Light - If you view the Earth from far enough away can you observe its past?

On a related note:<p>A 3GHz computer (ignoring multi-core, hyperthreading, other issues) executes 3 instructions every nanosecond. Light travels 1 foot per nanosecond. Your head is about 2 feet from your monitor. In the time it takes the light from this post on your monitor to reach your eye, your computer performs 6 instructions. Scale as appropriate.<p>When I first realized that, I wandered around in a slight daze thinking "wow, light is slow..."

I remember thinking of this one time as a teenager. Could we see a top down view of the rise of man? The huge caveat is that you have some capability of travelling faster than the light leaving earth, so that you can then look back and absorb the rays. It makes perfectly good sense that if you <i>could</i> travel faster than the speed of light you could use a massive telescope to peer back at the earth and see it's past geological events -- and I mean millions of years even, not just something trivial -- or given a strong enough telescope perhaps more detail could be seen. All of the information that left the earth as light is still out there, all over the universe.

There must be a calculable limit to the size of the event that you can see at a given distance. There are finite number of photons which are emitted from an event, these spread out with the inverse square law, so as your distance increases the probability of collecting sufficient photos to reconstruct an image of the event decreases and I would think at a distance of one lightyear it would be guess hard to see something much dimmer than an atom bomb.

One pottential difficulty in achieving this in practice would be that the amount of information (photons) available to measure would rapidly attenuate with distance, so you'd need to mitigate this by building larger and/or more sensitive detectors.

Fascinating question and fantastic answer.<p>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.<p>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.<p>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?<p>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.<p>Light travels 5,878,499,562,554 miles (5.78 trillion) a year[1].<p>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.<p>Or I could stick a mirror in space 335,530,797 miles away (~ 540,000,000 km) from earth and stare at it.<p>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.<p>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.<p>Or build something on earth with millions of mirrors in it to accomplish the same thing.<p>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?<p>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.<p>(DOH)<p>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.<p>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 :)<p>[1] <a href="http://www.universetoday.com/45047/how-far-does-light-travel-in-a-year/" rel="nofollow">http://www.universetoday.com/45047/how-far-does-light-travel...</a><p>[2] <a href="http://www.universetoday.com/14824/distance-from-earth-to-mars/" rel="nofollow">http://www.universetoday.com/14824/distance-from-earth-to-ma...</a><p>[3] <a href="http://www.enotes.com/science-fact-finder/space/how-far-moon-from-earth" rel="nofollow">http://www.enotes.com/science-fact-finder/space/how-far-moon...</a>

What? Nobody has mentioned a worm hole yet? Screw mirrors to cut the distance needed in half (although requiring the same amount of time).<p>How about this:<p>Step 1) open up a worm hole at a point along the path of light emanating from Florida on June 16th, 2008. 2) install a video camera with a giant lens around the same point pointing back at us 3) send the wireless ;) signal back through the worm hole. 4) Determine if the Anthony jury is a bunch of idiots or not.

This is something I have always wondered about.<p>Can the reverse be true too? i.e. if you are on the moon and can see what happens 1000km away, before someone else on earth 2000km away from that event, is that the future? Is that possible?<p>Can that then be extended to 27 lightyears into the future too?

Somewhat of a tangent, but the exploitation of this very phenomenon is one of the things that endears the book "Battlefield Earth" to me. Despite whatever other issues the author had, he wrote a darned good science fiction book.

We definitively can, but there is an issue: The light can be absorbed by some substance or refracted. How do you account for that?<p>You can account for that by recording every atom/photon on the space and then using some kind of miraculous processing power, to calculate how everything goes and estimate/verify the past actions.<p>For the processing ability, certainly some day we'll get there. For the possibility of getting the coordination/nature of any atom/photon in the whole space, this will need a discovery of something faster (way too much or may be instantaneous) than the speed of light that gives us the ability to recognize particles.<p>This is not amazing. This is sick. We'll be able to watch Pharaohs with an infinite precision. See how the earth was billions of years ago and how life evolved.

one of the commenters: "the question seems to lead to the idea that traveling faster than the speed of light == traveling backwards in time"<p>It's just a point of language, but that's silly. Seeing a video of France is very different from traveling to France, so one would imagine that seeing the past would not be described as "traveling" backwards in time.<p>edit: although, you may think about going faster than the speed of light as traveling backwards in time because of the "effects" of time dilation when your speed is &#62; c

Strictly speaking, everything we see happened in the past :)

We cannot even resolve the lunar lander site on the moon with Hubble.<p>How the heck exactly are you going to take advantage of that distance or mirrors?

Or, you can videotape yourself and then watch the past whenever you want.

The question has been protected on stackoverflow, so answering here. Yes a mirror 13.5 light years away can do the trick. But there is some fundamental lack of understanding regarding relativity that has lead to such a question. In fact, the top answer is incorrect on several levels.<p>1. Technically the only way a person can travel 27 light years in 27 years is by travelling through out at the speed of light. Ignoring the infinite amount of energy required for such travel, there is another aspect of such travel that is not being considered. Time Dilation. So assuming you were born and take off at the speed of light. Yes you will be able to observe your birth on earth 27 years later, but you would still be a baby and not 27 years old. Time would have been effectively frozen for you as you traveled.<p>2. The top answer says that you are always viewing and hearing the past. You are hearing the past, but you are actually viewing the present. The light radiating out of an event you are observing are also causality horizons. In your reference frame you are observing past events depending on the distance. However thanks to relativity and Lorentz contraction, there are reference frames where the distance between you and the event can be arbitrarily close to zero, effectively making the events simultaneous i.e. when you see an event you are effectively watching it as it happens simultaneously, you are not watching the past. This is also an alternative explanation for 1. i.e. the baby cannot grow older if you put it 27 light years away in 27 years time. It will be able to watch its birth, because its only just been born.<p><a href="http://en.wikipedia.org/wiki/Minkowski_diagram" rel="nofollow">http://en.wikipedia.org/wiki/Minkowski_diagram</a>

If you could figure out a way to invert all the constants and principles in the universe, then we could relive our past, in reverse. tea cup fragments on the floor would assemble into a teacup then accelerate up into the air and then sit on the table. The universe would play out, right back to it's formative moments. Then you could watch yourself being born, though you would have to wait years for it to render.

I wonder if there is research into gravitational lenses or other possible sources of naturally occuring space mirrors or lenses that we could use to collect light rays from the earth's past, maybe to detect its spectral content.