General relativity

General relativity (GR) or general relativity theory (GRT) is a geometrical theory of gravitation and cosmology published by Albert Einstein in 1915. In this theory:

Predictions of GR

:(For more detailed information about tests and predictions of general relativity, see Tests of general relativity)

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Gravitational effects

Acceleration effects

These effects occur in any accelerated frame of reference, and are therefore independent of the curvature of spacetime. (Note however that spacetime curvature usually is the source the causative acceleration when these effects are being observed.)

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• Gravitational redshifting of light: The frequency of light will decrease (shifting visible light towards the red end of the spectrum) as it moves to higher gravitational potentials. Confirmed by the Pound-Rebka experiment.
• Gravitational time dilation: Clocks will run slower at lower potentials than the observer in a gravitational field. Confirmed by the Haefele-Keating experiment and GPS.
• Shapiro effect (a.k.a. gravitational time delay): Signals will take longer than expected to move through a gravitational field. Confirmed through observations of signals from spacecraft and pulsars passing behind the Sun as seen from the Earth.

Bending of light

This bending also occurs in any accelerated frame of reference. However, the details of the bending and therefore the gravitational lensing effects are governed by spacetime curvature.

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• The magnitude of this effect is twice the Newtonian prediction. Confirmed by astronomical observations during eclipses of the Sun and observations of pulsars passing behind the Sun.
• Gravitational lensing: One distant object in front of or close to being in front of another much more distant object can change how the more distant object is seen. These effects include
• Multiple views of the same object: Observed of quasars whose light passes close to an intervening galaxy.
• Brightening of a star due to the focusing effects of a planet or another star passing in front of it: Such "microlensing" events are now regularly observed.
• Einstein rings and arcs: One object directly behind another can make the more distant object's light appear as a ring. When almost directly behind, the result is an arc. Observed for distant galaxies.

Orbital effects

These are ways in which the celestial mechanics of general relativity differs from that of classical mechanics.

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• Non-Newtonian periapsis precession: The apsides of orbits precess more than expected under Newton's theory of gravity. Confirmed for Mercury and observed in several binary pulsars.
• Orbital decay due to the emission of gravitational radiation: This has been observed in binary pulsars.
• Geodetic precession: Because of the curvature of spacetime, the orientation of an orbiting gyroscope will change over time. This is being tested by Gravity Probe B.

Rotational effects

These involve the behavior of spacetime around a rotating massive object.

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• Frame dragging: A rotating object will drag the spacetime along with it. This will cause the orientation of a gyroscope to change over time. For a spacecraft in a polar orbit, the direction of this effect is perpendicular to the geodetic precession mentioned above. This prediction is also being tested by Gravity Probe B.

Black holes

Black holes are objects which have gravitationally collapsed behind an event horizon. In a "classical" black hole, nothing that enters can ever escape. However, Stephen Hawking has shown the black holes can "leak" energy, a phenomenon called Hawking radiation.

Related Topics:
Black hole - Event horizon - Stephen Hawking - Hawking radiation

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Cosmological effects

• Expansion of the universe: This is predicted by cosmological solutions of the Einstein Field Equations. Its existence was confirmed by Edwin Hubble in 1929.
• Cosmological redshift: Light from distant galaxies will be redshifted due to their movement away from the observer.
• Big Bang: The arising of the universe from a primordial singularity
• Cosmic background radiation: The remnants of the primordial fireball. Discovered by Arno Penzias and Robert Wilson in 1965.
• Dark energy: An unobserved energy that is spread throughout the universe. Recent observations of distant supernovae indicate that the expansion of the universe is accelerating. The Einstein Field Equations can support this type of universe only if 70% of its stress-energy is in the form of this dark energy.

Other predictions

• The equivalence of inertial mass and gravitational mass: This follows naturally from freefall being inertial motion.
• The strong equivalence principle: Even a self-gravitating object will respond to an external gravitational field in the same manner as a test particle would. (This is often violated by alternative theories.)
• Gravitational radiation: Orbiting objects and merging neutron stars and/or black holes are expected to emit gravitational radiation.
• Orbital decay (described above).
• Binary pulsar mergers: May create gravitational waves strong enough to be observed here on Earth. Several gravitational wave observatories are (or will soon be) in operation. However, there are no confirmed observations of gravitational radiation at this time.
• Gravitons: According to quantum mechanics, gravitational radiation must be composed of quanta called gravitons. General relativity predicts that these will be spin-2 particles. They have not been observed.
• Only quadrupole (and higher order multipole) moments create gravitational radiation.
• Dipole gravitational radiation (prohibited by this prediction) is predicted by some alternative theories. It has not been observed.