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Stars and Time

  1. May 17, 2013 #1
    From my understanding the image we see of a star isn't a reflection of how that star is now. The image we are seeing of those stars is millions, or billions of years old because it's taken that long for the light from the star to reach us. Is this correct?

    But, lets say we could travel to one of these stars, one that still exists. If we could travel there in an hour, and looked back at the earth through a powerful telescope what would we see? Would we see the past on earth?

    Would it be true that further we travel back from the earth, the more of the past we would see because it's taking longer and longer for the present light from earth to reach that point in space where we would theoretically be looking from?
  2. jcsd
  3. May 17, 2013 #2


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    Yes, all of those things are true. Of course, you could only do that, look into the earth's past, if you could travel faster than the light which is theoretically impossible.
  4. May 17, 2013 #3


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    There is a causal barrier that prevents us from seeing distant objects as they 'currently' appear due to the finite speed of light. If you could somehow magically travel to alpha centauri in one hour, you would see earth as it was about 4 years ago. Needless to say, that would invoke a variety of paradoxes.
  5. May 17, 2013 #4
    Very interesting. Thank you for the replies.
  6. May 17, 2013 #5


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    welcome to pf!

    hi imageek! welcome to pf! :smile:

    if there was a mirror near a star that was a million light years away,

    there would be no problem whatever seeing the earth as it was two million years ago! :wink:

    (well, apart from all the obvious problems, of course)
  7. May 17, 2013 #6
    ... getting the mirror that far lol!

    Has there been any attempt to put a mirror into space for this purpose? Or anything similar?
  8. May 17, 2013 #7


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    well, i suppose the dinosaurs may have done it!

    but we won't know unless you go out and start looking for it! :smile:

    quick! :biggrin:

    EDIT: ooh, wait a mo …

    you may need to look in the infra-red

    assuming the dinosaurs didn't provide any means of slowing the mirror down, it could still be moving at nearly light-speed, and the reflection will therefore be highly red-shifted :redface:
    Last edited: May 17, 2013
  9. May 17, 2013 #8
    I love your answers tiny-tim :p I just realised how stupid my last question was.
  10. Jul 14, 2013 #9
    So if we put a huge mirror on mars we could look at it and see ourselves 16 minutes in the past?
    Oh my god I want to do that and start betting on horse races. Lol. Oh wait. Hahaha.
  11. Jul 14, 2013 #10


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    he he :biggrin:

    solar71, you'll make more money if you're running the betting-shop! o:)
  12. Jul 14, 2013 #11
    Lol so true so true.

    But still wouldn't it be cool to see ourselves in the past?
    To observe ones self in the past would be very enlightening.
    Not the mention the perspective gained.
  13. Jul 16, 2013 #12
    And if you travel away from the Earth, you would see the Earth in 'slow-motion', the faster you go the slower it would appear, until when you reach the speed of light it would appear forever frozen. Only if you could travel faster than light would the Earth start to appear to 'go backwards' towards the past.
    Now, that would be looking to your back towards the Earth. Looking in front of you, in the direction of your motion it would be the opposite, things would appear sped-up in fast-forward motion, the faster you go the faster the would appear to happen. Now, when you reached the speed of light, how would you see the objects in front of you? :confused: infinitely sped-up?
  14. Jul 24, 2013 #13


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    As a matter of fact there has! And a successful attempt at that. :smile:

    During the Apollo space program reflectors were set on the moon as part of the Lunar Laser Ranging Experiment. But one can only "see" about 2 ½ seconds into the Earth's past. And one can only see very bright and directed sources like pulsed lasers from Earth. [Edit: and you'll need a really big Earth based telescope too, to "see" the reflection.] But hey, it's better than nothin'.


    The main purpose of the reflectors and experiment was to measure the distance between the moon and Earth with great precision.

    But when you think about it, it's really the same thing being discussed here.

    ALICE: Ready the laser, Bob.
    BOB: Roger that, Alice.
    ALICE: Three, two, one, fire!

    [2 ½ seconds later.]

    BOB: Hey Alice, look; somebody from Earth fired a laser at the moon two and a half seconds ago.
    ALICE: Huh. Imagine that. ​

    It would vary from around 8 minutes to around 40 minutes depending on Earth's and Mars' orbital positions.
    Last edited: Jul 24, 2013
  15. Jul 25, 2013 #14

    Jano L.

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    Yes, this is how we think of large distances and "now". What we see now not how things are now, but how they were in the past.

    Depends on whose hour it is. If it is hour of Earth's time, it is impossible to accelerate so fast according to special theory of relativity. But if we could transport by other means and the rest of the special theory is correct, looking back on Earth we would see past events.

    If it one hour of traveler's time, it is theoretically possible according to special theory. The traveler will have to move close to speed of light. When he arrives at his destination and looks back at Earth, he will see how it was short time of the Earth time after he left, so nothing spectacular.
    Last edited: Jul 25, 2013
  16. Jul 26, 2013 #15
    Could anyone reply to this one? TX
  17. Jul 26, 2013 #16
    You can't reach the speed of light.
  18. Jul 26, 2013 #17
    I know that, but if it seems sensible to assume how would things look in the hypothetical case of travelling at the speed of light when looking to your back (I believe that they would appear frozen in time), why shouldn't be able to do the same exercise for looking at the direction of motion?
  19. Jul 26, 2013 #18
    Since the situation is outside the scope of valid physics you get nonsence answers in both the forward and backward looking case.
  20. Jul 26, 2013 #19


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    @Gerinski: It is pointless to ask "what will theory X predict if I do something impossible in theory X".

    If you travel very close to the speed of light relative to earth (and away from earth), things will appear extremely slowed down. In addition, everything gets redshifted - what appears as visible light here might be infrared (or even further away in the electromagnetic spectrum) for the spaceship.

    Everything not directly behind you will appear as if it would be in front of you (or very close to that), and most of those objects will appear blueshifted.
  21. Jul 26, 2013 #20
    Well, even if the subject in question is not realistic we can still have fun in thought experiments, can't we? Photons travel through 3-D space at a certain speed, photons reach eyes, photons reach moving eyes, I guess nothing out of scientific scope yet.

    The fact that travelling at a significant portion of c, looking back to where you are coming from things will appear slowed down and looking in front they will appear sped-up is a sound accepted fact (I was not 100% sure myself but I was assured so in a separate recent thread). The faster we travel the more severe these effects will become. So far nothing out of scientific scope I hope.

    So the thought experiment is trying to see how far can we extrapolate this fact. For the "looking back to where you are coming from" case it seems that we can get a sensible suggested answer: if things appear more slowed down the faster we travel, it seems sensible to assume that on the limit of c, things would appear frozen in time (and perhaps in the even more hypothetical case of travelling at superluminal speed we may guess that we would see things happening backwards in time).

    So far, certainly speculation but I hope nothing un-scientific. So the question is, if for the "looking back to where you come from" we can find sensible (even if hypothetical) answers for what might things look like when travelling at c (and possibly even faster than c, with things looking "running backwards in time"), why shouldn't we be able to find similar sensible answers for the same question but looking forward in the direction of our motion?
  22. Jul 26, 2013 #21


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    Sure, but those thought experiments have to stay in the possible realm.

    Depends on the speed you propose.

    You can get as much redshift / blueshift / time dilation as you like.
    That limit is not meaningful. You can do it in mathematics (it exists for "exactly backwards" only, it does not exist for any non-point source), but it has no physical meaning. You try to find a physical meaning in a purely mathematical calculation.
  23. Jul 26, 2013 #22
    I'm not trying to find any physical meaning (if you mean by "physical" = realistic).

    I'm just saying: I believe that saying that travelling at very high speeds and looking back towards where you come from, things would appear slowed down. First point: do you agree with that?

    The slow-down effect is bigger the faster you travel. 2nd point: do you agree with that?

    If we expand the effect to closer and closer to the speed of light, things would appear ever more slowed down. do you agree with that?

    Following this line of reasoning it seems sensible to say that "at c things would appear frozen in time" even if we know that is not a realistic sentence. Do you agree with that?

    If we can follow this argument for what we would "see" looking back to where we come from, why can't we follow and equivalent argument for what we would "see" looking ahead?

    Kindly explain at which point in this chain of reasoning the logic is broken.
  24. Jul 26, 2013 #23


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    No, with physical I mean "in theory possible in this universe".

    For something happening exactly behind you. For something happening in any other direction, this is not true.
    But there is no "exactly" in real life. There are no ideal point-sources you could look at.


    That is easier, the apparent evolution of the universe in front of you will speed up more and more, without a limit.
  25. Jul 26, 2013 #24
    Thanks, that's much closer to what I was asking! :smile:
    Now may I ask, what may we guess "speed up more and more, without a limit" may mean? what might it look like at the limit? does it make any sense to say that "we would see the fate of the universe"?

    You said that you do not agree that the "limit" of the looking backwards case, where things slow down more and more, may be seeing things "frozen in time". May I ask why do you think so?

    My first question (what would we see ahead of us) is just the converse. Not trying to be smarter than Einstein here or getting metaphysical, just trying to figure out what do scientists think about this subject.

    This may be unrealistic questions in practice, but I don't see why they should not be physically theoretical sensible questions.
  26. Jul 26, 2013 #25


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    No. It sounds plausible until you try working through the math and then it falls apart. You don't get results that look like what you get at almost c except even closer, you get garbage.

    One way of thinking about it: When you ask "how does the world look according to someone traveling at speed v?" you're really asking"What does the world look like to someone who is at rest while the rest of the universe is moving backwards past them at speed -v?". Set v=c in the second form of the question and you get a violation of the speed of light limit, and if you ignore that you get even more inconsistent results.
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