Wikipedia:Reference desk/Archives/Science/2019 October 2

= October 2 =

angular momentum of primordial black hole
Are primordial black holes expected to form with a lot of spin? If a very rapidly spinning, but less than stellar mass, black hole existed, how would the spin affect observations of it?Rich (talk) 01:52, 2 October 2019 (UTC)


 * This is pretty technical, but the takeaway is that most primordial black holes would be expected to have little spin: https://arxiv.org/pdf/1704.06573.pdf Dragons flight (talk) 14:49, 2 October 2019 (UTC)

black hole
Mass can be neither created nor destroyed. Then, where does mass 'sucked in' by a black hole go ? — Preceding unsigned comment added by 223.191.1.234 (talk) 03:34, 2 October 2019 (UTC)


 * It doesn't go anywhere, it stays right there, just in an infinitely small point. The mass is maintained, but the volume is not. See black hole.


 * But, incidentally, mass and energy are somewhat interchangeable, see mass-energy equivalence. SinisterLefty (talk) 03:57, 2 October 2019 (UTC)
 * Well, it crosses the event horizon, and thus adds it's own mass to that of the black hole. What goes on inside of the event horizon breaks physics, so it's kind of hard to say what happens after that.  There's lots of good videos describing what happens to objects, like this one, both from their own perspective and from outside observers, as they approach and cross the event horizon.  However, by definition, observation of what happens inside the event horizon is impossible, and models predict impossible things like "infinitely small points of infinite density" which is the Gravitational singularity, as all singularities are, an indication that the theory has broken down.  As noted at that article,  "current knowledge is insufficient to describe what happens at such extreme densities." We can say fairly certainly, however, that whatever it is, it is heavier by the added mass/energy of the infalling object.  The mass doesn't go away, it becomes part of the black hole.-- Jayron 32 04:32, 2 October 2019 (UTC)


 * Can it really get down to a true singularity? Might QM, say the Pauli exclusion principle, prevent that? Bubba73 You talkin' to me? 04:56, 2 October 2019 (UTC)
 * Probably not. As I noted already, when I said "all singularities are an indication that the theory has broken down" what I meant by that is that "all singularities are an indication that the theory has broken down".  In other words, the theory predicts the singularity, and any time the math in a theory predicts a singularity, we know the theory that used that math is wrong.  In most cases, such mathematical singularities have produced the need for new theories, as in when the singularity we call the ultraviolet catastrophe led to the development of quantum mechanics.  In even simpler terms, when your answer is "infinity", and yet the thing exists and is not infinite, then the theory needs some work.  There's some hope that a workable theory of quantum gravity might help resolve some of these issues, but quantum gravity itself seems to break physics in other more problematic ways, which is why we don't have any good bit there. -- Jayron 32 11:52, 2 October 2019 (UTC)
 * I don't know the details, but they are probably only taking into account General Relativity. Even if the size is the Plank length, the density would not be infinite.  Bubba73 You talkin' to me? 22:26, 2 October 2019 (UTC)


 * As stated above, we only know that our current best theories are unable to cope, so you cannot rely on them (as of now) to state what might or might not be prevented Gem fr (talk) 08:26, 2 October 2019 (UTC)
 * The available techniques for modeling quantum systems make assumptions that are not valid near the predicted singularity, so no. Maybe it does, but that answer will need to wait for probably a new theory of physics. To give you a sense of the ah, let's say "intensity" of a black hole, let's make an operational definition of gravity. We'll say it's the force you would need to supply with a rocket to hover over a celestial body. On Earth, you would need 9.8 Newtons of force per kilogram of mass to stay in a hover. If you were to approach a black hole from outside the event horizon, the force you would need to fire out the back of the rocket would increase asymptotically. But it may surprise you to know the asymptote is not reached at the center - it's reached at the event horizon. That is, to hover at the event horizon, you would need the rocket to supply infinity newtons of force. Once you cross that horizon, there actually is no amount of force that can keep you in a hover - you are going down. If you try to compute the required force to even stand still, you get an imaginary number. In a schwarzschild black hole, there is no winning against gravity. But then, general relativity does not model nuclear forces, so this could be wrong. Again, need a new theory. Someguy1221 (talk) 08:39, 2 October 2019 (UTC)
 * I may add that such singularity is just another kind of infinity, mathematically some ]0,1[ bracket (instead of [0,1]), so, if you can imagine an infinite universe, then you should be able to imagine some "infinitely deep well, with infinitely shrinking diameter (but still a finite number above zero), with no bottom". Same with time and big bang, actually. Gem fr (talk) 08:44, 2 October 2019 (UTC)
 * "Turtles all the way down"? ←Baseball Bugs What's up, Doc? carrots→ 11:51, 2 October 2019 (UTC)
 * Ancient Indians were not afraid of recursion and infinity, so there may be more truth than joke in the story
 * The part that is funny is imagining that the Earth being carried on the back of a giant turtle somehow helps to explain the universe (if we imagine gravity pulls in a constant direction, not toward the center of a spherical Earth), when then you need to explain what supports the turtle, and what supports that, etc. Similarly, saying that God(s) created the universe doesn't help to explain it, as then you need to explain where the God(s) came from. Imagining an infinite series of turtles, or gods, doesn't help, either. It just adds more complexity. SinisterLefty (talk) 19:05, 2 October 2019 (UTC)
 * ye, well, you just first stated that you are finite-sort-of-a-man, who needs a First cause or nothing, and cannot be happy with an infinite string of causes. Just fine. And then you mention a first cause (even of a collective kind) is just as silly as well. Just fine, too, except that it contradicts your previous point. I think you should make make up your mind... Even though I am not sure this is the place Gem fr (talk) 21:14, 2 October 2019 (UTC)
 * I cannot remember where I first read this philosophical way of looking at it, but there is an argument to be made that the absence of a prime mover is more parsimonious. It is assumed that not only occam's razor, but also that "simpler" has a hierarchy of types of assumptions. Assuming the existence of more things exactly like the things that are known is the simpler than assuming the existence of things unlike what is known. And that still is simpler than assuming the existence of entirely new rules for what is allowed to exist. And among new rules, those which are most like existing rules are preferred still. We are thus left with a choice when we inevitably arrive at the beginning of the universe, or if that is explained, trying to figure out why the laws are what they are. We ultimately have a choice, "there exists an explanation that is based on a law of nature, but we do not or cannot know what it is" or "there exists a deity or other power that is completely unlike anything ever known, the end." Given that both are untestable cop-outs, but the former assumes no new types of rules, it is simpler, and should be preferred. Someguy1221 (talk) 22:07, 2 October 2019 (UTC)
 * your whole development rely on law of nature, and some reason for it to pop out, which as observed by Spinoza means nothing but god, so, the line of reasoning you mentioned is just hiding god-like creative power under the carpet and pretending it doesn't need it, when it does. You cannot make thing simpler just by pretending the elephant in the room does not exist...Gem fr (talk) 07:22, 3 October 2019 (UTC)
 * That's... ridiculous. If a new particle were discovered whose properties were not explained by any known law of physics, "I bet there is a law of physics that explains this particle" is a wee bit simpler than "Well obviously it's controlled by an incomprehensible deity." For that matter, there are a number of open questions in physics that are not yet proved to have human-comprehensible answers, but this does not mean the assumption that an answer exists in the form of physics is equivalent to assuming the existence of god. Someguy1221 (talk) 08:48, 3 October 2019 (UTC)
 * Human history of science is all about going from "Well obviously it's controlled by an incomprehensible deity" to "I bet there is a law of physics that explains this", meaning the former is a wee bit simpler to imagine than the latter (and, indeed, if it is just controlled by some deity, there just nothing to look after or to do, except praying. Way simpler!). And, mind you, this applies just as well when it to be explained is "law of physics", so, either you recursively and infinitely explain each law of physics by another, deeper, law of physics, endlessly, or there is some starting spark with god-like creative power to set them all. You say we should not call it god? well, just fine, I guess you know what a god is and isn't, and we should take your word for it, just like from a clergy member. Anyway each branch (prime mover, or no prime mover) is equally beyond understanding and untestable, so there is no way to prove or disprove your claim that the "no prime mover" solution is "simpler and should be preferred"; it just seems that for some reason most people on Earth settled for the "prime mover" solution, and my guess is they found it simpler and making more senseGem fr (talk) 10:22, 3 October 2019 (UTC)
 * The definition of "simple" that you appear to be using does not align with the definition of "simple" that is used by either philosophers or scientists. Someguy1221 (talk) 11:23, 3 October 2019 (UTC)
 * Pretty sure William of Ockham, and most of them philosophers or scientists actually, believed in "prime mover", though. But, whatever, then again your claim is not provable nor disprovable, and it doesn't matter to me, so, if you want to have it that way, so be it Gem fr (talk) 11:56, 3 October 2019 (UTC)
 * It's actually not my way, which I have not stated. I am explaining an actual argument that is put forth. I don't care if anyone believes it, I was simply trying to make clear what the argument actually is. Someguy1221 (talk) 12:19, 3 October 2019 (UTC)


 * Semi-reiterating what some others have said: most physicists expect that a proper theory of quantum gravity will describe without any singularities what occurs beyond the event horizon. A singularity is simply what general relativity predicts inside the event horizon, but we already know GR is not a complete description of the universe, because it's a classical theory and only describes gravity. --47.146.63.87 (talk) 03:08, 3 October 2019 (UTC)
 * Fixed the YouTube link. --47.146.63.87 (talk) 03:08, 3 October 2019 (UTC)

Ancestors of human beings during the age of dinosaurs
I was thinking, if any animal existed during the times of dinosaurs through which human beings could have evolved. I searched the net, and found that some rat was our oldest ancestor.

https://www.seeker.com/ancestor-of-humans-lived-with-dinosaurs-1768223125.html

https://www.irishnews.com/magazine/science/2017/11/07/news/ancient-rat-from-dinosaur-era-was-oldest-human-relative-1181112/

Did this rat evolved into a primate? — Preceding unsigned comment added by 2402:3A80:A9F:1A1B:C12F:6490:7F92:401 (talk) 14:45, 2 October 2019 (UTC)


 * You can go even further back, to the synapsids!
 * Your example wasn't a "rat", it was a "small furry proto-mammal". It evolved into us, it evolved into rats. But we didn't come from rats. Andy Dingley (talk) 14:59, 2 October 2019 (UTC)


 * (...after edit conflict)
 * Have you read our articles, like the § early evolution of primates, and Evolution of mammals?
 * Very early mammals weren't exactly rats, but they had superficial similarities to modern rodents. We have various scientific words to describe what they were, in various levels of specificity: for example, we might call one of our ancestors a basal primate to generically describe that they had already evolved the distinguishing basic features of modern lemurs, apes, and humans; or we might call an entire group of our ancestors mammaliaforms or cynodonts to include specific characteristics for a huge group of animals in the fossil record.  These are a lot more precise than the word "rat," which we generically use to describe certain specific types of animal that aren't in the direct evolutionary path toward the modern human.  On the other hand, we can use the plain-english language word "rat" to imprecisely describe any furry small creature - in which case, sure - we evolved from some kind of rat monster.
 * Nimur (talk) 15:02, 2 October 2019 (UTC)
 * First of all that animal is not a rat, though modern rats are descended from it. That animal noted in the article is the oldest known placental mammal, being Durlstotherium; whereas rats are rodents, a different type of placental mammal that evolved later.  That animal is classified as a Eutheria, from which all modern placental mammals descended.  The first rodents did not evolve until well after the time period noted in the article you cite, rodents date no earlier than about 66 MYA, while Durlstotherium thrived 145 MYA.  Since both you AND rats are a placental mammals, then the species noted in that article is both an ancestor of you and of rats. -- Jayron 32 15:07, 2 October 2019 (UTC)
 * If you want to go far back in time, then LUCA is your "man". If you want some reasonably significant ancestor, you want the (unknown, AFAIK) animal of Euarchontoglires which underwent whole genome duplication sometime during Late Cretaceous, not just some furry. Gem fr (talk) 16:00, 2 October 2019 (UTC)

Do all objects (different masses) feel equal force in the Equivalence Principle?
Here on earth

Both hammer and feather fall at the same rate.

Gravitational force = F = mg, where g = GM/R^2 is gravitational acceleration - constant for all objects.

But the gravitational force on hammer is greater than the gravitational force on feather - not constant for all objects

Equivalence Principle - Thereon the floor of elevator in free space

Both hammer and feather stay at their original position. It is the elevator which accelerates upward @ 9.8 m/s/s and catches them at the same time on its way up

The force at which the elevator is accelerating upward is the same on both the hammer and the feather initially - both are being pushed upward with the same force

Non-gravitational force = F = ma where "a" is linear acceleration - not constant for all object

Since F= ma, F is the same on both feather and hammer as said therefore

For Hammer; a = F/m1 = 9.8 m/s/s where m1 is the mass of hammer

For Feather ; a = F/m2 = value of acceleration but > 9.8 m/s/s - where m2 is mass of feather. This is because the m2 < m2 therefore shouldn’t feather fly upward with its final velocity and losses its connection with floor of accelerating elevator

So is this a contradiction if not please explain?Eclectic Eccentric Kamikaze (talk) 18:30, 2 October 2019 (UTC)


 * Re: "The force at which the elevator is accelerating upward is the same on both the hammer and the feather initially - both are being pushed upward with the same force." That's not true. The floor of the elevator exerts more force on the more massive object. Let's say the elevator is in intergalactic space, to eliminate any significant effects from gravity. Then F = ma applies, not F = mg. So, the acceleration provided by the elevator is the same on both objects, thus the force applied is proportional to the mass. That is, F is proportional to m. Astronauts encounter this effect in space, where moving more massive objects requires considerably more force, to overcome inertia, than moving light objects. SinisterLefty (talk) 18:37, 2 October 2019 (UTC)


 * Just to clarify your misconception here. F=mg, the g is equal, but the m is not.  A single feather has a smaller mass than a single hammer, so the feather has a smaller gravitational force on it than the feather.  Every conclusion you draw after that is incorrect.  Indeed, the importance of the equivalence principle is only that the "m" in F=mg is the same m as in F=ma, or that Newton's law of universal gravitation uses the same definition of mass as is used in Newton's second law of motion.  Prior to Einstein, these two masses were only conjectured to be the same.  Einstein showed that they were actually the same.  All objects do NOT feel the same force, according to the equivalence principle.  What is the same is that objects of the same mass experience the same forces regardless of whether they are accelerating due to gravity or accelerating due to thrust.  -- Jayron 32 18:49, 2 October 2019 (UTC)

wouldn't the aforementioned elevator decelerate upon meeting with the joint resistance of feather and hammer at the same time initially and then more due to the greater mass of hammer alone therefore i was thinking that the feather would fly upward at the time when feather cease its resistance but the elevator has still to face rest of resistance of hammer. — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 20:15, 2 October 2019 (UTC)
 * wot??? did you, to begin with, understand that F=ma means that F1=m1g is different from F2=m2g, as explained by Jayron32 ?
 * Do you have in mind some kind of rebound effect of the kind observed at billiard? This rely on the elasticity of objects, so a feather and a hammer would indeed react differently, but not because of their mass Gem fr (talk) 21:23, 2 October 2019 (UTC)

Sorry but wouldn’t the pounding effect be the same on both objects? Anyway

Aforesaid elevator decelerates less when it hits the Feather on its way alone as compared to when it hits Hammer on its way alone – if yes then would force on feather be still less than the hammer if hit by elevator together.

Similarly, if a person stands firmly on the bottom of the said elevator with his head down, would he still feel the same effect of equivalence principle?

The direction of the weight force of hammer is in the direction of "g" at which it falls towards to earth.

I’m having trouble with finding the direction of the weight force of hammer (not the direction of the development of stresses) in case of accelerating elevator in equivalence principle. Would it be in the direction “a” of elevator? — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 23:16, 2 October 2019 (UTC)


 * The problem is that you're trying to create a complex mechanics problem. You're saying you have a large moving object that collides simultaneously with two smaller objects, meaning you are dealing with a 3-body collision. In a high-school physics course, we would either only deal with 2-body collisions, or we would stipulate that the largest object (the elevator) is unaffected by the two smaller objects, allowing us to consider each separately. The reality is complicated, and it cannot be solved with just equivalence. Someguy1221 (talk) 00:11, 3 October 2019 (UTC)

Thanks for the responses - How about the declaration of the elevator - Acceleration of 9.8 m/s/s of the elevator reduced when it meet any resistance (from small to large) on its way up while acceleration due to due to gravity "g" is constant (or increases) when an object hit the ground. — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 01:15, 3 October 2019 (UTC)


 * It would all depend on what's driving the acceleration of the elevator. A motor would indeed be slowed down when it suddenly accelerates more mass, at least initially. It might well then increase the force by using more electricity to respond to such a slowdown. However, the reduction in the acceleration rate from adding a feather's mass to the system is likely so small it would not even be measurable. SinisterLefty (talk) 01:46, 3 October 2019 (UTC)

its not just feather or elephant but this applies to the concept — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 02:45, 3 October 2019 (UTC)


 * What you're asking about really is collision mechanics in the context of an accelerating elevator. So really, you're talking about an accelerating object colliding with something. What separates this from a typical invocation of the equivalence principle in a classical mechanics problem, is that you normally only do that when gravity or acceleration are uniform. You wouldn't affix your observer (conceptually) to the elevator's accelerating frame of reference if your problem assumes it is going to collide with something that alters its acceleration. You've actually just made the problem unnecessarily difficult, and would instead stay in a hypothetical inertial reference frame like that of the elevator shaft. The problem you present is also underspecified. What is accelerating the elevator? How heavy is the elevator? How is the elevator connected to the shaft? This is far from simple. Basically, your question is unanswerable because the problem inherently assumes the existence of unspecified forces acting on the elevator whose properties are unknown. Someguy1221 (talk) 03:04, 3 October 2019 (UTC)

Let there is already an observer in cabin/elevator (C) (on a rocket engine) which is accelerating upward @ 9.8 /m/s/s in free space. Two objects an apple (A) and steel sphere (B) are at rest (not absolute) and at distance of height h from the said approaching elevator in free space. The size of both A and B are the same. After some time both A and B finally hit the floor of C at the same time.

We all know that there is only one source of elevator, which accelerates it upward. So how would observer in C explain the whole event after A and B hit the floor? — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 17:25, 3 October 2019 (UTC)


 * You still never said what is accelerating the elevator, or given any information from which one could predict how the elevator will react to hitting something. If the elevator assumed to be a magical device that always has constant acceleration, the observer would see things the same as if the elevator were firmly planted on the surface of the earth. Someguy1221 (talk) 20:52, 4 October 2019 (UTC)

It is the same elevator Einstein used for equivalency purposes (Equivalence Principle). — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 23:37, 4 October 2019 (UTC)


 * That elevator accelerates at a constant rate forever no matter what happens, so the observer just sees two objects falling and hitting the floor like normal. Someguy1221 (talk) 00:08, 5 October 2019 (UTC)

it cant be forever anyway I'm not going to fall in that type of discussion but my question was "So how would observer in C explain the whole event after A and B hit the floor?" we all know what does AFTER means. Would the elevator decelerate after A and B touch the floor of C at the same time? since B is heavier than A, therefore, would A fly up as explained above? It's for interested people only. thanks — Preceding unsigned comment added by Eclectic Eccentric Kamikaze (talk • contribs) 04:56, 7 October 2019 (UTC)