Little green men

Time dilation and length contraction provide a way of travelling
intergalactic distances across space (which has turned out to be a
godsend for ufologists). In Chapter 3 I gave you some impression
of the enormous distances between the stars. Even our closest
neighbours are so far away that it takes their light several years to
reach us, while most stars are thousands of lightyears away. This
would appear to rule out any possibility of us ever reaching other
stars, along with their possible planetary systems, within a human
lifetime.
This is where relativity comes to the rescue. If a spaceship
could travel close to the speed of light, then it sees the distance it
needs to cover contracted. Ajourney to a star a thousand lightyears
away might only take a few years according to the astronauts on
board. The catch is that, due to the effects of time dilation, the
journey would still take over a thousand Earth years. After all,
as seen from Earth, the ship is moving below light speed. On
Earth we would see the ship having to cover the full uncontracted
distance, but we will also see the ship’s clocks slowed down so that
we would also agree with them on the amount of time the journey
has taken according to them.
Does this mean that space travel across the Universe is
possible? In principle, yes, you could travel all the way across the
Visible Universe covering billions of lightyears in one day without
ever reaching the speed of light. This is despite the fact that light,
which is moving faster than you, takes billions of years to make the
same journey. The trick is to always make the distinction between
the time the journey takes according to clocks in your rocket (that
measure one day) and the time it takes as measured by clocks that
remain on Earth (billions of years). Arather baffling consequence
of this is that for light itself time stands still. If you could attach
a clock to a light beam it would not tick at all. We say that to a
photon, time does not go by at all (maximum time dilation) and
the whole Universe has zero size (maximum length contraction)!!
Since it is possible to cover any distance across space in an
arbitrarily short time by travelling close to the speed of light,
it appears that I have, in theory at least, opened the door to the possibility of us having been visited by beings from other
worlds. The argument goes that it is at least possible for a visiting
alien spacecraft to have a propulsion system that is hundreds,
even thousands, of years more advanced than anything we could
conceive of, and could reach speeds close to that of light. They
may therefore be able to cover the vast distances across space in
just a few months or years. Much as I hate to pour cold water
on this particular fantasy, it is highly unlikely (but not impossible
of course) that UFOs are genuine flying saucers for a number of
practical reasons. Since any alien spacecraft must obey the same
laws of physics as everything else in the Universe, it cannot travel
faster than the speed of light. Even though the journey from their
homeplanet might only take a few years according to the travelling
aliens, many thousands or even millions of years will have elapsed
back on their home planet. So, assuming that their life spans are
comparable to ours, it would be impossible for them to ever return
and report back on their findings. Their contemporaries would all
be long dead. Of coursewhoamI to judge (a) what an alien lifetime
is and (b) whether they would have any intention of returning
home anyway