Wikipedia:Reference desk/Archives/Science/2024 October 31

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October 31

Does an accelerated body at (instantaneous) rest, emit gravitational waves?

According to LIGO, "every physical object that accelerates produces gravitational waves". But how can the GWs, emitted from the accelerated body, carry away momentum from the body being at (instantaneous) rest? HOTmag (talk) 14:30, 31 October 2024 (UTC)

Perhaps because "instantaneous rest" is a mathematical abstraction, not a real-world condition that applies to a real-world accelerating body? Others more expert can doubtless address this concept better.

My impression is that you are just making up puzzles using random concepts, as you have previously been doing under this and your previous User name HOOTmag for more than ten years. I'm not intending to play anymore. {The poster formerly known as 87.81.230.195} 94.6.86.81 (talk) 15:39, 31 October 2024 (UTC)

Sorry, but my question is serious (like all of my questions here). I really don't know how to anwser it correctly (I don't remember I ever made up puzzles using random concepts). HOTmag (talk) 16:11, 31 October 2024 (UTC)

Zeno of Elea c. 490 – c. 430 BC tried "seriously" to divide time into instants. In his arrow paradox, Zeno states that for motion to occur, an object must change the position which it occupies. He gives an example of an arrow in flight. He states that at any one (durationless) instant of time, the arrow is neither moving to where it is, nor to where it is not. It cannot move to where it is not, because no time elapses for it to move there; it cannot move to where it is, because it is already there. In other words, at every instant of time there is no motion occurring. If everything is motionless at every instant, and time is entirely composed of instants, then motion is impossible. Philvoids (talk) 16:18, 31 October 2024 (UTC)

Zeno's paradoxes are well known, but the mistake hidden in them has already been discovered, actually after Calculus was discovered. The way to remove Zeno's paradox is achieved by the concept of mathematical limit.

But my question has nothing to do with those old paradoxes, because as opposed to them, I can phrase my question without using any instantaneous velocity, but rather with the rigorous concept of mathermatical limit. I've only used the concept "instantaneous rest" for letting you grasp my question intuitively. If I had used the concept of mathermatical limit, I would have phrased my question otherwise, but the question would have still remained. For example I could ask: What's happening to the GWs emitted by the body, when the body's velocity approaches to zero? Does the GW emission approach to a zero radiation emitted by the body? HOTmag (talk) 16:48, 31 October 2024 (UTC)

Given that the emission of a GW is continuous process, the question does not really make sense. Note that for a continuously accelerating body, the instant of zero velocity has a length of precisely zero seconds, and hence the amount of momentum transferred during this instant is also zero. But that is true for every (length zero) instant. You need to integrate over a non-zero time period if you want to see a real transfer or momentum (or energy). --Stephan Schulz (talk) 22:31, 7 November 2024 (UTC)

Some days ago, I already referred to integration: See below, my paragraph beginning with the (green) words: "the accelerated object". HOTmag (talk) 02:17, 8 November 2024 (UTC)

Yes, but if you integrate over a zero time span, the result is zero. If you integrate over a non-zero time span, the object is not at rest during that time. --Stephan Schulz (talk) 14:17, 8 November 2024 (UTC)

The statement is simplified and not strictly true. A linearly accelerating body without rotation (or with rotational symmetry around the axis of rotation) does not emit gravitational waves — as discussed in another thread the quadrupole moment of the mass distribution needs to change. This is true for almost any real-life body or mass distribution, and this argument could be used to justify making that statement in a non-technical web page for the lay public. The instantaneous rest thing is fine, by the way and in a frame-dependent way, but not particularly relevant here. --Wrongfilter (talk) 16:50, 31 October 2024 (UTC)

Thank you for this important clarification. I'm quite amazed. Previous threads mentioned the quadrupole moment of the mass distribution, but none of them mentioned what you're caliming now, that "A linearly accelerating body without rotation...does not emit gravitational waves". On the contrary, some users claimed that an acceleration was sufficient for emitting GWs, and nobody disagreed, so I thought they were correct. Now you're surprising me.

Anyway, according to your clarification, I wonder now why our article Gravitational wave claims "An isolated non-spinning solid object moving at a constant velocity will not radiate". Aren't the words "moving at a constant velocity" redundant? HOTmag (talk) 17:02, 31 October 2024 (UTC)

"A linearly accelerating body without rotation (or with rotational symmetry around the axis of rotation) does not emit gravitational waves ... This is true for almost any real-life body or mass distribution." (emphasis mine).

This made me curious. There are exceptions to this rule? Would you be willing to give some examples, please? (Asking as a member of said lay public.)

Thanks! -- Avocado (talk) 12:51, 1 November 2024 (UTC)

It's a word of caution. If I had written "all real-life bodies" somebody would have blasted me for that. If you wish you can read it as almost every. --Wrongfilter (talk) 13:06, 1 November 2024 (UTC)

Yes, this is also what I wondered about, but I finally didn't ask you about that, because I guessed you had only wanted to use a word of caution, as you say now. So it seems you don't rule out NadVolum's reservation "a constant linear acceleration doesn't generate gravitational waves", do you? HOTmag (talk) 13:18, 1 November 2024 (UTC)

Just a little extra on that - a constant linear acceleration doesn't generate gravitational waves. NadVolum (talk) 18:40, 31 October 2024 (UTC)

Now you add: "constant". But if the acceleration is not constant, then my question in the header comes back... HOTmag (talk) 18:45, 31 October 2024 (UTC)

A single, accelerating body doesn't even exist: conservation of momentum says that there must be at least second body, accelerating in the opposite direction. And although a single body has no quadrupole moment, the pair of two bodies has. So the issue is avoided. PiusImpavidus (talk) 20:24, 31 October 2024 (UTC)

This thread hasn't mentioned a "single" body, and I can't see how a universe containing more than one object avoids the issue. My question is actually: how can the GWs, emitted from a given body accelerated by a jerk (i.e. by a non constant acceleration), carry momentum away from the body being at (instantaneous) rest? HOTmag (talk) 08:52, 1 November 2024 (UTC)

The question is poorly phrased and possibly based on a misunderstanding.

"But how can the GWs, emitted from the accelerated body," The GWs are produced by the body, but the body doesn't do that on its own. The GWs are emitted by the space surrounding the body and its reaction mass. The waves are, as usual, a far-field approximation. The near-field is a bit more complex. "carry away momentum from the body" Who said that? In the discussion a few topics up on spinning rods I mentioned angular momentum. "being at (instantaneous) rest" Here you make the same error as Zeno (good he was mentioned). The accelerated object is at rest for a time interval of zero, so it must emit zero waves during that time interval, as waves are a continuous phenomenon. You have to consider the emission of waves over a time interval equal to the inverse of the wave's frequency. That's rather basic.

BTW, you won't find a constant acceleration in the universe. Also, a speed of zero is physically irrelevant. You can always make the speed zero by coordinate transformation, which cannot change the physics.

Now I'm wondering, you ask questions on general relativity, which I consider academic master's level of physics, yet make such basic errors, third year secondary school. I can't squeeze seven years of physics education in an answer here; that's a pile of physics books. If you aren't making fun of us, then you started reading that pile from the wrong end. I like to assume good faith and love a good physics question, but that's why I don't always respond to your questions. PiusImpavidus (talk) 17:12, 1 November 2024 (UTC)

I like to assume good faith. Thank you, and please keep assuming good faith. Yes, I graduated secondry school not long ago, so I may make mistakes sometimes. Anyway, when I state any statement, I only rely on articles in Wikipedia. If you think my wording is wrong, don't hesitate and please tell where my mistake is, and I will thank you from the bottom of my heart.

The GWs are produced by the body, but the body doesn't do that on its own. When I wrote "GWs, emitted from the accelerated body", I used a wording used in our article Gravitational wave: "This gives the star a quadrupole moment that changes with time, and it will emit gravitational waves". Anyway, if you're trying to claim that the wording in Wikipedia is wrong and that a single body cannot emit GWs, then please don't hesitate to say that (I'm still not sure if that's the case because you haven't said this yet), and I will thank you from the bottom of my heart for removing this error - not only from Wikipedia - but mainly from me, because I've always thought that also a single body can emit GWs (provided that its quadrupole moment varies).

"carry away momentum from the body" Who said that? Again, I'm only relying on Wikipedia. Please see our article Gravitational wave: "Water waves, sound waves, and electromagnetic waves are able to carry energy, momentum, and angular momentum and by doing so they carry those away from the source. Gravitational waves perform the same function. Thus, for example, a binary system loses angular momentum as the two orbiting objects spiral towards each other – the angular momentum is radiated away by gravitational waves".

The accelerated object is at rest for a time interval of zero, so it must emit zero waves during that time interval, as waves are a continuous phenomenon. You have to consider the emission of waves over a time interval equal to the inverse of the wave's frequency. That's rather basic. AFAIK, you can always use the well known mathematical operation called integration, for "collecting (uncountably) infinitely many infinitesimals" of instants at which the accelerated system was at rest, and then you receive a non zero quantity of energy of GWs produced by the accelerated system at those instants of rest. The question is, where was the energy/momentum of those GWs carried away from? But maybe this integration is actually impossible, not only physically - because (as you say): "waves are a continuous phenomenon", but also mathematically - because the set of those infinitely many instants (at which the system producing the GWs is at rest), is always a countable set only. Am I right?

BTW, you won't find a constant acceleration in the universe. Yes, but AFAIK you can always conduct an experiment which can artificially create a constant acceleration. Additionally, AFAIK a constant acceleration is important in theoretical physics, so you can regard my question as a theoretical one.

Also, a speed of zero is physically irrelevant. You can always make the speed zero by coordinate transformation, which cannot change the physics. AFAIK, physics does consider a speed of zero, for many purposes, e.g for deciding whether there is some momentum, and whether there is some kinetic energy, and the like. Additioanlly the speed of zero is important for establishing many relativistic concpets, e.g. proper frame, proper reference frame, proper length - being the longest length a given body can have, proper time - being the shortest life-time a given body can have, rest mass - being the smallest mass a given body can have (for those physicists who make a distinction between a rest mass and a relativistic mass), and likewise. HOTmag (talk) 19:03, 2 November 2024 (UTC)

The multiverse of science

Superhero fiction loves the concept of a "multiverse", that is, infinite universes that exist somewhere and that completely similar to the main universe, except for some details. It can be something big (as in, all heroes are villains instead, something turned the world into a dystopia or a post-apocalyptic wasteland) or something minor (as in, the radiactive spider does not bite Peter Parker but someone else), but in the grand scheme of things the history of the Solar System, Earth, life on Earth and human history are all basically completely the same. Needless to say, that's just a narrative device, one that has led to some awesome stories (and other so-so ones), but no more than that.

But lately I have noticed in actual science publications people who talk about the multiverse, in the real world. And we do have an article about that, Multiverse. Before breaking my head trying to understand the fine details, just a quick question: would the real multiverse be, at least in principle, similar to the multiverse as seen in fiction, or is it a completely different idea that got distorted? And if there are parallel universes, where would they physically be? I dismiss such a question with fiction because of the willing suspension of disbelief, but in science you can't get away that easily... Cambalachero (talk) 18:40, 31 October 2024 (UTC)

Try and wrap your mind round things like Elitzur–Vaidman bomb tester and if you really can explain it all well I'll be glad to listen! :-) NadVolum (talk) 19:06, 31 October 2024 (UTC)

In Multiverse § Types you can see different – sometimes very different – meanings of this term. Authors of pop-science articles who bandy the term accordingly do not all use the term with the same meaning. All of it is purely speculative and in most versions has the problem that the theory is unfalsifiable because it makes no testable predictions that differ from current theory, and is therefore generally deemed to fall outside the scope of science proper.  --Lambiam 20:04, 31 October 2024 (UTC)

It is among the Interpretations of quantum mechanics#Influential_interpretations.  Card Zero  (talk) 23:33, 31 October 2024 (UTC)

If there was only one universe, where would it physically be? For multiple universes, we can use the same answer. If you own a car, where do you keep it? It's a deep question, but it's not an obstacle to the concept of a person owning two or three cars.  Card Zero  (talk) 23:47, 31 October 2024 (UTC)

So the multiverse would be like a multi-car garage? ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:56, 31 October 2024 (UTC)

OK, if you have more than one parking spot it raises the question "how does one parking spot relate to the next, in physical space?", and that's a reasonable question if you know the first parking spot's location in physical space. But when the parking spots are universes, the answer might be "they don't", because nobody ever said the first parking spot was located anywhere anyway.  Card Zero  (talk) 00:08, 1 November 2024 (UTC)

Yes. It would be in some other dimension beyond mere physicality. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:23, 1 November 2024 (UTC)

They are all in your mind.  --Lambiam 07:06, 1 November 2024 (UTC)

I recall one professor contradicting the famous "Cogito ergo sum / I think, therefore I am" as "Maybe he only thinks that he thinks." ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:04, 1 November 2024 (UTC)

As a few people have said, "The problem with thinking about the universe is that there's nothing to compare it to". That's based on the assumption that universe = "everything that can possibly exist, anywhere". And yet, the human mind can comprehend the concept of an infinite number of different universes, each containing everything that can possibly exist, anywhere. It's no more difficult to work with such an idea than to make great use of the square root of -1. But is it actually true? I'm glad you asked ... -- Jack of Oz ^{[pleasantries]} 17:29, 1 November 2024 (UTC)

Now imagine a (strongly) inaccessible cardinality of universes.  --Lambiam 20:46, 1 November 2024 (UTC)

I reckon the multiverse concept was inspired by the failure of the universe that we know to contain everything that can possibly exist. This assumes it is bounded and doesn't contain, for instance, versions of itself at all other ages, and versions of itself where the laws of physics are different such that life is impossible. Then the other universes are the other possibilities. They're often synonymous with moments of time, in which case we are constantly moving through universes (or perhaps "featuring in a causally related series of moments" rather than moving - same difference). The parallel universes are moments of time that we don't go to, or in many cases couldn't possibly go to. On the other hand, especially in fiction, the term more often means a causally related series of moments, a timeline or "environment", where subsequent moments are constantly branching and diverging but share a common history.  Card Zero  (talk) 21:44, 1 November 2024 (UTC)

Is there a common term indicating that the current value of every derivative (of any order) of the position over time is zero?

That is, the current velocity iz zero, and so is the currect acceleartion, and so is the current jerk, and so forth...

I thought about "static" as a sufficient condition, but I'm not sure, so I'm also asking: Should every "static" body be considered to satisfy the property mentioned in the header? HOTmag (talk) 21:56, 31 October 2024 (UTC)

Something that doesn't move at all is described as a fixed point or fixed object. This usually means that the object won't move even if a force is applied to it. --Amble (talk) 23:34, 31 October 2024 (UTC)

Having all time derivatives zero at a particular instant is not the same as having constant position. The standard example is $e^{-{\frac {1}{t^{2}}}}$, but there are lots of other possibilities.

This is important to know when formulating differential topology, as it enables finding bump functions and partitions of unity in the smooth ($C^{\infty }$) category. --Trovatore (talk) 05:31, 1 November 2024 (UTC)

A term used by mathematicians for the function giving such a position as a function of time is "flat function".  --Lambiam 07:03, 1 November 2024 (UTC)

So my original question can be phrased as follows: Is there a common adjective, describing an object, and indicating that the object's location with respect to time is a flat function? Additionally, can the body's adjective "static" be a sufficient condition, for the above location to be a flat function? HOTmag (talk) 08:28, 1 November 2024 (UTC)

Extended content

1. immovable 2. immoveable 3. fixed 4. immotile 5. unmovable 6. fast 7. nonmotile 8. stiff 9. firm 10. stabile 11. amovable 12. unmoveable 13. static 14. immoble 15. moveless 16. irremovable 17. rooted 18. stationary 19. nonmobile 20. standing 21. dead 22. nonmoving 23. rigid 24. motionless 25. unbudgeable 26. inamovable 27. unimmobilized 28. unshiftable 29. nonimmobilized 30. unmoving 31. staid 32. immoved 33. nonmutable 34. nonchangeable 35. undeposable 36. untransmutable 37. unrelocatable 38. nonremovable 39. inflexible 40. untranslocatable 41. unfluid 42. nonchanging 43. nonrotatable 44. non-mobile 45. unmigratable 46. unmobilized 47. unstationary 48. non-stationary 49. nonflexible 50. incommutable 51. nonmodifiable 52. nonrelocatable 53. unresizable 54. unfixed 55. unmodifiable 56. unchangeable 57. untransformable 58. untransportable 59. unadjustable 60. intransmutable 61. unflexible 62. nontransportable 63. sedentary 64. invariable 65. nonmigratable 66. nonvariable 67. non-animate 68. noncommutable 69. nondisplacable 70. nondisplaceable 71. immalleable 72. unvariable 73. unmechanizable 74. inanimate 75. unlocomotive 76. confined 77. unmanipulatable 78. nonadjustable 79. undisplaceable 80. uninclinable 81. nondetachable 82. unalterable 83. undislodgeable 84. intransformable 85. unmutable 86. inanimated 87. restagnant 88. nonstationary 89. torpid 90. semistationary 91. unfluidizable 92. unfixable 93. unmaneuverable 94. nonportable 95. unbending 96. nonrotative

Philvoids (talk) 17:16, 1 November 2024 (UTC)

You need a preferential reference frame to get zero velocity, so the property is not intrinsic but observer-dependent. Have objects with this property been the subject of studies in theoretical physics? If not (and I can't think of a reason why they should be of interest to physicists), it is very unlikely that there is a term of art for the property.  --Lambiam 20:00, 1 November 2024 (UTC)

Time crystal? 176.2.70.177 (talk) 18:03, 2 November 2024 (UTC)