On Fri, Dec 29, 2017 at 05:55:20PM +1100, Rob O'Connor wrote:
> Tim Little wrote:
> > You can get a bit of both worlds if instead of truly reactionless
> > drives, you have a reaction drive with tachyonic exhaust.  That
> > would satisfy all the usual conservation laws.
>
> How about other non-FTL particles? Photons? Neutrinos?

All non-FTL particles satisfy |momentum| <= |energy| / c.  The amount
of energy in the exhaust has to be enormous for even modest delta-V.
(For most types of rockets, almost all of it is in the form of rest
energy of massive particles ejected)

FTL particles necessarily have |momentum| > |energy| / c.  As the
exhaust velocity increases, the energy in the exhaust per unit
momentum delivered goes toward zero.  Despite this, the usual
relativistic conservation laws would continue to apply.  The downside
is that as with pretty much everything related to FTL in relativity,
there are serious problems with causality.

- Tim