1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Travel back in time, with the help of light?

  1. Mar 21, 2010 #1
    I'm very sorry for probably posting this in the wrong place, but I'm kind of a newbie when it comes to physics.

    this is a question I've had for a long time, and it is really giving me problems sleeping, as i keep wondering if it(at least in theory) could be possible..

    here is my question:

    do you think it would be possible to somehow gather in the light that has been sent off earth while it has been here, and somehow use it to piece together a "film" of what has happened on the planet while it has been here..

    so that we would send something out there to gather/watch the reflected light and the light we have produced on our own, to find out anything about the planet? and I realize that to do this we would have to send something faster than light itself to see this.. but what if some of the light(or images, if I may call it so) have hit some obstacles on the way, and then been reflected against ourselves again, could we then possibly find something in this light?

    I hope I expressed my self clearly enough to get a good answer to this.
    thanks for all opinions on this, I wish I had a broader vocabulary, then i guess it could be a lot easier to explain what I'm trying to ask.
  2. jcsd
  3. Mar 21, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I don't know if I understand you correctly, but does your question go like this:
    if there were a monster mirror somewhere out in space far far away, would it be possible with a telescope to look at that mirror and see the earth "in the past" ?

    The answer is yes of course. If that monster mirror was, say, at 400 light years from earth, we would now be looking in that mirror at the earth as it was, 800 years ago.

    The practical problem is that there is most probably not a monster mirror in space that moreover is correctly oriented (perpendicular to the line of sight). And from random scattering of light of the earth by objects in space, that won't cut it I would guess, because I would guess (one should calculate it) that there are simply not enough photons that bounce off in the right direction for a potential image to be formed, even if we could somehow "reconstruct" an image from that in principle.

    So, for all practical purposes, WE will never see an old image of the earth.

    Of course, if there is life on far away planets and they are watching us, they might tape it and send us the tape one day :smile:
  4. Mar 23, 2010 #3
    What format will they send it in? DVD? BluRay? Or a simple .avi file?

    Jks lol
  5. Mar 23, 2010 #4
    vanesch: thanks! :D I really needed a good answer to this. and I wasn't thinking of a mirror, but more of the light reflecting the planets in our solar system.
    but you understood my question quite perfectly. and now i got a more reasonable explanation, and it was satisfying :D I will sleep perfectly tonight ;D

    BL4CKCR4Y0NS: silly you, of course they will have gotten YellowRay by that time xD
  6. May 30, 2010 #5
    360 days in a year times 2 (back and forward ) times 24 hours
    divided by the speed a rocket goes in km/h

    (1 light year / (365*2*24) ) / 57936 km/h = 9320 hour

    9320 / 24 = 388 days a rocket with a mirror should travel in order to see earth back 1 hour.

    if i got everything correct

    if you would be able to catch the light of earth though.

    A mirror on Pluto would already see 10 hours back i guess, since that is 5913520000 km away
    Last edited: May 30, 2010
  7. May 31, 2010 #6
    hmm :D cool ^^
    thanks for the answer citegrene! :D very helpfull ^__^
  8. May 31, 2010 #7
    Really cool idea, by the way. Completely impractical but the idea that our history is playing out as a "movie" in all directions is fascinating.

    Thank you for the question! The answer is good too :P
  9. Jun 2, 2010 #8
    @Brin yeah I know :P it's sort of fascinating though a bit sad :P because it's not possible to watch x) if it was.. then I'd definitely look to see if I found someone like the doctor :P (from BBC - Dr Who).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Travel back in time, with the help of light?