Intuition for time dilation in a cesium clock

[msumm21]

TL;DR

Wondering if there's an intuitive way to see why cesium clocks slow with velocity, analogous to the common example of a "light clock"

A common way to introduce time dilation is to show the example of a "light clock" which bounces photons back/forth and ticks each time a photon passes a certain point. Wikipedia does it this way, for example. From such a clock, it's easy to see why the constancy of the speed of light would cause the clock to appear to tick slower by exactly the formula given in SR.

I was wondering if there's any way to "see" why a cesium clock (or energy level transitions in atoms) would also slow with movement, due to the constancy of c, analogous to the above clock.

 

[PeroK]

TL;DR Summary: Wondering if there's an intuitive way to see why cesium clocks slow with velocity, analogous to the common example of a "light clock"

A common way to introduce time dilation is to show the example of a "light clock" which bounces photons back/forth and ticks each time a photon passes a certain point. Wikipedia does it this way, for example. From such a clock, it's easy to see why the constancy of the speed of light would cause the clock to appear to tick slower by exactly the formula given in SR.

I was wondering if there's any way to "see" why a cesium clock (or energy level transitions in atoms) would also slow with movement, due to the constancy of c, analogous to the above clock.

Put the cesium clock next to the light clock. They remain synchronized in a reference frame in which they are at rest. Hence, remain synchronized in any frame.

 

[Ibix]

Synchronise the caesium clock with a light clock.

Drive past it at high speed. You know the light clock will tick slowly as measured in your test frame. You know the caesium clock is synchronised with it. Therefore you know the caesium clock is ticking slowly.

 

[Dale]

You could also use Doppler shift.

 

[Vanadium 50]

To turn the argument of @PeroK around, if the two clocks went out of sync, I would be able to use the difference between them to determine my absolute speed. Relativity forbids that.

 

[Orodruin]

While I agree with the light clock argument, it doesn’t seem like what the OP is after. The wording of the OP suggests to me that they are asking for an intuitive description of why the Cs clock slows in the same vein as the light clock argument.

I am also not sure there is a Doppler shift argument accomplishing this.

 

[msumm21]

Put the cesium clock next to the light clock. They remain synchronized in a reference frame in which they are at rest. Hence, remain synchronized in any frame.

I know SR says experimental results are = in all frames and hence the transitions (and everything) must slow like the "light clock", but what I'm looking for is something analogous to the light clock argument, you might call it a more "direct" explanation (if it exists). Edit: might also call this the "why" the Cs clock slows as SR predicts. I know such a "why" question can't always be answered.

While I agree with the light clock argument, it doesn’t seem like what the OP is after. The wording of the OP suggests to me that they are asking for an intuitive description of why the Cs clock slows in the same vein as the light clock argument.

Yes.

 

[robphy]

I would think the answer to the OP is to find a model of the mechanism of the device which can be drawn on a spacetime diagram. That’s what I did in my “Relativity on Rotated Graph Paper” approach. I drew spacetime diagrams of ticking light-clocks.

The advantage of the light-clock is that it is easier to draw than other clocks (for example. a mass on a spring or a pendulum).

 

[PeroK]

what I'm looking for is something analogous to the light clock argument, you might call it a more "direct" explanation (if it exists). Edit: might also call this the "why" the Cs clock slows as SR predicts.

First, you have to explain how a Cesium clock works?

 

[Ibix]

you might call it a more "direct" explanation (if it exists).

A plausibility argument is that it is based on electron transitions in an EM field, and it would be very strange if they didn't obey relativity, what with EM being the driving mystery behind the development of SR.

 

[robphy]

Possibly interesting:

How an atomic clock works, and its use in the global positioning system (GPS)
by engineerguy


So, I’d suggest constructing a spacetime diagram of the essential mechanism.

 

[Dale]

I am also not sure there is a Doppler shift argument accomplishing this.

There certainly is, but you have to start with the experimentally observed relativistic Doppler. Once you have that it is just a bit of algebra to get the dilation of a caesium clock.

 

[Orodruin]

There certainly is, but you have to start with the experimentally observed relativistic Doppler. Once you have that it is just a bit of algebra to get the dilation of a caesium clock.

But that is an additional observation on top of the invariance of the speed of light that the OP wants to start from.

 

[russ_watters]

First, you have to explain how a Cesium clock works?

Why does it matter how a cesium clock works? This interpretation of the question (which I believe to be correct) implies different clocks might be expected to behave differently under Relativity so each different mechanism requires a unique explanation and evidence of how it obeys Relativity. Isn't the whole point of Relativity to avoid that?

 

[PeroK]

Why does it matter how a cesium clock works? This interpretation of the question (which I believe to be correct) implies different clocks might be expected to behave differently under Relativity so each different mechanism requires a unique explanation and evidence of how it obeys Relativity. Isn't the whole point of Relativity to avoid that?

Those would be my thoughts on the subject. But, if the OP wants to explore the specific question, then understanding how a cesium clock works is the first step.

 

[Herman Trivilino]

A common way to introduce time dilation is to show the example of a "light clock" which bounces photons back/forth and ticks each time a photon passes a certain point. Wikipedia does it this way, for example. From such a clock, it's easy to see why the constancy of the speed of light would cause the clock to appear to tick slower by exactly the formula given in SR.

I think you're missing the point. The purpose of examing the light clock is to demonstrate that time dilates. You might be able to do the same with cesium clocks, but then you might ask the same about pendulum clocks, or any other clock. And note that not all clocks are man-made. We use Earth's rotation and its revolution as clocks.

 

[robphy]

From the video I posted, it seems to me that
[from the viewpoint of special relativity] the “cesium clock” is a fundamentally a clock ( a world line partitioned with a equal-intervals of proper-time) that was well-engineered to be self-correcting to approximate an ideal clock over the course of a long time. While the mechanism of the clever engineering between the ticks is difficult to model, the essence is the equal-intervals of the ticks of proper-time on its world line.

For a clock and its mechanism, it may be easier to analyze a mass on a spring, compared to a cesium clock.

---

update: Once one has equal intervals of proper time on an inertial worldline,
one can use methods as I describe in https://www.physicsforums.com/insights/relativity-on-rotated-graph-paper-a-graphical-motivation/ to motivate special relativity.​

---

• Is the goal to understand how the mechanism of such a clock in motion still works in detail? (As opposed to merely comparing it to an adjacent light clock and invoking the relativity-principle?)
• Or is the goal to motivate special relativity from the clock and its mechanism?
  The light-clock has an advantage that its features are in-tune with the radar method of making measurements (because it cleverly encodes them).
• Or something else?

 

[Vanadium 50]

I think we can agree that "I don;t know how a cesium clock works, but I want an intuitive explanation of how these workings change" would be a very odd question to ask, so you are seeing people concluding the actual question must be something else.

 

[vanhees71]

Those would be my thoughts on the subject. But, if the OP wants to explore the specific question, then understanding how a cesium clock works is the first step.

The Cesium clock's working (i.e. the hyperfine transition defining the secondin the SI) is described by QED, which is a relativistic theory, and thus describes time dilation in the way the Oorentz transformation does.

Seen from this fundamental perspective of the "new SI" time dilation indeed is a consequence of the relativistic Doppler effect for the Cs hyperfine-transition radiation.

 

[hutchphd]

TL;DR Summary: Wondering if there's an intuitive way to see why cesium clocks slow with velocity, analogous to the common example of a "light clock"

I was wondering if there's any way to "see" why a cesium clock (or energy level transitions in atoms) would also slow with movement, due to the constancy of c, analogous to the above clock.

While I agree with the light clock argument, it doesn’t seem like what the OP is after.

I agree that this is the request. To which I would give two answers:
1 A clock must agree with all clocks at that location as a matter of practicality. If the readings diverge, one of them cannot be a clock.
2 Understanding the workings of the Cesium atomic clock mechanisms requires Quantum Electrodynamics which obeys all the tenets of General Relativity. Therefore any detailed explanation of the clock must also.
There has to be consistency. If you want a more detailed treatment, I recall somewhere there is a comparison treatment of wristwatch (with a coil spring) done in detail. Perhaps someone (else) can remember where it is published.

 

[FactChecker]

I was wondering if there's any way to "see" why a cesium clock (or energy level transitions in atoms) would also slow with movement, due to the constancy of c, analogous to the above clock.

IMO, you are missing an important point. It is time, itself, that is changing. Anything that changes with time will show those effects with no other rhyme or reason. Many people hated that idea, but eventually the experimental proof was overwhelming and it was undeniable.
I think that if you are looking for non-SR reasons for the slowdown of particular things like Cesium clocks, biological functions, planetary orbits, lifespan of subatomic particles, etc., you will be forever frustrated.

ADDED: In fact, the opposite tends to be true. SR explains some things that nobody could find a non-SR explanation for (including the effects of GR): the strange orbit of Mercury, why electrical current creates magnetic effects, the amount of curvature of light around massive objects, etc.

 

[msumm21]

Thanks all. Some of you mentioned a spring mass system and I can see how time dilation for that would naturally follow from relativistic mass increase, analogous to the light clock using constant c.

There are many posts about time itself changes and therefore everything changes, understood that the theory says everything slows, but what I'm looking for is an alternate, maybe "more direct" explanation for the specific phenomenon. So I think have one now for the light clock and a clock based on a spring/mass system. The Cs clock is overkill I guess, what I'm really after is why spontaneous transitions between energy levels would slow (again, if such an explanation exists). Reading about spontaneous emission now.

There certainly is, but you have to start with the experimentally observed relativistic Doppler. Once you have that it is just a bit of algebra to get the dilation of a caesium clock.

Could you elaborate on this, or do you have a reference? I'm not seeing it at first.

 

[PeroK]

what I'm really after is why spontaneous transitions between energy levels would slow (again, if such an explanation exists).

You may have fundamentally misunderstood the special theory of relativity (SR). What SR does not say is that "when something moves near the speed of light, its mechanical composition and function physically change". Which seems to be what you are assuming.

SR says, in fact, something that is almost the opposite of this: that the laws of physics are the same in all inertial reference frames. That means that all (inertial) motion is relative and there is no such thing as "moving near the speed of light". Only moving near the speed of light relative to something else. An object moving near the speed of light relative to you remains, in its own rest frame, physically and mechanically unchanged. In the same way that you, moving at near the speed of light relative to it, are not physically changed by that relative motion.

SR is, in fact, a theory of spacetime (space and time). The point already made that SR is about "time itself" is very relevant here. You should review your understanding of what SR says at this point, before your confusion becomes deeper.

Note also that "relativistic mass" is a hangover from the early days of SR - although it is still inexplicably fervently promoted in popular science presentations of SR.

I can see how time dilation for that would naturally follow from relativistic mass increase, a

In fact, we have an Insight on why relativistic mass is not a useful concept in SR:

https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

Our advice is to forget about relativistic mass, generally. When I learned SR (from an undergraduate textbook) relativistic mass was not mentioned - except as a historical footnote.

 

[Herman Trivilino]

Thanks all. Some of you mentioned a spring mass system and I can see how time dilation for that would naturally follow from relativistic mass increase, analogous to the light clock using constant c.

But there is no mass increase. (Relativistic mass is an outdated term, a relic of the way physicists thought about relativity in the early 1900's).

And anyway, the period of such an oscillator is not proportional to $m$, it's proportional to $\sqrt{m}$. So what you think naturally follows doesn't, when you work out the details.

The reason clocks run slow is because time dilates.

You are thinking time dilates because clocks run slow. Your logic is inverted. Clocks are just measuring devices.

 

[Sagittarius A-Star]

Some of you mentioned a spring mass system and I can see how time dilation for that would naturally follow from relativistic mass increase, analogous to the light clock using constant c.

The terminology "relativistic mass" is out-dated. A synonym is $E/c^2$.

So I think have one now for the light clock and a clock based on a spring/mass system.

Here is a calculation of a spring mass system:
https://www.mathpages.com/home/kmath068/kmath068.htm

Here is a calculation of a balance wheel clock:
https://www.physicsforums.com/threa...ausing-it-to-tick-slower.1011025/post-6584697

 

[msumm21]

But there is no mass increase. (Relativistic mass is an outdated term, a relic of the way physicists thought about relativity in the early 1900's).

And anyway, the period of such an oscillator is not proportional to $m$, it's proportional to $\sqrt{m}$. So what you think naturally follows doesn't, when you work out the details.

For the 4-force in SR you need a 1/gamma in the spring constant, the result is gamma^2 under the sqrt, hence the gamma overall, matching the time dilation equation.

 

[FactChecker]

SR says, in fact, something that is almost the opposite of this: that the laws of physics are the same in all inertial reference frames. That means that all (inertial) motion is relative and there is no such thing as "moving near the speed of light". Only moving near the speed of light relative to something else. An object moving near the speed of light relative to you remains, in its own rest frame, physically and mechanically unchanged. In the same way that you, moving at near the speed of light relative to it, are not physically changed by that relative motion.

Excellent. And I would add here that it is only in the other reference frame that the "moving" reference frame's time seems slow. Looking for some non-SR explanation for the slow down would make the symmetric nature of the slow down (each inertial reference frame thinks that the other is running slow) very hard to accept.

 

[Herman Trivilino]

For the 4-force in SR you need a 1/gamma in the spring constant, the result is gamma^2 under the sqrt, hence the gamma overall, matching the time dilation equation.

But I was responding to your claim about mass. You didn't mention the spring constant in that claim.

 

[hutchphd]

The reason clocks run slow is because time dilates.

You are thinking time dilates because clocks run slow. Your logic is inverted. Clocks are just measuring devices.

I dislike the notion of inference. The parameter t in our natural philosophy is what a clock indicates. They are equivalent by our design. Just a quibble that this seems facile to me.

 

[hutchphd]

For the 4-force in SR you need a 1/gamma in the spring constant, the result is gamma^2 under the sqrt, hence the gamma overall, matching the time dilation equation.

So this satisfies you as an explanation for the spring clock? If so, then we are successful (?)