Time Travel: What are the Theories & Factors?

[DaveC426913]

Dave, I'm still muddy on non-locality so I have a question here. I thought that non-locality said that you not only can but do have simultaneous cause/effect events separated by space, just that you can't send information that way since the "cause" is that you measure a property of one particle and the "effect" is that the property of another particle then instantaneously takes on its characteristics which are determined by the results of your measurement on the first particle. What am I missing here?

Thanks

I'm not really sure what you're getting at.

Two events happening simultaneously at opposite ends of a brick cannot have a cause/effect relationship.

Non-locality does not violate this cause-effect relationship because one event is not the cause of the other.

 

[DaveC426913]

A light-year is the distance light travels in one Earth year

Isn't that exactly what I said?

You seem to have twisted my words to say that I said; a light year is equally in distance to the distance Earth covers in its orbital circufrence.

ract/actr is correct. I misread what he said in post 26:

A light year = The distance light has traveled in the time it takes for the Earth to orbit the sun.

I retract (or should I say aetrctr) the assertion of his ignorance and apologize for that.

Unfortunately, it would not have prevented the banning. ract was banned for breaking the rules many, many times with unfounded assertions and zany ideas. As I said, it was only a matter of time.

 

[phinds]

Non-locality does not violate this cause-effect relationship because one event is not the cause of the other.

OK, that's where I'm confused. I was under the impression that the measurement on the first particle causes the instantaneous setting of the characteristic of the entangled particle. Is that not cause and effect? I AM fuzzy on all this so perhaps it's just the terminology that has me confused. I find non-locality mind-boggling.

 

[phinds]

I saw it. I haven't composed a reply yet. Hopefully, we can get the thread back on track before
a] the thread gets locked and cleaned up
b] ract gets banned
c] both

P.S I am not frustrated. Quite the contrary, I feel like a missionary with a lost soul that might, just might, be saved.

Well, I see ract finally got banned. Gosh I'm shocked, shocked I say. I would never have seen that coming.

I admire your abiilty to take that point of view in the face of someone like ract. I understand it and admire it, but can't really manage it myself. I just get swamped by the frustration and I have a deepseated dislike of people who just seem to not WANT to learn. I'm good with working all day with someone who seems to want to learn but in the face of that kind of willfull disregard for all evidence, I just can't maintain the equinaminity that you and Ben have.

I appreciate any help you can offer on non-locality

 

[DaveC426913]

OK, that's where I'm confused. I was under the impression that the measurement on the first particle causes the instantaneous setting of the characteristic of the entangled particle. Is that not cause and effect? I AM fuzzy on all this so perhaps it's just the terminology that has me confused. I find non-locality mind-boggling.

There is no cause and effect.

If there were, we could use it to signal.

We don't really know how it works (certainly I don't).

 

[DaveC426913]

I admire your abiilty to take that point of view in the face of someone like ract. I understand it and admire it, but can't really manage it myself. I just get swamped by the frustration and I have a deepseated dislike of people who just seem to not WANT to learn. I'm good with working all day with someone who seems to want to learn but in the face of that kind of willfull disregard for all evidence, I just can't maintain the equinaminity that you and Ben have.

He doesn't have access to evidence because he hasn't read up on anything. If he were science-aware yet still willfully ignorant, that would be one thing, but he's not. So all he's ever known are shadows on the cave wall - his whole world. But if someone unshackled him and let him out of the cave, he'd burst into tears at how vast and wondrous the real world is.

 

[phinds]

He doesn't have access to evidence because he hasn't read up on anything. If he were science-aware yet still willfully ignorant, that would be one thing, but he's not. So all he's ever known are shadows on the cave wall - his whole world. But if someone unshackled him and let him out of the cave, he'd burst into tears at how vast and wondrous the real world is.

Again ... admirable.

Perhaps my problem is that I'm too willing to believe that folks like that HAVE made some effort to understand things but have become set in their point of view early on and just aren't willing to change it even in the face of evidence. He certainly didn't seem at all interested in listening to any of us. You and Ben are great role models so I'll try to do better, but at my age, I am myself unfortunately somewhat set in MY ways. I spent a lot of time during my career mentoring programmers and engineers but they all WANTED to learn so it was relatively easy.

 

[rede96]

Well, I see ract finally got banned. Gosh I'm shocked, shocked I say. I would never have seen that coming.

I miss all the fun! :) Seems like I need to stay up later or move to a different time zone lol. But just in case Ract is still reading I thought I’d have stab at answering.

I didn't see all the posts, but I guess what Ract was saying is we all age, at the age we age, in our own frame. This is a biological process and 'moving' doesn't change that.

Our hearts will beat the same number of beats they were intended to beat despite 'moving around' (As an aside, there is an interesting theory that all animal’s hearts beat about the same number of beats. It is just that some beat a lot faster, hence they die sooner.)

What he didn't seem to get was the concept that moving relative to someone else means that the two people will age a different rates relative to each other, (i.e. the time between heart beats will seem to pass differently between the two frames.) but they will age the same in their own frame.

I think you're missing the point. What I'm saying is that it is just as valid to have a causality violation by linking xyzt with x'yzt as it is linking xyzt with xyzt'.

(Forgive my ignorance Dave, does x' just signify another point on the x-axis or is it also directional, as in -x?. Also, what do youn mean by 'linking'?)

So I may be missing the point, it depends on how I interpret your response. If you were saying that any two events in 4D space time could have a causality violation, regardless of coordinates, then yeah I get that.

In any case, what I was trying to say is that we have to treat the time axis differently the spatial ones and not assume the axis share properties.

EG: I can arbitrarily set direction for xyz. The direction for t is already set. I need to do something to move through xyz, I move through t without doing anything. etc.

So for me I don’t see us living in a 4D universe, as all the dimensions aren’t symmetrical. (Like a 1D, 2D and 3D universe would be without the time element.) I would say we live in a 3D universe with a time element. But that’s just me.

As regards time travel:

If I pick a point in time and space where I am, I can put everything I see in some spatial geometry, where other frames have a value along the xyz relative to me. However, other frames will not have a time value relative to me, t will always be 0.

And it is that last point that I get hung up on. I see how I can move between other frames along the spatial axis, as they have a value relative to me. But I can't see how I can move in time relative to another frame, as the time value is 0.

As all frames are valid, then no frame can move along the time axis relative to another frame.

So I conclude that time travel can't happen.

 

[bcrowell]

Folks, let's try to restore a decent signal-to-noise ratio to this thread by confining discussions to physics. If you see more posts that appear to be sockpuppets of ract, just report the posts and let the mentors deal with it.

As all frames are valid, then no frame can move along the time axis relative to another frame.

So I conclude that time travel can't happen.

You seem to be concluding that just based on mathematics, there can't be any such thing as time travel. But that is isn't so. GR allows spacetimes such as the Godel metric http://en.wikipedia.org/wiki/Gödel_metric that contain closed timelike curves (CTCs) http://en.wikipedia.org/wiki/Closed_timelike_curve . "CTC" is basically a relativist's euphemism for "time travel." One problem with your reasoning is that you're talking about frames, but GR doesn't have global frames of reference, only local ones.

In addition to the resources that I listed in #7, there is a very nice popular-level book, Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time, by Gott https://www.amazon.com/dp/0618257357/?tag=pfamazon01-20 .

I think part of the reason for the poor initial quality of this thread was that the loudest voices were the voices of those who hadn't taken any time to learn about the topic. Since they hadn't taken time out to study up on it, they were able to post immediately and copiously. A constructive thing for folks to do at this point would be to look at one of the print or online resources, find one that is at the right level for them, read it, and ask questions about things they don't understand.

 

[DaveC426913]

I didn't see all the posts, but I guess what Ract was saying is we all age, at the age we age, in our own frame. This is a biological process and 'moving' doesn't change that.

Our hearts will beat the same number of beats they were intended to beat despite 'moving around'

This is captured by saying in one's own frame of reference, one is always at rest. Thus, proper time is experienced.

What he didn't seem to get was the concept that moving relative to someone else means that the two people will age a different rates relative to each other, (i.e. the time between heart beats will seem to pass differently between the two frames.) but they will age the same in their own frame.

Perhaps. We are left to guess, since he insisted on (incorrect) assertions rather than questions.

(Forgive my ignorance Dave, does x' just signify another point on the x-axis or is it also directional, as in -x?. Also, what do youn mean by 'linking'?)

Yes. x' simply means some other point along the x axis.

A cause/effect link.


Poking an atom at x at time t cannot be a cause for the movement of an atom at x' at time t. The fact that they ar both at time t means they are separeted in space but not time, so for one to cause the other would require superluminal interaction. As soon as you have that, you can make causality violations.

So I may be missing the point, it depends on how I interpret your response. If you were saying that any two events in 4D space time could have a causality violation, regardless of coordinates, then yeah I get that.

Yes. If you set them up right. (i.e. farther apart in space (at c) than in time).

In any case, what I was trying to say is that we have to treat the time axis differently the spatial ones and not assume the axis share properties.

They are not identical, but for many purposes, they can be treated the same.

EG: I can arbitrarily set direction for xyz. The direction for t is already set. I need to do something to move through xyz, I move through t without doing anything. etc.

Yes. There are three space-like dimensions and one time like dimension. A timelike dimension is one that, for us, we can only move in one direction and we are doomed to move through it at a speed not in our control.

So for me I don’t see us living in a 4D universe, as all the dimensions aren’t symmetrical. (Like a 1D, 2D and 3D universe would be without the time element.) I would say we live in a 3D universe with a time element. But that’s just me.

The point is to look at the similarities rather than the differences. You wouldn't try to measure a brick using feet for width, light years for height and furlongs for length.

As regards time travel:

If I pick a point in time and space where I am, I can put everything I see in some spatial geometry, where other frames have a value along the xyz relative to me. However, other frames will not have a time value relative to me, t will always be 0.

Why? If you are moving relative to them at all, you're moving through spacetime at a different rate. One of the vlaues that will be in flux is time, just like any other dimension might be.

And it is that last point that I get hung up on. I see how I can move between other frames along the spatial axis, as they have a value relative to me. But I can't see how I can move in time relative to another frame, as the time value is 0.

As all frames are valid, then no frame can move along the time axis relative to another frame.

But we do it all the time when objects move relative to each other. Their passage through time diverges.

 

[rede96]

A constructive thing for folks to do at this point would be to look at one of the print or online resources, find one that is at the right level for them, read it, and ask questions about things they don't understand.

I agree totally, but unfortunately I don't always get time during the week for any in depth reading. But I can usually manage a post or three. (BTY I am looking for a good distance learning course. When I work away from home, I have plenty of time on my hands and would love to be able to appreciate this topic fully.)

So my approach will be to read and absorb what I see in the threads here and ask questions based on that. It isn't always the best way but it has helped my understanding greatly.

When I do get time, I do read up on things. But the best way I learn is to question and challenge. That often means thinking about things in a different way in order to check out my thought process and the theories. Particularly on matters that have not been proven conclusively, such as time travel.

Look Ben, with the greatest of respect, I think you should allow people a bit of slack from time to time. I know how frustrating it must be for you, but you've got the full range of people here and we all learn in different ways. As a trainer, that is one thing I do know from experience. As long as people are being respectful, there shouldn't be a problem with some slightly speculative posting, as long as it is in the spirit of learning and they are not asserting that they are right.

You seem to be concluding that just based on mathematics, there can't be any such thing as time travel. But that is isn't so. GR allows spacetimes such as the Godel metric http://en.wikipedia.org/wiki/Gödel_metric that contain closed timelike curves (CTCs) http://en.wikipedia.org/wiki/Closed_timelike_curve . "CTC" is basically a relativist's euphemism for "time travel." One problem with your reasoning is that you're talking about frames, but GR doesn't have global frames of reference, only local ones.

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!

 

[nitsuj]

So for me I don’t see us living in a 4D universe, as all the dimensions aren’t symmetrical. (Like a 1D, 2D and 3D universe would be without the time element.) I would say we live in a 3D universe with a time element. But that’s just me.

Not just you rede96. If I understand your interpretation that it's a "3D universe with a time component", than I agree as well that it is a better description of spacetime then 4D from a conceptual perspective.

The time "element" encompasses the other 3D's [STRIKE]as you pointed out[/STRIKE]. (personally, I feel time is merely an observation/derivative of the 3Ds)

I am unsure how you can "conclude" time travel via time dilation is not possible when considering your preferred description of "3D universe with a time element".

Your comment "...as all the dimensions aren’t symmetrical." implies you know that time dilation not only occurs, but is a typical quality of spacetime. Add to that, people can travel safely (proven) through slightly asymmetric spacetime and viola, time travel.

I don't find it a too great a leap to assume people could travel through remarkably distorted spacetime, however that may cause cancer so I don't recommend it be abused .

 

[rede96]

Yes. x' simply means some other point along the x axis.

Great, thanks for that.

Poking an atom at x at time t cannot be a cause for the movement of an atom at x' at time t. The fact that they ar both at time t means they are separeted in space but not time, so for one to cause the other would require superluminal interaction. As soon as you have that, you can make causality violations.

Cool, got it thanks. Although I am not sure how quantum entanglement fits into all this.


As regards time travel:

Why? If you are moving relative to them at all, you're moving through spacetime at a different rate. One of the vlaues that will be in flux is time, just like any other dimension might be.

But we do it all the time when objects move relative to each other. Their passage through time diverges.

Ok, let me try and explain my thought process here.

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.

So what I was doing was taking a snapshot of the universe from my frame. Trying to visualise what one of the chunks was like.

So as it is a snapshot, nothing is moving, don't have to worry about the differences in time etc., just mapping where everything is in space and time relative to me at that moment.

As I was looking at it from my frame, all time would be the same. Hence t = 0. Now obviously, if we keep indexing the chunks we would observe time dilation, sure. But at this snap shot I took, time would = 0 wrt my FoR.

So next I imagined that I wanted to move through space time to the next 'chunk' as it were.

I can see that there are lots of options for me to choose in the spatial directions. But there are no options for me to move anywhere in time.

All I can do is move to the next chunk, where time resets to 0 again from my frame.

So in anyone plank unit of time, there is no future or past to go to for my FoR. The best I could do maybe is to move about in 1 plank unit of time.

So that lead me to think maybe that is why we can't travel in the time dimension. Although we still would experience differences in time due to time dilation, but the two are different in my mind.

And, looking at it this way, even if I could move backwards, chunk by chunk, all I would do is put the universe and everything in it in reverse. So even if I was to travel back with it, it would be pointless, as my thoughts would be running backwards too. (If that makes sense!)

What I mean by that is maybe the way we think requires forward movement in time, and maybe we can't go back and forward in time simultaneously.

Also, as an aside, I also wondered if this was how a photon would see life. (Bad terminology I know, photons don't have a frame a reference!) As they travel at the speed of light they are not bound to move through time as we do, so they are free to move through space. Hence from the photons point of view, instant travel. A photon would move spatially in this chunk of space time until it was absorbed.

Anyway, as I said, this is just my way of thinking about.

Now for the fun bit! Where all the people that really understand this tell me what a crock of it is. Which is fine, because that actually really helps me.

So thanks in advance!

 

[rede96]

Not just you rede96. If I understand your interpretation that it's a "3D universe with a time component", than I agree as well that it is a better description of spacetime then 4D from a conceptual perspective.

The time "element" encompasses the other 3D's [STRIKE]as you pointed out[/STRIKE]. (personally, I feel time is merely an observation/derivative of the 3Ds)

Cool! Thanks

I am unsure how you can "conclude" time travel via time dilation is not possible when considering your preferred description of "3D universe with a time element".

Sorry, my bad. That wasn't what I was trying to say at all. Maybe I explained myself a bit better in my last post.

 

[Dale]

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.

While there are some theories that have quantized time or space there are to date no experimental observations that support them, so the effects are too small to matter at any scales that we can reach currently. So basically, although they exist, they don't actually explain anything that isn't already explained, including time dilation.

So that lead me to think maybe that is why we can't travel in the time dimension.

In this post and your earlier posts in this thread I think you are over-emphasizing this point quite a bit. 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 (-+++).

If there were three dimensions of time and one of space (---+) then you could have closed timelike curves in flat spacetime and you could not have closed spacelike curves. In a 1+1D spacetime with one dimension of time and one of space you cannot have closed curves of either natures, a timelike path can never "return" to the same time and a spacelike path can never "return" to the same position.

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.

 

[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.