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Time Dilation and physical processes

  1. Sep 25, 2012 #1
    If you travel at a sufficiently fast speed, then according to relativity, you will age at a slower rate compared to your slower moving contemporaries. This implies that biological functions (say, of a person) and mechanical functions (say, of a machine) are literally functioning at a slower pace. What is going on atomically that allows this? Are the atoms literally spinning slower?
  2. jcsd
  3. Sep 25, 2012 #2
    This is the infamous twin paradox again. I don't know what is the most widely accept solution to this, but i know there are different opinios among the experta. Here is a website that interviwed a bunch of professoonals on this:


    I warn you that the author of the website's opinion is NOT accepted, and i don't agree with it. I post it here just so you can see differing opinions on the subject, and i advise you to consider only the experts opinions.
  4. Sep 25, 2012 #3
    To the observer watching the relative mover, the mover's mechanics, biology, everything is going slower based on the observer's clock... but he will notice that the mover's clock is also going slower, so the mechanics etc appear slower, but so does the moving clock appear to be slower by the same amount.

    This means if the observer measures himself locally to see how far a neural signal is conducted along a neuron's axon in 1/1000 of a second, he will get a small length.
    If he then observes the same thing in the mover, he will see the same relationship because of the effects of time dilation and length contraction - the same relationship that the mover himself would get if doing this measure locally on himself.

    Running a video at different speeds makes it look like things are happening faster and slower to the person watching the video... but to the people doing things IN the video, everything will seem normal no matter how fast of slow - the relations between time and everything else will always match and correspond consistently. If the video was of a baseball game, everything would match - the pitcher throws very slow, the batter swings slow, the hit ball goes slow, the batter runs the bases slow... etc. Or everything happens fast, but matched and in balance. To the players, everything works and makes sense in spite of what it might look like when viewed at different speeds from the outside.
  5. Sep 25, 2012 #4


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    According to Special Relativity, the laws of physics are the same in all inertial reference frames. Also, according to SR, you can transform coordinates between reference frames using the Lorentz Transformation process. When you say that your time goes slower because you are moving fast, you mean that with respect to an inertial reference frame in which the other objects are at rest or moving more slowly. If you switch frames so that you are at rest, then the other objects are the ones for which time is going slower. Nothing is happening physically when you describe the same situation from one reference frame or another so we can't say absolutely that the atoms are spinning slower in any particular reference frame.

    Your question is really no different than asking if you are literally traveling at a sufficiently fast speed compared to your slower moving contemporaries. What we can say is that when you accelerate in one direction, your speed literally changes because that is true in all inertial reference frames but we cannot say that it literally increases because in some frames it is decreasing. Similarly, we can also say that your atoms literally change their spin rates when you accelerate in one direction but we cannot say that they necessarily slow down because in some frames they speed up.

    The point is that as long as the laws of physics remain unchanged when going through the Lorentz Transformation process, then we know that whatever happens will follow the rules of Special Relativity.
  6. Sep 25, 2012 #5


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    I think the widely accepted point of view is:

    1) SR cannot be distinguished from an appropriately constructed preferred frame theory. A majority find a preferred frame a distraction; a minority find it useful.

    2) SR as the limiting case of GR argues against preferred frame, since there is no such thing as a global frame in GR at all. There is also no such thing as relative velocity for distant bodies.

    3) There is a physical phenomenon of differential aging, that occurs both in SR and GR. In sufficiently simple gravity situations, the GR phenomenon can be understood as the SR phenomenon plus a contribution from change in gravitational potential. However, in the general GR case, it is impossible to construct gravitational potential, so there is little alternative than to treat it a purely an effect of the geometry on different paths.

    4) The SR differential aging has multiple valid explanations that all emphasize different features of SR (or, in the case of principle of equivalence explanation, a bridge to GR). They are not competing explanations but complementary explanations.
  7. Sep 25, 2012 #6


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    If you're trying to understand what's going on with the twin paradox, a better site is probably http://www.physics.adelaide.edu.au/~dkoks/Faq/Relativity/SR/TwinParadox/twin_paradox.html; in particular this one demonstrates that the "differing opinions" are not disagreements but different ways of explaining the same phenomenon.

    However, the twin paradox isn't OP's question; he's asking about the mechanism behind time dilation.
    Last edited: Sep 25, 2012
  8. Sep 25, 2012 #7
    Nice link, thanks.

    I am aware that was the question, but the way he formulated it (the aging part) seemed like he was considering something closely related to the twin paradox.
  9. Sep 25, 2012 #8


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    I don't seem to be able to access this link. It says I don't have permissions.

    [edit: I see the link just has trailing semicolon you must remove. The link should be fixed.]
  10. Sep 25, 2012 #9


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    Instead of saying that the biological and mechanical functions are functioning at a slower rate, you could say that they're functioning at the same rate as always but for a different amount of time. It's not at all surprising that I age exactly one year between my birthdays, even if someone else moving relative to me (same as saying I'm moving relative to him!) is measuring time differently than me, so sees two of his birthdays pass.

    That answers the "what is going on atomically?" question... Nothing, so no problem.

    However, it raises another question, namely how it can be that we don't all experience the same amount of time passing between two events (such as my two birthday parties)? There are two answers:

    First, the technical answer: Apply the lorentz transforms to see how events in one frame are related to events in the other frame. Note that the relativity of simultaneity means that we do not have to agree that our respective birthday parties are happening "at the same time". There are plenty of good textbooks, online explanations, worked examples - and a bunch of other posters here can explain further. Learning to draw spacetime diagrams is a big help too.

    Second, an analogy (and this is an analogy, so if it doesn't work for you don't let it confuse you, just forget it): I have two cars parked side by side. Two different drivers drive them over two different routes to two different destinations. You don't find it even slightly surprising that they burn different amounts of fuel, wear their engines differently, record a different number of miles covered (and you do not wonder about what might be going on atomically, in their engines and odometers, to account for the discrepancy). Even if they end up at the same destination, you aren't surprised; you just conclude that the drivers took different routes.

    So if it doesn't bother you seeing different distances for different paths between two points in space... Why should different times for different paths between two points in spacetime surprise you?
  11. Sep 25, 2012 #10


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    (and thanks for the quick diagnosis of the problem)
  12. Sep 25, 2012 #11
    Nurgatory, you make a good point, I suppose I shouldn't be surprised, but like most laymen I have a lot of trouble imagining time distortion, so when I think of someone appearing to age slower because of their extremely fast acceleration, I can't help but picture the constituents of their body moving slower as well.
  13. Sep 25, 2012 #12


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    It is indeed hard to imagine; it is at odds with the intuitions we've developed over a lifetime of living with speeds far less than c.

    But since my analogy worked so well so far, I'm going to press my luck... :smile:

    We've got these cars driving on different routes between two points in space, and because they're taking different routes (maybe even to different points in space) we're not surprised that their odometers record different distances driven, and we expect that amount of aging/wear that the engines have experienced and the fuel consumed correlates to the miles driven. Same engines, same physics, just different number of miles covered.... So far, so good.

    So why is so hard to think about time the same way when we're looking at two different paths through spacetime? It's because we're assuming (a natural assumption, given our lifetime experience in non-relativistic conditions, but still an assumption) that time is the same for both drivers. In the back of our minds, we have this idea that we could put a giant clock in the sky where both drivers can always see it; and that when the local AM radio station announcer broadcasts a message saying "Hey guys - look at the clock in the sky" they'll both look up and see the same time.

    But suppose that clock couldn't exist, even in principle? Instead, our two intrepid drivers have only their own wristwatches to track the passage of time. Each driver knows that his wristwatch is clicking off his passage through time just as surely as the odometer of his car is clicking off his passage through space; and if his speedometer registers 30 miles per hour he'll reasonably expect that his odometer will advance by one mile as his watch advances by two minutes. But without that clock in the sky he knows no more about what the other guy's watch is doing than he knows about what the other guy's odometer is doing.

    (As I said earlier.... This is an analogy. If it doesn't work for you, don't let it confuse you, just forget it)
  14. Sep 26, 2012 #13
    - Time dilation is not a function of acceleration but of speed.

    - One can imagine matter consisting of some kind of standing waves (there are obvious problems with that, but it serves to illustrate a simplistic wave model of matter. For more see http://en.wikipedia.org/wiki/Matter_wave).

    - Make sure to understand how a "light clock" ticks slower when the clock as a whole is moving. For that https://en.wikipedia.org/wiki/Time_dilation can be helpful; go to the section "Simple inference of time dilation due to relative velocity".

    Does that help to picture time dilation as relating to physical processes?
  15. Sep 26, 2012 #14
    Err, maybe not. I think I'm now getting confused as to the exact differences between "speed," "velocity" and "acceleration." I know I learned the differences in a rudimentary physics course from college, but my memory is cloudy.
  16. Sep 26, 2012 #15
    I'm afraid that a clear (fresh) understanding of such basic concepts is necessary for a useful discussion of the kind you started here. In some texts "velocity" is synonymous with "speed", but usually it refers to speed in a certain direction. And acceleration is rate of change of speed or velocity (such as in gravitational acceleration: every second you fall ca. 10 m/s faster).

    For refreshing your memory about such things wikipedia is not bad:

    In a certain way hyperphysics is perhaps better:

    Success :smile:
    Last edited: Sep 26, 2012
  17. Sep 26, 2012 #16
    Speed is a number that describes the distance something travels during some unit of time, like "100km/h" (you cross 100 miles in one hour).

    Velocity is pretty much the same, but when you know the direction, like "100km/h to the left", that's why it's called a vector – it is an arrow, a velocity pointing in a specific direction. Both velocity and speed are linear, you can't have a curved velocity (on more complex situations you can, but don't think about this now).

    An acceleration is a variation of a velocity, or a velocity over a velocity, like this: you are waking at 10m/s on a shopping mall and than you get on an escalator, but you keep walking at the same speed 10m/s. If the steps of the escalator are moving at 5m/s, you will have increased your speed to 15m/s. The period where you go from 10 to 15 m/s is called an acceleration, just like when you accelerate your car. A jerk is also an acceleration, just a negative one.

    Any time you vary your speed or your velocity (including the direction of the velocity), some acceleration has taken place. If you are going in a straight line at constant speed, you need to apply some kind of force to change your direction (like turning your wheels, you will turn because the friction between the ground and the wheels describes a force on the tires that keeps the car from going straight forward). Every applied force generates an acceleration. If you are standing still and I push you, I just applied a force, and you will move some distance away from me. That is a change in velocity, you were at 0m/s next to me, and I changed you velocity to, say, 5m/s away from me. From 0 to 5m/s represents an acceleration, and is represented by 5m/s2. 0m/s or 5m/s are speeds. 5m/s AWAY from me, is a velocity.

    There's more to it, but I think that will suffice for know, did you get it?
  18. Sep 26, 2012 #17
    Despite Nugatory saying the question "what is going on atomically?" is answered with relativity imo the way you ask the question leaves it still open.

    Comparatively there is an difference, and that's distance.

    Easiest way to visualize this is the classic SR light clock, - photon and two mirrors deal.

    I would consider that "visual" when thinking "what is going on atomically?".
  19. Sep 26, 2012 #18
    Altergnostic, thanks for the lesson on speed/velocity/acceleration lesson. I'm not sure if I'll ever fully understand the effects extreme acceleration has.
  20. Sep 26, 2012 #19
    That is sufficient for this discussion (no need for extremes here).

    Meanwhile I gave in post #13 a way to picture time dilation as a physical process (complete with a link to click on, with pictures), and nitsuj referred to the same. It was that example (of matter composed of waves, and understanding how a "light clock" has time dilation) that I meant when I asked if it helped.

    If those things are new to you, then you should probably take an hour or so to read it and think about it. :tongue2:
  21. Sep 26, 2012 #20
    Depending on how specific you would like know, "Rigidity" & "Speed of Sound" of solid objects would shed light on "understanding" the topic of "extreme acceleration".
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