamin K, iron, manganese, vitamin A, etc., these all have toxicity ranges that are easy to step in to when presented in concentrated form. With nutrient intake from whole foods, as opposed to supplements, you're drastically less likely to dose yourself into ",
"hypervitaminosis ranges",
" or ",
"adverse interactions with other drugs",
".",
"I'd add that supplements are generally (very generally) almost entirely peed out of your system. Toxicologists joke about vitamins being very expensive pee pills, and the like. I've looked for presentations of his online, which I can't seem to find, but if you ever get a chance to go to a lecture by Dr. Tee Guidotti, he has a few funny stories about this.",
"Others have mentioned important things to consider, too, like the bulk and fiber issue, but, frankly, there are many unknowns. We don't exactly know every aspect of how our diet impacts our health, but the health disciplines seem to have come to a central agreement: eat whole foods, because ",
"they do you a lot of good",
" even though we don't know ",
"all the mechanisms",
" behind that yet. If you're healthy, take low dose multivitamins if you must, but ",
"don't expect them to impact your health in the long run",
" because ",
"the odds are they aren't going to do anything beneifical for you, and they may do just the opposite",
". Otherwise, consult a doctor and get some bloodwork done if you think you have a reason to take specific vitamins or other supplements--those are the instances where supplements can really help.",
"Let me close on a regulatory note, since that's my field now. Pharmacopeias (like the USP-NF in the US and the European Pharmacopoeia in the EU) try to standardize quality and purity of plant and vitamin supplements, but supplements aren't regulated in the same way that pharmaceuticals are--they aren't required to have any demonstrable efficacy. This is all outlined in title 21 of the code of the federal register, in section 111 (",
"21 CFR 111",
"), but it might be simpler to read through the information ",
"here",
"."
] |
[
"explain the like i'm 5?"
] |
[
"Well, vitamin C is an anti-oxidant, and without it there's the risk of scurvy.",
"However, there are many other anti-oxidants, and there have been many different claims as to their wonderful abilities - as far as I know, none of those (anti-ageing, anti-wrinkle, better memory, etc, etc.) have been proven."
] |
[
"Are humans supposed to be monogamous?"
] |
[
false
] |
[deleted]
|
[
"There is a benefit to monogamy. It assists in the rearing of children by encouraging stable, well defended family group, especially important if the offspring require many years to grow and reach reproductive stage. They will need protection from both parents for much longer."
] |
[
"Before the first societies, humans were known to not practice monogamy.",
"If males don't know who the father of a child is, they all have an interest in supporting it. Hence, each child has the support of many parents."
] |
[
"does monogamy solely produce these conditions? Can't other groups like extended family or polygamous families provide safe environments for children? Surely it's worked in the previous societies that were not monogamous, else we may not be here today."
] |
[
"Why does pressing the sharp edges of two knives together not create such high pressure and temperature that it fuses the edges together?"
] |
[
false
] |
If I have two knives, and I put the sharp side together in an X shape and press the edges into each other with all of my strength, why does it not cause such a drastic increase in pressure, (very, very small area and conservatively moderate force) that almost reaches infinity, that the atoms at the edge get compressed and heat up to the point where they can melt together, which is only at 3000F. Surely my infinite applied pressure can cause a localized temperature increase to this degree?
|
[
"Your large pressure goes into yielding the metal, which will cause dents in the sharp edge. Since you are not doing very much work with your muscles you should expect very little temperature increase in the knives as well."
] |
[
"The maximum pressure that you can exert in a setup like this is the ultimate strength of the material (more realistically, for a person pressing things together, it is the considerably lower yield strength). This pressure is quite low in the cosmic scheme of things - only on the order of 1 GPa - and it doesn't actually take very much energy to deform a metal, either elastically or plastically. Even in commercial plastic forming operations you don't get much hotter than a few hundred degrees.",
"E: basically the edges will flow like a liquid but in a solid state long before they get hot enough to melt or fuse"
] |
[
"Basically, yes. To fuse the knifes together, you have to add lot of energy, using friction by example (like we do in friction welding).",
"EDIT: There are some extreme cases where you can weld metals together using only a choc or high pressure, like space mechanism in vacuum (mostly seen on aluminium, due to the destruction of the protective Al2O3 coating or when using similar steel alloy). You can avoid that by using different metal or crystallographic structure (eg. austenitic steel for one part and martensitic steel on the other). "
] |
[
"How can lack of power physically damage computer hardware components?"
] |
[
false
] |
I've heard of 'black outs' and 'brown outs' damaging computer hardware components (ie graphics cards) beyond trivial repair. I understand how a surge can fry stuff, but how can a sudden drop in power destroy anything?
|
[
"every hard disk built in the last 10 years uses a magnet to pull the head into parking position once power is cut. this is no longer an issue. (",
"http://en.wikipedia.org/wiki/Harddisk#Landing_zones_and_load.2Funload_technology",
")",
"also, the 5-second power button push is just an ACPI override signal that isn't interpreted by the operating system but directly by the hardware. it causes an immediate power down after 5 seconds; pretty much the same as pulling the plug. there is no power off signal sent to the hard disk; it's not even possible to do so. (ACPI spec section 4.8.2.2.1 at ",
"http://www.acpi.info/DOWNLOADS/ACPIspec50.pdf",
")"
] |
[
"every hard disk built in the last 10 years uses a magnet to pull the head into parking position once power is cut. this is no longer an issue. (",
"http://en.wikipedia.org/wiki/Harddisk#Landing_zones_and_load.2Funload_technology",
")",
"also, the 5-second power button push is just an ACPI override signal that isn't interpreted by the operating system but directly by the hardware. it causes an immediate power down after 5 seconds; pretty much the same as pulling the plug. there is no power off signal sent to the hard disk; it's not even possible to do so. (ACPI spec section 4.8.2.2.1 at ",
"http://www.acpi.info/DOWNLOADS/ACPIspec50.pdf",
")"
] |
[
"This man explains it better than I ever could.\n",
"Brownout explanation"
] |
[
"I know visible light can't escape a black hole but can more energetic electromagnetic waves (gamma rays) escape its gravity?"
] |
[
false
] |
Envisioning this in a differnt way: Say we have two super high powered flashlights fused together pointing towards a telescope in space. The telescope can detect vIsIble lIght and gamma rays. One flashlight emmits visible light and the other emmits gamma rays. If the fused flashlights enter a black hole will the telescope lose trace of the the visible light source first followed by the gamma ray source? Or will the telescope lose trace of both sources simotaneously??
|
[
"All frequencies of electromagnetic radiation will be affected in the same way by a gravitational field: they will get redshifted by the same factor, and the paths their photons take will get curved to the same degree.",
"Now, if you drop two lights (a regular one and a gamma-ray one) into a black hole and you observe them from a safe distance, you will see that their emission will get progressively redshifted (the electromagnetic waves get 'stretched out') before they cross the event horizon (and disappear completely). Because the gamma-ray emitter sends out photons with a much shorter wavelength than the visible-light emitter, we will be able to detect radiation from the gamma-ray emitter that is redshifted to, say, infrared at the same time that the emission from the visible-light emitter will be redshifted far into the low-frequency radio regime. Effectively, while both emissions make it towards you, you will fail to detect the radiation from the visible-light emitter before you'll lose the emissions from the gamma-ray emitter."
] |
[
"A different way of thinking about the question in your title: no, nothing can escape a black hole, not even gamma rays. The reason is that there are literally ",
". It's kind of analogous to how there are no paths leading backward in time.",
"Of course, in the experiment with the dropped flashlights, you would stop detecting the radiation from each flashlight before either one actually entered the black hole, because of the redshift that entropyjump mentioned."
] |
[
"Great response! Thank you!!! "
] |
[
"Does Monk Fruit and other \"natural\" sugar alternatives have the same health risks as artificial sweeteners?"
] |
[
false
] |
There have long been concerns about the health risks from artificial sweeteners. For example, recent studies indicate that artificial sweeteners could elevate blood sugar levels and raise one's risk of heart disease or stroke. There have also been reports that they alter our gut biome in harmful ways. For example this quote ( ): " Now, working in the lab of Weizmann Institute of Science immunologist Eran Elinav, Suez and colleagues have taken an array of common artificial sweeteners—saccharin, sucralose, aspartame, and stevia—and filled that gap in the literature by testing them in humans. Their findings, described today (August 19) in , suggest these sweeteners do indeed alter the gut microbiome, as was seen in prior mouse work. This, the researchers say, adversely impacts glucose tolerance, a measure of how readily the body moves sugar from the blood into muscle and fat, possibly leading to weight gain and diabetes. " My question is, are "natural" sweeteners such as those based on Monk Fruit (which gets its sweetness from a compound called, "mogroside V" and is often mixed with erythritol) chemically similar to artificial sweeteners and do they carry the same health risks? I have been using monk fruit for almost two years - and recently found my blood sugar levels are too high (despite being slender and extremely fit). Could Monk fruit be the cause?
|
[
"I would read better sources on the safety of common artificial sweeteners. ",
"According to the FDA, common artificial sweeteners like aspartame have been extensively tested in multiple studies and no risk has been found."
] |
[
"Not to pile on too much, but the fact that artificial sweeteners alter the gut microbiome does NOT make them unhealthy. First off, to a certain extent everything we eat alters the gut microbiome. Different microbes in our gut consume different things, and much like any ecosystem, as you feed some things at differing rates than others (which is going to happen constantly unless you're on some kind of lab-controlled, super-precise diet), the balance of the overall population changes.",
"Further, the body is always trying to achieve homeostasis, i.e. balance, and regularly adapts. One researcher noted \"by only recruiting individuals who did not consume sweeteners in their typical diet, we cannot be certain if we would see the same unfavourable effect for people that typically consume sweeteners, due to possible adaptation effects.\" The study was too short to observe possible adaptations (which isn't a knock on the study, since it wasn't intended to do that). While the study showed acute changes in glucose tolerance, long-term health outcomes (which is what we really care about) can't really be drawn from this. That's not to say they're not there, but all this study tells us is \"look into it.\" We DO however know that long-term consumption of actual sugar DOES have long-term negative health outcomes. Indeed, one of the researchers from this study emphasized this distinction.",
"So...in short, we can't really say artificial sweeteners are bad for you (and that's also ignoring the fact that the different sweeteners used all had different effects, so blanketing them with one term is also wrong). If someone wants to play it safe, sure, go for it, but do so understanding the limits of the current data available."
] |
[
"Perhaps people should know that about 99% of all this research that shows artificial sweeteners are bad, is paid for by the sugar industry. It's been less than honest and more about keeping people eating sugar, which we know causes serious illnesses. Making artificial sweeteners the bad guy is the problem. The real problem is sugar."
] |
[
"If the so called \"periodical cicadas\" (the infamous 13 and 17-year cicadas) emerge and reproduce in 13 and 17 year cycles, does that mean that every 221 years there will be a year when both types of periodical cicada will emerge?"
] |
[
false
] |
I was just wondering if there would ever be a situation when both the 17 and 13 year cicadas emerged at the same time and caused the worst cicada swarm ever, or at least emerged at different times in the year and produce two separate swarms. If this is so, what sort of ecological impacts would this have had or has had? Also, does anyone know when the next 13 or 17 year cicada swarm will be?
|
[
"You need to note, periodical cicadas are all synchronized, but not across the whole species. It's not the global population emerging at the same time, but 'only' the population of one region. So first, you need both species in one region and then, you need to be lucky enough that the years (and even hatching times) match.",
"For both of them to emerge at the same time, it's a massive amount of food for any animal thriving on them. Usually there are so many cicadas that birds can't even keep up catching them and eventually, they start covering the forest floor. For any animal surviving on insects, a year of both cicadas might be the best time to produce plenty of offspring. From what I know, there aren't any other serious environmental impacts, just a lot of noisy insects."
] |
[
"To add on to the \"birds can't even keep up catching them\": One find the same principle e.g. in forest trees like oaks, which have so-called mast years: Inbetween mast years, a lower fruit yield can only sustain a certain number of foragers, like mice and squirrels. In a mast year, the trees produce much more fruit, and a lot of them will not be eaten by foragers, as there are just too few of them. Of course, forager numbers can rise due to such plenty, but still, they have to catch up..."
] |
[
"It can happen! Mostly they just get very very loud and all the birds lie around going \"Oh, I couldn't possibly have another.\"",
"http://stlouis.cbslocal.com/2015/05/07/two-groups-of-cicadas-ready-to-emerge-across-missouri/"
] |
[
"What is the furthest distance we could have a satellite orbiting earth whilst still remaining useful to us here?"
] |
[
false
] |
I'm thinking for purposes such as GPS, but am interested to hear about others. My satellite knowledge isn't the best! Related questions. Are current satellites we'be put into space a similar distance away? Or do they vary dramatically? And do they share similar orbits/behaviours? Also, I'm on a phone and unable to add a flair, sorry!
|
[
"Depends of what you take as useful. ",
"The Kepler Space Telescope",
" is in a heliocentric orbit. It orbits the sun but follows the earth closely. It is pretty far away. If you you are talking about within the earths orbit then it would be the ",
"Sphere of influence",
"."
] |
[
"30 miles isn't even out of our atmoshphere."
] |
[
"While 30 miles up would technically be low earth orbit, some aircraft can get pretty close to it. Orbits of less than 80 miles aren't really practical due to drag. You need to get above 120 miles before atmospheric drag inhibits long term orbiting. GPS satellites are at roughly 10,000 miles up. DirecTV, TV broadcasting (think live news feeds), communication satellites are up near 22,000 miles. "
] |
[
"High Fructose Corn Syrup is about 55/45 fructose to glucose, and sucrose is 50/50 but a disaccharide. In a mildly acidic environment (the stomach), the disaccharide is cleaved. Is there any merit to hating HFCS?"
] |
[
false
] |
Inversion (cleaving the disaccharide): Glucose can be used anywhere in the body, but does fructose metabolism in the liver produce more dense storage molecules?
|
[
"The process by which our body turns sugar into energy is called glycolysis. It turns glucose into ATP which is energy we can use to do work. It essentially uses chemical gradients that are always trying to reach equilibrium (but never do because if they did we could no longer produce energy and we'd die). Anyways, at the beginning of glycolysis, glucose is phosphorylated at it's 6th carbon and then turned into fructose-6P. If body needs energy, then this fructose-6P is phosphorylated again by an enzyme called phosphofructokinase1. This is glycolysis' commitment step. Once this step is performed, the entire cycle goes to completion whether the body needs energy or not. Normally, when a body does not need energy, it stores the extra glucose as glycogen (mainly in the liver). Insulin is the body's chemical signal which tells it to store glucose as glycogen and not turn it into energy.",
"When high fructose corn syrup enters the body, it is read as a sugar that has already passed the commitment step of glycolysis so it gets turned into energy whether your body needs it or not. If your body doesn't need the energy, it sends out insulin to in an attempt to store this fructose as energy. However, HFCS can't get stored as glycogen, so it keeps getting turned into energy. This makes your body send out EVEN MORE insulin. Still no change. Well eventually your body has so much insulin that your insulin receptors become desensitized to it making. Then your body more or less says 'fuck it' and stops making insulin. This causes type 2 diabetes. This is the danger that HFCS holds over normal sugar (glucose)."
] |
[
"But as the OP points out, stomach acid cleaves sucrose into what is basically HFCS (but with 50/50 fructose to glucose). When does your body read whether the sugar has already passed the commitment step, if the conversation takes place before your intestines (with your taste buds)? It is unclear whether you really addressed the OP's question: how is table sugar any different from HFCS (besides the minor 55/45, 50/50 difference)?"
] |
[
"I learned this in my biochemistry class about a year ago which was built around this textbook: ",
"http://www.ncbi.nlm.nih.gov/books/NBK22395/",
". I remember this topic specifically because my professor was an angry Dutchman that brought it up often and included this very question on our final exam. ",
"To address rational's question. Food is turned into energy by breaking the bonds in complex molecules and using that energy to charge ATP. After the 3rd step of glycolysis, glucose and fructose both become a fructose with a phosphate group on the first and sixth carbon. Glycolysis takes this sugar and turns it into 2 ATPs and two pryuvate molecules (",
"http://en.wikipedia.org/wiki/Pyruvic_acid",
") through a series of steps). ATP As humans, we have mitochondria which then can break down this pyruvate even further by using oxygen. ",
"ATP: ",
"http://en.wikipedia.org/wiki/Adenosine_triphosphate",
"ATP is used as energy because there is an extreme amount of energy in the phosphate bonds which can be broken and used to do work. ATP is also used as a raw building block for RNA and loses and oxygen to become the letter A in DNA as well."
] |
[
"What would be the effects of removing excessive amounts of heat from the earth through the harvesting of geothermal energy?"
] |
[
false
] |
I understand it's implausible, but theoretically, what would happen if human efforts caused the mantle/core of the earth to cool? I suppose that the heat is caused by gravitationally-induced pressure, but doesn't thermodynamics demand a reaction of some kind if we were able to influence the temperature enough? Forgive my simplicity. I'm a social scientist but curious about the natural sciences.
|
[
"I studied this subject the past weekend.the heat in the center of the earth is created by the gravitationally-induced pressure,radio active elements and a great deal of it is left over heat from when the earth condensed.The earth radiates about 44terawatt of energy through it's surface mantel.this energy can be mined or used without a great deal of side affects.The core ought to grow if you otherwise remove heat beyond the point where it can sustain it's current state(size of the solid core,amount of magma surrounding it).if this keeps on it will upset the Coriolis effect happening in the magma which along with the \"conductive fluid\" is responsible for creating the magnetic fields.when this happens the earth will cease to be protected from the solar winds and it will start blowing the top layers of our atmosphere off.when the ozone layer is gone earth will be bombarded with gamma rays etc. from the sun and will become a radioactive wasteland.eventually there will be a very thin atmosphere only kept in place by gravity.This in affect will create an atmospheric pressure too low for water to be in a liquid form and thus all the oceans will be turned into vapour and blown off into space by the solar winds.most of this already happened with Mars "
] |
[
"This is absolutely correct (though it would take quite a long time for the core to melt again if all thermal energy were extracted), and should not be downvoted. There is a truly absurd amount of energy in the Earth's core, so \"excessive amounts of heat\" is rather more than the OP was probably thinking of.",
"To give you an idea, magma has a specific heat of about 1400 J/kg K. The mantle, with an average temperature of 2000 K and 84% of Earth's mass (so 5.01x10",
" kg) has about 1.4x10",
" J of energy that could theoretically be harvested from the mantle. The core has an average temperature of 5000 K and 12% of Earth's mass (the remaining 2% is the Lithosphere, atmosphere, and everything humans care about). This means the core has additional 0.5x10",
" J of energy. By comparison, humans use a puny 15TW of power, so the energy in the core would last us 1.25x10",
" seconds, or 40 billion years, at current consumption rates. It's not an immediate problem.",
"Although in terms of \"bye-bye life\", I think using the oceans as a heatsink (that's really the only usable heatsink) would kill everyone first."
] |
[
"Check out faul_sname's answer. If you replace 15 TW with the 44 TW noted above by moominza, you get something like 10 billion years before the core completely cools naturally. Of course, the real question would be \"how cool would the Earth's innards have to be for things to really change on the surface\", and that takes a bit more work since it would be well before the 10 billion years the above calculation implies. Still, it's likely to be on the order of several hundreds of millions of years at the very least, so talking about the timescales in millenia isn't meaningful..."
] |
[
"Does it get significantly harder to heat water another degree as temps go up?"
] |
[
false
] |
So say I put a pot of water on the range. If it takes x amount of time to heat up from 50 to 60 will it also take x amount to go form 60 to 70? Or does it get harder to heat up as it goes up?
|
[
"To add to what ",
"/u/DCarrier",
" said, in practice it gets harder, because the hotter the pot of water gets, the more heat it ",
"radiates",
", and the stronger the convection currents around the pot (causing heat lost to the air). Furthermore, if you use an electric kettle, the resistivity of the resistive heating element goes up with temperature, so the wattage of the heating element will go down a bit as the temperature increases."
] |
[
"According to ",
"this",
" it stays pretty close to 4.2 J/kg K. Though once you get it to boiling, it will take a lot of energy to actually boil it. More than it takes to go from freezing to boiling."
] |
[
"There will also be more evaporative cooling."
] |
[
"Why can't we see in colour in a dark room?"
] |
[
false
] |
Looking around your bedroom at night when it's almost pitch-black you can still make out shapes, but everything seems to be monochrome. Why is this?
|
[
"Your retina has two types of cells: rods and cones. Cones see in colour, but their number is much smaller and they aren't very sensitive. Rods are much more numerous and sensitive, but they don't see colour.",
"Therefore, when you're in the dark you only see with your rods: you can make out shapes that your cones would never pick up, but you lose the capacity to distinguish colour.",
"On a side note, rods are also responsible for peripheral vision. ",
"http://en.wikipedia.org/wiki/Human_eye",
"http://en.wikipedia.org/wiki/Rod_cell",
"http://en.wikipedia.org/wiki/Cone_cell"
] |
[
"It's probably worth noting too: the reason cones can differentiate colors is because there are three types of them which each have ",
"different sensitivities to different wavelengths of light",
". This means cones can relay ",
"three dimensions of color",
" to the brain, while the one type of rod would only ",
"give one dimension",
". "
] |
[
"interesting how the sensitive frequencies for the green and red cones are so close, could be why red/green colourblindness is so common."
] |
[
"Why is the sum of angles in a triangle more or less than 180 degrees in elliptical or hyperbolic geometry?"
] |
[
false
] | null |
[
"In mathematics, any \"fact\" that is \"proved\" actually depends on a base set of assumptions called ",
", which are usually taken for granted. However, if you are interested in alternative forms of mathematics, you can change what these axioms are, and you'll often get different (and often surprising) \"facts\".",
"This is a good example. The \"fact\" that the sum of angles in a triangle adds up to 180 degrees depends on the axioms that define Euclidean geometry (see: ",
"https://en.wikipedia.org/wiki/Euclidean_geometry#Axioms",
").",
"Four of these axioms were pretty intuitive: it would appear to be hard to do any sort of geometry without them. However, the fifth axiom was a bit controversial, because it seemed like you should be able to prove it with the other four, but no-one could. This is the famous Parallel Postulate: that parallel lines (i.e. two lines that are at right angles to the same third line) never meet.",
"It turns out (fast forward to the 19th century) that you could also do geometry if you took as an axiom that the parallel postulate was ",
", and this is how non-Euclidean geometry was developed. When you do this, the proof that the sum of angles in a triangle add up to 180 degrees no longer works, and in fact, it could take different values (as explained in another post, either greater than or less than 180 degrees depending on the whether the geometry was hyperbolic or elliptical).",
"Notice that you can make parallel lines meet on a sphere (like the longitudinal lines), meaning that triangles on the surface follow the rules of non-Euclidean geometry.",
"(TL;DR: The proof that angles in a triangle add up to 180 degrees depends on the parallel postulate, which doesn't hold in elliptical or hyperbolic geometry.)"
] |
[
"You got one thing backwards. Triangles in hyperbolic geometry ",
" have straight lines (by definition), but their projection onto a plane results in bended curves."
] |
[
"When you refer to a triangle in hyperbolic geometry, it is important to remember you are not referring to a triangle in the classical aspect. That is, the sides of the triangle are not straight lines and define a single plane, but a projection of a triangle on a surface. ",
"In hyperbolic geometry it is always going to be less than 180 degrees because the projection will cause the triangle to be 'bent inward.'",
"In elliptic geometry, such as the projection of a triangle onto a sphere, the projection will cause the triangle to be 'bent outward'. This makes the sum of the angles greater than 180 degrees.",
"Let me know if this helps."
] |
[
"Is there a fundamental difference between tornadoes and dust devils, or is it *just* a matter of size?"
] |
[
false
] |
Are they formed by similar (yet differently scaled) causes, or are they, at their root, different beasts that merely each other? Does there exist a smooth spectrum (continuum) of possible sizes, or is there a clear jump from dust devils to tornadoes, with nothing in between?
|
[
"Tornadoes begin a rapid rotation of extremely powerful super cells. The rapid rotation concentrates and accelerates, and wind shear causes it to descend to ground level. Dust devils are small low pressure vortices caused by rising thermals and the backfill of air along the ground. Fairly benign, but in theory could be the initial vertical development in an unstable atmosphere which leads to super cells. "
] |
[
"They are quite different.",
"Tornadoes must be attached to clouds; dust devils are not attached to clouds. Tornadoes require storms; dust devils form independently of storms. Tornadoes ",
" from the sky due to colliding air masses; dust devils ",
" from the ground due to heated updrafts."
] |
[
"At their root, they are different beasts that merely resemble each other.",
"Dust devils are driven by surface convection; warm air rising from the ground leaves low pressure behind it, and pulls in surrounding air to make up the difference. If there's even a tiny random rotation to that mass of surrounding air, that suddenly spins up as the air is pulled inwards - very similar to an ice skater pulling in their arms and spinning faster. Fundamentally, dust devils are a bottom-up phenomenon.",
"Tornadoes, on the other hand, require a lot \"helicity\" to form, which is a measurement of horizontal wind shear. In other words as you head upwards, how does the horizontal wind direction change? A lot of helicity means you can get horizontal rolling tubes of air between a lower mass of air heading one direction and an upper mass of air heading a different direction. Add in some strong convection, and you can start tipping these tubes from a horizontal orientation to a vertical one, allowing a funnel cloud to touch down. Fundamentally, tornadoes are a top-down phenomenon.",
" to add: Per your second question, there is sort of something \"in between\" tornadoes and dust devils - at least in terms of being a localized vortex with wind speeds somewhere between the two - but caused by a third entirely different reason: the ",
"gustnado",
". Strong thunderstorms have outflows; much the same way that rising air in a dust devil sucks air to the center, thunderstorm downdrafts can push air out around it at ground level. That can cause all kinds of turbulence at the edge of the outflow that can lead to vortices that spin off."
] |
[
"How does electric current flow through human body?"
] |
[
false
] |
For simplicity let's say that I touch two cables from electric grid. How these electrons moving in cables translate to, I guess, motion of ions in my body? Do these ions flow across cells, or in fluid surrounding?
|
[
"Ions wouldn't flow through the body, I'm not sure what you're getting at there.",
"Firstly, voltage is just potential energy to move electrons from one point to another. If you have a lot of voltage, the electrons have enough energy to flow through you, and that the high resistance of your tissue and flesh would cause a lot of heat. This presents a problem in that you'll probably become fried. ",
"Also related to voltage is current, the electrons themselves are the most dangerous part. There is a lot of current in the power grid, with a low enough voltage, it is fine, because the current will never be able to penetrate your high resistance skin to reach the important bits. But give those electrons energy, and they will affect your muscles among other things, most importantly your heart, the power in the grid is more than enough to send you into a lethal death grip, in which your heart fibrillates (irregular rhythm) and ultimately stops completely. That is if you are holding both ends of the cable in each hand, in theory electricity always takes the shortest path possible.",
"The reason why it does this is because your nerves that control your muscles work through a small electrical impulses, which cause them to contract. You get the picture."
] |
[
"But how do you imagine electrons passing through the body? Skipping from one molecule to another? Over so much distance? That's highly improbable. "
] |
[
"But the ions don't just move, they are a medium for the electrons if you like. They gain an electron at one side of the cable, and lose one at the other.",
"The salt content also makes you pretty conductive because under high voltage, they can generate a lot of current - as you might have expected, tissue and flesh itself has too high a resistance to generate significant current."
] |
[
"Does listening to music while studying makes studying less effective?"
] |
[
false
] |
I heard both recommendations of abstaining from listening to music while studying, and finding a song you know by heart and listen to it on loop while studying. Can someone give an answer for sure?
|
[
"So I actually did a research project on this in undergrad so I'll give a quick summary, but I'll have to dig through some old folders to find research to link.",
"So the main reason why listening to music can be bad is you encode (putting into memory) the sounds you're listening to while you're also encoding the information you are studying. When you go to retrieve the information you're looking through your memories for something that exactly matches the question you're reading. If you read the question with no music on, then you're trying to access information from memory but you're missing some of the information (the music) that was encoded originally. That being said, when you listen to music while retrieving the information your retrieval is actually better because you now have cues of music to help you find those memories as well. Its called a ",
"context effect",
".",
"I can expand on this answer later and add references, but I dont have time at the moment."
] |
[
"I've been listening to music to help me study for years. ",
"The best way I can explain it is that I need to have something that distracts me enough that I can ignore other things, but not so interesting that I need to think about what I'm hearing. This means music that I know really well, or instrumental music that isn't very complex. ",
"For the last year and a half-ish I've been listening to a Chopin station on Pandora, and Brown Noise ~20% Volume at ",
"SimplyNoise.com",
" to cover up the noise the music doesn't. ",
"If something complex or unfamiliar comes on my concentration gets broken and I need to change it, so I agree with you there. ",
" very familiar or simple music helps maintain focus, other stuff takes your attention away from studying. ",
"NOTE: didn't realize this was askscience, so no links, and sorry for the anecdote."
] |
[
"This next part will contradcit my first post a little, but it'll clarify which type of music is better to listen to. So my experiment in undergrad compared the effects of classical music, modern music (pop, rock, etc.), and no music. Due do a design flaw our results didn't turn out perfectly, but basically they showed listening to no music and classical music score roughly the same on our test, but listening to modern music which includes people singing harmed performance the most.",
"The theory behind why music with a person singing in it is worse to study to has to do with oscillatory synchrony and wave interference. Basically the idea is when we study we use an internal voice to rehearse the information we are reading much like we were speaking it out loud. This causes the wave patterns in our brain to look similar to the patterns seen when actually speaking. So when we also have music with language in it suddenly our brain has two language inputs to try and process at the same time, the one from the music and our internal one from studying. So when these language inputs create waves of activation in the cells of our brain that need to be transferred to memory. When there is a single wave it is simple, but when two waves come in together they begin to interfere with each other. ",
"video explaining wave interference",
". Basically the interference causes both language inputs to become somewhat scrambled on their way to being stored in memory leading to you not actually storing the information you're studying and thus you cannot recall that information at a later point in time. ",
"One source, can find others if requested",
"Listening to music with no singing in it can actually increase performace (even though my study didnt show this) by increasing arousal level slightly which can increase performance on memory tasks. This is based on whats known as the ",
"Yerkes-Dodson law",
".",
"tl;dr No music or music without singing in it are in theory the best to use when studying. Music with singing can interfere with your brains ability to remember the information you are trying to study."
] |
[
"How can we tell if a person is related to another by comparing their DNA?"
] |
[
false
] |
Hi there, I'm currently working on a programming project that compares two sets of DNA and determines their relationship(if any). Problem is I don't know exactly what I'm looking for to determine that relationship. Any help is greatly appreciated.
|
[
"Hi, I'm sorry no one has responded yet, but if you're still stuck, I hope the following is helpful:",
"From your data set, it looks like the columns from left to right are the specific genes, the chromosomes the genes are located in, the position of the gene in the chromosome, and the individual SNPs (single nucleotide polymorphisms) that differ between the parental alleles. For convenience, we will call these columns 1, 2, 3, 4A and 4B, respectively.",
"You are probably familiar with how gene transmission between parents and child works, but briefly: Each parent donates one allele of a chromosome to their child, who then has a total of two. But the alleles might not necessarily be the same, which is indicated by a different letter in columns 4A and 4B. The different letters in the columns are SNPs.",
"So when a parent passes genetic information to their child, they pass on 50% of their DNA. And if they have multiple children, each sibling should share 50% of the same genetic material with each other. So when studying inheritance and examining SNPs, we have primary familial relations (mother, father, siblings, children) that we share 50% of our DNA and, therefore, our SNPs with.",
"Then looking at your data set, to determine relationships between individuals, you would have to look at columns 4A and 4B and compare them across the same column 1 values. For primary familial relations, you would expect to see 50% identical values. For secondary familial relations (aunts, uncles, grandparents, nieces, nephews and half siblings), you would see 25%. For tertiary familial relations (first cousins), you would see 12.5%.",
"Hope this helps! Best of luck to you on your project!"
] |
[
"Were you given this task in university and nobody explained to you what to do?"
] |
[
"Sort of. My professor has a tendency to be vague in his project descriptions. "
] |
[
"Would someone who is exposed to germs more often have a noticeably better immune system over someone who is very hygienic?"
] |
[
false
] |
For example, someone who works outdoors all day versus someone who works in an office building. Or, someone who doesn't wash their hands and still eats food that is dropped on the ground versus someone who washes their hands frequently and never touches anything that might be dirty.
|
[
"This is in essence, the idea behind the Hygiene Hypothesis. It's the idea that the \"cleaner\" we've become, the less \"used\" our immune system is, and hence it becomes \"bored\". Of course, this is incredibly simplified.",
"Here are a few things I've seen in papers over the years that I can remember off the top of my head. You can ask me for references, but I won't guarantee that I'll remember everywhere I've read this stuff. I'm a graduate student in symbiosis research, so I read about this A LOT, but I don't always save the papers associated with what I've read.",
"\n* People who grew up on farms, or garden regularly, seem to have fewer problems with allergies.",
"\n* People who are given more antibiotics, especially earlier in life, are more prone to allergies.",
"\n* Allergies are more uncommon in \"third world countries\", and some speculate that this is because they have more problems with parasites. The major component of the immune system that causes allergies is also what combats parasites.",
"\n* Recent work has suggested that shifting the bacteria in our intestines towards one profile leads to inflammation, and could be associated with not only obesity, but a number of other ailments.",
"\n* I seem to remember reading something about people with multiple siblings being less prone to allergies, along with people who attended day care. But the memory of what I read is pretty fuzzy.",
"The gist that I gather isn't that people who are exposed to more \"germs\" have a better immune system, but rather that people who have less exposure have more problems. And not necessarily problems that we've always associated with our immune system."
] |
[
"almost completely absent in the 3rd world.",
"I think we should be cautious about saying this. Low diagnosis rates may be more due to a lack of medical care than actually low incidence rates. "
] |
[
"Does the \"more exposure leads to a better immune system\" idea only apply to children? Or are our immune systems dynamic enough to become stronger once we've grown into adults?"
] |
[
"Is there a maximum possible magnetic flux density?"
] |
[
false
] | null |
[
"There is the ",
"Schwinger limit",
": Above 4 billion T things get strange.",
"And then there is the Planck magnetic inductance, 2*10",
" T. That's at least the maximum where our known laws of physics have a chance to work."
] |
[
"Photon+photon -> lepton pairs are routine interactions in particle accelerators, more recently light-by-light scattering has been observed, too.",
"https://cds.cern.ch/record/2220773",
"https://arxiv.org/abs/1810.04602"
] |
[
"Photon+photon -> lepton pairs are routine interactions in particle accelerators, more recently light-by-light scattering has been observed, too.",
"https://cds.cern.ch/record/2220773",
"https://arxiv.org/abs/1810.04602"
] |
[
"What gives Polyester it's useful properties?"
] |
[
false
] |
I know that when Polyester is made into fabrics, the stretching forces the molecules into parallel patterns, giving the material more tensile strength, etc. What I want to know specifically: Why is it shatterproof? Why is it hydrophobic?
|
[
"\"Polyester\" is a broad class of materials. You're probably referring to polyethylene terephthalate (PET or PETE) which is the polyester used for soft drink bottles and most fabrics.",
"It is hydrophobic because its chemical structure is largely hydrocarbon based, with only weakly polar carbon-oxygen bonds at some intervals. Water is a polar molecule and there is not enough polar functionality in PET for it to interact with (i.e. the PET is hydrophobic) which is why water tends to bead up on untreated polyester surfaces. ",
"The weak polarity that ",
" present in PET is enough to help polymer chains \"lock\" against each other when the molecules are aligned (semi-crystalline), which helps provide it with strength.",
"In contrast you have polymers like polyethylene (plastic bag material) and polypropylene (yogourt tub material) which are semi-crystalline but have no polar functionality at all. They are highly hydrophobic but mechanically very weak unless they are cross-linked. "
] |
[
"I would agree with you until the last paragraph. Depending on which source you use then PET is about only 20-40% stronger than semicrystalline PP or PE. It is much easier aligned (by say drawing) so its effective stiffness and barrier properties can be much better though. Cross-linked amorphous PE is quite weak - semicrystalline PE is effectively crosslinked by the crystallites anyway and does not benefit from extra cross-linking."
] |
[
"Thank you for the clarification."
] |
[
"Please explain this Hytadid cyst in the brain"
] |
[
false
] |
It was submitted here: There's very little background or text. The doctor claims he was going to publish this in the journal. There are really dozens of questions about this, but if someone could explain this or has seen the journal article and could sum it up, that would be great as that would answer most, if not all of them. Thank you.
|
[
"You're being awfully vague on what you would like to know about this. It's a bacterial infection which forms cysts in the locations where bacteria have invaded, the cysts are fluid filled and patient's can be asymptomatic or symptomatic. Symptoms range due to the different areas in which the infection can manifest. There is no vaccine, it's generally resistant to normal antibiotic treatments and can manifest in people with vitamin/mineral deficiencies, generally of lower socioeconomic statures."
] |
[
"That's the sort of explanation I was hoping to get started, but it seems as though this one didn't make it to the front page."
] |
[
"Here's a link to the ",
"wiki",
" article for this disease. "
] |
[
"Would a laser sword cut a diamond sword or vice versa?"
] |
[
false
] | null |
[
"Well, they DO use lasers for cutting diamonds, but that doesn't mean that every laser can cut a diamond.",
"So the short answer would be: depends on the laser. If it was designed for cutting a diamond i suppose it would destroy the diamond sword, but if it weren't then the diamond sword would refract the laser beams and be unharmed."
] |
[
"So, would the refracted laser beams then become laser \"bolts\", of a sort, turning them into weapons themselves? Or does the refraction change the laser beam into some less \"powerful\" form of light. ",
"I know this is getting more and more sci-fi, but I know there are science-based explanations on how theoretical things would likely work if we had the technology.\nThanks again!"
] |
[
"Hmm, I suppose that would depend on how the beam gets split. For instance if it gets cut perfectly in half and each beam still transmits properly, then I guess it would burn two holes about half as large as the original burn hole would have been.",
"Keep in mind that the power of a laser beam is the intensity of the light, AND the fact that all the beams are traveling parallel with each other. You know the supposedly common practice of burning ants with a magnifying glass. At the point of focus, the sunlight is strong enough to burn. Basically, at every point along a laser beam the light is \"focused.\" So, if the diamond sword refracted the beam and messes with the focus, it could come out as a cone instead of a straight beam, then its power would be reduced dramatically with the distance traveled.",
"I suppose now is a good time to mention that Lasers is only like, two classes I took in college, so by no means am I well versed in laser sword physics."
] |
[
"Can the expansion of space be proved in other ways than the observation of distant things moving away?"
] |
[
false
] |
I took IB HL Physics (Highschool AP equivalent), and what I know is that Edwin Hubble discovered that distant stars, galaxies etc were moving away faster than the speed of light(measured with the Doppler effect), and things further away were moving faster proportionally. My question is that does this constitute proof of the expansion of space? Or is there other proof? Is there room for the alternative hypothesis that as distance increases there is an enhancement of the Doppler shift by some other factor, possibly some relation between space and EM waves that is insignificant at distances we can measure without it?, that explains the measured velocity of these distant objects?
|
[
"Remember that there's no proof in science - all we can do is accumulate more and more evidence for something.",
"An expanding universe is implied by a pretty wide range of observations today, like the irregularities in the [cosmic microwave background](en.wikipedia.org/wiki/Cosmic_microwave_background_radiation), the formation of structure like galaxy clusters, and the ",
"primordial abundances of light elements",
". These things would look very different (and disagree with observations) if the universe weren't expanding. Thankfully we're long past the days when we could only probe the history of the Universe using the measurements of galaxies moving away from us!"
] |
[
"Ah, that's a long and complicated story :) The CMB contains a ",
" of information. One of the simplest things we can do to the CMB map is to statistically analyze it by looking at how common hot or cold spots of a certain size are. The picture you get looks like ",
"this",
", using the latest data from the Planck space telescope. The x-axis is angular scale (from large to small), and the y-axis is roughly how many hot or cold spots of that size the CMB has.",
"Originally, this plot would have been a horizontal line (this is a key prediction of the theory of ",
"inflation",
"), which posits that the expansion of the very early Universe was rapidly accelerating). But between the earliest moments and the time, a few hundred thousand years later, when the CMB was emitted, the distribution of temperatures in the Universe underwent gravitational evolution and changed as the Universe expanded. Actually calculating how the horizontal line evolves into something bumpy like the plot I showed you is quite involved, but suffice it to say that when you input the standard cosmological model (expanding universe, dark matter, three neutrinos, etc.), along with parameters which are supported by other observations, you beautifully reproduce the data."
] |
[
"Not quite - energy isn't actually conserved in an expanding universe, for one thing. Photons, for example, lose energy as the Universe expands."
] |
[
"Can plasma electrocute you?"
] |
[
false
] | null |
[
"When it comes to lightning, are you sure it's the plasma that electrocutes you and not the electric current that created the plasma? The other answer given here says that plasma can't electrocute you, but the process which creates it can. So wouldn't the same electrical discharge that turned the air into plasma be the culprit for your electrocution and not the plasma itself?"
] |
[
"When it comes to lightning, are you sure it's the plasma that electrocutes you and not the electric current that created the plasma? The other answer given here says that plasma can't electrocute you, but the process which creates it can. So wouldn't the same electrical discharge that turned the air into plasma be the culprit for your electrocution and not the plasma itself?"
] |
[
"Plasma is a gas composed of positive ions and electrons in roughly equal amounts. By itself it can't electrocute you, but the process which created it could. Man made plasmas often use high voltage. A small neon bulb needs around 90 volts. A neon sign uses several thousand. A static electric shock creates a tiny, brief plasma. "
] |
[
"What happens to concrete on a molecular level when it cures?"
] |
[
false
] |
I know it does not just dry out the water, otherwise buildings would melt when it rains. What exactly happens? Thanks.
|
[
"I'll keep this simple, but here we go:",
"Cement is a mixture of chemicals which undergo a reaction called Hydration. In the first stage, the powder particles react with water and forms a gel layer. Out from this gel layer crystals grow into a spine structure, like a microscopic hedgehog. These spines interlace with local particles doing the same thing and thus we get a binding effect. Adding aggregates to the mix makes it harder for cracks to propagate through the solid concrete and thus increases strength. ",
"The hydration reaction continues for about 100 days but most of the concrete strength is achieved in three weeks. The reaction is irreversible which is why you don't see concrete dissolving in rain. ",
"The more important thing to remember is buildings are generally made from ",
" concrete. The reinforcing is sometimes fibres which prevent cracking just like aggregates but more commonly it's steel bars. Steel is good at resisting tensile loads (\"stretching\") and concrete is good at resisting compressive loads (\"crushing\") and so with these two combined, we get a synergy and the resulting structure more cost effective than just using heaps of steel.",
"Edit: Source materials science papers in university. Concrete chemistry is an ongoing object of study but I've given our current understanding."
] |
[
"Also notable is that concrete and steel tend to expand and contract at nearly the same rate with thermal variation."
] |
[
"Concrete doesn't dry out, that's correct. Instead, it sets, or cures. Portland cement (what pretty much every concrete building uses) contains Calcium Silicates which react with water to form mineral hydrates, which are what give cement it's desirable properties as a binding material in concrete."
] |
[
"How do we choose anything \"randomly\"? Does our brain have tendencies or patterns, or can we achieve true randomness?"
] |
[
false
] |
For example, asking to randomly pick a number between 1 and 10. You can choose one and not give a reason why. You just randomly choose a number from thin air. Is there a pattern or reasoning our brain goes through when choosing something randomly?
|
[
"Random number generators do not produce random numbers either, they are algorithmic, and they rely on the assumption that you don't know the input(s) to the algorithm that output the number."
] |
[
"There is a way to avoid this. Just reference the state of some external thing that you don't already know the state of.",
"I'm saying roll a dice."
] |
[
"Even if the universe it deterministic we could have apparent randomness depending on what we know about the current state and what we observe from the result set we get from our randomness generator. ",
"From our own brains if we spat out a list of random numbers with some bound we would almost definitely see a non-random distribution. "
] |
[
"If, using todays technology, we wanted to craft a set of Melee weapons and armor (sword, shield, armor), what materials and techniques would we use?"
] |
[
false
] | null |
[
"The best blade steel today is pretty much the same as it's been for hundreds of years...the difference is that today we know why it's good. A sword needs to be incredibly malleable (not brittle), as well as hard enough to hold a good point. The balance can be achieved by layering carbon inclusions, which harden the steel, in a matrix of mild steel. An accessible (though advertising) article about knife steel can be found here: ",
"http://www.kershawknives.com/aboutus.php?brand=shun",
". The biggest difference now is the amount of control and knowledge that we have. We know better how to make a blade that would hold its edge a long time, take a crazy sharp edge, or withstand lots of impact, although these three factors are usually tradeoffs. ",
"As for shields and armor, we have many many more options! Titanium and composites (carbon fiber) stand out to me. Some polymers might also be useful (kevlar), to line those gaps and protect you from a piercing rapier. The figure of merit here is simpler - we want a high strength to weight ratio, without being brittle, and these new materials beat steel plate here handily. "
] |
[
"Yes",
"To make a functional sword you need something that is durable and can retain an edge. Steel really is the best material for the job at this time. All you need to do is look at the knife industry. ",
"There are plenty of companies that make knives with ceramic blades. These blades will maintain their edge for ages, but can be shattered simply by dropping them. ",
"Occasionally you'll see a knife that's completely carbon fiber. Carbon fiber is incredibly durable, but you won't be able to get a usable edge on it."
] |
[
"You must remember that melee armour was really intended for battlefields only (and ceremonial tournaments).",
"In duels, fighters did not want to use armor, and preferred light blades like rapiers, optionally with a very small shield (like under 30cm diameter). The speed and mobility reduction from full plate and an arming sword or greatsword would enable a fencer to tire you out, then stab you when you were flagging.",
"On a battlefield where you could be shot or beset upon from an unexpected angle, armor was used and as a result, large, heavy armor piercing sword was required.",
"If you look at the difference between the japanese katana, with a curved blade - able to cut in one dirction only, not very good at stabbing, this blade was not designed to penetrate full plate armor, rather to produce murderous cuts vs light armors or unarmored flesh.",
"The european swords on the other hand, had straight blades with a sharp point that could be stabbed through plate armor. Such a sword would not cut flesh as \"easily\" in that, it could not do draw cuts (where you draw the blade along the flesh and the curve of the blade makes it cut deeper) but it was better verus full plate",
"I would ask you therefore - for what purpose were you making the weapons?",
"There's not really been any noticable increase in sword tech: katana are still good today, and while ",
" european swords were mass produced for soldiers, the special ones made for nobles were close enough to a katana that the difference would be the man, not the weapon. Europe had martial arts, and they were good, but abandoned all except fencing when crossbows and guns arrived. Japan banned guns as dishonourable and thus ",
" its martial arts.",
"I'm not up on current knnifeproof vests, can't comment on that. But it has to work vs spiked warhammers too.",
"EDIT: PS: These guys have some nice videos of recreations of the old european martial arts - greatsword with plate armor for example. It looks very different from what you see now in games and films, because modern stuntmen use modern eastern styles. The reason why they seem to have so little mobility, is basically, plate armor is not as flexible as it looks. ",
"http://www.thearma.org/"
] |
[
"Let's say in each hand I have half a sphere made of Uranium-235 such that together they are supercritical. What happens if I slowly bring them together?"
] |
[
false
] | null |
[
"You will see blue light, feel a little bit warm, and die an agonizingly, excruciatingly, horrible, painful death from radiation poisoning. Under no circumstances should you do this.",
"When you bring them together you are eventually going to hit a point where the atoms are going to begin undergoing fission chain reactions and you are going to get hit with a burst of neutrons, along with a wave of heat and a flash of blue light. The blue flash in criticality incidents is often incorrectly attributed to Cherenkov radiation, which is characteristic of some reactors and is produced when charged particles travel faster than the speed of light in the medium (Edit: ",
"See further explanation here",
"). In this case it's actually caused by ionization of air. The burst of neutrons will release a lot of energy and will cause cascades of particle collisions, doing tons of damage over a wide area, particularly inside you. During this exposure, you may experience a sour taste in your mouth and an intense burning sensation in the hands holding the cores, following which you will likely vomit and be hospitalized due to acute radiation exposure and there will be nothing the doctors can do for you as they watch your organs fail in the coming week. In reddit parlance, this kills the nuclear physicist. ",
"This is actually what killed 2 scientists working on the Manhattan project- they slipped up and let a core they were working with go critical for a brief time, and it killed them, which is how I have such good specifics in the above paragraph. And yes, they were literally tinkering with what would be cores of ",
" with the same sort of nonchalance that you might play with a Rubik's cube. Anyway, this core was named the Demon Core for killing these scientists, and you can read a lot about it ",
"here",
". It's amusing to note that the Demon core was used in a test after the war had already ended, and so it probably has the lowest nonzero body-count of any detonated nuclear device. ",
"But I really can't stress this enough: please do not slowly bring together two half-spherical Uranium cores to produce a supercritical mass of Uranium. This is a very bad idea. "
] |
[
"about 500 pounds of explosives slamming the two pieces together with high energy."
] |
[
"A certain mass-to-surface ratio is required for the Uranium-235 to attain the proverbial 'critical mass'.",
"Uranium-235 atoms naturally decay and shoot out a few neutrons when it does. In order to start a chain reaction, at least 1 of those few will need to hit another Uranium-235 atom, which will then also decay, shoot out neutrons, etc, etc. The more atoms there are in a small area, the more likely one of them will be hit by a neutron; the bigger the surface of the core, the more likely a neutron will scamper off before hitting another U-235.\nThe best shape to maximize mass-to-surface ratio is a sphere. The easiest way reduce the mass-to-surface ratio of a sphere is to cut it into 2 halves.",
"TL;DR: 2 half spheres of less-then-critical mass, put together to make one sphere of more-than-critical mass."
] |
[
"Why does liquid nitrogen freeze when placed in a vacuum?"
] |
[
false
] |
I teach science classes to kids at a camp, and in one of our experiments we put liquid nitrogen in a simple vacuum chamber. After a few seconds, it stops boiling and freezes. I was looking at the phase diagram for nitrogen and doing a bit of Googling, but I didn't find an adequate explanation. Can anyone tell me what's going on exactly?
|
[
"Think of it this way: when you have a liquid that is boiling, the parts of it that turn into gas are the parts that have the most energy (or are the hottest). The nitrogen is made up of a bunch of different particles all jiggling around with different temperatures. By applying the vaccuum pump, you are basically sucking away the hottest particles. This leaves the remaining particles cooler.",
"As the nitrogen gets cooler and cooler, it eventually gets cold enough to freeze.",
"This applies to evaporation as well, not just boiling.",
"Edit: there are some objections to my answer, scroll down and read more. I was trying to give an understandable/accessible answer, and it's not completely correct."
] |
[
"Why the downvotes?",
"It's not \"atmospheric water\" as the top voted comment suggests. It's a well known phenomenon that you can freeze liquid nitrogen by putting it in a vacuum chamber. There's tons of youtube videos demonstrating it, here is one (you can think of it as a source): ",
"http://www.youtube.com/watch?v=FReIs6rQbPM",
"Here's another source: ",
"http://www.youtube.com/watch?src_vid=OIcFSHAz4E8&feature=iv&v=1RJKnbIvbMg&annotation_id=annotation_471526",
"This is the old documentary about superfluid helium. You'll see that they cool the liquid helium down to the lamba point by using a vaccuum pump exactly as I described."
] |
[
"That assumes that the temperature is held constant during the reduction in pressure. Lowering pressure induces evaporation, which will remove heat from the bulk, the same way that evaporation of sweat cools you down. ",
"The enthalpy of vaporization of nitrogen is pretty low, so you'd need a lot of evaporation to freeze nitrogen, though."
] |
[
"Do we know whether Hash functions produce all possible outputs?"
] |
[
false
] |
Looking at SHA-256, it outputs 256 bits for any input. Due to the pigeonhole principle there are guaranteed to be collisions among them. My question though is if we have actually proven that every variation of the 256 bit output string is actually part of the result space or is there a possibility that one string is actually impossible to produce?
|
[
"Just from looking at the definition of a cryptographic hash function, it is computationally infeasible to generate a message with a given hash value. When we try and compute these, we are doing what is called a \"pre-image\" attack, and the fastest algorithms we have for these have complexity around 2",
" where n is the number of bits. ",
"So, theoretically, given a hash value, we could compute the preimage in O(2",
") time. This doesn't quite answer your question. ",
"There is no direct proof (at least that I can find) that states that any hash function is surjective (i.e. produces all possible outputs). ",
"However, there is a potential drawback to a hash function which can be proved to be surjective. If we can always find some string S such that Hash(S) is some given 256-bit string S', then we can generate collisions just as easily. This is very similar to the pre-image attack I described earlier. ",
"Furthermore, this proof would depend entirely on the algorithm used to generate the hash value, e.g. MD5, SHA-256, Keccak, etc.",
"So to answer your question, no, we have not actually proven that SHA-256 (or any other hash function) is surjective, and this is mostly due to computational complexity. Due to the complicated nature of the hash algorithms, my intuition says that this is not possible to do without doing a preimage attack on each of the possible 256-bit strings (for SHA-256). It would take a computational complexity of at least (2",
") to prove this. ",
"This stackoverflow thread might be helpful to read : ",
"http://stackoverflow.com/questions/2658601/do-cryptographic-hash-functions-reach-each-possible-value-e-g-are-they-surject"
] |
[
"However, there is a potential drawback to a hash function which can be proved to be surjective. If we can always find some string S such that Hash(S) is some given 256-bit string S', then we can generate collisions just as easily.",
"What if you had a non-constructive proof that some hash function is surjective?"
] |
[
" Nothing really changes. ",
"This answer is specifically for SHA-256, so some numbers will be different depending on which hash function we are talking about. Surjectivity means that given any y in {0,1}",
" , I can find x (any string of length 2",
" - 1 bits) such that SHA256(x) = y. ",
"The other way to prove surjectivity (other than generating x with given hash value y) would be to say that the image of SHA256 is equal to {0,1}",
" , which would require the calculation of the hashes of all possible strings (2",
" = 10",
" ) (roughly). ",
"But, let's assume for a moment, that starting with the string 000...0, and then moving on to the next string 000...01, then 000...10, etc. that we move through all possible hash values (no collisions for the first 2",
" calculations). Even if we were to calculate all of these hashes, it would still take 1.15 x 10",
" calculations to prove that we hit every string in {0,1}",
" . For a frame of reference, there are approximately 10",
" atoms in the observable universe. So we are still limited by computational complexity. ",
"So, to answer your question, if the hash function is proven to be surjective, ignoring for a moment how it is proven, then not much changes. In fact, it is probably true that most hash functions are surjective, just based on the sheer size of the inputs compared to the outputs. However, it still is computationally infeasible to generate a pre-image for a given hash value, and collisions are still computationally infeasible to generate. ",
"I would say that the act of proving surjectivity would be more impressive than the fact that the hash function is surjective. ",
"In Eurocrypt 2015, there is a rather abstract proof that almost every hash function is surjective: ",
"here",
" is a link to the google book page. "
] |
[
"dentist redditors. Is an electrical toothbrush really better than a \"normal\" manual one?"
] |
[
false
] | null |
[
"Dentist here: There are a few main advantages to using an electric toothbrush.",
"Most have a built-in timer. This more of a plus than you might think. The majority of my patients simply don't brush enough, and when they do, they don't brush for long enough. It is recommended that you brush for around 2 minutes, and although it may seem like forever, you've probably brushed for only 45 seconds.",
"Most brands will oscillate or pulse (depending on brand), and despite what the dental student says (aka what the Oral B rep has told him/her) it wont make much difference between the two. The main point is that slight vibration that you can't recreate manually. Now I'm told that this disruption does break up plaque colonies that are just out of the reach of the brush head, and I know that's mostly marketing too (but there's a bit of truth to it too).",
"Patients who are disabled or lack the dexterity NEED electric toothbrushes. It does the work for you. The handles are usually bigger and easier to grasp, and from there all you do it apply it to the tooth. And if you think technique doesn't matter, try just rinsing your mouth out with water and toothpaste and see how effective that is (bachelor frogs weigh in here?)",
"Last but definitely not least, I'll make this argument because fellow redditors need to know this. I can only assume it is stress that causes this, but people are brushing TOO HARD. This can cause severe recession and sensitivity problems. (Yes neuromuscular peeps, I know occlusion causes abfraction as well.) I know recommend a soft bristle toothbrush too nearly all patients and an electric if they can spare the 55.00 dollars I charge for one. With an electric toothbrush, you feel like you don't have to push as hard to get that clean feeling. For manual brushers, I tell them to hold the brush with only 2 fingers, or brush imagining that they are polishing a tomato.",
"TL;DR - Habit and personal hygiene are most important, but YES electric toothbrushes do help and I recommend them to most patients"
] |
[
"Study 1",
" compares Sonicare to manual toothbrushes for orthodontic adolescents.",
"I did read another study on normal people, but I can't find the link in my 15 seconds of google searching. What makes sonicare more awesome is the fact that is moves the water/saliva/toothpaste mixture around so fast that the brushes don't even need to contact the teeth to remove plaque. Rotating brushes don't move nearly fast enough to make the fluid do any actual work. I've had the same original Sonicare for over 10 years now, and it still works wonders to my teeth. I haven't had a single cavity in those 10 years and I go to the dentist 1 per year.",
"Don't bother spending extra money on the fancy-pants brushes, just go with the cheapest Sonicare you can find. You can even get AA-powered ones for under $30."
] |
[
"A 3-month clinical investigation comparing the safety and efficacy of a novel electric toothbrush (Braun Oral-B 3D Plaque Remover) with a manual toothbrush.",
"The effect of a new electric toothbrush on supragingival plaque and gingivitis. ",
"The long-term effect of an oscillating/rotating electric toothbrush on gingivitis. An 8-month clinical study.",
"The role of the electric toothbrush in the control of plaque and gingivitis: a review of 5 years clinical experience with the Braun Oral-B Plaque Remover [D7].",
"Excerpt from the last source...",
"A review of the findings from these various studies indicates that while both the D7 [an electric toothbrush] and manual toothbrushes effectively remove plaque and improve gingival condition, the efficacy of the D7 is clearly greater than that of the manual toothbrush. It is suggested that this advantage is related in particular to greater efficiency in interproximal, vestibular and lingual sites. ... It is concluded that the greater efficacy of the D7 over that of a manual toothbrush, together with the motivational effect that has been shown to be associated with electric toothbrush use, should offer potential for improved oral hygiene control.",
"If you don't feel like reading them most of the studies show a 10% advantage in reduction of plaque, gingivitis, and bleeding for the electric, especially in hard to reach areas like behind the incisors where it's hard to get pressure with a manual. It's funny that all the studies enforce brushing twice a day and the manual brushing groups health all improved as well. What were they doing before haha.",
"edit:\nFor In_The_East's question below.",
"A comparative clinical investigation of the safety and efficacy of an oscillating/rotating electric toothbrush and a sonic toothbrush.",
"Conclusion: No difference."
] |
[
"How do snowflakes form so symmetrical?"
] |
[
false
] |
[deleted]
|
[
"Snowflakes are ice of course, which is crystalline water. Snowflakes grow and take their shape the same way that any other crystalline substance does, whether it's steel or quartz or diamond. Crystals are substances where the atoms in them are ordered in a repeating pattern, called a lattice. It's very similar to Escher's tesselation drawings, if you are familiar with those: one shape (a particular cluster of atoms), in a particular orientation, repeats over and over. The exact shape and orientation of the cluster of atoms determines what (macroscopic) shape a crystal can take.",
"For snowflakes, if all the water droplets they start from are about the same size and the temperature and humidity of the air they move through are fairly constant, I would expect them to generally grow into similar shapes. I expect it was a nice calm day, without a lot of wind."
] |
[
"Thanks for the reply. Yes there was no wind and the flakes were rather large. It was very soothing. Especially since I live in a place that only sees a little snow once or twice a year, if that. "
] |
[
"A non-aggregated snowflake often exhibits six-fold radial symmetry. The initial symmetry can occur because the crystalline structure of ice is six-fold.",
"\n",
"http://en.wikipedia.org/wiki/Snowflake#Symmetry",
"\nYou have the world of information at your fingertips but no motivation to search it yourself."
] |
[
"If Earth has an elliptical orbit, why can't we feel its acceleration?"
] |
[
false
] |
In an elliptical orbit, the orbiting body is much faster as it is nearer to its sun(What's the generic term for the center body?). Why don't the people on the leading side of the earth get pushed downward when the planet speeds up?
|
[
"That is accurate though. We are orbiting the sun."
] |
[
"No.",
"We would not experience acceleration relative to Earth even if it were on a highly elliptic orbit. See other answers on this page."
] |
[
"Even in an elliptical orbit, you are still in freefall at your apogee, perigee, and everywhere inbetween. The best way to get an intuitive idea of how orbital mechanics work is to play Kerbal Space Program for a few hours."
] |
[
"Is heated food in the microwave radioactive?"
] |
[
false
] | null |
[
"Microwave radiation is orders of magnitude less energetic than something as simple as visible light.",
"Artificial creation of radioactive isotopes is something that is typically limited to nuclear reactors, particle accelerators or similar devices. It's not just a matter of simply scaling up a microwave oven."
] |
[
"No."
] |
[
"How come?"
] |
[
"Proton size problem?"
] |
[
false
] |
A recent experiment showed that the proton size of a muonic hydrogen atom was ~0.84 fm, however regular protons have a size of ~0.88 fm. Couldn't a muon be "pushing" a proton together making it smaller while a electron be "pulling" the proton making it expand. Can someone explain why this has to point to a "new" science not predicted by the standard model and not one already know?
|
[
"The point is not that the proton must be .88 fm or .84 fm, but rather that these measurements of the proton size are inconsistent with each other -- and ",
" is what requires explanation. Within the Standard Model, there is no effect that would cause the proton size to vary in this way depending on whether it is in an ordinary hydrogen atom or a muonic hydrogen atom. We know in very precise ways how electrons and muons and quarks interact, and there is nothing that would cause an effect like this. So the possibilities are that the discrepancy is not really there (could be a systematic error, could be a theoretical subtlety in comparing the measurements, etc.) or that there is some unanticipated physics causing this discrepancy.",
"This ",
"image",
" from the link in my previous answer indicates to me that this is a discrepancy that will go away, given that there seem to be two categories of results, but that is just a hunch on my part. High precision spectroscopy is an incredibly mature and sophisticated field, and so one should not take such a discrepancy lightly."
] |
[
"Can you link to a claim that this must require new science?",
"In ",
"this article",
" from members of the CREMA collaboration from 2010, several possible explanations are offered:",
"First of all we need to understand the origin of the observed discrepancy. It may be a computational mistake of the energy levels in muonic hydrogen or hydrogen, a fundamental error in bound-state QED, an unknown effect related to the proton or the muon, or a contribution which has been neglected. In addition it may be that the Rydberg constant, the most precise constant in physics, has to be slightly corrected."
] |
[
"Sorry, was reading ",
"this ars technica article",
" and this line",
"That shouldn't be the case, and if it actually turns out to be, then it's a sign there must be some new physics out there. ",
"is what I am going off...",
"Also ",
"this phys.org article",
" states that ",
"Very interesting proposals explain the discrepancies by physics beyond the standard model.",
"I was just wondering (not stating) why people think that this HAS to require something beyond the standard model since I see nothing that states that a proton must be exactly 0.88 fm. Only that a proton must have three color quarks. Is there something I'm missing from repeated wikipedia/Wolfram-Alpha searches that says quarks MUST maintain a certain distance from each other?",
"Why couldn't it be like a balloon, filled with air to the point of almost bursting on top of a mountain and then taken down to sea level where the balloon is now quite visibly smaller? The amount of mass, energy, charge, spin and anything else of the proton didn't change, only the size of the proton."
] |
[
"Why does meat not contain any carbohydrates when muscle tissue stores glycogen?"
] |
[
false
] |
Do animals (e.g. cattle) not store any relevant amounts of glycogen in their muscle tissue, or why does meat have no carbohydrates?
|
[
"Ah! Found the textbook(for humans anyway; ",
" by McArdle et al.). ",
"It's a distinction between concentration and total. For humans, the body might have 400g of muscle glycogen and 100g of liver glycogen, so you're right, in total, most of the glycogen is in the muscle.",
"But in terms of concentration (important when we're talking about eating a 1/4 lb of meat or liver as a \"serving\"), the liver weighs 1-2kg, and muscles might weigh 40kg. So the liver is 5-10% glycogen (100g/2kg), the muscle is 1% (400g/40kg). For nutritional labeling, this low % is the important number.",
"No idea about effects of slaughter; should have sounded like I was speculating there."
] |
[
"Most of the body glycogen is stored in the liver. Muscle only contains around 1-2% glycogen, which is also broken down quickly so even less remains in meat (low enough to not count for nutritional labeling).",
"(this is memory from a textbook; source not directly at hand but Wikipedia article on glycogen kind of implies this)."
] |
[
"Much of the intramuscular glycogen is lost when meat is cooked. Raw meat contains dietarily significant amounts if carbohydrate. "
] |
[
"Is quantum tunneling of DNA molecules the origin or one of the origins of DNA mutations ?"
] |
[
false
] |
[deleted]
|
[
"Short (pedantic) answer, no, tunnelling of DNA molecules is not an origin of mutations. Tunnelling is when something goes through a barrier that it seems like it shouldn't be able to (think of running at a wall and finding yourself on the other side). If a DNA molecule did this, that would not lead to a mutation (change in its chemical structure).",
"Longer answer: kind of, maybe. Tunnelling is one mechanism that can cause chemical changes, e.g. a part of a molecule (a proton, atom, group of atoms etc.) can tunnel to a different place, causing a change. A mutation mechanism was hypothesized (the Lowdin mechanism) by a which one nucleotide (part of a DNA sequence, think of as a DNA letter) can be changed into another. In this mechanism, a proton tunnels out of a nucleotide, changing it, and when the DNA is replicated, the old, modified nucleotide doesn't copy properly, leading to a change in the sequence. For details, I recommend googling the Lowdin mechanism. As far as I know, it hasn't been accepted that it actually happens, but there are a bunch of papers that suggest that seem to support it (that see mutations that seem likely to be caused by the Lowdin mechanism but I haven't seen any strong evidence that it is in fact that mechanism and not any other). ",
"More generally, quantum biology is definitely a thing and is accepted. Quantum mechanics describes how small stuff like atoms/electrons behave and their behaviour is the basis of chemistry and thus biology, so on some level quantum mechanics must be relevant to biology. In that sense (and likely others that can be derived from it) Schrodinger is right that quantum mechanics is responsible for mutations, because it is responsible for chemistry and mutations are chemical changes. But not in any sense that one cannot say about any chemistry. The role of entanglement in magnetoreception and olfaction are my usual go-to examples of quantum mechanics affecting biology, but there are many many others. "
] |
[
"Glad to be useful. PhD in physics and BSc in biochemistry. I currently do research in polymers and biophysics. "
] |
[
"Thanks for the very good answer. For curiosity, what is your background ? Biology ?"
] |
[
"Hey, scientists: who decides what gets researched?"
] |
[
false
] |
Every day there are new headlines about the latest cool discoveries. Obviously these discoveries generally follow years of intensive research by scientists. Who decides what projects to research? I imagine some projects happen because DARPA, a nonprofit foundation, a corporation, or some other group offers funding for specific research. I imagine other projects happen simply because some professor somewhere has a cool idea. What percentage of science's overall direction comes "top-down" from funding sources? What percentage comes "bottom-up" from the collective interests of each separate academic team? Are there other significant contributing factors in deciding which science gets done? (edit: massive wall of text)
|
[
"Professors come up with research ideas. They write up a grant application based on this. They submit it for funding, and then a panel looks it over and decides to give it money or not.",
"Generally, a grant panel has a certain amount of money to allocate and a certain number of projects, and they have to decide what goes where. "
] |
[
"I'm probably not the most qualified person to answer this, but I'll give it a go. As for the first, it would probably be better to discuss your ideas with a knowledgeable friend first and see if he or she can put you in contact with a professor with similar interests, because most people who go to professors with their ideas are totally insane.",
"As for the second, you could donate the money to a university or to a charity (my old research was supported by the Terry Fox Foundation, a Canadian cancer charity) and say \"I want this to go towards quantum computing\" (or whatever) and hope that it finds itself in a worthy place.",
"Another option would be to set up a scholarship fund for bright up and coming students, so that there will be more smart people with novel ideas who are able to get the education they need."
] |
[
"A huge percentage of biomedical research funding comes from the NIH. In 2003, according to Wikipedia, it was 28%, or 26.4 billion dollars. Most of the rest of the funding is locked up in industry and not available to academic scientists.",
"Every year, the NIH puts out requests for grant applications. Many of these are targeted in specific ways, which may be general or specific. For example, they might allocate X dollars to general cancer research, or Y dollars to people working on vaccines for a specific disease. The Principal Investigators (heads) of academic labs then write grant applications, laying out their research plan and detailing what they plan to do with the money.",
"To answer your question, yes, this process absolutely affects which science gets done. The NIH director and subgroups of people below him decide how money gets apportioned. (sometimes congress also gets sneaky and mandates that X dollars go towards AIDS, or cancer, or their favorite disease X). If you're running a lab, you make sure that your research fits in with these goals, or you don't get funding, your lab does no work, and you'll quickly find yourself out of a job. With only around 15% of NIH grant applications getting funded, competition is fierce. Established investigators with a strong track record gobble up a huge percentage of the money, which makes it very difficult if you're a young faculty member trying to get your start.",
"As for who actually decides who gets the funding, senior scientists in a field are invited to sit on a \"study section\", which reviews all of the grants, triages them, scores them, and then awards funding based on the rankings. There is much debate about how the NIH and these sections may be primarily funding \"safe\" research, which is guaranteed to make small advances, rather than betting bigger on high-risk, high-reward research. In response, they've earmarked some money towards things like \"innovator\" grants, which are supposed to help close this hole.",
"It's all really quite fascinating, inside baseball stuff, and the dirty secret of science is that running your own lab means that you have very little time to actually do science. You spend most of your time writing grants, dealing with paperwork, and supervising your postdocs and graduate students, who do pretty much all of the hands on work. It's easy to understand why some people have to desire to go that route."
] |
[
"Does healing from injuries (bruises,cuts, ect.) \"burn\" calories?"
] |
[
false
] |
Does the act of healing from an injury use a significant amount of energy? Would eating more help you heal faster? Sorry if this is a dumb question, just wondering.
|
[
"Yes, it does burn calories. Unless the injuries are VERY severe, it won't be a noticeable amount. For example, the calories burnt from healing a broken leg are probably less than the calories you would burn through walking/running/exercising with that leg.",
"In some cases, severe burns for example, caloric requirements can get quite high"
] |
[
"Interesting? Have any links for that? (Just wondering, would love to read up more on it!) And what about bruising and internal injuries such as joint hyperextension or tears? "
] |
[
"I am actually our resident hematologist",
"No you aren't.",
"http://www.reddit.com/r/MDEnts/comments/23mkht/baltimore_county/cgyl9jm",
"http://www.reddit.com/r/AskReddit/comments/1ootpl/how_old_are_you_and_how_much_do_you_earn/ccu1wc4",
"http://www.reddit.com/r/philosophy/comments/1qbv44/internshipsjobs_for_philosophy_undergrads/",
"Also, everything else wrong with what you said."
] |
[
"What happens to human remains in shipwrecked submarines?"
] |
[
false
] |
The discovery of the USS Grayback prompted me to wonder: what happens to human remains in shipwrecked submarines? First, is it necessarily true that all compartments of a submarine flood if it wrecks? If so, I would assume normal ocean decomposition. If there are air pockets, what happens to the bodies of the crew in those compartments? Are the microbes we live with capable of consuming our entire bodies? Would they eat only soft tissue? Would the bodies mummify?
|
[
"u/somewhat_random",
" touches on some important factors - even if a submarine has a sealed compartment, these seals will eventually fail. Water is quite capable at, with enough time, dissolving, corroding or otherwise wearing away at ",
". Plus, submarines are rated for a particular maximum depth - below this depth, their hulls can't withstand the pressure, and fail. Surface ships are not intended to be underwater at the best of times and so will fail much sooner.",
"A large, and not always appreciated, factor in what will happen to organic matter in the ocean, is the ",
". The depth that a submarine or ship has sunk to will have marked effects on what sort of biological breakdown processes are able to act. Depth affects three controls: bacterial concentration, water temperature, and oxygen concentration.",
"For example, if you check ",
"this link",
", it gives you what is called a 'thermocline' - the average temperature of the ocean at particular depths. The ",
" is about 426 metres below sea level (1,400 ft), so from the chart we can see that its local ambient temperature is around 15 - 20 degrees C. This is pretty good for microbial life. I'd say we could expect quite the community of life at this depth that are equipped to deal with surface organics. ",
"There's still plenty (relatively speaking) of oxygen at ~400m depth as well. ",
"Finally, microbe populations are quite depth-dependent too. ",
"This article",
" is an intriguing, yet dense, article that looks at microbe population in the ocean as a function of depth. Long story short, organic breakdown occurs to one extent or another at pretty much all depths (it turns out bacteria are ",
" adaptable), but the rate of decomposition is quite depth-dependent.",
"What all of this translates to is that the ",
" that the ship sinks to will affect how quickly the organic matter onboard (i.e. the sailors, provisions, etc.) will be broken down. In the deep ocean, where oxygen is lower and temperatures are colder, life is, to an extent, slower. Breakdown happens over a greater period of time. Many of the bacteria that we carry with us will likely perish because the environment is too damn different, but local strains will replace them and break down the bodies. Various scavenging creatures will assist in this process too. Again, though, how ",
" this process happens will depend on how deep the vessel sinks.",
"As far as I am aware though, there is no depth that provides 'perfect preservation'. As far as we can tell there is life at all depths of the ocean, so no matter how deep we sink we will end up as food for ",
"."
] |
[
"I know this thread is about ocean water, but interestingly, water in the Great Lakes gets so cold decomposition doesn’t always happen. The average temperature of Lake Superior is about 36 °F, inhibiting bacterial growth. This is why bodies on the wreck of the Edmund Fitzgerald are still there and intact.",
"OPs question relates to submarines though, and as far as I’m aware none of them operate in the lakes."
] |
[
"They found the body of a diver in Lake Tahoe who had been missing since the 90s that was really well preserved. He was floating upright a few hundred feet below the water, had no idea he was a corpse at first."
] |
[
"Does the probabilistic nature of quantum mechanics extend to non-quantum phenomena?"
] |
[
false
] |
My understanding of this topic is very basic, but from what I can gather, everything (?) at a quantum level is described in a probabilistic sense rather than a deterministic one. Not because of measurement error, but for more fundamental reasons. My question is this: does the uncertainty we observe in quantum mechanics manifest in any way in non-quantum phenomena? Are phenomena at a non-quantum level deterministic, as far as we can tell?
|
[
"What happens as things go from one to a few to many particles is that the total uncertainty for the system goes down relative to anything you would want to measure. So, simplistically speaking, the total amount of momentum could go up with the total number of particles, while the uncertainty of the momentum goes up much more slowly. By the time you get to macroscopic sizes, in the hundred of trillions of particles and more, the uncertainly due to quantum effects is smaller than the error of any measurement you could make. So for all practical purposes these things are deterministic with respect to quantum uncertainty. However, there are other things that determine uncertainty, such as an inability to accurately model large systems due to an inability to get all the details correct, hence the uncertainty in weather forecasting."
] |
[
"Signals in the brain are as much chemical as they are electrical. Action potentials progress an electrical potential across their surface by depolarizing-- the cells keep Na ions outside itself and K ions within, creating an electrical potential and, when the gates for these ions open, a voltage. Theres a lot of ions and a lot of gates so the actual mechanisms behind an action potential are to a scale where uncertainty of any individual particles is negligible.",
"You start getting major quantum effects in modern microprocessor technology, though. Here the electrical charge is based on the movement of electrons themselves and said electrons are packed into tiny channels so bunched together that quantum tunneling becomes a measurable effect that needs to be managed."
] |
[
"Once you get up to lengths, times, masses, etc. well above the scale of Heisenberg's uncertainty principle, things behave deterministically. The ħ/2 of the uncertainty principle is the scale factor that tells us at what point we have to switch from thinking deterministically to thinking probabilistically.",
"There are lots of processes that we can observe on the macro scale that arise from the probabilistic nature of quantum mechanics. Radioactive decay, nuclear fusion and fission, and semiconductors, to list a few. But on the macro scale, we're dealing with so many particles that the probabilities average out and become very predictable."
] |
[
"Why can't we manipulate the pituitary gland to grow taller?"
] |
[
false
] |
You read articles about people with tumors in or on their pituitary gland that ends up causing gigantism. How does this work and why can't we reproduce this phenomenon artificially for a short period? Disclaimer; 5"9 wouldn't mind being taller haha
|
[
"So what if a scientist gave a pubescent child a little bit of GH. Would that lead to some sort of feedback loop that results in gigantism or would it make them grow just a little bit more than they otherwise would have?"
] |
[
"It is my understanding that very short people are given GH as teenagers so they get a little extra height."
] |
[
"At the end of the pubertal growth \"spurt\", the ",
"epiphyseal plate",
" fuses and the long bones of the body stop growing. After this time, only bones such as in the skull and hands continue to grow in response to GH. Before/during puberty excess GH results in gigantism and after results in acromegaly. Look at the before and after pics of Barry Bonds for an example of acromegaly. "
] |
[
"According to chemistry, ionic bonds are strongest, but in biology, covalent bonds are. How can this be?"
] |
[
false
] |
I just completed AP chemistry in high school, and in chemistry it is taught that ionic bonds are strongest. However, my AP bio friends informed me that in biology, covalent bonds are strongest. Can someone explain why the two classes teach this differently?
|
[
"Covalent bonding is dominant in organic chemistry, but ionic bonds generally have higher dissociation energies. I'm not sure why your friends have been led to believe that covalent bonds are stronger, as that's generally not true. Certainly more common in biology, but being more common obviously doesn't mean that they're stronger.",
"Interestingly, the phenomena which cause covalent and ionic bonds to form are not mutually exclusive, so actual chemical bonds derive their strength from both forces simultaneously. When we call something a covalent bond, it means that covalence is the primary reason for the bonding, not that the bond has no ionic character."
] |
[
"As a chemist I am going to say the way this is taught in biology is sort of right and the way it is taught in chemistry is confusing. Ionic bonds are stronger. It takes more energy to pull the two atoms apart to infinity than it does in a covalent bond. But, that is an energy measured in a vacuum. In the presence of solvent, such as water, this changes drastically. This is because the two atoms in an ionic bond (as other commentators mentioned, there is no such thing as a purely ionic or covalent bond, it is just what we call the extremes of different ends of a continuum) once separated make ions. These ions are massively stabalized by the water molecules through non-covalent interactions. What this means in physical terms is that when you break a bond, you not only have to look at the stability of the bond, but also the stability of the products. When the products are very stable, there is a smaller energy difference between the the bond and the atoms. So, it is really hard to break a NaCl bond in a vaccuum. It is very easy to do it in water. So in biological contexts, always in water, ionic bonds are more suggestions than bonds-a convenient way of describing ion pairs, and are thus much much weaker than a covalent bond that doesn't dissociate in the same way. But in a vacuum, the ionic bond is much harder to break than a simple covalent interaction."
] |
[
"Its a matter of stable against what? ",
"Ionic and covalent bonding are rather good explained in MO-theory. ",
"Look at ",
"this",
" picture. When two atoms with half occupied orbitals encounter each other, the orbitals form an bonding (lower) and an antibonding orbital. Both electrons from the half occupied orbitals afterwards occupy the bonding orbital. ",
"The difference between covalent and ionic bonding is the energy difference of the initial obitals. A small difference leads to covalent bonds (picture left side) a large difference leads to polarized bonds/ ionic bonds(picture right side). Now the interesting thing is: the energy difference between the lower initial orbital and the bonding orbital (Delta E covalent in the picture) becomes smaller the larger the inital energy difference becomes. ",
"So when chemists break a bond they put back one electron to each inital orbital. \nFor covalent bonds this means 2*Delta E covalent for ionic bonds this is Delta E covalent +(Delta E covalent + Delta E inital).",
"Although Delta E covalent is smaller the total energy term is larger for ionic bonds. ",
"When biologists break a bond, they just take the smallest energy difference so 2*Delta E covalent for both ionic and covalent bonds. \nAnd for ionic bonds Delta E covalent is smaller. ",
"Why don't chemists also take the smallest energy difference? ",
"Because there is something the above picture doesn't show. When you put both electrons to one atom, you get two ions. (one atom has one electron more than initally, one has one electron less.) To separate those two electrons an additional energy is necessary to overcome the couloumb attraction. ",
"Biologists consider a bond broken, when they are separate parts in an aqueous solution. So the ions profit from all those ion stabilizing effects of water and aren't very far separated either. Chemists consider a bond broken (more exactly the bond breaking enthalpie is defined against) two non-interacting particles. For ions this means infinitely far apart and not interacting with anything else so gasphase. ",
"For any ground state molecule (even CsF) in gasphase this couloumb energy is just too much. At some point the electron snap back to form two radicals. "
] |
[
"Detecting the solstice: how much angular resolution?"
] |
[
false
] |
If I wanted to make a device that could detect the date of the solstice by measuring the elevation of sun every day, how much angular resolution would it need? By 'detect the date of the solstice' I mean, in the case of the summer solstice, to be able to say something like 'Yesterday was the day on which the sun reached its greatest elevation'.
|
[
"That makes no difference. The OP wants to measure the ",
" of altitude of the sun for a few days before and after the solstice, which is exactly the same regardless of location, so long as the Sun is visible at all."
] |
[
"You can use ",
"an ephemeris",
" to see that within a day or two of the solstice, the sun moves somewhere in the range of 0.3 to 0.5 arcminutes in declination per day. So you would need at least that resolution to be able to say anything.",
"At this desired level of accuracy, the experiment, of course, would be to observe the altitude of the Sun at noon, which changes by the same amount as its declination each day."
] |
[
"You can use an ephemeris to see that within a day or two of the solstice, the sun moves somewhere in the range of 0.3 to 0.5 arcminutes in declination per day. So you would need at least that resolution to be able to say anything.",
"Excellent, thank you.",
"At this desired level of accuracy, the experiment, of course, would be to observe the altitude of the Sun at noon, which changes by the same amount as its declination each day.",
"Yes, exactly. I'm thinking of detecting the day on which the sign of the rate of change of the noon altitude changes from positive to negative (summer) or negative to positive (winter)."
] |
[
"What would a large-scale experiment to simulate the origins of life look like?"
] |
[
false
] |
Assume you have one billion dollars and an army of scientists ("the right stuff") to solve this problem. seems like it was pretty small-scale... what if they had had much more resources and a modern understanding of molecular biology? and what kind of results would it be reasonable to expect?
|
[
"Okay, so this is not exactly going to be a grant proposal, but here are some thoughts...",
"Firstly, what questions are we trying to answer here? The broad question is : \"What conditions are necessary to produce self-sustaining chemical replicators?\". We assume that there are various stages, and that we are not willing to wait for millions of years for the experiment to finish.",
"A possible experimental setup might be to use '",
"high throughput screening",
"' to try out various combinations of temperatures, pressures, and chemical mixtures. Simply put, we could use a large array of small vessels that each have a different combination, and react them for a while. Successful candidate conditions are then selected for, and used to refine the search space.",
"Since we don't have millions of years, this would have to be forced (chemical) evolution. What this means is that you would not be seeing a realistic timescale for the evolutionary steps, but a quickened version. I'm thinking here of something like ",
"SELEX experiments",
" for evolving RNA that can bind to small molecules. Of course, the set of conditions in each reaction vessel would have to be encoded as some kind of artificial 'gene', perhaps for use in a genetic algorithm. Well any optimisation method would do I suppose.",
"The actual conditions themselves might have to include both small molecules and geological materials. Many of the theories on life's origins rely on the interaction between minerals and simple chemical precursors. Mineral surfaces may have acted as catalysts or porous minerals as tiny reaction chambers. It might be a bit much to try every combination of every mineral, however. Probably better to stick to the more likely ones to be reactive.",
"So, essentially, a scaled-up Miler-Urey might be an array of mixtures of gas/liquid/rock/temp/pressure passed through rounds of selection for desired products. The expected outcome would be a particular set of conditions (or range of sets) that affords the generation of life-necessary chemicals."
] |
[
"We don't know how life originated and therefore can't simulate it. Even if we could create life artificially it would not necessarily be how life began on Earth, and has the pitfall that human life was required to create it (albeit you did say simulation and this is more of a philosophical issue).",
"Regardless there is no point in concocting a billion dollar large scale experiment before small scale experiments indicate it would be helpful and yield useful information"
] |
[
"I don't think a billion dollars and an army of scientists would come close to being enough resources to create what you have in mind.",
"One has to remember that the entire ocean was essentially a giant organic chemistry experiment for a ",
" long time before life emerged.",
"One could imagine computational experiments taking the place of practical ones; but, in my experience, such computational studies are in their relative infancy right now and I'm skeptical that such work could be done practically in the near future without a more-thorough understanding of what the initial conditions of such a simulation should be."
] |
[
"My buddy's dad found this rock on their farm. Apparently a meteorite may have hit the region, could this rock have been affected by that collision?"
] |
[
false
] |
This was found near the Flint River in middle Georgia. Some scientists are starting to believe a rather big meteorite hit this area causing a pretty big topography change for the area. Apparently the river should not run the way it does through the topography it goes through unless a change happened after the river already was going through the area. In person this rock is looks awesome, especially if you put some water on it. EDIT: Found , which discusses this possible meteorite crash.
|
[
"First off, it's probably not a meteorite. ",
"Hullabazhu linked to a good site",
" that describes what a meteorite looks like.",
"The only rocks that are created by meteor impacts that I know of are ",
"Tektites",
". However, tektites usually look very smooth due to being melted by the meteor impact. ",
"My guess would be that the colors are caused by a pyrite deposit. Pyrite is fairly common in almost any part of the Earth. Although it's quite cool to see it when it forms rainbow iridescent colors. ",
"Here's some pics I found of pyrite deposits in a few different types of rock.",
" If there's a significant amount of pyrite within the rock, a powerful magnet should feel attracted to it. Although the colors seem pretty faint, so it might just be a trace deposit. "
] |
[
"I haven't found a lot of peer-reviewed journal articles about the impact, but what I've found seems to indicate it happened 83 million years ago. At that time, that part of Alabama/Georgia was under a shallow sea. I'm rather skeptical that a rock that large would be found in overlying sediments, especially a farm. However, it could be.",
"I'm not a geologist, but the rock is very interesting. It's hard to tell, but that almost looks like a fossil leaf or something, but if the rock is igneous, then there would be, of course, some other explanation.",
"You might want to dig up a reliable peer-reviewed citation as to the bolide impact before others answer. "
] |
[
"That is definitely a volcanic or metamorphic rock. I would guess it is a metamorphic rock and most likely an amphibolite (",
"http://geology.com/rocks/amphibolite.shtml",
")",
"The color change you see around the outside is called a \"weathering rind\" (like the rind on a watermelon) which is the weathering or alteration of the minerals- probably from sitting around for millions of years. The inside part of the rock looks like the same color as the outside, so my guess is there was a crack or something that let water get into the middle of the rock and weather that as well. This may have been why it split open easily when it was hit by farm equipment. It would not have been able to be split open by farm equipment if it was a meteorite (a meteorite would have most likely split apart when it hit on impact). ",
"It would be more helpful if you knew the geology of the area. It may be possible to figure out what it came from from this geologic map: ",
"http://geology.about.com/library/bl/maps/n_statemap_GA3100.htm",
" "
] |
[
"What is the physical meaning of Fourier analysis concerning limited-time lasers?"
] |
[
false
] |
Hi everyone I learned this term that a laser emitting at a frequency x but for a limited amount of time was actually not considered mono-chromatic because when the Fourier analysis is done, we observe more than just the frequency x. How is this physically possible? If you emit at precisely 440Hz from t=0 to t=5, where does come the full range of frequencies that we observe by the Fourier transform?
|
[
"If you emit at precisely 440Hz from t=0 to t=5",
"These requirements are mutually incompatible. If you want a signal which is a perfect ",
"step function",
" in the time domain, its Fourier transform is not a perfect delta function in the frequency domain.",
"And if you want a signal with a perfect frequency (a Dirac delta function in the frequency domain), it will not be a pure sine wave in the time domain."
] |
[
"Because the signal starts and stops, which represents a low frequency effect. If you had a 440Hz sine wave signal that spread backwards and forwards in time infinitely, that would be truly monotonic. A signal that starts and stops over 5 seconds clearly needs a 0.2Hz component, plus all the other frequencies near the on off transition. "
] |
[
"And if you want a signal with a perfect frequency (a Dirac delta function in the frequency domain), it will not be a pure sine wave in the time domain.",
"Are you making the distinction between sine waves and plane waves or is there some typo here?"
] |
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