To explain how gravity
can behave in this way, Einstein began by demonstrating that it is not
necessary to think of gravity as a force. According to Newton's theory,
an apple falls to the floor because of the force of gravity pulls the
apple down. Einstein pointed out that the apple would appear to behave
in exactly the same way in space, far from the Earth's gravity, if the
floor were to accelerate upwards. In other words, the floor comes up to
meet the apple.
A hallmark of gravity
is that is causes the same acceleration no matter what the mass of the
object. For example, a baseball and a cannon ball have very different
masses, but if you drop them side-by-side, they accelerate downward at
exactly the same rate. To explain this, Einstein envisioned gravity as
being caused by a curvature of space. In fact, his general theory of relativity
describes gravity entirely in terms of the geometry of both space and
time. Far from a source of gravity, like a planet or a star, space is
"flat" and clocks tick at their normal rate. Closer to a source
of gravity, however, clocks slow down and space is curved. A useful analogy
is to imagine that the space near a massive object such as the Sun becomes
curved like a surface in figure.
One of the first things Einstein did with his new theory was to calculate the orbits of the planets. Einstein realized that if his theory was correct, it should be able to predict accurately the well-known motions of the planets about the Sun. According to general relativity, space far from the Sun is almost flat and objects thus travel along nearly straight-line paths. Near the Sun, planets and comets travel along curved paths because space itself is curved. Einstein found that where gravity is weak, the general theory of relativity gives exactly the same results as the classical theory of Newton. But in stronger gravity, such as is found very near the Sun's surface, the general theory of relativity predicts that there will be noticeable effects upon space and time, and the Newtonian theory of gravity is no longer accurate.
Even before Einstein, scientists knew that the motions of the planets closest to the Sun do not agree with Newtonian mechanics. During the mid-1800s, French astronomer Urbain Le Verrier (famous for his prediction of a planet beyond Uranus) pointed out that Mercury was not following its predicted orbit. As the planet moves along its elliptical orbit, the orbit itself rotates or processes.
As shown in Figure, the long (major) axis of Mercury's orbit slowly changes orientation. (Somewhat confusing, the term precession is also used to describe how the axis of rotation of a top or a planet changes direction. Most of the Mercury's precession is caused by the gravitational pull of the other planets, as explained by Newtonian mechanics. But once the effects of all the other planets had been accounted for, there remained an unexplained excess rotation of Mercury's major axis of 43 arcsec per century. Although this discrepancy may seem small, it frustrated astronomers for half a century. Some astronomers even searched for a missing planet even closer to the Sun that might be tugging on Mercury; none has ever been found. But Einstein showed that his theory could account for the excess precession of Mercury's orbit. It was a spectacular confirmation of his general theory of relativity.
To help validate his theory, Einstein made other predictions that could be tested. With his calculations he showed that light rays passing near the surface of the Sun should appear to be deflected from their straight-line paths because the space through which they are moving is curved. In other words, gravity would bend light rays, an effect not predicted by Newtonian mechanics because light has no mass.
Einstein also made
a third prediction. Stated that because gravity causes time to slow down,
clocks on the ground floor of a building would tick more slowly than clocks
on the top floor, which are farther from the Earth (Figure a). A light
wave can be thought of as a clock; just as a clock makes a steady number
of ticks per minute, an observer sees a steady number of complete cycles
of light wave passing by each second. Hence, if a light beam is aimed
from the ground floor to the top floor of a building, an observer at the
top floor will measure the light to have a lower frequency, and thus a
longer wavelength, than an observer on the ground floor (Figure b). Because
an increase in wavelength is a red shift, this effect is called the gravitational