Is Time Necessary?

[swansont]

I seem to have contradicting answers now from swansont and fredrik

 

Fredrik's response looks much like the Lorentz response to the Michelson-Morley experiment, about the ruler shrinking in the direction of motion. It was discarded not because it was wrong, but because it was ad-hoc and had no mechanism that could be tested. Einstein came along with a mechanism, and that allowed testable predicitions. That's what is needed by science.

[fredrik]

> I seem to have contradicting answers now from swansont and fredrik

 

I'm not sure I see the contradiction? Perhaps we made different interpretations here or I misunderstood your intention. The discussion was a bit fuzzy after all (these "counters" and all) ?

 

/Fredrik

[fredrik]

I seem to have contradicting answers now from swansont and fredrik

 

I read again I suspect we are discussing two things here. I can identify at least two questions

 

1) What is time? or more specifically, how is time properly defined?

 

2) How does different times compare, when defined in different settings.

 

I was talking about (1), but I now suspect perhaps you were thinking of (2) - the relativistic time dilation effects. A clock in motion, and a non-moving clocks are different.

 

My suggested concept that I tried to explain is that time is defined by the relative change of your information relative to the information of your clock device. In the general case the information of other clock devices are not a valid replacement. Because you can certainly distinguish a clock at rest, and a moving clock. So they is two different clocks. And to make a proper comparasion you need to know their relation. This relation is what Einstein solved. But Einsteins did IMO not in detail explain (1) - what is the proper definition of time in the local frame.

 

In terms of my fuzzy talk in the previous posts, the relativistiv time dilation (relation between different observers) can be understood loosely like this:

 

The flow of time that appears to slow down in a near light speed frame or in a gravity field, can be explained if that environment is in general higher in order, thus explain why the similar "translated" event is less likely there as compared to your reference because the flow of time is related to *relative* order/disorder. And if your reference is more ordered, your relative disorder is higher, and the probabilistic driving force of "time" is lower. So there is no such thing as absolute disorder, only relative disorder, and relative time. But that's not to imply it's not real.

 

Maybe this complicated it further, if so, ignore this for now.

 

/Fredrik

[JohnF]

1) What is time? or more specifically, how is time properly defined?

 

This is the answer I am after

 

Take the Hafele-Keating experiment as an example. The experiment proves that speed slows down time, or does it.

 

The clocks were out of sync at the conclussion of the experiment so either...

 

1. Time was distorted.

 

2. The clocks just worked, counted events, at a different rate.

 

Even though the time dilation was predicted, which tends to confirm the validity of the experiment, is it possible that the same equations could in some way be applied to the atomic structure of the cesium atom?

 

Or if not applied to the cesium atom then to some other underlying particle that has yet to be discovered or fully understood.

 

Is there an observable change independant of time?

 

If the clocks had been travelling much faster, and assuming their atomic stucture was slowed down rather than time, would they have got colder? Even if they would get colder could such an observation be of any use?

[Robonewt]

Did the Hafele-Keating Experiment prove anything?

 

In the experiment the moving Clock was travelling at height and speed at the same time, is there any way you could differentiate how much of the effect was due to the speed, and how much was due to the reduced gravity at height. It could have been that there was no effect due to speed, only due to the reduced gravity at height.

 

Also, as you say, it didn’t necessarily prove that the rate of time changed, only that the mechanics of the clock was affected.

 

Could the effect also have been due to the clock travelling across the magnetic field of the Earth?

 

Could or have other experiments been done? such as flying north south to discount any magnetic field effects or floating a clock stationery at 10,000ft.

 

Is there any other clock type that could be developed to show the effect isn’t only relative to atomic clocks?

 

Practically, nothing would change, even if the effect is only mechanical, we would still need the relativity equations to allow the atomic clocks to be of use, all that changes is our reasoning behind it.

[swansont]

This is the answer I am after

 

Take the Hafele-Keating experiment as an example. The experiment proves that speed slows down time, or does it.

 

The clocks were out of sync at the conclussion of the experiment so either...

 

1. Time was distorted.

 

2. The clocks just worked, counted events, at a different rate.

 

Even though the time dilation was predicted, which tends to confirm the validity of the experiment, is it possible that the same equations could in some way be applied to the atomic structure of the cesium atom?

 

Or if not applied to the cesium atom then to some other underlying particle that has yet to be discovered or fully understood.

 

Is there an observable change independant of time?

 

If the clocks had been travelling much faster, and assuming their atomic stucture was slowed down rather than time, would they have got colder? Even if they would get colder could such an observation be of any use?

 

 

But you get the same effect if you use a rubidium clock instead of cesium. Or a hydrogen clock (maser). So the response of the atoms scales with the transition — it's not e.g. a constant effect for an electron (i.e. it's not consistent with just a certain force acting on the electron, causing a constant energy change). The effect is cesium is about 1/5 as large in hydrogen and 2/3 as large in rubidium, because they oscillate at different frequencies.

 

Not to mention that it's completely consistent with an effect that has to be there in order for electromagnetic waves to exist in the first place (wave equation solution to Maxwell's equations)

[swansont]

Did the Hafele-Keating Experiment prove anything?

 

In the experiment the moving Clock was travelling at height and speed at the same time, is there any way you could differentiate how much of the effect was due to the speed, and how much was due to the reduced gravity at height. It could have been that there was no effect due to speed, only due to the reduced gravity at height.

 

But the effect has been seen using different values of v and h. All consistent with prediction.

 

Also, as you say, it didn’t necessarily prove that the rate of time changed, only that the mechanics of the clock was affected.

 

But the effect was not symmetric with the motion with respect to the earth. How, mechanically, do the clocks know they are going east or west?

 

Could the effect also have been due to the clock travelling across the magnetic field of the Earth?

 

Atomic clocks are shielded to protect them against fluctuations in magnetic fields.

 

Could or have other experiments been done? such as flying north south to discount any magnetic field effects or floating a clock stationery at 10,000ft.

 

Is there any other clock type that could be developed to show the effect isn’t only relative to atomic clocks?

 

Practically, nothing would change, even if the effect is only mechanical, we would still need the relativity equations to allow the atomic clocks to be of use, all that changes is our reasoning behind it.

 

GPS satellites confirm this continually, as do clocks on other satellites of differing orbits, which are subject to different conditions.

 

Other (non-atomic) clocks are simply not precise enough to show any effects, but it has been demonstrated with different atomic and nuclear transitions.

[JohnF]

Since the Universe is expanding is it reasonable to assume that somewhere out there, in another Galaxy, there is a planetary system that is moving away from us at a very high velocity?

 

And if this is true, is it also correct to assume time is going slower there than it is here?

By slower I mean in absolute terms not just relative to us observing it.

[insane_alien]

By slower I mean in absolute terms not just relative to us observing it.

 

there is no absolute frame of reference. the only measurements possible are relative measures. thats the whole point of relativity.

[JohnF]

there is no absolute frame of reference. the only measurements possible are relative measures. thats the whole point of relativity.

 

That's what I thought. It's just the Twin Paradox always describes the return journey which makes it a little confusing.

 

So if you could place, in an instant, an atomic clock on such a distant planet and after a year or so retrieve it, in an instant, it would show the same time as any clock it had been synchronised to beforehand?

[Klaynos]

That's what I thought. It's just the Twin Paradox always describes the return journey which makes it a little confusing.

 

So if you could place, in an instant, an atomic clock on such a distant planet and after a year or so retrieve it, in an instant, it would show the same time as any clock it had been synchronised to beforehand?

 

The problem here is you can't return it or place it in an instant. That makes the question unphysical.

[JohnF]

The problem here is you can't return it or place it in an instant. That makes the question unphysical.

 

I know it can't be done. But what if it could?

 

You can't accelerate from zero to 1000 mile per hour in an instant. But if you could it would take exactly 1 hour to travel 1000 miles. There are two sets of calculations; one for the acceleration and one for the constant speed.

 

So if you could place, in an instant, an atomic clock on such a distant planet and after a year or so retrieve it, in an instant, would it show the same time as any clock it had been synchronised to beforehand? And if not would it have gained or lost time?

[fredrik]

To not run into issues of unphysical assumptions, one thing that is clear in a very general sense.

 

1) Take two, from your point of view, as identical clock devices as possible.

 

2) Synchronize them.

 

3) Then separate the clocks, and simply let the two clock devices exists under non-identical conditions (in a wide sense). Sending the clock away can also be thought of as a transformation of the clock. Because technically it's not the same clock in the information theoretic sense.

 

4) Then after a while collect the clock devices and bring them back to one place for comparasion.

 

In the "general case" these two clocks will now be out of sync, which can be expected because they have evolve in different environments. But the exact amount will depend on the exact differences between the two clocks. Ie. a clock in motion is clearly different from a clock at rest, because you will in general not mix them up.

 

The cases where they agree should IMO be considered a special case, or exception, due to the symmetries of the transformations. The general conclusion is that the clock device measures the time relative to it's own references. If you send a clock on a journey, the general case is that they will disagree upon a later comparasion, except for cases where the symmetry in the journey.

 

/Fredrik

[Klaynos]

I know it can't be done. But what if it could?

 

You can't accelerate from zero to 1000 mile per hour in an instant. But if you could it would take exactly 1 hour to travel 1000 miles. There are two sets of calculations; one for the acceleration and one for the constant speed.

 

So if you could place, in an instant, an atomic clock on such a distant planet and after a year or so retrieve it, in an instant, would it show the same time as any clock it had been synchronised to beforehand? And if not would it have gained or lost time?

 

But if your assumption is unphysical any result you get will be meaningless and unphysical. But if you magiced it there it would depend on your magic I would imagine.

[JohnF]

The cases where they agree should IMO be considered a special case, or exception, due to the symmetries of the transformations. The general conclusion is that the clock device measures the time relative to it's own references. If you send a clock on a journey, the general case is that they will disagree upon a later comparasion, except for cases where the symmetry in the journey.

 

So the clock you send to the distant planet that is moving away from you will be slower than the one kept here when it's returned.

 

If you had swapped clocks with the other planet then swapped them back again, both clocks would be running slower than their synchronised counterparts that had been left at the origin. This would be because, as far as the control clocks were concerned, their travelling clocks would have been moving away from them at a significant speed for the period of separation.

 

What I'm trying to envisage here is just the effect of speed on the clocks without having to take in to account the journey and acceleration.

 

It is possible to think that on the distant planet time goes slower which would lead to the logical conclusion that from the point of view of the distant planet time here would go faster. But clearly this could not be the case. From both planets, time on the other planet would go slower.

[Klaynos]

So the clock you send to the distant planet that is moving away from you will be slower than the one kept here when it's returned.

 

If you had swapped clocks with the other planet then swapped them back again, both clocks would be running slower than their synchronised counterparts that had been left at the origin. This would be because, as far as the control clocks were concerned, their travelling clocks would have been moving away from them at a significant speed for the period of separation.

 

What I'm trying to envisage here is just the effect of speed on the clocks without having to take in to account the journey and acceleration.

 

It is possible to think that on the distant planet time goes slower which would lead to the logical conclusion that from the point of view of the distant planet time here would go faster. But clearly this could not be the case. From both planets, time on the other planet would go slower.

 

You must remember that because from the point of view of the distant planet you are moving and it is stationary. It's all relative...

[JohnF]

You must remember that because from the point of view of the distant planet you are moving and it is stationary. It's all relative...

 

I appreciate that, hence the reason for the instant transport of the atomic clocks. It gives a measure of the difference without relying on observable phenomenon; theoretically. If it was just a quirk of observation then it seems to be as valuable as understanding the doppler effect with sound.

[fredrik]

What I'm trying to envisage here is just the effect of speed on the clocks without having to take in to account the journey and acceleration.

 

I'm not sure if I missed your point here but we discussed this earlier in the thread we talked about muon decay. The observed half-life of "high speed muons" is longer. (But the half-life of the muon in it's own resting frame is always the same.)

 

There is nothing magic about the clock devices as such, so if you prefer, you can loosely think of decaying muons a moving "muon clocks".

 

Thus one can picture it so that speeding clocks "run slower", when compared _on the fly_ to a resting clock.

 

/Fredrik

[Mowgli]

The observed half-life of "high speed muons" is longer. (But the half-life of the muon in it's own resting frame is always the same.)

 

I wonder whether there is a trivial explanation for the longevity of high speed muons. The muon lifetime (half-life) is computed assuming that the measured decay time is dilated according to SR time dilation. Thus, the computed half-life is significantly smaller than the measured decay times.

 

When the atmospheric muons in cosmic rays are studied, their half-life is assumed to be this smaller value computed in accordance with SR. When we see that they do reach sea level, we realize that their decay time is much longer than the computed lifetime. In other words, the decay time appears dilated. We then present this as evidence that SR is correct. This cyclic dependency in the “evidence” in favor of SR is not usually highlighted.

[swansont]

I wonder whether there is a trivial explanation for the longevity of high speed muons. The muon lifetime (half-life) is computed assuming that the measured decay time is dilated according to SR time dilation. Thus, the computed half-life is significantly smaller than the measured decay times.

 

When the atmospheric muons in cosmic rays are studied, their half-life is assumed to be this smaller value computed in accordance with SR. When we see that they do reach sea level, we realize that their decay time is much longer than the computed lifetime. In other words, the decay time appears dilated. We then present this as evidence that SR is correct. This cyclic dependency in the “evidence” in favor of SR is not usually highlighted.

 

What's cyclic about it? The half-life in our frame is longer when they are moving than when they are at rest. That observation is independent of SR.

[insane_alien]

correct me if i'm wrong here but don't muons have a mean-life instead of a half-life? i'm pretty sure i read something about the detection of muons at groundlevel (something: that old 100% reliable source ;P) where the muons had an average lifetime. which isn't the same as a half life.

[timo]

Lifetime and half-life time are two different measurements of the same thing, namely how fast something decays. A decay follows [math] N(t) = N_0 \, \exp (-\kappa t) [/math], with [math]\kappa > 0 [/math]. Lifetime is [math] \tau = \frac{1}{\kappa} [/math], half-time is the time for which [math]N(t) = N_0/2[/math], i.e. the solution for t of [math] \frac{N_0}{2} = N_0 \exp (-\kappa t) [/math].

 

Iow, both quantities aren´t exactly the same but they both are different expressions of the same thing (kappa).

[Mowgli]

What's cyclic about it? The half-life in our frame is longer when they are moving than when they are at rest. That observation is independent of SR.

 

Okay, let's say the decay time of the muon measured in the lab frame is Tm. We would conclude that the decay time in the muon rest frame is a smaller value tm because of SR time dilation. We measure a large number of tm's and take the half life as the muon lifetime, say tau_m. But, if we had taken the distribution of Tm's and fitted an exponential, we would've gotten a larger lifetime, say Tau_m.

 

We then go and look muon decay in a cosmic rays and find that decay time is Dm (say). We argue that the fraction of a muon lasting Dm is too small using a lifetime tau_m and so the decay time must be dilated. But, if we use the larger lifetime Tau_m, the slow atmospheric deay may not look improbable.

 

So, the cyclicity is that while measuring and veryfying the lifetime, we make the SR correction and call it a proof. Of course, if you can measure the decay time of muons that are at rest in our frame, (and get a lifetime equal to tau_m), we could really call it a proof of SR. The point is that muons are never at rest.

[swansont]

Okay, let's say the decay time of the muon measured in the lab frame is Tm. We would conclude that the decay time in the muon rest frame is a smaller value tm because of SR time dilation. We measure a large number of tm's and take the half life as the muon lifetime, say tau_m. But, if we had taken the distribution of Tm's and fitted an exponential, we would've gotten a larger lifetime, say Tau_m.

 

We then go and look muon decay in a cosmic rays and find that decay time is Dm (say). We argue that the fraction of a muon lasting Dm is too small using a lifetime tau_m and so the decay time must be dilated. But, if we use the larger lifetime Tau_m, the slow atmospheric deay may not look improbable.

 

So, the cyclicity is that while measuring and veryfying the lifetime, we make the SR correction and call it a proof. Of course, if you can measure the decay time of muons that are at rest in our frame, (and get a lifetime equal to tau_m), we could really call it a proof of SR. The point is that muons are never at rest.

 

 

It's not cyclic, since the lifetime will be dilated by a factor gamma that has a speed dependence, i.e you can make a specific prediction and measurement. A lifetime increase that was e.g. half as big or twice as big would falsify, not support, SR.

 

And the muon lifetime has indeed been measured for muons brought to rest by capture in a plastic scintillator.

http://web.mit.edu/c_hill/www/muons_paper.pdf

[Mowgli]

And the muon lifetime has indeed been measured for muons brought to rest by capture in a plastic scintillator.

http://web.mit.edu/c_hill/www/muons_paper.pdf

 

Thanks for the article. I read it rather carefully. I see some issues:

1. The lifetimes fitted using two different fitting methods were off by more than 7 sigma, indicating that some systematic errors were not taken into account.
   
2. The muon decay time is defined from the time the capture in the scintillator rather than the pion decay generating the muon. One could use the stationarity property of exponential distributions to argue that the lifetime measurement is independent of the shifts in the time origin, which would work if the shifts were constant.
   
3. Is it obvious that the cross section of charged current decay is independent of the presence of dense matter as in the scintillator?
   

Besides, one sentence in the article bothers me: "Following our calibration, we checked that the high voltage supply and discriminator settings were fixed such that the observed count rate agreed to a good approximation with our theoretical prediction for the rate of muon decay events in the cylinder." To me, it sounds like they tweaked the apparatus to match the expected rate. I should check with the author what it really means.