Time travel has been a staple of science fiction since, well, since H.G. Wells. Physicists have debated the real-world possibility of time travel for almost as long, ever since it has been shown that Einstein’s equations do not rule it out. However, one of the biggest objections to time travel (besides the “kill your grandfather” paradox) is that we don’t see time travelers. Even Stephen Hawking raised this objection, claiming that if time travel was possible we should be seeing tourists from the future. Since we don’t, it must be impossible.
There are two enormous logical fallacies in this line of reasoning. First, it assumes that time travelers would announce themselves as such, or at least that they would be obvious. This seems to me as a very naive assumption. Announcing that you have come from the future would be the dumbest thing you could do, in any era. In times of superstition, it would be a ticket to the stake, fire included. In modern times it would pit the hapless traveler against the avarice and ideology of those who would seek future technology to gain an advantage. Either that or the traveler (or his machine) would be hijacked for a trip to the past to change the hijacker’s current financial circumstances, or for a dozen other reasons. A time traveler who identified himself as such would be hard pressed to get out alive, whatever the year he or she popped up in.
The second fallacy assumes that humans will be around long enough to develop the technology, or at least that advanced technological civilization will be around long enough. There is no logical reason to make this assumption. In fact, based on the current state of affairs on this planet, humans could be extinct in very short order. Runaway global warming has caused the mass extinction of most of the life on Earth more than once, and it could do so again (this time thanks to us). Add to that the constant threat of nuclear terrorism and nuclear war, rampant pollution of the environment by toxic and carcinogenic chemicals, overpopulation, habitat and ecosystem destruction on a massive scale, and, of course, the ever present possibility of an asteroid or comet-induced extinction event, and it seems the likelihood of us surviving to the end of the century is not very great.
Time travel might very well be physically possible and technologically feasible, but if we don’t survive long enough to figure out how to do it, we’ll never know. And even if we do survive, no intelligent time traveler would ever reveal himself or herself as such. I think either of those is a more reasonable explanation for the lack of future tourists than to claim their absence is proof that time travel is impossible.