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Wednesday, April 23, 2008

This article or section is in need of attention from an expert on the subject. WikiProject Physics or the Physics Portal may be able to help recruit one. If a more appropriate WikiProject or portal exists, please adjust this template accordingly.Gravitational redshift In physics, light loses energy when it moves away from a massive body such as a star or a black hole; this effect reveals itself as a gravitational redshift in the frequency of the light, and is observable as a shift of spectral lines towards the longer, or "red," end of the spectrum.
Light coming from a region of weaker gravity shows a gravitational blueshift.

The gravitational weakening of light from high-gravity stars was predicted by John Michell in 1783, using Isaac Newton's concept of light as being composed of ballistic light corpuscles (see: emission theory). The effect of gravity on light was then explored by Laplace and Johann Georg von Soldner (1801) before Einstein rederived the idea from scratch in his 1911 paper on light and gravitation.
Einstein was accused by Philipp Lenard of plagiarism for not citing Soldner's earlier work - however, given that the idea had fallen so far into obscurity before Einstein resurrected it, it is entirely possible that Einstein was unaware of all previous work on the subject. In any case, Einstein went further and pointed out that a key consequence of gravitational shifts was gravitational time dilation. This was a genuinely new and revolutionary idea.


The receiving end of the light transmission must be located at a higher gravitational potential in order for gravitational redshift to be observed. In other words, the observer must be standing "uphill" from the source.
Tests done by many universities continue to support the existence of gravitational redshift.
Gravitational redshift is not only predicted by general relativity. Other theories of gravitation support gravitational redshift, although their explanations for why it appears vary.
Gravitational redshift does not assume the Schwarzschild metric solution to Einstein's field equation - in which the variable M; cannot represent the mass of any rotating or charged body. Gravitational redshift Important things to stress
Gravitational redshift was first observed in the spectral lines of the star Sirius B by Adams in 1925, although this measurement was criticized as possibly flawed, since it was difficult to rule out a shift of the spectral lines in the atmosphere of a white dwarf by some other (possibly unrecognized) effect.
The Pound-Rebka experiment of 1959 definitively measured the gravitational redshift in spectral lines. This was documented by scientists of the Lyman Laboratory of Physics at Harvard University.
More information can be seen at Tests of general relativity.

Initial verification
Gravitational redshift is studied in many areas of astrophysical research.

A table of exact solutions for gravitational redshift consists of the following:
The more often used exact solution is for gravitational redshift of non-rotating, uncharged masses which are spherically symmetric. The equation for this is:
z=frac{1}{sqrt{1-left(frac{2GM}{rc^2}right)}}-1, where

G, is the gravitational constant,
M, is the mass of the object creating the gravitational field,
r, is the radial coordinate of the observer (which is analogous to the classical distance from the center of the object, but is actually a Schwarzschild coordinate), and
c, is the speed of light.

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