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Putting Relativity to the Test

Though Einstein based his theory of gravitation on deep theoretical principles, he and others proposed a number of experimental tests of the theory soon after its publication.

Bending Light

The first prediction put to test was the apparent bending of light as it passes near a massive body. This effect was conclusively observed during the solar eclipse of 1919, when the Sun was silhouetted against the Hyades star cluster, for which the positi ons were well known.

Sir Arthur Eddington stationed himself on an island off the western coast of Africa and sent another group of British scientists to Brazil. Their measurements of several of the stars in the cluster showed that the light from these stars was indeed bent as it grazed the Sun, by the exact amount of Einstein's predictions. Einstein became a celebrity overnight when the results were announced.

The apparent displacement of light results from the warping of space in the vicinity of the massive object through which light travels. The light never changes course, but merely follows the curvature of space. Astronomers now refer to this displacement o f light as gravitational lensing.

But the Sun's gravity is relatively weak compared with what's out there in the depths of space. In the dramatic example of gravitational lensing below, the light from a quasar (a young, distant galaxy that emits prodigious amounts of radio energy) 8 billi on light years away is bent round by the gravity of a closer galaxy that's "only" 400 million light years distant from Earth.

The Einstein Cross
Image of Einstein's Cross

Four images of the quasar appear around the central glow formed by the nearby galaxy. The Einstein Cross is only visible from the southern hemisphere.
JPEG Image (25K); Credits and Copyrights

More on Light Benders
image of light bending by black hole

What might you see if you were to orbit a black hole? Computer simulations show that light near the hole gets so bent that the myriad stars behind it would appear as a series of concentric rings.
JPEG Image (33K); Credits and Copyrights

Want to see some movies that explain this strange effect? If so, here's a sample.

Movies of A Black Hole Bending Light

Mercury's Changing Orbit

In a second test, the theory explained slight alterations in Mercury's orbit around the Sun.

Daisy petal effect of precession

Since almost two centuries earlier astronomers had been aware of a small flaw in Mercury's orbit around the Sun, as predicted by Newton's laws. As the closest planet to the Sun, Mercury orbits a region in the solar system where spacetime is disturbed by t he Sun's mass. Mercury's elliptical path around the Sun shifts slightly with each orbit such that its closest point to the Sun (or "perihelion") shifts forward with each pass. Newton's theory had predicted an advance only half as large as the one actually observed. Einstein's predictions exactly matched the observation.

Gravitational Redshift

According to General Relativity, the wavelength of light (or any other form of electromagnetic radiation) passing through a gravitational field will be shifted towards redder regions of the spectrum. To understand this gravitational redshift, think of a baseball hit high into the air, slowing as it climbs. Einstein's theory says that as a photon fights its way out of a gravitational field, it loses energy and its color reddens. Gravitational redshifts have been observed in diverse settings.

Earthbound Redshift

In 1960, Robert V. Pound and Glen A. Rebka demonstrated that a beam of very high energy gamma rays was ever so slightly redshifted as it climbed out of Earth's gravity and up an elevator shaft in the Jefferson Tower physics building at Harvard University. The redshift predicted by Einstein's Field Equations for the 74 ft. tall tower was but two parts in a thousand trillion. The gravitational redshift detected came within ten percent of the computed value. Quite a feat!

Solar Redshift

In the 1960s, a team at Princeton University measured the redshift of sunlight. Though small, given the Sun's mass and density, the redshift matched Einstein's prediction very closely.

White Dwarf Redshift

Take a star like the white dwarf star, Sirius B that is 61,000 times denser than the Sun. Its gravitational field is correspondingly much stronger and so is the redshift for the light it emits: 30 times greater, ac cording to earlier observations from the Mount Wilson Observatory taken by W.S. Adams way back in 1924. Still larger redshifts have more recently been detected in studies of so-called neutron stars -- collapsed stars that are even denser. What about the redshift caused by a black hole? It can be thought of as infinite. In other words, photons inside the hole are so redshifted they can never get out!

The Ultimate Test

There have been dozens of other tests of General Relativity. The scoresheet is pretty impressive. However, there is one prediction that has never been confirmed directly. Einstein's theory predicted that disturbances in spacetime should generate a differe nt kind of radiation in the form of gravitational waves. Moreover, since black holes are by definition virtually "invisible," the only way to confirm they exist is to measure the gravitational wav es emitted as they form or interact with other massive objects.

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Copyright 1995, The Board of Trustees of the University of Illinois

NCSA. Last modified 11/22/95.