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James Bradley's observation of aberration (could have) anticipated Michelson-Morley?

  1. Oct 31, 2012 #1

    bcrowell

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    I'm reading Eisenstaedt, The curious history of relativity: how Einstein's theory of gravity was lost and found again. In ch. 1, Eisenstaedt says that in the early 18th century, James Bradley at Oxford tried to observe parallax, failed, but detected a much larger effect, aberration, which showed up as an annual variation. Eisenstaedt's discussion isn't very clear to me, but it sounds like Bradley's observations weren't compatible with Galilean relativity. This suggests that Michelson-Morley could have been anticipated by a century, if only Bradley's observation had been interpreted correctly. Is this right? Is there a natural interpretation of Bradley's observations in the ether theory? But the ether theory post-dated Bradley by 60 years, so I guess the only contender was Newton's emission theory?

    http://en.wikipedia.org/wiki/James_Bradley

    There are also some interesting remarks about Arago getting a negative result in attempts to detect a difference in refraction of starlight from the same star at morning and evening. According to Newton, such an effect should have existed.
     
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  3. Oct 31, 2012 #2
    Re: James Bradley's observation of aberration (could have) anticipated Michelson-Morl

    what is the connection between aberration which is a result of the transverse motion of the observer relative to the propagation speed and angle from the source and an ether???
    Wouldn't aberration be expected to occur with or without a static medium??
     
  4. Oct 31, 2012 #3

    jtbell

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    Re: James Bradley's observation of aberration (could have) anticipated Michelson-Morl

    According to

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

    Fresnel explained stellar aberration in 1818 by assuming that the aether is "partially dragged" by a material medium. The same hypothesis also explains Fizeau's later experiments on the speed of light through a moving medium (water). It turns out to be mathematically equivalent to a first-order approximation of relativistic "velocity addition".
     
  5. Oct 31, 2012 #4

    PAllen

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    Re: James Bradley's observation of aberration (could have) anticipated Michelson-Morl

    Bradley explained aberration by a corpuscular theory of light + Galilean relativity [in which invariant c is taken to be false; within this theoretical framework, aberration is a true one way measurement of non-invariant light speed (assuming you know observer velocity changes)]. This is normally referred to as classical or non-relativistic aberration. Since Einstein, 1905, want to eliminate any need for aether, and rescue both Maxwell + relativity, he needed a new model of aberration, which I think had no predecessors. It differs second order from Bradley's. My understanding is that the difference between Bradley aberration and relativistic aberration remains outside the precision of current observations.

    So, I don't see a way Bradley could be called an anticipation.

    Note, the various 'in between' explanations involving aether were created based on near unanimous rejection of corpuscular light theory, yet not having SR. Again, I don't see an anticipation.

    The beauty of the 1905 paper (and I don't believe any partial predecessor, e.g. Poincare, had this) was an almost trivial, conceptually clean derivation of aberration with no additional assumptions. [And, in this framework, aberration isn't really a measure of lightspeed at all (one or two way); it is measure of of the transformation of null vector angle; that is, it measures changes in observer velocity, only]
     
    Last edited: Oct 31, 2012
  6. Nov 1, 2012 #5
    Re: James Bradley's observation of aberration (could have) anticipated Michelson-Morl

    The groundwork for SR was definitely laid out by Bradley's experiments with aberration. They showed that the speed of light is isotropic regardless of the motion of the distant stars or the motion of the Earth around the sun, so regardless of the motion of the source or receiver, apparently where the second postulate comes from. Bradley even measured the speed of light quite accurately from aberration. Later experiments related to Fresnel's equations and Fizeau's experiments were performed by filling telescopes with water, still showing the same aberration.

    Much could have been determined from these experiments alone. For instance, if the speed of light is isotropic in our frame (considering it to be mostly inertial), and there is nothing special about our frame, then the speed of light is isotropic in all inertial frames. The experiments already established that the speed of light in inertial frames is non-ballistic. If we had then simply assumed frames to still be relative, just non-Galilean relativity and with no absolute frame for a medium, perhaps we could have established that the physics is the same for all inertial frames and measuring the same speed of light, for if they did not, there would be such an absolute frame, so frames could not be purely relative, the same as each views the other.

    But since isotropic speed is a property of a medium, it was assumed that such an absolute frame of a medium exists, the difference found within some small margin of error, which is what the much more precise M-M experiment set out to detect. So perhaps we could have had SR earlier, but that would have been a lot to ask at the time, and difficult to accept even after the null result of M-M confirmed the isotropy of light in all inertial frames more precisely, again considering there to be nothing special about our own frame. Also, M-M by itself could have been simply a ballistic result, with light travelling isotropically only from the source of the apparatus and back over the same distance to a receiver at rest with the source, so aberration was the real ground breaker for SR, not M-M as is often stated.
     
    Last edited: Nov 1, 2012
  7. Nov 1, 2012 #6
    Re: James Bradley's observation of aberration (could have) anticipated Michelson-Morl


    According to Albert the key things were Maxwell's and Fizeau.
     
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