On 2020-01-05, casagiannoni optonline.net wrote:
Not surprizing, even considering that intelligent apace faring species must
abound throughout the universe, considering the vast interstellar distances.
Even at near light speed, which is extreemly unlikely, travel times would be
prohibitively long.


Would they really be 'prohibitively' long, however? We know that
propulsion systems capable of reaching the nearest stars within a human
lifetime are possible (e.g., nuclear pulse as in Project Orion, or laser
sails). Given the age of the universe there may be civilizations
who have existed for billions of years: even travelling at a fraction of
the speed of light that's more than enough for a civilization to cross the
Milky Way several times, albeit at a slow pace and over many generations.

Nothing in physics or engineering prevents it, more so for a spacefaring
civilizations which, you'd assume, can command enough energy to do this
at little relative expense (without any breakthrough in physics). Time
and propulsion systems are no obstacle, unless you believe that a
closed-cycle life support system capable of working for centuries is
impossible (not just from the perspective of present human technology
but for everyone in all circumstances), or that no living being can
adapt to prolonged life in space (even within envisioned settlements
such as O'Neil cylinders) and thus cannot be away from a planetary
surface for more than a few years.

Setting aside the 'UFO' question, towards which I'm a skeptic, I think
it's easier to explain the Fermi paradox in terms of not having observed
long enough. We have not been around (or been paying attention) long enough
to claim that no intelligent species capable of interstellar travel /ever/
existed.