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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Exactly how does light get red shifted in a gravitational field?

Light carries energy because E = h x c / wavelength where h is Planck's Constant. But in any local reference frame, the speed of light in a vacuum is always fixed to be c = 299,792.5 kilometers/sec. Now, as light escapes a gravitational field, like all physical systems, it must lose energy as it works against the gravitational field. Since its speed may not change in any local 'freely falling' reference frame because of special relativity, this means that for its energy to change in the above equation the wavelength of the light is the only factor that can change according to relativity. So, light escaping a gravitational field will be shifted to longer wavelengths as it loses energy, and this is what distant observers see as a redshift.

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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.