QotW - Would a mirror in space show us our past?

21 February 2017

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Question

David - If we put a mirror millions of light years away and reflected back the Earth, could we see what Earth looked like millions of years ago?

Answer

Graihagh Jackson answered David's cosmic question...

Graihagh - A million years ago, only very early species of humans had begun to evolve and much debates around what they were doing - like could they control fire, and did they have language?

By putting a mirror in space, could we find out? I put this to Cambridge’s Hannah
Harahan. First though, let’s just unpack this astronomical question. What is a light year?

Hannah - When we look up at the night sky, we’re actually seeing the stars as they were sometime in the past because the light that's reaching us from these stars today has left the stars possibly hundreds of thousands of years ago.

Graihagh - Does that mean then, if there are some aliens a million light years away and they happen to glance in Earth’s direction, they wouldn’t actually see us here talking, they’d see Earth a million years ago?

Hannah - Exactly.

Graihagh - That’s because the light took a million years to get there. That’s assuming they have light sensitive eyes and use them as we do.

Hannah - If we could magically put a mirror a million light years away in space then, in theory, if we were able to reflect an image of the Earth that we could see back on Earth, the light would have left the Earth two million years ago.

Graihagh - Okay David. It’s theoretically possible but there’s one big hitch here. We can barely image planets a hundred light years away, let alone if you times that distance by ten thousand. But the other thing to contend with is the mirror itself.

Hannah - The size of the mirror that we would need would be absolutely huge. I haven’t actually done the calculation but it is possible that the size of the mirror would be so big and it would be so massive that it could actually collapse to form a black hole. So it’s definitely not technically feasible at the moment but, in theory, we could look back into the past.

Graihagh - There you have it David. I hope that answers your question. Next week we’ll be getting our knickers in a twist over this…

Stephen - I always seem to go for a pee within thirty minutes of drinking a cup of tea. And when I’m using the toilet I often say to myself is that the same cup of tea I’m getting rid of? How much of that drink was absorbed into my body so if I go for a pee within an hour of drinking a cup is this the same liquid I’m getting rid of?

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