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Question about light speed and something I'm having trouble understanding

  1. Feb 22, 2010 #1
    Ok, looking at it from the point of view of the photon due to time dilation the time at which its emitted to when it finally strikes an object is instantaneous, regardless of distance, from its own frame of reference because its traveling at the speed of light.

    The twin paradox tells us that if you take one twin and he/she goes 99% the speed of light he/she will come back younger than the other twin. However, if that twin was to travel right at the speed of light (which i know is not possible) then all time itself outside of the light speed vessel would be instantaneous, time would go by infinitely, everything would be different nothing that twin knows would exist when he/she finally slows down.

    So, my question is when a photon is emitted from the sun, how do I exist when this photon experienced time go by infinitely instantly?

    infinitely instantly, i like that.
  2. jcsd
  3. Feb 22, 2010 #2


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    I'll buy it, for a photon, time is naught.
    this is true regardess of the first twin's speed...he/she'd come back younger than the other twin even if he/she travelled at normal speeds....pretty hard to measure, though, it would be such a ridiculously small amount of time difference
    right, why don't you say 99.99999..% of lightspeed
    what do you mean infinitely, there would be virtually no time elapsed in the travelling twin's frame of reference
    I don't follow, when the travelling twin returns, he/she would barely have aged at all, and the other 'stationary' twin, depending of the length of the journey as measured by the stationary twin, would have aged a very certain significant amount, or have died, but the travelling twin would live on upon his/her return, enjoying whatever still exists, assuming mankind has not destroyed itself.....
    take it up with the experts in the special relativity forums. I'm certainly not one of them, so I apologize if I said something wrong
  4. Feb 22, 2010 #3
    what i mean is that the photon wont experience time. But when its traveling at the speed of light time outside of its frame of reference would go by instantly. at least that what i believe time dilation is talking about.
  5. Feb 22, 2010 #4
    It isn't possible to define a reference frame that travels along with a photon. If all events had the same time coordinate (i.e. they all happen at the same time), they'd also end up with the same space coordinate in the direction the photon is going: the universe would be squashed to a plane, having no extent in that direction. So, yes, from that perspective, the journey takes no time... but then neither have you gone anywhere. How fast are you going? 0/0 metres per second--can't say. This isn't a meaningful way to describe the universe we live in. Events can happen at different times, and we need more than 2-dimensions to describe the geometry of space.

    Photons always travel at speed c as measured in any inertial reference frame; it's no more possible to slow down from c than it is to speed up to c.
  6. Feb 23, 2010 #5
    A more detailed discussion here, with some better informed answers than mine here. See especially #8 and #14.
  7. Feb 23, 2010 #6


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    The most you could ever say for this is that the photon travels for a finite time in our reference frame, and (very iffy on the technicality of this next statement) for no amount of time in its own reference frame.

    If you want your infinity to come from somewhere, the time dilation is indeed infinity-ish. A finite number divided by zero (is undefined) but tends to infinity from the right side. In other words, your time dilation is ostensibly "infinite" but that only reduces the time felt by the photon to zero, and has no effect on our reference frame. Nothing ever happens in the reference frame of the photon then.

    Don't quote me on any of this...this is purely a layman's explanation. The actual science is far more rigorous.
  8. Feb 23, 2010 #7
    I said it wan't possible to define a reference frame in which a photon is at rest, but after reading the thread I linked too, I'm not sure if that's generally true. I'll leave it to more knowledgeable people to comment on that. It seems clear, at least, that it isn't possible to define an inertial reference frame with this property.
  9. Feb 23, 2010 #8
    No need to worry about lightspeed of a photon...just think about,say, 90% light speed, 0.9C..

    What does a "stationary" observer see aboard a fast moving (0.9C) space ship?? Time is slowed!! What does a space ship observer see at the "stationary" observer location? time is also slowed!!

    As you get relative frames moving closer and closer to lightspeed difference, time appears slower and slower in the other frame....
  10. Feb 23, 2010 #9
    And yet, having accelerated something to 0.9999999999999999999 c, or however close you like, you're still infinitely far from accelerating it to c, in the sense that, if you switch to an inertial frame travelling at that speed relative to the one you started with, light still travels at c in this new frame. So there is an absolute difference between any inertial frame and this hypothetical, undefined "light's eye perspective".

    Also, having made the switch to new inertial coordinates, you can always make the inverse transformation back to the frame you started with. But you couldn't do this if you made an infinite boost, because then you'd lose information about the various ways that events can be ordered in time in inertial frames, and where they're located along the direction of movement.

    And so I don't think we can say: don't worry about the difference between a massless and a massive particle, just imagine a really big boost and that'll show you what the photon sees. You'd need an infinite boost, and there's no such thing as nearly infinite.
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