I Is time dilation a real effect?

[Ibix]

Surely all time dilation exam / homework questions have a single correct answer that all people will agree to.

This is kind of the point I was trying to lay out in #13. Once you specify a frame, which clocks are time dilated is unambiguous. The problem with time dilation is that you do have to specify the frame: without that, there is no answer to "which clock is ticking faster". This is different from the twin paradox/differential aging case where there's a uniqe answer to who ends up older.

 

[Herman Trivilino]

I would put it that there are two different things being considered: time dilation, which is not invariant, and differential aging (as @pervect called it in post #26), which is. Differential aging is what has invariant consequences.

I was talking about the muon count being invariant, and that it's a consequence of time dilation.

 

[Herman Trivilino]

The count is invariant in the sense that any frame assign to it its proper time along the spacetime path being followed.

It's invariant in the sense that all observers will agree on its value, regardless of their state of motion.

 

[Grinkle]

The problem with time dilation is that you do have to specify the frame

The existence of frame-dependent time dilation and distance contraction is a feature of our universe. By saying something exists, one implies one thinks of it as real. Of course, you didn't say that, I did, and you may choose different phrasing than I did.

Perhaps hearkening back a bit to post 15, I have never seen a debate over whether Galilean relativity demonstrates that velocity is not real, and to me that seems a parallel line of reasoning to what is being discussed in this thread.

For me frame dependent quantities are quite real. That may be more an engineering perspective than a physicist perspective.

 

[jbriggs444]

I was talking about the muon count being invariant, and that it's a consequence of time dilation.

Or a consequence of the equally real or unreal length contraction. Or a consequence of the invariant space time interval between the creation events and the detection events.

 

[PeterDonis]

I was talking about the muon count being invariant, and that it's a consequence of time dilation.

If time dilation is not invariant, no invariant can be a consequence of it. That's why the distinction between "time dilation" (not invariant) and "differential aging" (invariant) is important.

 

[cianfa72]

Once you specify a frame, which clocks are time dilated is unambiguous.

Yes, they are the moving clocks w.r.t. the given inertial frame (i.e. w.r.t. the set of clocks at rest each other and in the given frame and Einstein's synchronizated in that inertial frame).

 

[Jaaanosik]

Time dilation can be calculated through electromagnetic field and its transformation between the inertial frames.
It can be shown that Lorentz forces acting on a charge have different magnitude in different inertial frames that have a relative velocity between the frames.
For example, an electron moving between cathode and anode plates, a cavity, can be considered as a clock.
The time dilation of the electron flight follows from the transformation of Maxwell equations as done by Einstein in his 1905 paper.

 

[Renato Iraldi]

I don't see what you mean. Surely all time dilation exam / homework questions have a single correct answer that all people will agree to. The amount of time dilation that will be reported by two observers looking at each other will be described the same way by anyone who is asked to describe that time dilation and has the information about the observers needed to do so.


I don't know what you mean by this - perhaps that is why I am not getting your point - please clarify?

My idea is difficult to express, what I think is that the measurement of a property that is real cannot be modified by the presence of a reference system that does not have any interactive phenomenon with it.
For example, we are on a train and we see the station clocks. The train accelerates and now from the train we see that the station clocks dilate, they go slower, I think that this dilation of time is not real, what is real is that the speed of the train has changed the observer.
In the same way that if I have kinetic energy and I get into a car at a certain speed, it is not the real thing that the energy has changed. What is real is that the observer has changed.

 

[Renato Iraldi]

As was pointed out earlier, energy does not meet this criterion of reality either. Neither does momentum. That means that, for example, if a car hits you and injures you, its energy and momentum that injured you were frame-dependent, therefore they weren't real. Does that seem reasonable to you?

As was pointed out earlier, energy does not meet this criterion of reality either. Neither does momentum. That means that, for example, if a car hits you and injures you, its energy and momentum that injured you were frame-dependent, therefore they weren't real. Does that seem reasonable to you?

You're right, I was confused. What I mean is that if a car has kinetic energy. I get on a train and go at a higher speed, it is not real that the kinetic energy of the car has changed but it is the observer who has changed.

 

[Grinkle]

In the same way that if I have kinetic energy and I get into a car at a certain speed, it is not the real thing that the energy has changed. What is real is that the observer has changed.

Thanks for taking to time to expand on this.

I think you would agree that whether the car has changed or the observer has changed is a matter of picking a reference frame. How can you justify saying it is real that the observer has changed? There are no preferred frames in our universe.

 

[Vanadium 50]

If time dilation is not invariant, no invariant can be a consequence of it.

I don't understand that. p^\mu is clearly not invariant. It transforms as a vector. But p^\mu p_\mu is a consequence of it and is invariant.

 

[Jaaanosik]

...
I think you would agree that whether the car has changed or the observer has changed is a matter of picking a reference frame. How can you justify saying it is real that the observer has changed? There are no preferred frames in our universe.

Acceleration, change of observer.
Changing observer is like jumping from one inertial frame to another, the idea of momentarily comoving inertial observers.
The jumping observer leaves photons behind that cannot reach him.
The jumping observer will be hit with new incoming photons that were not in his view before.
That's real.

 

[Orodruin]

You're right, I was confused. What I mean is that if a car has kinetic energy. I get on a train and go at a higher speed, it is not real that the kinetic energy of the car has changed but it is the observer who has changed.

Energy is conserved in an inertial frame. It is not an invariant that is the same in all frames. This is true in special relativity as well as in classical mechanics.

 

[Herman Trivilino]

If time dilation is not invariant, no invariant can be a consequence of it.

Interesting. So the invariant muon count is not a consequence of time dilation. Rather, time dilation is something, among other things, that can be used as part of an explanation of the muon count.

 

[Grinkle]

Acceleration

Yes, thanks - @Renato Iraldi is not talking about inertial frames here.

 

[PeterDonis]

What I mean is that if a car has kinetic energy. I get on a train and go at a higher speed, it is not real that the kinetic energy of the car has changed but it is the observer who has changed.

The kinetic energy of the car relative to you has certainly changed--and that will make a real difference to you if the car hits you.

 

[PeterDonis]

I don't understand that. p^\mu is clearly not invariant. It transforms as a vector. But p^\mu p_\mu is a consequence of it and is invariant.

If we want to be more precise, we could say that Lorentz scalars are "invariant" while vectors, tensors, etc. that can be contracted to obtain Lorentz scalars are "covariant".

Time dilation, however, is neither.

 

[PeterDonis]

So the invariant muon count is not a consequence of time dilation.

That is what I would say, yes.

Rather, time dilation is something, among other things, that can be used as part of an explanation of the muon count.

No, because you can't use A as part of an explanation of B if B is not a consequence of A. The explanation of the muon count has to be made in terms of invariants.

 

[Jaaanosik]

No, because you can't use A as part of an explanation of B if B is not a consequence of A. The explanation of the muon count has to be made in terms of invariants.

Time dilation is a factor in explanation of muon count.
There could be 10 muons created in the atmosphere but we might count only 8 at the Earth surface.
So B - muon count on the Earth surface is partly consequence of A - time dilation.
Two muons might have smaller speed and they will not make it down.

 

[PeterDonis]

Time dilation is a factor in explanation of muon count.

No, it isn't, because, as has been pointed out, it's not an invariant.

What explains the muon count is the proper time along the muon worldline, which is an invariant, compared with the muon half-life, which is just a physical constant.

Two muons might have smaller speed and they will not make it down.

That is not why some muons don't make it. Some muons don't make it because muons decay; they have a finite half-life. There is no difference in speed between the muons.

The actual issue is not that some muons don't make it; it's that more muons make it than "naive" Earth observers who don't take relativity into account would expect. That aspect is where "time dilation" is usually invoked as an explanation, but "time dilation" is not an invariant. As @pervect pointed out in an earlier post, a better name for the actual invariant involved would be "differential aging".

 

[Orodruin]

Time dilation is a factor in explanation of muon count.
There could be 10 muons created in the atmosphere but we might count only 8 at the Earth surface.
So B - muon count on the Earth surface is partly consequence of A - time dilation.
Two muons might have smaller speed and they will not make it down.

You are missing the point made. The point is that time dilation is not the physically invariant explanation. If you go to the muon rest frame, it plays no part whatsoever in the explanation. The invariant explanation refers to the invariant proper time of the muons.

 

[Jaaanosik]

Let us assume there is no time dilation.
How many muons we would count?

 

[PeterDonis]

Let us assume there is no time dilation.

Making an assumption that is contrary to the laws of physics is pointless. If you have a contradictory system you can deduce anything.

 

[PeterDonis]

Thread closed.