'If time-travel is possible, where are all the time-travellers?'

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:

1. If time-travel is possible, then at some point in the future, humans will time-travel. 
2. If humans will time-travel, they will travel back to our time. 
3. If humans will travel back to our time, then we should have encountered them. 
4. We have not encountered any time travellers. 
5. 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 

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. 

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.

See also: 'Time Travel and the Grandfather Paradox.'

The supposed passage of time

Time seems to pass - at least, that’s what plenty of people (some philosophers included) seem to think. However, on reflection, it’s pretty baffling what this claim is supposed to amount to. Taking it literally, we might suppose that time flows literally like a river, with moments passing by like water passing from one section of the river to the next.

'The Persistence of Memory' by Salvador Dali

However, if this is the way in which time flows, it makes sense to ask the question ‘how fast does time flow?’ The problem is, we cannot answer this question without talking nonsense. A river, let’s say, flows at x cubic liters per second - there’s nothing problematic about this observation. In the case of time, the only answer we can give to the question is ‘one second per second.’ But this is not a proper rate at all - it doesn’t tell us anything about the rate of time’s flow, since it is trivially true. It reduces to 1/1, which is not a rate; it’s just ‘1’. We are none the wiser.

We should therefore look to describe the passage of time in some other, perhaps metaphorical, sense. But to do this, we need to get a grip of what exactly we mean by the claim that ‘time passes.‘ Talk of events zooming out of the future, past us through the present, and into the past is all well and good, but seems pretty vague when considered in any depth. We see objects moving about, changing position - and ‘watching the world go by’ just means watching lots of these objects move about. But that this should indicate a great sweeping movement of all spatial events into the past seems unclear. 


Everyday talk about time’s passage seems puzzling from a philosophical perspective because the notion seems so ill-defined. However, we still think it’s true to say that ‘world war two happened’, and that ‘Fidel Castro will die’ (eventually). Descriptions of past and future events seem to require the passage of time, for what else could it mean for an event to become past if not to recede into the past, replaced by a moment that was once future?


There’s a strong case for arguing that time does not flow at all - that events are just laid out in a big static block, called ‘space-time.’ On this view, time is just like space - one dimension of this four-dimensional block of reality. If the universe is ‘static’ in this way (and most physicists already think that it is), then there is no reason to suppose a passage of time exists at all. Rather, time’s passage is a subjective illusion in our consciousness. A good analogy is one of a reel of film, which though giving the illusion of real motion, is just a series of still images all following each other. Life is similarly a series of stages, ‘sped up’ to look like passage in the minds of conscious observers. 

The proponent of this view, which I think is probably true, must give an account of what it means to say that time passes. Various theories have been proposed concerning the inner workings of the human mind, and how it might produce a sensation of passage, which does not exist in reality. However, for me, the biggest conceptual task in thinking about this view is the notion that time is like space. As noted, this is uncontroversial in physics, but radical in everyday talk about the world. It takes a bit of getting one’s head around. Still; think about what it means to say that 'time passes'; and if, like me, you are puzzled by the notion, then a static universe without real passage starts to make much more sense.

(More posts to follow on this view, sometimes loosely labelled ‘The B-theory’, or ‘four-dimensionalism.’)

Time travel and the grandfather paradox

Marty's mother would have married his father even without Marty's
heroic efforts to make sure it that it happened.

Is time travel the stuff of legend? Actually (and rather pleasingly), contemporary philosophers take the possibility much more seriously than you might think. Since it is generally considered that modern physics doesn’t conclusively rule out the possibility of time travel, room has opened up for philosophical speculation about what the possibility of time travel would mean for us as human beings. If time travel is possible after all, much of what we ordinarily assume about free will, causation, and time itself will turn out to be false.

Could you go back in time and, if you were so inclined, kill your own grandfather? Let’s assume that time travel is possible - that a human being could, given the right circumstances and resources, ‘travel’ back in time to events in the past. Let’s further suppose that such a person, Dave, is on a particular mission to kill his grandfather. Dave travels back into the past, and arrives on a sunny day in central London. Dave sees a vaguely familiar figure walking down the street - it is his grandfather, as a young man. Dave pulls a gun out of his back pocket and crouches behind a post-box, waiting for the perfect moment to strike...

But wait. What happens if Dave successfully kills his grandfather? Remember that we are in the past, and that Dave’s grandfather has yet to have a child (Dave’s father), who is in turn yet to have Dave. If Dave is successful in his assassination attempt, then his grandfather (being dead) will never have Dave’s father, and thus Dave will never be born. But if Dave is never born, then Dave could never have returned from the future in the first place to kill his grandfather in the past! Something has gone wrong.

We must ask the following question: is it within Dave’s power, given that he can travel back in time, to kill his grandfather? The answer is both 'yes' and 'no'. Insofar as Dave has a motive, willingness, and adequate resources (the gun, etc) to carry out the murder, he is able to kill his grandfather. However, in the grander scheme of things, Dave’s very existence in the future guarantees that he will not, in fact, succeed in his task. Crouching behind his post-box, Dave will suddenly have a change of heart. Or he will be grabbed by a passing policeman. Or he’ll be knocked over by a cyclist, allowing his grandfather time to escape. The specifics of the event that prevents Dave from firing the killer shot aren’t important - what is important is that whilst it is physically within Dave’s power to kill his grandfather in the past, it is logically impossible that he succeeds.

Philosophical reflection tells us that we should not try to kill our grandfathers. But then, we all knew that anyway.

(This puzzle is from David Lewis, The Paradoxes of Time Travel).