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Does time dilation affect only the measurement of time?

  1. Aug 31, 2015 #1
    As I understand, 'Einstein's mirrored light clock' shows us that a vessel (containing said clock) travelling at any speed greater than that of a duplicate, at rest will observe a slower passage of time. I understand that this is because of simple newtonian physics that shows us that in the case of the clock in motion, the light actually traverses a diagonal path before coinciding with either of its two mirrors, thus registering 'one second' or 'one hour' in a greater amount of time (as perceived by the clock in the non-inertial frame of reference.

    MY problem with this is.... everyone makes out that this is because time itself has slown down. But I cant see this. What I see is that time (this mythical thing) has done absolutely nothing, it has not sped up nor slowed down. What has happened is that our mechanism for measuring this parameter has been flawed by the fact that the measuring device is ill-equipped to measure the unit of time under the circumstances. And this is simple newtonian physics that shows this!

    For example, in the twin paradox, the twin on board the space craft allegedly will observe the passing of one day, whereas the twin on earth will observe the passing of 1 year (in the same amount of time). BUT, surely the twin in the space craft would notice that his 'light clock' seems to be slowing down (in an extreme example where the time dilation effect is anything but negligable? - for example, when your watch is running low on battery but not enough to stop all together, we readily notice that it loses time - we dont lose all concept of time!

    In addition to this, the concept that (in the twin paradox) the twin who journeyed into space would come back younger or older is silly, they would be exactly the same - only that the space bound twin would return possibly a bit perplexed and may even state that he felt as though his clock had been running slowly for part of the journey.


    Thank you very much, and apologies if this is in the wrong thread etc, I am new to this forum business :)
  2. jcsd
  3. Aug 31, 2015 #2


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    I can answer your question if you can answer mine.

    Suppose you have a 5 meter long ladder leaning against a garden wall. Suppose that it starts out with the upper end at a height of 4 meters up the wall. Later it slips down a little so that the upper end is 3 meters up the wall. The ladder has not changed, only rotated a little. But has the height of the ladder really changed?

    Measurement of time is a projection of a real thing onto coordinates. Just as the height of the ladder is a projection of the length of the ladder onto the wall. The thing being projected is a series of events in space-time.

    Changing your speed relative to a thing is somewhat similar to rotating the ladder so it changes where it contacts the wall. It can even be written very much the way a rotation is written.


    If two twins each had a 5 meter ladder, would they necessarily be able to climb to the same height on the wall? Suppose one rotated his ladder to a different angle.
  4. Aug 31, 2015 #3


    Staff: Mentor

    This explains why the light in the moving light clock travels a longer distance. By itself, though, it doesn't explain why this difference must be interpreted as "time dilation" of the moving light clock. For that you also need the fact that the two-way speed of light is the same in all inertial frames. That is not true in Newtonian physics; it's only true in relativistic physics.

    The problem with this argument is that Newtonian physics is wrong. Physicists knew it was wrong even before special relativity. The reason they knew it was wrong is that it made incorrect predictions about electrodynamics.

    The equations that govern electrodynamics, Maxwell's Equations, are not consistent with Newtonian physics as they stand. They predict a finite speed of light that is the same in all inertial frames. They also make other predictions about how electric and magnetic fields will appear in different inertial frames. Those predictions had been verified in the late 19th century, and they are inconsistent with the predictions that Newtonian physics makes about the same phenomena. (For example, Newtonian physics predicted that the Michelson-Morley experiment would give a non-null result, whereas Maxwell's Equations predicted that it would give a null result. In fact it gave a null result, consistent with Maxwell's Equations but inconsistent with Newtonian physics.)

    What special relativity does is rework our understanding of ordinary kinematics and mechanics to be consistent with a finite speed of light that is the same in all inertial frames, as predicted by Maxwell's Equations and verified by experiments. Time dilation is a consequence of that.
  5. Aug 31, 2015 #4


    Staff: Mentor

    No. He would notice that clocks moving relative to him appeared to be slowing down; but he would see no difference in his own clock. But there are a number of complications here that you are not considering; you should read, carefully, the Usenet Physics FAQ article on the twin paradox, which goes into the subject in detail and addresses the complications:


    This has been falsified, at least for subatomic particles (muons) with known decay rates. Muons confined in storage rings and moving inside them at relativistic velocities have their half-lives extended (i.e., their rate of decay slows down) exactly in accordance with the SR predictions. If your statement above were correct, this would not happen; the muons would decay at the same rate in the storage rings as they do at rest.
  6. Aug 31, 2015 #5


    Staff: Mentor

    If what you are saying were true then you could build a device which could measure absolute speed simply by comparing a light clock to a type of clock that is robust to absolute speed. This would violate the principle of relativity.
  7. Aug 31, 2015 #6
    Thanks for your reply, so I think I understand your point but...
    Ok I think I partly understand, thanks for your reply. So really what we have is a twin on earth who's measurement of time is as it is by virtue of their particular circumstance (eg rotation of earth) but not necessarily correct, rather an arbitrary measurement because it is circumstantial. Equally the space bound twins measurement is also circumstancial. Neither are wrong because both measurements suffer from the fact that neither twin is truly at rest, as such any measurement of speed/time can only be arbitrary.. ?
  8. Aug 31, 2015 #7


    Staff: Mentor

    The "rotation" he is talking about is not the rotation of the Earth. What he's saying is that, in spacetime, changing your speed relative to something is the same as rotating your point of view in spacetime; doing that changes how "long" other objects appear to you in both dimensions (space and time), just as rotating your point of view in space changes how long (or wide or tall) other objects appear to you in space.

    The underlying concept here (spacetime geometry) is a powerful one, but it's a lot to grasp if you're just starting out. You might want to stick with simpler concepts until you've at least gotten your feet wet.

    For the light clock, for example, a good exercise is to consider a "double" light clock that is set up like this: we have a light source that sends two beams of light, perpendicular to each other (when viewed in its rest frame). Each beam reflects off a mirror and returns to the source; the distances to both mirrors (again, when viewed in the clock's rest frame) are the same, so both beams return at the same instant.

    Now, what does this "double" light clock look like in a frame in which it is moving? The key point is this: both beams returning to the source at the same instant is an invariant--it's a direct observable, so it has to be the same in all reference frames. Try modeling this scenario and see what you come up with.
  9. Sep 1, 2015 #8
    Your post is much about the understanding of "time". In physics, time is defined by means of clocks (including the Sun-Earth clock); such clocks are a measure for the progress of physical processes. "Time dilation" is a term that is meant to describe the measurements that "moving" clocks (but also chemical processes, etc) advance less fast. Others already explained that this has nothing to do with light rays or Newtonian physics.
    Indeed, the clearest and unambiguous prediction of the so-called "twin paradox" is that a clock that has moved around more will be behind on a clock that has moved around less. And this prediction has been verified by a number of experiments, see for example https://en.wikipedia.org/wiki/Hafele–Keating_experiment
  10. Sep 1, 2015 #9
    Thanks for your help with this! Ok so I can picture the perpendicular beams of light. My first instinct would be to say that in the frame where the system is in motion, the beams would not return at the same time as the motion may affect the distance each beam is required to travel in order that it can be reflected back (or at certain times the system is actually travelling toward the reflected beam on its return journey to its source, thus reducing the distance required to travel (i suppoose this would be the newtonian conclusion ?) However, as you stated, the result whereby the beams return at the same time is a direct observable so must be the same in all reference frames, therefore the beam must return to the source at the same time even when the system is in motion (even though a greater distance has been covered and the speed of light was constant) - thus meaning that time itself must have been altered for the system in motion?

    I know its hard work trying to teach a noobie! :P thanks again!
  11. Sep 1, 2015 #10
    Hi thanks very much for your post, it brings me on to a very important aspect of time dilation that I am specifically interested in. Is it (as far as we can tell) the case that time dilation would almost definitely slow down ageing and bodily processes, e.g. nerve relapses to the bran, tissue decay, time taken to think, blink, walk.? would the effect of time dilation quite literally slow or speed up everything within the frame experiencing the dilation? (as opposed to merely changing the measurement of the time elapsed and having no actual effect on bodily processes etc).

    Look forward to your response, many thanks :)
  12. Sep 1, 2015 #11
    Yes, it is a famous predicted result of the "twin paradox" scenario that the very fast traveling twin will at his or her return be literally younger in a biological sense than the stay-at-home (for a very fast traveling twin the effect of the Earth's gravitation on the stay-at-home twin is comparatively small and can be neglected).

    PS here is how it was originally clarified:
    "Returned to Earth he has aged two years, then he leaves his ark and finds our world two hundred years older, if his velocity remained in the range of only one twenty-thousandth less than the velocity of light. The most established experimental facts of physics allow us to assert that this would actually be so."
    - https://en.wikisource.org/wiki/Translation:The_Evolution_of_Space_and_Time
    Last edited: Sep 1, 2015
  13. Sep 1, 2015 #12


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    Does anyone know any experiments that have been done that are prof of chemical processes,etc also slow down?
    I know there's experiments done that show atoms decay slower but how this effects chemical processes would be interesting to understand.

    [Mentor's note: Some erroneous and misleading stuff has been removed from this post, leaving only the question]
    Last edited by a moderator: Sep 1, 2015
  14. Sep 1, 2015 #13
    Such tests are difficult to perform; only the easiest tests as you mentioned have been done so far.

    [mentor's note: misleading/wrong stuff quoted from the previous post has been removed]
    Last edited by a moderator: Sep 1, 2015
  15. Sep 1, 2015 #14


    Staff: Mentor

    Yes. More precisely, a system in motion must appear to have a different "rate of time flow" from a system at rest. But if you were riding along with the moving light clock, to you it would appear to be ticking perfectly normally, since to you it would be at rest. Saying "time itself must have been altered" makes it seem like there is some absolute sense of "time" that is changing. There isn't.
  16. Sep 1, 2015 #15
    Ok excellent that makes sense! My next question is simply how can we determine an invariant scenario just through postulation? For example how did you know that the light beams would return to source at the same time regardless of which system they were in? Many thanks for all of your help! Also is an inertial frame simply one not at rest?
  17. Sep 1, 2015 #16


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    We did not know this "just through postulation." It was the result of experiments. Experiments which, at the time, were very surprising and frustrating to the people doing the experiments. They did not see how it was possible for the thing described to happen. They did not see how it was possible for light to travel at the same speed in all inertial reference frames. They were looking, very carefully they thought, for the effects of inertial motion on the speed of light. They didn't find it.


    There were some other experiments that contributed to the conception of special relativity. For example:


    and you can read more here.


    An inertial reference frame is one that is not undergoing acceleration. It's a little more complicated in general relativity where space-time is not necessarily flat. But for special relativity that will do. The term "at rest" only makes sense in reference to a particular reference frame.
  18. Sep 1, 2015 #17


    Staff: Mentor

    We constructed the scenario so that both beams returned at the same time in the clock's rest frame. But here "at the same time" refers to an invariant, a physical event: both beams arrive at the same place (the source) at the same instant. (In geometric terms, the worldlines of the two light beams and the worldline of the source all intersect at a single point in spacetime; such an intersection is an invariant, just as the intersection of curves in ordinary geometry is an invariant.) You can't change physical events by changing reference frames; that is a fundamental axiom of Newtonian physics as well as relativistic physics, so it's not in dispute here (since we're talking about why relativistic physics is right and Newtonian physics is wrong).

    No. "At rest" is not a concept that applies to frames; it applies to objects with respect to frames--i.e., an object can be at rest in one frame but not at rest in another.
  19. Sep 2, 2015 #18

    I think I confused myself... i think i understood that all 4 beams would return to their source simultaneously, even though 2 out of 4 of the beams had an altered path (which would otherwise lead you to believe that the time taken would alter hence offsetting the return time). Looking back, I don't think this is what you meant... Looking at your reply again, I think what I should have interpreted your answer as was... in each system, both beams will return at the same time. However if there were an absolute measure of time, there would be a time difference between the 2 beams returning to source for the stationary system and the system in motion. is that right? thanks again!
  20. Sep 2, 2015 #19
    I suppose my conclusion would still be the same.. that i would expect the beams in the system in motion to return at different times as they travel different length paths as a result of the motion. The only way to explain the fact that they do in fact still return at the same time (within that particular frame) is to assume that time has slowed down within the system.

    However, if time had slowed down within the system, then surely so would the light within this frame of reference. Thus indicating that the light had a greater distance to travel and also it was being slowed down. However, light cannot be slowed down, and as such if the system slows but the light does not, this allows the light to cover a greater distance. is this vaguely correct?
  21. Sep 2, 2015 #20


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    No, light speed is c in any frame. This might help:

  22. Sep 2, 2015 #21


    Staff: Mentor

    There aren't 4 beams. There are only two beams. Changing your frame of reference doesn't change the beams; it only changes the coordinates you use to describe them. We are not talking about two different light clocks, one at rest and one moving; we are talking about one light clock, described in two different frames, one in which it is at rest and one in which it is moving.
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