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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: sci.physics.relativity
Subject: =?utf-8?Q?Re:_Paper_Series:_Shift-symmetry_in_Einstein=E2=80=99s_Universe?=
Date: Thu, 11 Jul 2024 10:53:05 +0300
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On 2024-07-11 02:57:35 +0000, Eric_Baird said:

> An unusual "take" on relativity theory, the series examines Einstein’s
> universe in terms of symmetry relations.

That is not new and not really unusual. Already when there was no Einstein's
universe, Poincaré had analysed those symmetries. The first postulate of
Special Relativity, the equivalence of inertial frames, is a statement
about symmetry, although that is not always pointed out.

Note that the term "Einstein's universe" is ambiguous: it may mean either
Special Relativity or General Relativity. They have different symmetries.

> Part I – Introduction
> Shift-symmetry in Einstein’s universe: Einstein’s quest for mathematical
> perfection
> http://dx.doi.org/10.13140/RG.2.2.33448.34566
> 
> Einstein's special relativity has the unique set of relativistic
> equations required for velocity-symmetry, perfect energy-conservation,

Perfect conservation of energy is not essential to relativity. However,
relativity requires that if energy or momentum is conserved then both are.

> the absence of round-trip gravitational redshifts, and time-symmetry.

There are no gravitational redshifts in special relativity.

The time direction symmetry is not an essential part of relativity. The
full Lorentz and Poincaré symmetries contain it but both have subgroups
that don't contain it. Only the minimal Poincaré group is essential to
Special Relativity.

-- 
Mikko