Nicole asked:

Time travel is an extremely interesting subject, but is it really conceptually possible?

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My straightforward answer is no, it is not possible, no matter how you bend it. But if I left it there,
someone else will say, it is conceivable under such and such circumstances. So I'm going to have to
invite you along on a little journey of problems, just two or three of them, but all bristling with way-out
complexities. I'll try and make them as easy as possible, because it's worth thinking about these
matters, and also because our lives are so much under the influence of science and science fiction
today that the average person can hardly make out what to believe. And by golly, time travel is part of
the fare! You must have noticed how much it is taken for granted, as if there were no argument about
it!

Well now, since we have to start somewhere, let's take a peek at the 'space of all possible things/
events/ ideas'. Somewhere in this space you'll find time travel and no doubt millions of other ideas,
thoughts, objects, events and possibilities that have been dreamt about. They are all in this 'space' as
potentials, waiting to be realised. Yet the first thing to note about the 'space of all possible things etc.'
is this: there is no such space; for even the 'space' itself, the concept of this 'space', is part of the
'space of all possible things etc.'! Hence it is not a real space, not a finite, three-dimensional volume,
where things happen. So you understand that I'm talking about a conceptualspace, an infiniterealm
with infinitepossibilities that (so to speak) travels along with our finiterealm of real things and real
possibilities. It is the realm of the 'Maybe'.

The importance of this concept of infinity is not well appreciated, certainly not by time travellers. They
tend rather indiscriminately to toss finite and infinite states around as if they were lego blocks. They
talk about 'worm holes' and 'black holes' and 'big bang', and of 'string theory' and 'quantum flutters',
which are all entangled with infinity. But consider that infinity means, by definition, that you can't count
what's in it. So when you ask, how many atoms in the universe, you are immediately defining the
universe as finite.

Having got this far, what about time? Well, it's really the same problem all over again. Is the universe
in 'time' or not? Is time 'in' the universe or independent? Astronomers want to convince us that time
was created with the big bang, but there is a big chink in that logic. For if the spread of time is finite,
then of course the universe must be finite. And vice versa. But if the universe is finite, then we've only
pushed the problem of infinity out of the way, because we are then supposing another universe which
must contain ours; and that universe is probably contained in yet another: Russian doll universes all
the way down. In philosophy this is called 'infinite regress'.

We're obviously getting ourselves into a huge mess. Let's narrow down our focus and note down a
sort of definition: 'God invented time to prevent everything from happening at once.' This gives us a
vital first clue to what's wrong with time travel. On this definition, time is a concept of simultaneity.It
means that if two separate objects/ events occur such that third parties observing them agree in their
happening at the same instant, these parties then have a means of plotting the events on a graph,
marking their lines of approach and departure and assigning values (seconds, hours, days) to all
changes in position. This graph is a 'frame of reference', which can now function as a tool for
establishing the simultaneity of all events that fall within its scope. Evidently to make this work, a point
at rest has to be presupposed, called the 'residual observer', around which the other events revolve.

Now another difficulty comes up. When you have three, four, a thousand, a billion frames of
reference, practically all of them unknown to us because of the sheer size of the visible universe, the
notion of simultaneity suddenly runs amok; our little graph just can't cope any more and you'll find that
a second residual observer becomes necessary, then a third, a fourth...and in an infinite universe...?
You guessed it: an infinite number of residual observers. Where does that leave our simple concept
of time? Doesn't it mean there are as many times' as residual observers? True again.

So this doesn't get us anywhere. We're attacking the whole problem back to front. To find out 'what
time really is', we need to put ourselves in the seat of time itself. We need to ride along with time on a
beam of light. So let's now confront this issue with a 'practical' example.

Let's say you've been despatched from Earth to Alpha Centauri. In earth terms that trip is going to
take four years at the speed of light; that's not time travel, but it will serve for an opener. When you
last looked back, you might have seen your parents standing there, waving goodbye. A couple of
days later, you look again and still they're there. Patient people! But when you look again a year later
and find they haven't moved, you are suddenly jolted into the realisation that, of course, their image is
travelling at the same speed with you. Time is standing still for you in relation to that scene.

Now difficult as it may be, try and draw a sound conclusion from this. These are not your parents, but
merely their image. What then, if you could suddenly double back and return? The point is: nothing
changes;and when you arrive, to your parents you will only have hovered in the stratosphere for a
while and then come back down.

Now clearly this is nonsense. You've been en route for a year! Consequently there is an irresolvable
contradiction: you cannot, as a physical body, be in two places at the same moment, but this is what
the story entails.

It gets worse when you really start time travelling. Imagine yourself accelerating beyond the speed of
light. As you gaze out the porthole, you'll see start seeing things you shouldn't: ice ages, continental
drift, the earth aflame like a drop of molten iron etc etc. On our diagram of Earth, Alpha Centauri and
yourself, your numbers are running into the negative: you've reversed the time relation between you
and planet Home.

Now there is another side to this story. To observers on earth you would first dissolve and then
disappear. Conventionally we take this to mean that the speed of light can only be attained by
electromagnetic radiation (EMR), accordingly your acceleration has the effect of converting you and
your craft into EMR. But this in turn means that, in relation to Earth, you have ceased to exist. You
cannot therefore simply double back and hurtle back to Earth. She won't be there when you arrive.
On your diagram, where Earth and Alpha Centauri comprise a frame of reference in close
simultaneity, you have removed the residual observer, yourself.

But ah! you cry, even if I can't return to Earth, yet this is time travel, isn't it? Can't I now connect with
another frame of reference?

Well, I promised you this was going to be complex, mind-boggling and irritating. For while you may
conceivably exceed the speed of light in relation to your own system, you cannot exceed it in relation
to light itself. Here the equation is EMR = Time. The grain of EMR in the universe is also the grain of
time, and the best or simplest way to make sense of this is to reverse the notion of speed. To attain
the speed of light means, in this context, for you to become decoupled from any frame of reference
whatever because you have become connected to the stream of time/ light itself. But this 'stream'
being the grain of time itself, means you are standing stillagain, only this time in relation to the whole
universe. Then the objects of the physical universe, galaxies and nebulas and novas, will be fizzying
around you in a bewildering torrent of criss-cross patterns across the entire 'sensurround' horizon.
Indeed some or many of these objects may actually 'collide' with you, at the speed of light (!).

One last question: could you not 'decouple' from this unwished-for state and return to a definite
existence? Unfortunately the answer, once again, must be 'no'. I keep saying 'you', as though there
was a 'you' in this EMR stream. But of course, there's not: you have become a beam of light, pure
EMR, which contains not the thinnest thread of information. Once upon a time, in your real life, 'you'
were (among many other things) a packet of information; this is now gone, terminally erased. And this
is of course the real crux of the matter.

Simultaneity is the coincidence of objects (information) in a frame of reference: and all these frames
of reference are finite entities which might all, in principle, be co-ordinated in a network of finite
observers. But 'behind' this structure is the structureless grain; picture it like a single dew drop
somewhere in the midst of the Sahara desert. And in this structureless space all events occur
simultaneously, just as the sand in the desert 'occurs' all at once; but for us, who have a finite
perspective on them, these events occur in sequence and under conditions to which the concept of
simultaneity can be fitted.

I hope all this makes sense to you! If you wanted to put it into a nutshell, you could say that time
travel cannot happen because time is not real: it's not a road or a space or a field where you can
identify Point A and Point B in relation to one permanent, unchanging residual observer. It is (as I
said) the idea of some things occurring measurably simultaneously. So the crucial component (you
might have picked this up when you recognised your parents as only an image) is this: that light
waves bearing images are not physical reality. On this discrepancy the whole fancy breaks apart.
Time travel, so understood, is mistaking a 'report' for the event itself; and of course a report can long
outlast the event which has meanwhile ceased to be.

And this brings us back to the 'space of all possible things', where we started. Here simultaneity is
meaningless, because in an infinite space nothing is simultaneous with anything else, there is no
frame of reference and no residual observer; and indeed, there is nothing whatever in this 'space', not
an atom, not a breath. Just dreams of finitude, of finite possibilities. Dreams of being, for nothing in
this 'space', nor the space itself, has being.

Jürgen Lawrenz

Sydney

It Depends what you mean by 'conceptually possible'. I would say that time-travel is logically possible
because there seems to be no contradiction in the concept (which is obviously very different from
sayings its physically possible in our world.)

The interesting questions, as far as I can tell is what known as the grandfather problem. Suppose that
time-travel is possible. Now, suppose you go back to the time when your grandfather is in his youth
and you kill him — this would mean that in the future, there will be no you. But then how could you
have come back from the future and killed him?

Here I agree with David Lewis. He reckons that time-travel is possible but you can't change anything
in the past. This is because he thinks of time as a big line and each point is equally real. Consider
time T, when you travel back to point T*. Now, Lewis wants to say that point T* is equally real when
you travel back as when you are there at point T*. The only difference is your perception of T*. The
answer Lewis gives to the grandfather problem is that you can't kill your grandfather or change
anything for that matter, for the reason that you were there already. This sounds weird but if you think
about it it makes sense.

Rich Woodward

Travelling through time is something we all appear to do every day, this morning I was in the past but
now I'm in the present which was the future! I assume however what you are talking about is when an
individual travels to a time outside of the ordinary scope. There's an interesting article in Le Poidevin
& McBeath's book The Philosophy of Timeon the subject but I can't remember who wrote it, however
here are two key issues.

First if we were to travel back in time it would appear possible that we could change the past, possibly
causing a causal loop whereby our actions in the past affect the way we are in the future. Second
there is the ontological status of the past and the future.

To deal with the first problem, consider the 'Back to the Future' scenario where the character
potentially stops his mum meeting his father and therefore prevents his own existence. If this were to
happen however it would not be the case that in the future that he could go back and prevent his own
existence. The argument therefore entails that if he can prevent his own existence then he can't
prevent his existence. The other apparent way to avoid this problem is to suggest that you can't affect
the past when you go back, but this is somewhat strange. The way around this problem is to say that
the Time traveller can affect the past however he can't change it. the 'past'' is already a determined
system which the time traveller may cause an event in but any event that he causes will have already
happened. He is therefore free to affect the past but he cannot change anything that happened in it.

The second issue is whether there is anywhere to travel to. There are two main positions on time
which broadly are the tensed view and the tenseless view. Without going into the positions too much
the tenseless view of time is that there is nothing ontologically privileged about the 'present' that we
perceive, all times are equally real, thus this position is somewhat analogous to the conception most
of hold on space where there is nothing special about 'here' rather it is just the place we happen to
occupy. If you are a tenseless (b-theorist) theorist then there clearly is a 'place' to go to when you
time travel.

The second position that is held is the tensed theory (a-theory) of time whereby there is something
privileged about the present, namely it is the only time that is present. Time flows from the future into
the present, and the present to the past. One of the main motivations for this position is that it allows
us to hold that the future is open and allows for a non-deterministic position of the world. The
a-theorist has more work to do than the b-theorist at this point as for the a-theorist three main
positions are viable:

a. Only the present exists.

b. The past and the present exist.

c. The past present and future exist.

Now depending on which of a—c you accept you're potential to travel to those places is affected,
clearly if you hold a then time travel is a priori impossible, if b then you can't go to the future.

There are other issues but I feel these are the main two. As I say if you have an interest in time I
strongly recommend Le Poidevin and McBeath's anthology [The Philosophy of Time.Oxford
University Press 1993].

Mike Lee

David Gerrold in his classic 70's sci-fi novel The Man Who Folded Himself(new edition published by
BenBella Books, 2003 forthcoming) describes a version of 'time travel' where the time traveller hops
to alternate time streams. For example, you could hop to a time stream where it is September 10,
2001 and foil the terrorist attack on the Twin Towers. That might make you feel good for a while. Until
you realize that all you have succeeded in doing is prevent the attack in an alternative universe. In the
actual universe, what happened happened, and can't be made to unhappen.

For more on Gerrold's time travel universe see myAfterword to The Man Who Folded Himself.

Geoffrey Klempner