If time travel is invented some time in the future, we would have already seen time travelers. And no, they couldn't have just been careful. Despite what conspiracy theorists think, humans are really never very careful. But just think about it for a moment. Imagine there was a great event you wanted to witness, and you had a time machine. Say you went back to September 1776 to watch the First Continental Congress meet. Already you have time travelers squeezed into a small room along with the 55 delegates. For the sake of argument, say only one time traveler was there and he was invisible. And let's say that there is a rule that he is the only one that gets to visit that time period and that place. Somebody else can't show up while he's there. Even if that is the case, sooner or later, somebody is going to want to go back and verify his findings. Or maybe recording technology has advanced so much that they want to go back and get better footage of the event. Now you have two time travelers there. Even if a time traveler only went back to that particular time every 500 years, within 10,000 years of time travel you would have enough travellers to outnumber the delegates four to one. But I expect that people would travel more often than that. Tourists would want to see the First Continental Congress. And if it was really interesting, they would want to see it again, and bring the kids. Soon, Independence Hall would explode at the seems. We should not only have seen time travelers already. We should be seeing many multiples of them.