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What causes time to progress?

  1. Dec 6, 2006 #1
    Hello,

    I'm new to this forum and come here with a simple question.

    I am just starting to understand special relativity and read up on the Twin Paradox which overloaded my wtf-o-meter.

    Basically from what I understand is that time does not progress at the same rate everywhere, the rate at which time progresses from one person's perspective relative to another's depends on each person's velocity relative to the speed of light (?)

    So if someone leaves earth traveling at the speed of light, comes back later at the speed of light, they will have aged more slowly than the people on earth. So if twins are born on earth, and one travels to the nearest star and back at almost the speed of light, the travelling twin will be 5 years old but the earth twin will be 10 years old at the time they meet again. Not just an illusion, they will actually be different ages.

    I understand that this is fact, but can somebody please explain why.

    More fundamentally, what exactly is time and what physically causes time to progress? Why is the rate of time different in different places and why does the speed of light have anything to do with anything?

    Can someone please help me understand? If you can explain without having to use math more advanced than basic calculus (I have 2 quaters under my belt) that would be ideal.

    Thanks ahead of time!
     
  2. jcsd
  3. Dec 13, 2006 #2

    paw

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    Nobody really knows. All we can say with certainty is that time appears to be a property of the universe we live in and that it progresses at a rate dependant on the velocity relative to an observer. Anything else is speculation at present.

    Again, nobody really knows. All we can say is that the speed of light in a vacuum appears to be a constant in any inertial frame of reference.

    These questions are among the most important unanswered questions of modern physics.
     
  4. Dec 14, 2006 #3
    This still isn't an explanation why, but what I find interesting is that the four-velocity vector of some object will always have a magnitude equal to the speed of light. This means that an observer 'at rest' is actually hurtling forwards through the time direction at the speed of light (he is obviously not moving in the other 3 dimensions so the length must all be 'in time'). If you do not want to move through time you need to do 'the opposite' and move through space at the speed of light, which is exactly what light does, and in fact from the perspective of a photon hurtling through space, time has indeed stopped 'moving forwards'.
     
  5. Dec 14, 2006 #4

    Demystifier

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    Although nobody knows with certainty what causes time to "progress", the dominating point of view among physicists is that it does not progress at all. Instead, the progress an illusion caused by the fact that entropy increases with time.

    Your second question, why time depends on the observer. The simplest explanation is by viewing (it requires a certain mental effort for a beginner) time just as one of geometric dimensions like 3 dimensions of space. For example, the length of a rod looks shorter if you watch the rod under an angle. For a similar reason, time also may depend on the observer.

    Any more advanced explanation requires mathematics or at least drawing of spacetime diagrams.
     
  6. Dec 14, 2006 #5

    Garth

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    "the fact that entropy increases with time"
    But is that not "progression"?

    Our primary conscious experience is one of the "progression" of time; it is the 'environment' in which all other measurements and experiences are set.

    We may interpret physical reality non-temporally, and use untensed language, but that is not the perspective, or frame of reference, from which we observe the physical universe.

    Garth
     
    Last edited: Dec 14, 2006
  7. Dec 14, 2006 #6

    paw

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    Well, I don't want to argue with the <i>'dominant point of view among physicists'</i> but that is a diffucult statement to accept as is. Consider, for a moment, that in the absence of time we couldn't, by definition, measure any change in any system, even in principle. After all, change is a function of time. If we can't measure something, in principle, then it isn't within the scope of physics.

    Entropy is change and therefor a function of time. I suspect it's more likely that entropy is an effect of time rather than the cause of time.

    Which leads me to a question for the community, which I hope is in the spirit of the OP. Has there been any research into a geometric interpertation of time comparable to the geometric interpertation of gravity as in gtr? Can you suggest any links that would make interesting reading?
     
  8. Dec 14, 2006 #7

    Garth

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    You might find Self Creation Cosmology - An Alternative Gravitational Theory , (published in Quantum Cosmology Research Developments, Nova Science Publishers Inc., New York), Section 9, page 27 interesting.

    Garth
     
    Last edited: Dec 14, 2006
  9. Dec 14, 2006 #8
    No. We can describe how it happens, or link two separate phenomena with the same underlying principle. Questions of "why" only lead to myths about turtles all the way down.
     
  10. Dec 14, 2006 #9

    paw

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  11. Dec 15, 2006 #10

    Garth

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    The heart of GR is that there is no "pre-geometry". The geometry of space-time is determined dynamically by the distribution of stress, energy, mass and momentum within it.

    In SCC this is also the case. However, the presence of an interacting Brans Dicke-type scalar field, constrained by the "Principle of Mutual Interaction", results in a highly determined cosmological solution that does have a definite geometry. In the theory's Einstein conformal frame the spatial geometry is a hyper-sphere in which atomic time is the radius. This leads to the conclusion that the expansion of the universe and the 'passing, or progression, of time' are two separate observations of the same phenomena.

    The theory is being tested at the moment.

    Garth
     
    Last edited: Dec 15, 2006
  12. Dec 15, 2006 #11

    Demystifier

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    Consider a static configuration in which entropy increases from the left to the right. Is that a "progression" too? If yes, then the above is also a "progression". Otherwise, no. The idea of relativity is to treat time on an equal footing with space, so I only persistently apply such a view.
     
  13. Dec 15, 2006 #12

    Demystifier

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    I strongly disagree.

    First, static configurations, or states at a given time irrepective of that on other times, are also parts of physics.

    Second, even change is possible without time. Consider, for example, a physical variable f(x), where x is the space koordinate. Clearly, this variable changes with space, but not with time.
     
  14. Dec 15, 2006 #13

    russ_watters

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    There is nothing special about static situations. The time rate of change just happens to be equal to zero. In fact, saying that the time rate of change is equal to zero requires that time is progressing because if time were not progressing, you couldn't measure the time rate of change of the property.
    That's like saying roads can exist without cars. Sure, they can, but that doesn't have anything at all to do with whether or not cars actually exist. The fact that time is not the dependent variable in every equation doesn't mean time doesn't exist, it just means time isn't the dependent variable in every equation.

    Also, as a corollary to the first point, the fact that time does not appear in the equation does not necessarily imply that a change is happening instantly. Ie, the spring-mass equation (f=kx) doesn't mean that springs expand instantly, it just means tha time is irrelevant for that particular application. In fact, such equations are often simpler forms of equations where time does matter - if you take the equation for spring-forced movement and set dv/dt equal to zero, f=kx falls out of it.
     
    Last edited: Dec 15, 2006
  15. Dec 15, 2006 #14

    Garth

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    Your phrase "the fact that entropy increases with time" uses a tensed verb: "progresses"; the use of such a verb itself assumes the temporal perspective of 'passing time'. I was simply commenting on the self-contradiction in that whole sentence.

    In fact you can consider a tensed time that 'passes', or you can consider untensed 'block time', it is simply a matter of convention, or perspective, from which you interpret observations of the universe.

    The convention you choose says nothing about the central mystery of our ubiquitous experience of time, which is bound up with the problem of our consciousness.

    Garth
     
    Last edited: Dec 15, 2006
  16. Dec 15, 2006 #15

    paw

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    I understand that GR doesn't assume any pre-geometry. The way I understand GR though is that a spatial geometry appears at any given time from the distribution of stress, energy, mass and momentum within it. That is, each geometric picture of 3D space is static at a given time.

    My question is whether any extension of GR exists that models time in a geometric fashion. In other words, provides a fully dynamic, geometric picture of 4D space-time. It would seem to me that a geometric picture of time would provide the driving force that makes particles 'move' on the spatial geodesics.

    I caught that, very interesting idea. What I liked about SCC was that it may be readily falsified.

    I'm eagerly awaiting the results from Gravity Probe B. I understand the results should be public in the coming year. It will be interesting if there is a null result for the geodetic precession.:biggrin: Although I somehow doubt it.
     
  17. Dec 15, 2006 #16

    Garth

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    Thank you for your comments paw.
    Actually everything is "static at a given time". :smile:

    I don't think I understand the question - GR itself does provide "a fully dynamic, geometric picture of 4D space-time" - the subject is called geometrodynamics - space-time curvature tells 'matter' how to move, 'matter' tells space-time how to curve. I don't understand why you think particles need a driving force to make them move on their geodesics.
    As the measurement has never been done before under laboratory conditions anything is possible! :wink:

    Garth
     
    Last edited: Dec 16, 2006
  18. Dec 15, 2006 #17

    paw

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    Ah, the limitations of language (and probably of my understanding). Let me try it a different way.

    Consider a single body problem. A single massive particle occupying an otherwise empty universe. I understand GR has difficulty describing a single body problem in that there's no way to tell if the body is moving, rotating etc. In other words time and space can't exist (or at the very least it's not a subject for physics).

    If, on the other hand, time could be described geometrically (the only way I can describe what I'm thinking is 'downhill'), some of the problem disappears since you could at least say the body is constrained to moving along the time axis. At least that would grant 'existence' to the body. That's what I meant when I said a 'driving force', a geometric reason for the passage of time.

    Does that make any sense?
     
  19. Dec 16, 2006 #18
    If there is only one body then moving seems entirely irrelevant, since there is nothing to move in relation to. With regards to rotation, how do you conclude that there is no way to tell rotation? I completely disagree, you can tell if a body rotates or not.
     
  20. Dec 16, 2006 #19

    Garth

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    Ummmm...

    When you consider a space-time diagram it is by definition static, there is no movement, for movement requires time and time is already included in the diagram. The space-time diagram perspective is one that observes both space and time from 'outside' in some sense.

    So the body does not move along its world-line, it just 'is'. The geometry of the space-time, itself determined via the Einstein Field Equation, determines how different objects' world-lines might converge or diverge at succesive values of their proper time parameter.

    In our temporal '3D + time' experience of the world we see objects on these world-lines being attracted or repelled by each other and call it gravity or Dark Energy. Our experience of the 'passing of time' is our primary perception by which we interpret observations of the world 'out there', which is bound up with the problem of our consciousness. Consequently we have a choice about which point of view or perspective to take, a temporal '3D + time' or an atemporal 4D space-time interpretation.

    The question about a rotating solitary body in an otherwise empty universe is an interesting if highly hypothetical one. In GR such a body would feel centrifugal and coriolis inertial forces, however a fully Machian POV (which I find convincing) would question whether in the case of a supposedly rotating body, what is it rotating with respect to? It may be the case that, in such a body, centrigual and coriolis inertial forces disappear.

    The problem is we cannot test either hypothesis, that is unless you can find me a nice empty universe........

    Garth
     
    Last edited: Dec 16, 2006
  21. Dec 17, 2006 #20
    The 4-velocity does not behave like a 3-velocity (the spatial velocity component is [itex]\gamma v[/itex], not [itex]v[/itex]). So it is not entirely correct to make this interpretation in Minkowski space.
    Apart from that, the idea is attractive. But if you consider the constant 4-velocity the sum of velocities in space and time, you actually move the problem towards a higher dimension. After all, what is it then that makes this 4-velocity tick? A fifth dimension? If so, we could repeat the whole story for the 5-velocity, on its turn being the sum of velocities in 4D space-time and the fifth dimension. And so on, and so on.
    Although this doesn't solve the question on the "why", it does give a system where time as a separate dimension with its own unique and mysterious properties no longer needs to exist. Motion is all you need.
     
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