We have had some detailed discussions on wormholes on this list in the
past, and there is a good article in back issues of Extropy. Without
wormholes, it would probably still be possible, but expensive.
Years ago, people proposed ramjets which could get up to near lightspeed
by fusing interstellar hydrogen. Today we might envision using nanotech
to upload all the passengers into some kind of solid state form, perhaps
suspended, so that higher accelerations could be used. In one design
I saw, the interstellar craft was basically a gun able to shoot its
payload at close to the speed of light. The whole gun was launched at
near the speed of light from the home system (maybe with a light sail
or EM launcher so the g load is relatively moderate). Then on arrival,
it would shoot out the tiny "bullet" backwards, opposite to the direction
it was travelling. This would slow the payload down so it would enter
the target system at a moderate speed and crash into some body where it
could rebuild the passengers. But the bullet would pull a lot of g's!
Once arrived, there would not be much point in travelling back and forth.
EM waves could be used to communicate any results as fast as travelling,
and perhaps even to send compressed brain descriptions so travellers could
"beam over" between star systems.
All this would suggest that UFO's, if they existed, would be
interplanetary rather than interstellar craft. They might even be
strictly for use in the near earth region. They would be built and
launched somewhere nearby, either on earth or perhaps the moon.
However this is really just entertaining speculation; actually the
ostensible characteristics of the craft (their ability to crash, for
example) don't seem consistent with the kind of technology we are talking