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Gerry Quinn
Nov14-06, 05:00 AM
P: n/a
In article <>, says...

> Given a massless particle (rest mass = 0), other than a photon or
> graviton, what would be the relativistic explanation for why it can
> travel faster than c? What aspect of electromagnetic radiation make
> its speed of propagation in free space an upper bound on speed? I
> suppose the answer lies with the derivation of the sqrt(1-v2/c2) term
> in SR, but I could use some insight from those of you that have a
> strong understanding of relativity and its derivation.

First, there's nothing special about electromagnetic radiation in this
context, except that it's the easiest force to work with, and seems to
be of infinite range, indicating a massless force carrier.

[Also, the electromagnetic force is strong enough that we can
straightforwardly observe quantum phenomena associated with it - so
people often think of photons as being almost as real as electrons,
even when they have doubts about gravitons, which are too feeble to be
individually observable.]

The only other particle currently thought to be massless is the gluon,
although that is confined to a short range at normal temperatures. But
any real massless particle, if another is discovered, will have the
same speed limit of c. [Particle physicists associate masslessness
with certain symmetries, rather than infinite range as such.]

Neutrinos have only a tiny mass - a neutrino flux was detected from a
distant supernova (SN 1987A) at just about the same time as the light
signal from it. Energetic neutrinos travel at a speed so close to
light-speed that the difference in arrival time over 150000 years of
travel was not enough to demonstrate any difference between their speed
and that of light. [They were subsequently shown to be massive because
they can undergo certain transformations during flight.]

Historically, special relativity was defined in terms of light, because
of the ease of measurement of phenomena associated with light. But it
could have been defined in terms of a more general speed limit, with
light being at that time the only example of a phenomenon whose speed
can easily be measured, and that presses so closely on this limit that
it seems to achieve it.

As for the actual reason for this, the speed limit is basically an
axiom of special relativity, and thus special relativity can do nothing
to explain it. For that a more fundamental theory than special
relativity would be required.

- Gerry Quinn

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