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Light and Time!

  1. Mar 8, 2007 #1
    I was having an arguement and i need someone to clear it up

    Here is my side,

    If we can look at the sky with a telescope and see events that happened millions of years ago, then surely if an alien had a telescope at the right distance away from earth, looking back at us,

    that alien could see the lights of things in the past, like the world cup final in 1966, for example. Obviously this is just in theory, but someone clear all this up!
  2. jcsd
  3. Mar 8, 2007 #2


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    Note that observing distant objects for us only works when: the objects are: extremely large (such as stars or galaxies) and our "shutter speed" is quite long (minutes, hours, days).

    If the alien were on a planet 41 light years away, and
    If the alien had a telescope with nigh-magical resolving power to be able to see things in real time on the scale of metres or less.

    Then ... well, yes but no.

    Consider the following: the photons from Earth (and the players on the field at the World Cup game) diverge as the inverse square of the distance (i.e. the surface of an ever-expanding sphere). After 41 light years of exapnasion, just how many photons do you think will be available for the alien to pick up in his telescope. He has to pick up enough photons to a) resolve individual objects and b) resolve the objects simultaneously (i.e. you can't take 3 days or even 30 seconds to retrieve enough of an image like we do when observing very dim, distant objects).

    You could calculate an image resolution simply on the number of photons that would arrive at the alien's planet during whatever "frame rate" you'd need to see what's happening.

    It is possible ideally, but there will be very real limits on the resolving power.
    Last edited: Mar 8, 2007
  4. Mar 8, 2007 #3
    Yes, the alien could see us if he had a big enough telescope, but the laws of optics when applied to resolving power lead to the conclusion that if you can see me then I can see you, i.e. if the alien had a telescope that could resolve items smaller than our largest telescopes then those telescopes should be able to see his telescope.
  5. Mar 9, 2007 #4


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    Which would make his telescope the size of a star. (the smallest thing we can resolve optically). This is assuming the alien doesn't have some advanced technology that improves the resolving power of his scope.
  6. Mar 9, 2007 #5
    I think we might be reading too much into the OP's post. All (s)he wants to is whether it would be looking "into the past," so to speak, when at a certain distance from Earth. If there had been HUGE nuclear war in 1966, at the whole Earth just blew up giving out tremendous amount of light, then the alien would've probably detected a slight change in the intensity from the Sun's neighbourhood.
  7. Mar 9, 2007 #6
    Well the http://www.nidsci.org/essaycomp/cmaccone.html [Broken] proposes to put a probe at the gravitational focal point of the sun, using it as a gravitational lens
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  8. Mar 9, 2007 #7
    That supposition entirely depends how you clock-synchronize your frame of reference and thus construct your plane of simultaneity.

    Perhaps this topic is more suitable in the relativity section of PF.
  9. Mar 10, 2007 #8


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    No, he is not suggesting anything about simultaneity. He is simply talking about observing events in the past.
  10. Mar 10, 2007 #9
    Suggesting or not, this has everything to do with relativity and simultaneity. An Einstein clock synchronized frame will not view the incoming photons as something coming from the past.
    Is it progress in education to selectively ignore special relativity and revert to Galilean relativity?
  11. Mar 10, 2007 #10


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    I think you're making this more complex than it is.

    An alien 41ly away looks at our planet with a telescope and sees the 1966 World Cup.

    Granted, the alien's reference frame and our reference frame can't be reconciled because of, as you point out, simultaneity - but so what?

    The question is simply "can the alien watch the World Cup?" The answer is yes.

    I think what you're doing perhaps is anticipating a second question such as "can the alien tell us about it, or for another example, what the dinosaurs looked like?" - to which the answer is no - but the OP hasn't asked that.

    [ Addendum ] OK, I'll concede that the OP needs to follow through on his question to ask about the second phase of the thought experiment, since the first phase is basically inconsequential. (There is no "real" time with which to compare the alien's time or the Earth's time.)
    Last edited: Mar 10, 2007
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