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Jesus on a video

  1. Aug 30, 2008 #1

    Since we see a 1 lightyear -away from us- star how it was 1 year ago, Is the light emitted from surface of earth travelling the same way? What about the effects of atmosphere?

    If first is afirmative, Is it physically possible (with the proper technology) to capture that light in a coherent way (at some point on universe) that allow to reproduce a video of what happened on this planet x time ago?

    Thanks to all replies.
  2. jcsd
  3. Aug 30, 2008 #2


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    Yes, light travels at the speed of light no matter where it is emitted from (and the effect of the atmosphere is negligible) - but the problem is that nothing else can, so there is no way to get ahead of that light to catch it. Not to mention, it is also impossible to build a telescope of sufficient resolution to see anything useful from a few light years away.
  4. Aug 30, 2008 #3


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    Yes, the only hope would be that a distant civilisation has been filming us for hundreds of years (by coincidence) and is willing to send us the tapes.
  5. Aug 30, 2008 #4
  6. Sep 9, 2008 #5
    Sorry to bring an old thread back to life... Well it does mention Jesus so perhaps we shouldn't be surprised...

    Just reading this and thought about what my Physics teacher was saying today about time dilation... He was basically talking about as you approach the speed of light then time slows down and he then implied that if you were actually able to achieve the speed of light then time would actually stop.

    This thread got me thinking that since light must be traveling at the speed of light then surely the time that the photons have travelled for has then slowed down - or even stopped!

    Therefore when we view stars which are say ' 5 light years' away we then not actually seeing the star 5 years ago but instead much more recently, maybe even seeing it as it is at that moment???

    Hope that made sense, look forward to hearing your comments...
  7. Sep 9, 2008 #6
    you could do one thing...

    Light travels at a slower rate in a medium say water. Its speed in water is c/n. So if you are under the sea, and somehow manage to travel at a speed greater c/n (its possible but difficult) then you can catch up with the light ray, and see something that happened in the past.
  8. Sep 10, 2008 #7
    Those photons still took 5 years to leave the star and travel to us so we're still seeing the star as it was 5 years ago.
  9. Sep 10, 2008 #8
    "Those photons still took 5 years to leave the star and travel to us so we're still seeing the star as it was 5 years ago. " - Quote from Haroldholt.

    I'm not happy with that answer because although it took the photons five years to get to us we're not moving relative to them so they would have experienced time dilation. Please correct or slate me if i'm incredible wrong on this but wasn't there an experiment with two atomic clocks on space shuttles or has my teacher made this all up???

    The clock that had been travelling faster showed that it's journey time was less than the time the stationary clock said it had been travelling. Time had slowed down for the moving clock. Is this story a myth??

    If light is travelling really fast then it must be experiencing massive time dilation and therefore not as much time passing for it as there is for us observing it??? Therefore the light from stars many light years away may not have taken many years to get to us...

    Is there anyway of proving a star that we see 5 light years away is actually as it was 5 years ago?? The more i think on this the more i think it is wrong. Help!
  10. Sep 10, 2008 #9

    Andy Resnick

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    I just wanted to add something, since the OP concerned *imaging*.

    Detection of light is much different than image acquisition- yes, we see distant stars, but we do not image them. In fact, simply imaging satellites from the ground is complicated and requires a lot of technology.

    One thing works in favor of a distant civilization- first, the aberrating effects of our atmosphere will have less of an impact to them rather than it does to us. However, in order to properly image something, sufficient spatial extent of the wavefront must be acquired in order to deconvolve any intervening degredation.

    More fundamentally, the resolution limit of an optical system is related to the numerical aperture, which is related to the physical size of either the entrance or exit pupil (depending on which is convenient- for telescopes it's the entrance pupil). For the sake of argument, in order to resolve a 1 meter object (a 2 meter person will then be two pixels, and stand out above the background) at 1 light year requires resolving 6*10^-15 degrees of arc... it ain't gonna happen- grinding out the numbers is pointless and a waste of time.
  11. Sep 10, 2008 #10


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    You are mixing frames, which is the cardinal sin of all who attempt to understand relativity. It is meaningless to ask whether or not photons experience time, or whether or not 'time stops' for them. (That in itself is a misconception, but I'll leave it alone.) You're trying to compare the experience of one who rides alongside a photon (were it possible) to the experience of others who merely observe the photon moving from one place to another, but that's mixing reference frames. To any observer, anywhere, it takes light 5 years to travel 5 light-years.

    First, you're making this too complicated. Second, yes, it's a myth. Time does not "slow down" as you move more quickly. All that happens is that another observer moving in relative motion to you will see your clock running slowly, and you will see his running slowly. Your own experience of time never changes. If you were on a starship moving at 99% the speed of light, your own wristwatch would continue to tick just as it always has.

    Again, you're mixing reference frames. And we're pretty sure we have this right, you know -- we've sent spacecraft well beyond Pluto now, and have been in constant contact with them via radio. We know exactly how long it takes signals to travel between them and Earth. Even the astronauts who landed on the Moon had to deal with a time delay in their communications -- live human beings have experienced the finite speed of light directly. It takes time for light to travel over long distances.

    It's not wrong.

    - Warren
  12. Sep 10, 2008 #11
    Yes, thanks Warren. Thinking of it in terms of what we observe with radio waves helped. It's a shame though.. It would have been good to imagine the stars up there are as they are that very moment. It'll tell me teacher tomorrow he was wrong. - He also said that if two people synchonised their watches and then one agreed to run in circles round the other near the speed of light for 5 minutes then the one running would stop before the stationary one's watch had hit 5 minutes. Please tell me that is not rubbish too!!
  13. Sep 10, 2008 #12


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    From the sound of it (it's a little unclear from your posts), your teacher is "correct," though being sloppy with his language and oversimplifying to an unacceptable degree. When you look at a star that's 5 light-years away, you see it as it was 5 years ago.

    The "running around in circles" idea is the twin paradox rephrased in, again, an unacceptable way. Running around in circles involves only tangential motion. The distance between the twins is not changing at all, and thus there will be no relativistic effects experienced between them.

    - Warren
  14. Sep 10, 2008 #13
    Thank you. I'll read through the twins paradox. I've printed off this discussion - can't wait to show my teacher tomorrow that he's been described as 'sloppy' and 'unacceptable'!
  15. Sep 10, 2008 #14


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    Don't be rude to him -- there's no point in that. He's trying to teach you things with the simplest language possible, which is admirable. The problem is that the simplest language often leads to the greatest ambiguity. Perhaps his students are just not ready to be taught relativity in a more precise way? If so, it'd be better if he refrained from teaching it at all. It's very easy to fill students' heads with misconceptions with language like "time slows down as you go faster."

    - Warren
  16. Sep 10, 2008 #15
    Back to the original topic, (seeing the past) - this reminds me of the story "Light of Other Days" where "slow glass" is set out to see pleasant scenery and then turned into windows, which you an look thru to see these pleasant scenes. The light takes years to pass thru the slow glass, so when you look into the window, you see h=what was on the other side years ago...
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