subreddit
stringclasses 11
values | text
stringlengths 246
28.5k
|
|---|---|
askscience
|
I think the other thing going unconsidered in a lot of this is just how much more dense everything is there, especially further down. It's not like thinking about gas and liquid as we understand it on Earth. I don't understand hydro-physics or anything, but I did stay at a Holiday Inn Express last night and I'm pretty sure that stuff is closer to a solid on Uranus than it is here.
|
askscience
|
It's impossible for objects in stable orbits on different planes to be going parallel; the orbital planes would be tilted at different degrees and cross over each other. Every stable 2 body orbit, be it circular or elliptical, has to have its major axis pass through the center of gravity of the system (barycenter) - which in this case is effectively the center of the planet.
So in this example, if you were take an object orbiting at the equator and place another copy appreciably further north or south in space with the exact same mass/velocity/direction, the pull of gravity is no longer perpendicular to the prograde (forward) direction of its orbit. It will continue to be pulled straight towards the planet's center of gravity at every point on its path, which is in a NEW direction relative to its old orbit, so it will course correct and end up in a stable orbit in a new plane that is higher on one side and lower on the other with the axis passing through the barycenter again - and this will inevitably cross over the old plane at 2 points and have potential for more collisions.
|
askscience
|
> With your hand moving through the air, the air just gets pushed out of the way.
Eh, it gets pushed out of the way *faster* than this happens with a meteor hitting an atmosphere, it's deflected over a shorter distance, but there's still air being compressed in front of your hand, and that *is* the mechanism for feeling resistance when you stick your hand out the window. The difference is one of magnitude, not kind.
|
askscience
|
With telescopes, yes, definitely.
A large enough impact on Mars could cause it to brighten temporarily so that it might be noticeable to the naked eye. But that would be a catastrophic event. In 30 to 50 million years, Mars' moon Phobos will impact Mars, which might be visible to the naked eye, depending on if Mars is even visible from Earth.
Most impacts on our moon aren't visible to the naked eye, though many are witnessed by astronomers and amateurs with telescopes each year... but there have been day sightings of lunar impacts, with one of the first recorded happening in 1178.
Beyond just seeing impact events, there are unlikely to be any other effects that anyone would notice from Earth. Space is big. Other planets, even our moon, are very far away.
|
askscience
|
Yes, much as two roughly Mars-sized objects once collided and were destroyed, but only the most energetic debris could leave the orbit they were in, so most of the whole mass of crap was still in roughly the same orbit around the sun and eventually formed two objects: one large one that had most of the iron found in the two original planets, and one without much iron; the Earth and Moon, respectively.
|
askscience
|
Impact on an atmosphere is basically an extreme form of aerobraking; the heat of the compressed atmosphere plus the high rate of deceleration both contribute their own hazards to something doing the aerobraking. For that reason, Apollo spacecraft coming back from the Moon had a narrow range of angles at which they could hit the Earth's atmosphere. Too shallow they either skip off or fail to slow their descent and are cast up into another orbit (not to be lost forever, but up their much longer than they need to be, most likely) and too steep, they'd have to deal with both heat and deceleration. The heat would kill them first, but a perfect heat shield won't save you if you hit so fast the atmosphere is compressed into something of a higher density than you.
|
askscience
|
I see your flair so I may be digging my own grave here, but yet I'm surprised you only touched on the reason why things like Saturn's rings, the solar system, or galaxies are all flat, and instead gave a misleading answer of 'collisions'. The reason this is misleading is that if no 'collisions' could occur - ie hypothetically imagine particles/debris could pass right through each other - you'd *still* end up with a flat ring system. The reason I keep quoting the word collisions is that you should have specified that *the vast majority* of all of these collisions are elastic non-contact collisions, not regular physical collisions where 2 things smash into each other.
The real answer has to do with conservation of angular momentum, which you briefly mentioned but didn't elaborate more on (and you should have). Fundamentally, in a 3D world the vector expressing total angular momentum of a system of objects in 3D space will lie on a 2D plane. I don't know much more about this other than that is just how the mathematics work, sorry. In 2D the angular momentum is still on a 2D plane, but in 4D the angular momentum is on 2x2D planes. The dimension of the total angular momentum of a system will always be in *n-(n%2)* dimensions for an *n*-dimensional system.
[This](https://www.youtube.com/watch?v=tmNXKqeUtJM) video should help clarify more by what I mean and why the total angular momentum is the key to flat (2D) systems of objects in 3D.
|
askscience
|
Which is exactly what happens when you stick your hand out of the window. You compress the air in front of your hand and you feel an increase in pressure. Heat is also formed, just not to the level of an object in space interacting with a planets atmosphere. It's a better example than water because *it's exactly what's happening* just on a lesser scale.
If you had your hand out the window and accelerated a car to 17,500 mph your hand would incinerate. /u/Treshnell is correct.
|
askscience
|
Collisions alone will not produce an equatorial orbit for rings or constituent debris.
This must rely on some interaction with the centripetal equatorial bulge and/or solar tidal bulge of the planet. I think that an object in a random orbit with a polar component will be travelling fastest north/south over the equator because it is accelerated towards the bulge. Collisions will tend to remove more north/south momentum than east west and eventually the resultant gravel will coalesce into equatorial rings over the bulge.
Not 100% sure about this explanation, but something along these lines.
|
askscience
|
If it were large enough it would have a gigantic amount of momentum. Momentum is mass times speed, so at a constant speed the momentum will increase ()^3 =cubic. The atmospheric drag will be related to the surface area so it will probably increase ()^2 =square. (I am not sure if this still applies at this scale). So there could be a size of an object that is big enough (not necessarily bigger than a gas giant) that could go through. But then again gas giants (may?) have solid cores (metallic hydrogen?) that would impact with the large object. Fazit: i dont know.
|
askscience
|
Yes. By knowing how fast an object orbits. The heavier a planet (star, black hole), the faster things go at specific distances away.
For instance, an object orbiting 42,157km from the center of Earth will orbit once per day (geosynchronous orbit). An object the same distance from the center of Saturn will orbit once every 2.5 hours. If we didn't know the mass of Jupiter, but we saw that an object 42,157km from the center of Jupiter orbits once every 1.34 hours, we can calculate that Jupiter is 318 times more massive than Earth.
|
askscience
|
Not disagreeing, just pointing out that while sticky fish eggs is a theory it does not appear to have been scientifically proven one way or the other. I.E. These researchers were not able to find any scientific studies that had been conducted to prove the theory.
They do point out that it has been tested on aquatic invertebrates
[Dispersal of Fish Eggs by Water Birds – Just a Myth?](https://www.unibas.ch/en/News-Events/News/Uni-Research/Dispersal-of-Fish-Eggs-by-Water-Birds-Just-a-Myth.html)
"Small lakes with a surface area of less than 100 m2 represent the majority of global freshwater ecosystems. Many of these lakes are found in remote, often mountainous areas with no inflow and outflow. Yet in most of these lakes, there are fish. So how do fish reach lakes and ponds that are not connected to other bodies of water?
This question was already addressed by some of the leading natural scientists of the 19th century such as Charles Darwin, Alfred Russel Wallace and Charles Lyell, who all came to the same conclusion – water birds must be responsible for fish dispersal.
And they had a plausible explanation for this: fish eggs of some species are sticky and can survive for some time out of water. The theory is thus that the fish eggs stick to water birds’ feathers or feet; the birds then fly from one body of water to the next, where the fish hatch from their eggs.
A study carried out by environmental scientists from the University of Basel has now shown that although the research community considers this to be a proven theory, no studies have been published to confirm it.
To objectively measure the lack of evidence, the Basel research team conducted a systematic literature review. The result shows that no in-depth scientific studies exist to prove that water birds disperse fish eggs.
To rule out the possibility that the unsuccessful search was due to their method, the researchers also used the same approach to look for evidence of the dispersal of aquatic invertebrates. In this case, they found numerous scientific publications supported by experiments and field studies."
|
askscience
|
It initially seems that the article is supporting the opinion/theory of water birds carrying the eggs, yet focusing on the fact that no studies have been done to prove or debunk that theory. So the article offers literally no opinion and no explanation for the lack of opinion, ie: it is too difficult to determine which birds might come in contact with which species of fish egg, then track that bird, then find the isolated body of water in which the eggs are released, then track said, specific eggs to ensure they didn't come from another source, and then determine if the eggs are viable and if the species of fish hatched from those eggs are in an environment suitable to that species, etc. Why not treat it like the theory of gravity? Eggs and fish can't walk. Therefore to occur in isolated water bodies they must be carried. Barring humans hiking into the deepest mountain forests with home depot buckets full of Frye, or bears carrying pregnant fish from lake to like in a bucket brigade, seems birds are the most likely and only reasonable answer.
|
askscience
|
Here is an answer on this topic from archaeologist [Morley Eldridge](https://independent.academia.edu/MorleyEldridge)
In western North America, there is incontrovertible evidence that aboriginal people were stocking high mountain lakes. The anthropologist James Teit, a Franz Boas protégé, was married to a N’tlakapamux (Thompson) woman in the southern interior of British Columbia, Canada, during the late 19th century. A Scottish mountain man/rancher, he spoke several “Indian” languages fluently. He lived with aboriginal people while working as a packer and big game hunting guide. He was passionate about what we now call traditional knowledge, and wrote reams on this for the next 40 years. Here is a quote I found buried in a text where he rather tediously lists all the superstitions that regulated life after one’s spouse died (this is from memory as I am at home, I’ll edit to put the exact quotes in tomorrow _now done_):
pg 333 “If a widower transplanted a trout into another lake, before releasing it he blew on the head of the fish, and, after having chewed deer-fat, he spat some of the grease out on its head, so as to remove the baneful effect of his touch. Then he let it go, bidding it farewell, and asking it to propagate and become plentiful.”
pg 348: “Indians have a custom of taking live trout from lakes or streams, and transplanting them into lakes where that are none. Sometimes the fish propagate and become plentiful where introduced. The fish thus treated are supposed to be caught and handled by a person who is clean or not tabood….”
This was published in 1900 in “The Thompson”, a Jesup North Pacific Expedition publication from the American Museum of Natural History. It was written long before any economic claims of aboriginal people gained the legal and political significance they have now. Its not written in his sections on the economy, but in the section on birth, childhood, marriage, and death. He has several other references to this practice. Few other anthropologists spent sufficient time to gain the knowledge he had, and his work is exceptionally detailed.
While glacial isostatic depression or ‘fish rain’ might be possible explanations, its a much simpler and obvious explanation that they were carried live by people and over thousands of years, where environmental conditions allowed the fish to survive, they did. People are exceptionally clever at this sort of environmental modification.
|
askscience
|
Given that birds constantly have to preen themselves, especially waterfowl to maintain the oils necessary for waterproofing, I find it extremely unlikely that an egg would stick to a ducks leg longer than one or two cleaning cycles.
So an egg would have to survive that, getting dried out between lakes and somehow be in water at the time of hatching.
Unless there is evidence of it occuring, I think we should leave this as myth spread by the uninformed.
|
askscience
|
Maybe I'm oversimplifying, but this doesn't seem like a particularly difficult hypothesis to test. Choose some birds, see if they fly between lakes. If they do, that's step one. Step two is to occasionally capture some birds and examine their feet/feathers for some roe. If some is found, great. That would lend credence to the possibility that viable eggs could be transferred from lake A to lake B.
Another avenue, though, would be to study exactly what species of fish are more abundant. Are the more pervasive species all helpful to humans, or are there many species of fish that humans don't use that are also spread around? If it's the former, then the ancient human-dispersal hypothesis would hold water (ha!), but if it's the latter, the bird-dispersal hypothesis would be more logical.
|
askscience
|
>Barring humans hiking into the deepest mountain forests with home depot buckets full of Fry
But we in fact know this happens, there's no shortage of historical examples. Though it was usually done before home depot buckets were around, they often used milk cans and similar tins in the 1800's for example. Another good piece of evidence for this is that what you often find are trout...and trout are a fish people love to eat, and also a fish that does not have sticky eggs.
|
askscience
|
Not at all. Seasonal overflows offer crayfish the opportunity to migrate up system, I've watched it in the jungle, they carry eggs under their carapace and could easily carry sticky fish eggs as well. The only other theory holding water is fresh-water mussel spawn stick to gills of stickleback fish as they migrate upstream during seasonal overflows, that's why you find these sessile species far up stream systems. But they have to stay wet and be oxygenated, and bird feathers repel water.
|
askscience
|
I would choose ponds frequented by birds, then introduce modified fish populations (some harmless genetic marker). Continously harvest and check fish in nearby bodies for the offspring of those marked fish.
At the same time, introduce fish with a different marker in ponds devoid of aquatic or semi-aquatic birds, and look for the same result. If your fish marked for propogation by birds spread and the others don't, it may be more likely.
|
askscience
|
I see what you mean. Could we instead create the conditions? Artificial ponds, controlled fish populations of promising candidates, trained, trackable birds. It might be doable efficiently, since fish can be grown in very controlled environments.
Accelerate the process by choosing species that procreate ideally and frequently, birds that are most suitable, and maximize exposure. Try to show that different species of fish and birds DON'T transfer eggs. If eggs or fish show up in control pond/s, you've found something suspicious. You can compare it with bird tracking as well.
|
askscience
|
Certainly. These have to be controlled ponds, ideally artificial or at least terraformed for our purpose (i.e. for our chosen fish, birds, stability, and reproduction). Not impossible.
In that sense, you wouldn't expect people to be introducing their fish into these ponds, and if they did, you wouldn't expect them to be those marked fish from your other ponds. Another check is bird tracking. If you can show that there were no tracked birds who had gone from another pond to that pond, you know it was contaminated somehow, however unlikely.
|
askscience
|
The premise is that fish can be found in very remote lakes without inlet/outlet streams, though. Certainly many lakes in the Adirondack region have been stocked by adventurous humans, but what of spring fed lakes and ponds in Oregon, Washington, Colorado, Alaska...areas where a hike-in stocking would take weeks of trudging and carrying and caring for fragile eggs or Frye? If you've ever flown over such wilderness, you'd see the impossibility of such stocking. Even with today's technology the only way to stock certain lakes is via airplane. Which would further support the bird theory, keeping in mind a trout carried by a bird of prey like an eagle, hawk or gull, could easily be dropped while carrying eggs, if initially caught from a nearby lake or stream.
|
askscience
|
It's absolutely the case that people spent days hiking up into the mountains, or using mule trains, to get fish to remote bodies of water throughout the late 1800's and early 1900's in the mountain west. Fish are also dropped by airplane.
Here's an article on fish stocking in remote areas https://thefisheriesblog.com/2012/01/16/high-sierra-trout-stocking/ and a map of the part of the sierra nevada that was historically fishless http://www.highsierrahikers.org/fig2fishless.gif
Also, trout don't carry fertilized eggs, so the "bird transport" theory for adult fish also requires both male and female fish to accidentally dropped into the lake at about the same time.
However, it is worth noting that some apparently isolated bodies of water are actually connected during floods
|
askscience
|
You can not see an object going inside the event horizon of a black hole. From your point of view, it would take an infinite amount of time to reach the event horizon, because it would seem to be slowing down.
However, the actual reason why you would not see it, is because the light that comes from that object reaches you with lower frequency (gravitational redshift), until a point that the wavelength is sooo large that no instrument would be able to detect the light.
|
askscience
|
I've never understood the 'will take forever for an outside observer' statement. That only works if they accelerate infinitely toward the event horizon. Take a look at galactic cores. We think they're supermassive black holes that formed by swallowing other mass. If it took forever for things to cross the event horizon to grow the supermassive black holes, then from external observation points we should see no supermassive black holes.
I also thought that as far as time dilation goes, if entity A is under increased gravitational forces and entity B is under significantly lower gravitational forces, then entity B's observation of entity A is that it moves slower. See the plot line of the movie "Innerstellar."
|
askscience
|
We do not actually "see" black holes. We just detect their gravitational interaction with other elements close to them (e.g, Sagitarius A\*), or the material that is starting to fall into them (e.g, accretion disks). Also, the objects are not being accelerated by themselves, they are free-falling into the black hole.
Second statement: indeed some of the ideas of Interstellar are correct, and that is the direct cause of the redshift that I explain before. If you have a laser with certain frequency/period emitting in a intense gravitational field , for an external observer the measured period is higher, just because apparently the time runs slower when looking at the emitter.
|
askscience
|
"See" was the wrong word, but the point stands.
The problem that I'm trying to point out is that it doesn't make sense to me for it to take forever for an object to cross the event horizon, as viewed by any observer, which is how the original comment was interpreted. This seems entirely wrong...
Time is relative, but its flow remains fixed for the distant, stationary observer. We can say that time is moving normally for the external observer and is dilated for the one accellerating toward the black hole, increasingly so as they approach the singularity or the speed of light, whichever comes first.
For the observer falling into the black hole, my understanding is that their perception should be as if they are traveling faster than the speed of light (if spaghettification wasn't a concern) and that everything around them should seem to happen incredibly fast and/or blueshifted, though warped is probably a better word.
For our external observer, they will see the falling observer move faster and faster as it accelerates toward the event horizion. Additionally, photons emitted by the falling observer are more and more redshifted to distant observers at less dilated spacetime. This continues until the spacetime at the event horizon prevents any photons from escaping at all.
...so what part do I misunderstand?
|
askscience
|
Doesn’t it simply mean that the information that an object fell into the black hole can never reach external observer?
If so doesn’t it mean that from the point of view of such observer (eg us) a black hole could never ‘grow’ after its initially formed ?
I always wondered about this:)
also won’t uncertainty principle mean that at some point this object can cross the horizon ?
Not a physics grad so my thinking is most likely naive
|
askscience
|
>Doesn’t it simply mean that the information that an object fell into the black hole can never reach external observer?
Indeed that is what it is happening.
>If so doesn’t it mean that from the point of view of such observer (eg us) a black hole could never ‘grow’ after its initially formed ?
That it is a really good question that I have asked myself so many times and I do not have a clear and convincing answer. But my guess is that we actually see a black hole grow because, although it would take an infinite amount of time, the material that is falling into the black hole already adds mass to it, if you look from far enough. However, I do not feel that this is a good answer...
>I always wondered about this:) also won’t uncertainty principle mean that at some point this object can cross the horizon ?
No, because the object apparently is frozen, but from its point of view (where this quantum consideration should be done) it does not feel any special effect after entering the horizon. Furthermore quantum effects in the proximity/interior of a black hole is not easy/possible to study with our QM and GR theories.
​
|
askscience
|
I was rewatching walking with dinosaurs a few months back, and they suggested that sauropods did it from behind with the Male on top - like most quadrupedal animals today. I think they suggested that females had reinforced hip and spine bones to help them support the males weight. Also that it didn’t last very long. Though this was a TV show made nearly 20 years ago (which took an amount of artistic license) so the scientific consensus may well have changed since then.
Edit: here’s the link to the clip (https://youtu.be/-mv_v4ltSrY). Again this is a 20 year old show, but it positioned itself as a natural history documentary programme.
|
askscience
|
This post has attracted a large number of anecdotes, puns and simple joke
comments. The mod team would like to remind you that comments on r/askscience are expected to answers questions with accurate, in-depth explanations, including peer-reviewed sources where possible. If you are not an expert in the domain please refrain from speculating.
So far 80% of the comments have been removed. If you are going to comment with "carefully" or "loudly" you will be pleased to know that you are the 50th user in that thread who is trying to make this joke.
|
askscience
|
Well then. Birds are dinosaurs, so everything we know about birds falls under the purview of your question. However, for extinct forms, we can also make inferences using a technique known as [phylogenetic bracketing](http://en.wikipedia.org/wiki/Phylogenetic_bracketing).
Dinosaurs are [archosaurs](http://archosaurmusings.wordpress.com/what-are-archosaurs/), the two living representatives of which are crocodylians and birds (see also our [FAQ on why birds are dinosaurs](http://www.reddit.com/r/askscience/wiki/biology/birds_are_dinosaurs)). If there's a character that both groups have, it was likely present in their common ancestor. Things like a four chambered heart (which evolved independently from the mammalian heart), [unidirectional airflow in the lungs](http://mappingignorance.org/2013/03/29/triassic-lungs-unidirectional-flow-in-alligators-breathing/), and nest-building/parental care are present in both birds and crocodylians, so they were probably present in their common ancestor. That means extinct dinos likely had those traits or lost them secondarily. We have fossils that confirm these some of inferences, like brooding of nests.
Interestingly, we've also recently found that alligators are monogamous over multiple mating seasons, as are many birds, so that could have implications for how we look at extinct archosaur behavior. Alligators will also show nest site fidelity, coming back to the same or nearby areas over multiple nesting seasons. Many crocs have [complex mating rituals](http://www.wired.com/wiredscience/2011/05/alligator-mating-physics/) as well, so these also seem to be ancestral to archosaurs.
As far as dinosaur reproduction goes, we've found a lot of similarities between the reproductive tracts in birds and crocs. For example, [alligators and birds form eggshells in similar ways](http://www.ncbi.nlm.nih.gov/pubmed/1547315).
Most ["reptiles"](http://en.wikipedia.org/wiki/Sauropsida) have [hemipenes](http://en.wikipedia.org/wiki/Hemipenis), which are paired copulatory organs that are everted for mating. This is not true of archosaurs. Most birds have lost their penis, but some retained it (ducks and ratites like ostriches and emus are two examples). I don't know of any fossil dinosaur genitalia, but birds (those that have a phallus) and crocs each have a single phallus rather than the hemipenes of extant lepidosaurs. That's likely what other extinct archosaurs probably had. However, given the range in variation that we see in living birds alone, I'm sure dinosaur genitalia existed in all shapes and sizes.
In short:
- Dinosaurs probably ancestrally had penises similar to crocodylians and some birds, but they could have been lost in lineages like they were in many bird groups.
- At least some brooded their nests.
- They probably had mating displays like birds and crocs do.
- Some may have been monogamous over multiple mating seasons like many birds and crocs.
[This article](http://blogs.smithsonianmag.com/dinosaur/2012/02/the-anatomy-of-dinosaur-sex/) similarly covers these topics.
|
askscience
|
Extinct is one of those complex terms, I joked to a friend once that humans stopped evolving not because of any biological process but because we are to vain to call ourselves anything else.
My question would be more what dinosaurs did birds evolve from and did all birds come from one group. There is definitely a line that probably looks like gallimimus(butchered name), moa, ostrich and emu.
But what about chickens, peacocks etc. Did different group of birds come from different dino stocks.
Wait can different bird types mate?
|
askscience
|
Birds are dinosaurs, full stop. They’re theropod dinosaurs in the same way monkeys are all mammals. Not all dinosaurs are birds, so many groups of dinosaurs are extinct and don’t have living representatives. But yes, dinosaurs aren’t extinct. That extinction event wiped out a lot of things that weren’t dinosaurs, and it didn’t kill all the dinosaurs. It’s far more complex than is popularly portrayed, and we don’t fully understand the patterns we see.
|
askscience
|
Birds aren't descended from dinosaurs --- they *are* dinosaurs. They're the one group of dinosaur species that wasn't wiped out, and since then they've diversified and flourished.
Take a look at the second paragraph of the wikipedia article on birds (https://en.wikipedia.org/wiki/Bird):
> Reverse genetic engineering[3] and the fossil record both demonstrate that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs.
EDIT: Also, it's kinda cool that that means that some of the most intelligent living animals are dinosaurs.
|
askscience
|
There was a research topic posted at reddit some time ago, perhaps a year or two, that pointed out that genetic analysis and fossil evidence shows that several lineages of birds, at least four, survived through the K-P extinction event. It kind of blew my mind because I had always thought the diversification of birds happened later.
The big question, that we really don't have a good answer to, is why several species of birds, some of them not very flight-worthy (chickens, for instance), survived the extinction event, but non-avian theropod dinosaurs did not. What were the key differences that made the avians capable of surviving and the terror beasts not so much?
From the descent, it appears that only one species each of monotremes, marsupials, and mammals survived. That too is curious.
|
askscience
|
There was a research topic posted at reddit some time ago, perhaps a year or two, that pointed out that genetic analysis and fossil evidence shows that several lineages of birds, at least four, survived through the K-P extinction event. It kind of blew my mind because I had always thought the diversification of birds happened later.
The big question, that we really don't have a good answer to, is why several species of birds, some of them not very flight-worthy (chickens, for instance), survived the extinction event, but non-avian theropod dinosaurs did not. What were the key differences that made the avians capable of surviving and the terror beasts not so much?
From the descent, it appears that only one species each of monotremes, marsupials, and mammals survived. That too is curious.
|
askscience
|
Roosters and hens both have cloacas - one reproductive and waste tract. Based on the post above, we might infer something about dinosaur mating from chickens. Here's a description ([source, emphasis added](https://animals.mom.me/reproductive-system-of-a-rooster-7896371.html))
**Reproductive Process**
**When a rooster mates with a hen, he will climb on top of her back and place a foot on each of her wings, forcing her tail feathers upward so he can press his cloaca to hers.** The rooster ejaculates and transfers the sperm, and he hops off. *Often he will perform a victory dance of sorts, hopping around and strutting*, while the hen unruffles her feathers, flaps her wings and walks away
|
askscience
|
Aves is simply the crown group of birds. When you look at the larger group Avialae, you see an awful lot of birdlike creatures that are extinct, so it’s not as clean as *things that looked like birds did fine*. I’m not sure how other members of Avialae track with the K-Pg, but I wouldn’t characterize everything other than crown-group birds as “terror beasts” (which I realize is an English translation of “Dinosauria”). Other avialans are just as birdy as Aves, and in fact if you ran into a non-avian eumaniraptoran on the street, chances are, your brain would think “bird!”
Squamates survived, but groups like mosasaurs were lost. Pterosaurs had declined in diversity, but the remaining pterosaurs didn’t cross the K-Pg boundary. Lots of crocodyliform groups were lost.
Monotremes and marsupials are all mammals. We normally talk at higher than the species level for these events, because single species are generally too short-lived to be a good indication. Groups like monotremes, multituberculates, gondwanatheres, metatherians (including marsupials), and eutherians all cross the boundary, but I think some groups were lost and some regions were impacted more than others.
|
askscience
|
Yes, they could've only evolved from non-extinct species. There have actually been multiple mass extinction events (one of which was a global rise in temperature and release of greenhouse gas through natural means) throughout the history of the Earth where a majority of all species were wiped out, leaving all we see to have evolved from what was left. There was one actually where all but 5% of the world's species died out. It's very interesting and worth looking up.
|
askscience
|
I want to add that birds don't show up well in the fossil record because they have weaker bones that don't hold up as well over millions of years.
So the link between true birds and the larger group of dinosaurs is a little hazier than you would expect from another group, say mammals. We have a better record for the evolution of mammals (heavily based in jawbones and teeth) than we do for birds. In fact, teeth are so important that one group of proto-mammals, the cynodonts, are named after how they were first identified:
New Teeth. They have diverse teeth instead of a more reptilian single tooth which is just repeated over and over.
We could look at teeth for birds, too--when did dinosaurs start losing their teeth and getting true beaks? It's hard to tell because it's harder to find those teeth without a skeleton along with them.
|
askscience
|
I never said they were?
You were the one who used birds and crocodiles to talk about archosaurs and thus make inferences about dinosaurs.
Hippos and bats aren't humans either, but all three are examples of mammals just as birds, crocodiles, and dinosaurs are all (based on your comment) archosaurs.
I see from your flair that you are an actual scientist. I definitely am not, so I defer to your expertise of course, but my analogy seemed like a good way to contextualize what you were saying. Sorry if I was mistaken.
Tl;dr - I thought archosaur : bird/dinosaur/crocodile :: mammal : bat/human/hippo
Edit - thinking about it, crocodiles aren't dinosaurs, but birds are. So maybe a more apt comparison would be primates: apes, humans, and baboons? It's kind of interesting to think about it that way since humans are obviously apes so it seems redundant. Is that how you feel when people ask you questions about dinosaurs as though birds don't belong to that group?
|
askscience
|
Life was also far less diverse back then. What we’ve gathered from fossils so far shows a huge lack in biodiversity. Now whether that is a result of remains failing to be preserved or because of an actual lack could be up for debate. However we all share common ancestors. Ultimately we’re all descended from this one type of fish that got uppity and developed early limbs.
Though I would like to question the conclusion being based on crocodiles. Crocodiles existed at the same time as the dinosaurs so it’s possible that their habits and anatomy were very different from their far larger cousins.
|
askscience
|
Generally speaking, a chicken on a farm has little chance to demonstrate intelligence. People who have deliberately interacted with chickens to study them have had different results.
>[The birds were significantly more likely to peck at the second key, which offered a greater food reward but after a longer delay time. In other words, they showed self-control – a trait that some biologists think hints at a degree of self-awareness.](http://www.bbc.com/earth/story/20170110-despite-what-you-might-think-chickens-are-not-stupid)
There's an example of mindful pecking.
|
askscience
|
It seems like none of the living descendants do anything like how salmon reproduce. From what the OP said about phylogenetic bracketing, it sounds like it's unlikely that dinosaurs would have done something similar... but evidently, bird sex doesn't lack for variety.
That said, there weren't really aquatic dinosaurs. There were large reptiles like Plesiosaurs, or dinosaurs documented to be close enough to water to use it as a food source (Baryonyx). I'd suspect something would need to be primarily aquatic to have a water-based reproductive method.
|
askscience
|
> Dinosaurs are
> archosaurs
> , the two living representatives of which are crocodylians and birds (see also our
> FAQ on why birds are dinosaurs
> ). If there's a character that both groups have, it was likely present in their common ancestor.
Just to be clear, isn't it the case that it's only true phylogenetic bracketing when the extinct species is descendant from the last common ancestor of the two extant species? (E.g., there is a more recent node on the phylogenetic tree from which birds and crocodiles descend, and the argument applies to dinosaurs that are also descendants of that node.) In particular, it is *not* the case (as one might mistakenly infer from your language above) that if an extinct species has two extant descendants then anything shared by those descendants was also probably present in the extinct species.
|
askscience
|
I’m not sure what you mean. You look at taxa that are more basal and more derived with respect to the taxon of interest. If a trait is present in both taxa of your bracket, you can infer that it was present in the common ancestor of both groups *and* not lost in the descendants of that common ancestor. This allows you to generate hypotheses about traits that you can’t directly observe, and it can inform future work. One example would be parental care in non-avian dinosaurs. Both crocodylians and birds exhibit parental care of their young, and we now have [examples of this behavior in non-avian dinosaurs](https://naturalhistory.si.edu/exhibits/backyard-dinosaurs/how-did-dinosaurs-behave.cfm). Obviously this isn’t going to work all of the time, and with a group as diverse as Archosauria, we know there will be a ton of variation.
Beyond that, I’m not sure what you mean. What more recent recent common ancestor exists of crocodylians and birds?
|
askscience
|
In the context of the fiction /u/superflyguy87 is remembering (Halo), I think the device is more or less stationary and building up a sufficient charge to make a fictional "slip space transmission" (faster than light communication) to a set of predetermined coordinates. It also used that time to map out the stars and work out its currently location, it's a sort of homing device that was attached to an enemy (human) ship.
A real word analogue would be if we had something like an orbiter around Proxima Centauri that didn't have enough power generation to make constant transmissions to Earth and have them be distinguishable from noise, so instead it gradually stored up energy in a cell or capacitor and used that to fire shorter burst transmissions at Earth.
|
askscience
|
Every nucleus has some number of protons (Z) and some number of neutrons (N). The total number of nucleons is A = Z + N.
An element specifies the Z, but it doesn't specify the N (or by extension, the A).
Different isotopes of an element have different N, but the same Z.
Because the name of the element specifies Z, in order to talk about a specific nuclide, you have to specify the N as well. Or you can just specify the Z and the A, and the N is determined by N = A - Z.
In your example of uranium-238, the 238 is the A of the nucleus. Uranium always 92 protons. Uranium-238 has 238 nucleons. So that means that it's the isotope of uranium with (238 - 92) neutrons.
[Here](https://www.nndc.bnl.gov/chart/) is a chart where you can see every known isotope of every element.
>Are only nuclear elements capable of having a number?
I don't know what you mean by "nuclear elements", but every element has multiple isotopes.
>Could there be something like iron 222 or something?
No, there can't be iron-222. The stable isotopes of iron are around iron-56. If you try to add too many neutrons to an iron nucleus, you'll reach a point where any additional neutrons you add cannot form bound states with the nucleus. It's not known experimentally where this occurs for iron (it's known for elements up to fluorine), but it's almost certainly long before A = 222.
|
askscience
|
> I don't know what you mean by "nuclear elements", but every element has multiple isotopes
I meant like heavy elements that could be used in fission (IE uranium, plutionium, etc.) or light elements that could be used in fusion (IE hydrogen, helium, etc.); I don't believe that you could make a nuclear fuel out of iron so that's kind of what I meant.
> No, there can't be iron-222. The stable isotopes of iron are around iron-56. If you try to add too many neutrons to an iron nucleus, you'll reach a point where any additional neutrons you add cannot form bound states with the nucleus. It's not known experimentally where this occurs for iron (it's known for elements up to fluorine), but it's almost certainly long before A = 222.
But there could be an iron-x then? Let's say Iron 12 or something? Any element can be/have an isotope?
|
askscience
|
Think of it like trying to build a tower out of dry-ish sand - no matter how careful you are to pack your sand well and stack it carefully, eventually you'll get to a point where adding more sand means that it will just fall off the sides of the tower instead of actually making the tower higher. To remedy this, you can start with a larger base or build a shorter tower.
It doesn't matter how hard you try, eventually you'll get to a point where the nucleus will reject additional neutrons, and they'll effectively just get pushed out, which is why it's so difficult to do something like add 100 extra neutrons to iron.
|
askscience
|
You can't have Iron 12. Iron has 26 protons, so the lowest you could go would technically be 26, but the iron nucleus would become too unstable well before you got to a nucleus of only 26 protons. The lightest observed isotope of iron is Iron 45 which has a half life less than 2 milliseconds. Iron 54 is the lightest stable isotope. The heaviest is Iron 72 which also has a half-life measured in milliseconds.
|
askscience
|
To put it simply the number is the weight. An atom of U-238 weighs about 238 atomic mass units. A proton and a neutron weigh roughly 1 amu each.
So hydrogen would be H-1. Deuterium would be H-2. Tritium would be H-3. Goes all the way up to H-7 but H-4 and above the atom breaks apart in about .0000000000000000000001 seconds.
It's just a way of presenting the info that can be done with every isotope of every element, doesn't have anything to do with the isotopes stability.
|
askscience
|
Most marine life won't have a problem. Fish swim a little deeper. They are generally keep away from surf zones, if the zone gets bigger they move out and down.
In extreme winds the surface of the water turns to a froth layer a couple of metres thick, "too thin to swim in and too thick to breathe" according to my old oceanography lecturer. Sea mammals can't breathe would drown. They may be able to get to land and head inshore.
Things get really bad where the water meets the land. Here an enormous amount of wave energy gets dissipated destroying all sorts of stuff. Many thing die.
Sea birds can't cope with this very well. Old Navy sailor friend told me that you know when you are in a really bad storm when an albatross lands on the deck.
|
askscience
|
You can breathe before a couple minutes after going down. Maybe your instructor was talking about hurricane sized waves. A couple minutes is a very long time to not be able to breathe and people would die all the time if that were the case.
Edit: especially when you are doing a strenuous activity. Try running 100 meters then holding your breath for two minutes and see how that works out.
|
askscience
|
I don't think that is true unless the water has some sort of contaminant. This is Nazare one of the bigger waves in the world.
https://www.youtube.com/watch?v=uRJ87fzQ8Qc
There is a lot of foam but the guy's head is clearly visible and clear of the water.
Surfers do get held underwater by large successive waves sometimes but it's not the foam on the surface that causes them to drown, it's another wave and the circular hydraulics of the waves.
I've been held under surfing, it's scary but you are well below the surface of the water.
|
askscience
|
>In extreme winds the surface of the water turns to a froth layer a couple of metres thick, "too thin to swim in and too thick to breathe" according to my old oceanography lecturer. Sea mammals can't breathe would drown.
Does this still happen when the hurricane is out in the middle of the ocean? Do Sperm Whales have to get out of the path of hurricanes? Or does this mostly effect more coastal mammals like seals?
|
askscience
|
It has been done, see JATO pods and the Messerschmitt 163 for example. The issue is that it is very inefficient for planes.
Rockets have to carry their own fuel and oxidizer. But since planes by definition stay in Earth's atmosphere they can just use the oxygen in the air to burn the fuel in the engines.
The only advantage of rockets in the atmosphere (for missiles and such) is that they can be made relatively simple and can provide great acceleration which is useful when you are trying to catch up with another aircraft.
|
askscience
|
This is basically what is done right now. That's called a ramjet if you keep it simple. It has no moving parts but can only work at high speed. If you want something that works at lower speeds you need a compressor for the air which makes it a regular turbojet.
You can get fancy like with the SABER concept where you liquefy the oxygen out of the air before burning it but then it gets very complicated.
|
askscience
|
Depends on if it is sessile (attached) or mobile, and a dozen other factors. In Florida Bay, we tend to see the water literally get sucked out between the Florida Keys if the storm passes to the west. This exposes everything on the bottom and, if it’s long enough, itlll die.
We saw massive sponge die offs due to this [ETA: after Irma]. Hurricanes also move a ton of sentiment at times, which can scour the bottom life, or bury it, or just remain suspended and “choke out” corals and sponges and sea grasses by denying their ability to photosynthesize or suspension feed efficiently.
There were many reports of small fish kills due to surge pushing them up on land, again these were mostly the little critters that hide near the bottom nearshore (catfish, pin fish, sea horses, etc), not so much larger snapper or tuna or things like that. Lots of conch, sea stars, urchins, etc were washed ashore.
|
askscience
|
Yes! And this bias is something that we do have to take into account. Different methods for detecting planets have different biases, so we always have to account for that if we're trying to make statements about the population of planets as a whole.
The transit method works best when the planet is big enough to create a noticeable drop in the star's brightness, and when the planet's orbit is rapid enough that you can see lots of transits happen. You also need the planet's orbit to be lined up with our line of sight - i.e. the orbit is edge-on. So we're biased towards large planets that are close to the star. However, this method is quite sensitive, and we can actually see more than just the hugest and closest planets, but it definitely is easier to see the huge close ones.
Radial velocity measurements are even worse for this. For these, you look at how the star moves towards and away from you. We can see this with the redshift and blueshift of specific lines of radiation from the star. But again, this is easiest if the planet is huge and close to the star, so that it has a significant effect on the star. This method isn't quite as sensitive, so what you end up finding is a lot of "hot Jupiters" - i.e. massive planets really close to the star.
Another method is *direct imaging*, where you try to actually see the light of the planet directly. Here, it's easiest if the planet is *far away* from the star so that you can separate the images and not get blown away by the star's light. It's still easiest to see big planets, especially if they're hot and bright, but in terms of distance we get the opposite bias - it's easier to see distant rather than close planets. It's also easiest when the planet's orbit is *face-on*, because then we can see the planet go in a nice ellipse in the sky.
There's also microlensing which can be used to detect planets, but it relies on star systems just happening to line up at the right time, and while it's quite sensitive, it doesn't detect very many planets per year.
So yes, the various methods do create a skewed view of planets, and we really need to be careful what information we take from what we see. But bias is a universal problem in astrophysics. For example, the easiest stars to see at a large distance are the brightest stars, so we have to remember that the further away you look, the more dim stars you're missing etc. But in terms of planets, it means that we would still have difficulty finding a lot of the planets in our solar system, and instead find large planets that are really close to their stars.
|
askscience
|
That's pretty off-topic, but you can see some simulation results [here](http://www.illustris-project.org/media/) - what you get is a filamentary structure that's uniform on very large scales. You can see a map based on observations of real galaxies [here](https://www.sdss.org/wp-content/uploads/2014/06/orangepie.jpg) - this big black slices are regions that aren't visible because the Milky Way is blocking the view.
Basically, the universe is supposed to have this filamentary structure uniformly everywhere. There is no centre to the universe - the universe is actually thought to be infinite in size. It's best not to thing of the Big Bang as an explosion of stuff flying outwards from a point - it's better to think of it as the universe as a whole getting less dense over time, as everything stretches away from everything else.
|
askscience
|
Some suggest that cataclysmic events in the Earths prehistory could have wiped out the trace of possibly undiscovered ancient civilizations. Think about how quickly we went from Benjamin Franklin with a kite, simply theorizing about electricity being used for energy, to a few hundred years later (or 12-15 generations if you think about it like that) we have super computers, are on the verge of AI, and are able to genetically alter our DNA. I don't think its crazy to assume that there have been advanced technological advances in ancient history that have been lost with time. And I'm pretty sure the estimates for modern humans is at least 100,000 years, with new evidence coming out every day pushing that date further back. The peopling of the americas is even under scrutiny right now with finds in [California](https://news.nationalgeographic.com/2017/04/mastodons-americas-peopling-migrations-archaeology-science/) and more pushing it back to even past 100,000 years.
In the words of Graham Hancock, "we are a species with amnesia" and I believe the whole of our understanding of ancient cultures lies in the shadows.
|
askscience
|
You don't understand what I mean. Maybe their culture was less materialistic and more focused on knowledge and spirituality. Just because they don't have machines or fun little gadgets doesn't mean they weren't advanced in their thinking and abilities. Perhaps they valued oral tradition over the perils of written texts ability to fall into the wrong hands. There are many possible reasons for why an advanced civilization outside our realm of understanding wouldn't have left behind your idea of advanced technology. Also, how much evidence would be left in an area such as [this](https://www.youtube.com/watch?v=lzTRGECqgGY), where the landscape itself was devastated by flooding at the end of the last ice age?
|
askscience
|
>Satellites would be extremely obvious.
Yes, this is a good one, even though depending on how many thousands of years we're talking there might not be any left because they will have lost speed and burned up in the atmosphere or crashed back to earth where they got burried. Do you happend to know how long the average satellite today would stay in orbit once it isn't maintained anymore? I'm curious.
>You would expect to find various metallic relics on Earth.
Like what exactly and for how long?
>You would expect to find many exploited resources underground.
Mines collapse and disappear. You wouldn't know about the missing recources if you don't know they were there in the first place.
>A rapid change in CO2 levels in the past would be easy to find.
Yes, and if you look at the record we do find those too, but they can be explained through other natural processes as well.
I'm not arguing for OPs ancient advanced civilization btw, but I think it's impossible to disprove it. Just like it's impossible to really disprove anything. I don't really see any reason to believe that there ever was such a civilization of course, because there isn't any proper evidence for it.
|
askscience
|
Our geostationary satellites are expected to stay in orbit for millions of years at least. Not in a perfect geostationary orbit but still in an orbit.
> Like what exactly and for how long?
Everything, for a very long time (just gets harder to find over time). Remains of buildings, cars, tools, ... we would (will?) leave behind a geological layer of processed metals in many places.
> Mines collapse and disappear. You wouldn't know about the missing recources if you don't know they were there in the first place.
But you do. We never exploit the full deposit. We take out the core where it is worth the effort and leave the rest. Finding hollow deposit for various resources would be very curious.
> Yes, and if you look at the record we do find those too, but they can be explained through other natural processes as well.
Nothing as rapid as currently.
|
askscience
|
> Our geostationary satellites are expected to stay in orbit for millions of years at least. Not in a perfect geostationary orbit but still in an orbit.
Could you provide a source for that? I was of the opinion that they'd still be affected by ever so slight atmospheric drag. Impact with dust/debris/meteoroids might slow them down and push them off their orbit further over centuries. Wikipedia further says this: https://en.wikipedia.org/wiki/Geostationary_orbit#Orbital_stability
>Everything, for a very long time (just gets harder to find over time). Remains of buildings, cars, tools, ... we would (will?) leave behind a geological layer of processed metals in many places.
Yes, we are doing that now. Plastics for example last for a long time. Simple metal structures not so much. But things can fossilize and would then survive in the geological record for millions of years. Might be interesting to look into this, I only know of organic material fossilizing like wood and bones for example.
>But you do. We never exploit the full deposit. We take out the core where it is worth the effort and leave the rest. Finding hollow deposit for various resources would be very curious.
It's not as easy as it sounds. After sufficient time has passed it will be hard to tell what's natural and what man-made. These "hollow deposits" will collapse over time, or get filled up. Have a look at this: https://en.wikipedia.org/wiki/Dike_(geology) There are many ways something like it can and does occur naturally.
>Nothing as rapid as currently.
I'm unsure if that's true as of now (you're probably confusing it with the predicitons), but it's also not really important for the argument. A previous advanced civilization might have been smarter than us and stopped emitting so many greenhouse gases before they even got to the point of a feedback loop of a shrinking albedo effect, increased permafrost methane release aso.
|
askscience
|
Atmospheric drag is completely negligible at geostationary orbit. [LAGEOS](https://en.wikipedia.org/wiki/LAGEOS) is expected to stay in orbit for [about 8 million years](https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/current_missions/lag1_general.html) - at an altitude of just 5800 km.
> Wikipedia further says this: https://en.wikipedia.org/wiki/Geostationary_orbit#Orbital_stability
That part is about staying geostationary - above the same spot on Earth. The satellites won't do that long without fuel, but that is not what we are talking about.
> Plastics for example last for a long time. Simple metal structures not so much.
Unlikely that plastics will last so long. Bacteria are evolving to break down some types of it already. Metals can stay for a long time. We should add glass to the list.
> It's not as easy as it sounds.
I didn't say it is easy.
> A previous advanced civilization might have been smarter than us and stopped emitting so many greenhouse gases before they even got to the point of a feedback loop of a shrinking albedo effect, increased permafrost methane release aso.
Sure, not every civilization must have increased the CO2 level. Just like not every civilization must have launched satellites, exploited ores, invented plastics and glass and so on. They are still potential things how such an earlier civilization could be studied today.
|
askscience
|
OK, interesting. I somehow thought the orbit would be less stable.
>Unlikely that plastics will last so long. Bacteria are evolving to break down some types of it already. Metals can stay for a long time. We should add glass to the list.
Really depends on the exact material. There is no large scale breaking down of plastic by bacteria yet afaik. Most ends up in ocean sediment now. Some metals last long, others not so much. Glass isn't too bad.
In the geological record you'd also find traces of C14 and other radioactive isotopes if they had atomic bombs. And much more actually. If you find it an interpret it correctly that is. Geology is messy and we haven't come very far in that field yet.
|
askscience
|
Why can't you go through these infinitely many "halves" in a finite amount of time?
Let's say, for simplicity, that you're running to the bus stop at a constant speed, and you stop when you suddenly hit the sign, or something. So, you're running from position 1 to position 0 (note, I reversed this), say the units are meters. If you're running at, say, 1m/s, then the time it takes for you to go the first half, from 1 to 1/2, will be 1/2 a second. The time it takes for you to go the second chunk, from 1/2 to 1/4 is 1/4 of a second. The time it takes to get from 1/4 to 1/8 is 1/8 of a second. This pattern continues. The time it takes you to get from the chunk 1/2^(n) to 1/2^(n+1) is 1/2^(n+1) of a second.
What, then, is the total time of the trip? You would have to sum up all the times together. The times for each chunk are 1/2, 1/4, 1/8, 1/16, 1/32, etc seconds each, so the total time will be
* 1/2 + 1/4 + 1/8 + 1/16 + 1/32 +...
where the sum goes on infinitely. This is a very classic [Geometric Series](https://en.wikipedia.org/wiki/Geometric_series), and adds up to a finite value. That finite value, in this case, is 1. So after 1 second, you will have gone through all of these infinitely many halfway points and gotten to the end (which makes sense, because you're going 1 m/s for 1 meter). So, even though there are infinitely many halves to go through, it takes a finite amount of time to get through them all.
This situation is also known as [Zeno's Paradox](https://en.wikipedia.org/wiki/Zeno%27s_paradoxes).
|
askscience
|
The universe is not made up of Planck pixels. The Planck length is just the length scale at which our laws of physics break down. At such short distance and time scales, the uncertainty in momentum and energy becomes so huge that you are approaching black hole densities. Where quantum field theory relies on a flat spacetime, that can no longer be guaranteed. So yes, there's a lower measurement limit in our understanding of physics, it has no bearing on whether the universe is discrete or continuous.
|
askscience
|
If it approaches a finite values by a finite time, then it realizes that finite value in that time.
We can argue that this actually isn't the definition of infinity, and it is more subtle than this, but we'll gloss over that. Just because you have an infinite amount of things, doesn't mean you can't squeeze them all into a finite space. There is no end, but there is an "after" for them. There are infinitely many values in the sequence 1/2, 1/4, 1/8, 1/16, 1/32,... but these are all bigger than 0, so if we are at zero then we are "beyond" them.
Moreover, the universe isn't a computer game where you have to compute everything about every frame. It literally takes 0 time to go through each of these steps, so none of them have to be "done", we're just *describing* them using these infinite sums for our own understanding. Finally, it doesn't even make sense irl because you can't talk about infinitely fine slices of space or time this nonchalantly.
|
askscience
|
By the classical definition, what you’re really doing is taking a limit. You construct a sequence of partial sums, call it Sn. S1 is just the first term, 1/2 in this case, S2 is the sum of the first 2 terms (3/4) S3 is the sum if the first 3 terms (7/8) and so on. The sum of the infinitely many values is the limit of the sequence Sn as n grows large.
|
askscience
|
The problem arises when you try to shoehorn physical things like lightbulbs into abstract mathematics.
Either it is a physical lightbulb, or it is a mathematical, abstract lightbulb.
If it is a physical lightbulb, then its behavior is constrained by physics. At some point the switching back and forth will be rapid enough that the electric fields do not have time to propagate between the contacts and the lightbulb and drive a current; so there is a maximum upper limit to the rate at which bulbs switch on and off, which is set by the material properties of the bulb and the wire lengths etc. Once this maximum limit is reached, it will continue switching back and forth at that rate.
Otherwise let's consider an abstract lightbulb that is not constrained by physics. Then the question reduces to asking: what is the limit of the sequence (0, 1, 0, 1, 0, 1...) at infinity? The answer is: "undefined". This is a clear, precise answer, and since it is not a physical lightbulb, this answer is perfectly acceptable.
|
askscience
|
This is the [Thomson Lamp Paradox](https://en.wikipedia.org/wiki/Thomson%27s_lamp). There is a difference between the Zeno situation and Thomson's lamp. The main difference is that of convergence. Zeno's paradox represents a convergent sequence, and so we can talk about the end product. Convergence is basically synonymous with the existence of an end result. Thomson's Lamp represents something where there does not exist an end result.
It's kinda like that joke about mathematicians going to a pub. The first orders a pint of beer. The second orders a half a pint. The third orders a quarter of a pint. Etc. The bartender then just pours two pints and gives it to them. This works because there is an convergent "end result" to this infinite process. If a similar situation happened, with a mathematician coming into the bar at faster, and faster rate, asking for the light to be on or off, then the bartender would just have to throw them out because there is no end result that they are all gravitating towards.
So processes things like this are only mathematically valid if there is convergence. Of course, none of these things are physically valid, because you can't chop things up infinitely small like that. A similar situation is that it is mathematically valid to take a solid sphere, break it up into a few pieces and put it back together as two spheres of the same volume as the original one. Mathematically valid, but not physically.
|
askscience
|
As the others say, this fundamentally changes the problem. You could see it this way: It takes a finite amount of time to flip the switch, and so it would suddenly take forever to reach the bus.
Even if we consider the abstract, math-like situation where the flip is infinitely fast, you're basically asking if there is an odd or even number of terms in an infinite sum, which is like asking if infinity is an odd or even number. That doesn't make sense, since infinity isn't a number. It's like asking if the stegosaurus is an odd or even dinosaur.
Still, it's a good demonstration of the counter-intuitiveness and basic difficulty of understanding infinity.
|
askscience
|
Yes, it infinitely approaches 1 without reaching it throigh basic addition. The sum of all the numbers ultimately leads to (n-1)/n through direct addition, regardless of how large n is.
Using some fun maths, you can instead convert that infinite sum into a simple formula, and cancel out the infinitely recurring fractions. A simple way you look at it - set the first sum to a variable (x=1/2+1/4+1/8+...), pull out a common fraction (1/2) and get a second formula (x/2=1/4+1/8+1/16+...), and then solve for x (x - x/2=1/2) , (2x - x =1) , (x=1).
I'm hoping nothing is wrong there, I'm still groggy and don't want ppl up my ass about maths.
|
askscience
|
Then the answer depends on what you mean by "smallest possible".
For processes that take place over shorter timescales than the Planck time, our understanding breaks down. We cannot predict the results of an experiment that takes place over that timescale. In that sense, the Planck time is the "smallest possible" time, although it's not really a sharp boundary -- the lower and lower we go the worse our predictions get.
If you are asking: is time discrete and pixellated? And if so, what is the smallest time step? Then the answer is -- we don't know. All our current highly successful theories of physics are built on the assumption that time is continuous and not discrete, so there is no smallest time step. (Otherwise it becomes mathematically complicated). We do know that the theories break down at some point, but if we ever have a complete theory of everything, then it is not known whether this final theory will feature discrete time.
|
askscience
|
That just goes to the second point they made. At that point it goes to abstract mathematics and has no really physical meeting. It's the same as asking "what is the largest integer?" It has no real meaning because by definition there can be no largest number.
Asking if the "largest number" is even or odd goes against the axiom that there does not exist an upper bound to the number of integers. If you are working in a framework which is based on A, B, and C, asking yourself "how would this framework behave if A is false" is meaningless because if A is false, you are no longer working in the same framework.
|
askscience
|
Just for clarity: the *finite sum* 1/2 + 1/4 + ... + 1/2^(n) can be rearranged and subtracted like you say (since it's just normal addition!) to give us a value of 1 - (1/2)^(n) for any finite n. Indeed,
1/2 = 1 - 1/2,
1/2 + 1/4 = 3/4 = 1 - 1/4,
1/2 + 1/4 + 1/8 = 7/8 = 1 - 1/8, etc.
Now the *infinite* series is defined as the limit of all these partial sums. It is easy to see that the 1/2^(n) term can be made arbitrarily small; for any tiny distance d away from 0, we can make 1/2^(n) smaller than d by picking 2^(n) greater than 1/d, i.e., n greater than log_2(1/d) = -log(d)/log(2). So the limit is 1.
|
askscience
|
Eh, I'm not too sure this can be done meaningfully. Movement can occur on really small length scales -- we are already able to measure subatomic movements, and if you are liberal with the definition, much smaller movements. Even the Planck length isn't a limit, because it is only our understanding that breaks down at that point. If you ask the question: "Are movements smaller than the Planck length possible?" then the correct answer is not "no", the correct answer is "we don't know".
|
askscience
|
Not reaching the end, or approaching a limit, is different than a limit not existing. Some common reasons a limit would not exist are it's not bounded (think f(x)=x, it just gets bigger as x gets bigger) or repeating series (1, 2, 3, 1, 2, 3, .... for example has multiple converging SUBseries but does not have a limit for the series as a whole)
Also these concepts are definitely something that someone with a GED education could understand but math can be a bit counterintuitive and until you actually formally work out something, it can be easy to misunderstand or not totally grok a concept. I love the idea of people that haven't traditionally thought about these things pushing themselves to really think about the concepts.
|
askscience
|
I was coming from a position that since one cannot take an infinite amount of step (only approach a value) that theirfor time has to be discrete . Obviously I can't test my hypothesis since I don't know physics but all your answers definitely gave me a deeper understanding in a subject I'm not familiar with. Thank you. Now I know that planck time isn't what I meant. But it's an amazing concept.
|
askscience
|
I don't see the distinction to be honest and I will argue there is no distinction.
If I break down action A into discrete sub-actions D_1, D_2, ..., then there is no distinction between performing action A and performing actions D_1, D_2, .... They are both equivalent/identical.
The resolution to this seeming paradox is that the sum of an infinite number of discrete values can be finite and hence the sum of an infinite number of discrete actions can result in one single finite action.
|
askscience
|
You're right. The whole (or natural, as they're called) numbers have a certain (infinite) cardinality, which is often referred to as Aleph-0. The real numbers (which is the set of all possible numbers, decimals, the pi kind of strange numbers and all that) are more in number than this Aleph-0. This was proved by Georg Cantor, in what is called the Diagonal Argument.
The set of real numbers between 0 and 1 has the same size as the set of ALL possible real numbers (yes, this seems crazy, but it's true) so that number is indeed larger than the size of the set of all whole numbers.
|
askscience
|
Sort of. You're referring to the [cardinality](https://en.wikipedia.org/wiki/Cardinality) of the sets of those two types of numbers, which is basically the size of the sets. Whole numbers (integers) are [countable](https://en.wikipedia.org/wiki/Countable_set). Real numbers are [uncountable](https://en.wikipedia.org/wiki/Uncountable_set).
The intuition is that you can "count" all of the whole numbers without skipping any if you have an infinite amount of time. I.e. if you start counting at 1, you can pick a number and know that eventually you'll reach it. You can't do that for real numbers.
|
askscience
|
Think of it this way
Between 0-100 there are 100 whole numbers
If we also include all 0.5 numbers, there are twice as many numbers between 0-100
All fractions between 0-1 are 1 divided by one of these numbers. For the first set, there are 100 but for the second there are 200 fractional numbers. So if we limit the whole numbers 1-100 but say that the fractions can be 1/1.5, 1/2, 1/2.5 etc etc there are more numbers between 0-1 than whole numbers 1-100
Its the same thing for infinity, just with way more numbers. Not all infinite sets are equal in size if that makes sense. Hopefully this helps but im not sure i explained myself well
|
askscience
|
Adding to that, while irrational numbers are not countable the way that whole numbers are, that does not mean there are *more* irrational numbers than whole numbers. They are both still infinite and not traditionally comparable. All it is saying is that we can make up a rule where every rational number maps to a unique natural number, or every integer maps to a unique natural number, but we can't make up a similar rule for all irrational numbers.
Edit for correctness: it's easy to map any irrational to some natural number, but we care about that natural number being unique in the mapping/one-to-one.
|
askscience
|
There is no paradox
> If I break down action A into discrete sub-actions D\_1, D\_2, ..., then there is no distinction between performing action A and performing actions D\_1, D\_2, .... They are both equivalent/identical.
I should have worded what I said better, but here's what I was getting at:
Assume running to a bus stop is action A, where a1 is running halfway, a2 is running half the remaining distance, etc. Obviously performing all a\[1-infinity\] is identical to performing action A.
If you have an infinite amount of discrete divisions b1, b2, b3 that doesn't mean that there is necessarily any action B which corresponds.
There are some infinite series that have a definite value (1/2 + 1/4+ 1/8 ...) and there are some which don't (1-1+1-1+1...)
In this case there's selection bias. You know that we can run to a bus stop, because we have observed that phenomenon. You also know that there's an infinite series which corresponds to it because it's intuitive and easy to visualize. Your mistake is that you assume another infinite series must have a distinct end result
​
|
askscience
|
I never understood why this is a paradox. The infinite series approaches 1, but never arrives. The person walking 1 m/s is never constrained by that fact because at a constant speed you just blow through the infinitely large number of calculations at the end. And who is to saw he actually touched the sign? That type of measurement could never be made.
But the original post presumes you're spending a fixed time during each increasingly small distance, when in fact you're spending less and less time until you eventually stop or use blow through the limit.
|
askscience
|
Not sure if this correct but I remember doing this proof in my undergrad
Sum from 1 to inf( 1/2^n ) = (1/2 + 1/4 + ...)
Let S = Sum from 1 to inf( 1/2^n )
S = (1/2)(1+1/2+ 1/4 +...)
2S=(1+1/2+ 1/4 +...)
2S-1=(1/2+ 1/4 +...)
2S-1=S
S-1=0
S=1
Expand to any sum in the form 1/n
S=(1/n+1/n^2 +...)
S=(1/n)(1+1/n+ 1/n^2 +...)
S*n= 1+1/n+ 1/n^2 +...
S*n -1= 1/n+ 1/n^2 +...
S*n -1 = S
S*n-S=1
S(n-1)=1
S=1/(n-1)
The the trick of the proof is to recognize that you have the same sum on the right hand side after factoring out the fraction
|
askscience
|
No, it's more like the [Grandi Series](https://en.wikipedia.org/wiki/Grandi%27s_series). Turning the light on is a +1, turning the light off is a -1. The sum then oscillates between 0 and 1. Some of the more creative ways of assigning a value to the Grandi Series are invalid because of [reasons Thomson laid out himself](https://en.wikipedia.org/wiki/Thomson%27s_lamp#Mathematical_series_analogy). If a sum diverges by going to infinity, then it is still a valid sum with a value. A sum is "invalid" if it never approaches any value, even infinity.
It's also actually a little bit like the [graph of sin(1/x)](https://www.desmos.com/calculator/t8hohza1hs), which can have some [weird properties](https://en.wikipedia.org/wiki/Topologist%27s_sine_curve).
|
askscience
|
I'm going to use the more standard form of 1+r+r^(2)+r^(3)+...=1/(1-r).
What you have [is the idea of finding the value](https://en.wikipedia.org/wiki/Geometric_series#Formula), but a key, important part of this is [actually proving that S exists and is finite](https://en.wikipedia.org/wiki/Geometric_series#Proof_of_convergence). What you have done is show that if S exists and is finite, then it must be 1/(1-r), but there are cases where S is not finite. Take, for instance, r=2, which is the series 1+2+4+8+... In this case S is infinite, so it cannot equal 1/(1-2)=-1. But, given [different number systems with different geometries](https://en.wikipedia.org/wiki/P-adic_number), this S *is* finite and so it equals -1.
|
askscience
|
This kind of reminds me of the
.333333~ + .66666~ = 1
Thing. Similar concept that infinite numbers when added together can equal a whole number. The way the above made sense in my head was to think of it like:
1/3 = .33333~
2/3= .66666~
And because 1/3 + 2/3 = 1
.33333~ + .66666~ must also = 1
Might be basic to some in here, but that sort of blew my mind the first time I heard about it.
P.S. I couldn’t figure out how to do the bar over the numbers on mobile so the “~” is meant to mean infinite.
|
askscience
|
The only thing I’ve ever taken away from any discussion about Zeno’s paradox is that time and space must share either discrete units of measurement that correspond, or must both be infinitely divisible.
Otherwise you’d get a disconnect at the smallest units of distance and units of time.
Like, if the Planck length is the smallest unit of distance, then the smallest unit of time must be the time it takes to traverse one Planck length at c.
Perhaps that’s just really obvious and I’ve wasted everybody’s time.
|
askscience
|
It depends on the light bulb. If it's a florescent or another type with a significant start up time, it will appear to be off due to the fact that it was eventually in a state where the switch was being flipped faster than it could turn on. If the bulb has a negligible start up time, it will appear to be on due to our persistence of vision.
Our measurement of it affects our perception of it.
|
askscience
|
Using the set of most commonly used mathematical axioms, this series is divergent. There are other sets of axioms that can be used in which this series is convergent, and so it would make sense to assign a value to the final light, such as 1/2 or 1/4. None of the possible sets of axioms is more or less correct than any other, it just changes the nature of what things like equality mean within that set of axioms, and there is usually a trade off between the "value" of the results that can be obtained. Ie, within complex analysis, it would make sense to assign a value to the infinite sum of 1+2+3+4...=-1/12, and this ends up providing some pretty insane insights into the distributions of prime numbers among other things, but obviously conflicts with our "instinctual" idea of what the sum of the natural numbers would be.
|
askscience
|
You don't use the mappings to compare some set to another set, you use a mapping from one set to a cardinal to establish its cardinality. Then you can compare the cardinals with each other.
We don't even need that either, because there is clearly a relationship. The simple fact that there does not exist a surjection N-->R means that R is larger than N, because that's exactly how mathematicians define "larger." It exactly coincides with how we define size on finite values too, so it's not artificial or anything.
|
askscience
|
But even if you remove the countability distinction from it, it seems intuitive that there is still a significant mathematical difference between say, the number of whole numbers and the number of whole numbers divisible by 2. Both are countable, so that's not the deciding factor. Yet one is N (infinity) and the other is N/2... infinity/2, which is still infinite but in some meaningful way less than the other infinity. Or is all such distinction lost without the countable vs uncountable boundary?
|
askscience
|
The Planck length isn't the smallest unit of distance, it's just the unit of distance which comes naturally from the ratio of several universal constants.
That is, no matter what system of measures you use, physics has several universal constants. Whether you're using Metric units or Imperial Units or Thargoid Traditional Measures, you'll get the same value of c, the same value for the permeability of vacuum, the same value of Planck's constant, etc. If you take a ratio of these fundamental physical constants such that it works out to units of speed, you get the Planck length.
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.