Expert: Paul Wagner - 11/29/2004

Question
Hi
I heard that when you look at certain stars in the sky, it takes so long for the light to travel to us on Earth you are looking back into time. Like for instance if the Sun exploded right now, we wouldn't be able to see it for another 8 minutes or so. I know this deals with the Speed of Light, but is there any easy way you could explain it. Or give reference to a website that explains how this works, because I am confused.
Thank you very much!

Answer
Dear Stephanie:

I think you have a pretty good handle on this already.  Light travels at 186,000 miles per second--which is faster than you can imagine! But over the huge distances in astronomy it still takes time for light to travel across the vast distances of space.

I know that you have seen lightning strike...and then a few seconds later, heard the thunder crash.  Because light travels much faster than sound, the thunder is delayed---and it is delayed longer, the farther away the lightning stuck.  So you aren't really hearing "back in time" but what you hear is delayed.  

The same is true of light over the astronomical distances of the universe.  It takes light about 8 minutes to travel from the Sun to us.  Some objects in the sky are billions of light years away...and the light takes billions of years to travel to us.  In that case, the light we are seeing today left those objects billions of years ago..and we are just now getting the message.  So in that way, we can see something of what the Universe was like billions of years ago....and billions of light years away.

PauL Wagner