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4 newb qstns:ageless light,physical change,time,twins

  1. Jun 29, 2004 #1
    1) If light is timeless/ageless, how does it change. Even more confusing, if light moves so quickly that it lacks time, then, since it cannot have time, it cannot have velocity (since velocity requires time) and cannot change or move, and, if it cannot change, move or have velocity, then it cannot go through space, and if cannot go through space then it isn't moving and if it isn't moving then it must be hurtling through time as fast as possible --> how can light be both moving and not-moving, timeless and temporal ? The idea of timelessness seems to lead to a contradiction; does this mean that light is not actually timeless, but is instead just moving very very very slowly through time so that it might as well be timeless ? If that is the case, then isn't it fair to say that you can move faster than the speed of light (ie, if light still has time, then it is not moving at absolute speed, and so is not the fastest thing possible) --> again, assuming that time can be infinitely divided (and, why not), then you could get infinitely faster (ie, you can never run out of time, so you can always move faster) ?

    2) Do the physical properties of the pole or the barn in the barn-pole paradox actually change ?

    3) I am confused about what exactly time is. I had always thought of time as no more than a measure of motion. However, relativity seems to say the exact opposite. If time were a measure of motion, then the faster you move, the faster time moves, and the slower you move, the slower time moves (I suppose you could say that if you move faster, time slows down in order to equalize you with slower moving frames of reference, while for those slow frames, time speeds up in order to equalize them with faster moving frames -- but I doubt this is right, since things hardly become equal on account of relativity --> a fast moving person's two years are a slow mover's 20 years). If time does not measure motion, what does it measure ? What exactly is it ?

    4) consider an alternate twin paradox : the twin in space is still the twin in space, but not the twin on earth is cryogenically frozen. When the two twins meet up again, will the cryogenic twin have experienced even more years than if he had not been frozen ? So, could space-twin return after what she experienced as 3 years to find her brother still the same age (because frozen), but having now aged 500 years (ie, because, being cryogenically frozen, he became so incredibally still ?) ? Also, what about dead bodies ? I am confused about things like radio-carbon dating (something about which I know absolutely nothing) -- the dead body does not move at all, so it is in an even slower frame than living people on earth, so shouldn't it age more quickly than those people ? So, if I aged a dead bone with carbon dating wouldn't the displayed age be too great (ie, because the body once dead experiences time faster, it would act as if it had been decaying much longer, relative to the living frame, than it actually had --> taking the date given by carbon dating and accepting it would be conflating the time frame for the living and the dead. shouldn't there be an adjustment ?)

  2. jcsd
  3. Jun 29, 2004 #2
    1.) What do you mean by light "changing"? As far as I know a photon is a photon is a photon. If you mean how does it interact with other things, then I fail to see the point your trying to make.

    I get the impression you are trying to understand something classically, that cannot be explained that way, which is why you're running into these problems.

    2.) From my understanding, the physics dimensions are actually changing. In other words one observer is not just experiencing an optical illusion. If he tries to move his hand behind an object that has been contracted, he isn't going to hit an invisible object. To him the pole is definitely shorter than before.

    3.) Time is a non spacial dimension that measures how long actions in the spatial dimensions take to occur. Thats how I would define it at least, but I think someone else can provide you with a much more accurate definition. What Einstein did was to show that time is not absolute afterall, and the rate at which we move through time can be altered, just like the rate at which we move through space.

    Here is how Brian Green put it, and a really great way to think of the 4 dimensions. At all times, every object is traveling at c, THROUGH ALL 4 DIMENSIONS. This means that when you are at rest, you are traveling at c through time. However, as you begin to speed up, think of your motion being diverted away from the time dimension and into spacial movement. So now you begin to move slower through time, and faster through space. Hence time dilation as you move at high speeds.

    Hopefully this is useful ;).

  4. Jun 29, 2004 #3
    thanks for the reply -- clarification

    thanks for the reply. Regarding the first point, to clarify what I mean by "changing" --> changing refers to any alteration. For example, in order to move from one point in space to another, the photon must change its position; the problem is that change cannot occur without time to change through (note, here, I am not talking about absolute time, but any kind of time -- whatever it is. Part of my problem here is that I still don't know what time is. It cannot be a measure of motion (as you seem to suggest) or else we would end up (unless we went with the unlikely alternative that I presented in my question) with the opposite of the theory of relativity (faster space=faster time, which is not what relativity says) and cannot exist on its own). If light does not move through time then it cannot change; if you allow change without time, then there is not way to differentiate when those changes occur --> if light is truly timeless, then ther eis no way to order it states --> ie, if it moves from A to B to C, but has no time, A, B and C cannot be in a temporal relation to each otehr and so cannot come before, after and simultaneously with each other. Without time, there is no change (ie, no movement, no change of relation to other things, no decay, etc.), without change there is no motion, without motion there is no spatial speed, without speed there is no, as you said, energy diverted into motion through space, so all energy must go into motion through time, so light cannot be timeless --> granting timlessness seems to lead to the necessity of being temporal, which, obviously, is not what is intended. do you see my problem now ?
    Last edited: Jun 29, 2004
  5. Jun 29, 2004 #4
    3.) Time is a non spacial dimension that measures how long actions in the spatial dimensions take to occur. Thats how I would define it at least, but I think someone else can provide you with a much more accurate definition. What Einstein did was to show that time is not absolute afterall, and the rate at which we move through time can be altered, just like the rate at which we move through space.

    I am probably horribly wrong, but I would define time as the decay of order. I came to this conclusion because the only indicator of "time" is the decay of order (in a forward arrow of time) or the reunification of energy/the restoration of order (in a backwards arrow of time). In other words time is defined by the presence of entropy, or lack there of. Therefore time is a measure of entropy.
  6. Jun 29, 2004 #5
    Your first assumption is already wrong. You mustn't mix up the reference frame. Only an observer, not the photon itself, would see the photon's time as stopped, by time contraction. The photon itself would see time as if it were normal. And, the photon would see it's own speed as zero. Don't mix up your(the observer's) time and the photon's time.
    I don't know about no. 2.
    I'm no so sure about no. 3, but here's what I think: Time is another one of those "understood" and "commonly accepted" things without a definition. It is a measure so as to describe the relative interaction between two things: some object from a moment before can't interact with an object right now. That's why we distorted time so as to explain the interactions between things. It is also possible to think of this as a 3D coordinate graph: the x-axis is the time, y-axis is the position(Take it as 1D motion to make it simple) and the z-axis refers to various reference frames. At each different reference frame, the graph is not the same, just like a 3D plane may have a different graph for each z value.
    I don't think cryogenics has anything to do with the problem...dead or living has no relevance whatsoever with the passing of time. Radioactivity has only to do with time. Also, at non-relativistic speeds, the effect of relativity wouldn't have much of an effect on a person. If you have a fixed length of life, you can't live longer for 1 second, even if you run for all your life and never stop.
  7. Jun 30, 2004 #6
    kuenmao is right. You can't mix up the reference frames. Here is how I understand it. In our frame, photons experience no time, and in their frame, they experience no distance. Hopefully someone will clarify what I am saying here much more soundly for you.

  8. Jun 30, 2004 #7
    a new paradox (well , really the same old question)

    well, now I know the answer (thanks for the explanations everyone), that is, I am clear on what everyone is saying, but knowing the answer, I am even more confused (go figure that out). Anyway -- so, the photon experiences time but no motion, and we see it moving without time, so it is both timed and not-timed ? that is, it is both A and not-A --> is this possible ? Also, when you say that the photon only experiences time, does that mean that from its perspective, the universe changes around it but it goes nowhere ? Finally, if the explanation about diversion of energy into time or movement is right, then how can the two perspectives be accurate --> if light is diverting all its energy into spatial movement, then it has no energy left for temporal movement, and even from its perspective, it won't move throught time, because, according to the explanation, it just isn't moving through time (it has no energy to do so). A truck with no fuel just doesn't have fuel -- it won't move forward no matter how you think of it; so, while I can see how your answer solves the problem, I don't see how your answer can work (ie, how light can be timed, not-timed, moving and not-moving all at once), and thus I am left again wondering how a timless thing can change or move at all.
    thanks, and sorry if I'm being a dense dumbass,
  9. Jun 30, 2004 #8


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    1) despite what people have said, this is really a non-question, the photon does NOT have a valid refrence frame in special relativity, it makes no sense at all to talk about things from the 'photons point of view'

    2) They don't change they just 'are', but if you mean that in two different reference are do objects really have different physial dimensions, then yes.

    3) Time is what a clock measures

    4) There is no such things as 'slowere frames, speed is relative, the fdifference in the age of the twins is cause by the assymetry of the situation, i.e. one twin accelarting. Relativity only has noticeable effects at speeds that are appreciable fractions of c.
  10. Jul 2, 2004 #9
    ok, one more time

    alright, I will give this one final shot. I am going to ask the first and main question in three simple yes/no parts which, if not finalizing my understanding of the reasoning, should at least allow me to proceed with a definite understanding of what is going on :

    1) do photons experience time/do photons move through time ?
    2) do photons move through space, interacting with other entities, changing position, etc.
    3) does timelessness contradict the ability to change (whether in terms of spatial relations, existence at one moment but not at the next, or any other type of change you want to think about)
    that's it, and thanks,
  11. Jul 2, 2004 #10


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    1) it's a question with no answer, photons don't have a rest frame in Sr, so it's not valid to talk about them 'experincing time'

    2) yes

    3) see (1
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