Re: Is Time Travel is Possible?

[Keith Wilson <kw503@york.ac.uk>]

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Hi Nicholas,

I agree with your more sympathetic reading of the K&N article (which can be 
found at http://people.bu.edu/stk/Papers/Timetravel.pdf, for anyone who's 
interested) but think that Bernie is right that the presentist would deny 
that we travel through time at all. For the presentist, the only 
time-location that exists is the present, and although this is constantly 
undergoing a process of change (which is what we mean when we talk about 
time), this does not amount to any sort of travel in the normal sense of 
going from one place (or time) to another. (Defining 'time-movement' as 
something else would seem to reduce the concept to triviality.)

I'd like to suggest that our tendency to think of time as being analogous 
to space, i.e. as some kind of fourth dimension, is a misguided way of 
thinking that arises from our tendency to 'map out' time in terms of 
spatial relationships; e.g. on a calendar or a clock. Because we are used 
to thinking and talking about spatial relationships, and the fact that this 
is a very useful and practical (but metaphysically inaccurate) way of 
planning our busy lives, we extend the idea of space to cover locations and 
relations 'in time', and hey presto, we've invented the fourth dimension! 
However, there are good reasons to think that time is a different kind of 
thing entirely, and so the idea of 4D space-time may be more misleading 
that helpful in terms of providing a model for understanding time.

I realise that this flies in the face of the usual interpretation of 
relativity theory, not to mention Lewis's views on time travel, which 
envisage the world as being a 4D manifold of events laid out in space and 
time. However, as far as I know, relativity can equally be understood in 
terms of clocks running slower when they are moving rapidly through space 
or are in close proximity to a massive body, rather than things travelling 
at different speeds into the future as the four-dimensional view suggests. 
In other words, objects moving at close to the speed of light experience 
the process of change at a slower rate than objects that are at rest or in 
freefall. This is perfectly compatible with the presentist view that 
everything inhabits a single 'now', whilst allowing for the fact that 
different observers may experience the flow of time (i.e. change) at 
different rates. It sounds odd, but I don't think there's any 
contradiction.

I'm not sure what to think about K&N's presentist account of time travel. 
Perhaps it is reasonable to think that certain patterns of events could 
constitute something like time travel, even if the past and future don't 
actually exist. However, a more serious problem would seem to be that time 
travel either presupposes a strictly deterministic universe (which ours, so 
far as we can tell, isn't), or the existence of multiple alternate 
universes (thus allowing for the possibility that you could go back and 
kill your own grandfather without creating a contradiction). The latter 
scenario hardly seems to qualify as time travel, as you would actually be 
travelling into a *different* universe (possible world?), where things look 
much like this one except that the future could pan out differently, thus 
preserving the possibility of free will. However, I don't think that anyone 
(except perhaps David Lewis, who was a realist about possible worlds) 
really takes this idea very seriously...

It seems perfectly reasonable to me that time travel may in fact be 
possible, but that we will never observe it in nature or anywhere else. 
Conversely, any argument that tries to rule a particular metaphysical view 
of time (e.g. presentism) in or out on the basis that it doesn't support 
time travel seems rather tenuous given that the jury is still out on the 
issue of whether such a thing is possible in our own universe (which, after 
all, is the one we are trying to describe!).

- Keith


P.S. Can anyone recommend any good articles on presentism, and particularly 
on the presentist view of tensed facts? I wouldn't mind writing an essay on 
this at some point and would like to find out more.



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