# Relative simultaneity in Special Relativity: was it ever used in real world examples?

I'm working on researches in the area of Special and General Relativity (SR/GR) focused on time dilation and I have a question to pose. Lorentz transformations (LT) introduce a time transformation such that two events occurring at same time in one frame not being at same time in another frame (relative simultaneity).

A "special/singular" case for LT is when the clock in the frame at rest is also fixed. In this case the dependency on the position for transforming a delta time is dropped, and the conversion is only based on gamma (i.e. delta t' = gamma * delta t). This is the setup is fully equivalent to other relativity theories, based on absolute synchronization methods, and more specifically based on an absolute simultaneity. So this aspect is not a SR peculiarity anymore.

Looking at the SR literature for Earth and space time drift analysis real world experiments, all the models used are always based on the above "singular" case, i.e. an absolute-like frame is defined with a clock is at rest there. This allows to correlate delta times in different frames through coordinate vs. proper time comparisons, under certain assumptions for clock synchronization. But again it's not a specific feature of SR with respect to other theories! The same can be obtained with absolute frame & time models, with in some case much more easy and clear steps.

Does anyone know an example where the relative simultaneity is used for modeling a real experiment, i.e. where also the term -v*x/c^2 plays a role in the tranformation?

• The huge wall of text rambling about special relativity is not necessary. If you want a specific experiment where time-dilation is demonstrated see the muon lifetime observation. Aug 15, 2020 at 15:30
• Hi Charlie, thanks for the suggestion for question text editing. I've edited it removing non essential explanatory parts. Concerning the link, I knew that experiment already,. I've no doubt about the need for a relativistic effect in the model, but the on mentioned there is still another of the class of "singular" LT cases (i.e. absolute simultaneity like, no time transfer dependency on position). Aug 15, 2020 at 16:38

We investigate the speed and lifetime of cosmic-ray muons. The speed of cosmic-ray muons wasdetermined by measuring time-of-flight between parallel scintillator paddles for various separations

...

Relativistic kinematics was found to give a muchbetter fit, than Newtonian kinematics, between our experimental results and existing data on theenergies and momenta of cosmic-ray muons.

Actually all the analyses of the enormous amount of data in particle physics depend crucially on using relativistic kinematics.

• Hi Anna, than ks for the feedback and the link to the paper. It's interesting, but also that one still based on the "singular" LT form, i.e. τ = τ0 γ (no dependency on position). As said this relativistic model is compatible with different theories, not only with SR. I'm looking for a smoking gun for SR, with an time dilation experiment only solved by it. Aug 15, 2020 at 16:49
• @Gianni I have never heard of other kinematics fitting high energy particle physics data, so it is not necessary to separate time dilation from space contraction, they are taken care by the transformations used, in my opinion, and once the data is fitted , that is enough proof . Aug 15, 2020 at 18:06
• I fully support your point, but in those conditions it only proves that a class of relativistic theories (not only SR) is a valid model for high energy particle physics data analysis. Thanks anyway for the answer. Aug 15, 2020 at 20:15
• Only if they have the Lorenz transformation implicitly or explicitly, as GR .. Aug 16, 2020 at 3:57
• So far I'd rephrase in (... only if they have a relativistic kinematic time transformation implicitly or explicitly - not necessarily SR one - aside the GR based gravitational time drift part...) :-) Aug 16, 2020 at 9:28

I think you're asking for experiments in which there are two different physical reference frames, i.e., two arrays of Einstein-synchronized clocks in relative motion. The muon experiment doesn't count because there are no sets of comoving muons with Einstein-synchronized decay times.

You're right, it's hard to think of such experiments. Normally there's one convenient reference frame (the lab frame) and constructing another is prohibitively difficult.

It doesn't matter because relativity isn't about reference frames, even though they were a big part of Einstein's original paper and they've dominated introductions to it ever since. It's really about spacetime geometry. In Euclidean geometry, you can do some constructions without any coordinates at all. In many other cases it's easier to set up a Cartesian coordinate system. But it's rarely helpful to use two different Cartesian coordinate systems when solving the same problem. You don't need to, since one coordinate system gives you coordinates for everything, and that's enough to do the algebra.

The crucial thing in special relativity is the distance formula $$\sqrt{Δt^2-Δx^2-Δy^2-Δz^2}$$, which plays the same role as the Pythagorean formula. Lorentz transformations are important only because they preserve that formula. If you use rapidity (spacetime angle) instead of velocity (spacetime slope) in the Lorentz transformation, it looks like a transformation between Cartesian coordinate systems with a common origin. If you rewrite the Cartesian formula in terms of slope, it looks like a Lorentz transformation.

If we did an experiment with a second set of Einstein-synchronized clocks, it could still be analyzed from the perspective of a single reference frame (probably the lab frame). The results would be consistent with that analysis. And the results wouldn't prove anything, because you could still argue that the lab frame is the only true frame and special relativity only appears to be true because of distortions of the moving clocks, etc., just as everyone believed before 1905. The universe doesn't know what a clock is or what Einstein synchronization is, so it would just follow the same rules as always.

• Thanks for the feedback. You got my point, and in particular that with the approach of always defining a preferred frame with a clock at rest we can only say that those experiments can be solved with a class of relativistic theories, and not necessarily with SR. In turn we can still keep using separated time and space entities instead of spacetime events, as "mandate" by SR. As a consequence, if we can't find an experiment where SR is essential for its solution, then we cannot say the SR is the only right model for explaining the universe(!). Aug 15, 2020 at 20:22