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When we measure the speed of light we get the same answer in all directions. This is taken to undermine the aether or absolute motion hypothesis and give support to the proposal that the speed of light is invariant, from which derives the theory of special relativity.

But doesn't the fact that we only measure speed of light 'there and back' undermine this conclusion? Wouldn't we expect this result through an aether?

Qmechanic
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Andrew
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    Hi @Andrew, you said “Wouldn't we expect this result through an aether?”. Actually, no, if you work out the math the two way speed of light is not isotropic in a standard aether theory. – Dale May 19 '19 at 02:49
  • Why do you think we can only measure "there and back"? For instance, one of the first measurements of the speed of light was made by noting variations in the timing of eclipses of Jupiter's moons, which would seem to involve measuring only the "back" reflection of the sun's light: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwjQzue_9KbiAhUiPn0KHZ7qAUkQFjABegQIDBAE&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FR%25C3%25B8mer%2527s_determination_of_the_speed_of_light&usg=AOvVaw2rdZSwUqhnsN5h3zZntZYF – jamesqf May 19 '19 at 05:58
  • @jamesqf Wikipedia says that although Rømer's measurement of Jupiter's moons seems to give a one-way speed, careful analysis shows that those calculations assume that the travel speed is isotropic. However, I must confess I don't fully understand the arguments given on that page. – PM 2Ring May 19 '19 at 10:03
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    Careful analysis shows that Rømer's measurement says nothing about the one-way speed of light. https://physics.stackexchange.com/questions/456243/can-we-measure-the-speed-of-light-in-one-direction/456327#456327 –  May 19 '19 at 10:43

4 Answers4

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There-and-back measurements still show the effect of an aether, because you can compare the results in different directions. For example, there-and-back along the direction of motion would show a different speed from there-and-back across it. This was the approach taken in the Michelson-Morley experiment.

Bob Jacobsen
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The bottom line is that this is the wrong question to ask. You don't ever prove an axiom in physics.

You're not quite right about the ether: while the first order effect cancels out in "there and back again" experiments, the second order effect doesn't, which is why the Michelson-Morley experiment stood a chance of testing for the ether. But Michelson-Morley wasn't the end of ether theory, because you could always add fixes to account for the results. For example, the famous Lorentz contraction in special relativity was originally invented for ether theory; the idea was that flowing ether physically squeezed objects smaller. Einstein just took effects like these more seriously.

Ether theory limped on for another 40 years, getting progressively more complicated as more results came in. Ether was not abandoned because it was disproven by experiments or because special relativity was proven by experiments, because this never happens. It was abandoned by the 1930s it could only explain experiments using tons of epicycles, while special relativity just worked perfectly out of the box.

knzhou
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    To be fair, you don't ever prove an axiom. Thus is the nature of axioms. – tox123 May 19 '19 at 23:58
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    While it's true that you don't ever prove an axiom in the mathematical sense, you do attempt to falsify them in science. If, after many attempts to falsify it, you have failed to disprove it, then you may accept it as true. This is known as inductive reasoning, and it would typically be regarded as "proving" the theory in the common, non-mathematical vernacular. (Not to diminish the importance of understanding the difference, but we ought make sure we account for these things when dealing with misunderstandings.) – jpmc26 May 20 '19 at 05:45
  • @jpmc26 The point is that ether isn’t falsified either. You can almost always add tweaks to avoid falsification, so using that alone is not good enough. – knzhou May 20 '19 at 10:57
  • For example, you could even take an extremely weak formulation of the ether which is just identical to GR but picks out some frame as special, without any physical consequences. That is not falsified any more than GR is. We don’t use it because it complicates things for no reason. – knzhou May 20 '19 at 10:59
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That the one-way speed of light is not observable plays a large role in keeping various relativistically-correct aether theories alive. Like general relativity, these new aether theories locally reduce to special relativity in the absence of nearby massive objects. Unlike general relativity, these new aether theories have a preferred universal frame of reference, typically a frame co-moving with the cosmic microwave background radiation.

David Hammen
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    If such theories cannot be ruled out, then why shouldn't they be amongst the respectable opinions that thinkers and researchers may choose to adopt and work with? – Steve May 19 '19 at 00:40
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    @Steve “Choose to adopt and work with” is not a binary. People make different choices. Lots of people find GR useful; few find The Flying Spaghetti God hypothesis useful. Other theories are in between in popularity, i.e the MOND family. Nor is this decision time-invariant: theories and hypotheses are more and less popular as people show them more or less useful. – Bob Jacobsen May 19 '19 at 01:32
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    @BobJacobsen, useful for what purpose? Perhaps when working with aether concepts attracts comparisons to the Flying Spaghetti God, you make it dis-useful for professional scientists to adopt such views and perform such work? And my point is to rebut the tone in which David speaks of apparently tenable theories being "kept alive" - the writer's implication being that these theories ought to be dead. Like I say, if they cannot be ruled out, then they are fair game for further scientific enquiry. – Steve May 19 '19 at 01:53
  • There are lots of theories that can’t be ruled out. If you want to advocate for a theory on that basis alone, go ahead. But if that’s all it has going for it, why would anybody spend their time on it? That’s always a judgement call. – Bob Jacobsen May 19 '19 at 01:59
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    @Steve, general relativity is a simple theory. Its only arbitrary parameter is Newton's gravitational constant. Other viable theories carry additional arbitrary parameters. In the limit that these parameters approach some value, typically zero, these theories become equivalent to general relativity. To date, all observations are consistent with these free parameters taking on the value that makes the alternative theory equivalent to general relativity. – David Hammen May 19 '19 at 02:51
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    @DavidHammen, that's basically what I said above in my original comment to Bob's answer, that as it stands today (and as it stood in about 1920) Lorentz's aether theory (in its latter form) is equivalent to relativity - they are not mutually exclusive theories. It is only the general philosophical approach and concepts they employ which differ. – Steve May 19 '19 at 09:13
  • @Steve -- That philosophical difference is huge in the case of special relativity versus LET. Both theories postulate that the laws of physics are the same in all inertial frames. The two theories diverge from there. Special relativity postulates that the one-way speed of light is the same in all inertial frames and derives length contraction and time dilation as consequences. LET postulates an unobservable preferred frame and postulates length contraction and time dilation as the mechanisms that hide these preferred frame.This is WTF butt ugly and it fails the smell test (aka parsimony). – David Hammen May 19 '19 at 15:13
  • @DavidHammen, the philosophical difference is indeed significant, and neither approach has been falsified on any physical experiment. What is parsimonious is not easily established. In my experience of talking to many people who have strong views similar to yours, they actually know little of the philosophies involved and have little interest in examining them, and yet feel the need constantly to disparage alternative views or approaches (using subjective and ambiguous criteria like "smelly") which, as the record shows, certainly remain tenable. – Steve May 19 '19 at 15:46
  • What about these theories is more ether-like, above and beyond picking a special frame like the CMB already does? In other words, are these theories just GR but with a “thicker” layer of CMB-frame matter? – knzhou May 20 '19 at 00:02
  • I’m not a priori for or against these theories, I’m just legitimately curious here. Any references? – knzhou May 20 '19 at 00:03
  • @knzhou - Yes, I have references. ButI it will take some time to filter them down. My key reference is a paper published by a PhD candidate that we recently interviewed. He is smart enough to know that he has essentially no chance of getting a job in the field of his PhD thesis. – David Hammen May 20 '19 at 11:44
  • @knzhou - We interviewed him because we have other PhD physicists / astronomers who fare quite well in our field of work even though their educational background is a bit orthogonal to what we do. Based on my background, I interviewed him (he passed), and I was also asked to look over his papers from the perspective of can he think and can he program. – David Hammen May 20 '19 at 11:44
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The $x$ direction is not any different physically than the $-x$ direction. So light travelling towards $x-> \infty$ should travel in the same way it travels towards $x-> -\infty$ wether theres an aether or not. What should make a difference is if we measure the speed of light in the direction the body that emitted the light is moving through the aether and any other direction, for example, the direction perpendicular to this movement to see the difference a bit easier. However no difference in arrival times was measured.

Juan Pablo Arcila
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