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Ether and how it was disproved.

  1. Apr 3, 2009 #1
    Hello, i am relatively new to Physics and find the subject fascinating, really seems to give insights into the unvierse like religion/spirituality does.

    I was reading "A brief history of time" by Stephen Hawking and in the chapter "Space and time" he discusses Ether and how it was disproved.

    For any who has read that book, could you explain to me how it was disproved?
    It talks about relative speeds and how at right angles to this ether it SHOULD go at the speed of light, i have no idea what this means and whether it means the earths orbit of the sun or the earth spinning.

    I would really appreciate a baby step explanation of this as i would really like to get my head around the topic.
  2. jcsd
  3. Apr 3, 2009 #2


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  4. Apr 3, 2009 #3
    Yes thankyou, this helped me.
    But i am still unsure why light travelling at the same speed somehow disproves the existance of ether?

    Please elaborate, as the article cleared up some of my doubts but not fully.

    Thankyou for your reply :D
  5. Apr 3, 2009 #4
    Before Tam came here :)

    In fact, the story is not so straightforward. Special Realitvity technically had not disaproved the existance of ether: Lorentz had developed the theory called LET - it gives exactly the same predictions as SR.

    In modern terms, LET is an interpretation of SR with a hidden variable (the absolute rest frame of the ether). So LET is worse then SR for the following reasons:
    * ether is undetectable, remember about the Ocamms razor
    * ether has too many weird and isconsistent properties
    * hidden variables

    Finally even Lorentz had abandoned that idea.

    What is more important, SR is just a first step into GR, and there are no scientifically accepted analogs of LET which are compatible with GR.
  6. Apr 3, 2009 #5
    Suppose the solar system were filled with ether. Would our planet be moving in it? How fast? Would the speed of light always be c when an observer were at rest in the ether, and not when the observer were in a another moving inertial frame? Would the Earth ever be at rest in the ether? Would this be summer, winter, spring or fall? Is our planet dragging the ether along with it? Or is the ether always at rest in every inertial system? How can this be?
  7. Apr 3, 2009 #6
    Thankyou; but:
    Why does the experiment disprove/nearly disprove/whatever ether?
  8. Apr 3, 2009 #7
    The Michelson Morley experiment failed to provide support for aether.

    The scientific method goes a little like this (although it was not formalised as such at the time of Michelson and Morley):

    Think up a theoretical model for how things work.
    Create a falsifiable hypothesis from the model, or a number of hypotheses.
    Create, and then conduct, a repeatable experiment which tests those hypotheses.
    Report findings to your peers who can then repeat the experiment.

    So long as your model and hypotheses continue to pass repeated tests, it becomes a Theory.

    Michelson and Morely were testing for aether wind, which was a hypothetical consequence of the aether theory. They didn't find it and repeated experiments failed to find it.

    If for there to be aether, there must be aether wind, then the absence of aether wind disproves aether.

    As for LET (Lorentz Ether Theory, I prefer the older spelling version, since http://en.wikipedia.org/wiki/Ether" [Broken] refers to a chemical compound), LET is a model in which there is an aether, but it is undetectable because of Relativity. Some LET theorists could argue that there is no difference between aether and the "spacetime continuum" or the curved space that you see bent in many diagrams depicting the effects of gravity.

    The problem with aether, is that by making it undetectable, you make a step in the scientific method very very difficult, if not impossible. You can't falsify aether if it is undetectable.

    Einstein did make a comment at some stage to the effect that he hadn't necessarily destroyed aether (or the theoretical framework around it), but that he had reduced it to such an extent that it was as good as non-existent.

    So, really, aether is not disproved - it's not disprovable! - but aether wind was disproved. And as DaleSpam says on occasion, there are benefits in thinking about some things in terms of LET, just as there are benefits in thinking about other things in other terms.

    Last edited by a moderator: May 4, 2017
  9. Apr 3, 2009 #8
    What gets disproved is the notion of space being filled with a conventional like medium - one that acts like a gas or a fluid - when you try to measure the velocity of light with respect to this funny substance, the velocity always turns out to be the same in all directions - the null result was unexpected by M and M - and many interesting theories have been advanced to save the idea of an ether - you can read Einstein's own statments about a different kind of ether in some of his later writings - he attributes some of the properties of space as being ether-like ...the problem is, whatever space is, it can't be modeled like a conventional medium - the properties of space are sui generis - it has a characteristic impedance of 377 ohms as all antenna designers know - it has a capacity per unit length and an inductance per unit length - and it appears in some theories as having a characteristic that imparts inertia to particles - this would seem to be Einsteins's view - but others may interpret his comments differently
  10. Apr 4, 2009 #9


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    These numbers are just due to a mismatch of units in SI and have nothing to do with properties of space.
  11. Apr 4, 2009 #10


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    LET and the standard formulation of SR are not different physical theories. They are the SAME physical theory cast in different coordinate systems. LET does not have any physically "weird and inconsistent properties" that are not also shared by the standard formulation of SR. LET is just as physically compatible with GR as is the standard formulation of SR.
  12. Apr 4, 2009 #11
    Re Post #9 - Einstein would disagree with you - the properties of space are more than a units mismatch - You would also find strong disagreement from from Maxwell and a host of others. If space isn't something how is it curved - and how does it contain energy? To relegate space to nothing discredits much of what Einstein attempted to convey. The units ratio is historic - it does not detract from the reality of spacetime.
  13. Apr 5, 2009 #12
    Lets say LET and SR are different interpretations of the same theory, I mentioned it before
    So LET=SR
    Whats about GR? You say there is GLET=GR? Tam argued in the famous thread that GLET exists, but do you have any links?
  14. Apr 5, 2009 #13


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    Please read my post again. I wrote "These numbers are just due to a mismatch of units in SI and have nothing to do with properties of space." I read that to mean that space has properties that those numbers have nothing to do with.
  15. Apr 5, 2009 #14


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    In GR, physics is modeled with tensor equations. We don't need a different "GLET" because the tensor equations of LET are known to be valid (e.g., they are the same as those given by the standard formulation of SR in at least one inertial frame).

    I don't know what thread you are referring to here. There is no "GLET" afaik, but there is a metric tensor for LET (aka, GGT) that can be used in tensor equations.
    Last edited: Apr 5, 2009
  16. Apr 5, 2009 #15
    Specific models like Fresnel's (nearly) stationary aether were disproved by the Michelson-Morley-Experiment. Also models in which the Aether is fully dragged by matter were disproved by the Sagnac effect etc.
    However, as it was shown by Lorentz and Poincaré, one can create an aether model (LET) in which the aether is totally undetectable. For example, see Lorentz's famous 1904-paper:

    (Electromagnetic phenomena in a system moving with any velocity smaller than that of light)

    However, as it was told above, LET was superseded by SR because an undetectable aether is considered an unnecessary artifact within a modern scientific theory.

  17. Apr 7, 2009 #16
    True -the units for the electrical correlation are derived from basic units a la the SI - if these numbers have nothing to do with the characteristics of space - would you say the same with regard to a coax transmission line?
  18. Jul 4, 2009 #17
    I believe there has been some discussion as to exactly what Einstein meant as regards an aether.

    But anyway, leaving aside whatever Einstein meant, if as you believe, this new aether and spacetime have identical features, presumably we cannot tell them apart. So why not stick with spacetime.

    Last edited by a moderator: Sep 25, 2014
  19. Jul 4, 2009 #18
    Suppose we redefined ε0 and μ0 = 1. Then the speed of light in vacuum would be 1/sqrt(ε0 μ0) = 1, and the impedance of free space would be sqrt(μ00) = 1. But we would still have D = εE and B = μH (dielectric and magnetic materials). Maxwell's equations would not be fundamentally changed. We would always have inductance L and capacitance C. The velocity and impedance of a coaxial transmission line would still be 1/sqrt(LC) and sqrt(L/C) respectively. But in these units, the speed limit on our highways might be 9.5 x 10-8 c. We chose our units to be convenient in ordinary life. Cubits, stones, and days were all everyday units only a few hundred years ago, but none were very precise.

    α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω
  20. Jul 6, 2009 #19
    Not sure what happened to Latin of Lite's post that you quoted - I couldn't find any evidence of it.

    That aside, I think the reason for adopting a 'new aether' philosophy is that GR doesn't explain the mechanics of why space is curved in the vicinity of matter but is happy to regard it as a natural property of matter and stops there. The 'new aether', which seems to have an uncanny resemblance to dark energy, may take things a bit further.
  21. Jul 9, 2009 #20
    With respect to magnitudes - I would agree - but the ratio of units, irrespective of the magnitude(s) such as meters/sec squared conveys a physical thing ...farads/meter conveys a physical property
  22. Jul 10, 2009 #21
    Isn't the aether the same thing as the electromagnetic field? What defining property does the aether have which is not shared with the electromagnetic field?

    I think the only difference between them is that when we say "aether" we mean to communicate we are referring to an old view of EM which supposes there must be a preferred reference frame. However, this is not a defining property of the aether. By definition the aether is simply "the thing that waves to produce the wave phenomenon 'light'". Which is, again by definition, the EM field.

    It is often pointed out the aether had troublesome properties. For instance,

    Nevertheless, by this point the mechanical qualities of the aether had become more and more magical: it had to be a fluid in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It also had to be massless and without viscosity, otherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale.

    Well, does not the EM field have these very same properties?
  23. Jul 10, 2009 #22


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    Not it's not. The classical ether was the "medium" for EM wave propagation.

  24. Jul 10, 2009 #23
    Why don't we consider the EM field the medium for EM wave propagation?

    Ok. Wait. I think I got it. With, say, a sound wave the wave corresponds to the variation in time and space of the quantity pressure. The answer to the question "Pressure of what?" is "Air," the medium. With a surface water wave, the wave corresponds to the variation in space of time of the quantity height. The answer to "The height of what?" is "The water," the medium.

    With an EM wave, the wave corresponds to the variation in time and space of the quantity we call the EM tensor, or, if you prefer, the force per unit charge on an imaginary charge. But there is no "of what" underneath. There is only the EM tensor at each point. It is as if a wave could just be the variation of bare pressure with no associated fluid which it is the pressure of. Or a wave of varying height but not the height of anything.

    In these latter cases the absence of the medium makes it nonsensical. But when you think about, you see that the presence of a medium for EM, while not non-sensical, is just unnecessary.
  25. Jul 10, 2009 #24
    I too am reading this book and had a few questions about this chapter. all the replies so far seem pretty deep rather than "baby step" so i thought I would throw in what I thought of it because my "understanding" is very simple.

    so i thought the idea was that light waves would travel through the ether in the same manner that sound waves travel through air. so think about the doppler effect in the sense that the sound wave will appear to move at different speeds if you are moving toward the source as opposed to if you were moving perpendicular to the previous path, or not moving at all. but this was not the case for light, it travelled at the same speed during both instances.

    please correct me if i have made a mistake, as i too am trying to better understand it.
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