David, comments interspersed.

On 23/10/23 00:24, David Shaw - tihnessa at gmail.com (via tml list) wrote:
>
> I know that no-one has ever discovered anything that can travel faster
> than light speed, that it has been tested many, many, maaany times and
> that it is accepted as a fundamental axiom that this shall always be
> the case.  But...

I think that's nothing _with nonzero mass_ can travel _at_ c for longer
than a Planck time.

Massive, subluminal particles are (iirc) collectively called bradyons
(or, in humourous reflection of their superluminal cousins, tardyons);

Massless, luminal particles are collectlvely called luxons;

And (as yet undetected or conclusively ruled out) massive, superluminal
particles are collectively called tachyons (from the Greek _tachys_, for
swift).  As tachyons lose energy, they _speed up_.

>
> Does this absolute speed limit derive from the theories of relativity,
> or is it an assumption that feeds into them?

Strictly, neither.  By the time Albert had his _annus bloody
awesomeness_ in 1905 in/near the Swiss Patent Office, he was forty years
after James Clerk Maxwell, who introduced the idea in 1865.

Albert did propose the invariance of c with observer, position, frame of
reference, etc, as a consequence of the null results of
Michelson-Morley, which tried to detect the Earth's speed thru the
luminous aether.

Invariance of c with any inertial (non-accelerating) frame of reference,
and its consequences, was the basic conceit Albert explored in special
relativity - so the _invariance_ of c is a postulate.  Albert, along the
way, blew the lid off c and showed c was relevant outside light and
electromagnetism.

>
> And if it is an assumption on which the theories are founded, what
> would be the consequences if it were found to be a false assumption?

Given that relativity has withstood multiple challengers in the past 118
years (albeit with some extensions), and has generated usable, testable
predictions (that have held up in safety-of-life situations) across
masses of situations within increasingly-tight measurement error, such as:

the clock frequency offset required in the local frame of satnav
satellites to give a target clock frequency to a ground observer;

gravity wave emission and orbital plane changes of binary pulsars, at
least one over 40+ years, and binary black holes;

post-Newtonian precession of planetary orbits;

that sort of constrains where Albert's invariance postulate can't hold.

The _classic_ tests of GR (those proposed by Einstein himself, fully
understanding that theory stands or falls by experiment) have all held up.

If Albert's invariance postulate, despite all that evidence, turns out
to be bollocks, then we need something _else_ to explain all that
experimental evidence.  I'm not sure I can come up with those
consequences, or the alternate explanation.

Vernor Vinge explored something like this in the Zones of Thought
series, where he postulated the Milky Way was divvied up into the
following zones (with differing physical laws, affecting metal
intelligences more than meat), with boundaries subject to change over
time (unknown in-series whether they are natural or artificial), heading
outward from the galactic core:

Unthinking Depths - nearest the galactic core, where only minimal
intelligence, bio or otherwise, is possible.

Slow Zone - Contains the rock we're on.  Meat intelligence is quite
possible (see nearest mirror), but artifical _general_ intelligence
isn't.  FTL travel is a no-go (which sucks if you get stranded there by
cockup navigational).

The Beyond - AGI, FTL travel/comms, are all possible.  Efficiency of FTL
travel increases the further Das Boot is from the galactic core, both in
the drive itself becoming more efficient with radial distance, and
increased capability of AGI with radial distance.

The Transcend - Contains the galactic halo, where the native
intelligences are on the other side of a technological singularity from
us.  Inscrutable, incomprehensible, etc.  Does it matter to the ant if
you squash it maliciously or indifferently?

Boot-scooting to a parallel dimension with a differing value of c would
be SF-classic hyperdrive.

Alex