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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Why does the warping of space near the Earth make its orbit elliptical, but has no effect upon light?

The actual warpage of space near the Earth according to General Relativity is vanishingly small. For all intents and purposes, the spacetime near the earth is very flat to about 1 part in a million. This is why the motion of the Earth obeys newtonian physics so well. What counts is how close you are to the so-called horizon size of the Sun which is only a few kilometers. But since the Sun is vastly larger than its horizon size, the curvature effects are always very small and only seriously affect Mercury and light which grazes the Sun's surface at a small 'impact parameter'. If we were near a black hole so that we could probe the highly curved space near its event horizon, we would indeed see all sorts of interesting optical and mass effects.

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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.