Your complimentary articles
You’ve read one of your four complimentary articles for this month.
You can read four articles free per month. To have complete access to the thousands of philosophy articles on this site, please
Tallis in Wonderland
The Myth Of Time Travel
Raymond Tallis time travels merely by the power of thought.
There are many reasons why we should not waste our time speculating about the possibility of time travel. Some are obvious; others are less so. The most obvious problems arise from the contradictions that result when the traveller goes backwards in time – which I will focus on here – although they also apply to travel into the future.
The best-known objection goes as follows. Supposing I return to the day my parents first met, 6th June 1933. I distract one of them and they do not see each other; their relationship never happens, and I am not born. By interfering with the past, I have removed the most essential condition for my journey to the past – namely, that I should exist. More generally, if I really were able to return to an earlier date than the present, I could be in a position to disturb any of the conditions that resulted in my being able to engage in time travel. More generally still, any action that I take at my past destination date – not just events that have a direct and traceable influence on my personal history – will generate contradictions: it will have an impact on the subsequent history of the universe, and hence on the contents of the day from which I set out, for every event has a direct or indirect influence on every other event.
My departure date – let us say 5th September 2009 – would therefore have two versions: the one that I set out from, and the one that existed after I have interfered with one of its predecessors. How will these two versions of 05/09/09 relate to one another? Do they exist side by side, simultaneously as parts of parallel universes? Seemingly not, as they do not come into being at the same time – at least if time travel takes time (of which more presently). The version of the day I set out from will from my perspective precede the version of the day that will result from my arriving at its antecedents and disturbing the course of subsequent events. The same considerations will apply to the destination day 6th June 1933. It, too, will have two versions. Before I set out, Raymond Tallis will be present in it only as a future possibility not yet born; after I have landed, I will be present in it as a fully-grown actuality. In this case, the first version will be in place 76 years before the second version. And yet in the flow of history they will both lay claim to being the same day. This is, of course, unacceptable: the least we might expect of a day is that it should be simultaneous with, and identical to, itself!
Though we cannot get round the difficulty of having two occurrences of the same day, could we at least make sure that both occurrences would have the same contents? In order to arrange this, I would have to be prevented from doing anything at all when I arrive at 6th June 1933: it would have to be undisturbed, in order that its successors, including 5th September 2009, should be unaltered. Unfortunately this is an impossible requirement. Even if I were prevented from acting, and were weightless and held my breath, I would still have to interact with my surroundings for them to be present to me, for me to have arrived. For example, seeing those surroundings interferes with the light. In short, I could not be present at a destination day without affecting it.
This difficulty shows an important requirement at the heart of the notion of time travel: that the traveller should be causally insulated from the rest of the universe. In the traditional model, the traveller enters a machine and she and the machine leave the present for the past. To do this, the woman-machine complex has to break with the causal nexus it is embedded in before departure. This follows from the fact that it is going backwards in time, while the rest of the universe is moving forward in time. The only way to avoid this rupture would be for everything else to come along for the ride. If, however, the entire universe were wound backwards, there would be no experience, or indeed reality, of travelling away from the present into the past: there would be no clock or consciousness unaffected by the backward movement, able to register my journey. In short, time travel requires one small part of the universe – human plus machine – to break ranks with the rest.
There are therefore causal difficulties in both the departure lounge and the arrival hall. Here is another difficulty: how could we travel in time without travelling in space? ‘The same place’ in 1933 would not be at the same place in 2009. The planet, the solar system and even the galaxy are in different places relative to the universe at large. Moreover, given that time travel cannot be without causal imprint, this also means that even the spatio-temporal co-ordinates of the place would be altered.
That’s not the end of the barriers to time travel that should make us highly sceptical even of its logical possibility, never mind its practicability. How, for example, could the traveller alight at a particular target date? To do so, she would need to specify how far in time she is to travel, and hence the speed and duration of the journey. These parameters – speed and duration – are problematic, to say the least. The notion of the speed of travel through time, and of the duration of the journey, would make sense only with respect to a hypertime, a second-order time whose passage would time the passage of time as she makes a passage through time. This is an impossible conception. And if the journey did take time, the destination would recede further into the past even as the traveller advanced towards it. The only way to ensure arrival would be to time-travel backwards much faster than time unfolds. And even if we could get over these little difficulties, it would be interesting to know by what means the traveller would successfully alight on a particular target date. (The notion of travel as opposed to mere movement assumes that the journey is under our control.) How could the machine be brought to a halt at the chosen destination? Given that the time traveller has to shake off the causal bonds at her setting-off point, and is beyond the grasp of causation in all intermediate locations, what causal brakes could be applied?
These and many other problems with time travel show that it is an entirely incoherent idea. It hardly deserves a shrug. How then has it managed to capture so much attention? To understand why the possibility of time travel is taken seriously, we need to remind ourselves of the grip the notion of time as a quasi-spatial dimension has on our thinking. If time truly is comparable to space, then moving backwards and forwards in it should be as straightforward as moving backwards and forwards in space: to travel in time is thus simply to follow a path, which can be represented by a line joining one temporal location with another. If we want to rid ourselves of the myth of time travel, we must therefore critically examine the assumption that time is a dimension like up-down, right-left or back-front.
One thing should arouse our suspicion at once. If the dimensions of space and the dimensions of time are truly comparable, why are there three of the former (at least) and only one of the latter? And why do the spatial three look so similar, while the fourth looks very much like the odd one out? Up-and-down and back-and-forth and left-and-right are manifestly of the same kind, while before-and-after isn’t. This is evident in the way we group spatial dimensions as if they belonged together: we do not think of calling one of the spatial dimensions ‘the fourth dimension’. Time is manifestly the added-on party. What is more, time seems to be more substantial than the dimensions of space, such as length. Unlike time, length does not exist in-itself, except as an abstraction that has to be represented by a line; and that is a cheat, because the line has to have three dimensions to be visible, even if its length far exceeds its width and depth. Time seems more the equal of three-dimensional space itself than of any of the individual three dimensions of space.
A paradoxical consequence of thinking of time as a dimension on all fours with the three of space is that we are inclined to ascribe quasi-spatial properties to it which are not actually applicable even to spatial dimensions. For example, we talk about the passage or flow of time, as if it were moving, while we wouldn’t think of talking about the passage or flow of space. Indeed, we would not know what to make of the idea, because space is that within which things pass or flow. It is even suggested that, unlike space, time has a direction of flow. Philosophers have tried to find a basis for this, in, for example, the increasing overall entropy of the universe, or the relationship between cause and effect. That is to say, there is the notion of an ‘arrow of time’, which has a direction (although in what sense the arrow is in motion is never made clear) whereas the ‘arrow of space’ would not have much intuitive appeal.
The fact that time is attributed characteristics that are entirely alien to space should make one very dubious about regarding time as comparable to spatial dimensions such as height, width and depth. To say this is not to deny that time is a dimension in either the physical or mathematical sense. Nor is it wrong to represent it by a line on a graph, or to mark a point in space-time by a set of four figures related to four axes. This has been a very potent way of thinking of change, and of the mechanical laws that govern change. The mistake is to think that this quasi-spatial representation captures time in itself; to confuse a way of describing time with time as it is experienced. More precisely, those who think of time as a fourth quasi-spatial dimension are confusing Euclidean space, which has three dimensions, with Minkowskian space, which has four, but which is not the kind of space you can walk around, or travel, in.
So please let us have no more talk of time travel and its paradoxes – except in order to challenge the notion of time as a fourth dimension analogous to those of space.
© Prof. Raymond Tallis 2010
Raymond Tallis is a physician, philosopher, poet and novelist. His new book Michelangelo’s Finger: An Exploration of Everyday Transcendence is published by Atlantic.