# TIME DILATION. WHY do clocks that are

by abbott287
Tags: clocks, dilation, time
P: 152
 Quote by Janus Except that it has been shown experimentally that acceleration has no effect on time measurement. The set up is fairly simple" you put radioactive samples on a centrifuge, spin it up to high speed and then see how fast they decay. Now here's the trick. By varying the angular velocity and length of the centrifuge arm, you can set the experiment up so that the sample travels at different speeds but experiences the same acceleration or travels at the same speed but experiences different accelerations. Such experiments have shown that the resulting time dilation depends only on the speed at which the sample moves and is independent of the acceleration it undergoes.
I completely agree. However, the FAQ in this forum refers to a web site:
which states that:

"The so-called “twin paradox” occurs when two clocks are synchronized, separated, and rejoined. If one clock remains in an inertial frame, then the other must be accelerated sometime during its journey, and it displays less elapsed proper time than the inertial clock. This is a “paradox” only in that it appears to be inconsistent but is not. "

Now if people keep saying acceleration has anything to do with the elapsed time, it will confuse people... I do not know if this is the case this time... The link is actually referencing to jour example:

"“Measurements of relativistic time dilation for positive and negative muons in a circular orbit,” "
 Mentor P: 17,329 The acceleration does not directly affect time dilation. It only breaks the symmetry between the twins. There is nothing wrong with mentioning acceleration, because the broken symmetry is important (even though by itself it does not affect time dilation).
PF Gold
P: 5,060
 Quote by Janus Except that it has been shown experimentally that acceleration has no effect on time measurement. The set up is fairly simple" you put radioactive samples on a centrifuge, spin it up to high speed and then see how fast they decay. Now here's the trick. By varying the angular velocity and length of the centrifuge arm, you can set the experiment up so that the sample travels at different speeds but experiences the same acceleration or travels at the same speed but experiences different accelerations. Such experiments have shown that the resulting time dilation depends only on the speed at which the sample moves and is independent of the acceleration it undergoes.
But, in defense of EP arguments, it is worth noting that differences in 'pseudo-gravity potential' due to acceleration produce clock rate differences. I'm sure you're very familiar with the setup:

Two clocks set up to accelerate uniformly such from the view of the (e.g.) the back clock the distance between the clocks remains constant. There will be a clock rate difference proportional to the distance between the clocks and the acceleration, as if in a uniform gravitational field.

However, in an inertial frame, the above requirements lead to observation that the clocks get closer together, do not have identical acceleration or velocity, and that the velocity difference accounts for the difference in clock rate.
P: 152
 Quote by PAllen Two clocks set up to accelerate uniformly such from the view of the (e.g.) the back clock the distance between the clocks remains constant. There will be a clock rate difference proportional to the distance between the clocks and the acceleration, as if in a uniform gravitational field. However, in an inertial frame, the above requirements lead to observation that the clocks get closer together, do not have identical acceleration or velocity, and that the velocity difference accounts for the difference in clock rate.
How do you mean? The back clock is sending light towards a mirror next to the front clock and he measures the time it takes for the light to come back? Are you assuming that the light speed is c in the inertial frame and by somehow regulating the distance between the clock to accout for this and the time dilation, the measured round trip time of light will always be the same for the guy at the back clock?
PF Gold
P: 5,060
 Quote by Agerhell How do you mean? The back clock is sending light towards a mirror next to the front clock and he measures the time it takes for the light to come back? Are you assuming that the light speed is c in the inertial frame and by somehow regulating the distance between the clock to accout for this and the time dilation, the measured round trip time of light will always be the same for the guy at the back clock?
Look up the Bell space ship paradox. This is a variant of it, and any discussion of that will explain my other points. I don't have time to write up the details now. I'm sure Janus is completely familiar with all of this, just emphasizing different points. And there is no contradiction between what he said and what I said.

One key point is that if the distance remains constant from the point of view of the back clock, then length contraction demands that the two clocks get closer and closer in the inertial frame (as the back clock goes faster and faster). This requires that their speeds cannot be identical in the inertial frame. Thus two different explanations of the same observations: (pseudo)gravitational potential difference in the accelerating frame, simple difference in speed in the inertial frame.
PF Gold
P: 4,745
 Quote by Ernst Jan Light goes with the speed of light in SR, so the "problem" of movement stays.
"Problem"? What problem? What are you talking about?
 Quote by Ernst Jan I'm unaware my view of SR is incorrect. Please explain.
Well, you stated:

"Let's say this train accelerates further to c.
Now SR predicts time stops and in my view there no longer is an angle that will allow the observer to point towards the spot on the wall.
With time stopped it seems difficult to move, but in my view nothing changes."

This is an incorrect understanding of SR. As I said earlier, no matter how much the train has accelerated, it still is just as far from c as before it started. SR makes no prediction that there is any condition in which time stops. Rather, it states that time will be completely normal for any train, it never slows down or speeds up or stops, no matter how it has accelerated in the past.
 P: 19 [QUOTE=ghwellsjr;3508992 As I said earlier, no matter how much the train has accelerated, it still is just as far from c as before it started. SR makes no prediction that there is any condition in which time stops. Rather, it states that time will be completely normal for any train, it never slows down or speeds up or stops, no matter how it has accelerated in the past.[/QUOTE] According to my starting FoR, which I didn't change, you are wrong.
PF Gold
P: 4,745
 Quote by Ernst Jan According to my starting FoR, which I didn't change, you are wrong.
I made a lot of statements, which one(s) do you think I'm wrong about?
P: 490
 Did you mean to make that a capital G (the universal gravitational constant) or did you mean lower case g (the local gravitational field). Either way it doesn't have an effect to my knowledge, unless you have such strong tidal forces (changes in g) that the atoms are spaghettified.
Yes I did but it ain't absolute.
Well where do you draw the line, at which point do you decide where tidal(spagetiffication) forces and gravitational effects are distinct and apart.
P: 490
 Quote by Janus Except that it has been shown experimentally that acceleration has no effect on time measurement. The set up is fairly simple" you put radioactive samples on a centrifuge, spin it up to high speed and then see how fast they decay. Now here's the trick. By varying the angular velocity and length of the centrifuge arm, you can set the experiment up so that the sample travels at different speeds but experiences the same acceleration or travels at the same speed but experiences different accelerations. Such experiments have shown that the resulting time dilation depends only on the speed at which the sample moves and is independent of the acceleration it undergoes.
I am clueless to how they can do that.
How do accelerate a sample without making it move.
P: 152
 Quote by Buckleymanor I am clueless to how they can do that. How do accelerate a sample without making it move.
An object moving in a circle is always accelerating towards the center of that circle. Otherwise it would be moving in a straight line and not in a circle.
The acceleration goes as (v^2)/r.
Mentor
P: 17,329
 Quote by Buckleymanor Yes I did but it ain't absolute.
Huh? So was it the universal gravitational constant or the local gravitational field?

 Quote by Buckleymanor Well where do you draw the line, at which point do you decide where tidal(spagetiffication) forces and gravitational effects are distinct and apart.
That's easy. Gravitational effects can be removed through a coordinate transform and are not felt at all by a free-falling object, regardless of the strength of the gravitational field. Tidal effects cannot be removed through a coordinate transform and are still felt by a free-falling object.
P: 1,414
 Quote by Janus Except that it has been shown experimentally that acceleration has no effect on time measurement.
Differences in speed have an effect on time measurement. So it follows that changes in speed have an effect on time measurement.

But let me try to think through this out loud.

The general form of the Lorentz Factor, γ = (1 - v2)-1/2, remains unchanged wrt acceleration.

The value of the Lorentz Factor is directly affected only by the speed at which an oscillator is moving.

So we can say that the period of an oscillator is directly affected only by the speed at which the oscillator is moving. The faster(slower) an oscillator is moving, the greater(lesser) its period, and the lesser(greater) its frequency.

However, the speed at which an oscillator is moving is a direct effect of the oscillator's most recent acceleraton (assuming that the oscillator's speed hasn't remained constant throughout its entire history). That is, when the speed of an oscillator has changed during a certain interval, then we call the rate of change during that interval an acceleration. (Although an oscillator can presumably be accelerated, by changing its direction of motion, without in any way changing its speed, we're only concerned with the component of velocity that has to do with the oscillator's speed. And, a change in speed refers to, by definition, an acceleration.)

 Quote by Janus By varying the angular velocity and length of the centrifuge arm, you can set the experiment up so that the sample travels at different speeds but experiences the same acceleration or travels at the same speed but experiences different accelerations.
The speed of the sample is proportional to the rotational radius (r), the distance of the sample from the rotational axis (roughly the length of the centrifuge arm), times the RPM's of the sample. Increase(decrease) r while keeping RPM's the same and the speed of the sample increases(decreases). Increase(decrease) RPM's while keeping r the same and the speed of the sample increases(decreases).

I'm assuming that changes in either the rotational radius of the samples, or the RPMs of the samples isn't done on the fly. Otherwise, there are obvious accelerations involved. (Ie., if the arm is extended/retracted while keeping the RPMs constant, or if the RPMs are varied while keeping the rotational radius constant.)

I agree that the experiments you mentioned do show that the general form of the Lorentz Factor is unaffected by acceleration.

What I'm wondering about (with the understanding that the quantity of differential aging is a function of the time during which an oscillator is propagating at a certain speed), is when the change occurs wrt an oscillator whose frequency has been altered -- as it seems obvious that it can't be occuring while the oscillator is propagating at a constant speed. It follows that the changes in oscillator frequency must be occuring during intervals of acceleration.

Which means that speed accelerations/decelerations do directly affect (produce changes in) the periods of oscillators.

Thus, acceleration (involving variations in speed) affects time measurement.
Mentor
P: 17,329
 Quote by ThomasT Differences in speed have an effect on time measurement. So it follows that changes in speed have an effect on time measurement.
Yes, but acceleration is a change in velocity, not a change in speed. The whole point of using uniform circular motion is to have high acceleration without a change in speed. When you do that you find that acceleration does not cause time dilation, at least not up to about 10^18 g.

 Quote by ThomasT However, the speed at which an oscillator is moving is a direct effect of the oscillator's most recent acceleraton (assuming that the oscillator's speed hasn't remained constant throughout its entire history).
As you mention, it also depends on the initial velocity. Furthermore, if the most recent acceleration was centripetal then the speed does not depend on it. Your statement is not true in general.
P: 1,414
 Quote by DaleSpam Yes, but acceleration is a change in velocity, not a change in speed.
Speed is the magnitude component of velocity. While an acceleration doesn't necessarily involve a change in speed, a change in speed is, by definition, called an acceleration.

Wrt differential time measurement, it's the accelerations that involve changes in speed that we're concerned with.

 Quote by DaleSpam The whole point of using uniform circular motion is to have high acceleration without a change in speed. When you do that you find that acceleration does not cause time dilation, at least not up to about 10^18 g.
I agree. As the experiments show, time measurement depends only on the speed component of velocity. So, to say that time measurement is unaffected by acceleration because it's unaffected by the directional component of velocity is sort of misleading.

 Quote by DaleSpam As you mention, it also depends on the initial velocity. Furthermore, if the most recent acceleration was centripetal then the speed does not depend on it.
If the speed remains constant, and the rate of time measurement remains constant, then this shows that the rate of time measurement doesn't depend on accelerations that don't involve changes in speed.

But, again, a change in speed is, by definition, an acceleration. And time measurement depends on speed. Therefore it's incorrect to say that changes in the rate of time measurement don't depend on acceleration. It's just a semantic thing that needs clarification.

 Quote by DaleSpam Your statement is not true in general.
How about this? Changes in time measurement are a function of accelerations that involve changes in speed.

By the way, thanks also for your feedback on my previous concern. Since I don't have a mechanistic theory of relativistic differential time measurement to refer to (the extant ether theories aren't quite what I had in mind), then I'm left with the geometric account.
PF Gold
P: 4,745
 Quote by ThomasT Speed is the magnitude component of velocity. While an acceleration doesn't necessarily involve a change in speed, a change in speed is, by definition, called an acceleration. Wrt differential time measurement, it's the accelerations that involve changes in speed that we're concerned with. I agree. As the experiments show, time measurement depends only on the speed component of velocity. So, to say that time measurement is unaffected by acceleration because it's unaffected by the directional component of velocity is sort of misleading. If the speed remains constant, and the rate of time measurement remains constant, then this shows that the rate of time measurement doesn't depend on accelerations that don't involve changes in speed. But, again, a change in speed is, by definition, an acceleration. And time measurement depends on speed. Therefore it's incorrect to say that changes in the rate of time measurement don't depend on acceleration. It's just a semantic thing that needs clarification. How about this? Changes in time measurement are a function of accelerations that involve changes in speed. By the way, thanks also for your feedback on my previous concern. Since I don't have a mechanistic theory of relativistic differential time measurement to refer to (the extant ether theories aren't quite what I had in mind), then I'm left with the geometric account.
The point is, when discussing the Twin Paradox, that although, if one twin remains inertial and the other one experiences acceleration, we can identify that twin as the one that will have elapsed less time than the inertial one upon their reuniting, but we don't say that it is his acceleration that causes the change in age rate. It is his difference in speed over a period of time that causes it. Of course, the acceleration can cause a change in his speed which will then cause a change in his aging rate, but that by itself won't cause a change in his age when they reunite. He has to accumulate time at the different aging rate to acumulate a difference in age.

Consider a traveling twin that doesn't just decelerate to a stop at the turn-around point and then accelerate back to the home twin but rather maintains a constant speed and makes a loop back to turn around. Now he has never changed speed but he has accelerated.

Or consider Einstein's original introduction of the Twin Paradox in his 1905 paper in which one clock takes a circular path away from the inertial clock and every time it comes back, it has accumulated less time on it.
P: 133
 Quote by sisoev ghwellsjr, I'd like to give you credit for the explanations you gave in this topic. Earlier I said that the change of direction is not of any importance for the difference in the observations. I see now that I was wrong. Thank You :)
I second this. Thank you ghwellsfr for your illuminating examples and your great patience with some of the posters in this thread. I just wanted to make sure you understand that your efforts are appreciated!
Mentor
P: 17,329
 Quote by ThomasT How about this? Changes in time measurement are a function of accelerations that involve changes in speed.
That is OK as long as you add "and the initial speed". Although it will cause you communication problems for the following reason:

Do you know the difference between a partial derivative and a total derivative? When scientists say "X doesn't depend on Y" what they generally mean is $\frac{\partial X}{\partial Y}=0$. Basically, this means that X does not change if you change Y without changing anything else. So if X is a function of Y and Z then the partial derivative of X wrt Y is obtained by keeping Z fixed.

So, the type of centrifuge experiment described by Janus above is exactly the kind of experiment that would be used to investigate this type of dependence. That way you could change accelerations without changing speed and obtain $\frac{\partial \gamma}{\partial a}=0$ indicating that time dilation does not depend on acceleration, in the meaning intended by scientists.

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