Why Is Time Dilation Considered a Real Phenomenon?

[russ_watters]

And so, I want to please start by asking for a brief explanation of why anyone thinks that time dilation is real.

I see a lot of long answers and some irritation, but I haven't read them all, so I don't see if anyone tried the simple answer:

We know time dilation is real because it is observed to happen. It's really that simple.

That's a very different question from asking how or why it happens.

 

[robphy]

What about claims which have mountains of experimental evidence confirming them but which don't make logical sense to you? That appears to be the category into which relativity falls.

Speaking of mountains... Mt. Washington...

Time Dilation : An Experiment With Mu - Mesons (1962)
(D. Frisch and J. Smith)


http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/muonex.html
http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/muon.html#c1(Related and possibly enlightening, but probably not at the appropriate level for the OP,
my calculation and spacetime diagram of the muon experiment
https://physics.stackexchange.com/a/325582/148184 )

This might be more appropriate for the OP:
Bondi's Relativity and Common Sense
https://archive.org/details/relativitycommon0000bond

 

[Sagittarius A-Star]

Can you please explain exactly why GPS won't work?

Please see chapter "2.4.2. Relativistic Corrections" on page 22 and the related appendix F "Brief Derivation of Relativistic Corrections" on page 127 in:
https://www.ptb.de/cms/fileadmin/in...d_frequenz/pdf/Feldmann_2011_Dissertation.pdf

 

[robphy]

This may be useful:
Neil Ashby - Relativity in the Global Positioning System
https://www.aapt.org/doorway/TGRUTalks/Ashby/AshbyTalk1of6.htm
from
AAPT Topical Workshop:
Teaching General Relativity to Undergraduates
Syracuse University, July 20-21, 2006
https://www.aapt.org/doorway/TGRU/

 

[Dale]

Claims that have both experimental evidence and that makes logical sense are best. Claims for which experimental evidence may still be lacking but which makes logical sense comes a close second.

Looks like I missed a bunch of posts somehow.

SR makes logical sense. That is guaranteed by using a coherent mathematical framework. All of relativity boils down to the geometry of spacetime which can be written (globally for SR and locally for GR) as $ds^2=-c^2 dt^2 + dx^2 + dy^2 + dz^2$. Everything about SR can be derived from this one equation, and this equation is just as logical as the Pythagorean theorem. It simply describes the geometry of spacetime.

For your question specifically, time dilation can easily be derived from the above formula. The proper time is given by $c^2 d\tau^2=-ds^2$ and time dilation is $\gamma=\frac{dt}{d\tau}$. A few lines of algebra can derive the usual expression for $\gamma$

SR is also supported by a mountain of experimental evidence. My favorite resource for that is

https://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

In particular, see section 4 regarding your specific question on time dilation.

So SR in general and time dilation in specific is both logical and well supported by evidence, thus meeting your stated criteria.

By the way, I completely disagree that logical sense is subjective. If something is logical then it can be mathematically proven. The existence of a mathematical proof is not subjective. This is precisely why the mathematical framework is so important. It avoids the issue that happened here where you confused “makes intuitive sense” with “makes logical sense”. The math guarantees it is logical. The intuition has to be developed.

 

[Sunil]

And so, I want to please start by asking for a brief explanation of why anyone thinks that time dilation is real.

First, let's start with a quite common phrase: "Time is what clocks show". Feel free not to believe this phrase, thinking that time is something different. No problem. Let's use a different name. Instead of time, clock time. "Clock time is what clocks show" is already a triviality. Not?

And then translate all the claims about time you have heard in the context of relativity in this way, replace "time" with "clock time".

After this translation, the answer is quite simple: Clock time dilation is real because real clocks show it. All you need for this is to compare sufficiently accurate clocks.

 

[jbriggs444]

First, let's start with a quite common phrase: "Time is what clocks show". Feel free not to believe this phrase, thinking that time is something different. No problem. Let's use a different name. Instead of time, clock time. "Clock time is what clocks show" is already a triviality. Not?

And then translate all the claims about time you have heard in the context of relativity in this way, replace "time" with "clock time".

After this translation, the answer is quite simple: Clock time dilation is real because real clocks show it. All you need for this is to compare sufficiently accurate clocks.

We all agree that the clocks are real. But is the comparison real? One can make a strong argument that it is not.

 

[Sunil]

We all agree that the clocks are real. But is the comparison real? One can make a strong argument that it is not.

I'm not an experimenter. So I cannot judge about this. Of course, the comparison of the clocks has to be sufficiently accurate too. I simply assume that the experimenters know what to do with their devices and don't tell us fairy tales.

 

[jbriggs444]

I'm not an experimenter. So I cannot judge about this.

Let me explain it for you then. It is not a matter of experimental precision. It goes deeper than that.

In order to compare the relative tick rates of two clocks that are at some distance from another you need a standard of synchronization. You have to have some way of saying that "this tick over here happened at the same time as this tick over there".

But there is no unambiguous choice for a correct standard of synchronization.

As long as we are working within the flat space-time of special relativity, there is a natural choice of a synchronization standard: Einstein synchronization.

Einstein synchronization works by first picking a standard of rest. A frame of reference. You (in your imagination at least) set up a grid of identical, properly functioning clocks all at rest in this frame. You adjust these clocks so that a light signal from anyone clock to another is seen to travel at the speed of light. That is, if you subtract transmission time from reception time and multiply by the speed of light, you'll get the distance between the two clocks. If you turn around and send a return signal, you'll see the same.

If you pick a different standard of rest and look at this same grid of synchronized clocks, you'll see that they cannot possibly be correctly synchronized. From this viewpoint, the grid is moving. Light takes longer to go from a "downstream" grid member to an "upstream" grid member because it has to do a stern chase and is covering a greater distance.

So there is a different standard of synchronization for a different standard of rest. [Einstein's train experiment is another way to achieve this same insight]

You cannot call an effect "real" if it depends on your choice of reference frame. Many physicists reserve the adjective "real" to refer to things that are "invariant" -- quantities that are the same no matter what frame of reference you choose.

 

[mfb]

You cannot call an effect "real" if it depends on your choice of reference frame. Many physicists reserve the adjective "real" to refer to things that are "invariant" -- quantities that are the same no matter what frame of reference you choose.

Time dilation exists in the view of every inertial reference frame (and with a suitable definition also in non-inertial reference frames). The existence of the phenomenon does not depend on the reference frame.

Clock synchronization is trivial if the clocks are at the same place at the same time - and you can show experimentally that clocks can show a different time if they are put together again later.

 

[PeroK]

You cannot call an effect "real" if it depends on your choice of reference frame. Many physicists reserve the adjective "real" to refer to things that are "invariant" -- quantities that are the same no matter what frame of reference you choose.

That's not the meaning of "real" that was under question. From the OP:

And so, I want to please start by asking for a brief explanation of why anyone thinks that time dilation is real. You are welcome to provide either a purely theoretical explanation like a thought experiment, or links to peer-reviewed actual experiments that prove that time dilation is in fact real. Just saying 'otherwise satellites would not work' is not an adequate answer, I would need you to explain exactly why that is the case.

This was a question of whether anything other than absolute Newtonian time could be a valid physical theory.

 

[jbriggs444]

Time dilation exists in the view of every inertial reference frame (and with a suitable definition also in non-inertial reference frames). The existence of the phenomenon does not depend on the reference frame.

Clock synchronization is trivial if the clocks are at the same place at the same time - and you can show experimentally that clocks can show a different time if they are put together again later.

I get what you are saying and do not disagree. There is certainly something real going on.

Time dilation is as "real" as the fact that measuring the x extent of a ruler on a piece of graph paper depends on the angle of the graph paper. And as "unreal" as the fact that the ruler's length does not change as a result of the rotation.

 

[Sunil]

Let me explain it for you then. It is not a matter of experimental precision. It goes deeper than that.

In order to compare the relative tick rates of two clocks that are at some distance from another you need a standard of synchronization.

Yes, but there is no need for this. You don't have to compare tick rates, and you don't have to compare clocks at different places. Start at the same place, move one clock around, the other not, and make the final comparison again at the same place. The two clock times will be different.

Once you have done such experiments, you already know certainly that clock time depends on the path of the clock and that this is a real effect. And you can forget about Einstein synchronization, once it is not necessary to see this.

 

[jbriggs444]

Yes, but there is no need for this. You don't have to compare tick rates, and you don't have to compare clocks at different places. Start at the same place, move one clock around, the other not, and make the final comparison again at the same place. The two clock times will be different.

That is "differential aging", not "time dilation". Yes, differential aging is unambiguously real and has no requirement for a synchronization convention.

Time dilation is a coordinate effect. Differential aging is invariant.

 

[Vanadium 50]

It may be worth pointing out that the OP hasn't been back for a week.

 

[Sunil]

That is "differential aging", not "time dilation". Differential aging is unambiguously real.

Time dilation is a coordinate effect. Differential aging is invariant.

Nobody cares I don't care how you name it. Of course, all what you can measure is in independent of the choice of coordinates. If you want to name what can be measured with a different phrase, your choice. (Essentially I have made in my first post something similar, except that I have proposed to use "clock time dilation" instead of "time dilation", to cut away all that philosophy of time which often confuses people.) In this case, you restrict "time dilation" to some metaphysical claims of no interest for physics.

 

[weirdoguy]

Nobody cares about how you name it.

Actually most of us here do care.

 

[phinds]

Nobody cares about how you name it. Of course, all what you can measure is in independent of the choice of coordinates. If you want to name what can be measured with a different phrase, your choice. (Essentially I have made in my first post something similar, except that I have proposed to use "clock time dilation" instead of "time dilation", to cut away all that philosophy of time which often confuses people.) In this case, you restrict "time dilation" to some metaphysical claims of no interest for physics.

All nonsense

 

[Sunil]

All nonsense

Feel free to correct what you think is wrong.

Actually most of us here do care.

Ok, if you think jbriggs444 is such an authority that one has to care about his proposals how to name something, feel free to care and to follow his recommendation.

 

[phinds]

Feel free to correct what you think is wrong.

As has already been pointed out, your premise (time dilation = differential aging) is wrong, so unsurprisingly your conclusions are wrong.

 

[jbriggs444]

Feel free to correct what you think is wrong.

Ok, if you think jbriggs444 is such an authority that one has to care about his proposals how to name something, feel free to care and to follow his recommendation.

I am not an authority deciding these things by fiat. I have learned the conventional phraseology just like everyone else here. Word meaning is, of course, a matter of convention rather than a matter of physics.

A quick trip to Google pulls up this thread from 2016 in these forums which is directly on point.

 

[aclaret]

time dilation is to diffeo age-ing, as aubergine is to moussaka

 

[vanhees71]

We all agree that the clocks are real. But is the comparison real? One can make a strong argument that it is not.

One also has to take into account that you can only compare the two clocks when they are at the same place. There's no way to compare clocks at distant places without additional assumptions on how the information of the clock readings is transferred from one place to the other.

 

[RandyD123]

For example, time dilation never happens to you, so is it real?

So if it NEVER happens to "you" then is it true that it only happens to the observer?

 

[jbriggs444]

So if it NEVER happens to "you" then is it true that it only happens to the observer?

Time dilation never "happens" to anyone. It is the ratio of elapsed time (also known as "proper time") for a person or clock to coordinate time for a particular coordinate system. Mathematically: $\frac{d \tau}{d t}$.

In special relativity, the term "observer" is often used as a metaphor for a coordinate system -- a coordinate system in which that observer is at rest at the origin.

 

[RandyD123]

Time dilation never "happens" to anyone.

But yet something has happened. I leave Earth at near the speed of light, my clock ticks one second per second. But when I return, my twin brother is now older than me. So either time slowed for me or sped up for him. But yet both our clocks ticked at one second per second. It could be said that I never really left the Earth and instead the Earth moved away from me. In either case there is a "real" age difference for someone. It's very confusing!

 

[vanhees71]

It's not confusing if you just remember that everything that's measurable is given by frame-independent quantities. In this case it's the proper time of both twins measured by each twin's clock traveling with them. They can synchronize there clocks when you start your journey from the place where you and your twin brother are together at one place and then you can compare the readings of your and your brother's clock when you meet again at the same place. The two clocks will show in general different readings, i.e., the twins aged differently, and that readings of the clocks do not depend on the reference frame one uses to describe it, because the proper times of the twins are frame-independent physical observables.

 

[Nugatory]

But yet something has happened. I leave Earth at near the speed of light, my clock ticks one second per second. But when I return, my twin brother is now older than me. So either time slowed for me or sped up for him. But yet both our clocks ticked at one second per second. It could be said that I never really left the Earth and instead the Earth moved away from me. In either case there is a "real" age difference for someone. It's very confusing!

This is the “twin paradox”, about which we have many threads, and it is not explained by or even very closely related to time dilation.

 

[Sunil]

I am not an authority deciding these things by fiat. I have learned the conventional phraseology just like everyone else here.

Ok, I have thought this phrase is your own invention, I had never heard it before (I have learned GR in Russian language). I have corrected the "nobody cares".

Whatever, when I know all about that differential aging, I know all about the physics of time dilation, all what the experiment can measure about it. So, if somebody has understood and accepted that differential aging as a physical effect, the most important, most essential part of relativistic physics is already understood.

The next step would be to accept that given that differential aging we have no chance to measure absolute time with clocks, even if it would exist. What remains is metaphysics. That was my idea answering the OP.

 

[phinds]

The next step would be to accept that given that differential aging we have no chance to measure absolute time with clocks, even if it would exist.

No, not exactly. "Absolute time" DOES exist at anyone place. It's called "proper time". The time you measure with your wristwatch is your proper time and holds regardless of what other observers think.

Differential aging occurs because the proper time of different travelers does not measure the same AMOUNT of time, because they are taking different paths through space-time.