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I Could we detect an intrinsic change in the flow of time?

  1. Oct 28, 2016 #1
    In the principle study of the Pioneer Anomaly, John Anderson and Slava Turyshev suggested a speculative explanation for the apparent deceleration of the spacecraft: as an acceleration of the clock rate used for telemetry back on earth. To be clear, this explanation has been discounted- although it was explored further in this paper. The Pioneer Anomaly is generally considered solved, being due to asymmetric thermal radiation from the probes.


    My question is, if such a universal change in clock time did exist, i.e. that every subsequent second ticked by faster or slower than the one before it, how could we detect it? Can anyone imagine an Earth-based experiment? As I understand it, the effect that Anderson was proposing would be universal, effecting all clocks equally. Such an effect would be independent of special or general relativistic time dilations, occurring without any specific circumstances of motion or gravitation (although these effects would need to be subtracted from any experiment).The magnitude of the effect if it were to explain the Pioneer Anomaly, 2.9E-18 sec^-1, would exceed the uncertainty of an ordinary cesium atomic clock in a week. I suspect the most sensible comparison would be to compare the proper time of 2 observers at 2 different cosmic times.

    I'm not asking if this effect is real, I'm asking if we could make a practical experiment to detect something like it, or if it could even be logically possible for such an effect to exist. I proposed this thought experiment over at scienceforums.net, and we got as far as assuming light delay could put us in contact with a past rate of time (even of the very same clock, if a signal is bounced off something like the Pioneer probes), and so an interferometer with arms of very different lengths might register such an effect, but would require absurd precision for something on the order of magnitude of the Pioneer Anomaly.
     
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  3. Oct 28, 2016 #2

    PeterDonis

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    We couldn't. The idea is physically meaningless; it sounds plausible but when you try to make it rigorous, it doesn't work.
     
  4. Oct 28, 2016 #3
    I wondered if this might be the case, I get hung up on it when I think about reciprocity between 2 observers. But can you elaborate on where it fails to make sense fundamentally?
     
  5. Oct 28, 2016 #4

    PeterDonis

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    Because there's no such thing as "universal clock time" to begin with. There is only proper time along particular worldlines, as measured by particular clocks following those worldlines. So the question isn't even well posed, since it assumes that there is a meaningful "universal" definition of time, when in fact there isn't.
     
  6. Oct 28, 2016 #5
    change in proper time over cosmic time. We all refer regularly to the cosmic age of co-moving observers at rest relative to the hubble flow. Better posed?
     
  7. Oct 28, 2016 #6

    PeterDonis

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    Which is not well-defined either. See below.

    Yes, and what that means is proper time elapsed along the worldlines of such observers. There is no "cosmic time" other than that.

    No. See above.
     
  8. Oct 28, 2016 #7
    Ok, so can you imagine how Drs Anderson and Turyshev were referring to a clock acceleration in the original cited paper in my post? Were they and others involved referring to something that is physically meaningless? Or what is it that I am misrepresenting in their hypothesis?
     
  9. Oct 28, 2016 #8

    PeterDonis

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    As far as I can tell, yes. I think the point at which they go wrong is when they talk about a time-varying "gravitational potential" throughout the universe. The concept of "gravitational potential" only makes sense in a stationary spacetime (basically, a spacetime that you can slice up into a stack of spacelike slices that all have the same geometry). But our universe is not a stationary spacetime.
     
  10. Oct 28, 2016 #9
    ok then. Anybody disagree with PeterDonis?
     
  11. Oct 28, 2016 #10

    Chronos

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    Lacking an absolute clock, an idea that was ceremonially buried by Einstein, there is no grounds for disagreement.
     
  12. Oct 28, 2016 #11
    Often wondered if, during the 13.8B years since the BB, has "local earth time" always remained exactly the same, with 1s throughout those years always lasting 1s as we experience it now? If it hasn't then I presume that the often quoted 13.8B years might not be very precise?
     
  13. Oct 28, 2016 #12
    Thanks Guys.

    This should perhaps split into another post, but if it is meaningless to refer to a background gravitational potential in relation to the flow of time, why do we expect the overall density of mass to influence the expansion of space?
     
  14. Oct 28, 2016 #13

    PeterDonis

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    Because "gravitational potential" is not the only effect that density of mass (more precisely, stress-energy, which includes mass/energy density but also pressure, etc.) can have. "Gravitational potential" is just a way of describing one portion of that effect in a particular set of spacetimes (the stationary ones).
     
  15. Oct 28, 2016 #14
    Thanks Peter. In my mind, the stress energy tensor was roughly synonymous with gravitational potential, but I imagine this just betrays that I can't do the math of GR. Thanks for reviewing my question.
     
  16. Oct 28, 2016 #15
    I should probably stick to the question asking rather than answering, but I'll make an attempt here, and hopefully someone can back it up or refute it. The 13.8billion years is a cosmic time coordinate, extrapolated from our observations of the dynamics of spatial expansion. Because expansion has been close to constant over history, this age is pretty close to 1/H_o. It's the time it would take the universe to grow to it's present size at this observed rate. Any object on Earth, say a primordial proton, would have an age less than that if you could date it, because it has spent it's lifetime in various gravitational fields, which have an attendant time dilation.
     
  17. Oct 28, 2016 #16
    Measuring a rate for flow of perceived time; would that not need a second dimension of time to compare it with?
    Time apparently only has one dimension, and it's not easy to argue that there could be more.
    Occam's razor and all that.
     
  18. Oct 28, 2016 #17

    PeterDonis

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    Since the definition of "1 second" is based on an atomic process, which hasn't changed, then the second hasn't changed either. In other words, the 13.8B years is equivalent to a definite number of cycles of the atomic process on which the definition of the second is based.

    This is correct, but it doesn't quite address Tanelorn's question. He's asking (possibly without quite realizing it) whether it's possible that coordinate time in the standard coordinates used in cosmology could have been different from proper time along a comoving worldline in the early universe, even though the two are the same now. The answer to that is, first, that this is not possible if our current cosmological model is correct (or even approximately correct), and second, we have evidence that the physical constants governing the processes we use to measure proper time, such as atomic clocks, have not changed from then to now.
     
  19. Oct 29, 2016 #18

    Garth

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    I would like to make a few observations.

    In order to measure a clock drift you need two different types of clock that run on different physical principles.

    If part of the Pioneer anomaly is caused by a clock drift then there would be a discrepancy between the 'atomic clock' (the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom) behind the apparatus transmitting and receiver the radio pulses and the 'gravitational clock' (the period of the orbit of a test particle) used in Newtonian (and GR) gravitation determining the position of the spacecraft in its trajectory.

    In standard physics these two clock systems remain synchronised over cosmological time scales.

    However the frequency of the radiation emitted by a caesium 133 atom (and any other atomic process) is proportional to atomic particle mass, so standard theory assumes that these masses are constant, and the period of an orbiting test particle depends of GM being constant. A clock drift explanation of the PA (or part of it) would indicate that either particle masses or GM are actually varying.

    The nature of science is not to dogmatically insist on standard theory discovered so far but to constantly test it.

    Perhaps the PA is such a test.

    Note although the PA can be explained by 'normal' physics it has only just been so resolved by taking every possible source of acceleration on the spacecraft and vector summing the maximum values within their error bars together as if they were all were at these maximum values and acting in the same direction. IMHO a more reasonable assumption would be that they are not all at these maximum values and do not all act in the same direction so the sunwards component of the vector addition explains only part of the PA. The remaining acceleration seems interestingly close to cH: Hubble acceleration.

    We can define two physically significant ‘clocks’ as follows:

    Sample two photons, one emitted by a caesium atom the other sampled from the CMB radiation.

    The first definition of an ‘atomic’ second, is defined as the duration of exactly 9.19263177x109 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. This is the standard physical definition of a second.

    The second, a ‘photonic’ second, can be defined as the duration of exactly 1.604x1011 periods of the radiation corresponding to the peak of the CMB black body spectrum.

    Both systems of time measurement are physically significant and agree with each other in the present time, although they will diverge from each other at other times.

    If gravitational time actually follows the 'photonic second' then the clock drift between the two time systems would be cH.

    Just a thought...

    Garth
     
    Last edited: Oct 29, 2016
  20. Oct 29, 2016 #19

    Jonathan Scott

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    Surely if the rate of time were changing that is in principle detectable, because as we look into the distance we are looking back in time, so we can compare the time rate now with the past. The effect of a gradually increasing time rate would be similar to cosmological redshift, but in theory it would be possible to distinguish the effect because an object at a fixed distance would still show the redshift.
     
  21. Oct 29, 2016 #20

    PeterDonis

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    Only the first of these definitions corresponds to proper time as it appears in the mathematical model. So only the first corresponds to the time we use to define the "age of the universe". And our standard physics does predict that these two "seconds" will diverge as we look further into the past; so this "definition" is not in any way a change in standard physics.

    What is "gravitational time"? Meaning, what actual math are you referring to by this term? It has no meaning in standard physics.
     
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