When looking at distant objects, are we looking at the past?

01 May 2012

Question

I’d like to know about seeing things light years away. I understand that we see things when the light arrives to our vision on Earth, but I wanted to ask when we see objects light years away, are we looking into the past?

Answer

Chris - Effectively, yes you are. Light travels at 300,000 kilometres a second - very, very fast. Rather than report distances in terms of miles, kilometres, or metres in space, it's more convenient to say, "how long would light, travelling at the speed of light, take to go from where it started to us here on Earth?" And actually, it conveniently works out as about a billion miles an hour.

So if you sent a radio message, which is a form of light, between the Earth and Pluto, Pluto is about 6 billion kilometres away, so it would take about 6 hours for your message on your mobile phone or your signal to go from here to Pluto. If someone then sent a message back from Pluto, it would again take about 6 hours for that message to come back. So it'd be a pretty slow conversation.

So in other words, yes, there's been a very big delay in time between the time that your message was sent from one place to the other place and that means a time lapse must have happened because light has a finite speed at which it can travel.

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