If you look at the many-universe multiverse theory, going back in time would change the timeline, so that when you went back to change history, that would create another thread of time that is separate from the one you left.

Sort of like what happened in the last Star Trek movie.

Wow, great question.

While I believe in determinism, I believe that what it means is that, given perfect data and knowledge about a system, that system is perfectly predictable. Therefore, if the system input changes, the outcome itself may change.

There is the saying go back in time, kill your father. If your father died before you were born. How did you go back in time. You not existing.

The fact is. You went back in time and no matter what you do/did. Those events that you caused, being back in time. Are what caused the events in your life to happen. Up to when you got in your time machine.

Either you were adopted or did not kill your father. Because you do exist and did go back in time.

Remember, to go back in time. That being the past, you were already there.

If you go back in time, you would not be the same person who went through that time before. You would be your older self. So the second time through would be different. If the world is deterministic, then you can do anything you want because it’s been predetermined. So you can go back in time and kill your father or not. It won’t matter what you do because it’s already preordained what you will do,

In fact, there is no way to tell the difference between a world that is deterministic or one where you have free will. Since we can’t tell, we act however we act. It makes no difference.

Time travel… well, if it could happen, it would involve ftl travel. The travel would occur in a space ship, not some box like in the movies or novels. We move in space as well as time, so to get back to where we once were, we have to travel a significant ways in space, not to mention the issues involved in trying to go back wards in time.

The backwards in time novel is a thought experiment. It’s a way of going back and reliving history, which, of course, can’t be done in the real world. It allows us to think, “what if.”

These paradoxes about killing our grandfathers are just mental masturbation. The subtext is about who we would be if we weren’t who we are. If you could remake yourself—fix all the mistakes with the guidance of your future self, who would you end up as? When you think about it, it’s kind of a self-hating thing to do.

what choice do we have anyway?

You wouldn’t be able to go back and do things over again, because the past version of yourself is already doing them. Even if the present you were to replace the past you and cause a never-ending Groundhog Day loop, you could always go back to before you were born, live your life, and then die before you ever exist.

Does the time machine in this scenario only go into the past? If you can go into the future, can you destroy events that have already been lined up in the present?

@Rarebear Whether it is through String Theory of Strong Anthropic Theory I really have trouble buying the multiple universes. With Time travelers bopping back and launching new universe sin which other time travelers could bop back and lunch more universes, we would soon run into an infinity of universes. And since each universe must be completely undetectable from any other of the universes, I say that the Theory of Economy suggests we dispense with them till evidence shows one to exist.

@crisw But that takes us right back to the need for multiple universes if time travel occurs. Without multiple universes, in a fully deterministic Universe the boundary condition would never change, because the future me would have always come back to to the me of the past I chose to travel to, and would then always go right back to developing the time machine to do it again.

@ChazMaz That paradox is one that leads me to believe time travel is not going to be possible. That and the fact that time travelers from some distant future where the technology exists are not dropping in on us today. Some have suggested that the Universe conspires to make it impossible to set up a time paradox, but my reaction is why would the nascent universe have written laws of science to cover doing that?

@wundayatta Great points. But there are some posited methods of time travel that might make the classic sci-fi box type machine a possibility. Nothing more than wild postulates as we try to reconcile gravitation with the strong and weak force. But the hope is out there.

@iamthemob I don’t know what hope we have. I hope we have hope. I don’t know that the Universe is entirely deterministic. As @wundayatta noted, we haven’t yet established this. Certainly, macro structures act in an entirely deterministic fashion, but at the quantum level, things are quite different. We do not yet know whether they are truly stochastic, but they may be. And the activity that occurs in the human brain that amounts to thought relies on structures at the level of quantum mechanics. If that quantum uncertainty gives rise to an emergent I’ness” that can weigh facts, and either pick a rational direction or one that just satisfies the need for a lark, I would call that free will. How’s that for a long answer to a short question? :-)

@hobbitsubculture That brings up a fascinating question. Would the act of “going back” regress me to the me of today, before I invented my time machine. Or would the me of today exist alongside a me from many years in the future, an even more old and decrepit version of the me of today?

@ETpro

What is the theory of economy? Wikipedia suggests it is another name for Occam’s Razor? If so, how does this suggest that the multiverse explanation is not suitable? It is the only way time travel could be possible without creating paradoxes or without existing in infinite causal loops.

Have you ever seen the movie, Primer? It’s about time travel and deals with pretty much exactly what you are asking.

I think time travel forwards is already possible. Backwards, would likely not alter the universe, for one of two reasons. First if backward time travel was possible it would make everything happen as they did happen, therefor no change. The second is it wouldn’t be just time travel, rather time and dimensional travel. With the second you would end up in a separate universe or dimension, perhaps completely indistinguishable from your own, until you change it.

The multiverse theory seems logically sound to me.

@ETpro I have trouble buying the multiple universes also—there’s no shred of evidence that they exist. But it’s the string theorists bread and butter.

But then again, I have trouble accepting the premise of your question, time travel, as well.

@Rarebear

The multiverse theory is really the only way time travel is possible without creating paradoxes or without existing in infinite causal loops. Basically, since you cannot alter the past, you can only either relive it, or not actually change the past by changing the past of different universe.

@chocolatechip Right. That’s the first point I made, although you restated it more clearly than I did.

@chocolatechip & @everephebe Yes, Occam’s Razor AKA the Law of Economy. String theory supports possible multiverses if there are 10 or 26 dimensions. All but 3 of the spatial and 1 of the time dimensions have to be tightly curled into themselves so they are so close to a mathematical line (having no width) that they cannot be observed by humans. And yet they must exist and let beings pass through them even though they would have to be too tightly curved to permit that—and when someone did pass through one they would have to reach an alternate universe with 3 mostly flat spatial dimensions and one time dimension coexisting with our universe. Not my idea of an economical theory—to say nothing of the problem that we have never observed the universe we live in to have any such phenomena.

@Rarebear My best guess is the only way to travel back in time is FTL travel and the Universal COP or Causal Ordering Postulate won’t let us exceed the speed limit. But this is in fun. I also am not convinced that all human thought is causally ordered and thus could be fully predicted if only we knew the exact initial position of all the brain and environment’s particles and could do the almost infinitely complex math it would take to calculate it. But those debates are for some other thread.

@ETpro Are you talking about the geometric arrangement of the universes? Because outside of our universe, geometry has no meaning, since it is a property of 3 dimensions of our universe.

Furthermore, since we are presupposing that time travel is possible, and the multiverse theory is the only theory that supports time travel, how could it be false?

@chocolatechip There are actually multiple options that mathematically might make time travel possible. If there indeed are other universes, how do we know 3 dimensions have no meaning in them?

And talk about alternate universes, how come @ratboy has been drafting a response now for 73 hours?

@ETpro LOL

@ETpro What I’m suggesting is that the dimensions are properties of a universe, therefore outside of a universe, the properties of dimensions (time and space and whatever else) don’t exist. Each universe is a self contained space time continuum. Thus, humans wouldn’t be passing through dimensions in the geometric sense, that is, you’re not moving form one place or time to another, but from one continuum to another. This is the only way that multiverses in the context of time travel can exist as far as I know, since the idea of the multiverse theory is that you’re not actually traveling through the “time” dimension of our universe.

Also, what are these other mathematically correct options? Do they actually have any physical, interpretable meaning?

@chocolatechip To my knowledge, all the multiple universe possibilities are just that—possibilities. There are none supported by observational evince.

I actually came up with an answer to this in the car the other night, and it’s going to be a major plot point of my 2010 Nanowrimo novel.

Why would you go back in time, if there’s nothing you can change?

To see the past again. To see a better time in your life, or someone you loved who’s gone in the present. Because you’re so miserable in the present, that even seeing the past again is tempting enough, without the ability to change it. And because you happen to know someone who built a time machine for the purpose of killing Alexander Graham Bell, but they wimped out and didn’t use it.

@hobbitsubculture One very valid reason would be to live forever. Unfortunately, the fully causal single universe would only allow that if you were willing to live through the same set of happenings again and again forever. And since you had no knowledge of getting to your past via a time the first time through, presumably you;‘d have no such knowledge on each subsequent repetition either.

@ETpro Actually, what would probably happen in a causal loop situation would be that there would be two of you; one of you traveled into the past, and one of you has yet to travel into the past, since there is no mechanism to suggest you would “take over” the life of your past self. The two versions of you would live unique lives for the duration of time in which you coexist, until your past self travels back in time, leaving only yourself (at which point you would probably continue your life as it was before you traveled back in time by usurping the life of your past self).

Depending on whether you decide to communicate with your past self when you time travel, you may indeed have knowledge that causal loop is indeed occuring. Of course, whatever happens, your past self will always end up repeating the actions of your future self. Breaking the causal loop would require you to know all of the events of the loop, which none of the versions of you know in entirety until they complete their respective loops.

For an interesting movie on this subject, I suggest you watch Primer.

@chocolatechip There would have to be an mechanism in a fully causal universe. There is only one of me today. If a future me came back to this moment, the timeline would be shifted if there were suddenly two of me.

@ETpro If a future you came back in time at this moment, the timeline would not shift because it is a fully causal universe. Your future self traveling back in time plus all other events preceding it, is what caused you to travel back in time in the first place. If your future self walked into the room right now, that would be part of the chain of events leading to your eventual time travel. It’s a loop.

@chocolatechip That could only be true in a multiverse. If time travel ever becomes possible, and future selvees came back to hang out with their past selves, it would be happening around us right now, and it isn’t.

@ETpro It could be possible causally looping universe, and it wouldn’t be necessary in a multiverse. It’s an infinite causal loop. Event A leads to B leads to C leads to A.

How do you know future selves aren’t coexisting with us right now? My 20 years in the future self could be hanging out in Cambodia for all I know.

http://en.wikipedia.org/wiki/Causal_loop

Of course, as Wikipedia suggests, it is a paradox in the sense that causes become effects and effects become causes, but then again, doesn’t causality assume that time is linear? And if we’re travelling back and forth through time, clearly it must not be.

@chocolatechip Time is not linear. It is relative to the observer. And that does nothing to decrease causality. What rule would cause my future self to end up always separated by a great deal of space form me. And even if he is in Cambodia right now, wouldn’t the Cambodians notice a very old American guy suddenly materializing in their midst? And wouldn’t I be aware of all the Cambodians dropping into my current time zone from their future?

@ETpro I meant linear as in you can travel in only one direction, as opposed to the spatial dimensions in which you can travel in any direction, and therefore, that effects must precede causes. Since we are assuming time travel is possible, this isn’t true.

I never said there was any rule that your future self would have to end up separated from you. As I said, it is entirely possible that your future self could materialize in front of your eyes, and you two could go out for coffee, and talk about all the things that will happen to you in the future. This wouldn’t change anything. Your future self has already had these exact same experiences in their past when they met their future self.

I previously said, “Breaking the causal loop would require you to know all of the events of the loop, which none of the versions of you know in entirety until they complete their respective loops.” which now that I think about it is false.

Given the example I just gave, if your future self tells you everything that happened after you meet your future self, you would indeed have enough information to break the loop. However, doing so would create a paradox and thus I would assume it would be impossible to do. Remember that your future self had this exact same experience, and probably contemplated trying to break the loop as well, but for whatever reason (and those reasons will be the same as yours), decided not to.

Of course, there is the possibility that you could freely decide to violate causality and create a paradox, in which case who knows what would happen.

@chocolatechip Thanks for the clarification on your meaning of linearity. You are talking about the assumed arrow of time, which most of physics can’t demonstrate, but which the 2nd Law of Thermodynamics seems to enforce.

I am glad to see you grappling with all the conundrums that emerge when we assume time might somehow be reversible. But whether future selves are confined to showing up at far distant points or right here next to me, we still have to face the fact that we aren’t seeing any materialize. In all my life, I have never seen someone materialize out of thin air nor has any other seemingly sane person I know of. If it’s ever possible to travel back in time, we should be seeing some time travelers dropping in to our here and now, no?

Maybe.

You can think of any explanation you want for why we’ve never seen a time travel materialize, but that doesn’t invalidate the possibility that time travel in such a form is possible. This reminds me of something from Stephen Hawking. An excerpt from a quick Google search:

“Let’s imagine I’m throwing a party, a welcome reception for future time travellers. But there’s a twist. I’m not letting anyone know about it until after the party has happened. I’ve drawn up an invitation giving the exact coordinates in time and space. I am hoping copies of it, in one form or another, will be around for many thousands of years. Maybe one day someone living in the future will find the information on the invitation and use a wormhole time machine to come back to my party, proving that time travel will, one day, be possible.

In the meantime, my time traveller guests should be arriving any moment now. Five, four, three, two, one. But as I say this, no one has arrived. What a shame. I was hoping at least a future Miss Universe was going to step through the door.”

http://www.dailygalaxy.com/my_weblog/2010/05/stephen-hawkings-timetravel-instruction-kit-.html

@chocolatechip Chuckle. Hawking makes my point quite eloquently. The only ways I can see that time travel could be possible are if there are multiple universes, a new one splitting off on each time excursion, or if when you go back, you end up being the old you with no knowledge that you were ever in the future. In the second case case you would indeed be stuck in the never-ending Groundhog Day hell, only unlike Bill Murray’s character, you would never be able to figure that out and break out of the loop.

Either way, I’d have no interest in time travel. It wouldn’t do me any good. :-)