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Why was the Ether Theory refused by Modern Physics? If you please explain me, I just wanted to understand it more.

Rego
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It's not entirely true that ether theory was refused by modern physics, but seen as superfluous and over-complicated compared to special relativity's notion of time and space. There's a version called Lorentz ether theory which postulates that it's not possible to detect the absolute ether, and gives the same results as predicted by special relativity. Hence it's not possible to experimentally determine the difference between the two theories.

But whereas LET is constructed in an hoc way by adding time dilation and Lorenz contraction into the theory to make the ether undetectable, these follow from the two postulates of SR. So the sheer elegance and simplicity in favour of SR is why it's taken more seriously than LET.

John McAndrew
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It's because the luminiferous aether was, by definition, composed out of some particles or elementary building blocks with a well-defined location in space. Consequently, it picks a privileged reference frame, the rest frame of the aether. In this rest frame, the speed of light – vibrations of the aether – could be constant, $c$.

However, things moving relatively to this aether by the speed $v$ should detect a different speed of the light relatively to them – the speed would go from $c-v$ to $c+v$, depending on the direction. However, this modification of the light speed, the so-called aether wind, was shown to be non-existent by the Morley-Michelson experiment which measured the speed to be $c$ regardless of the source and the observer. This falsifies the existence of the aether.

The equivalent but even more robust refutation of the aether came from the theory. A physicist named Albert Einstein built a whole new theory of spacetime, the so-called special theory of relativity (a picture of this physicist is often being shown by the ordinary people as well), that also assumes/guarantees that the speed of light is always constant and there can't be any privileged reference frame. Relativity has been backed by the Morley-Michelson experiment as well as hundreds of much more specific experiments.

One of the things it guarantees is that light (electromagnetic radiation) has to be made out of disturbances of the empty space, the vacuum itself, and not a localized material carrier.

Luboš Motl
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    I'm going to argue with the wording "picks a privileged reference frame". I'm fairly sure you're implying that it is a well-founded expectation that physics does not pick a preferred reference frame. Physics, yes, but the universe, no. The reference frame defined by the co-moving coordinates (or stationary relative to the CMB) is preferred in some sense. I don't understand how anyone can argue otherwise. So I ask: what is the justification behind the requirement that no preferred reference frame exist, aside from experimental evidence relevant to E&M (a la Michelson and Morley). – Alan Rominger Nov 05 '11 at 22:52
  • Dear @Zassounotsukushi, the breaking of the local Lorentz symmetry by the cosmic microwave background is a spontaneous breaking. It's very tiny and what's more important, it can be completely undone. If you screen a vessel against the microwaves, the interior will be completely empty and no preferred inertial system may be identified by experiments inside the vessel. As long as you don't measure some very weak radiation that has nothing to do with typical other processes, it's true even in the Universe where the CMB is present. – Luboš Motl Nov 06 '11 at 07:58
  • "So I ask: what is the justification behind the requirement that no preferred reference frame exist" - It's the validity of a principle known as the "principle of relativity". It's one of the postulates of special relativity (the other's being the constancy of the light speed) but it's been a part of physics since Galileo. In non-relativistic mechanics, it implied the existence of the Galileo symmetry (one may add $\vec v$ to all velocities and nothing changes) which was deformed to Lorentz symmetry in the age of relativity. – Luboš Motl Nov 06 '11 at 07:59
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    Ultimately, the principle of relativity boils down to observations, and I don't mean Morley-Michelson's experiment. The relevant observations are that on the Earth's surface, we don't even detect that the Earth is moving 30 km/s in the solar system, by other speeds around its axis. In the moving train, we can't detect that we're in a moving train as long as the motion is uniform, etc. This is known from informal as well as very accurate observations and is summarized as the principle of relativity, a cornerstone of physics: physical laws have the same form if we add the velocity $\vec v$... – Luboš Motl Nov 06 '11 at 08:00
  • Ha-ha! Disturbances of the empty space! How convincing! – Vladimir Kalitvianski Nov 07 '11 at 10:52
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    @LubošMotl, my apologies in advance, but this one is just too much fun not to mention! Here's a quote from 1920: "...according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable." Does anyone care to guess who made this acutely heretical statement, one that voided Einstein's fundamental insight in special relativity that there is no ether? You can find the author and his 1920 address here. – Terry Bollinger Jul 07 '12 at 15:48
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    I agree with AE that in this sense, by its (vacuum's) having some qualities (e.g. values of fields in it), there exists aether. That's why I was carefully talking about the 19th century aether that had to be composed of physical particles. That was refuted by special relativity. – Luboš Motl Jul 08 '12 at 09:51
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    @TerryBollinger I think the penny has just dropped for me at age 50 why physicists object so violently to the word "aether". If the word means specifically "composed of non-vacuum states" or "made of particles", then I agree with the violence of the objection. I on the other hand have for a long time kind of liked the idea of calling the vacuum an "aether", because it reminds us that QFT (as well as general relativity, through your Einstein quote) teaches us that there is no such thing as a void (an idea that gave me nightmares as a child) - "empty space" is made of quantum fields. – Selene Routley Nov 21 '14 at 00:07
  • Aether may sound as anything that fills the vacuum, @WetSavannaAnimalakaRodVance. After all, the classical notion of "aether" - which used to be synonymous with "quintessence" - was some sort of a spirit. But since Newton, "aether" was used as a hypothetical material composed of particles. Newton was actually trying to explain gravitation via aethers of different densities etc. Those things culminated in the 19th century when the aether had very playful models with gears and wheels, by FitzGerald and others... – Luboš Motl Nov 21 '14 at 07:33
  • Even the claim that one needs quantum fields is debatable. One may describe the whole QFT Hilbert space in terms of particles only, so that the vacuum is really unique and empty, and one doesn't "need" to talk about quantum fields at all. – Luboš Motl Nov 21 '14 at 07:34
  • @TerryBollinger true: but why are there "quantum fields" in space ?? Either the "field" is equivalent to what was formerly called "aether", or there is something in space which produces the "fields" – PERFESSER CREEK-WATER Jul 28 '17 at 02:06
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There have been many attempts over the past century to answer this question, all of them incorrect as far as I can see. The most common refutation of ether (or aether) theory is based on the null result of the Michelson-Morley experiment (about which there is no doubt, the experiment having been refined and repeated many times with the same null result). M-M could be excused for it, but there is no excuse for modern physicists making the same mistake. What everyone seems to forget is that solid matter is mostly "empty" space - the separation between atomic nuclei is about 10^5 times their diameter, as discovered by Rutherford in 1910. Also, we now know that the atoms are held in place by electromagetic fields, so that the lengths of the arms of M-M's interferometer would be subject to the same effects of different light (e-m field) speeds as the interfering light beams themselves. This ("Lorentz contraction") is not an ad hoc effect, but follows from the finite speed of field changes within solids, limited to the same speed as the light beams bouncing to and fro among the interferometer mirrors.

This physical explanation for Lorentz contraction is set out well by Feynman in his Lectures on Physics, Vol II, Sec. 21-6 on retarded potential effects. The only physical constraint needed to explain relativistic length and clock/time effects in Lorentz Ether Theory (LET) is that light is the fastest signal in empty space. In practice, the ether as a unique reference frame for e-m field (e.g. light) propagation can be ignored because the experimental predictions of LET are always identical with those of Special Relativity (SR). But whereas SR offers no physical explanation for relativity beyond asserting the principle, LET does, which should surely be more satisfactory for physicists! Any inertial observer can assume that the speed of light is the same in all directions around him/her, but so can any other inertial observer, and they can not all be right, as is shown by the fact that they will disagree about estimates of the timing of simultaneous events, for example! LET shows exactly how this works in terms of the speed of light as an upper limit, as was known by Poincare in his 1904 St Louis World's Fair lecture "Science et Hypothese", in which he speculated about the collapse of relativity as a universal principle if anything can go faster than light.

By about 1920, Einstein had realised his error in declaring the ether superfluous as a concept, because he had realised that a unique, universal reference frame is implied by General Relativity in which the speed of light becomes locally dependent on gravitational potential around masses, instead of being absolutely fixed in value everywhere as in SR and LET. But noone seems to have been ready for that backflip. Instead, conceptual nonsenses like "curved space" are now preferred to a straighforward dependence of light speed on position in a gravitational field!

In summary, LET = SR in all predictions, but in my view LET is superior because it contains a physical explanation (light speed in a unique inertial reference frame as an upper limit), rather than relying on a statement of a principle - how do atoms know about principles?

The next interesting question is how does "empty space" (or "ether", to give a name for the concept of a space with physical properties) control the speed of light, between and inside atoms, and as slowed down near large masses? Does Higgs Boson theory shed any light on this (sorry!)?

(See trevm42 on Youtube)

Trevor Morris
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  • Trevor, I did not notice your third-to-last paragraph before adding a comment to the same effect to @LubošMotl's answer. The remarkable Leiden address you mentioned can be found online here. – Terry Bollinger Jul 07 '12 at 15:55
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    A great answer, Trevor. I find it fascinating that the Michelson-Morley experiment actually had two conclusions, the second of which is almost universally ignored. The alternate conclusion was that it is simply not possible to detect the ether drift because of Lorentz contraction effects. – dcgeorge Oct 07 '13 at 16:29
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    A scientific theory is only valuable when it is testable. Untestable components are superfluous, or even anti-scientific. Using your principle, any physical theory without a "why" clause would be improved by appending "because God did it (undetectably)". – Foo Bar Mar 15 '15 at 11:42
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    Great answer. My only comment is on the curved space "conceptual nonsense". I would agree that the intrinsic curvature of spacetime without embedding in a higher dimensional manifold is intuitively non-physical. However, there is nothing non-physical in space being curved in time (say, by energy). Reducing dimensions for clarity, if space is a 2D surface in a flat 3D spacetime, then there is nothing wrong with this surface having dents where massive objects are located. These dents would simply represent the fact that these areas of space are moving slower in time. – safesphere Jul 15 '18 at 18:34
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There are a number of experiments that disproved that there exists an inertial frame against which everything is moving, and electromagnetic waves propagate, the luminiferous aether. There exists a wikipedia article that covers these questions.

anna v
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    Anna, that's a really good reference, well worth reading. I liked in particular its matter-of-fact explanation of Einstein's later introduction of an updated ether concept, which often has not been discussed or even mentioned in courses on relativity. Open discussion is better, I think. – Terry Bollinger Jul 07 '12 at 16:31
  • I think the words you use are too strong. It cannot even be proven that light moves the same speed in all directions, which is intrinsically linked to the discussion of interial frames. The existence of an absolute rest frame is consistent with the experimental predictions of special relativity if you accept effects that slow clocks and contract lengths when moving relative to it (which can be made surprisingly non-ad hoc). I agree that this is seen as superfluous. But it is not disproven. – M. Winter Nov 20 '23 at 13:29
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There never was really any experimental or observation evidence that the luminous aether ever existed in the first place. It was merely an invention to pave over a gap in the Newtonian model of light.

Up until the mid-19th century everyone thought that Newton's experiments with prisms and slits had conclusively demonstrated that light was a wave phenomena i.e. it behaved just like waves in water. That, however, raised the question of "waves in what?" The answer was the "aether", something that couldn't be seen, touched or measured but whose motion transmitted light just like the motion of water transmitted the force of a wave.

Basically, the luminous aether was just a concept that the era's scientist pulled out of their collective… er, hats.

By contrast, Leibniz argued, purely on philosophical grounds, that light must be made of particles. Leibniz lost the argument because his hypothesis didn't have the experimental predictive power that Newton's did. However, if Leibniz's idea had been the dominate idea, the concept of luminous aether would have never been invented in the first place.

Prior to the late-19th century the idea of "subtle fluids" was evoked to explain many phenomena such as heat and electricity. That is why even today we talk of electricity "flowing" in "currents."

The history of science is filled with such inventions that paper over gaps in existing knowledge.

TechZen
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