Fact: Telescopes are time machines.
Ok yes, they can’t let you go back and kill Hitler. But in a very real sense looking through a telescope gives you the power to look millions of years into the past.
Light moves pretty fast – almost 300 million meters/second, to be precise – but it does not reach its destination instantly. Here on earth we never notice because the distances involved are so small, but when dealing with the vast distances in the universe the time it takes light to move through space can become very important. This gives us the concept of light years: a measure of distance equal to the distance light can move in space in one year (which, for the interested among you, is about 6 trillion miles).
For example, our nearest star (other than the sun, of course) is Alpha Centauri, at a mere 4.37 light years away. That means it takes light 4.37 years to get from Alpha Centauri to Earth. Therefore, if you look up at Alpha Centauri through a telescope, you are actually seeing the light that left that star 4.37 years ago – and so you are looking back 4.37 years into the past. Want to see further back in the past? Just look at something further away – the light will have taken longer to travel the further distance and so your telescope/time machine will be taking you further back into the past.
Of course you don’t actually need a telescope – even light from the sun takes a certain amount of time to get to us. In fact, if the sun exploded right now, we wouldn’t even see it for a full 8 minutes. Which means the sun could have exploded 7 minutes ago and we would be none the wiser. A good incentive to seize the day.
‘Another one of Mr Shaunak’s Little Bites of Science’
Image under creative commons license