One of the major objections to the idea that time travel is possible is the apparent fact that we haven’t been visited by tourists from the future. If travel to the past is possible, it’s likely that future historians may be tempted to take advantage of it, that terrorists or criminals may travel back in time to alter history, that someone would go back to visit their ancestor… and that, with those and so many other possible motives, it’s unlikely that the technology would never be used.
The most obvious answer to why we haven’t seen travellers from the future is that backwards time travel is either impossible or never gets invented. Maybe humanity dies out before inventing it, for example. But obviously we don’t want humanity to die out, and time travel is too cool to go uninvented, so what are we left with?
Fear not… all of your twisted sci-fi dreams may yet come to pass. Here are some possible explanations for why we haven’t met any time tourists yet…
We’re living in a timeline that will be erased by time travel.
I’m not sure this one makes sense, because if this timeline, before time travel’s initial invention, will be overwritten by the consequences of time travel, it would seem that we wouldn’t exist as we do today — that if time travel can exist, it must create some kind of self-consistency, resulting in a timeline in which the timeline created by time travel produces time travel at some point after the time travellers first arrive. Thus, there wouldn’t exist a timeline that wasn’t effected by time travel. However, at least one initial version of the timeline has to have existed before time travel was invented the first time. Maybe that’s this one, and somehow we’re to be forced out of existence when time travel exists.
I don’t like this option, because at worst we will have the totality of our existence erased, and at best, it just sucks. Let’s try to do better.
Backward time travel has occurred, or does occur, but we’re unaware of it.
This one has a few possible options. It may be that time travel has occurred but we’re unaware of it, because the resulting timeline has resulted in the human population, or at least most of it, lacking the knowledge of its occurrence.
One scenario to this effect is that, for reasons unknown, nobody goes back this far. Perhaps the time travellers have yet to arrive — they’re more interested in seeing 2012, or 3009, than 2009 or earlier. Maybe the human experience gets better, and they all think that experiencing our epoch would be torture. Not the strongest of these options but one of the possible ones.
If time travellers have gone back this far, or further, perhaps it wasn’t documented, or very few people are aware of it. Maybe tourists have only gone back to times in prehistory for scientific study, but wouldn’t risk contaminating human culture. Or, perhaps the only travelers operated in secret, using technology or mundane disguise to conceal themselves, not revealing the fact that they were from the future. (Perhaps you are the descendant of someone not born yet!)
If you want more of a stretch, perhaps the simulation argument is correct, and that its third possible conclusion is true. This would mean that we are almost certainly living in a computer simulation. Think The Matrix. Maybe we’re living in a simulation in which time travel is either restricted or not occurring, but in the external “real” world, of which we are unaware, time travel occurs.
Time travel occurs between, or creates, multiple universes
Some current theories and models in physics, including string theory and some interpretations of quantum mechanics, suggest that our universe may be one of many. The many-worlds interpretation of quantum mechanics, in particular, supports the idea that all possible outcomes of random events (and decisions) may occur in separate, ever-branching universes.
If this is true, we may simply be living in a universe that has not ye been, or won’t be, influenced by time travel. It may be that our universe is the preservation of the original time in the first proposed scenario above — that when time travel occurs, the traveller does not, in fact, travel back to their proper past, but to a copy of it. Time travel itself may spawn new universes, creating a new branch in the same way that other decisions would.
You can’t travel back to a time before time travel was invented
It may be that time travel requires some special conditions at the destination end. In other words, you can only travel to a location in space-time that is ready to receive time travelers.
Stephen Hawking was one to propose this option. He suggested that backwards time travel may require a special distortion or warping of space-time in the spatial location where the travel occurs, and as a result, time travellers won’t be able to arrive in a time where those conditions haven’t been created yet.
The movie Primer, which is one of my favourite movies, presents this option. In the movie, the characters create a time machine, which is essentially a box in which time flows backwards. They turn on the box, wait for a certain amount of time, climb into the box, and then wait for the same amount of time, emerging from the box at the point in which it was first turned on. If that’s the only way time travel works, the limitations are severe. It would prevent travel over long periods of time, unless the people in the time machine could be put into stasis, because you would need to exist within the machine for as long as the period of time you wish to travel. And, of course, it means you can’t travel back to any time before the time machine was turned on.
So there you have it — four general reasons why time travel may still be possible despite the fact that we haven’t, to our knowledge, witnessed visitors from the future. I like the fourth option best, as it seems to make the most sense and grants the most security to our timeline — until we invent time travel, that is.