Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time in the reference frame of the photon and aswell curved spac

  1. Mar 16, 2013 #1
    Time in the "reference frame" of the photon and aswell curved spac

    Hi there!

    I understand the tendancy of physicists to stick to the math, and the logic itself, and to often avoid attempting to conceptualize a process or law, but that's kinda the opposite of what I'd like to do here.

    I dont mind at all if you refer to the theory, math or logic, and id quite like that, but I am attempting here to conceptually comprehend the stuff. I am a little math freindly, and very concept, logic and word freindly.

    Here, I want to explore the implications of the math, in a thought experiment type manner, and I hope to understand, if, in this thought experiment, the conception valid for the proof/theory/math.

    According to what little understanding I have of special relativity, time dilution means that "time" (ie typical "change in the direction of entropy"), slows down as we near the speed of light, the potential implication being that at the exact speed of light, c, time is "fully diluted".

    Would it be valid to thus imagine time not existing for a photon, and perhaps all events occuring "simultaneously" for the photon, in its reference frame, according to the theory?

    Could we validly imagine light not experiencing change? Or is there some specific reason to think that a photon does "experience" time, while travelling at exactly c?

    (This may or may not have implications, it may or may not be testable, or valid, but on the surface it would seem to be an implication given a little thought).

    If indeed time does not exist for the photon, then could that imply that time does not exist at all, but only at "low speeds, or lower energy", much like potentially some of the feild forces like mass and magnetism?

    (Einstein does say that time is relative, and that motion is relative, but also implies that time-space is an objective, if flexable, absolute of sorts which seems slightly at odds)

    ......

    The one other thing I have trouble with in SR, is gravity. If "spacetime" is curved, in the analogy of a warped surface, with a ball rolling down into a hole - the ball would not roll down into the hole if there was not still a force like gravity acting on this warped space.

    If you placed a ball in this warped space, there is no force acting on the ball to start it rolling. So it would seem to me, that curved spacetime does not actually explain gravity. Without still have a force, everything would be static unless some force acted on something.

    The only verified "proof" of curved spacetime that I am personally aware of is the curvature of light around a gravity well, but that could equally merely be gravity effecting the light, no? Or are there other experimental proofs, outside of the math?

    Can anyone give a conceptual version of this theory of gravity, that explains the causation of gravity, in a way that will make sense to anyone?

    Thanks so much for your help guys!
     
    Last edited: Mar 16, 2013
  2. jcsd
  3. Mar 16, 2013 #2

    Dale

    Staff: Mentor

    There is no such thing as the reference frame of a photon. It is a self contradiction.

    https://www.physicsforums.com/showthread.php?t=511170
     
  4. Mar 16, 2013 #3

    Dale

    Staff: Mentor

    That is a famously bad analogy. Don't lose too much sleep over it. Just search PF for "rubber sheet" to see why.

    Regarding proof of curved spacetime, curvature is just tidal gravity, so any measurement of tidal effects is an experimental proof of curved spacetime.
     
  5. Mar 16, 2013 #4
    Okay fair enough, kinda avoiding the conceptual aspect of this question, but how about this then -is time 100% fully diluted at c?

    Or, ill try and put this better -Does "time" make sense, to use to refer to, or is it applied, for massless particles or photons travelling at c?

    Is "time" and "travelling at c", together, contradictory or meaningless? (Which really is the essence of the question).

    I'm not trying to pretend that light has some kind of experience, or that that would be a scientifically valid POV, I am trying to understand time, and SR in relation to c as a constant.

    If there is some state, speed, energy, particle, to which time does not apply, that would be interesting.

    It could in theory make time, roughly like potentially mass or magnetism, where it only exists in certain states/energies. Which in itself would seem to question the notion of space-time as even a flexible, absolute, if things exist outside it. Perhaps time being relative, means that time only exists as a relative notion. (It is after all the measure of change in the direction of entropy, it, like space, is a concept that is based on the relative). Of course, logically, the only subjective reference which could exist, would be one inside of time, by the nature of time being change, and the nature of observation being interaction, so that which is outside of time would be essentially permenantly invisible to everything - but it would still have implications, such as potentially determinism.

    If light, even though its reference frame is meaningless, because it is timeless, or without time, reaches every point, at the same "moment" it leaves (yes, temporal terms, lol), then that might imply that the entire history and future of the cosmos is already set, conceptually, which kinda flies in the face of uncertainty, even particle/wave duality, to some degree - as the path and decay and life of the photon is identical in respect to "time" as to its creation. Ie, this could have implications for frames of reference that matter.
     
    Last edited: Mar 16, 2013
  6. Mar 16, 2013 #5
    I will do that now, and see if I can understand where the "impetus"/force/causality comes from, conceptually.

    Okay, I did a search on "rubber sheet", and found a bunch of better analogies, diagrams, that dont require "gravity". However, still it doesnt seem to explain the impetus, or causality of the actual movement. If I were to say "put" or create, and object into any kind of model of curved space, theres no reason for it to move without inputing acceleration energy, or a force. Once its moving, then the curving starts to sort of make sense, but without energy or a force, I can see no cause for the acceleration/velocity/energy, in causation terms.

    You mean tidal as in the tides of the ocean? If not is there a google or link you could direct me at? Sorry to seem stupid :P
     
    Last edited: Mar 16, 2013
  7. Mar 17, 2013 #6

    Dale

    Staff: Mentor

    It isn't avoiding the conceptual aspect of the question at all. It is simply pointing out that the conceptual aspect is a self-contradiction. A self-contradiction is false, and any reasoning from a false premise is fundamentally fallacious. There is no point in pursuing this line of thought further, it cannot lead to new insights, it only leads to illogical conclusions.

    I know that you think that you have stumbled on some deep and mysterious idea that you want to discuss further. Unfortunately, that is not the case. You have simply made a common mistake. There is nowhere further to go in that direction.
     
    Last edited: Mar 17, 2013
  8. Mar 17, 2013 #7
    What are you talking about in this last part?

    I am simply trying to understand the conceptual implications of SR (also how the heck SR space-time gravity makes any sense if there are no forces involved, how does the object start moving ie.).

    I even re-worded this part of my question, so that it did not contain "frame of reference", and simply said, roughly, "how can time be talked about at c or can it not be?", but you just ignored that.

    Frame of reference is really irrelevant to the question, so far as I can see. Its impossible to have a timeless frame of reference anyway - a viewpoint requires time, as does concious observation, or normal comprehension. Frame of reference is basically a null concept regarding timelessness, outside of any specific scientific defination or equation, merely by basic logic.

    But that doesnt make it an invalid thought experiment, Einstein's rocket wasn't a real rocket either....Thought experiments don't have to be 100% literally real, in order to provide insight.

    Ultimately, the point is, not at all, whether light has a frame of reference or not, but how we can potentially relate the objects at speed c to the phenomena called time, or time dilation, OR not.

    And if its "not' thats pretty much an interesting answer in itself I think.

    If the line of questioning makes you angry, why not try not just not participate?
     
    Last edited: Mar 17, 2013
  9. Mar 17, 2013 #8
    Yes if you search for rubber sheet you will see this recent thread which discusses the limitations of the rubber sheet analogy.

    And shows a link to this one which might be more intuitive.

    https://www.physicsforums.com/showpost.php?p=4305064&postcount=13
     
  10. Mar 17, 2013 #9

    Dale

    Staff: Mentor

    Thanks for going through the exercise of looking at previous threads on the topic. It certainly helps to narrow down the question.

    In your quote I highlighted a key word. You mention curved space, but in GR it is not just space but spacetime which is curved. That is important for exactly the reason you mention. An object which is at rest in space is still "moving" through time. Since spacetime is curved, some of that motion through time can curve into motion through space. Thus curvature of spacetime can cause an object to accelerate from rest.


    Yes, the ocean tides are one type of tidal effect. A tidal effect is present whenever gravity varies from place to place. It is exactly that variation of gravity from place to place which causes the ocean tides.
     
  11. Mar 17, 2013 #10

    jtbell

    User Avatar

    Staff: Mentor

  12. Mar 17, 2013 #11
    If people in your original frame measure that you have accelerated to .99c, and measure that your clocks have slowed down to near stillness, you yourself measure light to traveling at c. You will not conclude you have come any closer to the speed of light or the "experience" of a photon.
     
  13. Mar 17, 2013 #12

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    No.
    Yes, contradictory and meaningless.
    Time does not apply at a speed of c.
    It cannot be.

    You're getting the picture.
     
  14. Mar 17, 2013 #13

    Nugatory

    User Avatar

    Staff: Mentor

    There's always going to be another "why?" question behind any explanation, so you may never find a reason that's completely satisfactory.

    However, here are some considerations that suggest ("considerations that suggest" is a fancy way of saying "hand-waving":smile:) that you don't need to look too hard for the impetus behind the actual movement to understand the physics:
    1) inertia. If a massive object is already moving, it needs no force to keep it moving.
    2) an object that is apparently at rest in space is still moving very rapidly in spacetime; its spatial position isn't changing but it is moving forward in time. (Your wristwatch measures the length you've moved in time the same way that a car's odometer measures the length the car has traveled in space).

    So you don't necessarily have to identify any force causing motion through space-time to understand the curvature animations. You just have to accept the unexplained but still quite plausible assumption that everything is already moving forwards in time and if left alone will continue to do so.
     
    Last edited: Mar 17, 2013
  15. Mar 17, 2013 #14

    Dale

    Staff: Mentor

    Then let me point out all of the places where you implicitly bring in a frame of reference by using a frame-dependent concept:
    Time dilation is a feature of the Lorentz transform, a transform between inertial frames.
    There are two kinds of time, coordinate time and proper time. Usually the unqualified word "time" refers to "coordinate time". Coordinate time requires a reference frame.
    Travelling at c refers to a velocity, which is a frame-dependent concept.
    Magnetic fields and energy are also frame-dependent concepts.
    At the same moment refers to simultaneity, which is a concept requiring a reference frame.​
    So clearly it is relevant. You are trying to ask the same illogical statement with different words to see if changing the terminology will make it logical. The problem is the underlying concept is self-contradictory, and those self-contradictory concepts are at the root of all of the other terms that you can use instead of "reference frame" to get the point across.

    It is possible to talk about time in any valid reference frame. In any frame a pulse of light in vacuum will have a distinct beginning and a distinct end, both in space in time. There is no such thing as a frame which is moving at c and there is no frame where light is timeless.

    But they do have to be self-consistent, otherwise it is invalid.

    My pointing out that it is a self-contradiction has nothing to do with anger. I don't know what would make you think that. It is a basic exercise of logic, devoid of emotion, to show that your idea is irrational.
     
    Last edited: Mar 17, 2013
  16. Mar 17, 2013 #15

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    The problem here as I see it is that curious45 is thinking and talking about physics in a Newtonian context, while trying (presumably) to understand special relativity.

    The assumptions Curious45 are making are incompatible with special relativity, presumably due to a lack of a knowledge curious doesn't realize this.

    Even after being told :-(

    One poster came up with a rather good way to make the issue manifest. That is to point out that when one is taking about the frame of reference of an object, one is really talking about a frame where the object is at rest.

    So the "frame" of a light beam woul be the frame where light is at rest. But we already know that one of the assumptions of SR is that light moves at C. So trying to ask about the "frame" where light is at rest just isn't compatible with special relativity.

    I don't think the issue can really be made any simpler than that
     
  17. Mar 17, 2013 #16

    Nugatory

    User Avatar

    Staff: Mentor

    In relativity, "frame of reference" has a specific meaning, and it's not what you're describing above.

    A frame of reference is a rule for assigning coordinates to points in space-time (often called "events"), and you're using one any time that you say things like:
    - "A and B happened at the same time"; that means that they have the same time coordinate in whatever frame you're using.
    - "This object is at rest"; that means that its spatial coordinates (x, y, and z if you're using Cartesian coordinates; but we could use latitude, longitude, and distance from the center of Neptune if we wanted) are not changing as a function of time.
    - "A happened one second after B"; that means that B's time coordinate is greater than A's and they differ by one second, using the time coordinates assigned by whatever frame you're using.
    - "This object is one meter long"; this is a statement about the difference between the spatial coordinates, in whatever frame you're using, of two points in space-time such that: both points in space-time have the same time coordinate in whatever frame you're using; the path through spacetime (sometimes called a "worldline") of one end of the object passes through one of the points; and the path through space-time of the other end of the object passes through the other point.

    You can see why people tend to take verbal shortcuts when talking about this stuff :smile:
     
  18. Mar 17, 2013 #17
    Thanks for all your in depth answers.

    Your two explainations here, do give me some understanding of how this may work. Of course, time being what it is, still a bit of a mystery, this isn't a highly conceptually satisfactory explaination, but logically I suppose it may work. Better than what I was working under already.

    Although wouldnt this mean that the conversion of energy/momentum from "time" motion, to "spacial motion" would cause the time motion to decrease? If we converted one spacial motion into another thats what would happen, yes?

    ....

    And I now understand why SR has nothing to say about objects travelling at c, in the way I am interested in. The terms and the lorentz equation simply don't apply to light, they are both used to describe the sub-light universe that is more direct to our particular experience, or experience in general. (understandably)

    Of course if light were technically "at rest" by being timeless, and we only experienced it as moving, from any form of frame of reference, this sort of lexical and mathematic conflict is one we would expect - and the equations would be specifically not designed to describe it.

    This might be perhaps implied by time dilution approaching c, but it is not stated by the current equations, so to infer directly that light is timeless only from the current equations is not appropriate. Got that.

    I think therefor I will try and ask another type of question, in another sub forum to the same implication, but without any SR involved. Time is after all change toward entropy, its not some phenomena only explained or understood through SR. Thanks for your time, and patience.
     
    Last edited: Mar 17, 2013
  19. Mar 17, 2013 #18

    Nugatory

    User Avatar

    Staff: Mentor

    Remember what I said about "hand-waving" in the earlier post.... What it really means is that the analogy isn't perfect and it will break down if you push it too far. At the beginning of this thread you asked for a conceptual instead of a mathematical explanation, and there are a bunch of people here doing a pretty good job of exactly that, but if you want something that you can build new conclusions on, there's really no substitute for the math.

    I'm inclined to think that "time is change towards entropy" is backwards; more accurate to say that entropy never goes backwards relative to time in large systems. If you want a definition of time that you can take to the bank, go with Einstein's definition: Time is what a clock measures.
     
    Last edited: Mar 17, 2013
  20. Mar 17, 2013 #19

    Dale

    Staff: Mentor

    I am with Nugatory on this. I have never heard the time is entropy definition, and I cannot think of a single clock that uses changes in entropy as its operating principle.
     
  21. Mar 17, 2013 #20
    Lol, what a meaningless and obtuse definition. Amusing. But then does einstein define time as a dimension of timespace, in SR?

    If the math is 100% persistantly accurate in all circumstances and situations (dunno if it is, maybe it breaks down in some situations), then perhaps we have the analogy/conception/interpretation wrong...

    If the math doesn't work all the time, maybe the whole thing is wrong, conceptually and mathematically, and just happens to work some of the time.

    But I agree youve been somewhat helpful, even if this doesnt really help me understand the conceptual implications of the math fully, its better that where I was at the start.
     
    Last edited: Mar 17, 2013
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Time in the reference frame of the photon and aswell curved spac
Loading...