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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What visual distortions occur as you approach the speed of light?

The first thing you will notice is that the light from objects directly ahead of you will become ever more blue shifted, while objects behind you will become redshifted. As the speed increases, the shifting will be so extreme that the light from the forward and rearward stars will be shifted completely out of the visible spectrum and they will appear to fade to black. But there is another kind of relativistic effect which causes an apparent displacement of the images as well. This is called relativistic aberration.

The relevant formula is:

                cos(Theta) +  (V/c)
Cos(Theta') =  ---------------------
                1 + (V/c)cos(Theta)

where V is the velocity of the spaceship, Theta is the angle between the star and the direction of travel when the spacecraft is at rest, and Theta is the same angle measured when the spacecraft is in motion.

What happens is that as you look in the direction of travel, the star images shift to smaller angles, Theta, in the direction of travel. At 75 percent the speed of light, the stars that are 90 degrees from the direction of motion have now shifted to a position 41 degrees from the direction of motion in the sky. At even higher speeds, all of the stars in the forward hemisphere will shift to positions within a degree or less of the direction of travel. At ultra-relativistic speeds, all the stars will merge together into one 'star-like' object located directly ahead of you. Because of the Doppler effect, the light will become increasingly blue-shifted and the 'star' will appear blindingly white. Because the dominant source of radiation in the universe is in the cosmic background radiation, it is this blue-shifted light that will dominate what you see over the light from individual stars.

In the opposite hemisphere, a large dark spot will appear opposite your direction of travel, and will expand in size to engulf the entire hemisphere

The details of what you would see at these speeds has been discussed by many individuals. For more information see:

Bernard Oliver "The View from a Starship Bridge", IEEE Spectrum, January 1964, 
p. 86.

S. Moskowitz, "Visual Aspects of Trans-Stellar Travel", Sky and Telescope, May 
1967, p. 290.

See also the book by Eugene Mallov, "The Starflight Handbook", 1989, John Wiley and Sons Publishing.

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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.