Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravitational Time Dilation and Twin Paradox ?'s

  1. Mar 2, 2013 #1
    Disclaimer: Im relatively illiterate on the subject so please try to keep explanations generally understandable. Aerospace undergrad programs get zero exposure to relativity I suppose.

    [Gravitational Time Dilation]

    First I want to know if im correct in saying that the gravitational time dilation is basically: gravity affects time? The closer to a gravitational source the slower the time?

    Now I know there is experimental confirmation from atomic clocks but the meaning of the results is what I wish to interpret. I understand that this often get's philosophical but I do not believe that it has to and any suppositions I make are only in effort to understand what is actually happening.

    I do not understand how having time implicit in the explanation of the result makes sense (not that anything in the universe has to I just believe that it can). Let me explain: If the interpretation was that radioactivity (or whatever mechanism the clock is using... it does not really matter) is affected by gravity and therefore the atomic clock will work relatively mechanistically....then that would seem reasonable. Analogous to temperature affecting the speed of molecules. Temperature is the conception; molecule speed is the physical nature -> time is the conception; radioactivity rate is the physical nature.
    I suppose bunching the underlying explanation of everything into time is reasonable if that was the underlying explanation but I do not think that's what gravitational time dilation is saying.

    [Twin Paradox]

    First what I know and is it right: velocity affects time -> twin traveling near light speed returns younger.

    Again I seek what is really happening in our universe not how we come to make predictions of it.

    Now I know the paradox is a thought experiment and not likely to actually occur but lets look at the interpretation of what this actually means.

    Trying to rationalize: If you are going to say Time is slower for the twin traveling near lightspeed you must be bunching everything that constitutes change in the universe into "time". If you are really going to say the twin comes back younger then you are really saying that every particle that makes up every atom is slowing accordingly and moreover, all different particles are slowing at the same rate (as a result of everything being bunched under the definition of "time"). That seems unlikely but is the only logical explanation I see in changing "time" from a conception to a physical aspect of the universe.



    Any thoughts on the subject or explanations on where I may be misinterpreting something are appreciated.
    Cheers
     
  2. jcsd
  3. Mar 3, 2013 #2
    You are correct in saying that.

    I do not quite understand what you want to say with the rest of your post, but in a travelling twin kind of scenario of course you also have to take the gravitational potentials at the locations the twins are positioned at into account. If the travelling twin escapes the earths gravitational field his clock will speed up, if he travels away from the sun his clock will speed up, if he gets closer to Jupiter or some star his clock will speed down.
     
  4. Mar 3, 2013 #3

    Bill_K

    User Avatar
    Science Advisor

    Apophenia, You seem to have the right ideas, just trouble accepting them!
    The idea that time dilation and Lorentz contraction are real physical effects acting on material objects was historically the first. Einstein's explanation that they were acting on space and time themselves rather than every atom equally was a great simplification.
     
  5. Mar 3, 2013 #4

    Dale

    Staff: Mentor

    Yes, clocks based on radioactivity slow down, but so do all other clocks. It doesn't matter if your clock uses the strong, weak, or EM forces, all of them slow down and they all slow down by the same amount.

    If clocks based on radioactivity slowed down but clocks based on EM did not, then we would probably say that gravity affects radioactivity. But that isn't what is observed. What is observed is that ALL clocks of whatever mechanism slow down. So I will turn the question around to you:

    What is the EXPERIMENTAL difference between time slowing down and all clocks of any mechanism slowing down?
     
  6. Mar 3, 2013 #5
    First I want to say that if this gets out of the discussion I welcome any moderators to point it to an appropriate forum.
    It seems impossible to test but are there any experimental confirmations of Lorentz contraction?


    Ok, very good point. I dont wish to muddy physics with philosophy but it may get that way so I apologize beforehand.

    Whenever I hear someone say "time changes" I think "what the hell are you even talking about". Does anyone really know what they are saying? (not being condescending to anyone; I will be the first to admit I do not). I think it was Feynman who said something to the effect of "if a physicist tells you he understands quantum physics, he is lying".

    Im sure they have, but are there gravitational time dilation experiments using clocks other than atomic clocks? Also, is the experimental difference between different clock mechanisms undoubtedly attributed to experimental error? Again, Im sure people testing these are cognizant of this.

    Now, let me respond to your question:
    If the mechanism behind the clocks all slow accordingly, then there is NO experimental difference between saying time slows or saying the mechanism slows as you would suggest. Then what is all the fuss about? you ask.

    The fuss is in the interpretation of reality. Saying time (as if it were a thing) changes seems like a huge misconception. There is a difference is saying time slows and the mechanism slows. If a clock measurement depends on temperature and two clocks in two different temperatures are clearly running at different rates saying that time is slower for one over the other when we know the mechanism is dependent on temperature does not seem correct. However, I suppose that if gravity does affect all matter saying time slows may be logical if it is all encompassing; i.e., all mechanisms of matter slow.... even specific to neurological processes (which is what Im doing to you all with this philosophical BS...haha).

    This could also have actual implications such as: if all the mechanisms of the universe were not slowed accordingly (only the mechanisms we measure or can currently measure are) then is it really correct to say time is what's variable? No, it is not. Specific example: if the cell processes in your body do not conform with slowing of mechanisms of clocks used to experimentally verify GTD then saying you would return younger is not correct. To say time is slowing it also MUST be the case that all mechanisms must slow at an according rate so that there is no experimental difference between saying time slows and the mechanism slows (like you were specifically referring to in your response).

    I may be off on this but was it not that Einstein did not like the idea of a pulling force at a distance? So, relativity get's rid of a pulling gravitational force and instead says: space-time pushes you or you just move through variable space time. I would argue that you can make variable time and space in the event of any pulling force? I would suppose a way to do this specifically with magnetic forces although there would have to be some ridiculous caveat to take into account that the magnetism is specific to electron/protons whereas gravity to all matter. It seems "magical" force at a distance is a fundamental aspect of the universe. Magic is only magic because there's something we are not seeing.

    Im like that evangelical astrophysicist who writes physics papers accepting theories that the universe is 13.5 billion years old but then privately believes it is 6000. LoL.....
    except that Im the ultimate skeptic.

    Thanks for the replies guys; I appreciate it.
     
    Last edited: Mar 3, 2013
  7. Mar 3, 2013 #6

    Dale

    Staff: Mentor

    The Michelson Morely experiment.

    Time dilation has been measured using gravity, the strong, weak, and electromagnetic forces. There are currently no other known forces to account for.

    Exactly. There is no difference. You are free to adopt either of the following two propositions:

    1) Time slows down as predicted by relativity.
    2) Some mysterious unknown mechanism which cannot otherwise be measured or detected (e.g. the luminiferous aether) causes EM processes to slow down and coincidentally it just happens to slow down by the exact same amount predicted by relativity, AND a second mysterious unknown mechanism which cannot otherwise be measured or detected causes strong-force processes to slow down and doubly-coincidentally it just happens to slow down by the exact same amount as the EM processes and relativity, AND a third mysterious unknown mechanism which cannot otherwise be measured or detected causes weak-force processes to slow down and triply-coincidentally it just happens to slow down by the exact same amount as the previous two processes and relativity, AND a fourth mysterious unknown mechanism which cannot otherwise be measured or detected causes gravitational processes to slow down and quadruply-coincidentally it just happens to slow down by the exact same amount as the previous three processes and relativity.

    You are free to believe proposition 2, no evidence will contradict you, but that is too many mysterious mechanisms and lucky coincidences for most scientists, myself included.

    Actually, if you believe 2 then you are quite far from being a skeptic. You are, in fact, a firm believer, willing to accept a belief in the constancy of time despite the lack of evidence supporting it, the abundance of evidence against it, and the many other fantastic things that you must believe in (without evidence) in order to maintain the constant-time belief in the face of the existing evidence.
     
    Last edited: Mar 3, 2013
  8. Mar 4, 2013 #7

    The Michelson-Morley supported Lorentz contraction as an explanation to a null result? Correct. Im not really familiar but there is no direct way to measure Lorentz contraction currently?

    Questioning makes me a skeptic. Postulating how this problem could be physical makes just makes me a thinker; I did not say I believe 2) as there is no evidence to support it. I accept 1) but think how it can translate to physical nature. Everything we know has a physical nature does it not? Massless particles...anything.

    However, if anyone is content with saying Time and not thinking what that would/should physically imply then that's fine; I understand perfectly the theory works and has progressed science. In this economy, I have time on my hands (unfortunately and fortunately)....but Time, it would seem, must physically imply something if you are going to say it is a variable aspect of the universe.

    Another way is to say: OK, space is variable via Lorentz contraction. Material shortens with velocity. So rather than saying or even acknowledging that it must have an effect on the physical nature of the material (atoms and any matter composing it) the explanation is bunched into saying space. That's fine and extremely elegant if so, but who would venture to say that this would have no effect of the material properties of the contracted object? Id suppose the explanation is space itself is something and it's contraction does not have any affect upon the material; a material shortens without changing its physical composition ! That is pretty mind blowing if true and understandably hard to rationalize. The universe does not have to be rational; yes I know, but I think most anything observable can be reduced logically. And logic makes sense. If space and time are somehow physical culprits in this problem I would hope that someday they are reduced logically. If they are physical, they should be reducible in the context that anything else in the universe is.

    We say light is something ; light is reducible -> made of photons or whatever we postulate, though it may be inconceivable in relation to what we commonly think or pure theory because it is currently unobservable.

    It's clear that this discussion in a general framework does not really lead anywhere but I do see the questions brought up in the discussion as 1) not philosophical and 2) essential to progress science.

    Thanks for the input and patience DaleSpam.
     
    Last edited: Mar 4, 2013
  9. Mar 4, 2013 #8
    This is not quite correct. Velocity is relative, and when two observers have a relative velocity approaching c, each sees the other as time dilated. So the travelling twin sees the stay-at-home twin's clock running slow as well.

    It is the whole combination of relative velocities and accelerations that result in the travelling twin having aged less when he returns. His deceleration/acceleration as he turns around plays a crucial part in this.

    Mike
     
  10. Mar 4, 2013 #9
    Would someone like to define exactly what "time" is and tell us what device you use to measure it? Scientifically speaking. Because I don't know the answer to either and I'd
    like to comment on the thread once I do.
     
  11. Mar 4, 2013 #10

    Nugatory

    User Avatar

    Staff: Mentor

    "Time is what a clock measures".
    Now all we need to do is to decide what a clock is.

    A pretty decent ideal clock would be two fixed mirrors facing each other and at rest relative to each other, with a flash of light bouncing back and forth between the mirrors, and with the physicist who is using the clock to measure time located at one of the mirrors (and hence also at rest relative to both of them). Now one tick of the clock is counted each time the light flash hits the physicist's mirror.

    We can argue about whether what this device is measuring is "really" time, but so far no one has found a better definition that is also physically measurable without sneaking in some hidden assumptions.

    You may also object that this device assumes that the speed of light is constant. It does, and that's OK because this assumption is one of the two explicit postulates of relativity. (The great success of Maxwell's equations and the negative results of Michelson-Morley experiments are what makes this postulate attractive).
     
  12. Mar 4, 2013 #11

    Nugatory

    User Avatar

    Staff: Mentor

    Length contraction provides one of the simplest and most elegant explanations of how moving electric charges produce magnetic effects; look in the FAQ at the top of this forum for a pointer to Purcell's work here.

    Of course, that's still an indirect effect, whereas to be really convincing, we'd have to take a meter stick, examine it and arrange a second stick of equal length, and then accelerate the first in such a way that it flies past the second at relativistic speed... And then live to tell about the observation. That's not a practical experiment with anything macroscopic, so we may have to settle for indirect results.
     
  13. Mar 4, 2013 #12

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Time dilation (due to relative motion) and Lorentz contraction are basically the same thing, just in different reference frames. So every measurement of time dilation (and that is done on a daily basis) in some frame is a measurement of length contraction in another frame.

    Length contraction in the lab frame has to be taken into account in the interpretation of heavy ion collisions.
     
  14. Mar 4, 2013 #13

    Dale

    Staff: Mentor

    What is indirect about measuring length using an interferometer? An interferometer is the most precise method of measuring length that we have.

    I have no idea what the term "physical nature" means. Please define it scientifically. I.e. what experiment can be performed to determine if something has a "physical nature" or not.

    The test for whether or not something is logical is whether or not it can be expressed in terms of a self-consistent set of mathematical statements. Math is the language of logic. "Makes sense" is not the correct criterion. Relativity is a prime example of how some features of the universe are emminently logical, but so far removed from ordinary human experience that it may not "make sense".

    And I am skeptical of both those claims. If you have two positions which cannot be distinguished experimentally then it sure seems philosophical to me, and I don't really see the questions as beneficial for scientific progress, let alone essential. I have my own opinion on what is most essential right now.
     
  15. Mar 4, 2013 #14
    For an atomic clock on or near earth experiment (Hafele-Keating for instance) tells us that the faster a clock moves in relation to the center of the earth the slower the clock will tick.

    Also the higher up from the earth the clock is situated the faster it will tick.

    Do you really need length contraction to tell how fast the clock will tick?
     
  16. Mar 4, 2013 #15

    Nugatory

    User Avatar

    Staff: Mentor

    In the rather weak sense that the formula you would use to calculate the change in tick rate doesn't include a contracted length, no. But you won't be able to (no one has ever succeeded and there are good reasons to think it's not possible) construct an internally consistent theory that includes time dilation but not length contraction - as mfb suggested above, they're two sides of the same coin.
     
    Last edited: Mar 4, 2013
  17. Mar 4, 2013 #16

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It is a tricky analysis with clocks (as their length is irrelevant), so let's consider another example: Cosmic muons. They have a lifetime of some microseconds. At highly relativistic speeds, but without time dilation, this corresponds to a decay length of about 500m. Many of them can travel through the atmosphere (>>500m) before they decay, however, due to time dilation.
    What happens if we analyze the problem in the system of the muons? There is no time dilation, so how can they travel through the atmosphere? Well, the answer is length contraction: For high-energetic muons, the atmosphere has a length of less than a kilometer.
     
  18. Mar 4, 2013 #17

    Dale

    Staff: Mentor

    Hi mfb, Nugatory, and Agerhell,

    mfb is correct, and the analysis he gave for muons is applicable to clocks as well. An atomic clock passes by two landmarks at some speed. Both a ground observer and a clock observer agree on the relative speed. The ground observer says the landmarks are a certain distance apart and the clock observer says it took a certain time. The ground observer distance divided by the clock observer time is not equal to the relative speed. The ground observer attributes that to time dilation and the clock observer attributes that to length contraction.
     
  19. Mar 4, 2013 #18

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    Here is at thread that deals with the tradeoff between Length Contraction and Time Dilation for an incoming muon.
     
    Last edited: Mar 4, 2013
  20. Mar 4, 2013 #19

    Nugatory

    User Avatar

    Staff: Mentor

    Of course he is - I don't know about agerhell, but I'm pretty sure that you, mfb, and I are all saying the same thing.
     
  21. Mar 4, 2013 #20

    Dale

    Staff: Mentor

    Yes, I just wanted to clarify it for Agerhell so that he wouldn't think that mfb was backpeadling or being forced to recant.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gravitational Time Dilation and Twin Paradox ?'s
Loading...