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Time traveling Light?

  1. Aug 27, 2011 #1
    So, time is relative right? Then how does this statement make sense:

    Light from the sun takes 8 minutes to get from the sun to the earth. So, if we looked at the sun we see the sun 8 minutes ago.

    If time is relative, wouldn't time move much slower for the light particle? So to us, it has been 8 minutes, but to the light it might be as few as 30 seconds. So we would see the sun 30 seconds ago, not what it looked like 8 minutes ago.

    Is this true or not?
     
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  3. Aug 27, 2011 #2

    Pengwuino

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    It doesn't make sense. I don't think you understand what it means for time to be relative. Learn a bit about special relativity and things should become clearer.
     
  4. Aug 27, 2011 #3

    Bill_K

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    In the rest frame of the solar system, 8 minutes has elapsed. There is no rest frame for the photon, so asking "how things look to the light particle" is meaningless. You cannot attach a clock to a photon.
     
  5. Aug 27, 2011 #4

    rede96

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    Although you won't get much joy imagining how things look from a photon’s point of view, there is another way to look your question.

    You could imagine that you have special space ship that can withstand the heat of the sun and travel at 0.9999978c.

    So you take a photo of the sun just as you set off to earth. When you arrive home, people on earth will think it took you 8 minutes to make the journey, but for you it would have been around just 1 second.

    You would obviously show people your photo of the sun which you only took a few moments ago. On earth, they took a photo of the sun as the observed you leaving the sun for earth. You look at the two photos and they are identical, showing the sun at the same time in its life cycle.

    However their photo will be over 8 minutes old and yours just over a number of seconds.
     
  6. Aug 27, 2011 #5

    Pengwuino

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    The photos will not be identical! At that speed there will be clear relativistic effects.
     
  7. Aug 27, 2011 #6

    rede96

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    I meant that he took the photo whilst he was at rest wrt to earth, just before he set off back. So they will be identical.
     
  8. Aug 27, 2011 #7

    K^2

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    But you can easily compute the proper time along the space-time path the photons took, which will always be zero. I suppose it depends on how you want to define things, but I would say it's perfectly fair to conclude that "from perspective of photon" not time has elapsed at all.
     
  9. Aug 28, 2011 #8

    ghwellsjr

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    Why don't you look up the FAQ called "Rest frame of a photon" where you will read
    "Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero. In the rest frame of a photon, the coordinates of any point in the universe at any time in the past, any time in the future is identically zero. That just doesn't make a bit of sense."​
    So your idea is not perfectly fair--it's meaningless and doesn't make a bit of sense.
     
  10. Aug 28, 2011 #9

    K^2

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    It's meaningless if you are trying to build a coordinate system and actually compare coordinates of different events in photon's frame. If your question is how much did the photon age, the answer is trivial. It does not age. "How much time passed from perspective of a photon" can imply either of these two questions.
     
  11. Aug 28, 2011 #10

    rede96

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    The "Does stand still for a photon" type of question crops up a lot. It was also one of the first things I tried to understand when learning about about relativistic effects.

    Personally, if I was learning SR from the start, I would find something like:

    much more satisfying and enlightening (No pun intended :smile:) then the standard answers of:

    type response.

    Why can't K^2's answer also be included in the FAQ?
     
  12. Aug 28, 2011 #11

    ghwellsjr

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    Photons are defined to travel at c. In Special Relativity, that's part of the definition of a frame. We cannot measure how fast a photon travels, just like you cannot measure the one way speed of light. It doesn't even make sense to talk about our perspective of a photon because we cannot see photons. So how can it make sense to consider that a photon has a perspective?
     
  13. Aug 28, 2011 #12

    rede96

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    Yes, no problem with that at all.

    Well, if science only discussed what was visible then I would suggest we would all still be in the dark ages. :smile:

    Look, the point is quite simple. If in explaining something to someone, whilst not being completely accurate in terms of current theories, it helps the person to understand then to me it is worth while doing and therefore the answer is not meaningless. (At least not for the OP.)

    So I guess it comes down to choice. When answering a question is the goal to dogmatically regurgitate the physics in its accurate but sometimes confusing form OR to allow for a little leeway and answer the question in a way that one believes will help the OP most.

    The choice, as they say, is yours.
     
  14. Aug 28, 2011 #13

    ghwellsjr

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    Rede96, your first post on this thread was completely accurate and yet I think may have helped the OP.
     
  15. Aug 30, 2011 #14
    Okay, the light starts at the sun at 8 minutes ago and then we see it now. The sun is on approximately the same time we are on with the speed of us and the sun moving through the space at the same speed(not including the speed of the orbit of the earth.)

    knowing that then understanding that the time that has elapsed for the light is shorter then the time which elapses for both the surface the light leaves and the light hits is where it may get confusing. The shines 8 minutes ago then then we see it now to us and the sun the light has existed for 8 minutes. The light only exists to itself for "30 seconds"

    earth perspective sees 8 minutes ago. o<=====0(sun)
    ^
    Light that is only 30 seconds old relatively to the suns
    state which is 8 minutes in the past relatively.
     
  16. Aug 30, 2011 #15

    ghwellsjr

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    Time does not elapse for light. It's a meaningless concept. Your statements, reiterating what the OP suggested:
    The light only exists to itself for "30 seconds"​
    and
    Light that is only 30 seconds old​
    are completely baseless.

    Please reread the FAQ and believe it.
     
  17. Aug 30, 2011 #16

    ZapperZ

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    This has nothing to do with being in the dark ages. It is knowing the RULES of the game. For example, the relativistic equations in which th idea of time dilation, etc., were based on an implicit assumption that the reference frames has light moving at c! So to consider a frame of reference in which light is at rest (i.e. to transform to that coordinate) and then say that at v=c, time is zero makes NO SENSE because you are now applying something where it doesn't belong and doesn't apply anymore.

    We teach physics by teaching students where many of these things come from. We don't just simply show them the end product. This is because they have to know the basic premise and assumption that are built into all of the physical theory and principles, so that they don't do exactly something like this - applying things where they shouldn't be apply.

    Zz.
     
  18. Aug 30, 2011 #17
    since photon or anythings that travel at the speed of light, time stands still.

    so the photons doesn't see any change at all.
     
  19. Aug 30, 2011 #18

    ZapperZ

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    It's as if you didn't even bother reading several posts in this thread.

    This is faulty. Read again things that have been written. What equation would you use to say that time stands still for photons? The time dilation equation? Why don't you figure out HOW it was derived? What was the implicit assumption on that derivation?

    Before writing such a thing, please read the FAQ sub-forum here.

    Zz.
     
  20. Aug 30, 2011 #19

    rede96

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    Well hopefully it helps, as do many other accurate posts. I wasn’t trying to argue against this. I completely agree that it is both important and necessary to explain the physics properly.

    However, I also think that sometimes the accurate can be accompanied by the abstract in order to help someone understand a principle. Particularly if they are from a non-science or academic background.

    (That is why it would be useful if people could put something about their physics knowledge / education in their profile so it shows up under their name to the left.)




    First off I should just say I have no intention to challenge the physics or SR specifically and agree that you are absolutely right from a mathematical standpoint.

    However, just because something makes no sense in one context doesn’t mean in can’t be useful aiding understanding in an other. (I can’t remember who said all models are wrong, its just that some are useful. It was relating to particle physics I think.)

    Imagining the life from the view point of a photon makes no sense from a mathematical point of view. But it could make perfect sense as an abstract concept, based as closely to the what we know as it can, if it helps someone understand the principles better.

    Even Einstein himself pondered on what it would be like to run along side a beam of light.

    Thinking in this way can also lead us down different paths where we may not have ventured if sticking strictly to the rules.

    If it was going to be constructive I would have liked see if we could answer the question from a abstract point of view of a photon, but I can see that perhaps in this case it wouldn’t.

    Anyway, although I find this debate really interesting, I guess this is not the right place to have it. I will start another thread in the appropriate forum and post a link for anyone that is interested.
     
  21. Aug 30, 2011 #20

    WannabeNewton

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    It doesn't make sense either way. You cannot lorentz boost to the frame of the photon so why do you think you are allowed to abstract what a photon "experiences". The fact that proper distance and proper time intervals on a null geodesic are zero doesn't state that time stands "Still" for a photon, or the similar. It is a statement of how trying to abstract what a photon's perspective would be like is just absurd in the context of GR.
    Yes and from this he postulated that it would be impossible to do so. He figured out that it was nonsense; he did the hard part for us. No point in stepping down a level.
     
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