On Sun, Jan 17, 2016 at 01:52:36PM +1000, B Kruger wrote:
> Given current and possible proposed technology, how long on average
> would it take to get between worlds 1 parsec apart at sub-light
> speeds?

It depends hugely on what you call "possible" proposed technology, and
what resources the launching civilization could devote to it.

For the near future, we currently have prototype thrusters that can
deliver about 200 km/s Isp, under lab conditions and with very
expensive reaction mass.  With nuclear reactors and truly enormous
reaction mass, something like that might eventually reach 300 km/s in
flight.  The travel time for a parsec would then be about 3000 years.

Another proposal would be a fusion-pulse system capable of reaching
about 0.02 c (maybe slightly more), with about 20 tonnes of fusion
fuel per tonne of payload.  We don't know for sure whether the fusion
reaction can work.  The travel time for a parsec would be about 150
years.

Light-sail systems are another possibility for even faster travel, but
the energy requirements for something on the order of size of a
generation ship would be staggeringly huge.  There have been no real
studies of the engineering feasibility either.  Even the tiniest
conceivable erosion rates would make the proposal totally unworkable
as the sail would be destroyed even before leaving the solar system.
Still, it's theoretically possible to reach decent fractions of c
without having to carry the energy supply along with the payload.  A
travel time of 10 years might be achieved, at an energy cost of about
10^17 J/kg.  A ship of 10,000 tonnes would require a continuous,
extremely tightly focussed beam of about 10^15 W.  For reference, our
civilization's total power use is a few percent of that.

There are plenty of other options, but I think these cover a suitable
range of times and plausibility.

- Tim