Special Theory of Relativity
5
No such shift was observed in the experiment.
The above result is based on Eq. (1.2) for the transformation of the velocity
of light, which was derived from the Galilean transformations (1.2). An attempt
was made to salvage the Galilean transformations by postulating that a hypothetical
medium called either, responsible for the propagation of light, is dragged along
by the earth as it moves in space. Then the speed of light with respect to an
observer on the earth would remain unaffected by the motion of the earth,
analogous to the speed of sound in which case the air is dragged along. This
would explain the null result of the Michelson-Morley experiment, but this is in
conflict with the observed aberration of starlight received on the earth. It is
found that in order to observe a star, the telescope should be titled in the direction
of the earth’s velocity (Fig. 1.2). This tilt would not be needed if the
light-propagating ether was dragged along by the earth. Actually, what is observed
is not the absolute tilt but the variation of the tilt as the earth changes it velocity
along its orbit around the sun. The either-based explanation became even less
tenable after Lodge (1892) showed that the velocity of light is unaffected in the
vicinity of rapidly rotating bodies.
Fig. 1.2 In order that starlight passes along a telescope moving with velocity v,
the telescope should be tilted at an angle of α = v/c.
1.4 POSTULATES OF SPECIAL RELATIVITY
Einstein (1905) proposed a radically different but, in retrospect, a simple approach
to the problem posed by the Michelson-Morley experiment. He started with the
principle of relativity but also postulated that the speed of light is the same in all
inertial frames of reference, thus giving it the status of a physical law. This
immediately explains the Michelson-Morley result but requires that the Galilean
5
No such shift was observed in the experiment.
The above result is based on Eq. (1.2) for the transformation of the velocity
of light, which was derived from the Galilean transformations (1.2). An attempt
was made to salvage the Galilean transformations by postulating that a hypothetical
medium called either, responsible for the propagation of light, is dragged along
by the earth as it moves in space. Then the speed of light with respect to an
observer on the earth would remain unaffected by the motion of the earth,
analogous to the speed of sound in which case the air is dragged along. This
would explain the null result of the Michelson-Morley experiment, but this is in
conflict with the observed aberration of starlight received on the earth. It is
found that in order to observe a star, the telescope should be titled in the direction
of the earth’s velocity (Fig. 1.2). This tilt would not be needed if the
light-propagating ether was dragged along by the earth. Actually, what is observed
is not the absolute tilt but the variation of the tilt as the earth changes it velocity
along its orbit around the sun. The either-based explanation became even less
tenable after Lodge (1892) showed that the velocity of light is unaffected in the
vicinity of rapidly rotating bodies.
Fig. 1.2 In order that starlight passes along a telescope moving with velocity v,
the telescope should be tilted at an angle of α = v/c.
1.4 POSTULATES OF SPECIAL RELATIVITY
Einstein (1905) proposed a radically different but, in retrospect, a simple approach
to the problem posed by the Michelson-Morley experiment. He started with the
principle of relativity but also postulated that the speed of light is the same in all
inertial frames of reference, thus giving it the status of a physical law. This
immediately explains the Michelson-Morley result but requires that the Galilean