Wikipedia:Reference desk/Archives/Science/2024 March 13

= March 13 =

Lake Kakhovka
What's the situation at the former Lake Kakhovka from a "rewilding" point of view? All satellite images I've been able to find online are close to the moment of the destruction of the dam, but I was curious to know if trees and plants are starting to grow and in general how nature is recoverinf there. Thank :) --LametinoWiki (talk) 00:42, 13 March 2024 (UTC)


 * Given that the dam was destroyed less than a year ago, we may not have much significant regrowth on "tree" scale yet (especially given that a significant amount of that time period was winter). Additionally, this is a war zone or territory illegally occupied by Russia, so I doubt a ton of scientific teams are able to properly investigate the location. Both the war and occupation conditions could also be an impediment to significant regrowth. --OuroborosCobra (talk) 02:28, 13 March 2024 (UTC)

Two beams of light start moving perpendicular to the initial distance between them. Will it become shorter, because of any gravitational curvature caused by them in spacetime?
I'm asking from an empirical point of view, rather than from a theortical one (as predicted in General Relativity). I was recently shocked to hear from a local astronomer, that beams of light frequently arriving from pulsars, are known to prove that the distance does not become shorter. As far as you know, is there any evidence that supports - or refutes - what I've heard from that astronomer, emprically speaking? HOTmag (talk) 11:04, 13 March 2024 (UTC)
 * That's not measurable. I don't know what your local astronomer was talking about but they may have thought of the pulsar timing array. --Wrongfilter (talk) 11:23, 13 March 2024 (UTC)
 * Thanks. Does the pulsar time array have anything to do with a (constant?) distance between two beams of light arriving from pulsars? HOTmag (talk) 11:43, 13 March 2024 (UTC)
 * Your astronomer friend is correct from the theoretical point of view though I don't know what they were referring to. Think of having an object going at near the speed of light alongside some light. The light would be deeply red shifted and have very little energy. NadVolum (talk) 12:52, 13 March 2024 (UTC)
 * As for your first point: Why was the astronomer correct from the theoretical point of view? Doesn't General Relativity predict both beams of light will curve spacetime because of their energy?
 * As for your second point: Please notice that according to General Relativity, the impact of a gravitational curvature on a given particle's momentum - does not depend on the particle's energy, whereas a gravitational curvature caused by a given particle - does depend on the particle's energy. That's why my current question is only being asked about the curvature (if any) caused by light, rather than about the impact of that curvature on light. HOTmag (talk) 15:29, 13 March 2024 (UTC)
 * I don't have the maths for that anywhere near my finger tips but it seems to me that such a prediction would lead to all sorts of problems. Intuitively I would guess that it's difficult to impossible to construct a non-zero curvature field that transforms covariantly. However, in another one of your threads somebody posted a link to a paper demonstrating some sort of attraction between light rays in the lab, I don't recall the details. Be that as it may, any effect is certainly much to small to be measurable in the presence of much larger perturbations that are ubiquitous and unavoidable on Galactic scales (this also precludes an empirical demonstration of a zero effect to any level that would satisfy you). --Wrongfilter (talk) 15:44, 13 March 2024 (UTC)
 * This is the link you've just mentioned. It mentions no "lab" nor any other empirical clues. All was "theoretically inverstigated" (as clearly stated in the last sentence of the abstarct). That's why my current question is being asked "from an empirical point of view".
 * As for the maths bothering you, please see our artice pp waves, indicating: The pp-waves solutions, model radiation moving at the speed of light. This radiation may consist of electromagnetic radiation, gravitational radiation, massless radiation...or any combination of these, so long as the radiation is all moving in the same direction....Penrose also pointed out that in a pp-wave spacetime, all the polynomial scalar invariants of the Riemann tensor vanish identically, yet the curvature is almost never zero. [They vanish]...because in four-dimension all pp-waves belong to the class of VSI spacetimes. However, this curvature "is almost never zero" because (as stated in our article about VSI) the solutions of the stress–energy tensor usually have also non-polynomial or non-scalar invariants, which actually do not vanish. HOTmag (talk) 15:59, 13 March 2024 (UTC)
 * "...light causes no acceleration to a co-propagating test ray." emphasis added. Thus co-propagating rays, like the light arriving from distant stars and pulsars, are unaffected. Modocc (talk) 16:17, 13 March 2024 (UTC)
 * Not in all cases. See again the last sentence of the abstract. Additionaly, please notice you are the person who presented this link for proving the photons "gravitate" (as you wrote). HOTmag (talk) 16:32, 13 March 2024 (UTC)
 * Yes not in all cases, nevertheless Wrongfilter and the astronomer are, in theory, both correct. Modocc (talk) 16:54, 13 March 2024 (UTC)
 * Please notice I asked user:NadVolum "why" the astronomer was correct. Doesn't General Relativity claim every object carrying energy curves spacetime? HOTmag (talk) 17:02, 13 March 2024 (UTC)
 * Yes and its gravity (GR's or any othr model's) depends on invariant mass. Co-moving photons appear to not have that with respect to each other. Modocc (talk) 17:34, 13 March 2024 (UTC)
 * Please notice, that General Relativity ascribes - the gravitational curvature - caused by a given particle, to the particle's energy, rather than to the particle's invariant mass. For more details about what properties of the particle curve spacetime, see Stress–energy tensor. HOTmag (talk) 17:44, 13 March 2024 (UTC)
 * Of course all particles gravitate (I've been consistent on this point) thus each photon contributes to the field (warpinging GR's spacetime). With a photon cloud their relative velocities also contribute to invariant masses which determine to what degree there are interactions between them if any. Modocc (talk) 17:59, 13 March 2024 (UTC)
 * Since "all particles gravitate" as you emphasize, was the astronomer correct?
 * I recall the astronomer has claimed - emprically [factually] speaking, that the initial distance between two parallel beams of light arriving from pulsars, does not become shorter, even after finishing the long journey to Earth. According to that apparently empirical [factual] claim, those beams of light don't curve spacetime, do they? HOTmag (talk) 18:13, 13 March 2024 (UTC)
 * Your astronomer cannot measure that. --Wrongfilter (talk) 18:30, 13 March 2024 (UTC)
 * I'm not arguing about whether the astronomer can measure it. Yet, the question still remaing is whether the astronomer could've been correct, had the astronomer been able to meausre it. Apparently, what the astronomer claims, from an apparently empirical [factual] point of view, contradicts General relativity, which claims that any given particle having non-zero energy (hence a non-zero stress–energy tensor) is supposed to curve spacetime. I thought, the astronomer's apparently empirical [factual] claim, really contradicted General Relativity, but I wanted to be sure, and that's why I posted this thread. HOTmag (talk) 18:58, 13 March 2024 (UTC)
 * Apparently you have no idea what "empirical" means. --Wrongfilter (talk) 19:14, 13 March 2024 (UTC)
 * Sorry for replacing "factual" by "empirical". Anyway, as I have already stated, "I'm not arguing about whether the astronomer can measure it". Whether because I "have no idea what 'empirical' means" (as you claim), or because - even if I do know what it means (me meaning "factual" rather than "empirical") - I don't want to ask about whether the astronomer can measure it, but rather about whether the astronomer could've been correct - had the astronomer been able to meausre it. I thought, the astronomer's claim contradicted General Relativity, but I wanted to be sure, and that's why I posted this thread. HOTmag (talk) 19:26, 13 March 2024 (UTC)
 * (ec Stop making all these microedits) Maybe the astronomer just wanted to end the conversation... You wanted empirical evidence one way or the other, now don't change that to "factual" (I don't even know what that's supposed to mean). I say the effect would be unobservable, even if there was an effect (I don't know - there are lots of subtleties in GR if one dives in deep), hence the question is empirically undecidable. But you will never be sure, no matter what anyone here says. --Wrongfilter (talk) 19:46, 13 March 2024 (UTC)
 * Just to be clear:
 * 1. By mistake, I replaced "factual" by "empirical" when I referred to the astronomer's claim about the beams of light arriving from pulsars.
 * 2. However, my original question was about any empirical evidence, any one of you knew of. Yes, I used the word "empirical" in my original question, on purpose, meaning empirical, because I do know what it means.
 * 3. Before I read your response about the immeasurability, I posted this thread asking whether any one of you - knew of any empirical evidence - which could support or refute the astronomer's factual claim about the pulsars, because I wondered whether the astronomer could be correct - assuming the astronomer's factual claim about the pulsars could be resolved by measurement. I thought, this astronomer's factual claim contradicted General Relativity, but I wanted to be sure, and that's why I posted this thread asking about any empirical evidence you knew of - which could support or refute the astronomer's factual claim about the pulsars.
 * 4. In your response to my original question, you've claimed all of that can't be measured. As I have already stated, I'm not going to argue about that. However, I still wonder whether the astronomer could have been correct - had the astronomer's factual claim about the pulsars been resolved by measurement.
 * Am I clearer now? HOTmag (talk) 20:43, 13 March 2024 (UTC)
 * The starlight is co-moving like racers in a dead heat so according to the citation the photons gravitate, but not each other. Modocc (talk) 18:27, 13 March 2024 (UTC)
 * Since "the photons gravitate" (me using your words), so they curve spacetime (don't they?), so how can they avoid gravitating "each other" (me still using your words), as long as the spacetime is being curved by them? HOTmag (talk) 18:58, 13 March 2024 (UTC)
 * Why they (the comoving photons) don't gravitate each other according to the research paper? Perhaps someone else better versed in GR can answer that question, but co-moving photons do not appear to have invariant mass with respect to each other (although their energies obviously contribute to the invariant masses of larger systems). Note that in reality no photon within our universe exists in isolation from it so the notion that they all contribute mass-energy to the field holds. Modocc (talk) 20:13, 13 March 2024 (UTC)
 * Plesse notice that also massless photons are influenced by the curvature of spacetime, that actually deflects their trajectory, e.g. when they approach the sun, so the question still remains: Why is neither of them influenced by this curvature when it's caused by the other photon, while you agree "they gravitate" - hence curve the spacetime. HOTmag (talk) 20:51, 13 March 2024 (UTC)
 * Their path's curvature (or spacetime's curvature) are dependent on the sun's mass. As for their affecting each other, that, will depend, according to the paper I cited, on their relative velocities which are vectors. As for measurement, I assume the strength of their combined field is proportional to their invariant mass and inversely proportional to the distance between them squared: in accordance with other masses, and subject to the same caveats (Newtonian approximation, quantum limits, MOND, etc). Comoving photons have zero invariant mass, but also add mass to larger bodies. Modocc (talk) 23:07, 13 March 2024 (UTC)
 * Two photons move in parallel paths. There is an electron between them. Do the photons curve spacetime (in addition to the curvature caused by the electron)? If the answer is positive, then do they gravitate each other (in addition to the gravitation caused by the electron)? HOTmag (talk) 01:35, 14 March 2024 (UTC)
 * At infinity the photons don't gravitate each other, according to the citation, but the electron is a gravitational lens causing their convergence: thus the photons' convergence should have an invariant mass that gravitates them, also in accordance with the citation. Modocc (talk) 02:01, 14 March 2024 (UTC)
 * What do you mean by "the photons' convergence should have an invariant mass that gravitates them"? I remind, we are talking about two photons co-moving in the same direction, so how can the photons' convergence have an invariant mass? HOTmag (talk) 08:23, 15 March 2024 (UTC)
 * Convergent rays carry energy and momenta towards a distant focal point (with a component velocity less than c). They are no longer propagating parallel to each other. Modocc (talk) 12:04, 15 March 2024 (UTC)
 * Can you think of any theoretical situation in which the invariant masses of two given photons will be zero, assuming that the whole world contains massive matter as well? HOTmag (talk) 13:50, 15 March 2024 (UTC)
 * As long as photons are moving along similar parallel geodesics they have energy, but, of course, it is not invariant. Invariant mass: "The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object or system of objects that is independent of the overall motion of the system." Modocc (talk) 17:37, 15 March 2024 (UTC)
 * Photons have energy and therefore, according to the Einstein field equations, curve spacetime just like other entities with energy. The predicted effect of two photons on each other is too small to be measured. --Lambiam 21:06, 13 March 2024 (UTC)
 * You've responded to a response of mine that was, both addressed to user:Modocc only, and assuming user:Modocc's assumptions only. Do you think you can also respond to my original question under the title of this thread? HOTmag (talk) 22:02, 13 March 2024 (UTC)
 * You have restored a contribution of mine that I had removed. That is a big no-no. When I originally posted it, I had overlooked the fact that the question had been changed: in the OP they were moving in orthogonal directions, not parallel. The latter is much more tricky, one immediate issue already being that the notion of geodesics being parallel in a non-flat continuum is ill-defined. --Lambiam 10:37, 14 March 2024 (UTC)
 * Wow, I'm quite surprised now. Of course I was not aware of the restoration made by me. I still wonder how it was actualy done, maybe because I had not received a warning about an ongoing edit conflict? Anyway it was not done on purpose. I guess you forgive me for this big mistake, for which I apologize from the bottom of my heart, don't you?
 * As for your second remark, I'm surprised again: What do you mean by "the question had been changed: in the OP they were moving in orthogonal directions, not parallel". As far as I remember: from the very beginning, the question indicated in the title (that has never changed) has always been formulated as follows: Two beams of light start moving perpendicular to the initial distance between them. Will it become shorter, because of any gravitational curvature caused by them in spacetime? In other words, from the very beginning the question has always been about two beams of light moving in parallel directions, hasn't it? HOTmag (talk) 08:09, 15 March 2024 (UTC)
 * Suppose one photon's trajectory is given by its position as a function of time as $$(ct,0,0)$$ while the other has $$(0,ct,\delta).$$ Then, when $$t=0,$$ each moves perpendicular to the line segment from $$(0,0,0)$$ to $$(0,0,\delta)$$ connecting them, and in orthogonal directions with respect to each other. I now think that also in this case they would not gravitationally attract each other. It is difficult to find reliable sources handling the issue, as any effect would be too small to measure, but nevertheless I found this: "This conclusion explains ... why photons do not attract each other during the long trip from the distant galaxies and collapse into a single clump, despite of the fact that photons have gravitational mass."  --Lambiam 19:00, 15 March 2024 (UTC)
 * Careful: that quote is from a paper published in Physics Essays, which looks like a rather dodgy journal. --Wrongfilter (talk) 19:19, 15 March 2024 (UTC)
 * I was trying to make it simple. There is no reason to invoke General Relativity, Special Relativity is quite enough to see why the rays don't attract each other. If two particles have the same direction and speed as each other then they are at rest relative to each other and only their rest mass counts for the gravitationl attraction between them - even if they are going at near the speed of light relative to us and have a huge mass relative to us. Now think of a photon as the limit of a particle getting a smaller and smaller rest mass but going faster and faster so the relativistic mass stays the same. in the limit we have a particle with zero rest mass but the same energy. Now two of those going along together won't attract each other because their rest mass is zero. NadVolum (talk) 20:57, 13 March 2024 (UTC)
 * Following your considerations, let's look at the whole picture - taking into account new considerations you didn't take into account, after making a distinction - as you've made - between two cases: A pair of massive particles, and a pair of photons.
 * 1. Two massive particles start moving, in the same direction, perpendicular to the initial distance between them. In their reference frame, the initial distance between them will soon become shorter, because of the gravitational curvature caused by their rest mass (as you have indicated). However [and this is my new consideration you didn't take into account], the distance will become much shorter, in any other reference frame, because the curvature is bigger in that reference frame, because the curvature in that other reference frame is also influenced by their kinetic energy (being their "relativistic mass" in that other reference frame, provided that we allow to use this concept, and I allow, and I assume you allow as well, but even if we don't allow we can still adhere to their kinetic energy instead).
 * 2. Two photons start moving, in the same direction, perpendicular to the initial distance between them. Had they had a reference frame in which they could have been at rest, then: In their reference frame the initial distance between them would have not changed, because the gravitational curvature caused by their zero rest mass - would have been zero as well - as you've indicated. However [and this is my new consideration you didn't take into account], the distance will become shorter, in any other reference frame, because the curvature is bigger in that other reference frame, because the curvature in that other reference frame is also influenced by their kinetic energy (being their "relativistic mass" in that other reference frame, provided that we allow to use this concept, and I allow, and I assume you allow as well, but even if we don't allow we can still adhere to their kinetic energy instead).
 * HOTmag (talk) 22:02, 13 March 2024 (UTC)
 * I'm having difficulty with what you said but apparent contraction of lengths only happens in the direction of motion, not at right angles to it. A super speed locomotive may seem shorter but the wheels will remain the same distance apart and stay on the track. Also while going any particular distance the particles will experience less time and a shorter distance than an observer and therefore will not approach each other even the amount one might assume from Newtonian gravity on their rest mass never mind with the extra energy. NadVolum (talk) 00:17, 14 March 2024 (UTC)
 * I was not referring to the Special relativistic effect of length contraction due to velocity, but rather to the General relativistic effect of difelcting a given photon's trajectory when the photon is in a gravitational field, e.g. when it moves near the sun, so the distance between the photon and the sun becomes shorter because of the effect of gravitational lensing, regardless of the Special relativistic effect of length contraction due to velocity.
 * That said, do you agree that when an electron and a planet start moving perpendicular to the initial distance between them, the direction will soon become shorter, according to the following three principles:
 * 1. The faster the reference frame moves relative to the planet, the bigger kinetic energy the planet has (hence the bigger relativistic mass the planet has).
 * 2. The bigger kinetic energy (hence relativistic mass) the planet has [in a given reference frame], the shorter the distance between the electron and the planet will become in that reference frame.
 * Logically, the following principle follows from the combination of the previous ones:
 * 3. The faster the reference frame moves relative to the planet, the shorter the distance between the electron and the planet will become in that reference frame.
 * If you agree, then try to read again my previous response, but now you should interpret the effect as a General relativistic one, due to gravitational lensing effect that obeys the third principle mentioned above (rather than as a Special relativistic effect of length contraction). HOTmag (talk) 01:22, 14 March 2024 (UTC)
 * Changing reference frames, in general, does not, I repeat, does not change physical interactions. For example, did you know that the Earth has arbitrary KE! Pick any train, rocket, plane or the like, the Earth has different KE due to the velocity of your chosen reference frame. Instead, g depends simply on the reference frame independent invariant mass of the gravitating body. That said, accelerations of objects indeed changes interactions: such as how Doppler is measured. In other words, physically changing one's reference frame (as opposed to only applying a Galilean transformation) is not trivial. For instance, surfers that ride the crests of ocean waves and then glide off them physically change their reference frames and the frequency of wave interactions increases. Hence, both kinds of change happen at the same time.      Modocc (talk) 02:30, 14 March 2024 (UTC)
 * AFAIK, in General Relativity, gravity is considered to be caused by curvature in spacetime, while this curvature is caused by the stress–energy tensor, while this tensor is defined by the relativistic energy E (and the momentum p along with some other components relating to these energy and momemtum) - of the entity (e.g. a planet) responsible for the gravity, while this relativistic energy E is equivalent to the relativistic mass E/C², rather than to the invariant mass E/C² devided by the gamma factor. HOTmag (talk) 07:52, 15 March 2024 (UTC)
 * Relativistic masses each contribute to the invariant masses of larger bodies, which in turn, gravitate in accordance with their invariant masses. So yes they gravitate. :-) Modocc (talk) 13:15, 15 March 2024 (UTC)
 * Please notice the current discussion is not about whether they gravitate but rather about your previous response in which you've emphasized: "Changing reference frames, in general, does not, I repeat, does not change physical interactions... g depends simply on the reference frame independent invariant mass of the gravitating body". In this previous response, you responded to a response of mine to user:NadVolum, in which I claimed "The bigger kinetic energy (hence relativistic mass) the planet has [in a given reference frame], the shorter the distance between the electron and the planet will become in that reference frame", so it seems your previous response meant to claim that gravity did not depend on relativistic masses. However, if you're claiming now that it does depend on them, then we agree, as far as our current discussion (about my response to user:NadVolum and about your previous response to my response) is concerned. HOTmag (talk) 13:50, 15 March 2024 (UTC)
 * I was careful to provide enough context with each of my points to avoid contradiction. Note that I said relativistic masses contribute to proper masses which constrains the problem of determining what curvature(s) are present within and outside the bodies under consideration. My first point was that unlike picking reference frames, any curvature present due to g is not arbitrary, and my second point was that for any fixed reference frame differences in invariant masses correspond to differences in curvature so yes we have to apply relativistic mass when calculating them. Modocc (talk) 21:59, 15 March 2024 (UTC)
 * There will be a gravitational wave associated with the two photons, it is generated when they are emitted. Like a single wave on water it has no effect if you're going along with it like a surfer but if you go across it has a larger effect the faster you cross it. NadVolum (talk) 13:23, 15 March 2024 (UTC)
 * Considering the indent of your current response, are you reponding now to my respose to user:Modocc? HOTmag (talk) 13:50, 15 March 2024 (UTC)
 * Irrelevant to what I said. You were not satisfied with a simple explanation using Special Relativity that fully explains what happens. As far as I can make out you want a General Relativity solution about why others would feel a gravitational attraction to the photons but the photons wouldn't feel any to each other. My description using a surfer is just descriptive, a fuller explanation would need you to read the papers and delve through the maths of General Relativity I believe. NadVolum (talk) 15:50, 15 March 2024 (UTC)
 * 3. The faster the reference frame moves relative to the planet, the shorter the distance between the electron and the planet will become in that reference frame.
 * If you agree, then try to read again my previous response, but now you should interpret the effect as a General relativistic one, due to gravitational lensing effect that obeys the third principle mentioned above (rather than as a Special relativistic effect of length contraction). HOTmag (talk) 01:22, 14 March 2024 (UTC)
 * Changing reference frames, in general, does not, I repeat, does not change physical interactions. For example, did you know that the Earth has arbitrary KE! Pick any train, rocket, plane or the like, the Earth has different KE due to the velocity of your chosen reference frame. Instead, g depends simply on the reference frame independent invariant mass of the gravitating body. That said, accelerations of objects indeed changes interactions: such as how Doppler is measured. In other words, physically changing one's reference frame (as opposed to only applying a Galilean transformation) is not trivial. For instance, surfers that ride the crests of ocean waves and then glide off them physically change their reference frames and the frequency of wave interactions increases. Hence, both kinds of change happen at the same time.      Modocc (talk) 02:30, 14 March 2024 (UTC)
 * AFAIK, in General Relativity, gravity is considered to be caused by curvature in spacetime, while this curvature is caused by the stress–energy tensor, while this tensor is defined by the relativistic energy E (and the momentum p along with some other components relating to these energy and momemtum) - of the entity (e.g. a planet) responsible for the gravity, while this relativistic energy E is equivalent to the relativistic mass E/C², rather than to the invariant mass E/C² devided by the gamma factor. HOTmag (talk) 07:52, 15 March 2024 (UTC)
 * Relativistic masses each contribute to the invariant masses of larger bodies, which in turn, gravitate in accordance with their invariant masses. So yes they gravitate. :-) Modocc (talk) 13:15, 15 March 2024 (UTC)
 * Please notice the current discussion is not about whether they gravitate but rather about your previous response in which you've emphasized: "Changing reference frames, in general, does not, I repeat, does not change physical interactions... g depends simply on the reference frame independent invariant mass of the gravitating body". In this previous response, you responded to a response of mine to user:NadVolum, in which I claimed "The bigger kinetic energy (hence relativistic mass) the planet has [in a given reference frame], the shorter the distance between the electron and the planet will become in that reference frame", so it seems your previous response meant to claim that gravity did not depend on relativistic masses. However, if you're claiming now that it does depend on them, then we agree, as far as our current discussion (about my response to user:NadVolum and about your previous response to my response) is concerned. HOTmag (talk) 13:50, 15 March 2024 (UTC)
 * I was careful to provide enough context with each of my points to avoid contradiction. Note that I said relativistic masses contribute to proper masses which constrains the problem of determining what curvature(s) are present within and outside the bodies under consideration. My first point was that unlike picking reference frames, any curvature present due to g is not arbitrary, and my second point was that for any fixed reference frame differences in invariant masses correspond to differences in curvature so yes we have to apply relativistic mass when calculating them. Modocc (talk) 21:59, 15 March 2024 (UTC)
 * There will be a gravitational wave associated with the two photons, it is generated when they are emitted. Like a single wave on water it has no effect if you're going along with it like a surfer but if you go across it has a larger effect the faster you cross it. NadVolum (talk) 13:23, 15 March 2024 (UTC)
 * Considering the indent of your current response, are you reponding now to my respose to user:Modocc? HOTmag (talk) 13:50, 15 March 2024 (UTC)
 * Irrelevant to what I said. You were not satisfied with a simple explanation using Special Relativity that fully explains what happens. As far as I can make out you want a General Relativity solution about why others would feel a gravitational attraction to the photons but the photons wouldn't feel any to each other. My description using a surfer is just descriptive, a fuller explanation would need you to read the papers and delve through the maths of General Relativity I believe. NadVolum (talk) 15:50, 15 March 2024 (UTC)
 * Considering the indent of your current response, are you reponding now to my respose to user:Modocc? HOTmag (talk) 13:50, 15 March 2024 (UTC)
 * Irrelevant to what I said. You were not satisfied with a simple explanation using Special Relativity that fully explains what happens. As far as I can make out you want a General Relativity solution about why others would feel a gravitational attraction to the photons but the photons wouldn't feel any to each other. My description using a surfer is just descriptive, a fuller explanation would need you to read the papers and delve through the maths of General Relativity I believe. NadVolum (talk) 15:50, 15 March 2024 (UTC)

Hump
In a freight yard, is there any advantage to putting three lead tracks on the hump, as opposed to the more usual multiples of two? 2601:646:8082:BA0:8014:354A:6A03:A76 (talk) 13:34, 13 March 2024 (UTC)
 * I don't see anything about what you are saying at that article. Have you something describing what you are talking about? NadVolum (talk) 21:12, 13 March 2024 (UTC)
 * If you look at the photos in the article Classification yard, the hump in these photos always has two leads, never three -- my question is, is there any particular reason why this is the case? 2601:646:8082:BA0:8014:354A:6A03:A76 (talk) 02:13, 14 March 2024 (UTC)
 * I don't know how to see which tracks are leads, but in any case, a larger number requires a more extensive hump. If the hump is a constructed hill, as suggested in, the extra cost and complexity may not be outweighed by an operational advantage. --Lambiam 10:19, 14 March 2024 (UTC)


 * I looked at several examples of hump yards and found three layouts for the leads:
 * A single lead going up the hump, after the downhill splitting multiple times into the bowl. For example in Cincinnati, Kansas City, Houston (all US), Odesa (Ukraine). This appears to be the dominant layout in the US.
 * Two leads, merging into one on the downhill before splitting multiple times into the bowl. For example in Vienna (Austria), Hamburg, Mannheim (south side) (both Germany), Riga (Latvia), Antwerp (Belgium). This appears to be the dominant layout in Western Europe.
 * Two leads with a scissors crossover on the downhill before splitting multiple times into the bowl. For example in Metz (France), Rotterdam (Netherlands), Mannheim (north side) (Germany), Shanghai (China).
 * All yards named after the nearby large city, not the name of the yard itself.
 * Now keep in mind the difference between Western European couplings and those used in other areas. Western Europe still uses screw couplings. Before humping a series of wagons, the train is first pushed slightly onto the hump, to compress the couplings. Then the couplings are loosened a bit, so that they can be unhooked. Then the wagons are pushed over the hump, whilst a worker uses a club to knock the link of the coupling off the hook, without stopping the train or getting between the wagons. This isn't needed elsewhere, as trains there use claw couplings. So having two leads is useful in Western Europe, as one train is pushed over the hump whilst the next is prepared.
 * Having a scissors crossover instead of a merge followed by a split allows sorting two trains at the same time, each into half the bowl tracks. In Metz, one can sort one train onto all 48 tracks or two trains onto 24 tracks each. You gain some flexibility for small jobs, at the cost of two sets of points and a diamond. I suppose that small jobs are more common in Europe, with shorter and more frequent trains, than in the US.
 * Three or more lead tracks gives diminishing returns, whilst rapidly making the required pointwork more complex. PiusImpavidus (talk) 11:02, 14 March 2024 (UTC)
 * Thanks! So, just as I thought -- adding a third lead gives you more than 50% higher construction costs (as well as operational costs, and per-capita aspirin consumption by the switchmen) in exchange for less than 50% higher throughput (and it gets worse from that point on)!  (Incidentally, the only hump I've ever seen with three leads was a fictional one at Knapford Yards -- or was it Tidmouth Yards? -- in Thomas & Friends (one of the old classic episodes, "Old Iron" "Pop Goes the Diesel" I think) -- and I have to remark, although these yards as a whole look impressive on the screen, they are actually laid out pretty poorly, and the hump seems to have been added pretty much as an afterthought!) 2601:646:8082:BA0:0:0:0:2EB (talk) 13:55, 14 March 2024 (UTC)
 * And two lead tracks with a scissors crossover gives redundancy. Any of the sets of points can be taken out of service for maintenance, still allowing the yard to operate at half capacity. PiusImpavidus (talk) 10:48, 16 March 2024 (UTC)

Would some like to point out where the leads are in this diagram just so I know. It never occurred to me before that you need a sorting algorithm to build trains despite it being obvious and despite having probably gone past these kind of places numerous times. Sean.hoyland (talk) 11:19, 14 March 2024 (UTC)


 * It's the track from the approach yard to the classification yard, leading up to the hump. The hump is located just before the first retarder. PiusImpavidus (talk) 11:48, 14 March 2024 (UTC)
 * Thanks. So the red rectangles are retarders I guess. Sean.hoyland (talk) 11:54, 14 March 2024 (UTC)
 * Just saw the legend which says in clear large font Retarders...Sean.hoyland (talk) 11:55, 14 March 2024 (UTC)

arrest, dearrest / de-arrest, arrested
What's the meaning of "arrest and dearrest" on page 684 of https://archive.org/details/proteintransfero0000unse/page/684/mode/2up?q=dearrest and "de-arrest of arrested" on page 3 of https://archive.org/details/arxiv-1206.2024/page/n1/mode/2up?q=%22de-arrest%22? Mcljlm (talk) 21:42, 13 March 2024 (UTC)


 * The verb to arrest means, "to keep (something) from moving". What is kept in arrest in these cases are molecules, in the first case specifically proteins; in the second case, those of which a glass is comprised. The verb to dearrest means, "to release (something) from an existing arrest". The form de-arrest is a spelling variant.  --Lambiam 10:06, 14 March 2024 (UTC)
 * Thanks . Until a few days ago I'd not come across dearrest/de-arrest at all {originally I was only intending to ask about that word}, and then only relating to people. Later I found it used relating to ships and goods in historical documents (the OED doesn't mention any use later than 1791). Is its use recent and common in a scientific context? Is the hyphen's use more or less frequent in scientific publications? Mcljlm (talk) 16:49, 14 March 2024 (UTC)
 * Regarding ships and boats: arresting a boat (in a legal sense) was certainly a thing up to the 1990s, and I would guess still is. In the early '90's my father, who worked at a legal firm, passed on to me an obsolete word processor (an IBM Displaywriter System) that had just been replaced at his firm; on the data disk were several form documents, including one for arresting a boat. 51.198.186.221 (talk) 00:43, 15 March 2024 (UTC)
 * It's dearrest/de-arrest which the OED doesn't have any mention for later than 1791. . Could it be that another word was used? Perhaps I should ask at the Humanities and/or Language reference desks. Mcljlm (talk) 04:52, 15 March 2024 (UTC)
 * The 1791 use actually uses the unhyphenated spelling dearrest. Here is a use of de-arrest, with a hyphen, from 1898. --Lambiam 17:46, 15 March 2024 (UTC)
 * Since it's an edition of a 1334 document it would be interesting to know if the hyphen is in the original and if not why it is in this edition. Mcljlm (talk) 10:05, 16 March 2024 (UTC)
 * Until 1905, the so-called atomic theory – the notion that matter was composed of molecules which in turn were composed of elemental atoms – was a hypothetical concept. There was in fact considerable opposition to atomic theory. This only changed with the publication of Einstein's article on Brownian motion. So most of the use of the term in this scientific sense of a molecule being arrested is indeed relatively recent. Proponents of the atomic theory did use the term arrest, though, already in the 19th century (e.g, in 1874, "the heat of arrested motion"). Nevertheless, this is not a term of art but the use of a word in an accepted common sense applied to a specific context. The term dearrest is rather rare, and it is hard to tell which spelling variant is more common. --Lambiam 20:58, 14 March 2024 (UTC)
 * Explained by the BBC (which appears to uses 'de-arrest' consistently): Who, what, why: What does it mean to be de-arrested? . -- Verbarson talkedits 13:49, 15 March 2024 (UTC)
 * I became aware of the use of de-arrest/dearrest relating to contemporary police action about a week ago when someone quoted from a Metropolitan Police statement. I then noticed its use in the other contexts I mentioned. Mcljlm (talk) 15:38, 15 March 2024 (UTC)
 * Having been taken into custody many times, I can confirm that "I'm de-arresting you" is one of the sweetest things a police officer can say, although not having ACAB tattooed on my knuckles probably helps. MinorProphet (talk) 00:39, 26 March 2024 (UTC)