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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 a High-Energy Astronomer"
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