Is time dilation a real effect?

[cianfa72]

TL;DR Summary

About time dilation effect: is it real or not ?

In the context of SR we know time dilation is basically the rate of change of moving clock's proper time w.r.t. the inertial frame coordinate time. It does mean that in principle the "tick events" of moving clock are compared against the time measurement of clocks Einstein's synchronizated in the "rest" inertial frame that happen to be at that "tick event" location.

Therefore basically time dilation is a "coordinate effect/artifact" and from this point of view the fact that it is a symmetric effect is not a problem at all.

So my impression is that time dilation is not actually "real" alike "differential aging" that is instead a real effect (take for instance the twin paradox).

What do you think about ? Thanks.

 

[anuttarasammyak]

So my impression is that time dilation is not actually "real" alike "differential aging" that is instead a real effect (take for instance the twin paradox).

I am not sure of the defiition of these "" words but I would like to draw your attention that SR and GR time dulation effect is well considered in designing GPS and other technologies.

 

[Orodruin]

What is your definition of "real"?

Time dilation is most certainly real as a statement about Einstein synchronized clocks.

 

[cianfa72]

What is your definition of "real"?

Time dilation is most certainly real as a statement about Einstein synchronized clocks.

My point is that for time dilation one does not compare the same clocks at same "spacetime points/events"; in principle one compares one clock (the moving one) against a "set" of Einstein's synchronizated clocks in the "rest frame".

 

[Grinkle]

One considers two events and then calculates the proper time of our reference observer that elapsed between those two events. The proper time for the reference observer will be the same no matter which other observer in some other frame calculates it. Its real - in what sense could it not be real?

 

[cianfa72]

The proper time for the reference observer will be the same no matter which other observer in some other frame calculates it. Its real - in what sense could it not be real?

Yes, the proper time along the spacetime path taken from that observer (i.e. the elapsed time of its own wristwatch) is real (all the frames calculate the same invariant value). The point is that the time dilation, instead, is not frame invariant.

 

[Grinkle]

If you want to say that only frame invariant quantities are real, imo that is semantics and I won't argue with you, I'll just agree to dis-agree on that being a useful definition of real.

 

[anuttarasammyak]

My point is that for time dilation one does not compare the same clocks at same "spacetime points/events"; in principle one compares one clock (the moving one) against a "set" of Einstein's synchronizated clocks in the "rest frame".

A counter example is the twin paradox. Twin brothers check their watches at start and at goal together. They observe that Earth brother's watch have ticked more than the pilot brother's.

 

[Lord Jestocost]

TL;DR Summary: About time dilation effect: is it real or not ?

In the context of SR we know time dilation is basically the rate of change of moving clock proper time w.r.t. the inertial frame coordinate time. It does mean that in principle the "tick events" of moving clock are confronted against the time measurement of clocks Einstein's synchronizated in the "rest" inertial frame that happen to be at that "tick event" location.

Therefore basically time dilation is a "coordinate effect/artifact" and from this point of view the fact that it is a symmetric effect is not a problem at all.

So my impression is that time dilation is not actually "real" alike "differential aging" that is instead a real effect (take for instance the twin paradox).

What do you think about ? Thanks.

The physical relationships can sometimes be better understood by thinking about experiments to confirm this prediction of the special theory of relativity.
For example, the muon experiments: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/muon.html#c1

 

[cianfa72]

A counter example is the twin paradox. Twin brothers compare their watches at start and at goal. They will see that Earth brother's watch have ticked more that the pilot brother's.

Yes, in that case the specific term to use is "differential aging".

 

[Dale]

As others have mentioned, you have to decide what “real” means for your question to have meaning. The term “real” in this context is not a scientific term, it is philosophical and in the philosophical literature there is (of course) some debate.

I prefer not to make a statement about whether or not it is “real” until someone explicitly gives their definition of “real”.

However, I will note that while time dilation is coordinate dependent, so is energy. If that is your basis for excluding time dilation as “real”, then energy is likewise excluded.

 

[cianfa72]

However, I will note that while time dilation is coordinate dependent, so is energy. If that is your basis for excluding time dilation as “real”, then energy is likewise excluded.

Ok, I got your point.

On the other hand, as @anuttarasammyak said GPS takes in account the time dilation between GPS's satellite onboard clocks and the time as measured by clocks here on the Earth.

 

[Ibix]

I agree that I don't think "real" is a useful word here.

You can say the following frame-invariant things:

Clocks A and B are at rest with respect to each other and moving inertially. They each directly observe the other to have the same lag compared to their own time (i.e., they're Einstein synchronised).

Clocks C and D are also Einstein synchronised, but have a different velocity to A and B.

If clocks A and C read 0 when they meet, clock C will read less time than clock B when they meet. Similar remarks apply to A when it meets C and D.

This is time dilation, in coordinate free language.

 

[Ibix]

On the other hand, as @anuttarasammyak said GPS takes in account the time dilation between GPS's satellite onboard clocks and the time as measured by clocks here on the Earth.

This is coordinate independent because there's a closed path - your clock, the satellite clock, and two null paths connecting them. What you are saying is that the proper time between emission events on the satellite is different from the proper time between reception events at your clock.

 

[cianfa72]

If clocks A and C read 0 when they meet, clock C will read less time than clock B when they meet. Similar remarks apply to A when it meets C and D

Yes, that was my point. Time dilation is symmetric (unlike differential aging).

 

[cianfa72]

This is coordinate independent because there's a closed path - your clock, the satellite clock, and two null paths connecting them. What you are saying is that the proper time between emission events on the satellite is different from the proper time between reception events at your clock.

Ah ok, so the above is invariant (i.e. coordinate independent). Is it a sort of "gravitational time dilation" ?

 

[Dale]

On the other hand, as @anuttarasammyak said GPS takes in account the time dilation between GPS's satellite onboard clocks and the time as measured by clocks here on the Earth.

That is correct. The GPS defines coordinates (that is its purpose), so it accounts for time dilation with respect to those coordinates.

 

[cianfa72]

That is correct. The GPS defines coordinates (that is its purpose), so it accounts for time dilation with respect to those coordinates.

Not sure whether it is the same thing @Ibix said in #14.

 

[Vanadium 50]

Two meta-points:

1. What is "real" is philosophy. Not physics. Is anything real? Maybe we're all just brains in vats.

2, OP, you sure are prolific. You write and write and write and write. A minute after someone posts and you have a wordy reply. And yet you are still confused. Perhaps you would make more progress if you spent more time reading what people are saying to you before jumping in with a reply.

 

[cianfa72]

Sorry, I read the answers. However sometimes things aren't entirely clear to me, also because sometimes I'm in trouble with English.

 

[jbriggs444]

Ah ok, so the above is invariant (i.e. coordinate independent). Is it a sort of "gravitational time dilation" ?

The result of any well defined measurement procedure is invariant. This particular number (the elapsed proper time between the receipt of two consecutive time-stamped signals from a particular satellite) is the result of a well defined measurement procedure.

That does not make it an example of "gravitational time dilation".

However, one can consider the observed discrepancy between the difference in the time stamps compared with the elapsed proper time and (approximately) factor that discrepancy into a kinematic component due to special relativity (relativistic doppler) and another component from general relativity (gravitational time dilation).

In that approximate sense, one can think of part of the discrepancy as being from gravitational time dilation.

However, what is "real" (invariant) is the discrepancy. What is not real is the arbitrary and somewhat artificial division of the discrepancy into two separate components. That is coordinate dependent.

 

[Mister T]

Yes, in that case the specific term to use is "differential aging".

Which, as opposed to time dilation, is invariant.

Does that distinction mean time dilation is not real? I remember as a student the profound effect the muon observations had on me.

The physical relationships can sometimes be better understood by thinking about experiments to confirm this prediction of the special theory of relativity.
For example, the muon experiments: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/muon.html#c1

I now understand the reason it was profound. The muon count is invariant. So time dilation, while not invariant, has invariant consequences. To me, that makes it as real as you can get.

 

[FactChecker]

My point is that for time dilation one does not compare the same clocks at same "spacetime points/events"; in principle one compares one clock (the moving one) against a "set" of Einstein's synchronizated clocks in the "rest frame".

It compares clocks in the most direct way possible in the circumstances.
It is very real, in the sense that if a "stationary" observer wants to understand what is really going on in a relatively moving frame, he must take time dilation into account. Consider the following example.

In high-energy charged particle generators, the charged particles are accelerated to tremendous speeds. In the charged particle's reference frame, they exist for the normal length of time. But in the "stationary" reference frame, they exist longer than that and can bombard targets that are farther away than they could be if the charged particles only existed for their normal "stationary" times. That is a very real effect.

 

[Renato Iraldi]

We don't have a definition of what real means, so I'll give a definition and then discuss whether time dilation is a real phenomenon or not.
A phenomenon is real if it is described in the same way for all possible observers, with the observer also understanding an object that feels the influence of the phenomenon.
Time dilation does not meet this definition of reality. This is why some consider time dilation to have the same reality as lens aberrations.
Other definitions of reality may include time dilation. But they could be more shocking

 

[PeterDonis]

A phenomenon is real if it is described in the same way for all possible observers, with the observer also understanding an object that feels the influence of the phenomenon.
Time dilation does not meet this definition of reality.

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?

 

[pervect]

TL;DR Summary: About time dilation effect: is it real or not ?

In the context of SR we know time dilation is basically the rate of change of moving clock's proper time w.r.t. the inertial frame coordinate time. It does mean that in principle the "tick events" of moving clock are compared against the time measurement of clocks Einstein's synchronizated in the "rest" inertial frame that happen to be at that "tick event" location.

Therefore basically time dilation is a "coordinate effect/artifact" and from this point of view the fact that it is a symmetric effect is not a problem at all.

So my impression is that time dilation is not actually "real" alike "differential aging" that is instead a real effect (take for instance the twin paradox).

What do you think about ? Thanks.

This caught my eye - I haven't read the other responses in depth. My basic response is that the definition of "real effect" is inherently a bit fuzzy, because of the imprecision of words and various philosophies of "what is real".

I often take "real" to mean "coordinate independent", and from the wording in your post, your thoughts run along similar lines to mine, but I am sure to many others (including casual readers), "real" means something different.

Certainly, time dilation is not the name of any tensor. It can be a piece of a tensor (the zero component of the 4-velocity, for instance), but it's not a tensor in and of itself.

My personal view is that the "real" effect is called "differential aging" rather than "time dilation", because differential aging is a coordinate-independent statement. Time dilation fails to be a tensor because it doesn't have enough information to be coordinate independent. Differential aging is the ratio of two tensors (the proper time of two different world lines that begin and end at the same two events), so it does have enough information to be independent of coordinates. You do, of course, have to specify the world lines in question to compute differential aging, but once you have this information coordinates don't matter.

 

[FactChecker]

There are innumerable "real" things that are so related to time, and time only, that I would consider time to be real. Then, when a relatively moving IRF measures those coordinate-independent events and indicates a different time, I would consider the difference to be "real".

 

[cianfa72]

The muon count is invariant. So time dilation, while not invariant, has invariant consequences. To me, that makes it as real as you can get.

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

 

[PeterDonis]

time dilation, while not invariant, has invariant consequences.

I would not put it that way. 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.

 

[Grinkle]

A phenomenon is real if it is described in the same way for all possible observers,

Time dilation does not meet this definition of reality.

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?

with the observer also understanding an object that feels the influence of the phenomenon.

 

[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.

 

[Mister T]

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.

 

[Mister T]

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.