Poetry of Physics and the Physics of Poetry

142 The Poetry of Physics and The Physics of Poetry

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 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.

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

following: the phenomena in an accelerating frame of reference are
identical with those in a gravitational field.
The equivalence principle forms the heart of the General Theory of
Relativity. Einstein exploited this principle to successfully predict that, a
gravitational field would bend a beam of light, i.e. a beam of photons.
This is somewhat surprising in view of the fact that the rest mass of a
photon is zero. Although the rest mass is zero, the photon still has
energy, and since Einstein showed that mass and energy are equivalent,
perhaps light can also be affected by gravity. To demonstrate this, we
turn to the propagation of light in an accelerating elevator car.
Let us consider a beam of light propagating perpendicular to the
direction of the acceleration and entering the elevator from one wall and
exiting the elevator on the opposite wall. The beam of light will appear
bent to an observer in the elevator, as shown in Fig. 15.2. The reason for
this is that, by the time the light beam has propagated from one wall to
the other, the elevator has moved upwards because of its acceleration.
The beam of light, however, is unaffected by the elevator’s acceleration
and hence, continues to propagate along the same straight line it was
moving along before it entered the elevator. Relative to the accelerating
elevator, however it appears to exit at a point below the one it entered.
The beam of light will appear, to an observer in the accelerating elevator,
to have been bent by the same gravitational field that causes the objects
she drops to also fall to the floor. If, instead of a beam of photons, we
had sent a beam of massive particles through the elevator, they would
behave exactly like the beam of light. An observer in the accelerating
frame will conclude that the paths of both the massive particles and the

Fig. 15.2