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I Arrow of time and travel at the speed of light

  1. Oct 2, 2016 #1
    If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct?

    Yet an observer watching this happen, sees the guy get on the rocket at point A, some time passes, and he gets off at point B? His progress has a past and this establishes an arrow of time for him from the observer's viewpoint. Is that correct?

    And if this is so, which way does the arrow of time point for the guy in the rocket? Aren't both the point of origin and destination equally plausible as either? That point A where he departed and point B where he ended his journey are one and the same and can be swapped around?

    In a possible real version of this problem, if a photon is exchanged between two particles (electromagnetic or strong force carrier photon for example), are two photons exchanged, one from particle A to particle B and one from particle B to particle A?
    or, is one photon exchanged that satisfies both particle A and B that they have both sent and received a separate force carrier photon (that is one photon between them, that appears to be both arriving and departing both particles simultaneously)?
    Or is it simply one photon moving from one particle to the other with no reciprocal exchange?
    Or is it the only other option I can think of, that one photon is sent out from one of the particles, it is received by the other particle and then it sends out a photon in exchange after it receives one.
    Last edited: Oct 2, 2016
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  3. Oct 2, 2016 #2


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    Incorrect, there is no frame that travels at the speed of light relative to the other frames. Light does not have a rest frame.

    The world line of a massless particle cannot be parametrised by proper time. However, it is perfectly possible to find an affine parametrisation that goes from earlier to later times in all frames, giving a proper time ordering to all events along that world line.

    First of all, you would do well to forget about exchange of photons as being throwing little balls between two charges. It is at best a popular scientific trick and at worst misleading. This can be described in a proper manner only with quantum field theory and it is not helpful to think about it in the way you have presented.

    Second, as I stated above, a time ordering can be imposed on s light like world line even if the proper time along it is zero.
  4. Oct 2, 2016 #3


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    Nothing with mass can travel at c, so the best you can do is "close to c" and "experiences very little passage of time"

    All frames agree on the direction of the arrow of time. In all frames the origin comes before the destination.

    Different events on a light pulse still have a frame invariant order. The math is more tricky, involving affine parameters instead of proper time, but there is no ambiguity about which event was emission and which was reception for a light pulse.
  5. Oct 2, 2016 #4

    Simon Bridge

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    [edit: oh wow - everyone jumped on this ... probably because you appear to have been thinking about it at length and otherwise quite well. Perhaps we need a sticky or insights article or something?]
    No - the statement "a guy travels at the speed of light" makes no sense in relativity.
    Time has no meaning and the reference frame of a photon also has no meaning.

    You can make this sensible by saying that the "guy" travels very close to the speed of light ... so that his travel time is too small for him to measure on his clock.

    No - the observer's clock establishes the arrow of time for that observer before the journey even started. This is how that observer can tell which event came first.

    In the same direction as his clock ticks.

    In the event of instant teleportation in a way that violates the postulates of special relativity - yes.
    What you are noticing here is what happens when you postulate something that makes no sense in physics even though it makes a valid English language sentence.

    The reference frame of the interaction is the particle's frame ... so the events are distinct, and occur in a specific order. The particles are not travelling at the speed of light, so the observation above does not apply.

    ... there is another option: your model is too simple.
    Charges do not "send out" photons ... instead there is a probability that there is a photon present to be interacted with.
    These photons are virtual - they are not "really" there in the sense that your chair is really there.
    They are probably best thought of as artifacts of a step in a calculation - like the intermediate steps when you do long division: you can use those steps to work out the final answer but none of those bits need ever exist as a stage in physically dividing an actual object.

    But there are experiments when you actually fire a photon at a target and the target responds - in that case, for the reasons you have noticed, it makes no sense to look at things from the reference frame of the photon.
  6. Oct 2, 2016 #5
    The rocket I was referring to would have been a massless rocket. Thought experiment one.

    So in regard to the clocks of the observer and the traveller travelling at C, who sets 'Now', the current moment? I think each sets it's own now, correct? As there is no universal clock keeping track of time?

    But I see an answer I was looking for that makes sense of a few things for me. That was the purpose, to gain understanding. Earlier I posted a longer model of the problem and got it zapped because it presented a non-allowed theory (actually my view of how I saw things with my current understanding and wanting people to show me where it was in error.)
  7. Oct 2, 2016 #6


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    What are you going to build a massless rocket and massless clock out of? You can't do it, I'm afraid.

    Trying to think about the perspective of something travelling at the speed of light will just give you a headache. There is no way to describe it in relativity. Light would have to be both stationary and moving at c for such an observer, which is obviously self-contradictory.
  8. Oct 2, 2016 #7
    It wasn't a problem for Einstein in his thought experiments.
  9. Oct 2, 2016 #8


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    I take it you are referring to the "riding a motorcycle next to a pulse of light". That's not a thought experiment so much as a fantasy. As Einstein later realised, it's impossible to describe doing such a thing.

    None of his actual thought experiments involve anything travelling at the speed of light except light itself. And he never describes anything from the perspective of something travelling at the speed of light because it's self-contradictory to do so, as I noted above.
  10. Oct 2, 2016 #9
    Here is his first: Source is http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/
    It says:
    How could we be anything but charmed by the delightful story Einstein tells in his Autobiographical Notes of a striking thought he had at the age of 16? While recounting the efforts that led to the special theory of relativity, he recalled

    "...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. For how should the first observer know or be able to determine, that he is in a state of fast uniform motion? One sees in this paradox the germ of the special relativity theory is already contained."

    So far as I know Einstein isn't massless and he speaks of pursuing a beam of light with the velocity of C. However that can be read that the light is travelling at C, though that would be redundant. So I read it as he is pursuing the light by moving the speed of light himself (how else would he get in phase with it).
  11. Oct 2, 2016 #10

    Simon Bridge

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    Doesn't matter ... it contains an observer able to tell the time. Therefore there is a clock at rest in the reference frame of the massless rocket. This is what does not make sense. The argument is not that it is impractical or physically unrealizable but that it is a logical contradiction with the postulates of special relativity.
    You cannot have something both travelling at some speed and also being stationary to the same observer.

    Appeal to authority: OK Einstein would be a valid authority on his own theory...please cite an example where Einstein postulated a massless clock in a thought experiment that accepted the postulates of special relativity?

    The "riding a motorcycle next to a pulse of light" thing? The story, you cited, did not involve a clock, pre-dated special relativity, and concluded that the situation contained contradictions so another theory may be needed. You will notice that, in the thought experiment, he posited that the light was stationary ... this is in violation of the postulate that the speed of light is the same to all observers so what is your point? It is possible to imagine things that are nonsense? I agree.

    Maybe we are not being clear - I admit it is tricky to wrap the mind around...
    Nobody is saying the sentence does not make sense as an idea in every possible framework, or that it does not make sense in terms of galilean relativity ... we are saying it is nonsense in terms of special relativity. The statement, after accepting the Einstein postulates, is a contradiction in terms. Just like the thought experiment you cited as an example.
    It you start from a nonsense assumption/assertion, you will get nonsense conclusions. Which is what happened.

    Have a look at the original paper:
    ... it is amazingly accessible. There is a popular treatment that should address some of the things you are interested in here:

    But you also asked about the relationship between time and clocks.
    In a nutshell: time is what is made by clocks. If it makes time it is a clock: that is what a clock is.
    For experiments we like clocks that also mark intervals of time that every inertial observer agrees are always the same as each other.
    The question of "now" is handled under 'simultaniety" and is a tricky subject that quickly turns into philsophy. You will find the concepts like "now" or "the present" do not get used all that much in relativity.
    The "arrow of time" stuff is usually related to how clocks make time - what is it about physical clocks that can be said to "make time" like I put it so glibly above and how come it is always one-way? That sort of stuff. This is not especially tied up with special relativity, despite the loose talk about FTL=time-travel, and gets into discussions about entropy, evolution of the mind, and how memories get laid down. All fun stuff, and all too big for this thread.

    But there is no Universal clock keeping a Universal proper time that we know of... we agree there.

    Which was? <all ears>

    That is fine - though with established theories you are better looking for ways you do not understand what the theory says and ask about that rather than try to postulate you own theory in opposition and ask people to shoot it down.
    Last edited: Oct 2, 2016
  12. Oct 2, 2016 #11


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    Did you read what you quoted? The man himself says that the idea is paradoxical. The fully developed version of this is that you can't travel at c nor describe the perspective of something travelling at that speed.

    Maybe I overstate my case to call the above a fantasy. At most, though, it is a proof by contradiction that it is impossible to travel at the speed of light.
  13. Oct 2, 2016 #12


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    Yes, and he also says in the same text that the result is a physical impossibility: "an electromagnetic field at rest though spatially oscillating", something that is forbidden by Maxwell's laws of electromagnetism. The point of the story that the assumption that you can travel at the speed of light is internally inconsistent; it leads to the logical contradictions that @Ibix points out in #6 of this thread.
    Last edited: Oct 3, 2016
  14. Oct 3, 2016 #13


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    As others have mentioned, you cannot have a clock travelling at c.

    However, even for a pulse of light, where there is no proper time between the emission and the absorption, the emission and the absorption events are still separate events with the emission occurring before the absorption in all frames. Because there is no proper time along the path, you cannot use proper time as the affine parameter, and instead you use some other affine parameter.
  15. Oct 3, 2016 #14
    For me learning requires that I can take what I have learned and extend the implications of it into an area that is unknown, and arrive at the proper conclusions. If I can't do that, then I haven't learned anything, I am just a parrot citing information in meaningless snippets of parroted script. If I do have an understanding that is correct, then I should be able to properly predict or explain other things that I currently don't understand.

    The answer you gave me was that I had missed an optional scenario, which was the apparently the correct one; that these photons are all over the place and are bumped into rather than being sent out like messenger photons. That in the process of bumping into one of these photons, the information between the particles is exchanged.

    Did I get that right? Or am I still missing an important point there?

    Perhaps then you can help me with this:

    When short lived particles sprayed down on earth from the upper atmosphere from gamma ray collisions in the upper atmosphere, reach the earth, due to the fact that they see the distance as shorter than we see it, the particles nonetheless get here. Our view seems to say they shouldn't get here. We reconcile it with time dilation in our frame. But the actual thing that got the particle from up there to down here was the length contraction, not the time dilation of time we observed. Time dilation that we observed didn't participate with the particle. Or did it?

    It's odd that of space and time, it is time that gets tossed around the most as possibly not being a real property, but possibly one made up by man. I don't see that with space. In the event mentioned above, the particle deals with space and gets to the earth without consideration of time. It is us as the observer to the event that has to play with clocks, and we call it time dilation so we can rationalize it, put it in harmony with our view or frame of reference.

    Is that right? But the particle didn't play with a clock, it spanned space. It apparently had less space to span. And our tinkering with time dilation didn't help it bridge that distance. So isn't the particles frame or viewpoint the only true valid frame for an event? UNLESS the observer is actually a participant in the event?

    The photon, a true point particle, sees no distance between the beginning and the end of it's journey. As with the subatomic particles reaching the earth, I would think the photon doesn't need to tinker with a clock anyhow to get from point A to point B and that clock tinkering is for us so we can rationalize it in our frame. So a photon is supposedly at the beginning and end of it's journey due to distance contraction? There was no distance to cover, so it's there at both ends simultaneously? But there's that word in the sentence 'simultaneously' which deals with time right? And our view of the event?

    How do I get it out of there? I thought like the subatomic particle, it experiences the ultimate in length contraction. That doesn't need time does it? So you see there where I have some huge misunderstanding and could certainly use being set right here. To get the proper view of these things.

    As one poster said:
    That doesn't work for me. Headache or not I feel I have a need to understand it on a conceptual level.

    Ultimately, all of this I feel, is required for me to understand the original problem I began with many many years ago, which is:

    Is potential an actual quality in this universe such as space and time? (Not potential energy or potential space or any other thing, but pure potential viewed as a substance or inherent property in the universe) Does it exist and is it required for other substances to exist (like energy)?

    It has been observed long before math came about. Potential is not the outcome of mathematics and mathematical theory, I think. If not, is it a substance? A thing? A property? A quality? Or is it possibly a construct of the mind, like time has been suggested to be?

    Is it useless even to try to answer the question? Perhaps it is impossible. But isn't it the journey that life is all about? Aren't we supposed to attempt to understand the incomprehensible?

    Potential is usually defined as that which may exist yet is not. I think maybe it is more likely, that which WILL exist yet is not.

    Is it required that a 'force' of potential precedes the existence of anything/everything?

    Or is this just another one of those things (as was said earlier) that makes sense in English, but makes no real sense in physics or the universe?

    Now don't think that this is a theory of mine. If it was, I would tell you how it worked and how it explains everything. It's not. It's a question, pure and simple. But I think it is inherently very hard to answer (at least it has been for me.) It goes along the lines of a discussion I had with a theoretical physicist at the University of Idaho, Moscow back in the late 90's, in which we discussed tachyons. I proposed to him that since tachyons apparently move in reverse through time, that one might put a detector at a distance away from the experiment that might generate them, and start it detecting before the experiment is turned on. Pretty simple right? haha. Well, his argument about detecting them was that until he knew their properties, he didn't know how to make a detector to detect them.

    And so in a way this problem of potential seems to fall into that same sort of trap. By nature it seems not to have any identifiable property. But does time or space?

    In recent history I have had some time to devote to try and answer this question for myself, and in the process have run across other dilemmas that relate to the answer I am looking for, like the photon dilemma I mention above.
    Last edited: Oct 3, 2016
  16. Oct 3, 2016 #15


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    No. Time dilation did get the particle "from up there to down here" in our frame. In the muon's frame it was length contraction that got the particle "from up there to down here". Neither view has any theoretical justification for being considered the "actual" reason. Both frames and their associated explanations are equally valid.
  17. Oct 3, 2016 #16
    Does the muon getting from point A to the ground (point B) require an observer, or the rest frame?
    If so what contribution does the rest frame make for the travel of the muon?
  18. Oct 3, 2016 #17


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    "No" for the first question, and "none" for the second.

    The physical facts are:
    1) A muon was created at a particular point in spacetime (which happens to be in the Earth's upper atmosphere).
    2) The muon's path intersected the surface of the earth at a particular point in spacetime.
    3) The proper time along the muon's path between those two points in spacetime has some particular value which is less than the muon lifetime.
    These facts have nothing to do with observers or reference frames; they'd be facts even if there were no observers around.

    A reference frame is just a rule that we use to to assign times and positions to these events. For a very prosaic example.... If I said that a firecracker exploded ten meters my left and you said that it exploded five meters to your right, we wouldn't be confused, we'd just conclude that you were standing 15 meters to my left, we're both right, there's nothing wrong with either of our perceptions of reality, we're just measuring distances relative to different starting points That's different reference frames at work.

    Relativity is less intuitive because it attaches different times as well as different positions to events, but the principle is the same - both pictures are just as good, and there is no physical content in the difference between them.
  19. Oct 3, 2016 #18
    2) A shared point that coincides with both the muon's length contracted spacetime and earth's non-length contracted spacetime?
    3) Proper time? Universal clock? Proper to who or what? Is this the observer frame? Local universe frame? I am not quite following.
  20. Oct 3, 2016 #19


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    Proper in the Latin sense of "one's own" (propio if you speak Spanish), like property. The muon's own time - or time measured by a clock moving with the muon if you want to be precise.
  21. Oct 3, 2016 #20


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    There aren't two spacetimes. There is only one. The Earth frame and the muon frame are different coordinate charts on the same spacetime, just as Cartesian and polar coordinates are different coordinate charts on the same Euclidean plane. Points in spacetime are independent of which coordinate chart you use, just as points in the plane are independent of which coordinate chart you use.

    Proper time is the arc length along a particular curve in spacetime, in this case the muon's worldline (the curve in spacetime that describes its history). Arc length along a curve is an invariant, independent of the coordinates you choose; this is just as true in spacetime geometry as it is in ordinary Euclidean geometry.
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