In article ,
(Jeffery) writes:

Interstellar travel is impossible since it would take centuries or
millennia to reach other stars.

A multi-generational starship is certainly possible, as are robotic
probes without living beings aboard.

This is the proof of
the nonexistence of UFOs. To claim otherwise is the same as believing
in UFOs.

This is logically wrong. It sounds like George W. Bush saying "If
you're not with us, you're against us", overlooking the fact that there
are other possibilities. I certainly believe that interstellar travel
is possible (via multi-generational starships) but don't think there is
any evidence that Earth has been visited by aliens (thus I don't believe
in UFOs). To claim otherwise is NOT the same as believing in UFOs.

No matter what our technology, we'll never travel to other
stars except maybe nearby stars like Alpha Centauri which you could
reach in a few decades. Otherwise the distances are just too vast. You
just have no comprehension of interstellar distances. If you claim
that aliens can travel to other inhabited worlds, then why not say
that an alien space craft crashed in Roswell, New Mexico in 1947?

Again, logically wrong. There is no point in setting up your opponent
as a straw man then knocking him down.

Absolutely no one in their right mind would suggest that Earth was the
only world with life in the entire Universe. It is unlikely but there
could even be other worlds with life within our own solar system, such
as on Mars, Ganymede, or Europa. I say "world" to encompass terrestial
planets, gas giants, moons, asteroids, comets, etc. There are about
100 billion stars in the Milky Way galaxy. If there was one planet or
moon with life for every 1000 stars, and one planet or moon with
intelligent life for every 1000 with life, that would be 100,000
worlds with intelligent life in the Milky Way, which is an average
sized galaxy out of 100 billion galaxies in the observable universe,
so you multiply 100,000 by 100 billion to get 10^16 worlds with
intelligent life in the observable universe. If you took a more
skeptical view that there was only one world with life for every
10,000 stars, and only one world with intelligent life for every
10,000 with life, that would still give us 1000 worlds with
intelligent life in the Milky Way galaxy, and 10^14 worlds with
intelligent life in the observable universe. No one would ever suggest
that Earth was the only world with life in the entire Universe.

They key factor is the lifetime of a technological civilization.
However you define it, we have had one for a very short fraction of the
lifetime of the Earth. So multiply this fraction by your numbers above.
Of course, we are young and we MIGHT last much longer, but that is not
yet proven.