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Sun's Gravity Bends Starlight

The primary message of this article is that all theories, even Einstein's, have to undergo testing before they become widely accepted. This test of General Relativity established not only his theory of gravity, but also Einstein's fame.

This article discusses the first confirmation of Einstein's Theory of General Relativity. He had introduced the theory several years earlier (1915); however, since General Relativity reduces to Newtonian Gravity except in cases of extreme speeds (i.e. close to the speed of light) or in strong gravity, the tests of General Relativity were somewhat limited.

The best test accessible to the scientists at the time was to look at starlight passing by a massive object. The closest object with sufficient mass was, of course, the Sun. However, in order to view starlight passing close to the Sun, observations had to take place during a total solar eclipse – otherwise, the light of the Sun drowns out the starlight.

Both Newtonian and Einsteinian gravity predict that the Sun will bend the starlight, but the extent of that bending is different. The test proposed by Eddington would observe how much the gravity of the Sun would cause the starlight to bend.

One possible point of confusion for students is why does Newtonian gravity predict the bending of starlight at all. Light is composed of "photons", and photons are massless. Therefore, since Newtonian gravity depends on the masses of the bodies involved, it is generally assumed that Newtonian gravity would predict that the Sun would not affect light at all.

To help understand this question, here is a brief history of how scientists viewed the possibility of the bending of light:

Based on this timeline, prior to the 1919 eclipse, astronomers could have expected one of three results: no deflection at all, assuming a massless photon and Newtonian gravity; some deflection, assuming massless photon that was still accelerated in a Newtonian gravity well; or full deflection, assuming a massless photon in General Relativity.

It's interesting to note that there is some question as to whether or not the equipment and results of the 1919 eclipse expeditions really had the sensitivity to detect the starlight deflections that Eddington claimed. It may be that the researchers injected some of their expectations into the reported results. However, many subsequent (and more robust) observations have been performed, all of which confirm the reported deflection of starlight as that predicted by General Relativity.

Scientists continue, even today, to put General Relativity to the test, and all of those tests have added further evidence in favor of General Relativity.

Cosmic Times is a product of the Imagine the Universe! website. Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan P. Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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Last Updated: Monday, 04-Jan-2010 09:51:56 EST