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B Existence of Light in time?

  1. May 17, 2016 #1
    Hey, guys! So, I've been wondering. Space and time are not only interrelated, time is the movement of space. So, time is space, yet space is not time. Now, my question. If light exists outside of time, and space is always moving, thus creating time, how does it exist - and how do we experience it, as we exist in time?

    What does this mean? I am perplexed.
  2. jcsd
  3. May 17, 2016 #2


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    Easy: light does not "exist out of time", so the entire problem does not arise.

    The easiest way to see this is to draw a spacetime diagram showing the path of a flash of light as it moves from one point in space to another, compare with the path taken by any object or person.
  4. May 17, 2016 #3
    Light does not exist out of time? So it experiences time?
  5. May 17, 2016 #4


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    You can't coherently describe what would be experienced by an observer travelling at the speed of light. So "does light experience time?" is a question like "what does green smell like?" It looks like a question but it makes no sense because it's based on faulty assumptions.
  6. May 17, 2016 #5

    Mister T

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    Yes, it would seem you've been exposed to some confusing conclusions drawn by someone intending to be helpful, but unfortunately that kind of help is pretty much useless for anything other than creating some kind of artsy-fartsy notion of spacetime.

    Things do move, and we use both space and time to keep track of that motion and to predict behaviors of objects. But space and time are not objects that move, they are constructs used to study motion. Time is not space, space is not time. Space doesn't move through time.

    The next time you take a trip somewhere divide the distance between your place of origin and your destination by the time you spent traveling. This will give you the size of your average velocity. That's really the most basic type of spacetime calculation you can do. If you found that your average velocity were a significant fraction of the speed of light, you'd find the measurements of distance and time to be strange and confusing. One way to explain those strange behaviors is by realizing that space and time are related in a more complicated way than we are led to believe when all we look at are low speeds.
  7. May 17, 2016 #6


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    Physical theories are sets of equations telling you how to make predictions. There exist descriptions of those equations in English and other languages, but those are not the actual theories, and they are never exact. Drawing conclusions based on those descriptions does not work because you are missing the actual theory behind those descriptions.
  8. May 17, 2016 #7


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    I don't understand what this means. It doesn't sound like anything that's actually in the theory of relativity.

    This doesn't make sense either.

    Neither does this.

    I think you would be better served, as mfb suggests, by trying to learn the actual underlying theory of relativity, instead of relying on ordinary language descriptions that can lead you astray.
  9. May 17, 2016 #8
    That is totally wrong. In priciple, space doesn't move, but motion occurs in space. [Unless we are talking about "relative motion" (between frames of reference) and "relative spaces" in the moving frames.]

    You could say however (and may be that's what your intuition was trying to describe) that time is "the measure of motion", via "duration" (which is a property of motion and of every process). A process of larger duration is longer in time etc. Thus by setting a "unit process" and comparing durations we measure time. Then all we have to do is to select (pick) an origin of time (or a point of origin in time), where t=0, and thus we obtain that way "the axis of time" (or the dimension of time).

    And in fact that's exactly what time is: a dimension, or a parameter [to study motion ... (and then it also becomes a dynamic variable)]. We cannot actually see time as an existing thing or object. We only see motion (+events and processes) [And since we inevitably participate in it, we only see the present (of motion and time)]. Thus time is not something tangible, that you can see and touch, although we do "feel" its passing ... . For instance, we do not see the past; we only remember it. [We always see the present, but also we are always moving towards the future.]
    Whereas in the case of space (+see ahead [below]), if you look behind you (while moving) you will see [the place] where you were ... .

    {In this whole discussion here, I assume everything with respect to a particular frame of reference.}

    Space on the other hand, is something more tangible. It is the "frame" or "framework" in which "all things happen". Motion takes place in space, and all events and processes happen there. The measurement of space is also of course done in the similar known fashion, resulting to 'length', 'area' and 'volume', as well as the 3 spatial dimensions [and x-y-z axis and coordinate systems].

    Now light exists both in space and time.
    Or better, light is a process, or motion [of photons], that happens in space and is measured with time. [These are to clarify the terminology.]
    I think that sais it all, to answer your questions and put things into perspective. Now I think you can also create your own perpective in further understanding the subject.

    Finally, since motion and all events and processes happen in Space and Time (or better: measured with time), Einstein (even in Special Relativity) came up with the idea to combine the two notions into one: Space-Time. But now instead of talking about "points of space" and "points of time" (moments) separately, one talks about "point events" [described by 4 parameters, coordinates or dimensions: (x, y, z, t)]. The resulting entity is called "the 4-dimensional space-time continuum". [But of course in a complete study e.g. of Special Relativity, that would also have to include the study of the Lorentz transformation, as well as the important notion or concept of 'Covariance'.] However, these may go beyond the original discussion. But they do give a good motivation and introductory entering into Relativity, which should be your "ultimate" goal.

    I hope these help to clarify things and put the discussion that you started into perspective.
    Last edited: May 18, 2016
  10. May 17, 2016 #9
    I totally agree with your comment, but I don't think he is ready to fully study and learn Relativity yet, unless he first studies and learns about basic notions of space, time, motion and how they are defined and measured, first in classical mechanics and geometry. If he went straight into relativity now I think there is a great risk of even bigger confussion.
  11. May 18, 2016 #10
    Although I liked and otherwise agree with your comment, I dissagree with the above statement.
    A more basic type spacetime calculation is comparing the duration of a process to a unit process (or a unit of time) and similarly for space ..., which leads to definitions and measurements of the corresponding quantities. [+see my other comment]
  12. May 18, 2016 #11
    Physical theories are definitely not just "sets of equations". That would be an extremely formalistic view! There is also (in the theories) concepts-notions (e.g. 'Space', 'Time', 'Matter'), sentences, Principles, Theorems etc. .
    The three dominant views in modern philosophy of science about the essence and structure of physical and scientific theories are: 1. The Syntactic View, 2. The Semantic View and 3. the Set-Theoretical predicates view. Of course, some combinations exist.
    However, since this is not a philosophy of science discussion, I will not say anything else about that.
    Last edited: May 18, 2016
  13. May 18, 2016 #12


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    All of these concepts only have precise meaning when they are matched up with the equations and the predictions those equations make for observables (and how those predictions compare with actual observations). Trying to understand our theories by using ordinary language descriptions of these concepts is a great way to get yourself confused.

    Yes, and please bear in mind that philosophy discussions in general are very likely to be out of bounds in this forum.
  14. May 18, 2016 #13
    It's probably not what he means but the passage of time does require motion. After all any measurement of time requires motion.
  15. May 18, 2016 #14


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    I would say any measurement of time requires change; motion is one type of change that could be used, but not the only possible one.
  16. May 18, 2016 #15
    I agree. I did not say that equations are not the important part of the theory. I said " ... also ... concepts-notions ..." (etc.). But I would put " ... and vice versa ..." in your argument. In other words, nor the equations can stand without first defining, supporting and understanding the concepts and notions that they involve or introduce. For example, if I just say E = mc^2 and F = dP/dt I mean nothing, unless I first define all the quantities and concepts involved (i.e. all the quantities in the equations and the concepts behind them that they represent).
    I also strongly agree with avoiding ordinary language descriptions and I cautiously never did that.

    I already knew the policy about philosophical discussions (in this forum), that's why I didn't start a discussion. But that doesn't mean that one can give false statements (even a mentor) without being criticised. I wasn't the one that made the risky statement ..., which actually falls into the territory of the philosophy of science (Structure of scientific theories, etc.).
    Note however that even in Physics there are different types (of physics) [tendencies or methods]: 1. Theoretical and Mathematical Physics 2. Experimental Physics 3. Phenomenology. [Which one do you subscribe?] All these three must eventually corroborate and collaborate to ultimately create: 4. The main-stream established and accepted textbook physics. And that's what we should try to accurately present here. [Please definitely tell me if you agree with this or not - since this is not an open research subject (that would mainly require references), but rather estsblished textbook physics (e.g. Relativity).]

    P.S. Please in your reply try to be specific and relatively detailed in what your real objection is, if any. I notice that sometimes you may answer selectively (or may be I am wrong). Also, if you agree with something please feel free to say so, as I do and have done several times (I also liked your other comment). These, I think, are for a better and more productive discussion, and they are also stated as advice to us in the guidelines.
    Last edited: May 18, 2016
  17. May 18, 2016 #16


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    Yes, but again, you don't define them by ordinary language descriptions--you don't define them by things like "E stands for energy", "m stands for mass", "F stands for force", etc. Strictly speaking, you define them operationally--for example, "E" stands for a particular measurement you are making in a particular experiment where "m" is another measurement (not necessarily in the same experiment, you might be using a value derived from another experiment). Often ordinary language words like "energy", etc. are used for brevity, when it is clear from the context what measurement "energy" corresponds to. But ultimately the measurement is what gives meaning to the symbols in the equations, at least as far as physics is concerned. That is what mfb meant in the post you originally responded to.
  18. May 18, 2016 #17
    I agree. For explaining relative issues see also my full discussion in the other first big comment of mine.
    Last edited: May 18, 2016
  19. May 18, 2016 #18
    (+see my other reply to yours) I add that I otherwise agree with the rest of your comment.
  20. May 18, 2016 #19


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    Okay, let me be more precise: sets of equations and descriptions how to use them.
    Doesn't influence the main point I wanted to make in my previous post.
  21. May 18, 2016 #20
    I mostly agree, but I think it also differs for Experimental and Theoretical physicists. For example, theoretical physicists tend to want to define everything (via equations) from primitive notions and quantities such as 'mass', 'charge', 'spin', etc., which can only, as you say, be defined and measured through experiments. Then e.g. the energy in classical, i.e. non-quantum, physics is usually a non primitive quantity defined via equations (e.g. E = 1/2 m V^2 for the newtonian kinetic energy and e.g. through the Stress-Energy Field tensor in General Relativity); while at the same time the theorist also accepts the existence of measuring rules and definitions via measurememts (coordinative definitions) even for those (non primitive) quantities, in order to compare, test and comply with experiment. However, for the theorist, e.g. classical energy (a non-primitive quantity) has a meaning independant of measurement (but eventually would coordinate or dissagree with measurement), because it is given by the equations or formulas (formulae).

    And these are not ordinary language descriptions, because they would involve equations, formulas, as well as measurements.

    The Experimentalists on the other hand may dissagree in the sense that they first define and measure all quantities via measurements and experiments and then discover the equations, formulas, laws etc. that correlate and relate them between themselves and govern the phenomena.

    These are I think two different views of viewing basically the same thing, something that we always do in regular physics, without telling them appart.

    But what probably justifies you completely is Quantum mechanics, where according to the Standard Interpretation, observer and instruments, as well as measurements, play the primary role ... (e.g. cf. Selection Rule or Principle ...).
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