Why Is Time Dilation Considered a Real Phenomenon?

[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.

 

[jbriggs444]

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.

I would say that the problem with absolute time has more to do with definition than with measurement.

The fact that the path length between two points on the Earth's surface depends on the path does not prevent us from measuring the distance between those two points with rulers. We just need to figure out a definition for a "straight path". [think "geodesic" or "shortest path on the surface"]

Nor does the ambiguity in path length to the equator or the prime meridian prevent us from measuring longitude and latitude with rulers once we have an agreement on placement for the equator and the prime meridian.

To bring this back around to General Relativity and the universe within which we live, I believe that it is possible, to define an "absolute time" for essentially every event in the universe based on the interval from the [imagined if necessary] initial singularity to the event in question. In principle, this "absolute time" would be measurable with clocks: the lowest clock reading on any clock that takes a geodesic path that starts in [some small region near] the initial singularity and ends at the target event. Sadly, I do not think this particular definition works. One could have a high speed trajectory to a black hole, a tight trajectory around the black hole and a high speed continuation to the target event, thus yielding an arbitrarily low "absolute time".

 

[PeterDonis]

At this point the OP question has been answered, the OP is long gone, and the only discussion happening is about words, not physics.

Thread closed.