|THE TEN BIG QUESTIONS
While I am a physicist, I am no expert on time-travel, but here is a link:
I am not sure that there are any paradoxes: If a system is accelerated, its clocks begin to slow down against the non-accelerated system, since acceleration makes a real difference to constant inertial motion and in this way is not "relative" anymore. If the accelerated system once more gets into contact with the place where it started, the clocks of the accelerated system are behind the clocks at the starting point. But they are not behind the starting time, since both clocks have advanced. There are no paradoxes of causality, since you never get behind your starting time. Then all the famous films on time-travel are physically nonsense while attracting to movie-directors by their bizarre logic. But I will check that.
Carl Sagan in the interview to which the second sublink points is not sure if travel "backwards in time" is possible. There seem to be ideas along that line, but not proven still. Maybe the "superstring theory" for which Jean asked will give an answer some day, but superstring theory is speculative too up to this time. For superstring theory generally go to http://superstringtheory.com.
And there may be some ride "backwards in time" after tunnelling through wormholes but in a parallel universe not causally connected to ours. Then too all the films on going "backwards in time" paradoxes, which like the "grandfather paradox" (you kill your own grandfather; then why then are you here?) depend on causality, would become nonsense once more.
In my opinion the most important "pedagogical" value of the theories of modern physics, astrophysics and biology is to force people to ask for strict mathematical argument from proven data. From a philosophical point of view this is a major progress from the the old speculative theories of alchemy, "vis vitalis", astrology, kabbala etc., which were self-generating systems without any "Popperian" check of validity. Even if "time-travel" and "Grand Unified Theory" are not proven yet, they at least are serious and falsifiable hypotheses to work on. But many people don't like this merciless insistence on evidence and proof.
This is really an interesting and extremely difficult question. Right now, as far as I know, there are a few physicists who think that some form of time travel (to the past) is possible in some limited fashion by using a "black hole" (which, remember, is not a hole, nor is it entirely black). I am have not looked at this in the depth necessary to evaluate it (assuming I could understand the math). But these physicists are in a minority, I believe.
Evaluating whether "time travel" is possible must wait on a good theory of "time"... and there is none. On the face of it, time travel is not possible, because it would require movement "through" time, which is the source of movement. What does that even mean? If you read Kant on time, you'll think that whatever is "out there", time itself is something we have created to make some sense of "it": the noumenon. But we don't know, really, that Kant is correct. If you look at fairly classical special relativity, then time travel would be possible if there were other dimensions to the universe which were inaccessible to us, so that whatever entities they were accessible to could employ our time dimension as a spatial one. Right. But all that would get those beings would be entry into what they would perceive (remember, this is all according to particular ideas in special relativity) as a sort of solid block of the four dimensions we inhabit. To them, our past and future would be all there, all fixed. So for them changing our "past" would entail no changes whatsoever in any other part of that block, just kind of chiseling out a small piece of a big brick. And we, as just a part of that brick, of course could not travel in time... whatever we've done is all there already. But this, as I say, is just one of a multitude of theories, none of which, as far as I know, have any experimental evidence for them which makes one preferable, really, to any other.
Given all that, what paradoxes are you talking about? There are none, because there is no theory from which to construct any.
Steven Ravett Brown
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 conceptual space, an infinite realm with infinite possibilities that (so to speak) travels along with our finite realm 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 still again, 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.
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.
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 Time on 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.
Now depending on which of ac 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].
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 my Afterword to The Man Who Folded Himself.
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