Philosophy Of... returns, after a short leave of absence!
Let’s assume that time travel is both logically and physically possible (contrary to the conclusions drawn in my previous post on time travel). A common objection that is raised to the prospect is really just a common-sense question: if time-travel is possible, then where are all the time-travellers? If time travel is possible, then we might expect future time-travellers to have arrived in our time. Or, more formally:
- If time-travel is possible, then at some point in the future, humans will time-travel.
- If humans will time-travel, they will travel back to our time.
- If humans will travel back to our time, then we should have encountered them.
- We have not encountered any time travellers.
- Therefore, time travel is not possible.
I don’t think this (the ‘future time-travellers argument’) is a very strong argument, so let’s evaluate it premise by premise. It appears that the argument is valid; that is, if all the premises are true, the conclusion cannot fail to be true (its truth logically follows from the truth of the premises). But is each premise true? I don't think so. This will be a common-sense investigation, rather than a technically philosophical one.
That humans would definitely make use of time-travel given its possibility can be easily questioned. We only have a finite time on the Earth (scientists tell us) before it is annihilated, and there doesn’t seem to me to be anything that guarantees we will have grasped time-travel by then. Perhaps it is too complicated to be grasped by humans even given 1000s of years. This certainly seems like a realistic possibility.
Let’s define ‘our time’ as the period between when human history records began and now. Even with such a general definition, this premise is also questionable. It might be, for example, that time-travel is developed, but only developed enough to allow random time-travelling, and not to specific locations in time which the future humans choose. If so, it seems perfectly possible that future humans, popping randomly into the past, never hit our time at all, and have to make do with some dinosaurs for company instead. After all, ‘our time’ thus defined is only a tiny region of the available pool space in which a time-traveller can plunge.
Even if time-travellers did travel to our time, perhaps they might have reasons for not revealing themselves to us. This response is less plausible than my responses to premises 1 and 2, but nevertheless must be considered. Science fiction makes much of the fact that small changes in the past can lead to large changes in the future; perhaps it would be dangerous for future generations for time-travellers to reveal their identities to present-day folk. Maybe they were briefed about this by their leaders before setting off!
The truth of this premise depends on how we approach premise 3. It may be that if time-travellers have travelled to our time, they prefer to stay hidden. This makes it likely that some present day folk have encountered them; they just don’t know it.
Since there are good ways to challenge all four premises of the argument, I think we must conclude that this argument does not convincingly demonstrate that time travel is not possible. The discussion has strayed into the realms of science-fiction, and because of this, tongues must be kept in the vicinity of cheeks. However, there is a serious point to be made: if the above critique is reasonable, then a popular argument for the impossibility of time-travel is rebutted. This is to say nothing, of course, of the possibility of time-travel more generally. Considerations from physics may show that it cannot happen anyway. But at least if we do want to rule out the possibility of time-travel, we cannot do it by means of the ‘future time-travellers’ argument.