Where are all the time travellers?

I hope I have given you an idea of the problems we must face if
we insist on the possibility of time travel into the past. The laws
of physics as we understand them do not rule it out, so where is
the flaw in the argument? You might feel that having to either live
with the time travel paradoxes or to accept the notion that there
is an infinite number of parallel universes is just too much. But
even physicists have failed to come up with a more convincing
argument to rule it out. One that you may have come across is
to ask where all the time travellers from the future are? If future
generations ever succeed in building a time machine then surely
there will be many who would wish to visit the twentieth century
and we should see these visitors among us today. I will therefore
list five possible reasons why we would not expect to see any time
travellers:1. Time travel to the past is forbidden by some as yet
undiscovered laws of physics.
2. A time machine can only take you as far back as the moment
it was switched on and no earlier. So if we figure out how
to build a time machine in the twenty third century we will
not be able to visit the twentieth century. The only way that
would be possible is if we come across a naturally occurring
time machine that has been around for long enough, such as a
black hole or a wormhole. Maybe there are none to be found
in our neck of the Universe.
3. Naturally occurring time machines are found and people do
use them to travel back to the twentieth century, but it turns
out that the many-worlds theory is the correct version of
reality. Our Universe is just not one of the lucky few which
visitors have visited.
4. Expecting to see time travellers among us presupposes that
they would, in fact, want to visit this century. Maybe for them
there will be much nicer and safer periods to visit.
5. Time travellers from the future are among us but keep a low
profile!
Much as I would like to think that time travel is possible, I
am afraid I would probably put my money on the first point. The
reason for this is really quite straightforward and I have mentioned
it before. For time travel to the past to be possible, the future—our
future—has to be already out there. I find this hard to accept.
Don’t despair. My advice to you, if you do not want to give
up on time travel, is to take comfort in the fact that there remain
loopholes in the laws of physics which allow it. As long as time
travel is not categorically forbidden we shall continue along our
journey.