Institute of Astronomy

Looking back in time

Published on 22/01/2013 
Question: 

When a picture is taken of deep space and it is said that it is from when the universe was 500,000,000 years old.  Mainly saying that you're looking into the past.  That doesn't make sense to me for the fact that you're able to capture a picture.  Distance and time can coincide but in this case i dont get how this theory works with space?  I understand at such a distance it takes time for light to reach us, the point I'm trying to make is that how can it be said that what we view from deep space is the past not the present?

The effects of large distances and time in astronomy can be a little confusing.  Take as an example Proxima Centauri, the nearest star to our Solar System.  This is 4.2 light years away, which means that it takes light 4.2 years to get from Proxima Centauri to us.  Now since the only way we can see something that has happened at Proxima Centauri is through light, this means anything we see at Proxima Centauri actually happened 4.2 years ago.  If there were a person on Proxima Centauri and they had an exceptionally powerful torch, which the flashed at Earth, it would take 4.2 years for the torch flash to reach us, so by the time we saw it the person would actually have flashed the torch 4.2 years ago.  Now as I said Proxima Centauri is very nearby, when we look at objects in the distant universe they are much farther away, billions of light years, so when we see them we are seeing light that left them billions of years ago, when the universe was much younger.  As a result we can in a way think of looking at objects that are very far away in the distant universe as looking back in time, because the light has taken so long to reach us that the universe has changed a lot in the time it has taken the light to get here.

Page last updated: 22 January 2013 at 16:56