<blockquote>
  <p>Imagine there is a supernova a hundred years ago. Lots of people see it but nobody has a decent telescope. A hundred years later we point a powerful telescope in the opposite direction, and find a huge alien mirror a hundred light years away, that allows us to watch the supernova.</p>
</blockquote>

<p>Light travels at a fixed speed so a hundred years later when we build our telescope we would see nothing in the opposite direction, because the light reflected from your alien mirror would only have reached that mirror, and has another hundred years to reach our telescope.</p>

<blockquote>
  <p>My question is, what is the limit of such a technique ?</p>
</blockquote>

<p>It's a thought experiment, not a technique.</p>

<blockquote>
  <p>Could we see right back to the big bang if the mirror was large enough and far enough away?</p>
</blockquote>

<p>Light takes time to reach us, so when we look at very distant objects they're already telling us about the distant past.  This is how we have built up an understanding of what happened in the distant past.</p>

<p>You might want to explore the subject of the <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="nofollow noreferrer">Cosmic Microwave Background</a> here.</p>

<p>In principle it would be easier to detect any radiation from the big bang directly (without a mirror) in a similar way.  But the universe is not a quiet place and radiation from the big bang has literally been through everything and won't be as easily detected (if it's possible at all).  We don't even have a proper theory to know what to expect to be able to detect, if anything.</p>

<p>All the mirror does is complicate things.</p>

CC BY-SA 3.0
2016-12-30T12:55:44.793Z
2016-12-30T18:20:25.177Z
2016-12-30T18:20:25.177Z
StephenG - Help Ukraine
<p>This is an addition to this question which is closed:</p>

<p><a href="https://astronomy.stackexchange.com/questions/1443/by-putting-a-mirror-in-space-would-we-be-able-to-see-into-the-past/8721#8721">By putting a mirror in space, would we be able to see into the past?</a></p>

<p>Imagine there is a supernova a hundred years ago. Lots of people see it but nobody has a decent telescope. A hundred years later we point a powerful telescope in the opposite direction, and find a huge alien mirror a hundred light years away, that allows us to watch the supernova.</p>

<p>My question is, what is the limit of such a technique?</p>

<p>Could we see right back to the big bang if the mirror was large enough and far enough away?</p>

<p><em>Please note this is not a question about whether the telescope or mirror are realistically possible.
Assume we are using some superior alien technology.</em></p>

CC BY-SA 3.0
2016-12-30T17:37:48.220Z
This has been done already, though not with artificial mirrors (by humans or aliens), but natural gravitational lenses: https://en.wikipedia.org/wiki/SN_Refsdal
chirlu
CC BY-SA 3.0
2016-12-30T18:55:01.643Z
I don't see how this asks anything that is not in the linked question. Questions about using a superior alien technology are hypothetical and probably off topic
James K
<telescope><light><big-bang-theory><mirror>
(Thought experiment) if we put a huge mirror into space, could we see back in time to the big bang?
thatsagoal
# (Thought experiment) if we put a huge mirror into space, could we see back in time to the big bang?

(Asked by: username_0 on 2016-12-30)

This is an addition to this question which is closed:

[By putting a mirror in space, would we be able to see into the past?](https://astronomy.stackexchange.com/questions/1443/by-putting-a-mirror-in-space-would-we-be-able-to-see-into-the-past/8721#8721)

Imagine there is a supernova a hundred years ago. Lots of people see it but nobody has a decent telescope. A hundred years later we point a powerful telescope in the opposite direction, and find a huge alien mirror a hundred light years away, that allows us to watch the supernova.

My question is, what is the limit of such a technique?

Could we see right back to the big bang if the mirror was large enough and far enough away?

*Please note this is not a question about whether the telescope or mirror are realistically possible.
Assume we are using some superior alien technology.*

**Tags:** telescope, light, big-bang-theory, mirror

> **Comments:**
> **username_1**: This has been done already, though not with artificial mirrors (by humans or aliens), but natural gravitational lenses: https://en.wikipedia.org/wiki/SN_Refsdal  
> **username_2**: I don't see how this asks anything that is not in the linked question. Questions about using a superior alien technology are hypothetical and probably off topic  

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## 1 answers

### Answer by username_3 on 2016-12-30. Score: 0

> Imagine there is a supernova a hundred years ago. Lots of people see it but nobody has a decent telescope. A hundred years later we point a powerful telescope in the opposite direction, and find a huge alien mirror a hundred light years away, that allows us to watch the supernova.

Light travels at a fixed speed so a hundred years later when we build our telescope we would see nothing in the opposite direction, because the light reflected from your alien mirror would only have reached that mirror, and has another hundred years to reach our telescope.

> My question is, what is the limit of such a technique ?

It's a thought experiment, not a technique.

> Could we see right back to the big bang if the mirror was large enough and far enough away?

Light takes time to reach us, so when we look at very distant objects they're already telling us about the distant past. This is how we have built up an understanding of what happened in the distant past.

You might want to explore the subject of the [Cosmic Microwave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background) here.

In principle it would be easier to detect any radiation from the big bang directly (without a mirror) in a similar way. But the universe is not a quiet place and radiation from the big bang has literally been through everything and won't be as easily detected (if it's possible at all). We don't even have a proper theory to know what to expect to be able to detect, if anything.

All the mirror does is complicate things.


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