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Evidence of Equivalence?

  1. Jul 16, 2007 #1
    In a G field, clocks at a lower potential (closer to the mass producing the field) are known to run slower. When the two clocks are brought togther, the upper clock should be found to have accumulated more time than the lower clock.

    A rocket accelerating at "a" is equivalent to a G field during the acceleration phase i.e., a nose clock in the rocket will observe the frequency of signals transmitted from a lower tail clock to be reduced in frequency, and the lower tail clock will see signals transmitted by the upper clock arriving at higher frequency. The two experients are equivalent during the acceleration phase. When the rocket stops accelerating, the two clocks are then in the same frame and their readings can be compared

    For complete equivalence - the nose clock and the tail clock should show a different reading after the experiment just as do G field clocks emersed for an extended period at different gravitational potentials. Can anyone point to a reference that deals with an experiment designed to measure accumulated time differences between rigidly separated clocks undergoing identical uniform accelerations
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  3. Jul 17, 2007 #2
    I don't think such an experiment has ever been performed. It would encounter substantial technical difficulties. A very long rocket and very high accelerations would be needed to see any visible effect.

    You can find an (incomplete) list of references to experimental measurements of the gravitational time dilation and redshift on Earth and in space in

  4. Jul 17, 2007 #3


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    In fact an experiment was performed in 1976 and called Gravity Probe A, although the two clocks were situated with one in the rocket and the other on the ground.

    Last edited: Jul 17, 2007
  5. Jul 17, 2007 #4
    Thanks Eugene. I am not surprised considering the degree of difficulty. I wonder if any meaningful data could be obtained using a large rotating disc with one clock mounted at the perhipery and a second clock at a radial position of 1/2 the perhipery - when the disc is rotated at a constant angular rate, the two clocks are subjected to different acceleration potentials as well as different velocities - after subtracting out the SR velocity effects, it would seem there should be a difference in the total amount of accumulated time between the two clocks. But if I remember correctly, these type of centrifuge experiments do not reveal time dilation other than that attributable to the velocity as per SR. Don't know whether that conclusion was arrived at by taking data while the disc was in motion or by comparing accumulated times logged by the clocks after the disc is brought to rest.
  6. Jul 17, 2007 #5
    Garth - that is interesting - it may be the closest thing that has been done along this line...but I may be not be correctly reading the result. As I understand the experiment, the measurements were made while the rocket was accelerating, specifically "The clock rate was measured for most of the duration of the flight and compared to theoretical predictions." This validates the equivalence principle during the dynamic phase. But is there a residual time difference between two separated clocks undergoing the same acceleration. In other words, we make a comparison between the times based upon gh/c^2 during flight - but this does not show that the two clocks are running at different rates - only that there will be an observational difference - not a permanent age difference as is the case with clocks in different gravitational potentials for extended periods. If the lower clock were actually running at a slower rate than the upper clock, the effect would be doubled since each pulse is delayed by gh/c^2 plus it would also have an added delay determined by the lower emission frequency of the lower clock.
  7. Jul 17, 2007 #6
    The question about the action of acceleration on the rate of clocks always puzzled me. The ultimate experiment of this kind was performed at CERN in 1970's. They accelerated a pulse of muons in a cyclotron ring and measured the increase of the muons' lifetime. They found that in full agreement with the velocity effect of special relativity the lifetime increased 27x. However, surprisingly, no effect of acceleration on the lifetime was found. This was in spite of really huge accelerations of the order of 10^18 g. Apparently
    acceleration had no effect on the clock's rate. I read in many places that this doesn't contradict the principle of equivalence, but I just can't understand why?

  8. Jul 17, 2007 #7
    Could you please explain a bit more why you think there should be a double effect?

  9. Jul 17, 2007 #8
    Eugene - the double effect could only happen if there was an actual alteration of the emission frequency at the source - and one would have to conclude that acceleration somehow affects time. But this doesn't occur, at least there does not appear to be any experimental evidence that the lower clock would be affected because it was subjected to a different acceleration potential (That is confirmed by the Gravity A experiment cited by Garth). Moreover, there does not seem to be any physical reason that would convey to the rocket clocks that they are in different gravitational potentials. So Equivalence in the sense of a closed elevator seems to be limited only to the duration of acceleration - as in SR each clock runs at the same rate in its own frame and the time difference is an observational one. What bothers me is the G field - an uncompensated GPS clock will gain 38 usec per day because of its altitude. This ongoing accumulation of additional time by the clock that is furtherest removed from the gravitational source would be measured as a real age difference between the two clocks when they are brought together for comparison at some later date.

    I guess I am questioning whether the mechanism that leads to time dilation in a G field is the same as that involved in other accelerations.
    Last edited: Jul 17, 2007
  10. Jul 17, 2007 #9
    You'll never obtain experimental evidence for this, yogi, because it's based upon a misconception. The principle of equivalence does not confer absolute equivalence. In the accelerating rocket, your two clocks experience the same acceleration. In the rocket standing on the surface of the earth, they do not. They can only experience the same acceleration if they're in what's called a uniform gravitational field, and in the real world, gravitational fields are not uniform.
  11. Jul 17, 2007 #10
    I think that gravitational time dilation is an absolute effect: all observers would agree that clocks in space tick faster than identical clocks on the Earth surface. If you bring the GPS satellite back to Earth, you'll see a real effect of extra aging of its clock.

    I know one experiment in which two sets of atomic clocks were used: one clock at the ground level and another clock high in the mountains. After some time the mountain clock was transported back to the valley and readings of both clocks were compared side-by-side. The gravitational time dilation was confirmed with the accuracy of 10-20%.

    L. Briatore and S. Leschiutta, "Evidence for the earth gravitational shift by direct atomic-time-scale comparison", Nuovo Cimento B, 37 (1977), 219

    Last edited: Jul 17, 2007
  12. Jul 17, 2007 #11
    That was my thought also - in the G field (rocket at rest on the earth's surface) there is not only a difference in potential, but there is a difference in force acting upon the upper and lower clocks. In the free space accelerating rocket, there is only a difference in potential. So does this lead to a proposition that says: Time dilation for the rocket sitting on the earth is real and permanent, whereas time dilation for the free space accelerating rocket is apparent only? If this is true, then, as you say, "there is no absolute equivalence." The observer in the sealed elevator can thus determine which kind of field he is subjected too by using two clocks - one on the floor - one the ceiling - if the operator monitors them from a midpoint and they read the same after a sufficient period, the elevator cannot be in a G field.

    Of course, this same result can be arrived at from the divergence of the G field - and while tidal and divergence effects distinguish uniform fields from mass created attractions, the force differences are usually regarded as indicative rather than causal. Here we seem to be dealing with a change in principle - in a G field the clock rate (and hence time) appears to be substantively affected.
  13. Jul 18, 2007 #12
    I think what he's saying is that because a real gravity field is a gradient, two clocks at different points in the gradient isn't the same thing as two clocks accelerating at the exact same rate. I bet you that if you varied the acceleration slightly between the two clocks - in fact, if you varied it in proportion to the ratio of the force of gravity on the two clocks in the gravity field - you would get the same amount of time dilation.

    You'd need a telescoping rocket. Or better yet, one that was like a Chinese yo-yo. But then you'd have to account for the angular acceleration, too. Hmm... two Chinese yo-yo's, wound in opposite directions, attached end-to-end! But someone would put their eye out.
  14. Jul 18, 2007 #13
    I wouldn't say "apparent", yogi, perhaps "relative" is a better word. If you and I passed each other in the dark depths of space I'd say your time was dilated and you'd say mine was, and we might get into an argument about who's time was really dilated. But as meopemuk says above, if I was on a planet and you were up in space, we'd both agree that my time was dilated. We'd say it was absolute rather than relative. As to whether it's correct to say that time dilation is real and permanent in a gravity situation, but is not in the accelerating elevator situation, is debateable. Personally I wouldn't describe this as a change in principle. I'd say the principle of equivalence still applies, and whilst it isn't a total exact equivalence, the time dilation still occurs and is measurable in both situations, even though one situation lacks some agreed baseline.

    PS: I believe the observer in the sealed elevator can perform a Pound-Rebka experiment to determine his situation, but please check this independently.
  15. Jul 18, 2007 #14
    I think this is a very good point. I think an observer in a sealed elevator cabin should be able to decide whether the cabin is accelerating or standing still in a gravitational field. To do that he would need to place identical clocks at the ceiling and on the floor of the cabin. Then wait for a while. Then bring these clocks together and compare their readings. If their readings are the same, then the cabin was accelerating. If the ceiling clock shows later time, then he was in the gravity field. If the floor clock shows later time, then he needs to wake up.

  16. Jul 18, 2007 #15
    In my understanding, Pound-Rebka experiment in an accelerated elevator cabin will show the same result as in the gravity field. This is usually shown by Doppler-effect-like arguments. I think yogi is right that Briatore-Leschiutta type experiment may do the trick and distinguish between acceleration and gravity.

  17. Jul 18, 2007 #16


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    Acceleration and gravity are exactly equivalent (in a differential way). They are the same. Different acceleration at different points changes the result only quantitatively. The clocks in the elevator would read different times, just as clocks on earth would.
  18. Jul 18, 2007 #17
    Noted, Eugene. Your comment also noted Ich. How does one get an expert to chip in on this?
  19. Jul 18, 2007 #18


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    This is introductory level SR, an area which I would claim to have mastered meanwhile.
    Of course you don't have to believe me, it is enough to draw a spacetime diagram and see that events of equal proper time are no longer simultaneous in a comoving frame.
    I might add that I don't know the "Briatore-Leschiutta experiment". I'm talking about my comment "The clocks in the elevator would read different times, just as clocks on earth would."
  20. Jul 18, 2007 #19
    The effect of a given spacetime curvature due to gravity - a given value of the Riemann tensor - and its equivalent rate of acceleration are the same. But gravity is a gradient - it shows up as a field and varies across space - and acceleration does not, at least not in an elevator.

    What do you mean "only quantitatively"? Quantitative things are all we're talking about - things which can be measured by numbers. Apart from what you're going to name your quarks, quantitative is all there is in physics.
  21. Jul 18, 2007 #20
    Special relativity is a constrained case of general relativity that does not involve acceleration. There's no changing between inertial reference frames in special relativity. This is GR.

    (I am not an expert either, BTW, but I know the difference between SR and GR.)
    Last edited: Jul 18, 2007
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