Help me understand Einstein's atomic clock experiments.

[EvanF]

Does a clock measure time, or does a clock measure itself?

 

 

 

 

To me it makes sense that gravity simply affects the clock in these experiments, gravity isn't effecting "time" itself.

Edited October 16, 2016 by EvanF

[Strange]

Does a clock measure time, or does a clock measure itself?

 

 

How would you tell the difference? If, for example, all clocks (and biological processes, etc.) are equally affected in what sense could you say that time is not affected?

 

And that is the case, to the extent that it can be confirmed. All processes where we can measure the effect, such as different types of atomic clocks, the frequencies of photons, the rate at which supernovae change brightness, the lifetimes of fundamental particles, etc. all follow the predicted behaviour.

 

So, as always in physics, we have a model which works to a high degree of accuracy. I guess you can argue about that is "really" happening (and there are always several lively threads about that) but that is philosophy / metaphysics rather than science.

[swansont]

 

To me it makes sense that gravity simply affects the clock in these experiments, gravity isn't effecting "time" itself.

 

 

Explain how it does that. Or, put another way, design a clock that is unaffected, or at least minimizes the effect.

[EvanF]

 

 

How would you tell the difference? If, for example, all clocks (and biological processes, etc.) are equally affected in what sense could you say that time is not affected?

 

And that is the case, to the extent that it can be confirmed. All processes where we can measure the effect, such as different types of atomic clocks, the frequencies of photons, the rate at which supernovae change brightness, the lifetimes of fundamental particles, etc. all follow the predicted behaviour.

 

So, as always in physics, we have a model which works to a high degree of accuracy. I guess you can argue about that is "really" happening (and there are always several lively threads about that) but that is philosophy / metaphysics rather than science.

The only way you could tell the difference is with some kind of literal time travel experiment.

 

 

 

 

But let me put this another way...I need someone to explain to me how gravity is affecting "time" instead of simply affecting the mechanics of the clock...

[Carrock]

It's velocity or acceleration from whatever cause that affects the clock.

 

In a tower accelerating at 1G a clock near the top of the tower will run faster than one at the bottom of the tower.

 

It doesn't matter whether the tower is sitting on the earth or out in space with a rocket pushing it.

 

(In both cases the clock at the top has more potential energy.)

[Strange]

But let me put this another way...I need someone to explain to me how gravity is affecting "time" instead of simply affecting the mechanics of the clock...

 

 

Well, it isn't really that gravity affects time.

 

Gravity is how we perceive the curvature of space-time caused by the presence of mass. Another visible consequence of that curvature is that two people at different locations will see each others lengths and times contracted or stretched.

 

So the effect on rulers (distances) and clocks (time), and the gravitational force are observable consequences of the presence of mass (or energy).

[EvanF]

 

 

Well, it isn't really that gravity affects time.

 

Gravity is how we perceive the curvature of space-time caused by the presence of mass. Another visible consequence of that curvature is that two people at different locations will see each others lengths and times contracted or stretched.

 

I'm not necessarily concerned with what physicists theoretically propose gravity actually is...I'm simply trying to figure out how a clock being changed is actually evidence of time itself being 'dilated' as it were, or if it's simply the clock's mechanics that are changing.

 

It seems to me that time cannot be truly "measured"...It seems like "time" is not an absolute physical property (in space-time.) "Time" exists but we don't actually measure t (time), but rather an object’s speed or frequency.

[Mordred]

There are plenty of examples of time dilation that does not involve a clock or even observers.

 

Muon lifetime decay for example, muons could not reach Earths surface, (their lifetime is too short) without time dilation.

 

Any falling particle that involves gravitational redshift is time dilated.

 

This isn't simply a case of velocity or frequency when a particle exceeds its mean lifetime. Nor does it matter what you use for a mechanical clock.

 

Time dilation affects all physical processes and information exchange between particles etc equally.

Edited October 16, 2016 by Mordred

[EvanF]

There are plenty of examples of time dilation that does not involve a clock or even observers.

 

 

 

 

Yes, but what I'm trying to figure out is the atomic clock experiments specifically...

Edited October 16, 2016 by EvanF

[Mordred]

What's to figure out. The time dilation is a consequence of spacetime curvature. All physical processes. Including decay rates is equally affected. The atomic decay rate just allows us to measure this change in time.

It is no different than the muon whose mean lifetime depends on its decay rate just like the atomic clock.

 

It literally doesn't matter what process you use. The time dilation will be the same. All information exchange is equally affected. In particle physics this is your force interactions and other interactions between particles. However time dilation doesn't restrict itself to information exchange rates, a particle by itself will undergo changes without interactions. These processes are also affected.

Edited October 16, 2016 by Mordred

[swansont]

It seems to me that time cannot be truly "measured"...It seems like "time" is not an absolute physical property (in space-time.) "Time" exists but we don't actually measure t (time), but rather an object’s speed or frequency.

Time is the integral of frequency, so if you affect the frequency you affect the elapsed time.

Yes, but what I'm trying to figure out is the atomic clock experiments specifically...

 

That all systems are affected is evidence that time is changing, rather than being a mechanical artifact in the clock.

[Tim88]

Does a clock measure time, or does a clock measure itself?

 

To me it makes sense that gravity simply affects the clock in these experiments, gravity isn't effecting "time" itself.

 

This most likely has to do with how you define "time". In physics, "time" is a measure of the progress (or relative "speed") of natural processes. If all natural processes of a system are affected by the same amount compared to an unaffected time standard, then we commonly say that its time has slowed down. And that's exactly what happens in a gravitational field, in theory it affects any kind of clock or natural process in the same way.

[Strange]

 

This most likely has to do with how you define "time".

 

Or how you define "clock"

[Tim88]

 

Or how you define "clock"

I doubt that's the issue- but one never knows!

[Janus]

The only way you could tell the difference is with some kind of literal time travel experiment.

 

 

 

 

But let me put this another way...I need someone to explain to me how gravity is affecting "time" instead of simply affecting the mechanics of the clock...

Mainly, you have to consider the fact that gravitational time dilation is related to a difference in gravitational potential rather than gravitational force. While a difference in gravitational force can be "felt" locally by each clock and thus might cause a mechanical effect on the clock's operation, a difference in potential can not.

An extreme example works would be two clocks at different heights in a uniform gravity field( one that does not change strength with height). Even though both clocks are experiencing exactly the same gravity force and the exact same physical conditions, the higher clock runs faster.

 

Another example involves comparing clocks sitting on the surface of the Earth and the surface of Uranus, the surface gravity of Uranus is slightly less than that of the Earth's, yet a the clock on Uranus would run slower due to being at a lower gravitational potential.

 

This indicates that there is something a bit more abstract than something mechanically altering the clock's operation going on.

[Carrock]

An extreme example works would be two clocks at different heights in a uniform gravity field( one that does not change strength with height). Even though both clocks are experiencing exactly the same gravity force and the exact same physical conditions, the higher clock runs faster.

The clocks must be accelerating and not in free fall when they would run at the same rate.

 

The same is true if there is no gravity field.

 

See http://www.scienceforums.net/topic/99633-help-me-understand-einsteins-atomic-clock-experiments/#entry949836

[EvanF]

Mainly, you have to consider the fact that gravitational time dilation is related to a difference in gravitational potential rather than gravitational force. While a difference in gravitational force can be "felt" locally by each clock and thus might cause a mechanical effect on the clock's operation, a difference in potential can not.

An extreme example works would be two clocks at different heights in a uniform gravity field( one that does not change strength with height). Even though both clocks are experiencing exactly the same gravity force and the exact same physical conditions, the higher clock runs faster.

 

Another example involves comparing clocks sitting on the surface of the Earth and the surface of Uranus, the surface gravity of Uranus is slightly less than that of the Earth's, yet a the clock on Uranus would run slower due to being at a lower gravitational potential.

 

This indicates that there is something a bit more abstract than something mechanically altering the clock's operation going on.

 

I think I might be understanding it a bit more at this point...

 

But yes, the whole concept of "time dilation" to me seems to be quite abstract and supernatural.

 

I still don't know if I even believe literal 'time' dilation is what's happening, as blasphemous as that sounds.

[Eise]

To me it makes sense that gravity simply affects the clock in these experiments, gravity isn't effecting "time" itself.

 

I am wondering if you understand special relativity, where we of course also have time dilation. I think if you understand time dilation in special relativity, then you would not ask such a question.

 

An easy analogy is the length of a line segment in 2-D space. Its projection on the X-axis can be longer or shorter dependent on its orientation in the plane: if it is parallel to the X-axis its projection is as long as the line segment itself; if it is perpendicular to the X-axis its length is 0. But you would not ask what the physical cause of the shortening of the projection is, would you?

 

Same with special relativity, except that the coordinate system is that of spacetime: dependent on the 'orientation' of the events in spacetime, which in this case means dependent on the relative velocity of observers, different lengths and time differences are measured. But there is no physical cause for these differences: it is a question of perspective.

 

Same with general relativity: dependent on the masses around, spacetime is bent differently, and so observers at different places (and different velocities) have different perspectives on events happening, including time (which of course includes frequency), and lengths. But for an observer at the same place, with the same velocity, a clock just normally ticks as usual. Physically, nothing changes. But other observers, because of their different perspective, see the events differently. So asking for some kind of mechanism that slows down processes, is just the wrong question. There is no mechanism at all, so not for atomic clocks either.

Edited October 18, 2016 by Eise

[Strange]

And that raises a good question: how could changing your own velocity (or gravitational potential) affect someone else's clock!?

[Eise]

And that raises a good question: how could changing your own velocity (or gravitational potential) affect someone else's clock!?

 

That is a very good question, indeed.

[Tim88]

The clocks must be accelerating and not in free fall when they would run at the same rate. [..]

 

With respect to the surface of the earth, a free falling clock is ticking slower due to both reducing gravitational potential (potential energy) and increasing speed (kinetic energy). This was verified with the Gravity probe A experiment. - Vessot 1980.

Yes, but what I'm trying to figure out is the atomic clock experiments specifically...

As I summarized here above, the frequency of a maser clock followed rather well GR's predictions.

[addendum: you can read about the experimental setup here]

And that raises a good question: how could changing your own velocity (or gravitational potential) affect someone else's clock!?

 

That's simple: as long as you are not very close, changing your own velocity cannot affect someone else's clock. Even your velocity won't affect someone else's clock.

Edited October 18, 2016 by Tim88

[Strange]

That's simple: as long as you are not very close, changing your own velocity cannot affect someone else's clock. Even your velocity won't affect someone else's clock.

 

 

And that was exactly my point. Therefore, time dilation cannot be a "mechanical" effect on the clock.

[Carrock]

 

The clocks must be accelerating and not in free fall when they would run at the same rate.

With respect to the surface of the earth, a free falling clock is ticking slower due to both reducing gravitational potential (potential energy) and increasing speed (kinetic energy). This was verified with the Gravity probe A experiment. - Vessot 1980.

 

I was referring to two clocks both accelerating or in free fall (neglecting tidal forces) and not comparing an accelerating clock with one in free fall.

[bvr]

And that raises a good question: how could changing your own velocity (or gravitational potential) affect someone else's clock!?

 

That's simple: as long as you are not very close, changing your own velocity cannot affect someone else's clock. Even your velocity won't affect someone else's clock.

 

And that was exactly my point. Therefore, time dilation cannot be a "mechanical" effect on the clock.

 

Of course you won't affect someone else's clock. Changing your velocity will affect your own clock. That's the principle of the twin's story, isn't it?

[Strange]

 

 

 

Of course you won't affect someone else's clock. Changing your velocity will affect your own clock. That's the principle of the twin's story, isn't it?

 

 

But you (and your clock) are always stationary in your own frame of reference. Your own clock always runs at the same rate (proper time). It is only someone else's clock you will see running slow - because you "made" them move.

 

The point is, it is just about observations / measurements made between frames of reference. Nothing "happens" to anyone's clock.

Edited October 19, 2016 by Strange