- #1
Erland Gadde
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 perceived 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
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 perceived 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
