Re: FW: starship-design: Interstellar travel-using vacuum..ur point?

[Steve VanDevender <stevev@efn.org>]

Since we have several new subscribers, and many new subscribers
immediately start wondering why our design project excludes FTL,
let me just make a few introductory remarks:

We exclude FTL drives from consideration (for now) for the same
reason we exclude a lot of other things like "vacuum energy" --
they're simply too speculative.  The main point of this list is
to consider interstellar travel as an engineering problem, and to
think about sound engineering designs.  That means that we limit
ourselves to things that are less speculative and known to be
physically possible.  Relativity, hydrogen fusion and antimatter,
as examples, are observable and experimentally verifiable.  Until
someone demonstrates FTL travel _of a massive particle_ or
successfully extracts "vacuum energy" in experimentally
verifiable ways, these things won't be considered because it's
impossible to make an engineering design without a real
understanding of such effects.

AJ Crowl writes:
 > Seems FTL is more likely than inertialess drives. At least we
 > know of physical processes that have involved FTL in the past
 > [cosmic expansion], whereas there's nothing known that can
 > alter inertia. Haisch and Puthoff's electromagnetic theory of
 > inertia is a start, but there's no guarantee that inertia can
 > be changed.

In general relativistic terms, superluminal expansion of
spacetime isn't at all the same as FTL travel of mass.  Even
during that inflationary phase there was no mass traveling faster 
than light through spacetime.

And just what other FTL processes do you think exist?  Theorists
have yet to come up with anything tenable for allowing mass to
travel FTL.  Things like quantum connectedness don't allow
transfer of mass or even information.  Attempts to theoretically
describe FTL effects using general relativity have so far always
run into physical impossibilities like negative energy densities.

 > > Hence my belief that FTL travel is
 > > necessary for serious and long-distance space exploration.
 > >
 > > Does anyone know what is (theoretically) meant to happen re. time dilation
 > > when you travel FTL? (eg. travelling back in time?)
 > 
 > Time reversal is supposed to occur, though I could never work
 > out why. "Events preceding their effects" - one result of
 > space-like motion [FTL], though I still don't see how. A
 > distant observer might see the events as occuring out of
 > order, but how does that then violate causality? Locally
 > nothing odd has occurred. No one has ever gone FTL so who can
 > say?

Actually, it's not a problem with distant observers; if you can
go FTL you can trivially travel into your own past.  Just do an
FTL jump, accelerate sufficiently, and then do another FTL jump
back to the location you started from.  The amount of
acceleration needed to do this depends on how much faster than
light you went, how far your jump was, and how far back you want
to travel into your own past.

Unfortunately it's difficult to include a spacetime diagram of
this effect here.