Did you know that the further we look away, the further we see back in time? #astrophysics #cosmology

Say you were waving to your friend on the Moon... the light signal would only take 1.3 seconds each way, so nothing would seem too strange.

But we see Mars as it was 4 minutes in the past because light takes that long to reach us. We see our friend's reciprocal wave 8 minutes later, by which time we are pretty sad at being ignored.

We see a neighbouring galaxy as it was approximately 2.5 million years ago. So a signal would take 5 million years for a round trip, by which time we would be very dead. Any alien in that galaxy observing us right now would actually see an archaic species of human.

So the further away an object is the farther back in time we see it. In my field of the first stars, we have built telescopes that detect light from an incredible 13 billion years ago. Cool!