Tests of General Relativity

There are three classic tests of General Relativity:

The precession of Mercury's orbit refers to the rotation of the axis of the orbit. The orbit is an ellipse, so it is meaningful to speak of the orbital axis rotating. There are many reasons for this precession, but all the known causes could only account for part of the effect. There was a small remaining precession that was unexplained when Einstein developed the General Theory of Relativity. The theory predicted that there should be an additional precession in about the correct amount to resolve the discrepancy.

The gravitational redshift causes the wavelength of light to move toward longer wavelength (redshift) as it climbs out of the gravitational field of a mass. This proved to be difficult to observe because of the many other effects that can cause wavelength shifts, most notably the Doppler effect.

Eddington first observed the bending of light by a gravitational field in 1919, during a solar eclipse. This was the first new effect to be observed that had been predicted by General Relativity. The telegram reproduced below reports the observations made on a subsequent eclipse in 1922.

Recent Hubble Space Telescope pictures dramatically illustrate the gravitational bending of light. This example below shows lensing of distant objects by a galaxy. The explanation follows.

There is another interesting gravitational lens here: galaxy cluster lens

These two objects represent a new distant class of quadruple, or cross-shaped, gravitational lenses which might eventually provide astronomers with a powerful new "magnifying glass" for probing a variety of characteristics of the universe.

The two gravitational lenses were discovered in about 100 fields of sky imaged by Hubble Space Telescope's Wide Field Planetary Camera 2. The first cross-shaped lens was discovered serendipitously by Eric Ostrander while processing HST images for the Medium Deep Survey, a Hubble key project led by Richard Griffiths. A second fainter and smaller lens was identified a few weeks later by Myungshin Im. Each configuration is in the form of four faint blue images situated symmetrically around a much brighter red elliptical galaxy. The distinctive cross-like pattern around an elliptical galaxy makes them unambiguous quadruple lens candidates, even before spectroscopic observations, which are typically used to confirm lenses.

Hubble's high resolution allows astronomers to extend the search to much fainter, and hence much farther lenses, than those few examples ground-based telescopes have uncovered relatively nearby. Hubble can explore a larger volume of space which could provide enough examples of this rare cross type of lensing to allow astronomers to address a variety of fundamental cosmological questions.

A gravitational lens is produced by the enormous gravitational field of a massive object which bends light to magnify, brighten and distort the image of a more distant object. Depending on the alignment between the objects and the mass distribution of the foreground lens, the more distant object can be smeared into arcs or split into pairs, triples, or even quadruple images.

Credit: Kavan Ratnatunga (Johns Hopkins University, Baltimore, MD)

Courtesy NASA. More pictures at NASA Image eXchange

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