A mirror one light year away.

[Motokid600]

My friend asked me something interesting. If a giant mirror was placed one light year away from us and with a giant telescope we looked at that mirror. What would you see? Could you theoretically see yourself two years in the past?

We're talking a hypothetical telescope and mirror. It was just a very interesting question that caught me off guard and I'd thought I'd share it.

[TheCanadianVendingMachine]

As long as the mirror was up there for > 2 years, then yes. You would see the past

[Kerbart]

While there is a resounding “yes, in theory†there are a few practical stumbling blocks that get too easily dismissed when you under-appreciate the simply vast scale of a lightyear.

What we “see†from ourselves is reflected light which is diffuse by nature and fairly weak to start with. Light intensity falls off with the square of the distance. There's really not a lot of light left of us after traveling for two light years. And if we have a magical amplifier that can beef up that light that came back to us (more about that later), it would likely be drowned out by light from stars (direct light) in the vicinity that is many magnitudes stronger.

Then there's the problem of aiming the mirror. It's late and I'm not in the mood to do the math, but given the distance I assume the alignment of the mirror only needs to be off by 0.0001° (and probably far less) and your reflected image will miss you by a couple of thousand miles, if not more.

Then of course there's also the implicit assumption, following the above, that the mirror is perfectly flat (everything, in reality, is "bumpy"--we just don't notice it). Of course we can aim for "perfectly flat" but if, given the distance, we're talking about being flat on an atomic scale it's going to be a challenge.

So, even as a theoretical exercise there are many roadblocks to be navigated.

[Motokid600]

Yea the parameters needed to achieve such a thing are purely fantasy, but I'd be lying if I said it didn't get my head turning. Thanks for the replies fellas.

I mean we're talking a mirror the size of a nebulae and a telescope with a mirror the diameter of a star. You know. Simple stuff

[Bill Phil]

If it gets there, you'd see 2 years ago. But it would take more than 2 years to get there....

[Kerbart]

I mean we're talking a mirror the size of a nebulae and a telescope with a mirror the diameter of a star. You know. Simple stuff

Ha! You're right! Those are totally reasonable assumptions!

[RainDreamer]

Are there any natural "mirror" in the universe? We have gravitational lens that let us see further by bending light itself. Are there anything that reflect light from us back to ourselves?

[Camacha]

As long as the mirror was up there for > 2 years, then yes. You would see the past

The mirror only needed to be there one year ago. You needed to be here two years ago, and here again now to look at the mirror and your reflection.

Edited November 4, 2015 by Camacha

[RainDreamer]

I wonder what it would be like if we keep a video of the mirror moving away... would it look like that, slowly, time are slowing down in the mirror view?

[Kerbart]

The mirror only needed to be there one year ago. You needed to be here two years ago, and here again now to look at the mirror and your reflection.

It gets more doable by the minute!!

[lajoswinkler]

Are there any natural "mirror" in the universe? We have gravitational lens that let us see further by bending light itself. Are there anything that reflect light from us back to ourselves?

Black holes' rims have entire sky visible in them, including the observer, but you probably realize it would be very difficult to pick one detail out and undistort it. Even if it's a thought experiment with everything perfectly set.