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.
Premise 1
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.
Premise 2
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Premise 3
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!
Premise 4
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.
Conclusion
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.