Environmental Engineering Reference
In-Depth Information
So where do we stand? The null (no ether) result of the Michelson-Morley
experiment
6
and the apparent need of an ether to explain stellar aberration have
led us into an impasse. It is an impasse which Einstein was ultimately able to
overcome.
5.3 EINSTEIN'S POSTULATES
Einstein was particularly concerned with the breakdown of the principle of rel-
ativity implied by the presence of an ether. Just a few years earlier, Maxwell had
written down the equations which define the classical theory of electromagnetism.
The equations are beautiful and encode the idea that light is an electromagnetic
wave. However, the equations taken at face value seem to predict that light travels
at a speed
c
1
/
√
ε
0
µ
0
independently of the motion of either the source which
produced it or the observer who measures it
7
. This circumstance seems absurd:
for a wave travelling through a medium the speed is indeed independent of the
motion of the source but it certainly depends upon the motion of the observer. Of
course one can sidestep this problem by supposing that Maxwell's equations are
only approximately correct and that the speed
c
which appears in them ought to be
replaced by the speed of light appropriate to the frame in which one wants to use
the equations. This attempt to hold on to the ether has unpleasant consequences,
for example Coulomb's Law would now be slightly different in different inertial
frames. With the evidence mounting, Einstein took the dramatic step of assuming
that the ether does not exist and that Maxwell's equations are correct. At a stroke
he could explain the null result of Michelson-Morley, restore the principle of rel-
ativity to its central role in physics and keep the equations of Maxwell without
modification
8
.
In 1905, Einstein therefore made the two postulates that define his new theory
of space and time, and which we can state as follows.
1st postulate:
The laws of physics are the same in all inertial frames
.Thisisa
strong statement of the principle of special relativity which we discussed above and
which was anticipated by Galileo. We talk about 'special relativity' to remind us
that it is concerned only with the equivalence of inertial frames, i.e. it says nothing
about accelerated frames of reference. By this Einstein insists that there should be
no experiment in physics which can allow any one inertial frame to be singled out
as special. It is this principle which implies that the ether does not exist.
2nd postulate:
The speed of light in vacuum is the same in all inertial frames
.
This statement saves the laws of electromagnetism since if the speed of light did
vary from frame to frame then Maxwell's equations would violate the 1st postulate.
It also explains in a trivial manner the null result of Michelson-Morley. We stress
=
6
Using masers, in 1959 Cedarholm & Townes constrained the speed of the ether relative to the Earth
to
<
30 m/s.
7
ε
0
is the permittivity, and
µ
0
the permeability, of the vacuum.
8
We shall also see that his theory is able to explain the effect of stellar aberration.
