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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Are we gravitationally affected by where an object is now, or where it was when the light we see started its journey to us?

Because all indications are that gravity travels at the speed of light, we can only be affected by where a body is at the SAME time we see the light from it. The light travel time from the Sun to the Earth is 8.5 minutes. When we look at the Sun NOW, we are seeing it where it was 8.5 minutes ago. It is also the position of the Sun NOW that determines the strength of the gravitational field that the Earth is feeling NOW. Where the sun actually is NOW has no affect upon us because this information is coded in the gravitational field which Earth will experience 8.5 minutes from now. This is why astronomers do not care where distant objects in the universe are RIGHT NOW. We only care where their light tells us they are NOW because it is this information that controls its affect upon the Earth and Sun.

Copyright 1997 Dr. Sten Odenwald Return toReturn toSpecial & General Relativity Questions and Answers page.