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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Why don't gravitational lenses also produce redshifts?

They do. The angular deviation depends on

           4 M G
  Theta = --------
             R c

For M = sun mass and R = sun radius you get 1.75 arcseconds. The redshift or 'radar echo delay' effect depends on
             4 G M
  Time =   ---------

and for the Sun this works out to be 72 kilometers/c = 240 micro seconds.

For larger systems such as gravitational lenses involving galaxies and clusters of galaxies, although the angular deviation can be seen, the redshift is completely undetectable given the uncertainties in galaxy distance and the absolute timing of phenomena. Even the spectral lines seen in the lensed images will not be shifted because the gravitational fields at the image locations are too weak to produce large scale spectral redshifts.

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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.