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[
"Is there a biological reason humans evolved to see the range we call visible light?"
] |
[
false
] |
Or in another way, is there anything special about the 400–790 THz band that makes it especially useful to see? Along the same lines, are there any organisms out there that perceive a different band?
|
[
"The majority of the energy put out by the sun per wavelength is in the visible spectrum. ",
"Spectrum Image"
] |
[
"It's high enough in energy to not be drowned out by thermal radiation, which is the case for a lot of the infrared (at least in daytime). And it's largely transparent to small molecules, so it penetrates air and water nicely, but it does interact with solid objects, making it pretty useful for what we use sight for. ",
"At even lower and higher energies there are other complications, but perhaps more importantly, I don't think there's much evolutionary advantage to 'seeing' radio waves or x-rays.",
"There are certainly animals that have eyesight farther into the UV and IR than we do, but I don't believe it's very far."
] |
[
"There's also a nice dip in the ",
"opacity of the atmosphere",
" around visual wavelengths and near infrared to mid infrared. The next big dip is at radio wavelengths."
] |
[
"The phrase 'dimensions' is used in science fiction all the time as another plane of existence; what does theoretical physics say about dimensions and whether they exist or in what terms the word 'dimension' is used for in science?"
] |
[
false
] |
Hopefully apart from length, width, and height.
|
[
"I'll try to provide an intuitive explanation of the concept of a dimension. A dimension is an independent coordinate that you can use to describe the location of a point in some kind of mathematical space. When we talk about an n-dimensional space, that means that we need n dimensions to specify a unique point in that space. For example, the surface of the earth is a two-dimensional space because we need two dimensions (latitude and longitude) to specify a location, e.g. a building. The entire planet is a three-dimensional surface because, in addition to latitude and longitude, we also need a third dimension to specify the height above the ground of the point, e.g. the floor of the building or so many feet above sea level. Scientists usually describe the entirety of the universe using a four-dimensional space called spacetime in which the fourth dimension is time. Time counts as an independent dimension because two events can occur at the same location in three-dimensional space but at different times. You need the fourth dimension of time to specify that you are having a Superbowl party in your apartment today at 6 PM but will likely be sleeping in the same apartment at 3 AM.",
"But the concept of a dimension goes beyond physical space and time. Anything that you can use to uniquely identify a point within a given space of points is a dimension. For example, the space could be the set of all people, and we would need many different dimensions to describe a single point (i.e. person) within this space. Dimensions in this people space would include height, weight, hair color, eye color, distance between eyes, length of nose, etc. Another example might be the space of musical notes, which we could describe by specifying the pitch, duration, timbre, and volume.",
"More pertinent to your question, you may have heard people say that there are extra dimensions in string theory. This sounds funny because I said above that we can specify a point in spacetime using four dimensions, three spatial and one temporal. The other dimensions only come into play at very small scales. To help visualize this, imagine stretching a perfectly taut tightrope between two buildings. If you are watching from far away, you could specify the position of an ant walking along the tightrope using one dimension: the length along the rope. However, if you get closer, you can see that there is actually another dimension: the angular position around the axis of the rope. The ant could be standing at the same position along the length of the rope, but could be in a different position around the axis, i.e. toward the ground or toward the sky or on one of the sides. This second angular dimension doesn't really matter until you get close enough to see the width of the rope, and so on larger scales (from far away) we only need the one dimension of length to specify the position of the ant on the rope.",
"In string theory, the extra spatial dimensions (six, seven, or twenty-two; depending on the version of string theory) are only significant on very small scales. On macroscopic and even atomic scales, we can adequately specify the position of a particle or an office using the normal four dimensions of spacetime. On smaller scales, thousands or millions of times smaller than the diameter of a hydrogen atom, these theories predict that there will be extra dimensions that we need to fully specify a unique position in spacetime. We don't currently have the technology to look at these very small scales to see if the extra dimensions exist, but they are predicted by the mathematics that describe string theory."
] |
[
"If everything you said is accurate to the topic (and I'm assuming it is since I have no reason to disbelieve you) this is the best simple explanation of extra dimensions I have ever read. Thank you. "
] |
[
"There are other arrangements which dimensions could theoretically take on, and those \"macroscopic\" ones are usually the sort to which sci-fi refers.",
"I.e., if our universe, in all four of its dimensions, could be metaphorically represented as a single page, some \"other dimension\" could be another page (usually in the same book) and the sci-fi technology is some magical means of transferring your \"words\" from your page to the other page."
] |
[
"How exactly does air traffic control radars work? (Civilian systems only if there’s a difference between civilian and government)"
] |
[
false
] |
Do planes need to be at a certain altitude to be tracked in the “blip”. Do they need certain FAA requirements to be seen on radar, do they need specific speed, or mass? All of the above?
|
[
"Do planes need to be at a certain altitude to be tracked in the “blip”",
"They need to be at a specific angle above the horizon with respect to the point-of-view of the tranceiver. otherwise the signal is lost amid noise from closer and larger objects on the ground like buildings and hills. Therefore planes flying at low altitude are difficult to detect. ",
"This is related to the radiation pattern of the beam from the tranceiver antenna.",
"In general the direction and design of the antenna dish is optimized to reduce reflections from the ground while at the same time maximizing their ability to pick up aircraft just above the horizon. ",
"Moreover modern ATC radars are able to take advantage of the Doppler Effect to detect objects that are moving relative to the tranceiver. This causes the frequency of the echo to be shifted slightly, while that of ground objects is not because they aren't moving. By using electronics that filter out the base frequency, echoes from moving objects can be distinguished much more easily. ",
"Do they need certain FAA requirements to be seen on radar, ",
"All except for some very light private aircraft are equipped with Transponders. These transmit a digital identifying code. They also usually transmit data on altitude, location, and airspeed to air traffic controllers. In the past x-ponders on small aircraft didn't have the latter functions, these days GPS is cheap and so it's a standard feature. If the aircraft isn't visible on radars for whatever reason the transponder signal can still be used by controllers.",
"Collision avoidance systems on larger aircraft make use of these signals as well.",
"Small aircraft without x-ponders are typically limited to daylight operation, limited altitudes and small airports.",
"do they need specific speed, or mass? All of the above?",
"It helps that most aircraft and constructed of aluminum which is very reflective to microwaves. Typical traffic control radars can detect an object on the order of 1-2 m",
" at a range of 100km."
] |
[
"First time replying on ",
"/r/askscience",
" so apologies for any unintended protocol oopsies...",
"/u/Diligent_Nature",
" provides a good summary response and I'd just like to expand on that a bit.",
"There are several different operational environments and associated radar/surveillance systems with their own requirements and characteristics. While your question likely refers to a traditional radar system I'm going to use the term surveillance system as some technologies used to detect and display an aircraft position are not really radar based. But more on that later. ",
"Surveillance systems are broadly split into three categories; independent non-cooperative, independent cooperative and dependent cooperative surveillance systems. Independent non-cooperative systems consist of the traditional rotating radar antennae, transmitters and receivers which transmits RF pulses and then detect and processes the reflections (further processing may be performed by a surveillance data processing system not located at the radar site but as part of the ATC system). This system is called Primary Surveillance Radar (PSR) and is widely used in ATC systems for ground and airborne surveillance in the approach area and in the en-route environment. The shortcoming of the system is that it does not provide identification information; radio communications and specific identification procedures are required to acquire identification of aircraft. Some non-cooperative target identification systems exist but to my knowledge they are only used in military applications as there are easier identification solutions in the civil domain. PSR systems have no difficulties in detecting civilian aircraft (even small GA aircraft), they will even pick up a flock of birds. So no reflectors are needed. In addition to the lack of identifying information PSR has other shortcomings though as radio waves usually travel in a straight line and shadowing/interference from other aircraft, terrain, etc. may occur. So altitude - as well as the placement of the radar antennae - does matter.",
"That covers your question really, the rest here is to provide some expanded information on surveillance systems and to the point mentioned by ",
"/u/Diligent_Nature",
".",
"Independent cooperative sensor systems include SSRs, Secondary Surveillance Radars. In an SSR system an aircraft’s transponder responds to interrogations sent by a ground station antenna enabling range and bearing from the ground station to be determined. The transponder in the aircraft may provide aircraft identification code (Mode A transponder) and pressure altitude (Mode C or Mode A/C transponder) in their responses. Mode A identification is based on a four digit code assigned by the ATC which is then coupled to the aircraft's flight plan in the ATC system. When the flight plan processing system detects a code selected by an aircraft it correlates an associated flight plan to that code and now the ATC has identification and other information concerning the flight available on the radar display. An evolution of the mode A/C transponder is the Mode S transponder which can use a unique 24-bit aircraft address (associated with a specific airframe and included in the flight plan) for identification and has a data link between the ground system and the aircraft. Mode S allows for e.g. air/ground speed, heading, vertical rate etc to be sent to the ground station (Downlink Aircraft Parameters - DAP). Use of Mode S transponders is mandatory in European airspace for aircraft operating under instrument flight rules (IFR), not sure about the situation in US. SSR ground stations can be independent or co-located with PSR antennae. Another type of independent co-opearative surveillance systems is Multilateration, MLAT. MLAT uses aircraft’s transponder signals detected by multiple receiving stations. MLAT uses time difference of arrival (TDOA) techniques to calculate aircraft position and the positioning can be done using any signals transmitted, usually SSR transponder signals. Minimum 4 receiving stations are needed to calculate a position if altitude information is required (3 if aircraft provides altitude information) but usually multiple stations are used to ensure coverage and redundancy (Wide area multilateration, WAM).",
"Dependent cooperative surveillance systems include ADS-C (Automatic Dependent Surveillance - Contract) and ADS-B (Automatic Dependent Surveillance - Broadcast. In ADS-C aircraft on-board navigation systems (generally GNSS) are used to determine position, velocity and other data and the ground system establishes a “contract” with the aircraft to send this data at regular intervals using data link. As ADS-C is not a real-time system it's mostly used in e.g. oceanic ATC but ADS-C will also be used in more advanced ATC solutions currently undergoing R&D (4D trajectory sharing, extended flight profile). With ADS-B the aircraft broadcasts its position, altitude, ID, etc. from the on-board GNSS. ADS-B does not require moving antennae like radars do but its not widely used in ATC due to needing to rely on information sent by the aircraft instead of determining the position with the ground systems. ADS-B spoofing is also a potential threat. Space-based ADS-B is being deployed to ensure surveillance coverage in remote oceanic areas.",
"Finally a note on drones and surveillance systems related to drone detection. While the systems detailed above are not ideal to drone detection (unless the drone is equipped with a transponder), there are systems capable of providing drone surveillance coverage. As noted, cooperative drones can be detected with even traditional systems if they transmit their own position but technologies like millimeter wave range radar will be required for non-coooperative drone detection. Such systems are not widespread in their use yet but drone detection systems are commercially available.",
"Some sources/references:",
"International Civil Aviation Organization (ICAO) Doc 9924: Aeronautical Surveillance Manual (not available online for free)",
"CANSO: ",
"ANSP Guidelines for Implementing ATS Surveillance Services Using Space-Based ADS-B",
"(pdf)",
"Skybrary: ",
"Transponder",
", ",
"Mode S",
", ADS-B: ",
"https://www.skybrary.aero/index.php/Automatic_Dependent_Surveillance_Broadcast_(ADS-B)",
"SESAR : ",
"i4D",
"Abstract on ",
"ADS-B spoofing",
"Information on ",
"drone surveillance systems",
" and ",
"here as well"
] |
[
"Air traffic control almost always uses ",
". Secondary radar works by the aeroplane having a radio transmitter and receiver known as a ",
". The radar sends out a \"who are you\" signal and the transponder replies with information such as aircraft callsign, altitude, and so on. Transponders are a legal requirement in some regions of airspace but not all.",
", relying on the aircraft passively reflecting radio waves, is used alongside secondary radar since it can track aircraft without a working transponder. However it can determine an aircraft's position and speed but ",
" its altitude. If secondary radar is unavailable, voice communication with air traffic control is needed to determine a plane's altitude.",
"There are also significant areas of airspace with no radar coverage at all, such as over oceans far from any land. In this case, air traffic control relies on radio communication with the pilot to determine a plane's position and direction."
] |
[
"What would an AM and FM radio signal look like if broadcast in the visual spectrum?"
] |
[
false
] |
Radio signals are just light, correct? If we rigged up a radio broadcast system such that it emitted a signal in the visible spectrum with brightness such that a human could easily see it, what would that signal look like? Would we see lots of blinking? Would we see rapid shifts in brightness and color? How would AM and FM signals differ? Would the entire antenna glow, or only certain points? Moreover, how would you do this? How hard would this be to accomplish if I wanted to see for myself?
|
[
"Radio signals are electromagnetic waves. What we refer to as light, or visible light, also consists of electromagnetic waves, but of a different frequency than radio signals.",
"If for some reason, we would use visible light to broadcast radio transmissions, what you'd see depends on whether you use AM or FM. Either way, you would see the broadcast tower light up as it broadcasts the signals across the area.",
"When using AM, or Amplitude Modulation, the signal is added to the carrier wave by changing the amplitude of the signal. That means increasing and decreasing its intensity. If you'd look at the broadcast tower, you'd see the light getting brighter and dimmer.",
"On the other hand, FM, or Frequency Modulation, alters the frequency (and wavelength) of the wave to encode the signal. Visually, this would look like colour shifts of the light coming from the broadcast tower.",
"There's one additional issue, with regular transmissions the change in amplitude or frequency would be too fast to really notice. So you wouldn't really notice the effect too much."
] |
[
"if you reproduced e.g. the FM broadcast radio (i.e. audible signals) spectrum (from 88 to 108 MHz) in visible light by scaling all the frequencies by a factor of about ten million, it would fit ",
" the visible spectrum; the ratio of the longest to the shortest FM radio wavelengths is about 1.22, while the ratio of the longest to the shortest visible wavelengths is about 1.75 (700 to 380 nm).",
"a factor of 1.22 is easily discriminable to the human eye - across most of the visible range, human wavelength discrimination is in the range of about 1-2% (i.e. wavelengths different by a factor of 1.01 are just visibly discriminable).",
"so, given this, different radio stations, especially far apart on the dial, would appear slightly different colors - the 'public' stations, down below 92MHz, would appear relatively redder (or yellower), while the other stations (up to 108MHz) would appear relatively greener (or bluer), depending on where you adjusted the radio range. the ridge outside of town with all the radio towers on it would look pretty weird (especially if you extend the range a bit to catch some of the TV channels just below 88MHz).",
"if you did AM radio instead, you probably would be able to see the flicker of the AM signal envelope (just as you can see the flicker of an LED hooked up to a speaker), since the signal weakens during each audio cycle, and the lowest audible frequencies (below 60Hz) are visible as flicker ",
". AM radio would also extend ",
" the bounds of the visible spectrum (covers a range from 540KHz to 1600KHz, a factor of almost 3), so some stations would be effectively infrared or ultraviolet."
] |
[
"First, even though it is all electromagnetic radiation light and radio, there are key differences between them.\nUsually the wavelength of the wave is similar to the size of the object emitting it. The electrical currents oscillate at frequencies and scales similar to the one of the emitted wave.\nWhen you get to the infrared and visible, still the analogy of currents applies, but the currents flow in the electron layer of molecules.\nThe radio broadcast like that would work if you actually shrunk the transmitter and antenna to the size of atom. Or similar. And then the antenna would radiate in the same way the big antenna would, only it would be much smaller.",
"Usually you cannot directly create light signal with any arbitrary waveform straight from the source. You need a light modulator.",
"And then, AM would be light changing its intensity depending on the modulating audio signal. And FM would be light changing the colour depending on the audio modulation. Which is just changing the frequency.\nBut actually doing that involves nonlinear optics."
] |
[
"How does the candle relighting trick work? the one where you light the smoke trail?"
] |
[
false
] |
As shown in this gif
|
[
"In order for something to burn, you need to vaporize the fuel. So when you light a candle, it takes a second or two to melt and vaporize the wax before it can ignite and become self-sustaining.",
"When you blow out the candle, the residue heat from the wick keeps vaporizing wax. The \"smoke trail\" you see isn't smoke - rather it is vaporized wax. So by bringing another flame close to the fuel trail, you can reignite the candle."
] |
[
"This picture",
" that hit the front page yesterday shows the wax that continues to come up after the candle is blown out."
] |
[
"You shouldn't pour gasoline on a fire at all, though, surely?"
] |
[
"How much uranium does a Russian made VVER-1000, 1000MW nuclear power plant use?"
] |
[
false
] |
Also follow up, does a nuclear power plant needs to be continually refueled everyday, or every few weeks/months/years?
|
[
"A typical Large nuclear reactor will have something like 160-200 tons of uranium in it. Typically 1/3 of the fuel is replaced every 18-24 months (depending on design). ",
"One notable exception is the candu reactor type, which loads small fuel segments in the front and drops them out the back into a pool of water every few Weeks/months. "
] |
[
"1 GW(e) reactor outputs around 3 GW(t) power. That equates to around 10",
" fissions per second. So 10",
" uranium isotopes used per second means around .04 grams of uranium used per second. That is around 1200 kg of U-235 used up per year. "
] |
[
"I got curious, although didn't find out the amount specifically in that reactor type but did find out quite a bit about fuel rod assembly from a promotional brochure from a company that supplies them. Pretty interesting PDF entitled \"Nuclear fuel for VVer reactors\" : ",
"http://www.tvel.ru/wps/wcm/connect/tvel/tvelsite.eng/resources/8b8b638047fdce049ce5ddc33fdd9f8b/brochure_nuclea_eng.pdf",
"The mention in there that each assembly has a 5-6 year cycle, which ties in with Hiddencamper's 18-24 month cycle for a 3rd of the fuel."
] |
[
"What happens to Helium?"
] |
[
false
] |
From my understanding there is a finite amount of He on our planet, and we extract it from where it is trapped in rocks... Then we fill up party balloons with it. What happens to the He once the balloon pops? Does it just go into space? Does it collect anywhere? What happens to it?
|
[
"Normally, the ballon shrinks and appears to gain wrinkles because the He atoms have leached out of the balloons skin. The size of the small particles can easily fit through the lattice-like structure of the balloon, and become smaller and more dense; and sinks down from the roof where you cousin let it go.",
"Once it is popped, it travels upward (because density=less than air) and would appear to spread out over the top of the atmosphere. Where solar winds, highly energised and ionised particles ejected from the sun smash into the top out our atmosphere and knock stuff out.",
"We lose our Helium that we get from decayed radioactive rocks to the top of the atmosphere where it is not economically feasible to retrieve it; and space. Which is even a bigger problem."
] |
[
"25%",
" of the mass of the Milky Way galaxy is helium.",
"There are \"pockets\" of He floating around, however, they are intermixed with hydrogen and form gas clouds and populate the interstellar medium.",
"The He atoms don't really accumulate since there is no force acting on them to do so. They are lighter than other molecules and atoms in the atmosphere and need less energy to escape the Earth's gravity well. Once in space they disperse into the rest of the Solar system, but I am not sure if they end up on the larger planets or not."
] |
[
"So the He way up in the atmosphere. Does it accumulate at all, or does the solar wind just 'blow' it away? Does it when it does get blown away get attracted to deeper gravity wells? Basically I am wondering if there are cosmic pockets of He just floating around."
] |
[
"How are old buildings built with asbestos demolished safely?"
] |
[
false
] | null |
[
"no need for \"moon suits\" or wrapping the entire building. You work section by section, room by room. You will seal off each room and pull negative pressure through a HEPA filter to catch all the fibers. And you wear a tyvec suit or another material that is fiber resistant, with a respirator to protect the lungs, and shower on your way out at the end of the day. You can even do it naked with a respirator, just so you don't carry fibers out with you!",
"Once the asbestos is gone, then demolish normally. Typically asbestos is in insulation, contained in fiberous building materials like ceiling and floor tile or used is binders and mastics."
] |
[
"This is how it ideally works but I worked in a demolitions crew for a bit and we specialized in removing asbestos (in reality we were always the lowest bidder) and we used absolutely no protection save for gloves and long pants. We’d just rip it up, shove it in a trash bag, pour water on top of it, tie and tape the bag closed, then haul it to a shipping container.",
"Our boss was crooked. He cut a lot of corners and paid inspectors and cops to look the other way. I’m due for my first chest x-ray next year, then I need one every five years."
] |
[
"asbestos was the wonder material of the last century, this century, not so wonderful. When toll booth operators were getting asbestosis just from car braking at toll booths, I knew we had a problem"
] |
[
"Can earthquake equipment detect underground tunneling ?"
] |
[
false
] |
[deleted]
|
[
"It won't necessarily be the best detector of active tunneling, since seismic equipment is picking up tiny movements all the time. But if it's used in a more ",
"targeted way",
", it may be able to detect anomalies underground, spotting already-made tunnels that way. This method, and others, have been used to find, among many more useful and common things, tunnels at borders, like US-Mexico, or Israel-Palestine."
] |
[
"Thank you"
] |
[
"Thank you"
] |
[
"Was Einstein's contribution to physics necessary for building the atomic bomb?"
] |
[
false
] | null |
[
"Not really, Einsteins contribution to phsyics with regards to building a nuclear bomb was minimal and restricted to Mass–energy equivalence (i.e E=mc",
" It gave nuclear physicists clues to refine their designs for an effective nuclear bomb. ",
"Its very likely that without Einstein the nuclear physicists involved in building the atomic bombs could have figured out this relation or intuitively refined it after many trial and errors eventually ",
"Einstein's politics played a more decisive role in the story of the atomic bomb than his physics. Following a request by the physicist Leo Szilard, Einstein signed a letter written by Eugene Wigner to president Roosevelt, explaining about the potential power of nuclear weapons and the possibility of Nazi Germany developing such weapons, and urging the president to take action. Einstein's letter played its part in setting into motion the political process that culminated in the Manhattan project - the development, construction and testing of the first nuclear bombs",
"source: ",
"http://www.einstein-online.info/spotlights/atombombe",
"However Einstein later regretted signing the letter because it led to the development and use of the atomic bomb against civilian populations, adding that Einstein had justified his decision because of the greater danger that Nazi Germany would develop the bomb first. In 1947 Einstein told Newsweek magazine that \"had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing.\""
] |
[
"But wouldn't they at least have had to have know that matter can be converted into energy, and isn't Einstein the one who theorised this?"
] |
[
"Even without focus on E=mc",
" , work on nuclear energy was conducted with experiments on radioactive materials since the early 19th century",
"Einsteins Mass–energy equivalence contribution only provided to be a tool in finishing the atomic bomb more quickly and efficiently like I said in a comment below",
"OPs question is like asking whether NASA could have sent men on the moon without the help of any computers. Its possible, if they took a lot of tools that helped them do calculations manually through cheat sheets or whatever. It would have been a pain in the ass but it wouldnt have been impossible to do so"
] |
[
"If/When humans make the trip to Mars, will we be able to walk or will we hop like the astronauts on the moon?"
] |
[
false
] | null |
[
"In between. The gravity there is about 40% as strong as Earth's."
] |
[
"Depending on the mass of a spacesuit, the total weight an astronaut would have to move would be comparable to the weight the astronaut has to move on Earth. The Apollo spacesuits weighted 180 pounds on Earth, as ",
"NASA reports",
". An astronaut who weighed 180 pounds on Earth wearing this suit on the Moon would feel a gravitational force of about 60 pounds. The same astronaught wearing the same spacesuit on Mars would feel a gravitational force of 140 pounds, so not so different from what the astronaut experienced on Earth. A spacesuit like that, then, would lead to astronauts walking, not bounce-hopping as they did on the Moon. Of course, the actual mass of spacesuits for a future Martian trip have not been determined!"
] |
[
"The gravitational field on the surface of a planet is primarily a function of its mass and radius. Larger mass, larger gravitational field; larger radius, smaller gravitational field. The gravitational acceleration can be computed to be ",
"G M / R",
"where G is a constant of nature, M is the mass of the planet, and R is the radius of the planet.",
"Now Mercury has about half the mass of Mars, but only about 3/4 the radius. These effects balance each other, so that the surface gravity of the two planets is about the same."
] |
[
"What is it about Mercury (Hg) that makes it so good for measuring pressure?"
] |
[
false
] |
I'm an American pilot, and although there is no actual mercury in my instrumentation, things like altimeter setting and manifold pressure are measured in inches of mercury (InHg). Why is this element used, and how does it compare to the accuracy of millibars (no other country has inches)?
|
[
"Other folks have mentioned the weight. Another benefit of mercury is that it's not prone to phase change at particularly low or high pressures. This is useful when you want to measure a vacuum; water for example would tend to boil or become water vapor when exposed to a vacuum, which would then feed into whatever it is you were measuring. Mercury comparably, remains in liquid form in a vacuum at normal temperatures."
] |
[
"Before we had electrical pressure transducers, measuring pressure was typically done by filling a U-shaped tube with some kind of liquid. One end of the tube is open to the atmosphere and the other end if attached to the pressure you want to measure. The positive or negative pressure you want to measure causes the liquid to move one direction or the other. You can make these types of gushes with any kind of liquid but mercury worked well because it's much more dense than other options and makes for a more compact measuring setup. "
] |
[
"water for example would tend to boil or become water vapor when exposed to a vacuum.",
"What you mean is, water has a reasonably high vapor pressure at ordinary atmospheric temps. Moreover it varies quite a bit from, 0 C to 35 C."
] |
[
"How can a photon even exist if it travels in no time and is absorbed in 0.0 seconds?"
] |
[
false
] |
It doesn't really exist for any length of time. Or how could it exist in one time frame and not in another?
|
[
"What he means is that in the rest frame of a photon, the elapsed time between two events is zero."
] |
[
"If you really want to be mathematically technical - and perhaps one needs to be in order to answer your question, a photon does not have a properly defined rest frame. ",
"You can choose any rest frame that you want, and the photon will be traveling at the same speed.",
"In the limiting case, the proper time one experiences along a journey from point A to point B gets close to zero as one approaches the speed of light. So one can imagine by extension that the proper time of a photon is zero in its 'rest frame'. However, one cannot boost into such a rest frame - it is undefined."
] |
[
"I should have also mentioned - if you have no mass, then it doesn't make any sense at all to have a rest frame.",
"The energy of a photon is dependent on the frame of reference in which you view it as it will be blueshifted or redshifted.",
"The energy of a photon (E) is related to its momentum (p) by p = E/c where c is the speed of light. In its 'rest frame' the momentum of the photon would be zero, and it would have no energy whatsoever. "
] |
[
"Why can't we relocate endangered species (such as orangutans) to countries that won't kill them?"
] |
[
false
] |
[deleted]
|
[
"We (the human race) have a long history of screwing up a habitat by introducing species that are not native to that area. The natural order of any given environment is rather fragile even when it doesn't seem so to us as individuals. ",
"We just don't know enough to do it without bad side-effects. We do know enough to not try (usually)."
] |
[
"We often do",
". However, there are rampant discussions on the effects some practices have, and I am not aware of any cases where an entire species was moved to captivity. However, there is a particular conservation status, ",
"extinct in the wild",
" which denotes an animal which, after being endangered, has gone extinct in its natural habitat, but thanks to conservation efforts, still continues to live in captivity."
] |
[
"While biologically inaccurate, \"human race\" is an everyday English expression used to refer to the human species. Nothing wrong with using everyday English."
] |
[
"How does s1=433, s2=333, s3=123 not break cantors diagonal proof?"
] |
[
false
] | null |
[
"I don't understand your question. Cantor's diagonal proof produces a number not on the list by constructing a number that differs from the ",
"th number in the ",
"th decimal place."
] |
[
"But the number constructed by those three is 433, which is equal to s1, a number on the list"
] |
[
"Hm? Cantor's proof is a proof that the real numbers are not countable. So it starts with a purported list of all real numbers. That is, an infinite number of numbers, which each have infinitely many decimal places. You've provided three 3-digit numbers. (Also, \"433\" does not differ from the second number \"233\" in the second decimal place.)"
] |
[
"What is it about the flu virus that makes it seasonal?"
] |
[
false
] | null |
[
"There are lots of explanations, but it's still not completely clear which of them is most right. (Probably, as with most biology, there are many different reasons adding up.) Some of the reasons put forward are:",
"Some of these are more convincing than others (I'm pretty skeptical about nutrition in particular). One of the most convincing explanations is ",
"It's more complicated than humidity alone, since flu still can transmit in tropical regions where it is less seasonal or not seasonal at all, but in temperate climates it's probably a strong influence.",
"Some references:",
"Global Influenza Seasonality: Reconciling Patterns across Temperate and Tropical Regions",
"Influenza Seasonality: Underlying Causes and Modeling Theories",
"Absolute humidity, temperature, and influenza mortality: 30 years of county-level evidence from the United States."
] |
[
"Never mind debunked, it was never bunked. It’s pure nonsense. Human influenza doesn’t work that way. Whoever told you that had confused the source of pandemic influenza (of which there have been four over the past 100 years) with seasonal influenza (which happens every year)."
] |
[
"I was taught years ago that it's because of seasonal bird flight patterns, and the flu DNA somehow getting remixed while the birds carry it into one of 128 possible strains..",
"Is that hypothesis debunked now?"
] |
[
"I heard we are running out of helium on Earth. How true is this, and is anyone working on a way to find/create more of it?"
] |
[
false
] | null |
[
"Helium is produced as a byproduct of natural gas. During the Cold War, the US subsidized helium production and created a massive stockpile. Over the past decade, they have been selling it off, which has made helium very cheap...so cheap, it hasn't been profitable to capture it from natural gas wells, so it gets released into the atmosphere instead.",
"This is the helium we are running out of, the cheap stuff. As helium becomes more expensive, it will once again become profitable to capture it from gas wells, and the price will stabilize at a higher level."
] |
[
"Scientists would love to find a way to ",
"make helium out of hydrogen",
". Their main goal in this isn't actually to have more helium, though."
] |
[
"Where in the world are you seeing highly reactive hydrogen gas just sitting around?"
] |
[
"Why does public transportation typically not have any seatbelts?"
] |
[
false
] |
[deleted]
|
[
"When a massive object collides with a small object, most of the damage is to the small object. So, when a car and bus collide, the bus passengers are much less likely to get hurt.",
"I was once in an accident where a car rear ended a city bus. The bus shook a little and barely had any damage. The car was totaled."
] |
[
"Everyone gets thrown around and gets hurt and it makes worldwide news. Luckily you don’t see that news that often. Per mile buses are the only transit safer than an airplane. "
] |
[
"But what if the bus collides with a wall?"
] |
[
"Black Hair and Blue eyes"
] |
[
false
] |
[deleted]
|
[
"Punnett squares don't work well here because there are multiple genes that affect your eye color (many people have more than one color in their eyes, or the color is not uniform)."
] |
[
"Just as an aside, punnet squares are good for really simple types of inheritance, but eye color isn't a simple trait that is readily conducive to looking at punnet squares. And I think that it's pretty obvious that it's possible, because I know people with naturally dark hair and blue eyes. "
] |
[
"All right so since she is hardcore Asian (genetically). Let us assume that she would have dominant dark genes in all six loci. There are three identified genes for basic eye colour (not including those that effect yellow rings etc. I am assuming she just wants blue and is not worried about rings.) Each gene has two copies or loci hence six. Her mate, if he is blue eyed will generally have all eight recessive blue. Any children they have will show dark eyes but be carriers of the recessive blue eyes gene. ",
"If her twelve offspring then have children with blue eyed men then a multitude of eye colour would probably show up. This happens because the genes are on different chromosomes. The fathers would all give the recessive traits but the mothers would would give one of the following combinations DDD, DDb, Dbb, bbb, bDD, bbD, bDb, DbD or any other one I missed, only the offsping in generation 2 with bbb will have blue eyes, the rest will have some other combination. ",
"Now this is being general we are not accounting for other random genes that could have an effect on eyes like the albino gene (if the father and husband of one of the daughters have it then the offspring in gen 2 could have both.)",
"Generally speaking yes there is a fairly good chance that if she had children with a blue eyed man and her offspring had children with more blue eyed men then she would have blue eyed grandchildren.",
"TL;DR: yes"
] |
[
"This may sound dumb, but why when a human gets shot in the brain its an instantaneous death?"
] |
[
false
] |
I understand a bullet to the brain would kill u very fast but why is it immediate and why can't surgeons retract the bullet and cauterize the vessels as any other part of the body? I'm not looking for answers like "because it's the control center". I would appreciate a detailed answer. Thank you.
|
[
"It's not necessarily instantaneous death, but it can very well be. The difference is in the degree of disruption.",
"A bullet passing through the skull imparts a great deal of its kinetic energy on the substance of the head. Depending on the closeness of the gun, expanding vapors may also accompany the bullet. The brain itself is very gelatinous and not all that sturdy. A bullet dumping a great deal of its kinetic energy into the brain, and into skull fragments that get dragged through the brain, will disrupt the structure of the brain pretty badly. Even apart from the actual bullet track, pressure waves traveling outward from the path will disrupt white matter tracts in the brain. Excess pressure will also have an overall negative effect because the skull presents an essentially rigid structure. What's more, the brain is very metabolically active and the lack of blood from damaged vessels can lead to irreversible injury in a matter of minutes.",
"In the end, what constitutes death is a fraught question. In places where brain death criteria are recognized, it's irreversible death of the whole brain. Getting shot in the head, unless with a projectile that essentially obliterates the entire brain, is unlikely to cause instantaneous death of the whole brain. Brain stem cells, for instance, would likely survive for some time. But consciousness is typically lost because the wide-scale projections of the ARAS (ascending reticular activating system) are disrupted, and the injury itself may lead to rapid death of most of the brain, herniation of the brain stem, and other effects, leading to an irreversible loss of function.",
"Some people do survive the initial gunshot. I've seen them in the neurosciences ICU. They do surgery, remove dead tissue, possibly leave the skull open in the event of swelling, and do aggressive intracranial pressure monitoring in the unit. The people who survive are often fairly devastated because of the brain that they lose."
] |
[
"For a visual on the amount of energy that is imparted into the brain by a bullet, ",
"here's a slow-motion video of a large handgun round (.45 ACP) going into ballistics gel.",
"This is a large rifle round (.308) for comparison.",
"In both of these, note how the \"tissue\" deformation is significantly larger than the bullet itself."
] |
[
"It's not always death. Many people have survived being shot through the brain, through surgery similar to what you have described. A very visible example from recent history is Congresswoman Gabrielle Giffords."
] |
[
"Can someone please explain counter-steering?"
] |
[
false
] |
As per from Thank you.
|
[
"In short, it comes down to generating the force needed to turn, without generating a torque that will topple you. ",
"The important fact here is that a tire generates force against the ground PERPENDICULAR to the angle that the tire is touching the ground at. With that in mind, realize that the side of the bike tire that touches the ground is rounded (opposed to a car's flat tire). When you lean sideways, it changes the angle of the (normal) force; the force of the ground holding up the tire. If you are leaning right, then you have a component of the normal force that is acting to the right. This is useful because to turn right, you need a force to act in the right hand direction.",
"How does that relate to countersteering?",
"If you are traveling straight and need to turn right, you need a way to lean right (to bend the angle of your tire amd generate the rightward force). You do this by steering left.",
"Why? Steering left will apply a force leftward onto the tire of your bike. This leftward force acts on the bottom of the front tire. Since it is not acting at the center of mass (of you and your bike, somewhere around your thighs) it exerts a torque and twists the angle of your bike to the right.",
"You are now leaning towards the right and ready to turn right.",
"Edit: If you don't countersteer, and try to turn by turning your wheel (with the bike straight up): the force acting right again acts on the bottom of the front tire. This will exert a torque on the bike and make you fall over left. ",
"You countersteer to lean over, which in turn generates the force needed to turn without generating a torque that will make you fall over.",
"Edit 2: It doesn't really pertain to the need for counter steering, but I should correct one thing: The Normal force is not what generates the sideways force that forces you inward as I said above. The Normal force is always straight up, perpendicular to the ground. The tire on a bike is rounded to allow the rider to lean side to side, but its shape does not affect the normal force between applied on it by the ground. ",
"The sideways force is generated by friction. Assuming you are turning right: once you have countersteered and as soon as you lean right, you then turn your wheel to the right. This creates a friction force that acts to the right. The sharper you want your turn to be, the more you have to turn your wheel right and the more you have to lean."
] |
[
"As a motorcyclist, that isn't what counter-steering is. Counter-steering happens when you're riding any two-wheeled vehicle above a certain speed limit (around 32 km/h or 20 mp/h or so). At these speeds and above, turning the bars right will actually turn the bike ",
" (or rather, 'cause it to lean left, thus causing a left turn).",
"It's something you learn either through courses or racing experience. Surprisingly few normal riders know they are doing this and are accidentally, or instinctively doing it, while trying to lean their bikes (but some do learn this through experience).",
"Edit: ",
"Wiki has an entire post on it."
] |
[
"I am both a sometime motorcyclist and, more recently, a mountain biker.",
"Should you apply the rules ",
"/u/kdeff",
" describes? Absolutely.",
"However the force you need to apply to the left handlebar to lean right on a bicycle (mountain or road) is so slight so that it's almost imperceptible. You are actually already doing it without realizing.",
"Now you know though, you can use the knowledge to help you setup for corner entries and exits."
] |
[
"FAQ Friday: What determines how fast a scent can spread? Find out and ask your questions about smells here!"
] |
[
false
] |
This week on we're exploring the amazing world of scents and smells! Have you ever wondered: What is a smell? When smelling something, are we inhaling molecules of what we recognize as a scent? How fast can an odor travel? What is the "speed of smell"? If I smell something is it possible to use up all of the scent? Read about these and more in our , or ask your questions here. .
|
[
"If I am briefly in a room that stinks and I don't want to breathe it - eg a trash room or a recently used restroom - is it better for me to take short shallow breaths or infrequent deep breaths? Should I breathe through my nose or my mouth? If the smell was somehow toxic, is there one breathing pattern that is safer for me to follow than others?"
] |
[
"How seriously do smell experts take the idea of ",
"quantum olfaction",
"?"
] |
[
"Wow! I just casually remembered that this FAQ would be up today, I click on it, and see that the number one comment, unanswered, is addressed by the very first paper I ever had published!",
"As I'd prefer not to throw my full name and publication history up there for all to see, I'll just give you the gist (a super interested party would be able to find it, but I doubt there are many of those). In short, I implanted mice with pressure transducers in their nasal cavities that could very reliably and accurately measure frequency of sniffing, looked at males and females, with and without hormone treatment, to see if they could perform an odor task better/worse. Basically, when the task was made more difficult, animals that decreased their sniff frequency performed better; the animals that did this happened to be the ones treated with hormone. So, quicker sniffs mean they have LESS sensitive odor detection--in other words, if you want to avoid taking in an odor, you should take short, quick breaths (my animals that took short quick breaths were less able to smell urine at low concentrations than animals that took slow breaths).",
"There are certainly caveats, like the sniff frequency could have been spuriously correlated with whatever other factor the hormone treatment affected, I am less certain of the transducers' ability to measure sniff amplitude (and didn't report it because of that), etc etc, but just anecdotally, I tried it out myself a number of times in the process of writing that paper, and it seemed pretty legit."
] |
[
"what is the plausibility status of parallel universe/s being the cause of unexplained phenomena attached to dark matter and dark energy?"
] |
[
false
] |
This is a very unscientific claim, but when watching the miriad documentaries on the subject, it always comes to me the idea of an unseen dimension/universe accounting for gravitational and momentum misteries currently labelled as dark matter or dark energy. Is there any plausibility or actual theory about this? ps . sorry if the wording reads weird, english is not my mother language.
|
[
"Not really. There are some scientific ideas kinda like what you're proposing, but postulating ",
" just to explain some nonradiating matter and metric expansion is a huge violation of parsimony."
] |
[
"The issue isn't scale, the issue is using a more complex explanation when a simpler one suffices. We have pretty good explanations for dark matter, and we have avenues of attack to figure out dark energy significantly less complex than \"so there's a universe which can interact gravitationally with ours except somehow ours can't interact gravitationally with it.\"",
"I guess that's the biggest problem, I should've mentioned it earlier. Because inverse square laws are valid, we know that gravity can't \"leak out\" of our universe, so it's quite complicated to explain how there could be something outside the universe pulling on stuff."
] |
[
"No. Inverse square laws are valid only if gravity propogates through three dimensions; however you add another one, you can't keep that inverse square law valid."
] |
[
"If black is the best absorber of light and white is the worst/most reflective, why are mirrors silvery/grey?"
] |
[
false
] |
[deleted]
|
[
"Describing the color of a mirror is a bit as tricky. In most cases we associate the color of an object with the reflection spectrum of that object in the visible range. So for example if you have an object that absorbs all incoming light and reflects very little visible light, we say that it is black. On the other hand, if the object reflects most incoming light with little spectral dependence, we would say that it is white. Finally, if the object ",
" absorbs light, but with little spectral dependence we would perceive it to be grey. ",
"However, the appearance of an object will depend not only on the overall degree to which reflects light, but also on the mechanism by which it does so. For example, if you have a smooth flat mirror, its reflection will be strong and spectrally flat (as for a \"white object\") but in addition it will be ",
"specular, or mirror-like",
", which means that an incident ray of light will be reflected at a specific angle ",
"as shown here",
". So now say you are in a room illuminated by a small white light bulb. The mirror will look bright and white in the part of the mirror where the light bulb is reflected, but it may look duller gray elsewhere. It is this combination of high and directional reflectance that we perceive as silvery. This is not a coincidence by the way, most common mirrors actually use a thin coating of silver as the main reflector since silver due to its electronic structure is ",
"highly reflective in the visible part of the spectrum",
".",
"In contrast, objects that appear white, also reflect light with fairly little spectral dependence, however they do so via what is called ",
"diffuse reflection",
". This situation arises when the inhomogeneity of the reflecting surface is of a length-scale of the order of or smaller than the wavelength of the incident light (as is the case for the ",
"fibers that make up paper",
"). In this case light will be reflected in ",
"all directions",
". This is why a piece of paper illuminated by the same light bulb would look white and uniformly bright compared to say a piece of silver."
] |
[
"Thanks for the informed and elaborate answer.",
"What is spectral dependence?"
] |
[
"\"...little spectral dependence...\" meaning the reflection happens substantially equally at all wavelengths. "
] |
[
"Question, may save soldiers lives. How much water would be needed to stop artillery shrapnel / concussion."
] |
[
false
] | null |
[
"Hey bud, AD Army w/ 2 tours. check DA PAM on survival, i know it gives the thicknessess of different materials to protect against nuclear blast / radiation, it might also give it for fragmentation. Good luck an Give em hell"
] |
[
"Bullets directly fired into water very quickly dissapate all of their energy. (within a foot or less). This is also why jugs of water are used as targets.",
"The problem would be that a system such as what you are proposing would only work once, as the water would then leak out."
] |
[
"Indirect fire would mean that they're not really having to deal with a constant stream of impact and/or the need for a reusable protection item. They just want a buffer for the one-off round that hits right where their bunk is."
] |
[
"When does the body know to release Insulin?"
] |
[
false
] | null |
[
"Essentially, the concentration of sugar (glucose) in the blood activates the beta islet cells in the pancreas to release insulin. The more glucose, the more the cells are triggered, releasing more and more insulin. ",
"The mechanism goes: glucose diffuses in to the beta cell, goes through the metabolic process of creating ATP. More glucose = more ATP. This ATP causes the concentration of potassium in the cell to rise. More ATP = more potassium. When the potassium concentration reaches a peak, calcium channels in the cell wall open. Calcium floods in to the cell and activates vesicles full of insulin to pop to the surface of the cell and release. The cell then resets ready for the process to go again. ",
"https://en.wikipedia.org/wiki/Beta_cell",
" "
] |
[
"So basically the islets are sitting there hungry and when they get sugar they start doing their job which is to pour insulin into the blood stream.",
"Kind of like a big lock system, the engine of the cell is switching the sluices open by the mechanism of potassium and calcium open the gate."
] |
[
"http://www.nature.com/nrendo/journal/v3/n1/images/ncpendmet0368-f1.jpg"
] |
[
"Is it possible to visit a fixed point in space multiple times?"
] |
[
false
] |
What I mean is it possible with all the planets, solar systems, galaxies and probably the while universe spinning and constantly moving, is it possible to determine exact coordinates in space and then return to those same coordinates again with 100% (or at least 99.9%) accuracy?
|
[
"There is no objective universal coördinate system with which to ask whether you've \"really\" been to the same place twice; any such statement will necessarily depend on the arbitrary choice of coördinates you assign to spacetime events. As such, it is just as \"correct\" to say that you've ",
" been at a fixed point in space while the universe moved around you as it is to say that you're on a rock orbiting a star orbiting the center of a galaxy."
] |
[
"Galaxy positions are typically defined in terms of the Right Ascention (RA) and Declination (DEC), a coordinate system analogous to latitute and longitude (but whose reference zero points are completely different). 3D coordinate systems are worthless due to the immense light travel times involved when talking about galaxies, so galaxy positions are essentially fixed on the 2D celestial sphere. ",
"Wikipedia has a decent overview with pictures and more details if you're interested.",
" This coordinate system changes because of the precession of the Earth's rotation axis; RA and DEC have to be given in terms of a particular ",
"\"epoch\"",
" and then updated to the current date to be useful when using large telescopes with a small field of view.",
"The same follows for stars, and it's worth noting that we don't really have concrete distances for most stars much past 500 parsecs (~1600 light years) so the same holds true. The most nearby stars have significant motions relative to our Sun, meaning that it's sometimes very likely that when using some bright nearby stars as reference observations, they're not actually in the same spots as their finder charts and archival images that were created 30 years ago due to their own motions relative to us. It can be annoying."
] |
[
"Does NASA have any system they use to identify a position in a solar system or galaxy?"
] |
[
"How fast do various microorganisms travel in MPH/KPH? For example, could a flagellated microorganism, if put in water and could only go forward, travel anywhere close to 1 MPH/KPH?"
] |
[
false
] |
Or simply travel any significant measurable distance?
|
[
"One of the best studied systems for microbial chemotaxis (motion based on chemical species concentrations) is E. coli. E.coli use flagella to move around, and reach speeds of about 20um/s, which is about 7.2cm/hour. Not very fast by our standards, but pretty fast compared to the length of E. coli (1um). ",
"Chemotaxis is very interesting and is an incredibly complex system though. And E.coli hardly ever swim in a line for a long period of time, instead they either tumble around, or swim in a random direction. If they are at a place with lots of nutrients, they will just tumble around, and if there is less nutrients, they will attempt single direction swims more often."
] |
[
"Put in perspective. If the e. Coli travels 20 body lengths per second, that's equivalent to a human swimming at ~80 mph through the water. Not bad."
] |
[
"A quick googling (searching for \"speed of microorganisms\") returns this page:",
"http://hypertextbook.com/facts/2000/ElaineKung.shtml",
"The fastest movement they list is 200 µm/s, 200 micrometers per second, which translates to 0.000447387 MPH."
] |
[
"Why can't we apply Hilbert's Hotel paradox to Cantor's diagonal argument?"
] |
[
false
] |
I've been doing a bit of reading and watching video's about infinity lately and two ideas about infinity really stood out to me: and . For those that don't know about Cantor and Hilbert, Michael from Vsauce does a really great job at explaining these concepts in Now to get to my question, and I hope I can explain it clearly enough: can't we make the real numbers between 0 and 1 countable by applying Hilbert's Hotel paradox to the diagonal argument? If we use Cantor's method of generating random real numbers between 0 and 1 and linking them to whole numbers until infinity we can still create a real number that exists between 0 and 1 that we haven't linked to a whole number yet by using Cantor diagonal method. But can't we treat this new unlinked real number as a new guest like in Hilbert's Hotel Paradox and just shift every randomly generated real number to the next whole number, opening up the whole number '1' for the new unlinked real number? And then we can start the process again by creating another diagonal number with the new set we just created, just to create a new set by shifting everything again and adding the new unlinked real number to 1 again, and then we just repeat this process for infinitely many times so that we link all the real numbers between 0 and 1 to all the whole numbers, making the real numbers between 0 and 1 countably infinite. What is wrong with my logic here?
|
[
"The process wouldn't be able to generate the real numbers in countably many steps because they are uncountable. You're missing a crucial part of Cantor's argument, it begins with the assumption that you have a one to one correspondence between the real numbers and natural numbers. Then it finds a real number that wasn't there, which is already a contradiction, you can't just addit in (because it was supposed to already be there). Basically Cantor did not have a process for generating real numbers, rather he assumed that he could enumerate the real numbers and showed that being able to do that gives you a contradiction, which shows his initial assumption that the real numbers are countable is wrong."
] |
[
"Cantor assumes that there exists a completed list of real numbers, then finds a number not on that list. You could alter that list to include the new number, but that would contradict it originally being a complete list."
] |
[
"More directly giving the fallacy in OP's argument, if the first two numbers in the list are .0000000000000... and .1000000000000000... respectively, OP's process will only ever construct numbers that contain at least one consecutive pair of 1s. However, there are lots (as it turns out, an uncountable collection) of numbers that don't have a consecutive pair of 1s."
] |
[
"Does sodium pentathol really work as a truth serum? And if so how?"
] |
[
false
] |
[deleted]
|
[
"Well, the theory is that it suppresses higher brain functions, sort of kills off your inhibitions to lie. The fact is it is more likely to make you a chatty cathy, but I really doubt its efficacy as a 'truth serum'. It is conceivable though that it could work. In my line of work I routinely administer potent sedatives, on rare occasions thiopental is one of those sedatives. What I do notice is that yeah, patients say realllly embarrassing things sometimes, usually though they aren't secrets or revealing lies or anything just more like \"I really think you're handsome\" to any of the people of the sex they're attracted to in the room. I say it like that but they do say more explicit things too."
] |
[
"Just to cause rapid unconsciousness."
] |
[
"I think he worded that poorly. Probably more, kills off your inhibitions towards telling the truth, or your reasoning on keeping up a lie."
] |
[
"Is every trait of every animal a result of evolution?"
] |
[
false
] |
Since every animal evolved from single celled organisms, it makes sense (to me) that each trait or characteristic came about because of evolution or natural selection. For example I presume I have two kidneys because it's more efficient than one big kidney (if one should die). Surely the fact that I have two kidneys is because my one-kidney'd ancestors died out due to natural selection? But then things like wings throws this idea out the window and the steps between a wingless animal and an animal capable of flight / freefall. Surely until the wing could actually do anything, it was just a hindrance and of no evolutionary benefit? What I want to know is if everything about every animal from the colour of my hair to the shape of my toes is 100% down to evolution and natural selection. If a trait wasn't caused by evolution, how and why does that animal have that trait? I hope my question makes sense and I'd love to hear what you thought about it. Thanks
|
[
"Technically, yes - every trait of every animal is a \"result\" of evolution.",
"Every lineage of organisms has been evolving for about 3.5 billion years.",
"The characteristics of human beings are the result of 3.5 billion years of evolution. (Of that species and its ancestors.)",
"The characteristics of a seagull are the result of 3.5 billion years of evolution. ",
"The characteristics of a lobster are the result of 3.5 billion years of evolution. ",
"Etc etc.",
"then things like wings throws this idea out the window and the steps between a wingless animal and an animal capable of flight / freefall. ",
"Surely until the wing could actually do anything, it was just a hindrance and of no evolutionary benefit?",
"This isn't true at all.",
"For example there are ",
"flying squirrels",
" (which are actually \"gliding squirrels\"), and even ",
"gliding snakes",
" and ",
"gliding frogs",
". ",
"These animals live in trees and use gliding to get from one tree to another without descending to the ground, to escape predators, and/or to avoid injury in falls. ",
"You can easily see how having \"some\" gliding ability might be better than ",
" gliding ability, how having \"good\" gliding ability might better than having \"mediocre\" gliding ability, how having \"some\" ability to fly might be better than having \"only\" the ability to glide, etc etc. ",
"---",
"To address your main question, every characteristic of every organism is \"a result of evolution\", but that doesn't necessarily mean that that characteristic has been selected because it's \"the best option\". ",
"If one group of organisms has any characteristic that makes them more successful than another, then the more successful ones become more common and the less successful ones become less common (and may die out.) ",
"On the other hand, if a characteristic is working \"well enough\", then it can persist, even if it's not noticeably better than some other characteristic. ",
"For example, hair color in humans: ",
"As far as we know, it's not much more advantageous for human beings to have black hair rather than brown hair, or brown hair rather than black hair, etc. ",
"Black hair, brown hair, red hair, blond hair all \"work okay\" for human survival. "
] |
[
"The newer traits that pop up and are selected are kind of random. So while some that stay are likely due to natural selection (color of certain animals, for example), not all were necessarily selected for. ",
"For example, let's say you have lots of fur and that works well because the environment is cold. However, you also have three eyes. The fur would be 'selected' for, but the three eyes would also get passed on because they both happened to be on the same animal. ",
"Evolution is kind of like trial and error. Species get a bunch of different characteristics and the ones that work well stay by virtue of being able to reproduce. It doesn't actually have a planned direction or anything (that is, mutations can be harmful to your chances of survival). "
] |
[
"Fun facts: \n- A trait (like 3 eyes) that comes about as a result of selection for another trait is a \"spandrel\".\n- If it used to serve a role but no longer does, it's a \"vestigiality\"\n- If its a throwback to an ancestral trait, its an \"atavism\""
] |
[
"What's outside the spectrum of EM radiation?"
] |
[
false
] |
Are there waves with a lower frequecy than radio, or higher frequency than gamma? If so, what makes them different than EM radiation? Is it possible that there are wavelengths that are light-years across?
|
[
"What happens when you move towards infinite on either end? "
] |
[
"What happens when you move towards infinite on either end? "
] |
[
"To expand on this, if you change your reference frame light can redshift or blueshift. If there was a minimum possible wavelength, that would imply that if there's light with that wavelength you can't change your reference frame to blueshift it more. Likewise, if there were a maximum wavelength you wouldn't be able to further redshift the low energy light. But under special relativity, all reference frames run on the same physics. You can always change your reference frame."
] |
[
"Fluid Mechanics: Is it possible to determine the viscosity of a Non-Newtonian fluid by sending it through a pipe?"
] |
[
false
] |
Say you have a Non-Newtonian fluid flowing through a straight pipe. You have a pump in one end and you can measure flow and pressure at both ends of the pipe. Would it be possible to determine the viscosity of the fluid? What properties of the fluid and pipe would you need to know (density, temperature, friction?)? I'm thinking it would be impossible, but I can't seem to put it into words.
|
[
"First of all, for a non-Newtonian fluid the viscosity isn't constant, but changes with the shear rate.",
"The only way I could see that you could do this would be if you know (or assume) that your non-Newtonian fluid is a ",
"power-law fluid",
". By testing different pipe sizes and pressure gradients, you should be able to use the results to solve for the power-law parameters K and n.",
"However, you would have to be certain that you have laminar flow, otherwise the analysis wouldn't be valid. Obviously the Re=2000 for turbulence isn't valid for a non-Newtonian fluid, you'd have to do some research to see if there's a well-defined critical point for a power-law fluid.",
"My guess is that while it's theoretically possible to do the experiment, the signal/noise ratio would be very bad, and your results would have huge error bars. It would be much simpler to measure the apparent viscosity at different shear rates with a cone and plate viscometer, and then fit the data to an appropriate model."
] |
[
"What if we had a Newtonian fluid though, and that I just wanted to see a trend (lets say that the temperature and flow is constant)? \nWould it be possible to use pressure at the inlet and outlet, along with the flow rate to find the viscosity of the fluid?"
] |
[
"Sure, that's simplicity itself. You just keep the Reynolds number below 2000 so that you have laminar flow, and use the ",
"Hagen-Poiseuille Equation",
". Knowing the flow rate and pressure gradient, then you solve for the only remaining unknown, the viscosity."
] |
[
"What effects do the solar cycle have on Earth's temperature?"
] |
[
false
] |
We currently experiencing a "solar minimum" meaning there is less electromagnetic activity and fewer sunspots. We are also experiencing record setting high temperatures do to high levels of green house gases in the atmosphere. Will the next solar cycle exacerbate the greenhouse effect, or have some other effect?
|
[
"The sun does get slightly brighter at the peak of each solar cycle, but the climate effect is very small compared to greenhouse gases.",
"The sun gets brighter and dimmer on a fairly regular 11-year cycle. This changes the amount of sunlight absorbed by the Earth by about 0.25 watts per square meter, about 0.1% of the total absorbed. In comparison, greenhouse gases like CO2 have increased the energy input by about 3 watts per square meter since 1880 -- ten times more of an effect. And unlike greenhouse gases, the solar cycle just repeats itself, with no apparent long-term trend.",
"https://commons.wikimedia.org/wiki/File:Changes_in_total_solar_irradiance_and_monthly_sunspot_numbers,_1975-2013.png",
"This graph shows the relative importance of greenhouse gases, solar brightness, and other effects:",
"https://data.giss.nasa.gov/modelforce/Fe_H11.gif",
"The maximum in solar brightness occurs when the Sun has the most sunspots. This may seem backwards, but sunspots have bright edges that more than make up for the dark spots. We are currently in a solar activity minimum: the last maximum was around 2014. The 2014 maximum was later and weaker than usual, for reasons we don't understand, but unless something weird is going on with the sun, the next maximum should be around 2023-2026.",
"https://en.wikipedia.org/wiki/Solar_maximum#/media/File:Solar_Cycle_Prediction.gif",
"https://en.wikipedia.org/wiki/Solar_maximum#/media/File:Sunspot_Numbers.png"
] |
[
"The sun gets brighter and dimmer on a fairly regular 11-year cycle. This changes the amount of sunlight absorbed by the Earth by about 0.25 watts per square meter, about 0.1% of the total absorbed. In comparison, greenhouse gases like CO2 have increased the energy input by about 3 watts per square meter since 1880 -- ten times more of an effect. And unlike greenhouse gases, the solar cycle just repeats itself, with no apparent long-term trend.",
"So during a solar maximum, our warming rate could increase to 3.25 watts per sq. meter, and during a minimum our warming rate could decrease to 2.75 watts per sq. meter?"
] |
[
"Half that change, I gave the full range, not the amplitude."
] |
[
"How I calculate the velocity of the water inside of a pipe through the pressure and the area?"
] |
[
false
] | null |
[
"It's often treated as ",
"Poiseuille flow",
"."
] |
[
"You should post the exact question on ",
"/r/AskPhysics",
", you'll get good answers there."
] |
[
"The problem is we don't have the volumetric flow rate. Actually the purpose of finding the velocity is to find the volumetric flow rate.",
"Edit: ΔP isn't avaliable as well."
] |
[
"Can /r/askscience help explain basic lightning to me? (link to a reddit comment that sparked this inside)"
] |
[
false
] |
I don't feel like I'm getting the complete picture here:
|
[
"The same convection based process that forms hail also helps build up electric charge separation in thunderclouds. This creates a voltage (electrical potential difference) between the cloud and the ground, much like a capacitor. The air between is an insulator but when the voltage builds up high enough insulators can break down. A small current begins to ionize the air in paths, this is the initial slowly growing tree that you see. Ionized air is conductive, so once one path is established to the ground the bulk of the electrical charge will discharge (this is the bright lightning bolt you see)."
] |
[
"Once a lightning channel is established through all of the small ionized paths (the dart leader process) that same channel is often used to multiple discharges. Initially there is one large pocket of charge in the cloud that creates the first discharge, then you get discharge channels reaching further into the cloud until they find another charge center which is then discharged back to ground through the same channel again. This is why you can sometimes see multiple stages to a lightning flash instead of one single bright flash.",
"Here is a ",
"diagram",
" and a ",
"photo",
" to help."
] |
[
"Yup. I assume that there are multiple \"pockets\" of charge in the clouds, when one of them comes into contact with the ground that changes the electric potential of the former pocket, encouraging a lightning bolt that connects another pocket to the discharged pocket and the ground."
] |
[
"Is it possible for a paper airplane to hover between two fans?"
] |
[
false
] |
This clip has been making the rounds lately, showing a paper airplane hovering between two desk fans: Would it actually do that? And if so, how?
|
[
"It doesn't seem possible for the paper airplane in that clip to hover as it does. The fans both seem to be pointing and blowing turbulent air at the airplane. A fan (approximately) generates a vortex tube, which can also be considered a turbulent jet (a tube whose surface supports a velocity jump). When two of these tubes collide head-on, even a little off-axis, they widen; and at the middle is a stagnation point. At the stagnation point, flow approaches from one axis and retreats along the other two axes (much like what you would feel if you stood on the ground directly beneath a hovering helicopter). The forces on an object at the stagnation point generate a stable equilibrium in only one direction: along the axis connecting the fans. In the other two directions, the equilibrium is unstable, with any deflection causing increasing velocity in that direction. I don't see how an object in the shape of a paper airplane can get around this, nor how this flow is substantially changed by the presence of turbulence."
] |
[
"I don't know if it is relevant, but in the MoMa entrance, there is an ",
"installation where a strip stays around air flowing between two fans",
"."
] |
[
"Art layman here. I have seen it first hand and unfortunately, there are two extremely thin wires going from one fan to the other keeping the strip from flying off. Nothing more than an optical illusion. So... sorry, but no."
] |
[
"Why do we windmill our arms when we lose our balance?"
] |
[
false
] | null |
[
"Conservation of angular momentum. If you're standing on the edge of a platform, your angular momentum is zero. If you start wheeling your arms in a clockwise fashion, your trunk will have to start turning in a counterclockwise fashion so that the total angular momentum of your body is still zero. This is what you want, to get your body to rotate AWAY from the edge. So you rotate your arms in the opposite direction.",
"You see the same thing with dragsters. The wheels start turning clockwise, and so the whole body lifts counter-clockwise."
] |
[
"Exactly correct, except:",
"Dragsters popping a wheelie aren't a good example of conservation of angular momentum because they are in contact with the ground; there is an actual torque applied to the body of the car from the traction force being not in line with the center of mass (the traction is in line with the ground). "
] |
[
"This is true. But then again, so is the case of someone wheeling their arms, because there is torque applied at the shoes, pretty much analogous to what's happening with the dragster."
] |
[
"As there are robotic limbs that can be controlled by the brain, is it possible in many years from now, for brains to be in essence a CPU for a whole mechanical robotic body?"
] |
[
false
] |
[deleted]
|
[
"Brains already are \"CPUs\" for a \"mechanical body\". The mechanical body just happens to be squishy, fleshy and often times smelly.",
"But, I believe you're asking if either a) we can slowly replace our bodies piece by piece until we are made of circuits and metal/fibers/plastics and leave the brain (much like Robocop) or b) become like Cain in Robocop 2.",
"There answer is a conditional no. Conditional on 1) why do this?, 2) brains and bodies don't like these kinds of things. The immune system gets very touchy and tries to reject things that don't belong and 3) the loss of your face (i.e., your perceived identity) may have severe psychological effects (see the people who got face-transplants; one requirement for them is psychiatric evaluations and treatments to cope with \"not being them in a mirror\")."
] |
[
"I think it's ridiculous to imagine that our medicine and technology won't have advanced sufficiently to allow such a thing in a hundred, a thousand, or even a million years in the future.",
"What's to say we'll even be around in 1000 or 1 Million years? It's silly to conjecture that far. In a reasonable time frame, this is unrealistic. "
] |
[
"In addition to this, we haven't actually solved the brain>computer interface issue yet. Yes, we can control robotic limbs \"with our brains\" but really, we're controlling the robotic limbs \"with part of our brain that has a slightly higher electrical potential in an area while we think really damn hard on moving our limb\".",
"We currently have prosthetic limbs that function by \"thought\", they read the electrical potential of the synapse for the nerve that used to control your arm. To successfully integrate electrical and chemical signals from the brain(nervous system) to an outside electrical would take quite a bit of work."
] |
[
"How does the Earth's outgoing radiation attain equilibrium with incoming radiation?"
] |
[
false
] |
Science has always been a difficult subject for me to understand, and I lack much of an education in even basic physics - which means that my decision to try and learn the science behind climate change through the internet was probably a bad idea, haha. One concept that's been explained to me by several people is that if the Earth had no greenhouse gases, the amount of radiation emitted by the Earth's surface would be in equilibrium with the amount of radiation absorbed by the Earth. But because the Earth's atmosphere have greenhouse gases, most radiation emitted by the Earth's "surface" can't escape into space. And so, instead, there's an altitude in the atmosphere called the "effective emission height", at which outgoing radiation is equal to incoming radiation. In order for the effective emission height to actually emit as much radiation as the Earth absorbs, it has to have a temperature of 255.20 K, which is known as the Earth's equilibrium temperature. But currently, the Earth's effective emission height is only 255.04 K, which means that the amount of radiation emitted from that altitude is actually somewhat less than the amount of radiation absorbed by the Earth. This is why the Earth's average temperatures are currently increasing, and the Earth's temperatures will continue to increase until the effective emission height warms up to the equilibrium temperature. Because the amount of radiation absorbed by the Earth currently exceeds the amount of radiation emitted by the Earth, the amount of thermal energy present on Earth is increasing. Some of this additional thermal energy remains around the Earth's surface, increasing surface temperatures, and some of this additional thermal energy is making its way up through the atmosphere, increasing temperatures at higher altitudes. Once enough thermal energy is added to the Earth to warm the effective emission height's temperature to 255.20 K, the Earth will regain equilibrium and stop warming - so long as the amount of greenhouse gases in the atmosphere doesn't increase (and obviously, the amount of greenhouse gases in the atmosphere increasing, which shifts the effective emission height to a higher, colder temperature, and so we're unlikely to attain equilibrium anytime soon). Or at least...that's how I understood what was told to me. But it's now starting to seem like I'm not putting this together properly. I was recently talking with someone who seems to have a very strong physics background. Early in our conversation, this person said: What happens as the land and oceans get warmer? They will emit more thermal radiation, which will raise the temperature at the emission height until it balances the equilibrium temperature. When this happens, the planet won't be banking thermal energy any more, and and the global mean surface temperature will stop rising. This to match-up with what I just summarized. But later in our conversation, this person said: The Earth doesn't gain equilibrium by using thermal energy to warm the air at a certain height. That's not how any of this works. When I pointed out that this second statement seems to contradict the first and asked for clarification, this person replied that they were unable to explain the distinction between their two statements, because my background in physics wasn't strong enough (and again, it's quite true that I barely understand any basic physics, aside from a scattering of things that have been explained to me on Reddit over the last few months). The person also recommended that I watch , paying special attention to the part about the "adiabatic lapse rate". So I watched the video and pointed out that the instructor makes the following two statements: You heat up the ground. The ground starts transferring that energy back up, some of it through convection, some of it through radiation, some of it through the water cycle. So basically the energy starts working its way up through the atmosphere. Eventually you reach a point where you're high enough in the atmosphere, there's little enough of that gas above your head, the infrared radiation can just escape out there into space...so this is the level of the atmosphere that you have to heat up to that minus eighteen degrees centigrade, in order to get the balance between the energy coming out and the energy coming in. Both of those quotes seem to support my initial understanding of the concepts, and also seem to line up with this person's first statement that I quoted. But when I asked the person again if they could clarify their second statement, they said that they couldn't. They also said: Of course the air at the emission height will have to get warmer before equilibrium is reached. If you want to take that super-literally, then yes, this happens by thermal energy being added to the air. That is literally a thing that happens. What I'm saying is that this completely misses the point of what is involved in equilibrium being reached. I have absolutely no idea what this person is trying to communicate to me, so I thought I'd start a new thread and see if someone else is following what the person is saying. Can anyone with a firm grasp on this topic understand what it is that I'm missing? Thanks!
|
[
"I don't know if the effective emission height really helps you understand the mechanism at work.",
"The equilibrium condition means that the Earth has to radiate the same amount of energy away that it receives from the sun. This always has to be true- if we radiated significantly less energy than the sun imparts to us then the Earth would start warming rapidly and not stop. If we radiated more energy than the sun imparts then the Earth would start cooling rapidly and not stop. What this means is that the Earth radiates about the same amount of heat regardless of whether there are greenhouse gases or not. What changes in the presence of greenhouse gases is the energy content of the atmosphere, with greenhouse gases trapping more energy against the planet. ",
"As it sounds like you know, all objects emit black body radiation based on their temperature. In our case, objects roughly \"human hot\" emit infrared radiation. This happens continuously, all over the world. ",
"Imagine that the atmosphere had no greenhouse gases, and assume that this means that infrared radiation does not interact with the atmosphere. The each ray of emitted radiation would then leave the surface of the Earth and go out into space without interference. It's easy for the infrared radiation to escape in this scenario.",
"Now assume we add greenhouse gases. In this scenario the rays of infrared light are emitted from the surface of the earth, but they don't travel out into space uninterrupted. Instead, if they strike a molecule of greenhouse gas that ray is absorbed for some time, and then re-emitted out in a random direction. That re-emitted ray may make it's way out into space, or it might hit another molecule and go through the process again. ",
"Given enough time, this ray of infrared light will eventually escape the atmosphere and leave the Earth. Greenhouse gases don't change that. The only thing that changes is the amount of time that it takes that ray to leave the Earth.",
"Now, what happens to the climate as a whole? It takes longer for the sun's radiation to be radiated back out into space, and more of that energy is trapped in the atmosphere. The energy content of the atmosphere rises, which causes the temperature to rise. If you increase the amount of greenhouse gas, the radiation takes longer to leave the Earth, and the temperature rises even more. If you decrease the amount of greenhouse gas, it takes that radiation less time to leave and the total amount of energy in the atmosphere drops, and the atmosphere cools.",
"An analogy might be a traffic jam. Suppose you've got a highway where cars are constantly entering and leaving the roadway. When traffic flows smoothly then the cars enter, make it to their destination, and leave the highway efficiently. When there's a traffic jam, you've still got the same amount of cars entering the roadway at the entrances, and you've got the same amount of cars leaving the roadway at the exits. The difference is that everything slows down, and the density of cars on the roadway goes up. The analogy to the climate is that the amount of energy radiated onto the planet by the sun is fairly constant, and the amount of energy radiated away from the planet is also fairly constant (if it wasn't, then we'd experience dramatic heating or cooling). The only thing that's really changing is the amount of time that the sun's energy spends bouncing around our atmosphere.",
"The other analogy is the blanket analogy. Ignoring the sun for a moment, consider what happens when you sit in a cold room versus sitting under a blanket. Your body emits a fairly constant amount of heat, but in the first situation you're cold and in the second situation you're warm. What's different? The blanket traps the thermal energy against your body so it takes longer for it to escape, it doesn't add or subtract energy from the situation, it just traps energy closer to your body so you stay warm. The same equilibrium condition must be true as above- the amount of energy you emit with and without the blanket must be the same. If you+blanket emits less energy than your body produces then you will continue to get hotter and hotter without a limit. If you+blanket emits more energy than your body produces then you will continue to get colder and colder without limit. ",
"Now this is an extremely simplified situation. There are lots and lots and lots of what we call second-order effects in the real atmosphere. For example, snow reflects a lot of light into space since it's white, and increasing temperatures means there's less snow cover, so more energy is absorbed and less is reflected. More snow cools the planet, and less snow warms it. What this means is that if the planet warms a little bit and we have less snow, then the lack of snow then causes the planet to start warming faster than it was before. There are dozens of such variables we haven't talked about here."
] |
[
"Right, the Earth's actual temperature is not constant. And neither is solar irradiance, which are the two big assumptions in the above analysis. ",
"If the Earth's temperature is constant for any length of time then it will be in total equilibrium. The amount of energy emitted will be exactly equal to the amount of energy absorbed.",
"But, we know that the Earth's (average) temperature is not constant. When the Earth is warming, as is the case right now, it is absorbing slightly more than it is emitting, and the total energy of the system is increasing. When the Earth is cooling it's emitting slightly more energy than it is absorbing.",
"I don't have any clue what the actual magnitude of the difference is. "
] |
[
"If only it were that simple: ",
"https://andthentheresphysics.wordpress.com/2014/03/05/effective-emission-height/",
"If this model is correct, then the effective emission height rises, but the temperature at the effective emission height does not.",
"Edit: or more specifically, the temperature at every height rises, but the effective height is moving up to where it had been colder before."
] |
[
"Is it true that when astronouts return they are a couple of inches taller thanks to the effects of zero g on their spinal discs?"
] |
[
false
] | null |
[
"No. They're taller in space because the cartilage in their spines acts as a spring. In space, there's no gravitational pressure compressing it, so they're taller. As soon as they're back in a normal gravity environment they re-compress."
] |
[
"Do they re-compress instantly, or how long does this take?"
] |
[
"After getting a good nights sleep, you are a tiny bit taller, as lying horizontal gives those discs a chance to expand. beeblebroxh2g2 comment is correct, but I disagree with his \"no\"... sounds like he confirmed it. It takes time for those discs to re-compress."
] |
[
"If you freeze blood, how long will the DNA in the blood maintain its integrity?"
] |
[
false
] | null |
[
". DNA on it’s own is a relatively stable molecule owing to the double helix and (in eukaryotes) chromatin structures. A pure, dry DNA sample with no contaminants will survive for quite a while at room temperature and in fact this is how DNA is typically shipped in the mail. However long term DNA preservation is not always trivial because of sensitivity to handling and environmental contaminants. ",
"DNA in buffered solution can survive indefinitely at -20 C or -80 C even in non sterile conditions. At these temperatures, enzymatic activity by DNA degrading proteins is essentially zero. The main issue with frozen DNA is actually the process of thawing it when you want to use it - repeated freeze/thaw cycles can cause excessive shear on the DNA strands that mechanically rips apart the molecule. ",
"As for the case of blood, where the DNA is contained in the nucleus of white blood cells as well as in solution, proper cryo storage will require extra measures to protect the integrity of the cells. This involves chemical preservatives like glycerol and EDTA. These frozen stocks are viable at -80 C for an indefinite amount of time, although you may see a decrease in extraction yield as time goes on. This is attributed to cell lysis, which is extremely slow at low temperatures and in a preserving environment, but nonetheless still occurs at a non zero rate. For short term storage, 4 C is sufficient.",
"Edit: several people have pointed out that DNA does actually exist in the blood outside of white blood cells"
] |
[
"Red blood cells do not contain any DNA,",
"Unless you're a bird, reptile, amphibian, etc. Or you have a strongly regenerative anemia with some residual DNA in it ;)"
] |
[
"Almost indefinitely. Source: I pull DNA from ages old blood samples regularly. ",
"Scientists have had very good luck pulling DNA from mammoths frozen in the tundra - you can read \"Bring back the King\" if you have any interest in that sort of stuff. ",
"DNA itself is very hardy. We pulled DNA from a decaying carcass, with just bone left behind. It had been in the elements for 6 months or more, and we were able to identify the carcass to an individual animal based on various DNA markers. That's without even trying, really. "
] |
[
"When laying a new undersea cable with a plow, how do they not accidentally dig up an old cable that needs to be crossed over?"
] |
[
false
] |
(Don't judge that it is quora). They mention that they bury the cables under the sea bed around a meter using a . Looking at a of where cables are buried, they obviously criss cross at certain points. How does the plow avoid digging up other cables?
|
[
"Existing cables are shown on our charts so we stop ploughing 500m before the cable and fly the plough over it till 500m after. From that point we continue ploughing the new cable in. A followup vessel will come along and use a trenching ROV to jet in the cable for the 1 km that is exposed. A guard vessel will usually be stationed over the exposed cable at the crossing until it can be jetted in."
] |
[
"Usually the path and margins on each side that the cable will be layed will be surveyed.",
"\nFYI, There’s thousands of miles of old analog cables that crisscross every body of water imaginable. Sometimes that gets snagged and has to be cut.",
"\nIt’s the live fiber optic cables that belong to someone else you need to be careful about damaging."
] |
[
"Only military cables are secret. In general we want everyone to be aware of the cable location so they don't accidentally drop an anchor in the wrong place. Governments now recognize the importance and vulnerability of subsea cables. Check out ",
"https://www.iscpc.org/"
] |
[
"Why are so many people scared of- or disgusted by spiders? Is it cultural, or is this 'disgust' found in many cultures?"
] |
[
false
] | null |
[
"It's found in many cultures, but to my knowledge the only animal universally feared by humans is snakes (see ",
"Human Universals",
"). Some cultures make a practice of eating tarantulas, which even their children will fearlessly capture.",
"This, coupled with the fact that any parent of a young child is aware that very young children do not share the same disgust of bugs (including spiders) that adults do, implies that this is a learned behavior. I can't find the reference at the moment, but I recall reading a study in which toddlers were offered cups containing a floating roach and, below a certain age, happily drank from it. Slightly older kids would express disgust, but happily drink the water once the roach was removed. Even older kids refused the water altogether, demanding a new cup.",
"So given that the aversion to spiders is not universal across human cultures and is not found in very young children of any culture, it is evidently a learned behavior. We then need to consider ",
" we would learn this behavior. Given that small arthropods such as insects and spiders may carry diseases or be venomous, it doesn't seem like a big stretch to assume that people have simply been teaching their kids (even if unconsciously) to avoid them for many thousands of years."
] |
[
"About what country/area of the world are you speaking?"
] |
[
"Given that small arthropods such as insects and spiders may carry diseases or be venomous, it doesn't seem like a big stretch to assume that people have simply been teaching their kids (even if unconsciously) to avoid them for many thousands of years.",
"In my country, killing a spider is bad luck; having a spider is your room is considered good. Webs are removed only if they get in the way, and even then, most people are careful not to remove it while the spider is on it.",
"Since there are no venomous or dangerous spiders in the area, I guess the people haven't been conditioned to avoid spiders."
] |
[
"Mixing different temperature liquids, differently"
] |
[
false
] |
I have 2 cups, one of just boiled water, and one of room temp (cool) water. If I pour one cup into the other, which results in the cooler mixture? hot into cold, cold into hot, no difference
|
[
"Water has a high heat capacity. If you mixed boiling water with room temperature water then you'd just end up with water with a temperature somewhere in between 100°C and 25°C. The order in which you mix the liquids shouldn't matter. "
] |
[
"It should not make a difference HOWEVER practical factors like heat exchange with the room and splashing will cause a difference that depends on the mixing and measuring procedure used.",
"There is a \"correct\" way order to add them, but that's to stop you splashing yourself with boiling water."
] |
[
"shouldn't matter.",
"hmmm, I'm going to try and test this and video it.\nMy common sense says you're right."
] |
[
"Would the altitude of a (American) football stadium make a difference in the distance of a kick/pass?"
] |
[
false
] |
Question came up while watching the Denver Broncos and St. Louis Rams game, curiosity got the best of me so I decided to ask you guys/gals.
|
[
"Yes, it will. The higher altitude will have a thinner atmosphere and there will be less wind resistance against the ball. ",
"One quick way you could check this would be to see if there are more touchbacks kicked in Denver than in other stadiums. If you look at NFL stats Denver has lead the league in touchback % at home for ",
"4 of the last 5 years",
". This is a not necessarily the best way to do it, since year-to-year it will depend on the actual kicker for the team, it just happened to be the quickest for me to find a comparison. I'll try to see if I can dig up more hard numbers.",
"Also, though only tangentially related, ",
"Coors Field",
" is the most hitter friendly park in Major League Baseball. This is in part because the ball has less resistance in flying and because ",
"curveballs move less",
" when thrown in the thin atmosphere. "
] |
[
"Yes, Balls in Denver/Phoenix do travel slightly higher and farther in the thinner air, but the effect is negligible. Far more important is the high altitude effect on the endurance of the players, which gives the home team a distinct advantage (as they are acclimated)"
] |
[
".",
"I worked this out a while back on a question on home runs at Coors field. The answer is yes, although the effect is a lot more pronounced in baseball where you're dealing generally longer distances (400 ft home run versus maybe a 50 yard football pass.) Let me work it out for you here:",
"Say you throw a ball at a given angle theta_0 and initial velocity V0, which continues to fly at V and some angle theta. The forces on it in the vertical direction give an acceleration",
"y'' = -mg -D*sin(theta)\n",
"where m is mass of the object and g gravitational acceleration. D is the drag we'll get to -- that's the component that will vary with altitude. The horizontal direction forces are just drag,",
"x'' = -D*cos(theta)\n",
"Drag is equal to",
"D = 1/2 rho * V^2 * CD * A\n",
"where rho is the air density, V is velocity, CD is a drag coefficient, and A is a reference area for that coefficient. Taking CD for an American football as 0.15 (I googled this) and it's cross-sectional area, and assuming a 60mph throw at 45 degrees, ",
"I rewrote the differential equation as 4 first order equations, threw em into matlab, and came up with this.",
"That's the different trajectories at 5000 ft vs sea level. The difference is significant! Around 28 vs 33 yards.",
"(Edit: I'd add that the CD of 0.15 for a football is apparently a bit high for a spiral throw -- and I'd expect a 60mph football throw to go farther than 30 yards. Saw another paper that tested it at 0.05. That would make the throw distance a bit more reasonable, but still, that altitude difference is significant.)"
] |
[
"Is there a satellite that provides a live feed of the earth?"
] |
[
false
] | null |
[
"Like, is there a satellite for public use where if I drove my car in front of my house, I could see my own house or car? "
] |
[
"No unless by accident a satelite happens to line up"
] |
[
"You can find the answer with a simple google search / on wiki. Please start there and come back with a more specific question."
] |
[
"While whatching a time lapse from the ISS, I noticed distinctive green lightning from the south part of México. Could someone explain?"
] |
[
false
] |
Different from the orange and yellow artificial lightning from the world at large and nothing to do (I think) with the northern lights, which are also featured on the video. It appears to be the only place on earh with this kind of luminosity. [0:26-0:30].
|
[
"I noticed that before and wondered the same thing. The only possible explanations that come to mind are: There is some thin aerosol or cloud layer there, that has unusual spectral extinction (meaning, it preferentially scatters or absorbs all but the green light); or there is something particular about those light sources.",
"The first option seems very odd; assuming the light sources are \"typical\", meaning, white lights and the yellowish sodium vapor lights, I don't know of any kind of aerosol that would have that property.",
"The second option seems more plausible to me. It is hard for me to get the country boundaries exact, since the view is \"upside down\" and curved, but I think the greenish lights are actually in Guatemala, not Mexico. Doing a few web searches, there appear to be a lot of non-profit organizations promoting solar powered white light LEDs for rural lighting; so some of this might be white light in green foliage? That does seem to be a bit hard to believe since that would require a lot of LED lights ... It could also be that mercury vapor lights are more common than the sodium lights in that region. I've never been there, so I'm speculating rather wildly - hopefully someone who has actually been there will read this and set me straight =)",
"One thing that came to mind - if anyone knows where there are more ISS timelapses like this - does that specific region have green lighting at other times? If it is consistently that color, then it would be clear that the lighting sources are just different in that region, and it isn't some atmospheric effect."
] |
[
"I can't see it, but judging from the video the lightning you're talking about at that time is mostly occurring in semitranslucent clouds over large cities. The most obvious answer would be the blue light from the lightning flashes is additively combining with scattered yellow light from the sodium D line at 589nm coming from the street lamps below. eg. yellow + cyan light combines in the efficient scattering medium of small ice crystals in a large thunderstorm cloud to produce the subjective impression of green light to an observer above."
] |
[
"If it's something related to the light sources, some more info should be somewhere on the Internet. If it isn't, perhaps the phenomenon should be observable in another time lapse (in another region, maybe). Yeah, I vote for more observation, that would discard some of the options given. Thanks for the response."
] |
[
"Since light is also a wave, can you use light to cancel out light as you can do with sound in noise-canceling headphones?"
] |
[
false
] | null |
[
"Sound is propegated by a mechanism of pressurization/rarefaction of air. The frequency of these cycles, or \"vibrations\" create sound, higher the frequency, higher the tone of sound created. By feeding a signal identical to the original, but 180 degrees out of phase, you can cancel the original. This is known (with sound) as active noise cancelling or also referred to more generally as destructive interference.",
"Although light does not use this same mechanism, because of it's wave-like properties it is still possible as demonstrated in the following clip:",
"https://www.youtube.com/watch?v=RRi4dv9KgCg"
] |
[
"Yes absolutely. This is an entire branch of optics called interferometry which deals with the wave interference of light. It is very often used in advanced and precision measurement techniques. LIGO uses interferometry to measure differences in distance smaller then 10",
" meters.",
"Interference is a property of all waves so anything that can be described as a wave can exhibit interference."
] |
[
"The magic happens because the fields pick up a minus signs on reflection only in certain places (it depends on the index of refraction of the materials on either side of the bounce)."
] |
[
"How can antimatter annihilation release so much energy?"
] |
[
false
] |
[deleted]
|
[
"The famous equation, E=mc",
" is called the ",
"mass-energy equivalence",
" relation. This means that mass has a certain amount of energy, so no energy is lost in the conversion (see more ",
"here",
"). The ",
"full equation",
" relates the total energy to both the rest mass and the momentum."
] |
[
"You can also use energy to create mass. One anti particle and one normal particle will be created. It is strange then that the universe have more normal matter than anti matter, wouldnt this imply the creation of the universe created mass out of thin air, violating the rule?"
] |
[
"Definitely right. Mass-energy should only be considered \"potential energy\" in a very casual linguistic sense (\"mass has the ",
" to be converted into energy\"). Kind of in the same way that we can consider Chemical energy to be the \"potential\" for a chemical to release energy due to a reaction."
] |
[
"Why does intense physical activity make you want to vomit?"
] |
[
false
] | null |
[
"The best theories about this are:",
"Exercise shunts blood flow to the skeletal muscles and away from the intestines. This causes peristalsis (the wavelike movement of your intestines) to decrease/stop and the food won't progress. This will cause nausea/vomiting if you have recently eaten or drunk too much. However I always found this answer lacking because it doesn't explain dryheave retching. If there's no food in the stomach why do people still retch?",
"Exercise that continues beyond the aerobic threshold and into the anaerobic area will produce large amounts of lactic acid. This decrease of blood pH is usually easily compensated with buffering and elimination mechanisms. If you continue to push yourself into to anaerobic area without allowing time for oxygenation your body will empty the stomach, and its acid, and its free hydrogen ions out of the body in an effort to reduce acidity. Of course this isn't a very effective way to increase blood pH so it's not done until other mechanisms have failed.",
"I don't have sources for these but they're the leading explanations I've heard.",
"I believe both are happening in various degrees in exercise. That's why it's easier to vomit exercising on a full stomach because the peristalsis slows down before the lactic acidosis kicks in and starts to get severe enough to cause vomiting. "
] |
[
"Two quick reasons, though there are additional factors:",
"Build up of lactic acid due to anerobic processes causes acidification of the blood. Acidification of the blood triggers receptors that lead to nausea.",
"The sympathetic system halts digestive processes while performing intense physical activity. "
] |
[
"Its been a while, but I believe you're correct with the lactic acidosis, at least that's what I learned in my systems physiology class. I believe it affects the Area postrema of the brain stem which controls vomiting once the buffering in your blood can no longer keep pH balanced at the systemic level. "
] |
[
"If I were a photon and I could see, what would I see?"
] |
[
false
] |
[deleted]
|
[
"You would see nothing. Though there is technically no inertial frame moving at the speed of light, as you approach the speed of light your clock slows relative to others. In the limit, your clock wouldn't tick at all so to you, in a way, no time would ever pass between being absorbed and being emitted. You can't see any events because those would necessarily take some amount of time."
] |
[
"The problem is that one of the postulates of relativity says that all photons travel at the speed of light, c, regardless of the reference frame. That means that even if you are moving away from me at c/2, and I shoot a laser beam at you, we will both see the photons of the beam travel from you to me at a speed equal to c.",
"Now when you ask \"what would a photon see?\" you're asking what the world looks like from its point of view. When we look at something from an object's point of view, we call that its \"reference frame\". An object's speed in its own reference frame is always zero, because the distance it is from itself is always 0.",
"Here's the problem with photons. We are looking at the world in a photon's reference frame, that means that the photon's speed must be zero. But our postulate from relativity says that the speed of a photon is c in ",
" reference frame. We have a contradiction! The speed of the photon whose reference frame we are trying to construct must be both zero and c. Thus, looking at things from a photon's reference frame is impossible. There's just no meaningful answer that physics can provide.",
"It's like asking \"but what would happen if we ",
" go faster than the speed of light?\" the question essentially boils down to \"Assuming that physics can't tell us what happens, what does physics say will happen?\" "
] |
[
"So... if I understand this correctly... by physics, a photon is technically blind because within the context of the photon, time has stopped and therefore light that I would see can't be seen because that would be a function of time? Or would everything be seen at once kind of like a bright light that never goes away?"
] |
[
"In the most scientifical way, how likely is it to have a zombie outbreak."
] |
[
false
] | null |
[
"Anything transmitted solely by bites should be relatively easy to deal with. An epidemic of biting attacks would be noticed, and quarantine procedures would be fairly straightforward. It would be stopped quite quickly."
] |
[
"Well, not unless you were going to anyway. I think you're ok. "
] |
[
"How much something like that spreads depends on the incubation period. If it is minutes or hours from the bite until the person becomes a zombie, it won't spread as well as if it takes several days, because people won't be able to travel as far carrying it. Epidemics where people are killed very fast tend to run through a population quickly, but the diseases kill their hosts before they can be passed on. If a disease takes longer to show symptoms, people will do more travelling with it, and it won't be confined to a specific geographical area. It's been said that the advent of international air travel has made combating infectious diseases far more difficult, as people can being a disease across the world before they realize that they have it. However, the danger could be mitigated depending on whether someone who is infected but hasn't been turned into a zombie is contagious, or if the disease can only be transferred by a zombie bite."
] |
[
"Are there enough nuclear weapons on the planet to kill every human?"
] |
[
false
] | null |
[
"[Citation needed]",
"The wikipedia article",
" (from which you can follow links to more credible sources if you like) isn't at all so certain about what would happen. In any case, it seems quite unlikely that a nuclear winter would kill every human being. An average temperature drop of -20 C (which is at the extreme range of what different studies have concluded) would absolutely disrupt the whole planet and cause a massive amount of people to die for various reasons but I can't see it killing everyone. Even without any special shelters a lot of people could survive. I mean people do live in pretty cold conditions at Arctic regions right now. Subtract 20 C from the temperature of the tropical regions and they're still above zero most of the year. Sure it's far from ideal conditions but I don't see why people couldn't survive in that."
] |
[
"For reference:",
"According to the Ploughshares Fund's ",
"Nuclear Stockpile Report",
", there are approximately 19,000 nuclear bombs worldwide (including 10,000 in Russia and 8,000 in the U.S.).",
"And the most powerful American bomb (Castle Bravo) has a payload equivalent to 15,000 kilotons of TNT, while the most powerful Russian bomb (Tsar Bomba) has a payload equivalent to that of 50,000 kilotons of TNT.",
"I'm too lazy to make a good estimate of the average bomb size and area of effect that would have, but here's a starting point."
] |
[
"There is enough potential nuclear destructive power to alter the environment for the entire planet by blocking out the sun with dust and radioactive fallout, so that anyone who does not have a special, protected long term shelter with years of supplies would not be able to survive. The radioactive fallout would only bean issue for a short time, but the lack of sunlight and the effects on vegetation and temperature would last years"
] |
[
"If elephants had gone extinct before humans came about, and we had never found mammoth remains with soft tissue intact, would we have known that they had trunks through their skeletons alone?"
] |
[
false
] |
Is it possible that many of the extinct animals we know of only through fossils could have had bizarre appendages?
|
[
"Trunks do leave visible attachment marks for muscles, ligaments, & such on the skull. However, from osteological correlates alone, it would be impossible to infer exactly what the trunk looks like. In what is perhaps a \"reverse-application\" of this line of reasoning, ",
"trunks can be rejected for sauropod dinosaurs",
".",
"EDIT: ",
"Another discussion of osteological correlates of trunks",
", this time applied to the giant rhinoceratid ",
"."
] |
[
"Wow. What a crazy amount of research on this exact topic. Thanks."
] |
[
"Except for the fact that we have fossilized webs preserved in amber, so we actually do know about ancient spiderwebs. ",
"Edit: fixing autocorrect"
] |
[
"Why does cold water seem to quench your thirst better than warm water?"
] |
[
false
] | null |
[
"I seem to recall this question popping up here not so long ago. Even without feeling really hot, a normal healthy amount of human warmth may still drive a little bit of sweat production. In its own small way, a chilled beverage may reduce this warmth and inhibit sweating. The end result is a sense of less need for hydration along with whatever actual hydration the drink provides."
] |
[
"The body handles excess heat through perspiration, which has the side effect of using up the water in the body. Cold water not only replenishes the body's store of water, it also cools it down directly. When you drink water, you get a positive feedback both from thirst satiation and the presence of a cooler substance.",
"At least this is the way it has been explained to me."
] |
[
"Frequently when you are thirsty it is in some party due to your body attempting to cool down. It could be via sweating out breathing hard. Either way your if your body temperature us high then bringing it down with cold water will bring a feeling of refreshment. Warm water might end up raising your body temperature. It may slake your thirst, but if you were thirsty due to heat in the first place then it behooves your body not to reward you for drinking hot water. "
] |
[
"Was there any reason for the dial up modem tone that we heard back in the 90s?"
] |
[
false
] |
That Dial-Up tone you hear whenever you wanted to connect back to the internet. It was a series of very memorable ... noises... so to speak. Was there any significance to this like what those sounds meant? I'm asking this because I remembered there was a period of time we could NOT connect at all and the tones kept replaying in a loop.
|
[
"Because dial up modems were trying to send digital information over a network made to carry human voice conversations, the modems would have to speak our language to make use of it. That noise you're hearing is the modem dialing into your ISP and then starting a connection. That noise is literally played over the phone line and received on the other end to establish your connection, very early modems would have a regular speaker that did the same thing in a box ",
"and you would have to place your phone's receiver onto it",
". More modern modems allowed you to skip using your phone handset and connected directly to a phone line."
] |
[
"That's right, the data in a dial up connection is sent over the phone line as sound essentially. You'll hear that same kind of sound if you've ever tried calling someone while they were sending a fax. "
] |
[
"Also, that ksssssshhhhhh PANG PANG PANG ksssssssssshhhh sound was the modems figuring out what protocol versions were supported. If the other modem didn't respond correctly, it could back down to a slower connection. ",
"I used to leave my modem volume up so that I could disconnect and reconnect if I didn't get the connection speed that I wanted."
] |
[
"Why do certain foods, notably meats and cheeses, taste differently depending on how finely they're cut or shredded?"
] |
[
false
] | null |
[
"They dont necessarily taste differently, but you experience them differently. ",
"A small piece of cheese that you can easily chew and taste and swallow would be experienced alot better than an entire cheese wheel that gets all cloggy and hard to chew and swallow."
] |
[
"Increased surface area means more interaction with your taste buds. A big chunk of cheese has to be chewed. This turns the once flat piece of cheese into small curdles or chunks. That takes a flat piece with a lot of surface area and changes it to small sphere-ish things with a much smaller surface area to interact with your tastes buds."
] |
[
"I’m not an expert, but my take is that exposed meats/cheese oxidize. When meats/cheeses oxidize their flavor profiles will usually change along with the amount of oxidation that has occurred; this is where the gamey flavor of lamb comes from for example. So different ways of cutting meats/cheeses will expose different amounts of surface area to our atmosphere leading to different amounts of oxidized surface area relative to overall volume of product. "
] |
[
"Can you actually gain weight from just a few meals?"
] |
[
false
] |
[deleted]
|
[
"People will say that a particular meal caused them to \"gain five pounds\" or that they gained x pounds over the weekend. The scale may actually register a weight gain, but it might not be permanent. They may still be digesting an unusually large meal. The entirety of the meal is still inside them and hasn't been processed to waste and eliminated yet. They may be retaining a lot of water which will shortly be discarded.",
"The only way to actually know would be to measure exactly how many calories they burned over the weekend, and how many they ate. Then blame the corresponding (well known, scientifically) weight gain associated with that caloric count."
] |
[
"What I meant is that you can gain more weight than you eat, if you include the effect of water loading. For each gram of carbohydrate, you store around 3 grams of water. You can gain alot of weight simply by storing water from eating a lot of excess carbohydrates."
] |
[
"You are neglecting water."
] |
[
"Why do we say electrons in a metal move around with velocities determined by kT instead of the Fermi level?"
] |
[
false
] |
It's all over solid state text books that the velocity of electrons in a metal (assume no applied field) is something like root(kT/m) (I don't care about the 2's and/or 3's that I'm missing). This perspective makes sense because it's sort of the amount of energy an electron might get from the thermal bath it's sitting in. But also, from QM, if we keep adding fermions (electrons in this case) to the metal, they'll occupy higher and higher energy states. Even at T = 0, the electrons will have nonzero energy, in particular, electrons at the top will be at the Fermi energy. In this case, the thermal velocity would be zero, but the Fermi velocity can get up near a percent of the speed of light (graphene for example). Clearly this is a huge discrepancy. So, why do we say electron speed is governed by kT instead of the Fermi level? It’s a quantum vs classical issue as pointed out. Dense systems of electrons should go by Fermi energy, sparse systems can use kT.
|
[
"They're both sort of right. In solid state physics it's important to remember we can't actually solve for the electron's motion. There are 10",
" electrons interacting with eachother and 10",
" nuclei. The 2- body problem is classical is hard enough. So every thing is an approximation. You use the simplest one that works. ",
"Sometimes you can assume the electrons are classical particles, then the energy is given by kT. It works pretty well in most metals. When that doesn't work you have to use quantum, so the energy is from the Fermi energy. \nTLDR. Math is hard so physicists simply everything and use whatever approximations we can get away with"
] |
[
"Thank you."
] |
[
"The 2- body problem is classical is hard enough.",
"The 2-body problem is trivial. It's the 3-body problem that is so difficult."
] |
[
"Anyone have a conceptual explanation of how fission and fusion works?"
] |
[
false
] | null |
[
"There is no simple answer or “conceptual” picture. There is a complicated balance of forcing which determine the binding energy of any give nucleus. It just turns out that if you plot BE/A versus A for nuclides near stability, it peaks at nickel-62."
] |
[
"Well concepts and visualization work great in introductory physics, but quantum systems are non-intuitive, and not easily visualized."
] |
[
"There are lots of different theories of nuclear structure that you could apply to model nickel-62. Most of them are quantum theories (the main exception being the liquid drop model). And in quantum mechanics, you describe things with quantum state vectors."
] |
[
"How does drinking water clear the bad stuff out of our bodies?"
] |
[
false
] |
Does distilled VS stilled water make a difference? How does it help our skin? Just curious how it actually works inside of our bodies. Always heard that drinking water can flush all the bad stuff out. Esp after a greasy, fat meal.
|
[
"Always heard that drinking water can flush all the bad stuff out",
"Pop culture has endless theories on how things work, probably because most people don't care to actually learn the biology or realize that biochem processes are exceedingly complex and try to simplify the knowledge."
] |
[
"I don't have any opinion on the pancreatitis, but asking for medical advice on Reddit is even in a best case scenario a waste of time. Go see a real doctor, or at the very least (if you don't have access to healthcare) find a reasonably trustworthy source. This could be a good place to start:",
"http://www.webmd.com/digestive-disorders/digestive-diseases-pancreatitis#1",
"Get well soon"
] |
[
"I've been drinking 1 USgal/day for a few months because I have pancreas issues. (Chronic Pancreatitis) which is miserable. It just seemed intuitive. So I'm wasting my time?"
] |
[
"In regards to \"nitrogen narcosis\", how does the gas we inhale while diving suddenly become narcotic to us at great depths?"
] |
[
false
] | null |
[
"You need to understand a little about how SCUBA works. You can snorkel at the surface, right? What if you had a 12 foot snorkel, would it work? No, but why? Because the weight of 12 feet of water is squeezing you body. You don't feel it because hey we are made out of water. However, it's more weight than your lung muscles can push against. You would not be able to suck in any air. ",
"With SCUBA an aluminium tank can be filled with 2000 to 3500 psi. That's more than the pressure of the water down to a couple hundred feet. The pressure is controlled by the regulator. Every time you suck in air the regulator gives it to you at a pressure ",
" to the pressure of the water around you. It's almost too easy to breath when you are on SCUBA, its fun. Sport divers, the ones who do it for fun, use normal, filtered air.",
"At the surface when you breath in you get about a half a liter of air and your lungs can hold about 4 liters total. At ten meters deep the pressure is twice as much, so you will actually suck in twice as much air! At that pressure you could have 8 liters of air squeezed into your lungs! At 20 meters it would be 12. So, as you go deeper you suck in a lot more air. At about 100 feet, or 30 meters the pressure is 4 times normal.",
"So, if you drink one cup of coffee at the surface that would be like drinking 4 cups of coffee under pressure. 4 cups of coffee is a lot more caffeine that 1 cup. So with normal air at 100 feet you would get 4 times the nitrogen, 4 times the oxygen and 4 times whatever else is in the air. If you had pure oxygen by the time you reached 60 feet the amount of pure oxygen would burn out you lungs! I had some friends get some bad air once, a small amount of exhaust from the pump motor got in the tanks. That was Carbon Monoxide. At the surface you wouldn't notice it, but when they reached 100 feet down it would be four times as much CO enough to kill you in minutes. They noticed the probable at about 30 feet down and surfaced, but in the three minutes it took them to dive down and come back up they had enough carbon monoxide that they had turned blue.",
"There are lots of things in air that can cause you to get drunk-like symptoms, including nitrogen. Dive deep enough and you get stronger concentrations of those gasses. So they can make you intoxicated. I have never experienced, nor has anyone in any group that I was with. That includes dives to 200 feet, so I think it's pretty rare.",
"No 5 year old could sit through my long winded explanations."
] |
[
"Most gasses are lipid-soluble, meaning that they can diffuse into fats, which includes cell membranes. At standard pressure, the amount that dissolves into tissue is negligible, but at higher partial pressures that amount increases and eventually it's enough to interfere with the chemical signaling of nerve cells."
] |
[
"This is spot on the answer i have been searching for. Thank you!"
] |
[
"How long does tap water need to be boiled for or kept at boil in order to sterilise it?"
] |
[
false
] |
Looking around online and I can't find any sources or scientific information on how long water needs to be boiled for in order to sterilise it. I'm seeing between 1 and 10 minutes?
|
[
"For clarification, the process you're referring to (Pasteurization) kills a large number of bacteria but absolutely does ",
" sterilize the milk. ",
"Here's an interesting document",
" that shows how many bacteria are allowable in milk products (TL;DR: a lot, as long as they aren't pathogens)."
] |
[
"For clarification, the process you're referring to (Pasteurization) kills a large number of bacteria but absolutely does ",
" sterilize the milk. ",
"Here's an interesting document",
" that shows how many bacteria are allowable in milk products (TL;DR: a lot, as long as they aren't pathogens)."
] |
[
"Depends on what the water contains. To kill most of living bacteria you do not even need to boil it. If it contains spores, it's the best autoclave it (~121 degrees C) for 10 min, but it reqiures equipment. When you don't have an autoclave, you can go old school and use tyndallization method.\n",
"http://en.wikipedia.org/wiki/Tyndallization"
] |
[
"How do transitions lenses work?"
] |
[
false
] | null |
[
"I am an optician and sell them every day but still had to goto web search to get the scientific terms for it.",
"Old glass photochromic lenses were embedded with silver chloride. Here you can see what happens to raw silver chloride when exposed UV light.",
"http://en.wikipedia.org/wiki/File:Decomposition_of_Silver_Chloride.jpg",
"So you could imagine what it would do if it inside of a clear lens. Eventually they would wear out and not change anymore.",
"Modern plastic transition lenses (which is the brand name for photochromatic lens) use ",
"Naphthopyran",
" which has the same effect as silver chloride. Reacts to UV radiation which in turn appears darker to our eyes."
] |
[
"Well, silver chloride decomposes, naphthopyran only changes its shape (photoisomerization being the fancy word for 'changing shape due to light). ",
"If I'm interpreting it correctly, the UV light is absorbed by the double bond between carbon atoms seen opposite oxygen in the ring. This excites the electrons in the double bond (specifically from the pi 'bonding' orbital to the pi",
" 'anti-bonding' one). Effectively this makes it a 'temporary single bond' instead of a double one, which allows the structure to rotate about that bond, ending up with the one seen on the right in that picture, which in turn causes a phase change in the liquid crystal it's in are good for, which changes how it polarizes the light passing through it, and in combination with a polaroid filter you get the darkening. ",
"But the original state has lower energy, so eventually, through random thermal motion, it finds its way back to the starting structure after a while. (which is a bit of a problem, since it means they'll not get as dark when they're warmer, such as on a hot and sunny day, which is when you'd want them to be dark!)",
"Chemically, this is essentially the same mechanism (although with different molecules and no liquid crystal) that is used by the ",
"chromophores",
" that do the light-detecting in your eye. There, a double bond absorbs energy, temporarily weakening it and leading to a twisting around the axis of that bond. And this change of structure sets off a whole chain of events that ends with your brain getting the message, while the chromophore eventually settles back to its original shape. These chromophores absorb in the visual range (red, green and blue) rather than UV, of course. ",
"(infrared detection that some species have works in a different way though, more related to how we detect heat. You can't get the excitation energy down far enough for this mechanism to work there)"
] |
[
"The auto-darkening welding helmets I've seen are electrically powered LCD screens. Usually a small solar panel keeps it charged (it requires very little power). It uses a common photocell to determine whether to switch to \"dark\" or \"light\" mode. There's a dial adjustment for that."
] |
[
"Where was all of the condensed matter prior to the big bang?"
] |
[
false
] |
This has always bothered me, and has made more than one science teachers angry at me when I asked it. But seriously, the big bang theory seems to never address this, or more likely I've never heard an actual explanation. If there was no galaxy prior to the big bang, then where was all the matter that caused the big bang?
|
[
"The equations describing the space-like separation between points in the standard cosmological model have a singularity at a finite time in the past (t = 0). Our best model right now says that at all times ",
" that time (t > 0), the universe was infinite, but ",
" that time the distance between any two points becomes zero.",
"Standard analogy:",
"Imagine a number line. The distance between any two points (numbers) x and y is given by |x - y|. This rule for measuring distance is called the metric. Now let the line undergo \"metric expansion\" in such a way that the distance rule is \"really\" given by t*|x - y|. Note that at t = 1 (which we'll call \"right now\") we get the original distance, |x - y|. Now, as long as t > 0, no matter how close to 0 it gets, there are an infinite number of points as far apart as you like. Give me any distance, and I will find two points at least that far apart. But when t actually gets to 0, suddenly the distance between all of our points has become 0."
] |
[
"i don't think you know how science works.",
"theories are more than just \"i have an idea\".",
"We can observe evidence for the big bang, that we can demonstrate and view over and over again.",
"It's not the same as, \"3 children saw the virgin mary appear, what a miracle, god exists!\"."
] |
[
"God isn't even an hypothesis, since it isn't based in any reliable observations whatsoever. "
] |
[
"Is donated blood cleaned in any way to prevent the spread of disease?"
] |
[
false
] |
I was watching Blade and that scene where he fell into a pool full of blood and healed himself and it got me thinking. Aren't blood transfusions from multiple donors? Meaning you'd get every single virus they carried combined?
|
[
"Blood is transfused (out of the donor and into the recipient) by the unit and each unit of blood is only from a single person. Also, it is all screened, in high income countries, anyhow, for a range of infectious disease including (These are what the Red Cross check for, other organizations test for other diseases): Chagas disease (T. cruzi), Hepatitis B virus (HBV), Hepatitis C virus (HCV 3.0), Human Immunodeficiency viruses, Types 1 and 2 (HIV 1,2), Human T-Lymphotropic virus (HTLV-I/II), Syphilis (Treponema pallidum), West Nile virus (WNV) ",
"Source",
".",
"World Wide blood testing"
] |
[
"Just an FYI for those who have never donated, in addition to testing, you answer ~50 questions relating to possible disease exposure, and talk about these with someone. Temperature, blood pressure, pulse, iron are also tested pre-donation to make sure everything's normal. I guess you could lie on the questions, but there's no incentive to..."
] |
[
"If you know you're going into surgery where you might need a transfusion, you and your family members and friends can donate blood ahead of time and they will set it aside for you, so only your blood will be put back into you.",
"In the United States, you used to be able to sell your blood if you were a rare blood type but they no longer do this because they don't want people to have a reason to lie and say they have no risk factors for STDs or other blood diseases. Honestly, I'm baffled they let people sell plasma but maybe that's easier to check for diseases.",
"I've never donated plasma, but donating blood is really rewarding. Do eet!"
] |
[
"Why does it take so long to develop aids from a HIV infection?"
] |
[
false
] |
[deleted]
|
[
"That's an excellent question, and, to the best of my knowledge, we really don't know. The problem is, although we know that HIV causes AIDS, we don't know exactly ",
" it causes AIDS. Or rather, we know the proximate cause -a massive reduction in the numbers of a very important type of immune cell, known as ",
"helper T cells",
" or CD4 positive T cells. Without those cells, your immune system is severely compromised, and you die from secondary infections like pneumonia. However, we don't actually know why the T-cells disappear, and, without knowing the mechanism, it's difficult to say why that disappearance is delayed.",
"There are a couple of obvious possible answers, however, that we do know are wrong. ",
"First, you might think that the delay is because the virus is just not replicating. Maybe it's sitting in your body, biding its time, basically quiescent. Then, one day, BOOM, the virus decides it is time, and starts infecting lots of cells and causing disease. Such latent infections are common in some other viruses, so it makes sense that HIV might follow the same pattern. This is a common misconception, however, which is exacerbated by the term \"clinical latency\" -the period in between the initial acute infection, and progression to full blown AIDS. However, clinical latency is not a time of quiescence for the virus at all. In fact, the virus is replicating wildly; billions of new cells are infected every day, and billions of infected cells die every day, in roughly equal numbers, so the level of virus in your body remains steady, for years.",
"So it's not latency, but rather, an active infection that, at first, does not cause disease. The second obvious answer to why the cells disappear is that HIV is infecting and killing the cells, because, well, it is. CD4 positive T-cells are HIV's preferred target for infection, its favorite food, if you will -and, as mentioned above, these are the cells that disappear, leading to AIDS. HIV does seem to kill the immune cells it infects, so maybe HIV eventually eats all the immune cells, and then AIDS develops? Unfortunately for that hypothesis, we know from studies of closely related viruses that infect monkeys but ",
" cause disease (known as simian immunodeficiency viruses, or ",
"SIVs",
") that the rates of T cell death in HIV infected people are similar to the rates in SIV infected monkeys, strongly suggesting that cell death is not enough to cause AIDS. ",
"There are a number of other hypotheses on why T-cell numbers eventually drop and AIDS develops, but we don't have really strong evidence for any of them yet. And without knowing why the cells disappear, it's tough to answer the question of why it takes so long for them to disappear. ",
"This paper",
" is a more detailed explanation of what we know about HIV and AIDS that I think is quite readable, even for a lay audience; the last section goes into some detail about the various competing hypotheses about why HIV causes AIDS. Or if anyone wants a summary of the hypotheses, feel free to ask, although frankly I don't really understand enough immunology to give more than a very basic overview."
] |
[
"Well, clearly it does something like that in some way, directly or indirectly, but it's highly unlikely that it does so \"on purpose,\" so to speak. Those T-cells are its food, it would go directly against the interests of the virus to inhibit production of said food. ",
"Fortunately for HIV (but unfortunately for us), progression to AIDS is slow enough for it to transmit itself to other people before death occurs. But, given the lack of pathogenic effects in the vast majority of known SIV strains, it's generally assumed that HIV's virulence is due to it being a recent crossover into humans, that hasn't yet fully adapted to its new host, or vice versa."
] |
[
"Well, clearly it does something like that in some way, directly or indirectly, but it's highly unlikely that it does so \"on purpose,\" so to speak. Those T-cells are its food, it would go directly against the interests of the virus to inhibit production of said food. ",
"Fortunately for HIV (but unfortunately for us), progression to AIDS is slow enough for it to transmit itself to other people before death occurs. But, given the lack of pathogenic effects in the vast majority of known SIV strains, it's generally assumed that HIV's virulence is due to it being a recent crossover into humans, that hasn't yet fully adapted to its new host, or vice versa."
] |
[
"Is it possible to quantify an emotion?"
] |
[
false
] |
[deleted]
|
[
"Sure, they just handed out surveys during my morning transit commute. On a scale of 1-5, How satisfied are you with the cleanliness of your train?",
"Replace \"satisfied\" with \"happy\" and congratulations, you've just quantified my happiness!",
"Edit: To all the downvoters, poll questions such as the example I have provided are widely accepted as standard in peer reviewed research in established fields. Everytime you go to the hospital they use such a poll to assess pain."
] |
[
"You are right. Concerning the pain scales, they often set a certain range where in people can place a dot relatively to the minimum and the maximum. People answer these on intuition."
] |
[
"Whereas surveys do contain specific questions for the patient to self-evaluate, they are most frequently accompanied by an array of other questions that address the point obliquely. The overall panel of questions are then used to generate a score that may not match what the patient self-evaluates as. i.e. a patient that scores themselves 10/10 for depression, but answers other questions positively about recent social interactions, ability to function day-to-day, events in the future they look forward to etc. would not be scored as '10/10 depressed' by the clinician."
] |
[
"Why is the twin paradox asymmetrical?"
] |
[
false
] |
We've heard it a million times, every time relativity is discussed: a twin goes out of the earth for a trip at near-light speeds, and when the ship comes back, the twin in question is much younger than their earthly sibling. The part that confuses me is this: according to classical mechanic laws, a person standing on a train going 50 m/s forward is equivalent to the whole world going 50 m/s in the opposite direction, depending on the frame of reference. So what would happen if instead of the twin in the spaceship accelerating forward, the entire earth (or universe) accelerated in the opposite direction, then accelerated back to the original position, with the spaceship remaining stationary? I mean, theoretically it should be indistinguishable, right?
|
[
"This is the whole \"paradox\" in the twin paradox. The resolution to be paradox is the fact that the motion of the twins is ",
" symmetric. In order for the twins to meet up at the end of the experiment, at least one of them must accelerate, and therefore ",
" in order to turn around.",
"In the standard formulation, the twin on Earth stays in the same (approximately) inertial frame the whole time. However the twin on the ship leaves in one inertial frame, turns around, and returns in a ",
" inertial frame.",
"Changing frames is not relative, there is no ambiguity in which twin accelerated in order to turn around.",
"That's why the symmetry is broken."
] |
[
"Ah, the clever topological twist to the twin paradox.",
"The resolution of the paradox in that scenario is that special relativity is ",
" with the topology you proposed. A flat torus of characteristic length L is constructed by the identifications:",
"t ~ t\nx ~ x + L\ny ~ y + L\nz ~ z + L\n",
"(The identification x ~ x + L means these two values represent the same physical point. For example, if L = 5m, then 2m ~ 7m, that is, the position x=2m and x=7m represent the same point.)",
"This boundary breaks special relativity because it puts time in a special place as the only dimension that doesn't wraparound, while special relativity requires time and space to be on equal footing. In fact, the above identifications aren't invariant under change of frames, because the Lorentz boosts used to switch frames mix time and space. Your time coordinate will also wraparound after the boost.",
"Because your topology broke special relativity, there is a notion of absolute space and time. A preferred global frame arises which can be used to determine who's truly at rest and who's truly moving; it's the (t,x,y,z) where the torus identification is the one above. A twin at rest in this frame ages more slowly than her twin in a different frame ",
".",
"At a physical level, it has to do with the time wraparound I mentioned. It causes issues when trying to synchronize clocks and results in some observers failing to measure time properly.",
"Note this effect is only observable when performing an experiment at length scale L or greater. You can't measure this in a lab. Locally, special relativity is still respected. It breaks down only globally. Since we haven't observed any such effect, it must mean that the intrinsic length L of the universe's wraparound must be larger than the radius of the observable universe, if it exists at all."
] |
[
"What you have described is the paradox. One would naively think they should be equivalent and they both give different answers.",
"The ",
"wikipedia article",
" gives a good explanation of the resolution."
] |
[
"If the speed of light is the maximum speed an object can move then isn't there a maximum temperature?"
] |
[
false
] |
From my limited experience I understand that heat comes from particles moving and bumping into each other. The faster the particles the higher the temperature. So since particles are moving arent they bound by the speed of light? And if that's true isn't there a maximum temperature since the particle speed is limited? Or maybe i'm just confused...
|
[
"Temperature is technically defined as the amount of internal energy/entropy of the system. Now let's just ignore entropy for the moment, as that's a much larger discussion. In the non-relativistic picture you are correct in saying that energy and velocity appear to be linked in a simple manner. E=0.5 mv",
" . But as we get to higher velocities, E",
" -p",
" c",
" =m",
" c",
" becomes the equation of interest. Let's for the moment, throw out the m",
" c",
" since it's just a constant term that shifts the energy up uniformly for each particle. E=pc, this E being just kinetic energy alone. 'p' is the particle's momentum. p=(1-(v/c)",
" )",
" mv. So as v->c the first term, called gamma, goes toward infinity. ",
"tl;dr, we can keep adding more energy even if the velocity doesn't continue to increase."
] |
[
"Agreed. Been on here for hours. I feel like a dumbass, but I enjoy learning these things. "
] |
[
"the energy going into mass is a largely outdated concept. What is true is that for massive particles, p=mv is only the low-speed limit of the fuller definition of momentum, p=gamma*mv. Somebody decided to smush gamma*m together and call it a \"relativistic\" mass. But that doesn't ",
" anything physically. It's more correct to just say that momentum is only linear with velocity for the low velocity limit."
] |
[
"Why does albino specimens have red eyes?"
] |
[
false
] |
I just saw a video of an albino owl and was amazed at the red color of the eyes. Why is it that albino specimen have red eyes? Can you have the mutation of discolored / white body without also mutating the color of the eyes?
|
[
"The melanin in the eyes are responsible for iris color. Without that melanin present, all that is visible are the veins at the back of the eye, giving it a red appearance. ",
"It's the same reason we get red eye in photos with the flash on: the light passes through the iris and bounces off of the blood vessels at the back of the eye, making the eye appear red in the photograph. "
] |
[
"To elaborate on sharltocopes answer, Albinos suffer from improper pigment production. This results in an inability to produce pigment in both skin/fur/scales and eyes. So the eyes are red for the same reason that the skin is white. No pigment. "
] |
[
"You can also have an animal that has a more specific pigment deficiency instead of albinism (total and complete lack of pigment production), which would make certain body parts, like fur, white without affecting others, like eye color. A blanket term for such animals would be \"leucistic\"."
] |
[
"is one divided by infinity zero?"
] |
[
false
] |
My mom posed this question to me and I don't know the answer. I would think it would be zero.
|
[
"For most practical purposes, yes. But the question itself is a bit improper.",
"The term 'infinity' is usually used as part of a limit as a shorthand for a less intuitive formal definition. We can readily say that limit (x->inf) 1/x =0. However, this is just shorthand for saying, \"for all epsilon greater than 0, there exists a delta greater than zero such that for any x>delta, 1/x<epsilon\".",
"Infinity doesn't exist as a value that we can manipulate in isolation. Or, to be more precise, if you are actually dealing with an infinite value, there are many different infinities. So dividing by an unspecified 'infinity' isn't well posed.",
"Infinity is either merely a shorthand for expressing limits or a large set of different values, depending on the context. In neither case does there actually exist a meaningful abstract 'infinity'."
] |
[
"If you're a scientist, 1/∞ = invent renormalization groups."
] |
[
"Infinity isn't a number, but usually when you see it in the context given folk really mean the limit of the expression as the quantity grows without bound. In that case, yes, the limit of 1/n as n grows without bound is 0."
] |
[
"What exactly is aliasing, and what does a nyquist frequency have to do with it?"
] |
[
false
] | null |
[
"In digital systems which interact with the physical world there are always three stages: signal acquisition, signal processing, and signal output. Signal acquisition is the use of sensors (microphones, cameras, accelerometers, etc.) to obtain a digital representation of a signal of interest, which you then process in some way to obtain the desired effect.",
"Since our signals in the physical world are almost always analog - that is, they display a continuous variation over time - and computers deal in discreteness, acquisition involves two main steps: ",
", which is discretisation of time (that is, you only \"see\" the signal at certain instants); and ",
", which is discretisation of the values of the signal (for example, from grayscale to integers in 0-255). The first step can generate what we call ",
".",
"As you probably know, all signals (within some mathematical restrictions which are of no consequence here) can be represented by their ",
": a pure audio tone, for example, would only have one frequency component, while a telephone conversation can range from 300 Hz to 3400 Hz. But these are continuous frequencies. What happens to the spectrum of a signal when we sample it?",
"Well, that depends on the ",
", i.e. the rate at which you take samples. Aliasing is what happens when this rate is too low to adequately represent the original signal. For example, if you were sampling a pure sinusoid, something like ",
"this",
" could happen: the black dots are the samples, the red wave is the original signal, and the blue wave is the signal that the samples appear to be representing. The problem is that the signal is varying too quickly for us to keep up with it, and we're missing the details of this variation. In more complex signals, the higher frequency components that we are missing actually appear as low frequency components, thereby ruining our sampled signal.",
"So what's the minimum sampling frequency we can get away with in order to avoid aliasing? That's given by the ",
"Nyquist-Shannon sampling theorem",
", and is called the ",
": we have to sample at least at twice the highest frequency present in our signal. This is somewhat intuitive if you picture a sinusoid: you need to catch all the valleys and peaks to get what's going on.",
"If not properly handled, aliasing can have very unfortunate effects, such as ",
"artifacts",
" appearing in improperly sampled images. In properly projected systems, the acquisition stage always includes a filter to block frequencies that are too high to be properly sampled (we call this a low-pass filter), so that they won't generate aliasing in the sampled signal. Also, in real systems the sampling frequency is never just the Nyquist frequency, but usually some ten times or so higher, so that the downstream processing and filtering are easier. Similar things go on in the signal output stage, but these are the important concepts."
] |
[
"if you picture a sinusoid: you need to catch all the valleys and peaks to get what's going on.",
" what I needed to understand. Thank you hexphreak. "
] |
[
"You might find the explanation in the second video ",
"here",
" very enlightening. The first video is also very good."
] |
[
"Having some problems understanding Torque Scientifically. Can someone explain to me how the relationship between lever length and force works?"
] |
[
false
] |
So we all know that trying to tighten with a bolt with your bare hand is MUCH harder than with a wrench, and it is NOT due to grip. But scientifically, why does this work? tl;dr, if I had a kilometer long indestructible rod, I could probably lift an elephant (or maybe elephants) easily. Why?
|
[
"Ok. Torque is basically the rotational version of work and Work = Force * Distance. So you can increase work by increasing either force or distance. Same thing with torque. You can increase torque by increasing the force applied ",
" increasing the distance that force is acting through.",
"When you're turning a bolt, friction is generating \"negative\" work (i.e. it's opposing the work you're doing). When you're using using your hand you're generating very little torque (because the radius of the bolt is so small). By using a lever (wrench) you're applying the same force but over a longer distance and therefore generating more work to overcome it. Spinning a 1cm bolt with your hand to turn it one full turn means you're only exerting force over a couple of centimeters (3.14cm). Using 30cm wrench means you're exerting the force of a much larger distance (94.2cm) so you're doing thirty times more work. ",
"It's the same thing with carrying something. Carrying a 50 pound weight close to your body is fairly easy because the weight isn't generating much rotational torque (the force in this case is gravity). If you extend your arms all the way out and try to hold it there, the weight is generating torque with your arms as the lever (so you have the weight times the length of your arms times the force of gravity you're working against), so you need to do more work to hold it level.",
"The famous saying attributed to Aristotle \"Give me a place to stand and with a lever I will move the whole world.\" is actually about torque. If you had a long (and stiff) enough lever, you could literally move a mass the size Earth with your own strength by taking advantage of the benefits of the lever arm."
] |
[
"Wait, wait! So the work to push the lever, say 30 degrees from the middle vs from the end of the rod is the same? So basically it's like lifting up a huge box of weighs as opposed to picking up the weights one by one? ",
"In other words, it's easier to do but requires the same total work? "
] |
[
"It might be easier to think of it \"backwards.\"",
"When you apply force at the end of the wrench, it moves through a certain angle. You exert X amount of work pushing the end of the handle and every part of the wrench is moving through that same angle. As you get closer to the bolt, the actual \"straight line\" distance that point of the wrench has to move is smaller. Since you're doing that X amount of work and the distance is smaller, the force at that point ",
" be greater. If it wasn't greater it would mean that from your X amount of work at the end, some of it \"disappears\" as it gets closer, conservation of energy won't let that happen."
] |
[
"How does signaling through a cable work (e.g, USB)? How can they obtain such high speeds with such great accuracy?"
] |
[
false
] |
USB 3.0 can transfer up to 5 Gigabits per second, that's 5 billion bits per second. I understand USB works by essentially signaling 0s and 1s through a cable (two cables if we want to get fancy and talk about redundancy/noise cancelling). But I cannot comprehend how a wire can send 5 billion electrical pulses so accurately for such a sustained amount of time? How does it synchronize itself? What if the signal "hiccups" and lags for a nanosecond, then wouldn't the subsequent bit be misrepresented and all the following bits would be incorrectly shifted? If it uses quartz, quartz's frequency is only 32768 Hz, a far cry from 5 billion. Even if we can upscale the signal of quartz by multiplying it by a factor, wouldn't it then be inaccurate overtime? This is only USB. I understand that fiber optics nowadays can obtain rates in the hundreds of terabytes! Wow! TL;DR: 1. How can cables signal bits so fast and accurately? 2. How do we synchronize / error check the signal to make sure there are no hiccups along the way? 3. Is there a theoretical physical limit to how fast we can make these things?
|
[
"2. How do we synchronize / error check the signal to make sure there are no hiccups along the way?",
"When there's no dedicated clock line in the wire, signals are usually encoded such that there are guaranteed signal edges every now and then for synchronization (so that e.g. a series of all 0s isn't just a flat line that makes it hard to say how many 0s there are).",
"An example for that is the ",
"4B5B",
" code, which encodes 4 bits of data into a symbol of 5 bits. This also takes care of error detection to some extent, since there are obviously more 5 bit than 4 bit combinations and the mapping is chosen such that there are no adjacent code symbols, so a bit flip or clock detection error will usually detect as an \"unused\" combination and therefore must be an error."
] |
[
"There will always be a chance of a bit error. That fact is simply unavoidable.",
"Let me start by answering how we signal though a cable (wired) or wireless medium.",
"When you boil it all down, we transfer digital information consisting of bits that are either 1's or 0's. We can modulate the data which will encode it to speed up the bit rate. Take 4-PAM (Pulse Amplitude Modulation) for example. It consists of 4 symbols and each symbol carries 2 bits of information. We can define the symbols as such: -10.5V = \"00\", -3.5V = \"01\", +3.5V = \"10\", and +10.5V = \"11\". We can go to the next level of PAM which would be 8-PAM that has 8 symbols and each symbol transmits 3 bits of information (-10.5V = \"000\", -7.5V = \"001\", -4.5V = \"010\", -1.5V = \"011\", +1.5V = \"100\", +4.5V = \"101\", +7.5V = \"110\", and +10.5V = \"111\"). As you increase the number of symbols, notice how the symbols get closer and closer together. In 4-PAM the voltage difference was 7V and then in 8-PAM it was 3V. With the addition of noise in the system, this will add/subtract voltage that is detected at the receiver. The more symbols we have, the higher the chance of the receiver confusing one symbol for another in the presence of noise, but we increase the rate of information that we can send. This tug-of-war between bit rate and noise is called the Shannon-Hartley Channel Capacity Theorem. All bandwidth limited channels have a maximum bit rate that we can transfer information.",
"There are other forms of modulation as well. If we send sinusoidal waves, we can modulate using the amplitude, frequency, or phase to encode information. Or a combination of any of them. Quadrature Amplitude Modulation (QAM) is a very popular and practical form of modulation that uses both the amplitude and phase of a sinusoid to encode information. QAM is used in military radio waveforms, wireless WiFi, and wired cable modems (eg. DOCSIS 3.0). DOCSIS 3.0 can use 1024-QAM or even 2048-QAM transmitting 10 to 11 bits respectively with each symbol. But the channel must have a low amount of noise compared to the signal in order to use these higher levels of modulation or risk a high amount of bit errors. There are other exotic forms of modulation like GMSK, CDMA, and OFDM for the curious reader.",
"Now on to how we do this accurately.",
"We can use differential signaling to help. Differential signaling uses 2 wires instead of 1. On the second wire we send an inverted replica of the signal at the same exact time as the first. The same noise will couple to each wire and at the receiver we take the voltage difference. Any voltage that is common will be removed and only the signal will remain. This practice is used in Ethernet and USB and other high speed wired interfaces.",
"We can use Cyclic Redundancy Check (CRC). This transforms the received and decoded message into a checksum (like a hash). If the checksum fails, then we know that the data is corrupted.",
"If the data is corrupted we may ask the transmitter to resend it. This is called ARQ (Automatic Repeat Request). But this is really inefficient. Imagine transferring 1 Gigabyte file and then when your done receiving it your off by one bit and you have to resend the whole thing again. You can send smaller packets of data. Maybe 1 Megabyte or 1 kilobyte at a time but there are better ways.",
"A better way is to use Forward Error Correction (FEC). This is common practice. We add extra bits of data to the message. At the receiver we use these extra bits to either detect/correct a corrupted message. There is a special class of FEC's called Turbo Codes. Turbo codes have allowed us to transmit information in a bandlimited channel at very close to maximum channel capacity! There are other FEC's worth exploring like Reed-Solomon and LDPC. When there is a lot of noise, we may do FEC on both packets of data, as well as a collection of packets (called a frame). DVD's use Reed-Solomon FEC by the way as a real world example.",
"Another very basic thing we can do is to use Gray Codes. Gray codes differ by 1 bit. Above when I gave the symbol mapping for 8-PAM I used \"000\", \"001\", \"010\", \"011\", \"100\", \"101\", \"110\" and \"111\". Notice that when we go from \"011\" to \"100\" that all 3 bits changed. So if we transmitted -1.5V \"011\" and we had +1.6V of noise and the receiver got +0.1V and decided that +1.5V \"100\" was what was sent, the receiver just made 3 bit errors. If instead we had mapped it like (\"000\", \"001\", \"011\", \"010\", \"110\", \"111\", \"101\", \"100\") we would have only made 1 bit error in this example.",
"We can also just increase the voltage. In the PAM examples above we went from +/- 10.5V. We could have made it be +/- 30V. Look into charge pumps.",
"We can also interleave the bits that we send. This is sort of like scrambling. Suppose we mean to transmit \"ENEMY CEASE FIRE AT 0100\" but because of noise we received \"ENEMY ???? FIRE ?? 0100\". Lets say we scrambled the same message \"REE YMSA00FTEE I1 E ANC0\" and we got the same errors in the same positions as before but we received something like \"E?EMY CEA?E FI?E ?T 010?\". At least it's still somewhat intelligible and you didn't miss the most critical part of the message.",
"And now on to synchronization.",
"So in wired communication you can have another wire with the clock on it in addition to wires that carry data. Waveforms that carry clock information are called synchronous. SPI, JTAG, and Ethernet are wired examples that have a dedicated clock line. You can also choose to send clock information in the data such as manchester coding but you sacrifice your bit rate. Asynchronous communication does not carry clock info and wired examples include serial (UART) and USB.",
"You must know when to sample the data (i.e. the precise sampling instant). With serial, you oversample the data at say 16x. And put the oversampled data into a correlator or a match filter. They help filter out Gaussian noise (AWGN) common to all electronics. For serial, you wait until the start bit when it transitions from high to low. When the output of your correlator/MF crosses a certain threshold, that's your clue to start sampling (at 1/16x in this example). That way you sample right in the middle of a symbol and not on the transition between symbols.",
"Another way of synchronization with asynchronous waveforms is to send a special message in the beginning called a preamble. It could be a unique sequence of 5, 10, 16, 32 symbols. You look at the output of your correlator/MF and when you detect the special pre-message, that's your clue that the main message is shortly to follow.",
"Digital communication is my forte. I'm building a FPGA based DUC/DDC for Software Defined Radio."
] |
[
"How does it synchronize itself? What if the signal \"hiccups\" and lags for a nanosecond, then wouldn't the subsequent bit be misrepresented and all the following bits would be incorrectly shifted? If it uses quartz, quartz's frequency is only 32768 Hz, a far cry from 5 billion. Even if we can upscale the signal of quartz by multiplying it by a factor, wouldn't it then be inaccurate overtime?",
"32,768 Hz crystals aren't the only crystals they make. In fact, 32khz crystals are just about one of the physically largest crystal oscillators you can buy. You can easily get 100+MHz crystals without a problem.",
"There are electronic devices called Phase Locked Loops (PLLs) and basically what this does is it compares the phase of a controllable oscillator with a reference oscillator, usually from a crystal oscillator. By dividing down the controllable oscillator, you can sync the controllable oscillator to a fractional amount of the crystal oscillator, allowing you to create all kinds of frequencies with incredibly low jitter.",
"wouldn't it then be inaccurate overtime?",
"Yes, but that doesn't matter. These devices operate with the assumption that these clocks are out of sync with each other, and there are ways to mitigate this, one of which is getting a PLL to lock onto the incoming signal's clock. The other guy explained how clocks can be embedded into a signal (clock recovery). ",
"Also to correct about 4B5B, this is what they call \"line codes\" which applies a code to correct for certain deficiencies in the channel. There are other codes called error correcting codes to add redundant information into the signal (a little bit cooler than that, but yeah) and uses math to detect and/or correct possible errors. ",
"But I cannot comprehend how a wire can send 5 billion electrical pulses so accurately for such a sustained amount of time?",
"Errors actually do happen. You either correct or detect, and you can also request the sender to retransmit.",
"There's a thing called an eye diagram which shows you how reliable a signal link is. Basically, it overlays all the signals received on the other end and sees how clean the levels are at the point of sampling the signal. ",
"Is there a theoretical physical limit to how fast we can make these things?",
"Yes, there is a thing called the Shannon channel capacity, which tells you the maximum amount of bits through a noisy, bandwidth-limited channel. band width limiting means you can only pass a certain width of frequencies through the channel. However, AFAIK for digital links we are nowhere near the Shannon capacity limit. This is mostly of interest for RF links, where the SNR is much lower."
] |
[
"What is happening on the cellular level when second degree burns occur, and why can't the body fully heal them?"
] |
[
false
] |
And why does a burn continue to feel warm for hours after it occurs? Surely it isn't storing heat for so long?
|
[
"The distinction between the various ",
"degrees of burns is based on the depth of the burn",
".",
"First degree burns are epidermal burns - i.e. they only involve the outermost layer of the skin. Because the underlying dermis is undamaged, they quickly look red (from capillary dilation), and heal readily as the epidermal layer is replaced. Because of the increased blood flow (and the release of inflammatory cytokines in the area), that area can feel warm afterwards. ",
"Second degree burns penetrate into the dermis, which lies under the epidermis. Superficial second degree burns tend to form large blisters from the fluid that escapes from damaged capillaries. Deep second degree burns look more dry/leathery, and are often not red.",
"Third degree burns involve all layers of the skin, and again may not look red or feel hot due to the damage to underlying vasculature. "
] |
[
"If the fluids in blisters is from the capillaries, then why doesn't it look like blood? And do blisters help the healing process, or are they just a by-product of the damaged skin? ",
"And why is the body, still having the DNA that tells it how to make things, unable to replicate the same structure and fully heal if the burn was bad?"
] |
[
"This paper",
" gives a good description of how the different components of your cells are impacted in a burn. The authors use mathematical modeling to describe the thermodynamics (reaction to heat) of different molecules in a cell (proteins, lipids, carbohydrates, nucleic acids, and subgroups thereof). As ",
"/u/arumbar",
" says, your second degree burn scenario is one in which cells deep in your skin but not in the tissues it covers are impacted, which traps heat in the area, so the deeper the burn the longer molecules get exposed to high heat and the more damage will result."
] |
[
"Which DNA, nuclear or mitochondrial, is more abundant in a human body? Has the total quantity been measured or is it only estimated?"
] |
[
false
] |
Mitochondrial DNA is rather small compared with nuclear DNA but there can be up to 1,500 mitochondria in a cell. Their number also varies in tissues and nuclear DNA is absent from red blood cells for instances. Perhaps some other cellular organelles have also some DNA.
|
[
"So far, DNA has only been identified within the nucleus and mitochondria. The human genome consists of 23 pairs of chromosomes within the nucleus that contain a total of ~6 billion base pairs. Mitochondria can have a range of 1-15 molecules of DNA where each molecule consists of ~16500 base pairs in any given human cell. Even though some muscle and liver cells each contain thousands of mitochondria, the majority of a cell's DNA will always be found in the nucleus. ",
"Edit: corrected mistake of amount of chromosome pairs and clarified amount of mtDNA. "
] |
[
"A human cell averages ",
"100 mitochondria",
", and each mitochondria averages 5 mt genomes (each 16569 bp in length). Thus a cell averages just over 8 million bps of mtDNA. ",
"A diploid human nuclear genome (present in nearly every human cell) contains around 6 billion bps (slightly less in males; slightly more in females), so regardless of how many cells are in the human body, there is >700x more nuclear DNA than mtDNA. ",
"Since mammalian red blood cells lack both nuclei and mitochondria, they do not affect the relative abundance of nuclear vs mitochondrial DNA. (For this rough calculation, I'm ignoring the relatively small proportion of germline cells, which are haploid.)"
] |
[
"What about bacterial DNA?"
] |
[
"If Dissociative Identity Disorder arises during early life when major skills are learned, could one personality learn a skill while another has no clue about it?"
] |
[
false
] |
Say a child develops DID before they reach kindergarten or so. If one personality is dominant during the time the child learns to read and write, would the other personality(s) know how to read/write? Or are knowledge and skill sets entirely compartmentalized?
|
[
"In short, yes. Sorry for how delayed this response is. I have DID. An example that relates to what you are asking about: I used to be quite good at viola. Haven't played in a couple years. However, I played in a quartet with three of my cousins at my grandfather's funeral a couple days ago. I and one of my alters are the only ones that can play -- the other two were useless. And yet a different alter sang for me so that I wouldn't cry in another portion of the service.",
"Sorry if that didn't make any sense; took a redeye flight last night and I'm clearly struggling with making words happen. :) Anyway, let me know if you have any other questions, I'll do my best to answer them. You can also PM me if you prefer."
] |
[
"In some people they do. I get the impression that Guardian ages with me, but Little One and Melody are stuck at their particular ages.",
"And yeah, that's pretty much what it is like! Some people have a literal \"head space\" that helps them to with communicating between alters, etc. If such exists with my alters, I'm not aware of it."
] |
[
"Interesting. So the alters, as you say, are aware of what's going on and can take over at any time if need be? Is there any dominant alter generally agreed upon by all?",
"And does that mean that Hollywood's version of DID where a person can not even realize that they have it actually not exist? Or is that just extreme cases?"
] |
[
"Does any organism's hair, other than humans, go white with age?"
] |
[
false
] |
I was looking at my dog and though to my self "I've never seen a pets hair grow white when it got older". Does this phenomenon only occur in humans?
|
[
"Sure, ",
"gorillas",
" and ",
"golden retrievers",
" both have fur that lightens as they age"
] |
[
"it's fairly obvious on black labradors' muzzle, here's a ",
"young one",
" and an ",
"older one"
] |
[
"Thus the term \"silverback gorilla\" for describing a older, male gorilla."
] |
[
"Are Wormholes a proven fact?"
] |
[
false
] |
I ask this in the light of Interstellar. I know black holes are a thing, like we have seen it happen and I've heard about wormholes my whole life and how they can bend time and space. But is that actually true? Or are just we still trying to get hard proof (I know we've never actually sent anything through what could be considered a 'wormhole')?
|
[
"Wormholes are purely theoretical at this point. We have no reason to believe they exist outside that the mathematics supports them under very particular circumstances."
] |
[
"I thought so. Thank you for clarifying. "
] |
[
"It should also be noted that even though there are theoretical models in which you can get wormholes, there are lots of problems with such models. Despite the possibility that comes up when you naïvely do the mathematics, many physicists believe that these objects are non-physical and could not occur in nature—there's good reason to think so."
] |
[
"Can the air become so humid a human can be hydrated from it alone?"
] |
[
false
] | null |
[
"Or would you drown?"
] |
[
"To really make sure it stays put, the partial pressure of water in air needs to be equal to the vapor pressure of water in your body.",
"Relative humidity is the ratio between the partial pressure of water vapor in the air and the vapor pressure of liquid water at that temperature.",
"If it is 37 Celsius outside (human body temperature) and the relative humidity is 100% then a pot of water that's also at 37 Celsius would never evaporate. ",
"Do note that your body would be completely unable to cool itself under these conditions. You can't cool yourself by sweating and the air is the same temperature as your body. Your body will still try really hard to sweat and you'll probably lose water that way eventually, or heat up until you reach a new equilibrium, or simply die of heat stroke.",
"There is a way around this. If the air is completely free of all dust particles and there are no surfaces for it to nucleate on then you can have air that is supersaturated with water and relative humidities well above 100%. At 37 C and 110% humidity, a pot of water would slowly fill instead of evaporating.",
"A bit below 37 C and a bit above 100% humidity you could finally have drinkable air. In fact, you'd be at the center of your own private rainstorm as your skin and hair would provide nucleation sites for water droplets. This isn't really a realistic state for weather."
] |
[
"So then another question would be, do you have to ingest water to hydrate? Or can you just absorb it from the atmosphere. If you can absorb it through the air, then there's probably a point of humidity where you start absorbing enough water, and then another point where you start drowning, and then if the drowning point is bellow the hydration point, you would probably drown while dehydrating.... But that is just how I look at it, I'm not basing that on any sources..."
] |
[
"If I'm in polluted air or there's an awful smell, is it better to breathe through my nose or my mouth, to have as few molecules reach my lungs as possible?"
] |
[
false
] | null |
[
"It honestly depends on what is in the air. Is is particles or chemicals? For larger particles (think dust) , breathing slowly through your nose would be best. Your nose is built as a rough filter. ",
"For a chemical, think paint thinner or another solvent, it wouldn't matter. You need specialty air filters to remove these (think gas mask). ",
"Smells, which are generally chemical, will enter your lungs through either your nose or mouth. Your body has no real method to filter out things like this. ",
"If you are concerned about particals, wear a N95 or N100 rated mask. For solvent or smells, which would not be considered particles, you will need a respirator rated for solvent. ",
"Hopefully this helps. "
] |
[
"For respirators, you would still need the correct cartridge for protection. ",
"More importantly: An old gas mask may not provide protection against anything especially if the cartridge has been opened for a while, more than about 30 days. The cartridge is often filled with charcoal and that stuff is always absorbing stuff from the air. The charcoal will become saturated and will not longer filter at that point.",
"Rule of thumb, at least for industrial VOC, if you can smell it, you are exposed, possibly over exposed, depending on the substance. Some VOC have exposure limits lower than when they become detectable by smell."
] |
[
"I worked with respirators for a while, where exactly did you get the 30 days rule? As far as I know the charcoal is just fine unless some adsorbents are getting through it in high concentrations, such as chemicals which undergo physisorption (for chemical sorption, which is necessary to remove volatile chemicals with low molecular weight an impregnated type of charcoal is needed). Whereas the impregnated charcoal may indeed suffer from high humidity and temperature, pure charcoal should stand much longer than 30 days. Practical example - if you have a kitchen hood, it is most likely equipped with charcoal-based filter, and it does need to be exchanged every month (unless you're frying bacon on high heat every single day). Besides some adsorbed molecules, such as water, have actually beneficial effect for removal of certain compounds, such as H2S (source: ",
"http://www.sciencedirect.com/science/article/pii/S0021979701979524",
")\nBTW H2S is a really fancy stuff when it comes to detection by sniffing it with your own nose. There are two cases where you cannot smell it - one when the concentration is too low, and the other, when it's sufficiently high to kill you. Otherwise I agree, the nature gave us pretty good detection system in our noses."
] |
[
"Why is it that a bad night of sleep makes my *everything* hurt?"
] |
[
false
] |
[deleted]
|
[
"yeah, my google-fu failed me too. I thought about that, but even if I spend 8 hours in bed, just laying there, still sore when I get out of bed with the alarm if i didn't fall asleep.",
"Thanks for trying though!"
] |
[
"Hmmm... What about the possibility of your brain only starting the repairing when it's on autopilot? Aka just your subconscious working."
] |
[
"Hmmm, I tried googling it, but I didn't find anything pertinent in 50 seconds, so my guess is that your body goes into a full on repair mode while you sleep so your muscles can regrow, joints can relax, etc. and when you don't get enough sleep, the process isn't completely finished and it still hurts. Kind of like a bad bruise. It might not hurt when you hit something, but when it's healing, it's stiff and a painful bastard. Just my speculative thoughts. I'm probably wrong though haha."
] |
[
"How is it that electromagnetic waves can travel through space without needing a medium? If they are able to do so because they are also particles, then why describe them as waves at all?"
] |
[
false
] | null |
[
"We know we can feel electric forces and magnetic forces at a distance. The concept of a field is the classical way to explain how these forces can work, since we need some way of describing the rules for how much force you feel as you move relative to a magnet or charge. ",
"Now, if someone moves a magnet or a charge at some distance away from you, you should be able to detect that motion by measuring the forces predicted by the field where you are standing. But it turns out the field doesn't change instantaneously - there is a delay related to the speed of light. Maxwell's equations explain how to calculate exactly what forces you will experience vs. time. A consequence of these equations is that an oscillating charge or magnet will produce ",
"radiation",
" carried by waves in the electromagnetic field. ",
"Now, sometimes it is easier to explain electromagnetic waves using photons. But the description of static electric and magnetic forces is very difficult using photons (you can sort of do it with ",
"QED",
" and Feynman diagrams, but it involves very counterintuitive ideas like virtual photons that can have negative energy and travel at any speed). So the idea of a field is more useful in describing static electric and magnetic forces. It isn't just used as a classical approximation either. ",
"Quantum field theory",
" treats the fields as the fundamental thing, and photons or other particles are just quantized excitations of these fields. It is very similar to the way vibrations in matter can only occur in quantized increments we call ",
"phonons",
"."
] |
[
"Electric and magnetic fields oscillate perpendicular to each other at the point where they meet they push each other and hence propagate without any mediums need.",
"Even in a \"perfect vacuum\", i.e. barring quantum fluctuations, all fields are present. Their field values just are zero, corresponding to no particles or electromagnetic fields present.",
"The fields are there. Therefore, just as a matter wave (which is nothing but a particle), light waves, i.e. waves in the electromagnetic field, can propagate through such a \"perfect vacuum\"."
] |
[
"In fields, a wave is a technical abstraction of a probability distribution.",
"You're thinking of wave ",
". A wave ",
" in quantum mechanics is a quantity that when squared gives you a probability distribution.",
"a ",
" is a change in amplitude of a particular field. In a water wave, the amplitude of the height changes, in an EM wave, it is the strength of the EM field that changes. Waves are how changes in field amplitude propagate."
] |
[
"How can gravitational waves be measured using light (optical path distance), when the space itself (including the light) is stretched?"
] |
[
false
] |
I'm sure I'm missing something here. It seems like you're measuring the length of some line of space with a rubbery ruler. When the space is stretched, the ruler is also stretched, so you're not able to actually tell the space is any bigger... Does it even make sense to talk about space being stretched? What is the space being stretched relative to?
|
[
"A beam is split into two beams that run orthogonally to each other (at 90 degrees), to a separate mirror and back. If the beams combine and cancel each other out, the two distances are the same.",
"Any variation in the distance causes the waves to not completely cancel out, and the resultant signal is a candidate event.",
"This works because the gravity wave compresses space in one direction, while stretching it in another. Making the beams orthogonal gives them a chance to detect the wave. Ideally there would be a third dimension involved.",
"Having two such detectors, thousands of miles apart, and due to the curvature of the earth, at different angles, means that the same event can be detected in two different physical locations, at very slightly different times, ie how long it takes a gravity wave to travel thousands of miles. This helps determine the direction the wave came from.",
"But you're right, the thing being measured is being measured by a ruler that stretches as well. But having two of them at 90 degrees means they won't always stretch the same."
] |
[
"You're absolutely right about rulers stretching with space - by itself, a single ruler could never measure any distortion in space, for the reasons you mention.",
"The trick to making such a measurement is to compare two different rulers in areas of space with different amounts of stretching. This is what LIGO does using a couple of very large Michelson interferometers: one laser path goes one way, while the other goes at right angles to it. When the beams are recombined, it's the ",
" between the two paths, not any single distance by itself, that tells you that space is distorting. That is, the ruler in one direction is stretched ",
" the one in the other direction, which is measurable even when the absolute stretching of a single ruler isn't."
] |
[
"The speed of light is the same in all reference frames. So if you sit at the laser beam junction, the time it takes for light to travel to the mirror actually depends on how stretched or squeezed the distance is. "
] |
[
"How does fluoride ion (HF acid) hydrolyze amides and esters, why is it so corrosive?"
] |
[
false
] |
[deleted]
|
[
"Shamefully you stumped me and I had to go look in Housecroft and Sharpe to figure it out! You probably know most of this, but to reiterate...",
"The Pka value for HF, as you rightly say, is high(ish), suggesting low dissociation in aqueous solution. The reasons for this are two-fold:",
"A good spontaneous reaction would be very exothermic and also lead to a big increase in entropy, but if you only have fulfilled one of those criteria, the reaction can still sometimes occur spontaneously (see salt + ice). We measure how spontaneous a theoretical reaction will be by calculating both of these and taking into account the temperature in a 'free energy' calculation (or a Gibbs' calculation). So, even though the H-F bond is about as polar as you get, the free energy of dissociation prohibits bulk separation into ions, as the overall energy cost is too great. Some molecules will still separate of course, as random pools of energy allow them to overcome the free energy 'cost'.",
"Then, how can HF be any good as an acid catalyst? Well, it IS a fantastic acid, just when it is on its own with water, there happens to be a balance of the two driving forces (entropy and enthalpy). If there is an atom which can accept a hydrogen ion with a stronger bond than water can, then the enthalpic benefit of making that bond begins to outweigh the entropic cost of also making a fluoride ion. Especially if the result is a nice stable organic cation, because that could block the water from getting as ordered around the fluoride ion, and also reduce your entropic cost. Both of these things would make the acid dissociate more, and make the acid stronger. There is also consideration to be made of the free energy gain of the hydrolysis reaction, which could also drive the dissociation of the HF molecules more positively towards ions.",
"There are other considerations you can also make; if you get high concentrations of hydrofluoric acid, dissociated ions can join with undissociated ions to form [HF2]",
" (in most cases) or [H2F]",
" (on rare occasions like on mixture with SbF5) which stabilises the ions, prevents them from making ordered cages of water around themselves, and increases the acidity of HF.",
"It seems to me it is most likely that HF's role in ester hydrolysis is the same as any other acid's, though I am happy to be proven wrong! The HF provides the initial proton for the protonation of the carbonyl group, and then everything proceeds as normal. I wouldn't like to comment on whether the F- acts as a better counter ion than most halogen anions. It would seem to me that if the F",
" acted as a nucleophile in the hydrolysis then it would be unlikely that you would ever displace it again, as fluorine forms strong bonds and is a poor leaving group. The resulting products would be halogenoalkanes, rather than the expected alcohols and carboxylic acids, so water must still be the main nucleophile in the reaction.",
"Hope that helps at least a little!"
] |
[
"Thanks for your reply! The problem is with the convention that we have set up. It isn't that HF is a weak acid, it's that it is a weak acid IN WATER. Because we have used water as our standard we also have large entropic effects to consider, it magnifies the entropic cost of making the resulting F- ion, which really strongly attracts the dipole on water. The extra enthalpic cost of the H-F bond is almost inconsequential (~10 kJ mol",
" ) compared to the entropic effects.",
"When there is ANYTHING else, especially a nitrogen atom as you would expect in a protein (or any biological system), the HF continues to act exactly as you would expect a strong acid to do. Even though biological systems are largely water, the amounts of proteins and other bases in there really pushes the dissociative equilibrium of the HF (HF <--> H",
" + F",
" ) into overdrive as they can 'mop up' the H",
" ions and so HF acts as a terribly strong acid (and you are probably aware of the horrible manner in which it corrodes the body). In some ways it is worse that there is a small delay in the production of protons with HF because that gives HF time to diffuse through the skin and fat and do most of it's damage in the bones and surrounding muscle. As long as there are enough basic atoms around the HF will eventually fully dissociate to reveal its full horror!",
"I'm certain a better inorganic chemist than I am would be able to give you a proper graphical interpretation. If you already take chemistry classes I would suggest asking a professor (or TA if you're in the US?). We love random questions like this, it gets the grey matter going!"
] |
[
"Thanks for your reply! The problem is with the convention that we have set up. It isn't that HF is a weak acid, it's that it is a weak acid IN WATER. Because we have used water as our standard we also have large entropic effects to consider, it magnifies the entropic cost of making the resulting F- ion, which really strongly attracts the dipole on water. The extra enthalpic cost of the H-F bond is almost inconsequential (~10 kJ mol",
" ) compared to the entropic effects.",
"When there is ANYTHING else, especially a nitrogen atom as you would expect in a protein (or any biological system), the HF continues to act exactly as you would expect a strong acid to do. Even though biological systems are largely water, the amounts of proteins and other bases in there really pushes the dissociative equilibrium of the HF (HF <--> H",
" + F",
" ) into overdrive as they can 'mop up' the H",
" ions and so HF acts as a terribly strong acid (and you are probably aware of the horrible manner in which it corrodes the body). In some ways it is worse that there is a small delay in the production of protons with HF because that gives HF time to diffuse through the skin and fat and do most of it's damage in the bones and surrounding muscle. As long as there are enough basic atoms around the HF will eventually fully dissociate to reveal its full horror!",
"I'm certain a better inorganic chemist than I am would be able to give you a proper graphical interpretation. If you already take chemistry classes I would suggest asking a professor (or TA if you're in the US?). We love random questions like this, it gets the grey matter going!"
] |
[
"If a 10w LED can output the same amount of light as a 100w incandescent, where does the added energy output from the 100w bulb go? Heat? If the full answer is that the additional 90w is output as heat, then is that output at the same efficiency as if you had a 90w electric heater?"
] |
[
false
] |
The bulb obviously wasn't made to emit heat, it was made to emit light, but the heater was made with the sole intention of outputting heat. So, does the efficiency of both match?
|
[
"The extra power is definitely lost as heat. And the resulting heat would be produced as efficiently as that produced by a resistive electrical heater.",
"Now... that's not the only type of home heating, and there are some complications as well. A heat pump will warm a house more than twice as much for the same power-- because instead of making heat, it pumps it in from outside. Gas heat has approximately the same efficiency as electric on site-- but gas heat loses less in transmission. Electric heat runs on power provided by a generator, and the very best of those are only about 60% efficient, with another 8% lost during transmission. So even though an electric resistive heater converts 100% of the power to heat, this happens after about half the power is lost in generation and transmission. Gas is simply piped to the house, and the losses are smaller because the only power lost is that used to pump the gas. The generation step with the biggest loss is eliminated.",
"And finally... light bulbs are generally just not well-placed to keep you warm. They're frequently up by the ceiling where much of the heat they produce will remain, rather than warming the room's occupants. So even though they produce heat about as well as an electric space heater, the space heater will deliver the heat to where you are more effectively.",
"But if you used a light bulb of the same wattage as the heating element and put it in the same place, the resulting warmth would be the same."
] |
[
"Not exactly. Incandescent bulbs put out two types of heat, local convective heat within the bulb which is what makes it hot to the touch, and infrared much like a heating element. Infrared is radient heat and is more desired than convection as a heating source. \nMore to the point: The latent heat of lighting isn't wasted when used inside a well-insulated space. Incandescent bulbs get a bad rep. in this regard. Also, they fill a room with diffused light much better than LEDs. What gets compared in efficiency testing is only lumens and color temperature. LEDs compare well since they put out little heat and laser-like directional rays that are picked up by the chosen photo sensors in most test labs.\nSource: Rayminder Engineered Lighting Tech."
] |
[
"Even a heater will not be 100% efficient because there might be sound or light or other energy released.",
"For example, your toaster might hum as the resistive elements heat up. This is sound energy which is not going to toast your bread. You can also see the elements, which is infrared, not all of which is going to toast your bread (because you can see at least some of it). "
] |
[
"How do plants like lemon trees and chilli bushes hope to spread their seeds and multiply?"
] |
[
false
] |
I ask because I was under the impression that that plants evolved fruit to be appealing to animals who would consume them and the seeds within. However with really bitter fruits like lemon and the spicy chilli, how in nature would these plants get animals to eat their seeds?
|
[
"Capsaicin does not affect birds. Hotness in Capsicum plants actually helps them spread their seeds because it makes them unpleasant for mammals, but still tasty for birds.",
"Moreover, you should keep in mind that the fruits you bring up are human-made cultivars, and they are often very different from their wild ancestors, if such are even identifiable any longer. It's not necessary to keep fruits appealing to wild animals if we are spreading and planting them ourselves.",
"Many chili peppers are considerably hotter than any wild variety, and I understand lemons have been cultivated for citric acid as well.",
"Edit: I've gotten many excellent replies that point out additional details, so I recommend checking them out. It would appear some wild capsicums are hotter than I thought."
] |
[
"The biggest thing that has changed by human cultivation of chili peppers is size. The wild varieties are small, blueberry-sized fruits and tend to be pretty spicy, encouraging distribution by birds. Over time humans have bred chili peppers to have less heat and be much larger than their original fruits."
] |
[
"This is tangentially related to this discussion, but it's a cool thought I read in a book about foraging for wild edibles once.",
"The very first chapter, the author spoke about how people tend to think of wild plants as \"poisonous\" or \"not poisonous,\" but that distinction appears silly when we look at domesticated food. Garlic, he notes, is certainly considered to not be poisonous, but go ahead and see what happens to you if you decide to eat a big plate of baked garlic for dinner. Or a half-dozen lemons.",
"Additionally, we've bred huge concentrations of chemical irritants into domesticated plants, but we seem to have different \"rules\" for those than we do for wild plants. The most interesting thing he had to say was something along these lines: \"Imagine what people would do if they chewed a wild plant and it did the same thing to them that a jalapeno did. What do you think they'd do? ",
"\"",
"Food for thought."
] |
[
"Are there illnesses for which bloodletting actually improves a person's condition?"
] |
[
false
] |
In the modern practice of medicine, are there genuine applications in which bloodletting is still regarded an effective treatment or even the most effective method of treatment?
|
[
"Bloodletting is used today in the treatment of a few diseases, including hemochromatosis and polycythemia; however, these rare diseases were unknown and undiagnosable before the advent of scientific medicine. It is practiced by specifically trained practitioners in hospitals, using modern techniques, and is also known as a therapeutic phlebotomy. In most cases, phlebotomy now refers to the removal of small quantities of blood for diagnostic purposes. However, in the case of hemochromatosis, bloodletting (by venipuncture) has become the mainstay treatment option. In the U.S., according to an academic article posted in the Journal of Infusion Nursing with data published in 2010, the primary use of phlebotomy was to take blood that would one day be reinfused back into a person."
] |
[
"In addition to the haemochromatosis mentioned here, bloodletting is also an effective, but seldom used treatment for one more condition. In heart failure with acute intravascular volume overload, the quickest way to relieve this is by letting a few hundred ml of blood. But normally diuretics and CPAP carry the patients through that phase, so few clinicians even think of the option"
] |
[
"Do you consider injury as a type of illness? \nBloodletting with leeches is sometimes used in treatment of replantation of digits after accidental amputation. ",
"Caution, some explicit photos of severe hand wounds in the article:\n",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4082525/"
] |
[
"How do the doctors figure out a prescription for baby glasses?"
] |
[
false
] |
I've seen a lot of videos of babies getting glasses. How do the doctors figure out what prescription the baby needs? As an adult, there's a lot of guess and check in getting my prescription right, and they need continuous feedback from me. Is it a less exact prescription for babies? I know the machines they use before the guess-and-check get the prescription somewhere close.
|
[
"There is a machine that can actually get your prescription exact, it shines a light in and measures how it comes out, they tend to not use it on adults because with feedback it's easier to use the current method.",
"There is also a form, wherein you use special cards to attract their attention, with smaller and smaller stripes, when the baby doesn't care it can't see them.",
"That's not really that accurate, but it can get pretty close.",
"Last, you can do the above, but measure the babies brainwaves to see if it can see something."
] |
[
"I've never heard of the brain wave machine, but autorefractors that measure how infrared light is focused on the retina as the eye looks at an image are common. They're often used for adults, too (this is the machine that you look into with the picture of a hot air balloon or house at the end of a road that goes in and out of focus) but usually only as a starting point for subjective testing (\"which looks better, 1 or 2\") because not only optics but also the brain are involved in clear and comfortable seeing."
] |
[
"Just curious where you got this information. A baby brain wave machine for eye testing? 🤔"
] |
[
"If C is the fastest speed an object can travel, how fast are objects going relative to each other if they accelerate at .999 C in opposite directions?"
] |
[
false
] |
[deleted]
|
[
"I'll answer this for two objects moving at a velocity of 0.999 * c ; object's can't accelerate at a speed, since acceleration is a rate of change in speed. The velocity addition formula for relativistic speeds is: ",
"s = (v + u) / (1 + (v*",
"u/c",
"For objects moving towards one another. Notice that, for low u and v, the second term in the denominator goes to zero and the formula becomes:",
"s = v + u",
"For u = v = 0.999, this gives",
"s = 2*0.999 / (1 + .999",
" ~ .99999995",
"Or 99.999995% the speed of light. This is very fast. "
] |
[
"To clarify, that velocity transform equation can be found in any intro university physics textbook, and is a result of Einstein's special relativity."
] |
[
"For the powers, you have to put what you want to become superscripted in parenthesis. like this ",
"(v*u/c^(2))",
", which displays like this: (v*",
"u/c",
")",
"If you don't, it displays like this (v*",
"u/c",
" with the closing parenthesis also superscripted, because markdown ends the superscript on a space. This can also be used to make a group of words superscripted ",
"."
] |
[
"If you were to throw a paper airplane in a vacuum would it fly or fall?"
] |
[
false
] |
Would if fall straight down because of gravity? Or in space would it just float there where there is no gravity?
|
[
"It would trace part of a parabola"
] |
[
"what these people are trying to say to you is that throwing a paper airplane in a vacuum would be exactly like throwing a tiny ball in a vacuum."
] |
[
"Apollo 15 Hammer and Feather Drop"
] |
[
"If the Earth had 2 moons."
] |
[
false
] | null |
[
"Here's a collection of relevant AskScience posts (in no particular order):",
"How would multiple moons affect a planet's tides?",
"If Earth had a second moon, how would it affect the tides?",
"Pretend we have a second moon, basically identical to our current one, orbiting perfectly on the opposite side of the planet as our own. Would we still have tides?",
"What would happen to the tide on a planet with more than one moon?",
"A few questions about tides",
"What would the effect be of two moons to a planet?",
"Multiple questions about having 2 moons"
] |
[
"The entire approach to coastal living would have to change, this is assuming that the the new, second moon, is in addition to our current moon. If our current moon were split in two it would depend on their distance from the Earth. The second moon would have to be further out so as not to be pulled into the closer moon, therefore having less of an effect on the tide. A second moon, depending on its luminosity could also screw with insects, but no more than man made lighting."
] |
[
"In a situation like that would you see towns like New Orleans and Copenhagen get flooded?"
] |
[
"What do you use to de-ice your -80C fridge? (X-post from r/biology)"
] |
[
false
] |
I used to use a hammer and the dinky scrapper that comes with the freezer, but is there a better way? What are some lab hacks that you use to keep your freezer from icing over? Our lab only has one -80C freezer. So it's not feasible to transfer the stuff to a second freezer and defrost. :(
|
[
"Eeee, don't use a hammer and scraper, it's way to easy to rupture a coil.\nI'll sometimes scrap areas where I know there are no cooling coils, but I never hammer. You don't want a shelf size piece of ice to suddenly move and kink something.",
"Looking over the ",
"r/biology",
" thread, I agree with everyone who said move everything onto a dry ice cooler and defrost the whole thing. Don't take the advice of the people telling to use localized heating to thaw one shelf at a time, that's going to create more problems than it solves."
] |
[
"we just had an issue with ours where ice had collected between the rubber gasket (think that's what it's called) and the frame so the door wasn't closing properly and we never got to -80C. the fridge guys came and looked and advised that you should defrost EVERY YEAR to prevent issues. also said once you see actually ice (not frosty love that covers everything so you can never find what you want), you have a problem. before that, deicing was a screwdriver and my finger....."
] |
[
"haha. i think those were troll posts. But I'm still thinking of just using a scraper and occasional hammering instead of moving everything."
] |
[
"Why do the stars appear fixed?"
] |
[
false
] |
In other words, I would think a bunch of unrelated stars at all sorts of distances (~4 to 8,000 LY for eye-visible stars) might change position relative to each other on a nightly basis. Are we in some sort of larger reference frame all together moving around the galaxy?
|
[
"The stars appear fixed because humans don't live very long and distances involved are mindbogglingly huge. When traveling along the highway, the median speeds by, the telephone poles move more slowly and the distant mountains hardly move at all—now consider the stars lightyears away. Those stars are indeed going about their own motion, here are two examples: ",
"https://upload.wikimedia.org/wikipedia/commons/6/6c/Barnard2005.gif",
" ",
"https://upload.wikimedia.org/wikipedia/commons/4/48/61_Cygni_Proper_Motion.gif"
] |
[
"Most of the stars we are noticing are much bigger than the sun and not very close. Those stars are in our galaxy and relatively close, like in our arm of the galaxy. Like our sun they're shifting about relative to us and the stars near them. The reason they appear fixed is that their vast distance from us mostly hides their proper motion relative to us. The stars in Orion are from 243 to 1360 light years. ",
"Rigel is one of the stars in Orion. Its 777 light years away but 51,000 times as bright. So lets say we could see it moving about the width of the disk of our moon in a year. The full moon is 1/2 of a degree. So Rigel would have to move about 1/720th of the sky in one year to travel a moon's width. In one year it would travel 2",
"777/720 light years, or roughly 6.4 light years. That's 6.4 times the speed of light. It took one year to go 6.4 light years. A normal star moves at about our star's speed of 240 km/sec. "
] |
[
"The yearly motion of Earth around the sun changes the apparent relative positions of nearby stars in the sky by a very small but measurable amount. This ",
"parallax",
" effect can be used to measure the stars' distances. It's completely indistinguishable to an unaided eye though."
] |
[
"Are there some good alternative explanations or hypotheses in place of the dark matter theory?"
] |
[
false
] |
The fact that something like dark matter exists in such a large quantity that is yet undetectable seems less likely to me than an alternative explanation like, maybe the laws of physics act differently in space in different places or this extra mass I see made up of a bunch of tiny black holes everywhere
|
[
"The fact that something like dark matter exists in such a large quantity that is yet undetectable",
"Dark matter is not undetectable, or we wouldn't know about it. Multiple ways of detecting it all give the same results. We see the gravitational influence of dark matter on the motion of stars in galaxies, in the motion of galaxies relative to each other, and in the deflection of light around galaxies via gravitational lensing, and also can determine how much dark matter there is from the ",
"properties of cosmic microwave background radiation",
".",
"Dark matter is dark, which means it does not reflect light, it does not absorb light, and it does not emit light. That should not be confused with being ",
".",
"By the way, detecting things by their gravitational influence is nothing unusual. Exoplanets (planets around stars other than the Sun) have routinely been discovered in this way (without being directly imaged), and in our own Solar System, Neptune was predicted via it gravitational influence on Uranus before it was directly observed."
] |
[
"Sure, tons. Everything from MaCHOs (numerous rogue planets, brown dwarfs, or black holes) to gas clouds to modified gravitational mechanics.",
"The thing you have to understand about the dark matter theory (i.e. cold dark matter in the form of WIMPs) is that it is the survivor. Every time scientists have conducted an experiment or performed some test they've put all of these theories \"into the ring\" and every single time the WIMP theory has come out victorious. It's the only theory that matches all the current evidence, everything else is much more questionable.",
"Astrophysicists never desired for most of the mass of the Universe to be in the form of particles we know almost nothing about. They've consistently come up with many different plausible scenarios to explain the phenomena in a seemingly more mundane way. And every time they've been disproved by observation. The process of science is to follow where the evidence leads you, not your preconceived ideas, and the evidence has led us to the theory of dark matter as weakly interacting massive particles of a type we currently lack the particle physics to identify."
] |
[
"/u/fishify",
" gave an excellent response, but perhaps didn't really answer your question. The answer is \"yes\", there are alternatives. The most common is MONDs (Modified Netwonian Dynamics), which basically suggests that maybe the behaviour of gravity is a little different at very small accelerations:",
"https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics",
"However, it doesn't explain things like the Bullett Cluster, which dark matter does:",
"https://en.m.wikipedia.org/wiki/Bullet_Cluster",
"and thus dark matter has risen to the top as the \"best\" theory."
] |
[
"When comparing the costs of developping, maintaining and operating reusable launch systems with those of single-use systems, are reusable systems worth the effort and if so to what extent?"
] |
[
false
] |
Not too sure if the tag should be Planetary Science or Economics
|
[
"http://www.zmescience.com/space/spacex-reusable-rocket-100-times-cheaper-0432423/",
"According to SpaceX’s Elon Musk, $200,000 in fuel and oxygen make up just 0.35% of his firm’s launch costs. The rest is in their $56.5 million Falcon 9 rocket."
] |
[
"To expand on this: rocket flight is ",
" rough on the rocket, so with past+current technology, even if we do recover the rocket, we need to spend huge amounts of money banging it back into shape, replacing tons of components, running diagnostics on all the parts, etc., before its next flight. The space shuttle, for example, cost somewhere around $1 billion per launch, despite being designed around re-usability. The Soyuz rocket, meanwhile, which has largely replaced the role the shuttle had, is a much simpler disposable Russian rocket and costs maybe $60M per launch.",
"Rocket fuel isn't that much more expensive then jet fuel, and rockets aren't that much bigger than large jets, so the total fuel cost of a rocket is of the same order as the fuel cost for a large jet - rather less than $1 million.",
"This disparity between refuel costs and relaunch costs makes it very tempting for smart inventors to try to figure out how to make rockets survive flight in better shape. This would make rocket flight as routine as jet flight, so that we can launch a rocket for the same cost as flying a jet, just paying for the fuel. However, we're not there yet, and aiming for re-usability has been hugely costly in the past.",
"(By the way, even if we could do all this, it still wouldn't make a ",
" on a rocket as cheap as a ticket on a jet, since the payload capacity of a rocket is much smaller than a jet - you could fit 5 people on a rocket, instead of 500 on a similar sized jet. So spaceflight would still remain perhaps 100 times as expensive as air travel.)"
] |
[
"Sure, but, ironically, with cheap lift comes the ability to lift ",
", making it cost-effective to attempt efforts in space that may lead to sustainability and a future in which we don't have to lift everything the astronauts need to survive. ",
"You make SpaceX sound like a marginal proposition, and that may well be the case, but I would suggest that Elon Musk's greatest contribution is not proving that cheap lift is possible, but attempting to ",
" whether or not it really is. This is necessary since, after all, the Space Shuttle does not even begin to serve as a reasonable example of reusability.",
"(Edit: WTF would anyone downvote the above post? Poyi expressed a well-educated opinion and a useful (and probably accurate) observation about wear and tear and the impact of cheaper lift on ticket prices. Yeesh.)"
] |
[
"Does there exist an animal with naturally polarized vision?"
] |
[
false
] |
[deleted]
|
[
"Some cephalopods can distinguish light polarization, although the purpose of this ability is not quite clear",
"This is not quite the same thing that happens in polarizing sunglasses, though. The sunglasses block all light except that which has a particular polarization. The cephalopod eyes do not block any light, but rather can detect the polarization angle of incoming light, if that light is already polarized."
] |
[
"Many arthropods, including most flying insects, are able to detect polarized light. ",
"In insects, there are usually two or three ocelli on the 'forehead' of the insect, in between the compound eyes. These are sensitive to polarized light, and are thought to be involved in navigation and predator detection. In flying insects, it's thought that the polarized light may act as something like an artificial horizon, helping the insect to maintain orientation with the sky above and ground below.",
"In marine environments, stomatopods (aka mantis shrimp) have eyes that sensitive to a wide range of colours (up to 16 different types of cones, compared with our 3), can pick up multiple spectra, including ultraviolet, and detect polarized light. What exactly they do with this supervision is still a matter of debate. "
] |
[
"Humans ",
"can detect",
" polarized light too, though the effect is weak and probably doesn't serve any evolutionary purpose."
] |
[
"How did Mars get sand dunes?"
] |
[
false
] |
posting here from seeing the image on Just wondering.. I thought it was through erosion? Water, and lots of it? Or are there other ways?
|
[
"Mars has an atmosphere, and therefore wind, just like Earth does. This wind carries sand particles, and these can, under the right conditions, form dunes. This image ",
"http://earthsci.org/education/teacher/basicgeol/windes/winddepos.gif",
" somewhat shows the process, in that a natural barrier (e.g. a rock) causes windspeed behind it to drop, which can then carry less sand and deposits the sand behind the obstacle. That makes the obstacle bigger, and thus more sand is deposited, and so the dune grows."
] |
[
"Erosion can happen via wind.",
"Think monument valley (",
"http://parkerlab.bio.uci.edu/pictures/photography%20pictures/2009_09_14_Monument%20Vly_SELECT/IMG_3402_tweak.jpg",
").",
"IF Mars had water in the past, some erosion may have happened via water as well. "
] |
[
"It is due to the wind, although the very weak atmosphere changes the physics a bit compared to dunes on earth. Can't find a peer reviewed link, but ",
"here is a nice discussion",
" from a radio interview."
] |
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