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From this answer, I get that an atomic clock, in principle, works as follows:

  • oscillator, cycles with tunable frequency $f$
  • counter able to count the oscillator's cycles
  • oscillator creates microwave beam with frequency $f$ which is sent towards some atoms, say Cesium
  • tune oscillator for maximum absorption, $f_c$
  • read out counter: at a defined count result, we have one second.

If I raise the clock up towards less gravitation (e.g. from Earth to satellite), what happens inside the clock with the frequency of the Cesium atoms:

When the clock is brought "up there", it runs faster, so the Cesium atoms' hyperfine transition frequency must gain.

Compare with an unrelated photon leaving Earth, it will lose frequency (gravitational redshift).

Of course, a hyperfine transition is not a photon; it just emits/absorbs photons of matching frequency, so is there an explanation of how/why the transitions happen faster?

Harald
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1 Answers1

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The heuristic here is the Principle of Equivalence.

(I refer to it as a heuristic to avoid giving the impression that it is safe to regard the principle of equivalence as an exhaustive representation. At present the principle of equivalence is the closest thing we have.)

Presenting a well-known thought demonstration:
A disk-shaped space-station, the space-station is rotating, so that the parts going round are pulling G's.

The further away from the axis of rotation, the higher the G-load.

Let a number of clocks be co-rotating with the space-station, at various distances to the axis of rotation. Clocks at larger distance to the axis of rotation have a larger velocity than clocks close to the axis of rotation. Thus for clocks further away from the axis of rotation a smaller amount of proper time elapses than for clocks close to the axis of rotation.

If the principle of equivalence holds good then the G-load can be represented as a gravitational potential, with corresponding gravitational time dilation.

If the principle of equivalence holds good then velocity time dilation and gravitational time dilation are intrinsically indistinguishable.


Specific to your question:
The most plausible interpretation, it seems, is that time dilation is a property of spacetime itself. That is, the most plausible interpretation is that the clocks do not undergo physical change as they ascend or descend in a gravitational potential. Rather, the parts of spacetime at different levels in a gravitational potential stand in a relation to each other such that at different levels in the gravitational potential a different amount of proper time elapses.

Then again, that could be an over-interpretation. The most cautious attitude here is to grant the effectiveness of the mathematical expressions of relativistic physics as is.


I'm referring to it as 'the most plausible' interpretation because there is also the interpretation of Special Relativity in terms of Lorentz Aether, which does entail accounting for time dilation effect in terms of physical change. But that interpretation is regarded as highly implausible.


Cleonis
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