Special & General Relativity Questions and Answers
Is it difficult to know what is happening now if all of our information is about the past ?
Yes. because light takes a finite time to travel from place to place, we never see things the way they are, but always the way they were some time ago. Astronomers hardly ever worry about what the universe looks like at this instant because this requires information we can never obtain. We can only deduce the current picture from our models. I know of no astronomical investigation that worries about what a distant galaxy we see a billion light years away, looks like today. There is no particular, question we can pose that would require this knowledge. In our solar system, where light travel times are no longer that an hour or two, we can be properly worried about what planets and asteroids are doing RIGHT NOW, because this could have some impact ( sorry) on the Earth in the near future. For this we use the predicted orbits of the bodies and make forecasts.
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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.