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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Is the cosmological redshift real, or is it an optical illusion caused by dust ?

The cosmological redshift seems to be real enough. For more about it see my article in Sky and Telescopemagazine, February 1993, which gives some background on how to interpret this phenomenon.

As the article explains, there are three ways that nature seems to be able to shift the wavelengths of spectral lines from atoms. The familiar Doppler shift is the one we all know from the passing siren. When it approaches, its pitch is higher than when it moves away from us. What works for sound waves also works for light waves, and we can see the Doppler shifts in the light from stars that orbit one another in space.

Einstein's theory of general relativity predicts two new ways of doing the same trick that don't involve motion as we normally think of it.

The gravitational redshift happens when light tries to escape from a gravitational field. This is actually a phenomenon that you can explain using ordinary newtonian physics. Thanks to Einstein's famous E= m c squared, and Planck's equally famous law relating the energy of light to its frequency, E = h x frequency, we can see that as a particle of light ( photon) moves out of a gravitational field, it must loose energy working against the gravitational field. Since photons always travel at the speed of light, the only place where this energy loss can show up is in a change of frequency. The frequency of the photon must decrease so that the energy carries by the photon is lower, and this corresponds to a 'red shift' to longer wavelengths. This phenomenon has been confirmed in laboratory experiments carried out by Pound and Rebka at Harvard University over 30 years ago. It's not a theory, its real.

Even more bizarre than the gravitational redshift is the so-called cosmological redshift. There is no known way to independently test for this because we cannot create a system as big as the universe to compare against. But we can look at other aspects of general relativity and see whether it gives the right results. All tests to date seem to suggest that general relativity is in pretty good shape, so that gives us confidence that something like the cosmological redshift might be a reality. Astronomical observations of distant galaxies reveal that we cannot explain their redshifts by either the Doppler or gravitational redshift mechanisms without ending up with unphysical answers. General relativity, in the guise of Big Bang cosmology, however, predicts just what we are seeing in terms of the redshift caused by the expansion of space. It's not an optical illusion any more than E=mc squared is. And it cannot be produced by any known mechanism that does not in some way have to do with gravitational fields.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.