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The Question

(Submitted November 14, 2001)

According to an answer to a previous question ( http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961102.html) photons do not respond directly to a gravitational field. Instead, they respond to the curvature in space-time, caused by the gravitational field.

If photons respond to the curvature in space-time, I would expect this curvature to be the same for all wavelengths of light. Is there any experimental evidence for this, so, does blue light follow the same path as red light in a gravitational field?

The Answer

Yes, you are right. According to Dr. Gregory Bothun of the Department of Physics, University of Oregon
( http://nedwww.ipac.caltech.edu/level5/Bothun2/Bothun4_6_6.html):

"Gravitational lensing is achromatic. There is no equivalent to the index of refraction for a normal lens as the behavior of light passing through curved space time is independent of its wavelength. This is the unique signature of gravitational lensing as the cause of the variability of the light output of a distant star." This is a fundamental property of general relativity and no violations have been measured. Some theories of quantum gravity do predict a small wavelength dependence at short wavelengths. This effect has not been seen either.

Hans Krimm for "Ask an Astrophysicist"

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