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Time, entropy and relativity

  1. Jan 23, 2008 #1

    I have been participating in a couple of threads here about time. It seems to be a widespread opinion that time has to do with change and the "arrow of time" with entropy.

    Can anyone here explain to me how the fenomenen of different flow of time in different frames of reference as proved by special and generel relativity is explained in this "time is change"-theory?

    Or maybe I should ask: If time is a consequence of change how does that explain the relativity of time?

  2. jcsd
  3. Jan 23, 2008 #2
    You ask a very wide ranging question with a lot os of aspects (some of them philosophical) so I will just add a few comments to the melting pot.

    When a glass shatters, the broken state has a higher entropy than the unbroken state. If we film this event and run the film backwards we see the shattered glass reforming to its unbroken state but that statisically unlikely to happen in nature. The arrow of time always seems to go in a direction of increasing entropy. Thermal dynamics seem to give us a direction for time not a rate.

    So why does time advance at a rate of one second every second? I am not sure there is a definite answer to this but it seems it is closely related to the constant speed of light. A photon moves forward a distance of one planck length in a time of one planck time interval. Although the speed of light is always constant when measured by a local observer, the apparent speed of light (the coordinate speed) as measured by an observer at a different gravitational potential can be different. If we syncronise two clocks and lower one into a gravitational well for a while we see that when we bring the clocks back together again that they have recorded different time periods. The difference is real. This raises the question "Are the planck units invarient?"

    Another aspect of space and time that is interesting is the invarient spacetime interval. It can be shown that any object is always advancing in space or time and cannot be stationary in both aspects. You could call this the "arrow of spacetime" :P An object that is stationary with respect to a local observer experiences the maximum possible proper time rate. An object that is stationary in time (experiencing a proper time rate of zero) has to be moving at the maximum spatial rate (the speed of light) relative to the local observer. In relativity, time and space are in a way interchangeable but combine in a way that is invarient.

    In special relativity two events that simultaeous to an observer in one inertial reference frame are not not simultaneous from the point of view of an observer in another reference frame. There is no prefered reference frame so there is no concept of universal time in SR.

    [EDIT] Zeno's paradoxes provide another interesting playground for the analysis of the real meaning of time and space. Classically the paradoxes are resolved by summing infinite series. Peter Lynds put a new twist on the subject in a paper that hit the headlines where he concludes the paradoxes are resolved by removing the concept of an instant of time or exact location of a moving object at any given instant.

    Just a few random thoughts :P
    Last edited: Jan 24, 2008
  4. Jan 24, 2008 #3
    Thank you.

    Yes. This is the explaination I've heard. My problem is that I can easily follow the idea of increasing entropy following the arrow of time.. what I have difficulty accepting is that there are any indications that it should be the other way around as well: That time is a result of change and follows "the arrow of entropy". And this is why I ask whether this theory is able to explain other properties of time than the direction - i.e. the properties you have listed.

    But was your answer that a non-invarient planck time and planck distance is needed for this explaination to work with relativity? Or is the answer simply: "No, the idea that time is a consequence of change and the direction of time a consequence of entropy offers no explaination for the other properties of time."?

    - Henrik
  5. Jan 24, 2008 #4


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    What does "time is a result of change" mean? That doesn't sound like any theory of physics, it sounds closer to a philosophical statement. And anyway, relativity seems more compatible with the philosophical view that time is a dimension, the idea known as eternalism, rather than presentism where the future doesn't really exist until it happens...I would think someone saying "time is a result of change" would be arguing for something more like presentism, although it's hard to tell for sure, and of course physics cannot actually resolve these kinds of philosophical questions.
  6. Jan 24, 2008 #5


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    I think he means that one possible answer to the question "what is time?" is that "time is the t in F = dp/dt, etc." Since those equations always involve some change "time is a result of change" or perhaps "time is a measure of change".
  7. Jan 24, 2008 #6


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    I think that's what he means as well.

    Another way to look at is, if we were in a universe where nothing changed, ever, how would we measure time? If you can't measure something, even in principle, then it doesn't exist, at least not from a scientific pov.
  8. Jan 24, 2008 #7


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    Maybe so, but it's still more of a philosophical statement than something that follows from physics...after all, I think pretty much any theory of physics can model a static unchanging universe perfectly well.
  9. Jan 25, 2008 #8
    Time and again...

    Hi again. Thanks for your answers. It's clear, that I am not.. clear. So I'll try again. What do I mean with ”time is change”. Well I am not sure. It is my interpretation of arguments that I've been met with in a couple of forums here. For example:
    Or this:

    And then my question is: If time is something we experience because there is change but not a physical reality on it's own then how is it explained that time is relative following a mathematical equation?

  10. Jan 25, 2008 #9


    Staff: Mentor

    I would disagree strongly with this statement. Each term in an expression like F = dp/dt has a physical reality, and a relationship to the other terms. I can't imagine what kind of justification you would use to say that one of these terms has "physical reality" and the other terms do not.
  11. Jan 25, 2008 #10


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    I don't think you should take the statements you quoted from people on forums as authoritative...it seems to me like these people were waxing philosophical rather then saying things that correspond to well-defined predictions of any theory.
  12. Jan 25, 2008 #11
    Physical this time

    Thanks, I needed that. Does that in your opinion mean that time must be a result of a physical process in order for it to work in relativity?

    - Henrik
  13. Jan 25, 2008 #12
    I don't know if this will help much but here goes anyway:

    It has already been said that time is change. If nothing changes, then we would not notice the passage of time. If you put your self in a sense deprivation chamber where nothing whatsoever happens, you will get bored and annoyed. Your physical processes will continue to change, they will continue to mark the ghost of time through there changes. Your perception of the rate of change of time will alter though.

    Have you tried the old trick of staring intently at the second hand of an analogue watch. In this case the rate of change of the clocks ticks remains the same but your perception of that rate alters. This means that the distance in time between two events is not reporting anything to your senses because if it did, staring at the watch would not make it feel as if time was taking longer.

    When you sleep soundly, you do not notice the passage of time because your methods for representation are curtailed. I guess we could say that time and awareness are intrinsically related. Without one, you can't have the other.

    In the physical universe, everything is interdependant. The rate at which change takes place is dependant upon the conditions surrounding the thing that is changing. Heat it up and it probably goes faster. Swing a clock with a pendulum round your head for an hour and it will operate at a different speed and so on.

    Time is relative following a mathmatical equation because everything is relatve to its own particular circumstances and that includes our bodies.

    Sit in a centrifuge for several years and who knows what will happen to your bodily processes, but you can be sure they will be affected.

    The only thing we can experience is change, if you can really get into a situation where nothing changes, and that would have to include your mental and physical processe in total, then you would have no experience, no impressions whatsoever, and you would not experience time.

    You cannot experience the temporal spaces between events, you can only experience the events, and that is why I called time a ghost!

    Physics and relativity maps the interrelationship of change, that is why it is mathmatically dependant.

    Few hope this makes sense!
  14. Jan 25, 2008 #13


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    What do you mean by "time must be a result of a physical process"? In relativity time is treated as a dimension in 4D spacetime...you can use relativity to model a completely empty spacetime where nothing ever changes if you want, although obviously this wouldn't be a realistic model of our universe.
  15. Jan 25, 2008 #14
    If nothing changes in the temporal spaces between events, then no time will have passed. No one will be older, nothing will have worn out a little more and so on.

    Time has to be the result of a physical process, yes.

    I am not convinced that there is any point to 'unrealistic' models but in a completely empty space time universe, relativity would have nothing to map and would be redundant.

  16. Jan 25, 2008 #15


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    These are not statements of physics but of philosophy. "Relativity" considered purely as a mathematical theory of physics can certainly model an empty universe, even if this model is not useful in a practical sense. If you want to say something along the lines of "in practice, there is no way to measure time without something changing" I certainly agree, but to go from this to "if nothing changes ... no time will have passed" is philosophy.
  17. Jan 25, 2008 #16
    Isn't Schrodingers cat philosophy?
  18. Jan 25, 2008 #17


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    If you draw any conclusions about what is "really" happening in the box before you open it, yes. Different "interpretations" of quantum mechanics would give different answers, and since it is impossible in principle to distinguish these interpretations experimentally, arguments in favor of one interpretation vs. another are philosophical ones.

    I'm not saying physicists should be forbidden from talking about philosophical issues, just that it's important to distinguish philosophical questions from questions that can be addressed by the scientific method.
  19. Jan 25, 2008 #18
    I do agree with you Jesse, on both of your previous points.

    The original question did drift a bit into the philosophical!

    I would say though that Descartes, often classified as the farther of the scientific method, was a philosopher.
  20. Jan 26, 2008 #19
    Yes, I see. I'm glad. I couldn't make the ends meet.

    I got carried away and my own crack pot ideas spilled over for a second. Sorry. It has no place here. Thanks for your answers, I really appreciate it.
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