Looking Back in Time

When you look into the night sky, you're seeing the stars because of the light that travels from them to you. It takes time for that light to reach you. Imagine you're looking at a star that is say 6,000 light years away. You're seeing that star as it was 6,000 years ago. Now imagine someone at that star looking at us. They'd be seeing us as we were 6,000 years ago.During a thunderstorm, the sound of the thunder takes longer to get to you than the light does, so you hear the thunder several seconds after you see the lightning. Echoes work the same way: you can hear the delay caused by the fact that sound takes time to travel. In a sense, you are 'hearing back in time'.

Now, we usually do not see a 'light echo' because light moves so fast, but it does take time to move. Sometimes you can hear a difference on a phone if you are making an international call where the signal goes up to a satelite and back down to earth. Again, the delay is very short (less than a second), but it is there. For the sun, the delay is a bit more than 8 minutes. For stars it is several years to hundreds of years for the ones we see without telescopes. For more distant galaxies, the delay can be millions or billions of years.