bei48482_FM

Gravity and Light

It follows from the principle of equivalence that light should be subject to gravity. If a

light beam is directed across an accelerated laboratory, as in Fig. 1.18, its path relative

to the laboratory will be curved. This means that, if the light beam is subject to the

gravitational field to which the laboratory’s acceleration is equivalent, the beam would

follow the same curved path.

According to general relativity, light rays that graze the sun should have their paths

bent toward it by 0.005°—the diameter of a dime seen from a mile away. This pre-

diction was first confirmed in 1919 by photographs of stars that appeared in the sky

near the sun during an eclipse, when they could be seen because the sun’s disk was

covered by the moon. The photographs were then compared with other photographs

of the same part of the sky taken when the sun was in a distant part of the sky (Fig. 1.19).

Einstein became a world celebrity as a result.

Because light is deflected in a gravitational field, a dense concentration of mass—

such as a galaxy of stars—can act as a lens to produce multiple images of a distant

light source located behind it (Fig. 1.20). A quasar,the nucleus of a young galaxy,

is brighter than 100 billion stars but is no larger than the solar system. The first

observation of gravitational lensing was the discovery in 1979 of what seemed to

be a pair of nearby quasars but was actually a single one whose light was deviated

by an intervening massive object. Since then a number of other gravitational lenses

have been found; the effect occurs in radio waves from distant sources as well as in

light waves.

The interaction between gravity and light also gives rise to the gravitational red shift

and to black holes, topics that are considered in Chap. 2.

34 Chapter One

Laboratory in Accelerated laboratory

gravitational field

a = –g

g

Figure 1.18According to the principle of equivalence, events that take place in an accelerated

laboratory cannot be distinguished from those which take place in a gravitational field. Hence the

deflection of a light beam relative to an observer in an accelerated laboratory means that light must

be similarly deflected in a gravitational field.

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