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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What does light travel time have to do with looking into the past or future?

If an object is 5 billion light years distant you are seeing what it looked like 5 billion years ago. You cannot receive an image from an object that is later than its light travel distance. In other words, the light from the distant object that was emitted 4 billion years ago, is still on its way to us, and the light it emitted 6 billion years ago already passes us by over a billion years ago. You can never look into the future. Honest. You cannot see light from an object BEFORE it emitted the light.

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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.