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We were asked a question to differentiate the difference between the idea of an Ether and the idea of Quantum Fields. When I really began to think about it I concluded that the ideas are the same. The two are essentially the same idea. They both consist of the idea that 'something' permeates all of space and acts on everything. Why is this wrong?

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    $\begingroup$ Personally, I think you are right. The ideas are the same with respect to both describing something that permeates all of space (and, time) and acts on "everything". I would not say "everything" for a single field though since Physics offers up different fields for different purposes: We have motion fields (describing the flow of a liquid for example), we have temperature fields, we have Electromagnetic Fields (ala Maxwell), we have QED fields, we have gluon fields, and W,Z fields, and so on and so forth. $\endgroup$
    – K7PEH
    Commented Jan 29, 2017 at 18:38
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    $\begingroup$ I disagree. As anna v says, the whole point of the ether was to pick out a special frame in which Maxwell's equations hold exactly. But the laws of QFT look exactly the same in every frame, so the core motivation for the ether isn't there. $\endgroup$
    – tparker
    Commented Jan 30, 2017 at 9:40
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    $\begingroup$ Oh my, I love your teacher/tutor :P You're looking at both aether and QFT in very simplified terms. Your "definition" applies to every force "acting at a distance" (which is every single force if you look close enough) - including inertial forces, static electric fields (pre-QFT)... it doesn't even distinguish aether and QFT from most of the other competing theories, much less from each other :) There are similarities between aether and QFT, but you need to try a lot harder at understanding both to see them. Obvious hint: think about locality and relativity. $\endgroup$
    – Luaan
    Commented Jan 30, 2017 at 10:05
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    $\begingroup$ i can't help but wonder: would the old timer aether folks from the 1700's, if introduced to QFT, be saying "Now that's the ticket, that's my aether theory i've been yammering about!" -- that is, would they recognize QFT as an advanced and refined aether theory, seeing that it corrects the holes and mispredictions of their original theories -- would they take QFT as validation of their intuition that light is a phenomenon which travels within a greater context? $\endgroup$ Commented Dec 29, 2019 at 9:04
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    $\begingroup$ As far as I know, ether was thought of a kind of substance/matter that, in fact, could be pushed or sensed as a "wind" if you move related to it. Therefore, it was expected that the speed of light (using ether for moving) would be measured to be different depending on the relative movement of observer and the ether. Michelson experiment disproved this. A field is, on the other hand, a mathematical tool to do accurate calculations (with the important property of invariance of coordinates) for which it is not clear what "physical entity" the field is the face of. $\endgroup$ Commented Jun 6, 2021 at 8:04

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A different point of view to Jamal's answer: I think what distinguishes quantum field theory, where each elementary particle in the particle table defines a field all over space time, from the luminiferous aether is Lorenz invariance.

The luminiferous aether theory was falsified by the Michelson Morley experiment because it was not Lorenz invariant.

In quantum field theory an electron traversing space time is described by a quantum mechanical wave packet (which means that what "waves" is the probability of existing at (x, y, z, t)), manifested by creation and annihilation operators acting on the electron field, and the expectation value defines the location of the electron as a function of (x, y, z, t). The same for a photon, riding on the photon field. The quantum fields though by construction are Lorenz invariant and thus cannot be identified with the luminiferous aether.

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  • $\begingroup$ Yes, but the point is this comparison of a quantum field to the aether is non-sensical in the first place; it's comparing two completely different notions, of a medium and of a field. $\endgroup$
    – JamalS
    Commented Jan 29, 2017 at 17:57
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    $\begingroup$ @JamalS a medium could be written up as a field as far as mathematics used in physics go. en.wikipedia.org/wiki/Field_(physics) . In terms of the wave packet of a particle the photon or electron field acts like a medium. $\endgroup$
    – anna v
    Commented Jan 29, 2017 at 18:18
  • $\begingroup$ Your answer is not actually 'in contrast' to mine by the way; we don't disagree but offer different points of view and they're rather separate. $\endgroup$
    – JamalS
    Commented Jan 29, 2017 at 19:56
  • $\begingroup$ @JamalS ok, english is not my first language $\endgroup$
    – anna v
    Commented Jan 29, 2017 at 20:00
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    $\begingroup$ It might be worth pointing out that Lorentz ether theory is a theory of an ether that has Lorentz invariance in some sense, but its ether is slightly different from the standard luminiferous ether $\endgroup$
    – ACuriousMind
    Commented Jan 29, 2017 at 22:33
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The aether or luminiferous aether was the medium postulated as that which light travelled through. For example, sound waves are mechanical waves, in that they travel through pressure and displacement of the medium, e.g. water or air.

Likewise, the aether was thought of as the 'material' which allowed the propagation of light. Now, if we look at the notion of a quantum field, it is understood as a field that propagates through all of space and time.

The physical interpretation of the quantum field itself may vary depending on what we are describing; in the case of electromagnetism, one quantises the 4-potential $A_\mu$. In the case of the Higgs, one quantises a scalar, $h$ (though it is more complicated than this in reality) which has dimensions $[h] = 1$, that is, of energy in natural units.

One can think of the excitations of the field $A_\mu$ as giving rise to quanta which we call photons, but in the general case of a quantum field, there is no notion of any wave propagating through it, and so it is a distinct notion from the aether.

In the case of the other quantum fields of the Standard Model, there is certainly no plausible relation whatsoever to the aether, as they don't have anything to do with electromagnetic radiation, but rather completely different particles.

In summary, just because two things are postulated to occupy all of space and time does not mean they are the same notion. Another example: a force field $\vec F$ is different from a scalar potential, $\phi$. In addition, the aether itself is a material and not a field; we could perhaps describe its distribution in terms of a mass distribution $\rho$ which is a field, but the aether itself is not a field.

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Theoretical physics isn't really concerned with ideas very much; the research is really into building a theory, that is, a set of mathematical instruments which can be used to make accurate calculations and predictions.

The idea of aether may be reminiscent of the ideas of QFT. The fundamental difference is that the theory that came with aether actually made incorrect predictions, and so didn't turn out to be very useful. The mathematical tools of QFT, on the other hand, when turned into a specific theory such as QED, have stood the test of time in terms of accuracy.

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    $\begingroup$ I very much disagree with your first paragraph: this is an "instumentalist" view of science that some scientists perhaps espouse during their working hours, but it is by no means what most do "after hours". Read e.g. David Deutsch "The Fabric of Reality" , ch. 1 - or N. David Mermin's "Boojums" book for a spirited argument against the "shut up and calculate" view. $\endgroup$
    – NickD
    Commented Mar 22, 2017 at 20:42
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Above theories have fields. Above theories can have Lorentz invariance for observable effects. The difference is that in an ether theory the fields have a physical interpretation - they describe some properties (say, density, velocity and so on) of the ether. Instead, the fields in QFT are simply fields, nobody even tries to interpret them.

To see how an ether theory corresponding to the SM would look like take a look at http://arxiv.org/abs/arXiv:0908.0591 or http://ilja-schmelzer.de/matter/ This is, of course, not a classical "luminiferous aether", it has to interpret all fields, not only the EM field, as different properties of some ether. But the main idea, namely that the speed of light is the speed of sound of the ether, remains.

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I'd say that the main property of ether is that it was introduced related to relativity, as a reference frame allowing to measure, for example, something as the speed of light, which was thought to be $c$ in some special reference frame, and $c\pm v$ in frames moving at $v$ with respect to that frame. The ether was then thought as something special and absolute, while quantum fields are thought to evolve in a space-time, and to have specific transformation properties under the Poincaré group.

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  • $\begingroup$ Ether theory predates Relativity. The problem of its physical properties started with Maxwell's theory and originally had nothing to do with reference frames. $\endgroup$ Commented Dec 16, 2023 at 6:26

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