Poetry of Physics and the Physics of Poetry

The General Theory of Relativity 143

The General Theory of Relativity 137

Fig. 15.2

On the basis of this argument, he predicted that starlight passing close

to the Sun would be bent by its gravitational field and hence, during a

solar eclipse, the position of a star close to the S un in the sky would be

displaced from its normal position. In Fig. 15.3, we illustrate how the

bending of the starlight by the Sun makes it appear that the position of

the star has changed position. The first opportunity to measure the effect

of the gravitational pull of the Sun on starlight came during the total

solar eclipse of 1919. The expedition of British scientists who traveled to

Africa to observe the eclipse, found that the position of stars near the Sun

had indeed changed, as Einstein had predicted. This measurement

verified the validity of the equivalence principle.

Fig. 15.3

Einstein further exploited the equivalence principle to determine the

effects of a gravitational field on the space and time perceptions of an

Fig. 15.3

light were bent because both the massive particles and the light were
attracted by the gravity. At this point, Einstein exploits the equivalence
principle. He argues that, since the laws of physics are the same in the
accelerating frame and the stationary frame sitting in a gravitational
field, then, one can expect the beam of light to be bent by a bona fide
gravitational field such as that generated by the Sun.
On the basis of this argument, he predicted that starlight passing close
to the Sun would be bent by its gravitational field and hence, during a
solar eclipse, the position of a star close to the Sun in the sky would be
displaced from its normal position. In Fig. 15.3, we illustrate how the
bending of the starlight by the Sun makes it appear that the position of
the star has changed position. The first opportunity to measure the effect
of the gravitational pull of the Sun on starlight came during the total
solar eclipse of 1919. The expedition of British scientists who traveled to
Africa to observe the eclipse, found that the position of stars near the Sun
had indeed changed, as Einstein had predicted. This measurement
verified the validity of the equivalence principle.
Einstein further exploited the equivalence principle to determine the
effects of a gravitational field on the space and time perceptions of an
observer. Let us consider from the point of view of a stationary observer
a clock in an accelerating elevator car. By virtue of the fact that the clock
develops a velocity with respect to the stationary observer, as a result of
the elevator’s acceleration, we expect the clock to slow down. Hence, the
stationary observer will observe a time dilation in the accelerating frame
of reference. By virtue of the principle of equivalence, we would also
expect that a clock sitting in a gravitational field would also slow down.
This gravitational time dilation has been experimentally verified
in two separate experiments, both of which involve ‘atomic clocks’.
An atom is like a clock. The electron orbits the nucleus of the atom