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Is this possible, IN PRINCIPLE?

  1. Oct 11, 2007 #1
    Is this possible, IN PRINCIPLE???

    Millions of years after we die, that an intelligent civilization in the Andromeda galaxy could possibly construct a telescope large enough to see us walking on the Earth, as we once had millions of years ago?

    The question obviously deals with light, and its finite speed and time to reach a distant galaxy. It also questions if somehow images of ourselves are traveling through space at light speed. We know can see starlight that has made a journey of several lights years, but what about an actual picture of something happening on a planet? Could that ever be detected just as easily as starlight, if there was a theoretical telescope capable of viewing that?
     
  2. jcsd
  3. Oct 11, 2007 #2
    In principal at least? Sure. It takes something on the order of 2.9 million years for photons emitted from earth today to reach Andromeda.

    In reality? It's not likely. That would have to be one very large telescope. :)
     
  4. Oct 12, 2007 #3

    Chronos

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    Possible, but unlikely. The deep space limit of magnification far less than necessary to view us walking aroung at the distance of say the Andromeda Galaxy. Quantum effects begin to veil images in the billion-x range. A single photon can only convey a very limited amount of information [see information theory] about its source, and the number of photons receivable from a distant object is constrained by the square of its distance.
     
  5. Oct 12, 2007 #4
    Photon noise would almost certainly restrict such a measurement, we're not so far from this limit even today. You would require some kidn fo method for compensating for the atmospheric turbulence too. Perhaps a version fo adaptic optics but applied in reverse could work or some very detailed atmospheric models that could predict the exact chaotic behaviour our atmosphere undergoes. But all in all I would say no!
     
  6. Oct 12, 2007 #5

    russ_watters

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    The biggest problem would be one of resolution. Resolution is dependent on the aperature of the telescope. I'll see if I can calculate how big the scope would need to be later (it would be an interferometer)....
     
  7. Oct 13, 2007 #6

    Chronos

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    Resolving the disc of even the nearst stars requires a 30 foot aperature on earth based telescopes. The scope necessary to resolve the disc of a star in the Andromeda galaxy would be . . . priceless.
     
  8. Oct 14, 2007 #7

    russ_watters

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    Thinking about this more, actually, the resolution is a theoretically solvable issue. With long baseline optical interferometry, you can make the telescope arbitrarily large. Plans are underway to make a telescope with an effective aperature equal to twice Earth's distance from the sun by placing a telescope on the opposite side of the sun from earth.

    Lets see if we can throw in some math, though. The equation for resolution is 138/diameter (mm)= resolution (arcsec)

    If we wanted to image, say, the Galileo space probe orbiting Jupiter, at a distance of 800,000,000 km and a resolution of 1 meter, that's 2.6E-7 arcsec. So that's an aperature of 500 km. Definitely doable.

    If we wanted 1m resolution on a star orbiting Alpha Centuari, that's 42.5 trillion km away or 4.8E-12 arcsec. So an aperature of 28 billion km, or about four times further out than Pluto. Theoretically possible, but highly unlikely considering we haven't sent any probe out to that distance yet.

    So there are no real theoretical limits here, but the practical limits are formidable.
     
  9. Oct 14, 2007 #8

    Integral

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    While, theoretically you can build a telescope with any resolution you wish, it is not clear to me how you could penetrate the barrier presented by our atmosphere. We are only now getting the technology to obtain a reasonable image of stars through our atmosphere. And that is with good knowledge of the conditions. It is not clear that observers distant in time and space would be able to reconstruct the chaos created by our atmosphere.
     
  10. Oct 14, 2007 #9

    JesseM

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    That argument doesn't really make sense to me, since once the light clears the atmosphere it isn't affected by it any more, so why should the Earth's atmosphere be more of a problem for observers in the Andromeda galaxy (or any other distant location in space) than it is for satellites orbiting the Earth? Some satellites in orbit today can resolve features on the surface just 60 cm apart according to this, so if we neglect interstellar dust and other effects like quantum randomness and gravitational lensing, any telescope in space should be able to get equal resolution with a sufficiently large lens.
     
  11. Oct 15, 2007 #10

    russ_watters

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    It's kinda hairsplitting, but the OP did say watching people, which are a little smaller than can really be recognized. But whatever - at 1m resolution, you can recognize a lot of objects that are unmistakeably products of intelligent minds.
     
  12. Oct 15, 2007 #11

    JesseM

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    Is this in reply to my post? If so, even if it's true that spy satellites can't recognize people at present, would atmospheric distortions make it impossible in principle, or is it just a question of the current satellites not being sensitive enough or not having large enough lenses? Is there a theoretical limit on the resolution of any possible future spy satellite?
     
  13. Oct 15, 2007 #12

    russ_watters

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    Current satellites are not limited by their mirrors, they are limited by the atmosphere. That is, those without adaptive optics are. But adaptive optics wouldn't work long distance.
    http://en.wikipedia.org/wiki/KH-11

    Atmospheric distortion is, of course, weather dependent, but it doesn't get much better than .5 arcsec looking up from the ground. Assuming it would be the same looking down from 150 miles (not sure that it would be), that's about 2 feet - pretty close to your 60cm (I didn't plan that).
     
    Last edited: Oct 15, 2007
  14. Oct 16, 2007 #13
    I was thinking that using something like the 'Lucky' camera would also help lessen the effects of atmospheric distortion.

    But still, aren't we talking about a telescope with an aperture of 100's of light years to see this kind of detail? Seems to be stretching the known laws of physics a bit, if you ask me.
     
  15. Oct 17, 2007 #14

    Chronos

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    At least by the known laws of economics, Vast. The cost of building such a monster would be . . . impractical.
     
  16. Oct 18, 2007 #15
    I think so

    I'm not a guru when it comes to this but since light travels at 186,000 mi/sec, then if you are in an area of space x amount of light years from earth, than you would see x amount of years into earths past. Galaxies that we view today could technically not even exist anymore, and dying stars that we see died along time ago. Also if you were able to travel faster than the speed of light, and you went far enough into space, you would be able to look into the earths past (Hey NASA start working on this one).

    Thats my response from pure logic, but leave it to the experts to corroborate.
     
  17. Oct 18, 2007 #16

    DaveC426913

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    Stretching technology sure, but not the laws of physics.

    It's happening as we speak: gravitational lensing.
     
  18. Oct 19, 2007 #17
    Indeed!
     
  19. Oct 19, 2007 #18

    DaveC426913

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    All true. Though immaterial to the question.


    Yes, if.
     
  20. Oct 21, 2007 #19
    How would you go about measuring how far into space you would have to travel ( x amount times faster than the speed of light) to look back into the earths past a certain amount of hours/days/years/etc.
     
  21. Oct 21, 2007 #20

    DaveC426913

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    Since you cannot go faster than c, you would simply make up your own answers about how fast you'd travel.

    But if you used somne sort of wormhole to travel the distance without exceeding c:

    If your wormhole dumped you out one light year away, you'd see Earth as it was one year ago. One light century = one century, etc.
     
    Last edited: Oct 21, 2007
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