# Seeing back in time?

I was thinking the other day, and a thought struck me;For the sake of argument, What if i built a mirror in space, 10 Light years away, and i built a large telescope to view the aforementioned mirror. If i was to look at the telescope 20 years later, would i see myself 20 years ago? If not, how come?

Just a quick disclaimer I’m not a scientist, i don’t have any degrees or qualifications or anything of the sort. I’m just a rather curious person.

Because first you or someone would have to actually go there and build the mirror* first*; and by the time you got there (a minimum 10 year trip at the speed of light; probably much slower) that past light would already be long gone. And then you’d have to go back to that telescope.

Lets gloss over how the mirror got up there then.
If i was to look at the mirror from earth would i look back in time?

Make it simpler…the mirror is already magically there 10 light years away from the earth.

If, from the earth, you looked at the reflection from the mirror you would see the earth as it was 20 years ago (light leaves the earth and travels 10ly to the mirror then takes 10ly to travel back to the earth).

So yes, in theory you would see yourself as you were 20 years ago (in reality I doubt you could even image the whole earth much less yourself).

It’s much easier to take a picture of yourself, wait 20 years, then look at it. It’s like the equivalent of a Star Trek transporter for images. And you don’t have to actually wait the 20 years, but then you would be amazed at how much better you looked back then.

Yes, you would be seeing “back in time”. This happens even when you look in your bathroom mirror - if you’re about half a meter away from the mirror then the image you see is about one three hundred millionth of a second in the past. The light travels from you, to the mirror, then back to you and it takes time to do that, the speed of light being about 300 million meters per second. The greater the distance between you and the mirror, the greater the distance the light has to travel, and the further back into the past you’re seeing.

If you put a giant mirror on the moon and looked at it through your telescope, you’d see yourself roughly 2 or 3 seconds in the past (400 million meters from the earth to the moon means 800 million meters round trip, speed of light is 300 million meters per second, so 2.6666… seconds)

You look back in time every time you look up at the night sky, in that you are seeing the stars as they were many years ago, not as they are now. So if the light that contains the image of you travels 20 light-years to get out, then back to you, that 20-year-old image is what you’ll see.

An interesting, and far more perceptible, example of this is provided by the (slow) speed of sound. Go out into a field and watch a woodsman chop wood. As you walk away from him, the sound of the axe-falls becomes more and more out-of-synch with the sight. At only a moderate distance, you can measure the discrepancy in terms of whole seconds.

You are “listening” back in time.

The problem of resolving a useful image of a person 20LY distant is not just a practical one, I believe - that is, it’s not just a matter of engineering it carefully enough - the resolving power is limited by physical laws.

AFAIK, the resolving power is only limited by the objective width. If a distance/width ratio of 1000/1 is acceptable then you only need a mirror 0.02 light years across, or more realistically two coordinated mirrors that distance apart. The main limit that I’m aware of is light-gathering power- an object emits a finite number of photons and you have to capture enough of them to make an image.

Yeah, wouldn’t the photons bouncing off of you tend to diverge over distance? How many of them reflecting off a human body would reach a reasonable sized mirror 10 light years away? And how many of those would be reflected back directly toward the earth?

Consider that when you look at a star that is 10 light years away, you only see a tiny speck of light. Not very many photons from that star are reaching your eye. But stars are very very bright compared to planets. We’ve never imaged a planet around another star. We can detect them due to their gravitational effects on the star they orbit, but even though we know those planets exist we can’t take an image of them, not even a dot.

Heck, we have a tough time taking an image of Pluto, and that’s in our solar system. So there’s no actual way you could “see yourself” in the past from 10 light years away, because your body is so small that none of the photons emitted from your body 20 years ago would impact your detector. You’d have to have a telescope much better than anything we’ve ever built just to detect Jupiter, let alone Earth.

Or more along the lines of the OP, shout into echo canyon and you can hear yourself a second or so back in time. This seems really more GQ than GD.

Wasn’t there a SF story where they had a special kind of glass? Light would take longer to pass through it, so the refection would be noticeable slower,

Make the glass thick enough and you could see your reflection from the day befoere.

The Light of Other Days by Bob Shaw.

I think the problem is: making that kind of glass at all.