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Understanding time dilation

  1. Jun 23, 2015 #1
    Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand.

    What I am curious most about it 'what is time' from a scientific standpoint. I can't seem to find a definitive answer to this, which maybe there is not one. It would however help me in understanding the next part of my question.

    I am thinking of this in term of GPS positioning satellites. I understand and accept that these satellites experience a time dilation relative to the earths surface due to the speed they travel and the gravitational difference. I understand that they use atomic clocks which are the most accurate way we have of measuring time. What I don't understand is how do we know that speed and gravity are actually affecting 'time itself', and not just affecting the atomic vibrations we are measuring instead?
  2. jcsd
  3. Jun 23, 2015 #2
    Time dilation was predicted by special relativity, and later proved to be real.
    As for what time actually is, Einstien said that it's the thing that gets measured by clocks.
  4. Jun 23, 2015 #3


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    Hi, kweagle. Welcome to PF!

    The usual definition of time used in physics is 'time is what clocks measure'. It is the definition Einstein adopted when developing his special and general theories of relativity.
    So there's no difference between saying that 'time slows down' and 'all physical processes slow down', since a clock is just some physical process (usually a multitude of those).

    We could posit a hypothesis that it's not time (all processes) slowing down, but just one specific type of a process (e.g., radioactive decay), but so far all observations agree with the 'all processes' assertion. The hypothesis would need to specify which kind of process is not affected, and then test it.

    But that would be a bit backwards, since the predictions of time dilation/contraction of SR and GR follow from the definition that treats time as encompassing all processes.
  5. Jun 23, 2015 #4
    So with the idea that clocks are just a physical process, is correct to say there a difference between 'time' and a 'physical process'?

    I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?
  6. Jun 23, 2015 #5


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    To define time as what a clock measures is an example of a philosophy called operationalism: http://plato.stanford.edu/entries/operationalism/

    But we don't have a more reliable tool.
  7. Jun 23, 2015 #6
    Because the physical processes change their (apparent) speed, in a way which is consistent with what relativity describes.
  8. Jun 23, 2015 #7
    Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?
  9. Jun 23, 2015 #8


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    If this approach were to be useful, you would need some way to define and measure this true or undistorted time. Nobody has any way to do that.
  10. Jun 23, 2015 #9
    Is my way of looking at it somehow flawed though? Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself? This has always been my biggest problem with understanding time dilation.
  11. Jun 23, 2015 #10
    One way of putting it is that there is no universal absolute clock which can be referred to.
    Relativity makes predictions (such as time dilation) which are useful in some cases (GPS), and more accurate than Newtons (equally amazing earlier) proposals, which make the assumption of 'time' as being a universal absolute constant.
  12. Jun 23, 2015 #11
    So can the following statements all be said to be true?

    -Clocks measure time
    -The definition of time is a measurement of a physical process
    -All physical processes are affected by speed and gravity
  13. Jun 23, 2015 #12
    1, Yes, 2,Yes, and 3.I don't know( but it seems to be so).
  14. Jun 23, 2015 #13


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    The first one is true but can be qualified to 'clocks measure their own time'. In SR the time elapsed on a clock has a clear mathematical definition and is an invariant.
    The nearest thing to a deifinition of time is 1. So probably 2 is redundant.

    Your third point is true as regards gravity. Relative velocities are important physically if two things collide or interact in some way. Otherwise relative velocity causes Doppler shifts and strange length measurements.
  15. Jun 23, 2015 #14


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    The third one is difficult because it lacks a mechanism or even a theoretical basis. According to the principle of relativity that has been a part of physics since physics was first invented by Galileo, speed is only relevant as measured between two objects. So at any time, any object can have an infinite number of different speeds depending on what frame of reference you choose to measure it in. That makes the clock rate an essentially arbitrary choice and not a single value for a particular clock.

    Even with gravitational time dilation, there is a difficulty in detecting an actual impact of gravity itself on the processes. That is wholly different from the way gravity affects a pendulum clock, for example.

    So the idea that gravity/relative speed affects time and not just individual physical processes is theoretically simpler -- the opposite view is highly problematic and has no evidence for it whatsoever. It isn't necessarily wrong, but it is scientifically inferior due to its complexity and lack of support (Occam's razor).
    The way you say that implies that you think that some other instrument, not yet invented, might be able to measure how time "really" works and not be influenced by gravity/speed. While not completely impossible, that would be a pretty huge coincidence that all current clocks show exactly the same "error" (if accurate enough to measure it), even though we have methods of measuring time that use vastly different operating principles.
  16. Jun 24, 2015 #15
    All those statements can be said to be true - but the third one is not generally accepted even though it works as a model (the theoretical basis is nowadays often called "LET"). As no absolute speed can be determined (measurements of uniform speed are relative, that's relativity in a nutshell), many people hold that it doesn't exist, and of course physical processes can not be affected by the speed with respect to your freely chosen reference system! Debates on this forum on that rather philosophical topic have been ended with a formal interdiction, see the FAQ here, https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/ [Broken]

    Concerning gravity, indeed according GR, a "clock goes therefore slower when it is placed in the neighbourhood of ponderable masses" - https://en.wikisource.org/wiki/The_...Perihelion-motion_of_the_paths_of_the_Planets. [edit: slight correction mine]
    Last edited by a moderator: May 7, 2017
  17. Jun 24, 2015 #16
    This is wrong because there are different tools for measuring time and they all yeild the same results for time dilation. Theres no way, I'm sorry, that its just our tools and not varying time. Your totaly disputing GR your saying Einstien was wrong and time is constant.

    Why would gravity make a clock go slower, I could use a sand clock and gravity would make time go faster.

    Think of time as changes, and we are in a high entropy universe so everything is changing. That the best way to think of time as rate of change .
  18. Jun 25, 2015 #17
    Hi Kweagle, welcome to PF forum.
    I'm no physicist either, much less scientist. But let me try to help you with a layman point of view.
    Grandfather clocks/pendulums measure time? Yes, but it depends on gravitation
    Digital clocks measure time? Yes, it doesn't depend on gravity but it depends on the power of its battery
    Bacteria fermentation measures time? Yes, it does not depend on electric force but it depends if you put it in a refrigerator or not.
    Atomic clocks measure time? Yes, but a moving atomic clocks run faster than stationary atomic clocks. SR theory.
    So I think the standard clock is atomic clock without acceleration force applied to it. It can move, as long as it does not experience acceleration.
    I'm at lost here also. What defines standard clock? The vibration of caesium atom or the time it takes for light to travel 1 metre? Or something else?

    1 second is the time it takes for the second hand in analog clock to rotate 60 clock wise (of course)
    caveat: If somehow the clock axis has rust and dragging the second hand, the time is slow according to the clock.
    1 second is the time it takes for the temperature of 1cc of water to be raised 10 Celcius if applied by 4.2 watt.
    1 second is the time it takes for caesium to vibrate 9,192,631,770 times. https://en.wikipedia.org/wiki/Caesium_standard
    The definition of time is a measurement of a physical process. But first, you have to define the physical process.
    You just can't put 4.2 joules in 1 cc of any water to increase it 10 celcius. It has to be pure H2O in certain pressure and in liquid state.
    But the definition of standard time is usually by atomic clock.

    Gravity perhaps..., But speed? I don't think so. Speed is relative. Of course if you move one object speed is affective, or in the case of kinetic energy. Perhaps someone else can answer...?
  19. Jun 25, 2015 #18
    Right. The original standard clock was the Earth's rotation in its orbit, giving the solar day as reference (divided in 24 hours->min->s). Sundials were replaced by pendulum clocks and nowadays as atomic clocks are more regular than the Earth's rotation, they have mostly replaced the solar clock for precise time keeping. [edit:] The basis for long periods remains the solar day, that's why we need to insert a leap second now and then - the next one will be in a few days from now!
    Those two statements sound contradictory to me (and "faster" should be "slower") :wink:. Several clarifications have been given already, mine is in #15
    Last edited: Jun 25, 2015
  20. Jun 25, 2015 #19
    The moving clock runs SLOWER wrt rest. Sorry, sorry, sorry :smile:
  21. Jun 25, 2015 #20


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    What is "time itself"? That is something pretty hard to pin down, if you don't want to use the operational definition given above.

    However, let's look at this alternative as a possibility. Suppose that there is a background "time itself" which is not subject to time dilation. If that were the case then we would need some mechanism to explain why clocks based on EM (e.g. atomic clocks) are coincidentally time dilated to exactly the same degree as we would expect from relativity, despite time itself continuing unaffected. OK, so we propose such a mechanism.

    But some clocks use the weak force as their clock mechanism, and we find that such clocks also are coincidentally time dilated by exactly the same degree as expected from relativity. But since the mechanism is different then we need a separate mechanism for the time dilation. OK, so we propose another mechanism which accomplishes that.

    But some clocks use the strong force as their clock mechanism, and we find that such clocks are also coincidentally time dilated by the same degree as expected from relativity. I'm sure you see the point.

    The number of theories that you would have to derive and have to tune exactly correctly for it to all coincidentally turn out to be the same is not something which is taken seriously. It is asking for coincidence upon coincidence upon coincidence, all to arrive at the same place as you get from the postulates of relativity.
  22. Jun 25, 2015 #21
    Can I ask a question here?
    Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.
    But what about speed? We could have move 300km/s from Andromeda, but all the physical/chemical processes are not affected. Is this right?
    Of course if you count the kinetic energy if Andromeda hits us is ##\frac{1}{2} * Andromeda_{mass}? or Milkyway_{mass} * 300000^2## (or I should calculate it with momentum equation?) and assuming Andromeda and Milkyway are just two supermassive blackholes, not two galaxies with sparse stars.
    Seriously, does speed affect physical process INSIDE a closed system that moves? Because motion is relative, right?
  23. Jun 26, 2015 #22
    That intuition is quite useless: clock frequency is a function of gravitational potential energy. It is not a function of gravitational strength.
    You cannot go as fast as light. According to a reference system in which you are moving almost as fast as light, your physical processes are very slow; and indeed, you also have a very high kinetic energy as measured in that system. This slowdown of your body clock to nearly zero at nearly the speed of light can in principle be used for intergalactic travel, so that it should be possible for an astronaut to arrive alive at a distant star that is hundreds of light years away according to us. Thus Einstein already mentioned in his 1905 paper that "we shall, however, find in what follows, that the velocity of light in our theory plays the part, physically, of an infinitely great velocity."
    Because our measurements of uniform motion are relative, no effect of constant linear speed can be detected inside a system that moves; no observation from a different perspective can turn a star into a black hole. :oldwink: It doesn't matter that the mass density goes to infinite according to you; the equations relating to black holes are wrt the centre of mass reference system.
    For calculating kinetic of very high speed objects (with always v<c), the classical equation is not accurate enough. For v=c, Ekin=infinite
    Last edited: Jun 26, 2015
  24. Jun 26, 2015 #23
    Hey , same with me and it is nice that even if we have no formal education we are still interested in science and particulary in physics. It is nice to try know and understand our universe. Well , time if I dont missremember , as Michio Kaku said : Time it is not the Tick Tick Tick of a clock , it is the Click Click Click of an electromagnetism itself and no wonder that it will always be affected by other forces like gravity for example. Time , spacetime is always there or here or just around and we just move in it or just live in it. Humans invented clocks or watches to measure it and remember , the very fist clock that was "invented" it is our Earth herself : day and night, day and night :) but again just to meausre it :)
  25. Jun 26, 2015 #24


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    All of physics is just about physical processes. If what you call "time itself" cannot by measured by physical processes, then it's not part of physics.
  26. Jun 28, 2015 #25
    Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.

    First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.

    Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.

    With that being said, here are my questions...

    If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

    If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

    If two people left earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?
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