Why do people sometimes say that when we look at stars that are very far away that we are "looking back in time"?
This is because of the finite speed of light. When we look at objects that are very large distances away from us, the light that is hitting us now will have started from the object quite a long time ago, so in effect we aren't looking at what the object looks like now but what it looked like some time ago (when the light was emitted).
For example, Proxima Centauri, which is the closest star to us (other than the Sun), is about 4 light-years away. This means that the light we see from it now left the star about 4 years ago. Something catastrophic could have happened to the star within those four years and we can't know about it yet (but that's unlikely, by the way!).
Even the light from the Sun takes about 8 minutes to reach us here on Earth, so when you look up at the Sun, you see it as it was 8 minutes ago! (PS: It is NOT advisable to look at the Sun too closely as you can damage your eyes.)
Similarly, even the light you see from nearby objects is slightly delayed, but since the speed of light is about a foot per nanosecond (billionth of a second!), the finite speed of light doesn't matter much in everyday life. However, you may notice it on TV broadcasts involving communication between people who are on opposite sides of the Earth. It takes time for the signal from one person to reach the other, so you may observe a slight delay between one person's question and the other's response.
This page was last updated on June 27, 2015