They say it's impossible to travel back in time, right? I'm sure I don't need to remind you why (like what's stopping you from locating Hitler's grandfather, pushing him under the wheels of a stage coach, thus preventing Hitler from being born? ).
But suppose I told you that it's theoretically possible to see what really happened in the past, say at Dallas, Texas in 1963, or to the Marie Celeste? You'd say rubbish, would you not? Certainly I would have, up until a couple of hours ago.
Well think again.
Now you've heard of those distant exoplanets, right? They are outside our own solar system, orbiting their own stars. We know they are there - a few hundred have now been detected. No, we can't see them directly, but they can be detected by the small amount of light they block out when they transit their sun.
Suppose now there's an exoplanet called Zog, and that it is, let's say, 25 light years from Earth (our own Sun, by comparison, is 8.5 light minutes away, and the nearest star, Alpha Centauri, about 4.3 light years away).
Suppose the Zogians are far more advanced than we are, technologically, and have not only detected our presence, but have gone on to develop high-resolution video ( unlikely, but not theoretically impossible). Supposing Earth's history is being stored on Zogian video tape right now. What it records right now is what took place on Earth 25 years ago - because that's how long it took the light to travel from Earth to Zog.
Now suppose we find a way of sending a radio message to Zog, saying "Hello neighbours. Please could you include us on your subscription list for Earth video footage?" OK, so it would take 25 years for Zog to receive our message, and - if they responded promptly- another 25 years for the footage to get back to us. So Zog's most recent footage would show what was taking place on Earth 50 years ago, albeit in Zogian "real time"!
For historians, it would be the equivalent of the "third umpire" in cricket - the video camera that is the final arbiter on what really happened... Now then, was there a second gunman, lurking behind that grassy knoll?