What is the cause of time dilation in Special Relativity?

[giulio_hep]

I answered your request for clarification of Nugatory's answer. This is the relativity forum and I'm 99.9 % sure that Nugatory based his answer on the relativity postulate:
We cannot detect absolute motion, the laws of physics are the same wrt any inertial frame.

The works of Lorentz and Einstein at the start of the 20th century focussed on EM but the relativity principle would be broken if for example a radioactivity clock would behave differently so that it does not work in accordance with the Lorentz transformations.

Also ZapperZ at #10 mentions the relativistically covariant form of Maxwell Equations, in my understanding as the theoretical reason for the invariance of c.
In Susskind's words [link to youtube lecture at that minute]:
the principle of Relativity is that the laws of physics are the same in every reference frame, that principle existed before Einstein and it was not invented by Einstein, Einstein added one law of physics [look also at the wikipedia paragraph], that the speed of light is c ...

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By the way, also in modern mathematics the idea of Special Relativity is associated to "the Lorentz force exerted by the electromagnetic field on a charged particle as the contraction of that 2-form with the tangent vector of the trajectory of the particle", where "If one models the electromagnetic field via the Kaluza-Klein mechanism as a field of gravity on a fiber bundle, then trajectories of charged particles subject to the Lorentz force in the base space of that bundle are equivalently just geodesics on the total space"

 

[giulio_hep]

Speed is determined by the choice of a reference frame, and doesn’t imply any past acceleration.

But then notice the Newton-force law for charged relativitic particles: the Lorentz-force law. it says that the acceleration bivector v˙v of the relativistic particle equals the (electric component of) the curvature 2-form.

Also, a nonlocal special relativity theory has been developed in which nonlocality appears as the memory of past acceleration [6].
[6] B. Mashhoon, “Nonlocal Special Relativity”, Ann. Phys. (Berlin) 17, 705 (2008) [arXiv:
0805.2926 [gr-qc]];

Anyway, as I've written at the beginning, this part of my question is maybe the least important to me...

 

[Nugatory]

Things with mass travels at different speed. Trains travel for example, 100Kmh, cars at 40kmh, air planes at 1000kmh. But, do things without mass ALWAYS always travel at "this" universal speed limit?

Yes.

 

[Nugatory]

Ehm... what I think I've asked in my first posts in this thread is:
let's start from the covariance of EM fields and derive that a clock based on EM fields is subject to time dilation, why should a clock based on another force (is gravity an example?) measure such a dilation?

That question was answered in post #3 and many subsequent posts in this thread: although the development of relativity was motivated by the observed behavior of electromagnetic phenomena, the theory does not assign any special status to these phenomena; the first postulate applies to everything, not just Maxwell's equations.

This thread is closed.