Wikipedia:Reference desk/Archives/Science/2021 February 25

= February 25 =

How much percent General relativity still need to be proven?
I know that General relativity is not completely proven by Eddington eclipse test at South Africa. How much percent General relativity still need to be proven? Rizosome (talk) 02:03, 25 February 2021 (UTC)
 * , why are you asking, and will you respond to any questions or to any answers given to you? Why did you tag me in your earlier post, then not answer when I asked you why you tagged me? As to your question, how does one even quantify how much of any hypothesis needs to be proved? Also, the concept of "proving" a hypothesis is a misnomer. Proofs are used in mathematics, but in science, we come up with theories containing models explaining things like observables. They aren't "proved," so much as demonstrated to match reality as we have observed it, and subject to change once further data comes along. Without knowing when, or even if, let alone how much data will come along, the idea of a "percent" of any hypothesis or theory that is still not demonstrated is nonsensical. I'll put it to you another way, quantum mechanics postulates the idea that gravity could have a relevant subatomic particle, the graviton, though we have not found one yet. What percentage of our understanding of gravity is missing by the fact that we have not found a graviton? How would one quantify that? Especially since the answer may be "there's no such thing as gravitons." --OuroborosCobra (talk) 06:01, 25 February 2021 (UTC)
 * Indeed, per OuroborosCobra, science doesn't "prove" things at all (despite the inaccurate language used by MANY people who like that word). Science, instead, is a process by which we alternate between making predictions about reality based on observations, then we make new observations to see if they match our predictions.  If they do, great!  If they don't, we refine our predictions.  That process is what science is.  The predictions go by various names (theories, laws, models, etc.), but basically it's just a formal way of saying "What happens when I do X?" or "Why does X happen when I do that?" or things like that.  I highly recommend familiarizing yourself with the work of Karl Popper, who was one of the foremost philosophers of science, to help get a framework for understanding what science is and what it does.  General relativity, like all theories, is an explanation of where gravity comes from and how gravity works.  It makes fantastically accurate predictions that, near as we can tell, work very well with matching observations.  -- Jayron 32 12:11, 25 February 2021 (UTC)

So general relativity is useless because of misnomer? Rizosome (talk) 14:11, 25 February 2021 (UTC)
 * Essentially yes . The question should be "how accurate has General relativity been in making predictions?" and the answer would be "very". Jules  (Mrjulesd) 14:18, 25 February 2021 (UTC)
 * Wait, what? No!  General Relativity has been fantastically useful in describing how gravity works.  It makes predictions prior theories of gravity don't make (i.e. Newtonian gravity) and those predictions match reality much MUCH closer than Newtonian gravity.  GR is far from useless.  All sorts of things like the proper operation of GPS satellites to understanding (or even knowing of the existence of) black holes, to being better able to describe orbital mechanics are all ONLY possible due to general relativity.  I don't see where either of you come to the conclusion that OuroborosCobra's answer means that general relativity is useless???  -- Jayron 32 14:23, 25 February 2021 (UTC)
 * Woops I meant the question "How much percent General relativity still need to be proven?" is useless! Apologies -- Jules (Mrjulesd) 14:27, 25 February 2021 (UTC)

very means how much percent? Rizosome (talk) 15:56, 25 February 2021 (UTC)
 * Look this is getting extremely circular. But in essence you can't say that scientific theories are "x% proven" because scientific theories are never proven. -- Jules (Mrjulesd) 16:32, 25 February 2021 (UTC)
 * "Very" in this case means that "if we use the equations and mathematics that General Relativity gives us to predict the motion of bodies as they move near massive objects like planets, stars, and black holes, we get answers from those equations that match observed motions to a degree of accuracy that exceeds our ability to detect any errors in the predictions". That's what "very" means.  It isn't a percent thing.  It's that the equations of General Relativity make predictions about motion, and those predictions match observations. -- Jayron 32 17:36, 25 February 2021 (UTC)
 * Those equations, by the way, are known as the Einstein field equations, and while they are fantastically complex and a real bear to solve (involving some really messy four-dimensional tensors!), they do a better job of predicting gravity under extreme conditions than does Newton's law of universal gravitation, which in terms of simplicity is MUCH easier to solve (it's a simple 5 variable equation). The extreme complexity of the equations, by the way, is why we only use them when we need to, i.e. when observations deviate from Newton's equation.  Those deviations only become measurable under fairly extreme conditions.  -- Jayron 32 17:42, 25 February 2021 (UTC)

GR breaks down at the center of a black hole, but it makes precise predictions about stuff that happens within the event horizon but away from the center. E.g. a baseball thrown into a black hole moves along a path that GR lets you calculate precisely, including after the event horizon is crossed. But, by definition, the baseball once inside the event horizon can no longer be observed, so GR's prediction of its motion cannot be checked. So that part of GR can never be experimentally tested (unless you follow the baseball into the BH yourself, but that means you can't report the results to the outside). So I'd have to say that 1) this part of GR can never be "proven"; 2) similarly, it can't be falsified; 3) it is still a perfectly good scientific theory (including the interior of BH part), despite what Karl Popper might say. Popper also takes some heat over unrelated issues in this article. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 20:14, 25 February 2021 (UTC)
 * Imagine the frustration of an observer who is so driven by curiosity that they venture into a black hole, because they can't resist the urge to find out if the field equations are still valid there, only to discover that they are not, but now having no way to report this to the outside world. --Lambiam 22:54, 25 February 2021 (UTC)
 * Maybe they could be tied to a balloon just outside the event horizon, and tug once if it's still valid, and twice if it's not. ←Baseball Bugs What's up, Doc? carrots→ 03:04, 26 February 2021 (UTC)
 * No, they couldn't. That's the point of the event horizon. --Wrongfilter (talk) 06:29, 26 February 2021 (UTC)
 * Also, although the observer learned Morse code as a boy scout he completely forgot it, and anyways, the watch he gave to his daughter is an old electronic watch – a collector's item – and the battery has run out. --Lambiam 08:06, 26 February 2021 (UTC)


 * If he could survive the plunge into the black hole, then anything is possible. ←Baseball Bugs What's up, Doc? carrots→ 11:48, 26 February 2021 (UTC)
 * All the observer needs to do is fly around inside the black hole inside her or his space ship (to make their observations), then scan the interior of the event horizon with warp particles to find the hole made when the ship crashed through the event horizon, and fly back out. EdChem (talk) 12:00, 26 February 2021 (UTC)
 * Yeah, that doesn't work. The whole point of the event horizon is that past the event horizon, spacetime has been warped to the point that the time dimension has been squished onto the space dimensions (or if you prefer, the other way around).  Regardless, inside the event horizon, all possible world lines lead to the singularity.  That's not merely coincidence, that's the definition of the event horizon itself, which is the boundary inside of which all possible world lines lead to the singularity.  Inside of the event horizon, every forward direction, including forward in time leads to the singularity.  It doesn't matter which direction you point your ship; any direction you point and fire your thrusters only moves you closer to the singularity.  Even moreso, you couldn't see behind you.  If you could even see, every direction you looked is straight at the singularity.  There is no possible you could just... way out.  Game over.  -- Jayron 32 12:20, 26 February 2021 (UTC)
 * Warp particles, if used properly, can locally warp spacetime in reverse by rotating over an imaginary Minkowski angle. However, this is mainly theoretical. For practical use, the traveller will have to bring a YeV hypercollider with them and figure out a way to focus the beam of warp particles without getting warped themselves. --Lambiam 16:00, 26 February 2021 (UTC)
 * How would it be possible to survive going insidet the event horizon? ←Baseball Bugs What's up, Doc? carrots→ 13:57, 26 February 2021 (UTC)
 * It depends on the size of the black hole. Paradoxically, a smaller mass black hole is likely more dangerous; given that gravitational forces will still obey the inverse square law, the event horizon as defined by the Schwarzschild radius is considerably closer to the gravitational singularity at the center of the black hole.  Tidal forces caused by the difference in gravitational attraction between different sides of you are likely to be so intense as to tear you to pieces in a process known as spaghettification.  For things like supermassive black holes, the event horizon is so far from the singularity you might likely not even notice when you cross it; indeed there's nothing particularly eventful for the observer crossing the event horizon to notice that they did indeed cross it.  For such an observer, there's nothing dramatic that happens specifically at the moment the event horizon is crossed.  Spaghettification can happen outside the event horizon for smaller black holes, and for the really big ones, you wouldn't notice much of anything.  To an outside observer, they never see you actually physically cross it.  You just slow down as you approach it, and what they see is your slowing and then frozen image slowly fading as the light that leaves you becomes gravitationally red-shifted beyond the visible range.  -- Jayron 32 16:31, 26 February 2021 (UTC)
 * Essentially, being inside a black hole isn't too different from being inside a universe undergoing a Big Crunch. It doesn't matter how you travel in space, as you can only go forward in time and every path forward in time leads to the singularity. Any message you want to send out, has to be send back in time, which is unfortunately a violation of causality. PiusImpavidus (talk) 15:30, 26 February 2021 (UTC)
 * What, if anything, would prevent you being crunched yourself? ←Baseball Bugs What's up, Doc? carrots→ 16:27, 26 February 2021 (UTC)
 * Nothing. Your future worldline has been constricted to a single path, which takes you to the singularity.  Your mass just becomes part of the singularity's mass as you are gradually crushed into it, and even the individual atoms of your body lose their identity as distinct particles.  -- Jayron 32 17:06, 26 February 2021 (UTC)
 * Then you wouldn't have any capability of even thinking about signaling, let alone actually signaling. ←Baseball Bugs What's up, Doc? carrots→ 21:17, 26 February 2021 (UTC)
 * If the hole is large enough, you may die of old age even in free fall before the local spacetime distortion starts to exert macroscopic effects on your wave function. For all we know, the observable universe is contained within a black hole. --Lambiam 09:29, 27 February 2021 (UTC)
 * and others, it appears that my attempt at humour, which I described in my edit summary as "a simple and solved problem, I saw it on TV," has fallen flat. I was referring to the Star Trek: Voyager episode Parallax in which the ship is flying along and experiences a massive jolt, which turns out to be them crashing through the event horizon of a black hole.  After flying around for a while and seeing a time-delayed reflection of themselves, they realise that they are trapped inside the event horizon, which is described by one character as a "very powerful energy field" surrounding the quantum singularity (IIRC).  They scan the event horizon with warp particles to locate the "crack" in the event horizon that they made on entry.  This has started to collapse but they widen it with decyons (IIRC) and then fly through the hole in the event horizon, escaping.  I mentioned it because this episode is (IMO) one of the most scientifically ridiculous in all of Star Trek.  As is explained above, the event horizon is not some great barrier that a spacecraft can crash into, nor is the space within it amenable to flying around to look for a way out and then escaping through a convenient hole.  I was attempting to mock the treatment of the region within the event horizon as a place from which to escape as opposed to being a region where gravitational effects will crush the ship.  The episode also includes the idea of seeing from outside the event horizon a ship trapped inside it and planning to use a tractor beam to pull the trapped ship free, yet the nature of an event horizon means that seeing anything inside it from outside is impossible.  SciFi can be scientifically plausible, but also scientifically flawed and (at times) unintentionally hilarious (at least to me), and this episode definitely fell into the latter category.  EdChem (talk) 00:24, 27 February 2021 (UTC)
 * I think it's fairly clear you were being funny, but thanks for providing the context. Star Trek is often more about the adventure than about pure science. It's instructive to an ignoranimous like me to wonder about some of this. For example, what if a ship inside the event horizon was tethered by a really strong steel cable to a ship outside the event horizon? Could the ship be pulled back out? Or would the outer ship get sucked in? That all assumes the inner ship doesn't get scrunched into oblivion, of course. ←Baseball Bugs What's up, Doc? carrots→ 02:18, 27 February 2021 (UTC)


 * My contribution about the theoretic potential of warp particles, if used properly, was meant to be in kind. --Lambiam 09:35, 27 February 2021 (UTC)


 * Speaking of the ultimates of Trekkian scientific ridiculosity (and subsuming Star Trek: Discovery within that universe), in Season 1, Episode 4, "The Butcher's Knife Cares Not for the Lamb's Cry", a ginormous tardigrade appears to possess, as part of its formidable abilities, the ability to travel the filaments between the stars (which are formed by mycelium), possessing an inbuilt navigation map of the galaxy. By entering the navigation chamber of a spaceship with a sporal drive, the ship's navigation system interfaces immediately and spontaneously with that of the tardigrade, thereby solving the crew's predicament. --Lambiam 09:53, 27 February 2021 (UTC)
 * Lambiam, don't give up hope! If the field equations are wrong inside the event horizon, then there might be a way to get the information out after all!  2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 19:33, 2 March 2021 (UTC)


 * I have proposed a test to isolate the effects of general and special relativity, but I don't think anyone has yet done the experiment or anything quite like it. The Earth rotates about its axis, so special relativity effects depend on how far you are from the Earth's axis of rotation, but in theory, you could choose two points on Earth's surface that are equidistant from the Earth's axis of rotation, but which have different gravitational strengths, for example, due to nearby heavy metal deposits, or by choosing one point at the top of a mountain at a high latitude, and another point at a lower latitude at sea-level. It would also be interesting to test whether or not time dilation behaves exactly as predicted when underground using the same idea of choosing a second point equidistant from the axis of rotation.MathewMunro (talk) 10:53, 27 February 2021 (UTC)
 * maybe there's something I'm not understanding, but what's the advantage of your proposal over something like the Hafele–Keating experiment (mentioned in both our Time dilation and Experimental testing of time dilation articles)? Nil Einne (talk) 17:18, 27 February 2021 (UTC
 * I would have thought that was bloody obvious. Flying clocks around in aeroplanes doesn't prove much other than that "something" causes time dilation, because it mixes up acceleration, speed & gravitational effects, whereas my idea would isolate the gravitational effect, and could be made as accurate as you like by simply leaving the clocks in place longer. And obviously, flying clocks in planes doesn't test the effect of being underground - ie it doesn't test whether it's the strength of gravity, or the theorised "depth within the gravity well" that causes time dilation.MathewMunro (talk) 21:21, 27 February 2021 (UTC)
 * I'm still confused here. If you want to "isolate" the gravitational effect, what is the advantage of your proposal over something like [//science.sciencemag.org/content/329/5999/1630] which tested the effects of gravity with less than 1 metre height change difference, and which again is something mentioned in our articles. They also tested the effects of a 10 metre/s difference in relative speed. (As mentioned in the abstract, the authors were fully aware there have been similar tests before albeit with greater velocities or height changes e.g. Iijima et al. The only reason their bothered seems to have been to show they can even with fairly small differences.) BTW, as I understand from our article, we also have good experimental evidence that acceleration doesn't affect time dilation. Also, let's not forget that the Hafele-Keating experiment was not simply comparing to clocks on the ground but also flying in opposite directions and was followed by similar experience. And was followed up by more detailed repetitions. Nil Einne (talk)
 * I don't understand how you could possibly not see the difference between what they did and what I'm proposing. Their test did not compare two clocks moving at the same speed in different gravitational fields like my proposal would. Their test also did not investigate the effects of having a large amount of mass above you partially cancelling-out the gravitational effect of the mass below you.MathewMunro (talk) 21:38, 28 February 2021 (UTC)