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Humans travelling at light speed?

  1. Jul 3, 2011 #1
    humans travelling at light speed?????

    just a little thought experiment. we know it is ludicrous to suggest that humans can travel at "c" because we have mass and it takes infinite energy to travel at c if you have mass. however, does this work for relative velocities: let us assume you are in a closed dark room standing completely still; suddenly a flash-light at the end of the room is turned on. we know that the flash light is expelling photons that are able to travel at c. my question is: with respect to each photon, what velocity do you (standing still) travel at?
     
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  3. Jul 3, 2011 #2

    Pengwuino

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    Re: humans travelling at light speed?????

    It makes no sense to ask what photons see. They do not have frames of reference where it makes sense to ask "what does a photon see". They don't experience time or space.
     
  4. Jul 3, 2011 #3
    Re: humans travelling at light speed?????

    thanks i guess that would make sense. but that's no fun, light can experience time (we see galaxies as they were millions of years ago), light can experience space (black holes and other massive objects can "bend" light) but light has no reference frame in that particular thought experiment - in short, i don't get it
     
  5. Jul 3, 2011 #4

    ghwellsjr

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    Re: humans travelling at light speed?????

    When we talk about a frame of reference associated with some object/observer, we define that object/observer to be at rest in that frame. We also define the speed of light to be c in any frame of reference that we are considering. A photon cannot both be at rest and traveling at c at the same time so it is meaningless to talk about a frame of reference associated with a photon.
     
  6. Jul 3, 2011 #5
    Re: humans travelling at light speed?????

    would the same principle apply if talking about two "photons" moving in opposite directions. can that be described using frames of reference?
     
  7. Jul 3, 2011 #6

    Doc Al

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    Re: humans travelling at light speed?????

    Sure it can be described. Just not from the frame of reference of a photon, which has no meaning.
     
  8. Jul 3, 2011 #7

    ghwellsjr

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    Re: humans travelling at light speed?????

    Yes, but remember, a photon, like any ray of light, is defined to travel at a speed of c. It makes sense in SR to talk about two photons being emitted in opposite directions simultaneously from a common source at rest in a reference frame so that they simultaneously hit a pair of targets equally spaced from the source. But when we use a different reference frame where the source and targets are in motion, the speeds of those two photons are redefined to be c in that new reference frame so that they do not hit the targets simultaneously.
     
  9. Jul 3, 2011 #8
    Re: humans travelling at light speed?????

    so in other words, looking at one frame of reference can have a completely different effect to another frame of reference. that makes me even more confused
     
  10. Jul 3, 2011 #9

    Pengwuino

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    Re: humans travelling at light speed?????

    No, they do not experience either. The reason we see objects as they were millions of years ago is simply because it takes time for light to travel. However, that's in our frame of reference, not the photon's. Same idea with space.
     
  11. Jul 3, 2011 #10
    Re: humans travelling at light speed?????

    now i'm super confused. if light has a numerically defined speed - 300,000km/s in a vacuum. then surely a photon travelling at that speed experiences time. in other words, it would take 20 seconds for light to travel 6,000,000 km. And in general relativity, light does get effected by space ignoring frames of reference, right? the medium light propagates through is space, right? that's why it can travel in a vacuum.

    so doesn't light get effected by space and time or am i completely misunderstanding your point?
     
  12. Jul 3, 2011 #11

    Ryan_m_b

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    Re: humans travelling at light speed?????

    Photons do not experience because they are non-conscious entities.

    However to suggest an answer to your question (which may be an inappropriate answer because I may be wrong about this) if we imagine a hypothetical, conscious entity that is able to travel at light speed it would still not be able to "experience" anything. This is because time dilation at the speed of light becomes infinite.
     
  13. Jul 3, 2011 #12
    Re: humans travelling at light speed?????

    sorry, what i meant by "experience" is the forces acting on it. i would say for example that a ball "experiences" gravity. i'm not crazy enough to say that the ball has a conscious or something. i (foolishly) mean that the ball is attracted to the earth through the force of gravity. Likewise, i am saying that the force of gravity has an effect on the path of light on large scales and equally time has an effect on light because it doesn't take an instantaneous amount of time for light to travel a light year. Am i wrong?
     
  14. Jul 3, 2011 #13

    bcrowell

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    Re: humans travelling at light speed?????

    FAQ: What does the world look like in a frame of reference moving at the speed of light?

    This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

    The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

    Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

    What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.
     
  15. Jul 3, 2011 #14

    Ryan_m_b

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    Re: humans travelling at light speed?????

    No your not wrong, photons are affected by gravity, matter etc.
     
  16. Jul 3, 2011 #15
    Re: humans travelling at light speed?????

    so in short, until we are able to hypothesise a situation where we have infinite energy to travel at "c", it is impossible to even begin to describe what can happen at a photon's frame of reference
     
  17. Jul 3, 2011 #16
    Re: humans travelling at light speed?????

    so using my basic (probably naive) concept of velocity and distance, surely then there is no infinite time dilation if travelling at the speed of light (hypothetically). this is because it will still take you time to travel huge distances.

    what confuses me is that if "c" is an actual numerical value, then how can time stop if travelling a distance? do you understand my confusion here?
     
  18. Jul 3, 2011 #17

    ghwellsjr

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    We don't "look" at a frame of reference, rather, we "define" an arbitrary reference frame from which we "look" at everything else. We can then use the Lorentz Transform to "see" what the same things "look" like according to a different frame of reference. Switching between frames of reference doesn't change the effect of anything that is happening, it only changes how we label what is happening.
     
  19. Jul 3, 2011 #18
    Re: humans travelling at light speed?????

    a-ah! but with the example of the light particles and the sources, two different effects are observed right?

    but i don't understand why light (photons) are exempt from reference frames. photons, as i have tried to explain are affected both by space and time. what "criteria" must you meet in order to be "allowed" a frame of reference
     
  20. Jul 3, 2011 #19

    JDoolin

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    Re: humans travelling at light speed?????

    The limit of acceleration.

    Say you are looking at a planet in front of you, and you accelerate toward it.

    The event that you are looking at moves into the past, and further away.

    In the limit as you accelerate "to" the speed of light, the event moves infinitely far away, and infinitely into the past.

    The image then approaches superluminally. In the limit, as you accelerate "to" the speed of light, the image approaches at infinite speed, and arrives in zero time.

    Also, the frequency of the light coming to you, of course, goes to infinity; the wavelength goes to zero.
     
  21. Jul 3, 2011 #20
    Re: humans travelling at light speed?????


    thats if you are accelerating to the speed of light. However, if you are already moving at a constant speed of c, are you saying that there is an infinite time dilation. in other words, when we "see" the sun as it was 8 minutes ago, in the frame of reference of the light from the sun, what actually happened was that it arrived on earth instantaneously but earth went into the past to make it appear as though there was a delay of 8 minutes?
     
  22. Jul 3, 2011 #21

    JDoolin

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    Re: humans travelling at light speed?????

    Well, there is a big difference between looking in front of you and looking behind you. If you look behind you while traveling "at" the speed of light, then it would appear that the earth was staying with you. The light from behind you is just keeping up.

    If you look at the variety of good relativity books out there, you'll see a place where this type of discussion ought to be, but is notably lacking. It's the part of the twin paradox where they show the situation from the perspective of the stay-at-home twin, but they do not show the situation from the perspective of the traveling twin.

    You can see it has been tried in Wikipedia here http://en.wikipedia.org/wiki/Talk:Twin_paradox/Archive_13#Specific_example
    but, even though it's a simple calculation, because it is considered "original research" Wikipedia can't publish it.

    I've read one version of the twin paradox even, where the traveling twin is put into a locked cell with no windows, to assure that you don't question what the traveling twin sees, because the traveling twin sees nothing at all!

    When the traveling twin turns around, they will see the image of home shoot suddenly away during the acceleration phase, before rushing back superluminally during the return.

    If you imagine zooming across a room, near the speed of light, the far wall will appear to be much further away, and the space across the room in front of you is likewise, far distant, but approaching superfast. The wall behind you would hardly appear to be receding at all.
     
  23. Jul 3, 2011 #22

    ghwellsjr

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    An observer can only be aware of the photons that hit his eyes or instruments located right next to him. For photons that he or his instruments emit, he cannot tell where they are at any particular time.

    So light (photons) are certainly not exempt from reference frames, in fact, this ambiguity in knowledge of their whereabouts is one of the main reasons why Einstein came up with his Theory of Special Relativity. In any reference frame, light is defined to travel at c.

    So, as I said before, there are not two different effects that are observed as a result of using two different frames. Things that are defined according to one frame because they could not have been observed, for example, the trajectory of a photon, also cannot be observed in any other frame, but are merely given a different definition.

    I don't understand your question: 'what "criteria" must you meet in order to be "allowed" a frame of reference'. The only criterion for a frame of reference in Einstein's Special Relativity is that it is inertial, that is, not experiencing acceleration, either a change in speed or a change in direction.
     
  24. Jul 3, 2011 #23

    DrGreg

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    Re: humans travelling at light speed?????

    There is also a second criterion, that the speed of light is the same (c) in all directions, and that clearly rules out a frame in which light (in one direction) can be stationary.
     
  25. Jul 3, 2011 #24
    Re: humans travelling at light speed?????

    Thanks that really explains it well.
    so in summary, light can be a frame of reference so long as it travels at c. time, sometimes, must be warped to hold this fact. light still travels at the same speed but time stops so light reaches a destination instantaneously. this must be true to obey Einstein's rules that c is the maximum speed. Correct?
    what confuses me is that the "photon" doesn't experience time. so each photon of light is actually light from the very beginning of the Big Bang?
     
  26. Jul 3, 2011 #25

    ghwellsjr

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    Please go back and read the posts at the beginning of your thread. I don't know why anything you have read in this thread would lead you to conclude that "light can be a frame of reference so long as it travels at c". All light travels at c in all frames by definition. You could say that any object with non-zero mass traveling at constant speed in a straight line in any direction "can be a frame of reference" but you cannot say that about a photon.
     
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