Stellar aberration and Fresnel ether drag

  1. I'm interested in the prehistory of the history of relativity. In this
    history the nature of light and the ether comes in. One phenomenon that
    was debated in this context was stellar aberration: that the positions
    of the stars in the sky change slightly, or, more precisely, the
    perceived directions of the light beams emitted by the stars change
    slightly, depending upon the motion of Earth around the sun. This
    phenomenon is equivalent to the percieved change of the direction of
    which falling raindrops hit a car, depending upon the motion of the
    car.

    Stellar aberration is thus easily explained if a light beam is
    considered to be a stream of particles (corpuscles or photons), but
    with a wave theory of light, stellar aberration shouldn't occur, unless
    we use the relativistic Lorentz transformation. Why is it so is
    explained in the beginning of this article:

    http://www.mathpages.com/rr/s2-05/2-05.htm

    Briefly: if we consider a medium denser than air/vacuo moving in a
    direction perpendicular to the propagation of incoming planar light
    waves, coming from vacuo, and the medium/vacuo boundary is also
    perpendicular to the incoming wave motion, then a wave front reaches
    every point on the boundary simultaneously. Then, Huygens's principle
    implies that the waves must propagate in the medium in a direction
    perpendicular to the boundary, that is, in the same direction as the
    incoming waves, independently of the velocity of the medium. Thus,
    stellar aberration would not be observed.

    In the above article, we are told that Fresnel, in 1818 (long before
    Lorentz and Einstein) proposed a way around this problem: that the
    medium drags the luminoferous ether slightly, so that the part of the
    ether which contains the medium is moving in the same direction as the
    medium with velocity (1-1/n^2)v, where v is the velocity of the medium
    and n is its refractive index.

    But I can't see that this solves the problem. On the contrary, the
    moving (dragged) ether has the the same boundary to the stationary
    ether, in which the incoming waves propagate, as the medium which
    drags it. Thus, by the same argument as before, the waves must
    propagate in the moving ether in a direction perpendicular to this
    boundary, that is, in the same direction as the incoming waves. The
    waves will then propagate in the same direction all the time, namely
    perpendicularly to the boundary between vacuo and the medium (which is
    also the boundary between the stationary and the moving ether), whether
    observed by an observer in rest relative to the stationary ether, the
    moving ether, or the moving medium. Thus, even with such drag, stellar
    aberration wouldn't be observed. So, Fresnel's hypothesis doesn't solve
    the problem it was intended to solve!

    So I wonder, have I misunderstood something, or this above article
    faulty, or what? What was the point with Fresnel's ether drag
    hypothesis?

    Grateful for all answers and comments,

    Erland Gadde
     
  2. jcsd
  3. Erland Gadde wrote:
    > What was the point with Fresnel's ether drag
    > hypothesis?


    I believe his ether drag hypotheses was intended to explain the
    measurements of Fizeau, in which the speed of light in moving water is
    increased or decreased. Today we know his drag coefficient is equivalent
    to the lowest-order expansion of the relativistic formula for velocity
    addition. See, for instance, the Wikipedia article on "aether drag
    hypothesis".


    Tom Roberts
     
  4. Erland Gadde wrote:
    > http://www.mathpages.com/rr/s2-05/2-05.htm
    > In the above article, we are told that Fresnel, in 1818 (long before
    > Lorentz and Einstein) proposed a way around this problem: that the
    > medium drags the luminoferous ether slightly...But I can't see that
    > this solves the problem.


    As I read the article you cited, the seeming lack of aberration of the
    wavefront is explained just by considering how an image is actually
    formed in a telescope. But then we should see different amounts of
    aberration depending on the index of refraction of the medium filling
    the telescope. According to that article, Fresnel's partial drag
    hypothesis
    was intended to explain why we don't see different amounts of
    aberration,
    even if we fill the telescope with water (for example).

    Tom Roberts wrote:
    > I believe his ether drag hypotheses was intended to explain the
    > measurements of Fizeau, in which the speed of light in moving water
    > is increased or decreased.


    Fizeau performed that experiment in 1851, but Fresnel's hypothesis was
    in 1818. It's true that Fresnel's hypothesis is consistent with the
    outcome
    of Fizeau's experiment, but it wasn't proposed specifically to address
    Fizeau. (Actually, I believe it was the other way around, i.e., Fizeau
    was
    trying to test Fresnel and the other dragging hypotheses.)
     


  5. You are totally right. Fizeau's article of 1859 at the Annalles de Chimie et de Physique make it clear the purpose of the experiment was to choose among three possible models for the relationship between matter and the aether: (1) the aether is unaffected by the motion of transparent bodies; (2) the aether shares the motion of transparent bodies; or (3) part of the aether is dragged along with a mobile transparent body. He claims his experiment with the moving water to be an experimentum crucis of some sort, but when confronted with the result that only (3) agrees with his experimental results, Fizeau asserts that more exploration on the subject is needed before concede this extraordinary hypotheses reflects the true nature of the phenomenon.
     
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