General relativity (GR) is a theory
of gravitation that was
developed by Albert Einstein
between 1907 and 1915.
According to general relativity,
the observed gravitational
attraction between masses
results from their warping of
space and time.
By the beginning of the 20th
century, Newton's law of
universal gravitation had been
accepted for more than two
hundred years as a valid
description of the gravitational
force between masses. In
Newton's model, gravity is the
result of an attractive force
between massive objects.
Although even Newton was
bothered by the unknown nature
of that force,[1] the basic
framework was extremely
successful at describing motion.
Experiments and observations
show that Einstein's description
of gravitation accounts for
several effects that are
unexplained by Newton's law,
such as minute anomalies in the
orbits of Mercury and other
planets. General relativity also
predicts novel effects of
gravity, such as gravitational
waves, gravitational lensing and
an effect of gravity on time
known as gravitational time
dilation. Many of these
predictions have been confirmed
by experiment, while others are
the subject of ongoing research.
For example, although there is
indirect evidence for
gravitational waves, direct
evidence of their existence is
still being sought by several
teams of scientists in
experiments such as the LIGO
and GEO 600 projects.
General relativity has developed
into an essential tool in modern
astrophysics. It provides the
foundation for the current
understanding of black holes,
regions of space where
gravitational attraction is so
strong that not even light can
escape. Their strong gravity is
thought to be responsible for
the intense radiation emitted by
certain types of astronomical
objects (such as active galactic
nuclei or microquasars). General
relativity is also part of the
framework of the standard Big
Bang model of cosmology.
Although general relativity is not
the only relativistic theory of
gravity, it is the simplest such
theory that is consistent with
the experimental data.
Nevertheless, a number of open
questions remain, the most
fundamental of which is how
general relativity can be
reconciled with the laws of
quantum physics to produce a
complete and self-consistent
theory of quantum gravity.
From special to general
relativity
In September 1905, Albert
Einstein published his theory of
special relativity, which
reconciles Newton's laws of
motion with electrodynamics
(the interaction between objects
with electric charge). Special
relativity introduced a new
framework for all of physics by
proposing new concepts of
space and time. Some then-
accepted physical theories were
inconsistent with that
framework; a key example was
Newton's theory of gravity,
which describes the mutual
attraction experienced by bodies
due to their mass.
Several physicists, including
Einstein, searched for a theory
that would reconcile Newton's
law of gravity and special
relativity. Only Einstein's theory
proved to be consistent with
experiments and observations.
To understand the theory's basic
ideas, it is instructive to follow
Einstein's thinking between 1907
and 1915, from his simple
thought experiment involving an
observer in free fall to his fully
geometric theory of gravity.[2]
Equivalence principle
Main article: Equivalence principle
A person in a free-falling elevator
experiences weightlessness, and
objects either float motionless
or drift at constant speed. Since
everything in the elevator is
falling together, no gravitational
effect can be observed. In this
way, the experiences of an
observer in free fall are
indistinguishable from those of
an observer in deep space, far
from any significant source of
gravity. Such observers are the
privileged ("inertial") observers
Einstein described in his theory
of special relativity: observers
for whom light travels along
straight lines at constant speed.
[3]
Einstein hypothesized that the
similar experiences of weightless
observers and inertial observers
in special relativity represented
a fundamental property of
gravity, and he made this the
cornerstone of his theory of
general relativity, formalized in
his equivalence principle. Roughly
speaking, the principle states
that a person in a free-falling
elevator cannot tell that he is in
free fall. Every experiment in
such a free-falling environment
has the same results as it would
for an observer at rest or
moving uniformly in deep space,
far from all sources of gravity