That being said, as Edgar added to Vintage's answer, if you put a mirror 13.5 light years away, and watched your reflection from earth, that would be the same as being 27 light years away.
So if I doubled the number of mirrors (2 in orbit, 2 on earth) and halved the distance to 6.75 light years I could accomplish the same thing.
If you take that example to it's conclusion, could I construct an (expensive/complicated/etc.) device here on earth that had so many mirrors it could let me look into the past at all?
The physics-answer seems "yes" so my question is "Why haven't we tried that?" and one obvious limitation, I suppose, would be just how many mirrors you would need.
Light travels 5,878,499,562,554 miles (5.78 trillion) a year[1].
Given that, if I just wanted to see an hour into the past, I think that means I would need to observe earth from:
5,878,499,562,554 / 365 days / 24 hours = 671,061,594 miles away.
Or I could stick a mirror in space 335,530,797 miles away (~ 540,000,000 km) from earth and stare at it.
Mars, at the widest distance from Earth, is 401,000,000 km away[2], which is close enough for my purposes (I'm not picky)... so I guess if I stuck a mirror on Mars and looked at the reflection of earth I could see something like 45 minutes in the past.
The Moon is almost exactly 1000x closer to the earth than Mars[3], so I wonder if I used it for my mirror array instead if I could just put a station with 500 mirrors on it to accomplish the same thing.
Or build something on earth with millions of mirrors in it to accomplish the same thing.
I would normally think something like this impossible, but I just watched a docu on the LHC and now I wonder if even at a micro-second scale, if we have tried building something like this and observed two points in space using a computer and seeing if the visual data coming in is micro-seconds apart from each other?
For example (assume I have a camera and visual-diff software sufficient for this and that my "mirrors" have sufficient magnification capabilities to make this seem like an easy setup), if I pointed one camera at a monitor drawing a unique pattern 2' away from me, then point another camera at a mirror that has bounced the image 10 miles before being displayed... I imagine, like sound, there would be a lag in that image if we bounced it enough times.
(DOH)
It suddenly dawns on me that using this method to look into the past is effectively the same thing as recording something with a video camera and playing it back later... you are literally capturing the light for review at a later date.
So as cool as this idea is, I think I just answered my own question as to why we haven't tried to build a million-mirror-array before... cause I can buy a video camera for $300 instead :)
Since you'd need to put all this stuff in place before the event you want to see actually occurs, wouldn't it be easier to just break out a video camera and record it?
I think this would have other uses. Presumably, this thing would not use energy; it would just pass photons from A to B. Make one a meter square with a twelve hour delay and point it to he sky, and you have instant lighting at night. It would he like a battery storing solar energy coupled with a lamp.
Unfortunately, even if we could fold a 12 hour light path into a tiny space, losses would probably be too high. Some of the light that got in would come out after 12 hours,but most of it would be converted into heat long before.
hugh3, you are exactly right. I updated the original post as I was thinking through how this amazing physics experiment could be helpful, and it dawned on me, it wasn't.
I could just use my Canon to accomplish the same thing.
I think my brain was more caught up in the fun of walking through the thought than accomplishing something with it.
Even so, I think it would be fascinating to interact with a device that consisted of mirrors, even if was just to experience a delay of a millisecond (or whatever is the minimum amount of time to perceive a delay).
Exactly. I think it'd be incredibly cool to walk in front of a thin transparent wall with this property, run around to the other side and see yourself walk along it.
I wonder if you could information in this fashion.
Not if you consider the resolution would be much higher then anything technically feasible. You might not now what you want to focus on before hand, but if the system is setup you can go back and decide what to look at.
Mars, at the widest distance from Earth, is 401,000,000 km away
A lot of the time, it's closer. And when it's the furthest away, both earth and mars are on different sides of the sun. A star between you and the mirror will make your mirror hard to use.
rmc, good point about the specifics of Mars. The range I found is 100-400 km. I think the (useless?) point I was making still more or less holds, but my calculations need asterisks next to them with a clarification at the bottom: "Would require you to be able to see through the Sun"
> The Moon is almost exactly 1000x closer to the earth than Mars[3], so I wonder if I used it for my mirror array instead if I could just put a station with 500 mirrors on it to accomplish the same thing.
Plus, since the moon has synchronous rotation, the mirors would at least always be facing us. Assuming all the other massive difficulties got worked out, it'd be a good place for those mirrors.
One problem with this plan is that mirrors are not perfect and some non-negligible fraction of the incoming light is absorbed and/or scattered away. Even if you could build perfect mirrors, you would still likely need to place the whole setup under vacuum to prevent the light from being scattered by air.
You are right that regular mirrors absorb or scatter light energy. However, to get 100% reflection it should possible with a setup where the light is getting refracted or bounced off critical angle.
http://en.wikipedia.org/wiki/Total_internal_reflection
This is commmonly used in Fibre optics.
Its more of an engineering challenge to come up with a setup to view your past than a theoretical one!
The scattering problem is one that confused me, too. At a light-year or several dozen away, there's no way you could find and piece together all the photons that bounced off the moment you're looking for. Am I misunderstanding something?
I don't get it - even if you use a mirror, the distance is still the same. Light needs to go to the mirror, reflect and come back to your eyes, there's no difference if you're n lightyears away or just n/2 and use a mirror.
farico, you are exactly right, and that is the point. You just add mirrors to make the distance needed shorter and shorter.
I was just taking the example to a logical absurdity to reason that we could potentially make a small array of millions of mirrors, here on earth, that could see back in time in "real-time" if you pointed it at something.
But as I soon realized, this is basically a video camera =(
I think this is still a worthy experiment to try out. My guess is that it would cost in the 100s of millions. Would be nice if there was a cheap way to see 1-2 seconds in the past ... cheap enough high school for science classes.
That being said, as Edgar added to Vintage's answer, if you put a mirror 13.5 light years away, and watched your reflection from earth, that would be the same as being 27 light years away.
So if I doubled the number of mirrors (2 in orbit, 2 on earth) and halved the distance to 6.75 light years I could accomplish the same thing.
If you take that example to it's conclusion, could I construct an (expensive/complicated/etc.) device here on earth that had so many mirrors it could let me look into the past at all?
The physics-answer seems "yes" so my question is "Why haven't we tried that?" and one obvious limitation, I suppose, would be just how many mirrors you would need.
Light travels 5,878,499,562,554 miles (5.78 trillion) a year[1].
Given that, if I just wanted to see an hour into the past, I think that means I would need to observe earth from: 5,878,499,562,554 / 365 days / 24 hours = 671,061,594 miles away.
Or I could stick a mirror in space 335,530,797 miles away (~ 540,000,000 km) from earth and stare at it.
Mars, at the widest distance from Earth, is 401,000,000 km away[2], which is close enough for my purposes (I'm not picky)... so I guess if I stuck a mirror on Mars and looked at the reflection of earth I could see something like 45 minutes in the past.
The Moon is almost exactly 1000x closer to the earth than Mars[3], so I wonder if I used it for my mirror array instead if I could just put a station with 500 mirrors on it to accomplish the same thing.
Or build something on earth with millions of mirrors in it to accomplish the same thing.
I would normally think something like this impossible, but I just watched a docu on the LHC and now I wonder if even at a micro-second scale, if we have tried building something like this and observed two points in space using a computer and seeing if the visual data coming in is micro-seconds apart from each other?
For example (assume I have a camera and visual-diff software sufficient for this and that my "mirrors" have sufficient magnification capabilities to make this seem like an easy setup), if I pointed one camera at a monitor drawing a unique pattern 2' away from me, then point another camera at a mirror that has bounced the image 10 miles before being displayed... I imagine, like sound, there would be a lag in that image if we bounced it enough times.
(DOH)
It suddenly dawns on me that using this method to look into the past is effectively the same thing as recording something with a video camera and playing it back later... you are literally capturing the light for review at a later date.
So as cool as this idea is, I think I just answered my own question as to why we haven't tried to build a million-mirror-array before... cause I can buy a video camera for $300 instead :)
[1] http://www.universetoday.com/45047/how-far-does-light-travel...
[2] http://www.universetoday.com/14824/distance-from-earth-to-ma...
[3] http://www.enotes.com/science-fact-finder/space/how-far-moon...