On Thursday, September 16, 2021 at 1:14:33 AM UTC-3, John Doe wrote:

<snip>

*Post by John Doe*True. I am not a physicist, but I like this topic. I would like to understand

why Einstein assumed the velocity of light as constant and started measuring

everything with it, and then started to get weird results like time dilation

and lenght contraction as a result. I am understanding this right?

John, I copy an explanation (mi idea) about why Einstein assumed the constancy of "c". It's extracted

from this thread: https://groups.google.com/u/1/g/sci.physics.relativity/c/eOuE7zI2iHQ

and now I'm modifying it a little bit:

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IMHO, SR has everything to do with light because at its foundations, tracing back to the first attempt to

describe relativistic behaviors due to the invariance of "c": Woldemar Voigt and his 1887 paper.

His goal was to find what were the formulae for a linear transform of E(x,y,z,t) into another domain E'(x',y',z',t')

in such a way that the WAVE EQUATION be invariant under such linear transformation. Voigt used these three

general wave equations in 1887, one for each dimension (x,y,z). The value ω is the universal velocity of the wave,

independent of any direction x, y, z (represents the modern c).

∂²u/∂t² = ω² Δu = ω² ( ∂²u/∂x² + ∂²u/∂y² + ∂²u/∂z²)

∂²v/∂t² = ω² Δv = ω² ( ∂²v/∂x² + ∂²v/∂y² + ∂²v/∂z²)

∂²w/∂t² = ω² Δw = ω² ( ∂²w/∂x² + ∂²w/∂y² + ∂²w/∂z²)

verifying that the gradient for the general wave function u(x,y,z,t) is:

∂u/∂x + ∂v/∂y + ∂w/∂z = 0

where ∇ is the Gradient operator, Δ is the Laplacian operator, and is

Usually, the wave equation is analyzed traveling only in one direction, and is usually u(x):

∂²u/∂t² = ω² . ∂²u/∂x²

You can read more about the wave equation here: https://en.wikipedia.org/wiki/Wave_equation

This apply for Maxwell's equation application for a light wave traveling on a unidimensional x direction:

∂²u/∂t² = ω² . ∂²u/∂x² = 1/(μₒ.εₒ) . ∂²u/∂x², where ω² = c² = 1/(μₒ.εₒ)

For Einstein 2nd. postulate of 1905 SR:

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"We will raise this conjecture (the purport of which will hereafter be called the “Principle of Relativity”)

to the status of a postulate, and also introduce ANOTHER POSTULATE, which is only apparently

irreconcilable with the former, namely, that LIGHT is always propagated in empty space with a definite

velocity c which is INDEPENDENT OF THE STATE OF MOTION OF THE EMITTING BODY. These two

postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of

moving bodies based on Maxwell’s theory for stationary bodies."

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is crucial (as it was for Voigt, Lorentz and Poincaré) that "c" be invariant under any linear transform, so

any beam light emitted from one frame reference E' be received and measured with "c" speed at E, and

reciprocally. Its independence of the inertial motion at a relative velocity "v" makes c independent of v.

So, Einstein claims that measurements of c+v or c-v are IMPOSSIBLE, otherwise Lorentz transforms

can´t be applied and the wave function of light would DEPEND on the selected inertial frame, moving

with any constant velocity v.

Accepting the 2nd. Postulate with the meaning stated above, everything else is valid within SR domain

of applicability.

Most physicists adhere to this constancy. Others don't agree and say that the measure value can be:

c' = c + v OR c' = c - v. The discussion is on for the last 100 years. If the constancy of c is disproved,

the entire theory of special relativity collapses, and everything in modern physics has to be reformulated.

Personally, I don't believe in the universal constancy of "c" value. Too big and too old is the universe for

this be hold as true in the last 10 billion years or at 10 bly away. But this is just me.

Also, the domain of applicability of SR requires the absence of gravity and a flat (euclidean) space, void

of any trace of matter or energy. And this is obviously NOT TRUE.

But relativists insist into applying SR here on Earth: mean life of cosmic muons coming down from 10-16 Km

from above, suffering permanent gravitational acceleration; kinematic time dilation in Hafele–Keating

experiment, CERN LHC and the huge presence of electric and magnetic energy, and many other experiments

which violate the domain of applicability for SR.

Special Relativity allows muons to make it to sea level (page 11)

https://physics.nyu.edu/NYSCPT/summer02/sb1.pdf

Hafele–Keating experiment

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment

Restarting the LHC: Why 13 Tev?

https://home.cern/science/engineering/restarting-lhc-why-13-tev