On Oct 11, 10:28*am, Doug Freyburger wrote:
Niklas Holsti wrote:

The explanation for the lightspeed limit that I was taught is that two
observers moving with a relative velocity larger than lightspeed could
disagree on the order of events that they both observe.

There are two issues here that I am aware of.

For one take us as an observer between the two moving objects. *The
transformations of Special Relativity say that I see both of them as
moving under (or over) C, but because the unverse is relative they also
see each other as moving under (or over) C. *Unlike in General
Relativity, in Special Relativity there's nothing to say objects can't
go above C there's just no way to transistion from below to above and
vice versa. *As photons can be made to decay into particles in principle
according to SR ignoring GR the process could create tachyons. *That's
way the recent neutrino news was so interesting. *The neutrinos were
newly created. *Tachyons created at the big bang will have been beyond
the observable horizon long agao.

This would mean
that while one observer would see that dropping an egg from your hand is
followed by the egg smashing on the floor, the other observer would see
the egg magically reassembling itself on the floor and then levitating
to meet your hand. Since special relativity postulates that all
observers are equivalent in that they see the same physics, including
causality, this would be a contradiction.

http://www.theculture.org/rich/sharp...es/000089.html

That's a very long write up on FTL. *It ends up with the conclusion that
either FTL is impossible or causality is wrong. *But the examples that
break causality involve objects travelling near C communicating with
each other. *I'm not worried about launching craft at 0.8 C looping back
towards Earth and telling us stuff.

So far special (and general) relativity agree with observations (I'm
told) so there you are. But I'm not sure if this really proves that the
equivalence postulate is true, always and everywhere.

So far. *Time will tell if FTL is impossible or causality is wrong.
Maybe eventually.

Quantum entangled photons might not care about the limit of +/- c.