The Science Book

220 ALBERT EINSTEIN

within a particular reference frame
is given by the equation E = mc^2.
This principle of “mass-energy
equivalence” was to become a
keystone of 20th-century science,
with applications that range from
cosmology to nuclear physics.

Gravitation fields
Although Einstein’s papers in
that annus mirabilis seemed too
obscure at first to make much
impression beyond the rarefied
world of physics, it propelled him
to fame within that community.

Over the next few years, many

scientists reached the conclusion

that special relativity offered a

better description of the universe

than the discredited ether theory,

and devised experiments that

demonstrated relativistic effects

in action. Meanwhile, Einstein

was already moving on to a new

challenge, extending the principles

that he had now established in

order to consider “noninertial”

situations—those involving

acceleration and deceleration.

As early as 1907, Einstein had

hit upon the idea that a situation

of “free fall” under the influence

of gravity is equal to an inertial

situation—the equivalence

principle. In 1911, he realized that

a stationary frame of reference

influenced by a gravitational field

is equivalent to one undergoing

constant acceleration. Einstein

illustrated this idea by imagining

a person standing in a sealed

elevator in empty space. The

elevator is being accelerated in one

direction by a rocket. The person

feels a force pushing up from the

floor, and pushes back against the

floor with equal and opposite force

following Newton’s Third Law.

Einstein realized that the person in

the elevator would feel exactly as

they would if they were standing

still in a gravitational field.

In an elevator undergoing

constant acceleration, a beam of

light fired at an angle perpendicular

to the acceleration would be

deflected onto a curved path, and

Einstein reasoned that the same

would occur in a gravitational field.

It was this effect of gravity on

light—known as gravitational

lensing—that would first

demonstrate general relativity.

Einstein considered what this

said about the nature of gravity.

If objects with mass

distort space-time,

this explains their

gravitational attraction.

Our experience of

gravity is equivalent to

that of being inside a

constantly accelerating

frame of reference.

The acceleration can

be explained by a

distortion in the

space-time manifold.

General relativity

explains gravity as

a distortion in the

space-time manifold.

According to general relativity,

mass creates a “gravitational well” in

space-time. The idea can be visualized

by representing three-dimensional space

as a two-dimensional plane. The gravitational

well of a massive object such as the Sun

causes light to be deflected onto a curved path,

altering the apparent position of distant stars to

an observer—an effect called gravitational lensing.

Real light

trajectory

Apparent light

trajectory

Real location

of the star

Sun

Apparent

location of

the star

Observer