Time Travel: What are the Theories & Factors?

[bcrowell]

No I wasn't concluding anything, just explaining my thought process as mentioned above. I have read and watched a lot on time travel, albeit pitched at a very 'layman' level. So I am aware and very open to it. There just isn't any empirical evidence to back it up, so it is all just theory. Therefore I see it as fair game to challenge, even at my elementary level. I would never assume to be correct, but the challenging part is the most fun for me and learning should be fun!

Relativity is "all just theory" in the same sense that Darwinian evolution is "all just theory." Relativity is not a field where you can say anything that strikes your fancy, and it's just as valid as anything else, because it's "all just theory." Special relativity is backed up by a great deal of empirical evidence: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html So is general relativity: http://relativity.livingreviews.org/Articles/lrr-2006-3/

CTCs have never been observed in our universe, but that doesn't mean you can just say anything you like about them, and it's just as valid as anything else. They have been heavily studied, and quite a bit is known about them. Some statements about "time travel," i.e., CTCs, are simply wrong because they are inconsistent with what we know about relativity -- and what we know about relativity is in turn firmly based on empirical evidence.

PF has rules against overly speculative posts. There is not an exception to those rules for cases where you feel that it would suit your personal learning style better.

 

[bcrowell]

First off, I agree that if I am moving relative to another frame then time dilates occurs. I think I've got a good handle on that now, and it was not my intention to challenge it. I whole heartedly support it.

I do still try and visualise just 'how' it happens. Which although is good fun, can get me into trouble leading me down the wrong path. (Such as this post!)

Whilst doing that (Trying to visualise) I was wondering if time or space time was quantised. I read some stuff on wiki about Plank time / length etc.

So it got me thinking that if time (or space time maybe) may be quantised, then we move through our world lines in discrete chunks, one after the other.

What you've done here is to make a leap straight from special relativity to quantum gravity. The more natural progression is SR->GR->quantum gravity. There is no "time travel" (i.e., no CTCs) in SR. GR does allow CTCs, and figuring out what CTCs mean in that context is hard enough without jumping ahead to speculation about quantum gravity. We don't have a theory of quantum gravity.

 

[rede96]

There are only two important differences between time and space, first they have different signs in the metric, and second there is only one dimension of time while there are three of space. This is completely summarized by the fact that the signature is (-+++).

What does that mean exactly?

So yes, time is different from space, and that is completely and fully recognized by describing spacetime as Minkowski, or pseudo-Riemannian, or (-+++) signature, or 1+3D spacetime. But that does not mean that they are not otherwise equivalent.

That bit I am finding hard to understand. Equivalent in what way exactly? Do you mean as metrics of space time?

 

[rede96]

Relativity is "all just theory".

I didn't see it that way. As you've stated there is lots of empirical evidence to back up Special relativity.

Relativity is not a field where you can say anything that strikes your fancy.

Agree. I kinda hoped I wasn't just saying what I fancied. Everything I've posted here has been my best effort to arrive at a logical conclusion based on what limited knowledge I have gained so far. I’ve thought long and hard about most of the posts I’ve made before making them. Really!
Obviously, as my knowledge on this topic is limited, my conclusions are sometimes in error.

CTCs have never been observed in our universe, but that doesn't mean you can just say anything you like about them.

I didn't realize I had said anything specifically about them.

PF has rules against overly speculative posts. There is not an exception to those rules for cases where you feel that it would suit your personal learning style better.

Nor should there be. If you believe that in my attempt to learn, my posts are over speculative, I am sure you will delete them.

 

[rede96]

There is no "time travel" (i.e., no CTCs) in SR.

Ah, right. That makes sense as I was trying to stay within the realms of SR

What you've done here is to make a leap straight from special relativity to quantum gravity. The more natural progression is SR->GR->quantum gravity. GR does allow CTCs, and figuring out what CTCs mean in that context is hard enough without jumping ahead to speculation about quantum gravity. We don't have a theory of quantum gravity.

I can see that although my intension was to stay within the realms of SR, I am jumping about a bit. I'll keep that in mind.

However, I wasn't aware that I was speculating about quantum gravity at all.

 

[DaveC426913]

Relativity is "all just theory" in the same sense that Darwinian evolution is "all just theory." Relativity is not a field where you can say anything that strikes your fancy, and it's just as valid as anything else, because it's "all just theory."

Point of order:
rede96 did not actually say "relativity is all just theory";
he said "time travel ... is all just theory".

Thus:

I have read and watched a lot on time travel, albeit pitched at a very 'layman' level. So I am aware and very open to it. There just isn't any empirical evidence to back it up, so it is all just theory.

 

[bcrowell]

One question I have about this topic is whether there is actually any link between CTCs and paradoxes, or whether the same paradoxes can arise in a universe without CTCs.

Historically, a lot of the motivation for wanting to prove things like the chronology protection conjecture was that it was felt that CTCs would lead to paradoxes such as the grandfather paradox, where you go back in time and kill you grandfather before he ever meets your grandmother. There was a group at CalTech about 20 years ago working on investigating whether this type of paradox really is a paradox:
http://authors.library.caltech.edu/3737/
http://authors.library.caltech.edu/6469/
I don't know whether this research program continued, petered out, or failed, or what, but the thrust of it was to show that CTCs don't necessarily lead to paradoxes. They worked with simple models like billiard balls going through wormholes.

There are also links between CTCs and the theory of computation. For instance, classical problems in computer science, like factoring large numbers, become easier if you have CTCs: http://www.frc.ri.cmu.edu/~hpm/project.archive/general.articles/1991/TempComp.html In terms of computation, we could hope that the laws of physics would allow perfect prediction of the future based on knowledge of initial conditions, in the sense intended by Laplace: "Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective positions of the things which compose it...nothing would be uncertain, and the future as the past would be laid out before its eyes." GR messes up Laplace's dream by allowing naked singularities such as the big bang, which are inherently unpredictable, and CTCs, which make it impossible to define the notion of initial conditions.

But say we live in a universe where there are no CTCs and no naked singularities other than the big bang, and suppose we have comprehensive initial data and can use a powerful computer to make predictions in the sense intended by Laplace. Then in theory it ought to be possible to predict that tomorrow I will eat an egg salad sandwich for lunch at the cafeteria and die of food poisoning. I get the prediction of this event out of the computer, so of course I call up the cafeteria and warn them not to serve any egg salad tomorrow, and I certainly don't eat any of it myself. This seems to me to be exactly equivalent to the time-travel paradox that arises if I die of food poisoning, my wife hops in the time machine and travels back in time, and she warns me. (Because she warns me, I don't eat it. But then because I don't eat it, she never gets the information that it was creeping with E. coli, so she never goes back in time and warns me.)

There is an interesting paper on this kind of thing by Wolpert, Physica D 237, 1257-1281 (2008), http://arxiv.org/abs/0708.1362 , where he claims to put certain limits on Laplace-style inference machines that are "independent of the precise physical laws governing our universe." One of his results, which he jokingly calls the "monotheism theorem," states that every universe can have at most one inference machine. (If there were more than one, then each could predict the other's behavior, and he shows that that's impossible by a Cantor diagonal argument.) There is a short and nontechnical discussion of Wolpert's work here: P.-M Binder, Theories of Almost Everything. Nature 455, 884-885 (2008), http://www.astro.uhh.hawaii.edu/PhilippeBinderResearchPage.htm . One thing that doesn't quite make sense to me about Wolpert's paper is that he seems to take time as a primitive concept, to assume that the real number line is a model of it, and that simultaneity is well defined. All of this seems relativistically invalid to me, which makes me doubt his claim that his results are "independent of the precise physical laws."

 

[bcrowell]

However, I wasn't aware that I was speculating about quantum gravity at all.

Quantum gravity is where we get things like the Planck time. The Planck time is \sqrt{\frac{\hbar G}{c^5}}, which has to do with both gravity and quantum mechanics, since it has both G and h-bar in it. There is no discretization of time in SR or GR. (I believe that some attempts at a theory of quantum gravity also have no quantization of time. In loop quantum gravity, I believe areas are quantized, but lengths and time intervals are not.)

 

[nitsuj]

Maybe I explained myself a bit better in my last post.

Wow, you explained it much more clearly there. From that perspective I'd agree with what you said regarding time travel. However, your description seems to take time itself out of the picture.

 

[Dale]

What does that mean exactly?

In natural units the Minkowski metric is:
ds^2 = -dt^2 + dx^2 + dy^2 + dz^2

So the time coordinate has a - sign in the metric and all of the space coordinates have + signs. Hence, they have different signs in the metric.

 

[edgepflow]

In natural units the Minkowski metric is:
ds^2 = -dt^2 + dx^2 + dy^2 + dz^2

So the time coordinate has a - sign in the metric and all of the space coordinates have + signs. Hence, they have different signs in the metric.

Some CTC solutions to the Field Equations happen when the sign of one of the space dimensions changes from positive to negative at the right conditions in space. This, for example, is how the Tipler Cylinder solution happens. This solution is for an infinite cylinder spinning between about 50% to over 90% speed of light at the surface. The radius away from the surface where light cones tip over horizontal and create CTC is a function of this angular velocity. Some say the solution may not be valid for finite cylinders. I have not seen the Tipler Cylinder cast into a form for a finite cylinder. Some also feel the cylinder would collapse by gravity under its own weight. I did a corrupt calculation and found it would not collapse for H/D up to about 300. Eventually, I would like to solve the EFE for a finite length Tipler Cylinder metric and see what happens.

 

[edgepflow]

But say we live in a universe where there are no CTCs and no naked singularities other than the big bang, and suppose we have comprehensive initial data and can use a powerful computer to make predictions in the sense intended by Laplace. Then in theory it ought to be possible to predict that tomorrow I will eat an egg salad sandwich for lunch at the cafeteria and die of food poisoning. I get the prediction of this event out of the computer, so of course I call up the cafeteria and warn them not to serve any egg salad tomorrow, and I certainly don't eat any of it myself. This seems to me to be exactly equivalent to the time-travel paradox that arises if I die of food poisoning, my wife hops in the time machine and travels back in time, and she warns me. (Because she warns me, I don't eat it. But then because I don't eat it, she never gets the information that it was creeping with E. coli, so she never goes back in time and warns me.)

This is an interesting point of view. The resolution likely follows those of time travel (REF: Black Holes, Worm Holes, & Time Machines by Al-Khalili).

a) The extent that the universe is deterministic is limited.
b) You do not have free will to avoid eating the bad egg salad.
c) As soon as your calculation is complete, the universe splits into two and you are now in the different one where you don't eat the bad salad.

 

[bcrowell]

a) The extent that the universe is deterministic is limited.
b) You do not have free will to avoid eating the bad egg salad.
c) As soon as your calculation is complete, the universe splits into two and you are now in the different one where you don't eat the bad salad.

A seems to me like a possible solution, but not really B or C because it's fairly straightforward to cook up versions of the paradox where free will and quantum mechanics don't play any role. You can make a version where the role of the human is replaced by that of a machine made out of gears and levers, which we'd all agree doesn't have free will. And you can make the machine a system that is described by Newton's laws to essentially a perfect degree of approximation, so that quantum effects aren't relevant.

Is the Al-Khalili book good?

 

[Fredrik]

I didn't see it that way. As you've stated there is lots of empirical evidence to back up Special relativity.

Yes, but that doesn't mean it's not a theory. I like to say that there are only two kinds of answers to questions about reality. Unfortunately I don't think the forum rules allow me to use the word that's appropriate for the kind that isn't a theory. So I'll just use the nicer word "garbage". There are two kinds of answers to questions about reality: theories and garbage. The distinction is essentially just that if it makes predictions about results of experiments, it's a theory. If it doesn't, it's garbage. No amount of empirical evidence can turn a theory into something other than a theory.

However, I wasn't aware that I was speculating about quantum gravity at all.

You mentioned quantization of space and time. There's no such thing in SR and GR, so you must have been talking about quantum gravity (even though you probably felt like you were talking about reality). People who haven't studied a lot of physics (and a surprising amount of people who have) often think of what they're saying as theory-independent statements about reality, but this doesn't make much sense when the terms they're using (e.g. "spacetime") are defined by theories. It will be easier for you to learn SR if you focus on what that the theory says, instead of speculating about what reality is "really" like.

 

[Fredrik]

A seems to me like a possible solution, but not really B or C because it's fairly straightforward to cook up versions of the paradox where free will and quantum mechanics don't play any role. You can make a version where the role of the human is replaced by that of a machine made out of gears and levers, which we'd all agree doesn't have free will.

You may not have the free will to build that machine.

 

[DaveC426913]

You may not have the free will to build that machine.

Niven's Law of Time Travel:
If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.

He wrote a short story where an attempt to build a time machine brought about the total destruction of the world where it was being built.

 

[DaveC426913]

From wikipedia:

Hans Moravec glosses this version of Niven's Law as follows:

"There is a spookier possibility. Suppose it is easy to send messages to the past, but that forward causality also holds (i.e. past events determine the future). In one way of reasoning about it, a message sent to the past will "alter" the entire history following its receipt, including the event that sent it, and thus the message itself. Thus altered, the message will change the past in a different way, and so on, until some "equilibrium" is reached--the simplest being the situation where no message at all is sent. Time travel may thus act to erase itself (an idea Larry Niven fans will recognize as 'Niven's Law')".

 

[rede96]

Wow, you explained it much more clearly there. From that perspective I'd agree with what you said regarding time travel. However, your description seems to take time itself out of the picture.

Maybe, I'm not sure to be honest! Although I did think that I was just taking a snapshot of the universe at a certain time. So time is included from that point of view I guess.

I think Ben answered my real question, which was does SR permit time travel. I gather from his answer here

There is no "time travel" (i.e., no CTCs) in SR. GR does allow CTCs

that it doesn't.

I've not dared to get into GR just yet, as I have a hard enough time with the calculations in SR. The concepts are sinking in some, even without the math. However I know to understand SR properly, I need both.

Anyway, I guess for this topic, I can't really add much more.

 

[rede96]

There are two kinds of answers to questions about reality: theories and garbage. The distinction is essentially just that if it makes predictions about results of experiments, it's a theory. If it doesn't, it's garbage. No amount of empirical evidence can turn a theory into something other than a theory.

You know what they say, one man's garbage is another man's gold. (Or something like that.)

I do take your point, but I find hard to see how we can go from nothing to theory without producing a whole load of "Garbage" Even Einstein himself must have produced his fair share before getting it right.

I'm sure many a good theory has come from garbage. I guess it is a necessary step.

However, a genuine question, asked with no malice or hidden agenda...is this PF forum for the discussion of theory or garbage as well?

It will be easier for you to learn SR if you focus on what that the theory says, instead of speculating about what reality is "really" like.

Yes, a very good point and I am beginning to realize that.

However, because of the type of person I am, I'm not looking for garbage or theory, I'm looking for answers. I want to know how things work, not just that they do. It is a really frustrating characteristic!

But I think I'll take your advice. (I Just have a couple more “garbage” questions I’d like to ask first! lol)

Does anyone know the best way for me to study SR, taking into account that I am a bit behind with the math?

 

[Fredrik]

I do take your point, but I find hard to see how we can go from nothing to theory without producing a whole load of "Garbage" Even Einstein himself must have produced his fair share before getting it right.

I'm sure many a good theory has come from garbage. I guess it is a necessary step.

Yes, this is certainly true.

However, a genuine question, asked with no malice or hidden agenda...is this PF forum for the discussion of theory or garbage as well?

I think the admins would agree that it's a forum for discussions about the established theories and the experiments that test the accuracy of their predictions. Most attempts to go beyond that would violate the rule against overly speculative posts. Of course, to some extent, what's considered "overly speculative" depends on the context. This is why the moderators allow a lot more speculation in the "beyond the standard model" forum than they do in here in the relativity forum.

Does anyone know the best way for me to study SR, taking into account that I am a bit behind with the math?

Learn to draw spacetime diagrams. Most questions about SR can be answered by drawing a few straight lines on a piece of paper. The book by Taylor & Wheeler has an approach based on spacetime diagrams, and not a lot of math (I think). I haven't read it myself, but it gets more recommendations than any other introductory text around here.

 

[bcrowell]

Does anyone know the best way for me to study SR, taking into account that I am a bit behind with the math?

Some good books about special relativity are (from easiest to hardest):

Takeuchi, An Illustrated Guide to Relativity
Mermin, It's About Time: Understanding Einstein's Relativity
Taylor and Wheeler, Spacetime Physics

The advantage of Takeuchi and Mermin is that they require very little background in math or physics. Their disadvantage is a total lack of connection to experiment.

 

[hanii]

does this mean that our heart rate would slow down...and all our body functions also slow down...if we were traveling at 90% of c so that our age doesn't increase compared to the person on the earth?

 

[rede96]

does this mean that our heart rate would slow down...and all our body functions also slow down...if we were traveling at 90% of c so that our age doesn't increase compared to the person on the earth?

No, not exactly. For you, wherever you are, (Called your Frame of Reference, or FoR as you might have seen posted by people.) you age at the same rate. It is only when you move relative to someone else that they will see you age slower.

The strange thing about Relativity is that you will also see them age slower too.

 

[rede96]

Some good books about special relativity are (from easiest to hardest):

Takeuchi, An Illustrated Guide to Relativity
Mermin, It's About Time: Understanding Einstein's Relativity
Taylor and Wheeler, Spacetime Physics

The advantage of Takeuchi and Mermin is that they require very little background in math or physics. Their disadvantage is a total lack of connection to experiment.

Brilliant, thanks.

 

[edgepflow]

Is the Al-Khalili book good?

Yes, in my opinion this one sticks out above the other popular books on the subject.

 

[DaveC426913]

does this mean that our heart rate would slow down...and all our body functions also slow down...if we were traveling at 90% of c so that our age doesn't increase compared to the person on the earth?

As rede96 intimates, yes and no.

Your heart rate and metabolism will be observed to be at a different rate than that of an observer moving relative to you. You experience nothing untoward.

But note: the time dilation is quite real; it is not an illusion. Time really is marching by at a different rate for the two of you.

 

[Tanelorn]

Hong Kong scientists 'show time travel is impossible'


http://news.yahoo.com/hong-kong-scientists-show-time-travel-impossible-150026913.html


Looks like the odds that any form of time travel exists are now extremely short.

 

[phinds]

Hong Kong scientists 'show time travel is impossible'


http://news.yahoo.com/hong-kong-scientists-show-time-travel-impossible-150026913.html


Looks like the odds that any form of time travel exists are now extremely short.

What does the speed of a photon have to do with the possibility of time travel?

 

[DaveC426913]

What does the speed of a photon have to do with the possibility of time travel?

Read here:
"The possibility of time travel was raised 10 years ago when scientists discovered superluminal -- or faster-than-light -- propagation of optical pulses in some specific medium, the team said.

It was later found to be a visual effect, but researchers thought it might still be possible for a single photon to exceed light speed."

It is a specific circumstance where it looked like time travel might be possible. That specific circumstance has been ruled out. It says nothing about any other possible solution to time travel.

 

[phinds]

Read here:
"The possibility of time travel was raised 10 years ago when scientists discovered superluminal -- or faster-than-light -- propagation of optical pulses in some specific medium, the team said.

It was later found to be a visual effect, but researchers thought it might still be possible for a single photon to exceed light speed."

It is a specific circumstance where it looked like time travel might be possible. That specific circumstance has been ruled out. It says nothing about any other possible solution to time travel.

Yes, I saw that statement but infered that the "ruled out" applied to ALL possible mechanisms somehow. Thanks for that clarification

 

[Tanelorn]

Well it seems like if we can't bend the rules enough for a single photon, a very tiny piece of information, then it doesn't hold much hope for real time travel with photons or anything else.

Time travel might involve having to leave and re-enter this universe entirely, but that doesn't make sense with any real Physics I know of.
This also assumes the past actually exists to be able to enter it and not a perpetual infinitesimally wide present.

 

[DaveC426913]

Well it seems like if we can't bend the rules enough for a single photon, a very tiny piece of information, then it doesn't hold much hope for real time travel with photons or anything else.

Well, that's like of like saying, if we can't bend the rules enough for a pig to fly, it doesn't hold much hope for American Airlines.

 

[Tanelorn]

lol. Well that is another way of looking at it I guess.

Another way of looking at my view is that time is more like a singularity than a dimension in which one can travel up and down. ie. Only the present ever exists because the present, which is where the 3Dimensions of space exist, is only infinitessimal width.

A similar way of looking at it is that the preset, where all the action is going on, is like the stylus playing a record. ie. narrow as a singularity. The past and future exist as potential information only, but the stylus isn't there so you can't travel there because the action of energy conversion (or music) isn't playing there yet.

A computer processing information in a large file is also similar to the above.

 

[DaveC426913]

lol. Well that is another way of looking at it I guess.

Another way of looking at my view is that time is more like a singularity than a dimension in which one can travel up and down. ie. Only the present ever exists because the present, which is where the 3Dimensions of space exist, is only infinitessimal width.

A similar way of looking at it is that the preset, where all the action is going on, is like the stylus playing a record. ie. narrow as a singularity. The past and future exist as potential information only, but the stylus isn't there so you can't travel there because the action of energy conversion (or music) isn't playing there yet.

A computer processing information in a large file is also similar to the above.

These are all examples of traveling though one spatial dimension at a constant rate through the time dimension; which is unfortunately not doing much to illuminate the issue.

 

[Tanelorn]

Dave, I think you are probably referring to the way in which time slows in a spaceship as one moves at near light speed through space?

My answer is that whilst I agree that the passage of time passes more slowly, the present time is still the only time ever being experienced inside the spaceship during the journey. So, for me, I still don't think I could describe time as being like a dimension of space that I can move forward or back, only the present ever exists and only in the present does matter and energy exist and interact. The rest is been and gone and exists only in history books and the future is yet to be.

The matter and energy of neither the future nor the past can interact in any way with the matter and energy of the present. We could today be occupying the same spatial coordinates as a supermassive black hole, but there isn't even a quantum of interaction on the matter of the present in the same location. I think we may well have all been overly influenced by H G Wells and Dr Who et al into seeing time in the wrong way.

 

[phinds]

My answer is that whilst I agree that the passage of time passes more slowly, the present time is still the only time ever being experienced inside the spaceship during the journey. So, for me, I still don't think I could describe time as being like a dimension of space that I can move forward or back, only the present ever exists and only in the present does matter and energy exist and interact. The rest is been and gone and exists only in history books and the future is yet to be.

I see your point, and it is reasonable, but it is impossible (I think) to discuss some relativistic effects without reference to time as a dimention and it is absolutely standard in physics to treat time as a dimension in the 4d existence that is space-time.

 

[DaveC426913]

So, for me, I still don't think I could describe time as being like a dimension of space that I can move forward or back,

Correct. Because it is not like that.

The key thing about a time-like dimension, as opposed to a space-like dimension, is that we are constrained to move through it only forward, and only at a fixed speed.

 

[Tanelorn]

So the basis for ever achieving true Time Travel is exceedingly weak? i.e. backwards and forwards at will and at any speed.

 

[phinds]

So the basis for ever achieving true Time Travel is exceedingly weak? i.e. backwards and forwards at will and at any speed.

I think "weak" is a very strong categorization of the basis. "Nonexistant" may not be true, but I think it would be closer to the current understanding.

 

[Cosmo Novice]

So the basis for ever achieving true Time Travel is exceedingly weak? i.e. backwards and forwards at will and at any speed.

Forward time travel - assuming the caveat that we have relatavistic travel, is entirely plausable. Future destination time could even be factored into the equation to arrive at a certain "when".

Backwards time travel is altogether different, barring CTCs, tachyons and the doctors TARDIS it is highly unlikely this is possible - you would need FTL. If we could go FTL, we could time travel but again I don't think it possible to ever achieve true FTL and I think it unlikely there is a mechanism which allows "percieved" FTL travel.

Just my two cents worth