Is SR-allowable time travel future-only ?

[MattAndMatthe]

Is SR-allowable time travel "future-only"?

Relativity theory allows for the possibility of time travel to the future, according to time dilation as illustrating by the twin paradox.

Wikipedia says, "it is currently unknown whether the laws of physics would allow backwards time travel."

My question is, doesn't relativity theory remove any uncertainty as to the possibility of past-ward time travel? What unknowns in SR give room for its possibility?

 

[torquil]

In a way, it does disallow it, but SR doesn't describe the universe perfectly.

In SR, backwards time travel can be represented as a worldline that goes in a loop. But since we know that no physical object can have such a worldline, this is not possible.

But another aspect is interaction between ordinary objects and tachyons (objects traveling faster than light). SR is a kinematical theory saying that no object can be accelerated up to or above the speed of light. However, it doesn't say that no tachyons exist. You can read about tachyons here:

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

If you can interact with tachyons, then you can influence events in the past.

But, if spacetime contains closed time like curves, then things are a bit different. I.e. if the topology of spacetime is such that a worldline can make a loop without every violating the rule of not accelerating past the speed of light, then it becomes a more complicated issue, because then relativity in itself doesn't rule it out, and people start to talk about quantum mechanics and entropy.

Usually, the existence of closed timelike curves is seen as unphysical, and such a spacetime manifold will not be taken as seriously as a possible spacetime of the universe.

Torquil

 

[Frame Dragger]

Generally speaking the emergence of CTCs in a theory is a real problem. Even the famous "wormhole" is never constructed (in this universe's history) in such a fashion as to be traversable.

Just think of it this way: You can always travel into your own future with the classic Time Dilation 'Planet of The Apes' scenario. Maybe not what you meant, but otherwise travel into the future is not possible in SR/GR. Traveling from the future to the past IS. Maybe a fine distinction.

You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be.

Personally I like Stephen Hawking's view, which is that any attempt to construct such a 'machine' would result in arbitrarily high energy densities and collapse (destruction) of the 'machine'.

This should be clear however: this is about a human being in THIS universe as described by SR and GR. QM and its interpreations have many more (conflicting) views.

 

[Altabeh]

Generally speaking the emergence of CTCs in a theory is a real problem. Even the famous "wormhole" is never constructed (in this universe's history) in such a fashion as to be traversable.

Just think of it this way: You can always travel into your own future with the classic Time Dilation 'Planet of The Apes' scenario. Maybe not what you meant, but otherwise travel into the future is not possible in SR/GR. Traveling from the future to the past IS. Maybe a fine distinction.

You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be.

Personally I like Stephen Hawking's view, which is that any attempt to construct such a 'machine' would result in arbitrarily high energy densities and collapse (destruction) of the 'machine'.

This should be clear however: this is about a human being in THIS universe as described by SR and GR. QM and its interpreations have many more (conflicting) views.

How would the bold-faced sentence be pictured in explicit terms of SR? I don't see anything in SR being capable of taking us to the future by any means! In GR, the most optimistic view over this issue is on the basis of finding a http://en.wikipedia.org/wiki/Closed_timelike_curve" put forward by Hawking which states that at large scales time travel is by no means possible to occur. Most physicists believe that the possibility of time travel even through this way is too beyond our technology and science! Quite a while ago, Hawking himself believed that if the universe changed its phase of expansion to contraction, then the arrow of time would possibly be reversed so the dream of time travel could come true because then we would go through time backwards and see what we did in the past. But Penrose proved him wrong and forced him to go back to his old-fashioned idea of microscopic possibility of time travelling!

The last thing to say, if I were you, I would go and study everything but this stuff!

AB

 

[George Jones]

For a little more on this, see

https://www.physicsforums.com/showthread.php?p=1420104#post1420104

and the links within.

 

[Frame Dragger]

How would the bold-faced sentence be pictured in explicit terms of SR? I don't see anything in SR being capable of taking us to the future by any means! In GR, the most optimistic view over this issue is on the basis of finding a http://en.wikipedia.org/wiki/Closed_timelike_curve" put forward by Hawking which states that at large scales time travel is by no means possible to occur. Most physicists believe that the possibility of time travel even through this way is too beyond our technology and science! Quite a while ago, Hawking himself believed that if the universe changed its phase of expansion to contraction, then the arrow of time would possibly be reversed so the dream of time travel could come true because then we would go through time backwards and see what we did in the past. But Penrose proved him wrong and forced him to go back to his old-fashioned idea of microscopic possibility of time travelling!

The last thing to say, if I were you, I would go and study everything but this stuff!

AB

Did you just not read the words immediately following, which were, "If you ignore all modern thinking on the issue". Did that sound like I was endorsing the possiblity to you, really? No.

Edit: To breakdown the sentence... "You could travel into the past in SR (if you ignore all modern thinking on the issue)..." and "While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be." Again... where on Earth does it seem like I'm endorsing the possiblity?

 

[Altabeh]

Did you just not read the words immediately following, which were, "If you ignore all modern thinking on the issue". Did that sound like I was endorsing the possiblity to you, really? No.

Edit: To breakdown the sentence... "You could travel into the past in SR (if you ignore all modern thinking on the issue)..." and "While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be." Again... where on Earth does it seem like I'm endorsing the possiblity?

Looks like you are really angry at me! The point of my early post was that I actually wanted to say "No" to the reality of your claim since I didn't get what exactly you were up to say by "If you ignore all modern thinking on the issue"! I don't get what this even has something to do with a paper job of revealing a possibility (you call it imagination, but I find it so delusive here since on one hand I don't even know how mathemtics of SR is able to get us such picture in which we can travel through time backwards, and on the other hand I got to say imaginations have shown us they could come true someday if assuming there is one that you are aware of in this case!) It's only, if possible, up to GR to decide whether we can go to past times or not! Period!

AB

 

[Frame Dragger]

Looks like you are really angry at me! The point of my early post was that I actually wanted to say "No" to the reality of your claim since I didn't get what exactly you were up to say by "If you ignore all modern thinking on the issue"! I don't get what this even has something to do with a paper job of revealing a possibility (you call it imagination, but I find it so delusive here since on one hand I don't even know how mathemtics of SR is able to get us such picture in which we can travel through time backwards, and on the other hand I got to say imaginations have shown us they could come true someday if assuming there is one that you are aware of in this case!) It's only, if possible, up to GR to decide whether we can go to past times or not! Period!

AB

I'm not angry, but I get the sense that English is not your first language, and you missed the point of what I was saying, or just parroted part of it. Again, perhaps this is a language-barrier issue, which I certainly don't hold against you.

 

[Altabeh]

I'm not angry, but I get the sense that English is not your first language, and you missed the point of what I was saying, or just parroted part of it. Again, perhaps this is a language-barrier issue, which I certainly don't hold against you.

That has nothing to do with language and I think you are trying to digress the main discussion! You say

"You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be."

Could you please explain to me (who has no knowledge of GR!);

1- What does "negative (scalar) curvature" have to do with time traveling and why positive curvature can't do this job?

2- How, again, can we assume that SR allows us to travel in time backwards?

Personally I like Stephen Hawking's view, which is that any attempt to construct such a 'machine' would result in arbitrarily high energy densities and collapse (destruction) of the 'machine'.

Completely believable and I also like that view!

This should be clear however: this is about a human being in THIS universe as described by SR and GR. QM and its interpreations have many more (conflicting) views.

Yes. But not all human-related stuff has entered in these sciences to make every dream come true for people! (My own idea)

AB

 

[Frame Dragger]

That has nothing to do with language and I think you are trying to digress the main discussion! You say

"You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be."

Could you please explain to me (who has no knowledge of GR!);

1- What does "negative (scalar) curvature" have to do with time traveling and why positive curvature can't do this job?

2- How, again, can we assume that SR allows us to travel in time backwards?



Completely believable and I also like that view!



Yes. But not all human-related stuff has entered in these sciences to make every dream come true for people! (My own idea)

AB

The question of negative curvature (like the classic "surface of a saddle" that is so often referenced) would allow for travel into the past without the creation of structures such as ERBs that only emerge in GR. As I was saying, it would be a radically different universe than the one who's history we seem to be a part of. Without that negative curvature travel in SR is only possible in the context of the Twin Paradox and other Time Dilation effects, or Tachyons (probably a mathematical artifact).

From a GR point of view (where my knowledge primarily lies, not in SR), the perfect example is AdS space embedded with (p,q) where q is >/= to 1, but that is definitely in GR, not SR.

I don't quite understand your last sentence however. I believe you're pointing out that everything we want to be true about physics isn't always true. If that is the case, I agree with you completely. If not, I'd love for you to rephrase it so I might understand.

When it comes to time travel/FTL in general however outside of the realm of SR, and in the universe we actually live in, we agree. Chronology Protection, conjecture or not, really does make sense and would expain the lack of temporal tourists and CTCs. The notion of infinite transit for some particles in the CTC causing an effect like the approach of a massive body to 'c' requiring arbitrarily high amounts of energy to accelerate seems sensible. Probably untestable, but fascinating.

Hmmmm... could you have an SR 'Alice' Universe? I realize you'd need GR to get there, but the notion of no universal definition of charge seems doable under SR.

 

[Demystifier]

Relativity theory allows for the possibility of time travel to the future, according to time dilation as illustrating by the twin paradox.

Wikipedia says, "it is currently unknown whether the laws of physics would allow backwards time travel."

My question is, doesn't relativity theory remove any uncertainty as to the possibility of past-ward time travel? What unknowns in SR give room for its possibility?

The claim that one can travel only to the future makes sense only if future can be distinguished from the past. However, the theory of relativity alone does not distinguish the future from the past. Instead, one must introduce a time arrow that points from the past to the future, which defines a preferred direction. On the other hand, a preferred direction seems to be in conflict with the theory of relativity. Nevertheless, there is no conflict, as explained in
http://xxx.lanl.gov/abs/gr-qc/0403121 [Found.Phys.Lett. 19 (2006) 259]
It turns out that travel to the past is impossible, but the reason for that has nothing to do with relativity. Instead, it has to do with thermodynamics.

 

[Frame Dragger]

The claim that one can travel only to the future makes sense only if future can be distinguished from the past. However, the theory of relativity alone does not distinguish the future from the past. Instead, one must introduce a time arrow that points from the past to the future, which defines a preferred direction. On the other hand, a preferred direction seems to be in conflict with the theory of relativity. Nevertheless, there is no conflict, as explained in
http://xxx.lanl.gov/abs/gr-qc/0403121 [Found.Phys.Lett. 19 (2006) 259]
It turns out that travel to the past is impossible, but the reason for that has nothing to do with relativity. Instead, it has to do with thermodynamics.

I've read similar arguments in the context of CPT symmetry and breaking, as well as a general examination of why we percieve a particular arrow of time being linked to thermodynamic processess.

 

[bcrowell]

The claim that one can travel only to the future makes sense only if future can be distinguished from the past. However, the theory of relativity alone does not distinguish the future from the past.

It seems to me that you're mixing up two different issues here: causality and the arrow of time.

Causality in SR can be expressed by saying that if observer 1 measures time t₁ between events A and B, then for any other observer, 2, the signs of t₁ and t₂ agree. You don't need an arrow of time in order to express this.

I think you can also have an arrow of time while violating causality, e.g., with CTCs.

Generally speaking the emergence of CTCs in a theory is a real problem.

But there are those who have made the case that CTCs aren't a big problem, e.g.: http://authors.library.caltech.edu/6469/

[EDIT] Removed mistaken word "proper" above.

 

[Frame Dragger]

But there are those who have made the case that CTCs aren't a big problem, e.g.: http://authors.library.caltech.edu/6469/

Ahhh, Kip Thorne. I respect the hell out of him, but I think this is yet another defense of wormholes. I respect the notion that there may be a huge number of 'safe' trajectories as described in the paper, but what if human beings were able to use such a wormhole? Immidiately you have causality issues separate from the issue of microscropic self-interaction.

The concept of CTCs that don't ACT like CTCs is fine in theory, but find me something without Kip Thorne's name on it that talks about traversable wormholes and CTCs as anything but a possible footnote.

 

[bcrowell]

For a little more on this, see

https://www.physicsforums.com/showthread.php?p=1420104#post1420104

and the links within.

I thought the Barcelo-Visser paper, http://arxiv.org/abs/gr-qc/0205066 , linked to from your #8 in that thread was very interesting. Basically they argue that classical scalar fields violate all known energy conditions, and allow (unstable) traversable wormholes. Because the result is *classical*, we can't wriggle out of it by imposing a high-energy cutoff, or by saying that semiclassical gravity is suspect. My personal reaction is that it would be really fun if the LHC failed to observe the Higgs. Presumably all hell would break loose among particle physicists, but if it leaves us with the ability to disbelieve the existence of scalar fields, then maybe it would be comforting for non-particle physicists.

 

[Altabeh]

The question of negative curvature (like the classic "surface of a saddle" that is so often referenced) would allow for travel into the past without the creation of structures such as ERBs that only emerge in GR. As I was saying, it would be a radically different universe than the one who's history we seem to be a part of. Without that negative curvature travel in SR is only possible in the context of the Twin Paradox and other Time Dilation effects, or Tachyons (probably a mathematical artifact).

Seems like we are gently transmigrating into a very different universe than the one SR creates. I was wondering if you could show me some article, paper or book regarding how negative curvature can create a universe where people go to past times and I believe strongly that time travel cannot ever be found in SR (meaning that all those scenarios e.g. twin paradox, tachyons and time dilation would never let one travel in time backwards; maybe if the question was "is traveling forwards in time possible" one would answered in the affirmative due to the twin paradox). An argument concerning time travel is given in this http://en.wikipedia.org/wiki/Time_travel#cite_ref-Gott_20-0" and for this reason the whole scenario has been put aside by physicists and myself. But now I'm probably getting the idea of your sentence "if you ignore all modern thinking on the issue" which can be this big paradox rising in SR version of time travel and why I'm telling you that SR is incapable of giving us an opportunity to see our past is this, right? Hope that you are no longer angry at me!

From a GR point of view (where my knowledge primarily lies, not in SR), the perfect example is AdS space embedded with (p,q) where q is >/= to 1, but that is definitely in GR, not SR.

Mine too lines in GR, and of course there are lots of examples which are left abandoned because of all those annoying paradoxes. One of the interesting possibilities is Alcubierre drive that GR offers for those who like to travel in time which is similar to the idea of a "tachyonic time machine" because it makes use of FTL (faster than light) scenario, thus again serious problems such as VoC (violation of causality) or violation of energy conditions in gigantic amounts happen to exist in this case, too! (For more info read http://www.arxiv.org/abs/gr-qc/9702026 and http://www.arxiv.org/abs/gr-qc/9905084).

I don't quite understand your last sentence however. I believe you're pointing out that everything we want to be true about physics isn't always true. If that is the case, I agree with you completely. If not, I'd love for you to rephrase it so I might understand.

I mean that, for example, Higgs bosons have not yet been observed but they exist in theory and maybe they are going to remain on papers for good. The dilation of "biological ageing" has not ever been shown to be true in twin paradox experimentally though we are used to making example of a twins, one on the Earth and the other on a spaceship traveling at the speed of .9c so after 40 years that the flying brother comes home he is way younger than his brother! But this, too, may rest in theories forever.

When it comes to time travel/FTL in general however outside of the realm of SR, and in the universe we actually live in, we agree. Chronology Protection, conjecture or not, really does make sense and would explain the lack of temporal tourists and CTCs. The notion of infinite transit for some particles in the CTC causing an effect like the approach of a massive body to 'c' requiring arbitrarily high amounts of energy to accelerate seems sensible. Probably untestable, but fascinating.

But I don't want to be much of a pessimist when it comes to possibility of time travel. I think someday one of us will show how to do it! Don't get me wrong!

Hmmmm... could you have an SR 'Alice' Universe? I realize you'd need GR to get there, but the notion of no universal definition of charge seems doable under SR.

Let me stay with my GR 'Alice' Universe! :)

 

[Altabeh]

The claim that one can travel only to the future makes sense only if future can be distinguished from the past. However, the theory of relativity alone does not distinguish the future from the past. Instead, one must introduce a time arrow that points from the past to the future, which defines a preferred direction. On the other hand, a preferred direction seems to be in conflict with the theory of relativity. Nevertheless, there is no conflict, as explained in
http://xxx.lanl.gov/abs/gr-qc/0403121 [Found.Phys.Lett. 19 (2006) 259]
It turns out that travel to the past is impossible, but the reason for that has nothing to do with relativity. Instead, it has to do with thermodynamics.

Causality has nothing to do with the arrow of time in general. This means that it would be imaginable to find a relation between them, as Stuckelberg in formulating the TCP symmetry of QFT puts forward, for anti-particles which appear to travel backwards in time and be detected before their emission, causality seems to be violated, but this is a matter of convention in physics and your claim is not generally correct!

AB

 

[Frame Dragger]

Personally I'm very much a pessemist when it comes to time travel/FTL. The only way I can imagine it working is if human beings changed ourselves in such a fashion as to no longer be human. Either we'd have to keep track of multiple times and histories, or construct wormhole that somehow (pure fiction I think) acts as 'tuner', delivering you to a distant place FTL, but adjusting for time as if the trip were made at c.

I don't think the universe works that way however. I think it's a vast universe of what are essentially islands.

By the way, it's an important distinction to make that while yes, many textbooks already have the Higgs in there, it's not the Boson but the MECHANISM which has such faith in the scientific community. Granted, that has led to a pretty high confidence that the Higgs Boson will be found, but a failure to do so would still leave the door open for underlying means by which mass arises. Of course, it would be an experimental and practical kick in the beans to not find it. :rofl:

Finally, and most importantly, YES, I do see what you mean about SR, and I am on the same page with you. Again, I really never was angry with you, I'm just a little harsh sometimes. You're correct however, I don't see any way with just SR, and that is the title of the thread. I should have given the matter more careful consideration and remembered the issues of gravity and multiple framees GR brought. Ah well, I live I learn.

 

[Frame Dragger]

Causality has nothing to do with the arrow of time in general. This means that it would be imaginable to find a relation between them, as Stuckelberg in formulating the TCP symmetry of QFT puts forward, for anti-particles which appear to travel backwards in time and be detected before their emission, causality seems to be violated, but this is a matter of convention in physics and your claim is not generally correct!

AB

In fact, Hawking Radiation can be described in terms of a single particle in which the emission is time-reverse of the other half of the formerly 'virtual' pair falling into the singularity and past the EH.

I have to say, I still remember how shocked I was when I first learned that Parity necessarily symmetric. It's odd at first, and definitely has lead to a lot of anthropic thinking.

 

[Rasalhague]

Nevertheless, there is no conflict, as explained in
http://xxx.lanl.gov/abs/gr-qc/0403121 [Found.Phys.Lett. 19 (2006) 259]
It turns out that travel to the past is impossible, but the reason for that has nothing to do with relativity. Instead, it has to do with thermodynamics.

Very interesting.

So this defines time's arrow, the thermodynamic arrow of time, "TA", as the gradient of entropy density (a scalar field, the magnitude of the entropy density vector field), and "physical TA" as the component of TA tangent to a curve. Nikolic raises the possibility that this curve needn't be timelike. What would it mean for TA to be spacelike?

Later he writes "physical time is defined globally, as a coordinate with respect to which the total entropy increases." And "for observers that are able to measure entropy in large regions of spacetime, it is natural to divide the spacetime into appropriately large regions and to assign the entropy to each region as the average entropy of this region."

So now, and please correct me if I've misunderstood, we have the following concepts, "TA" (local, invariant, a gradient), "physical TA" (local, frame-dependent, a tangent vector), coordinate TA (regional), "physical time" (global, the average direction of increase of entropy). I suppose, for a timelike curve, "physical TA" is the direction of proper time, and "coordinate TA" the (frame-dependent) direction of coordinate time.

At the bottom of page 4, what does "a constraint on the boundary condition" mean?

 

[Altabeh]

In fact, Hawking Radiation can be described in terms of a single particle in which the emission is time-reverse of the other half of the formerly 'virtual' pair falling into the singularity and past the EH.

I have to say, I still remember how shocked I was when I first learned that Parity necessarily symmetric. It's odd at first, and definitely has lead to a lot of anthropic thinking.

I'm very unhappy with what Feynman and Stackelberg give away as to the mechanism of working with systems containing particle-anitparticle interactions and how these systems are invariant under TCP just because arrow of time is something more weird than Bondi mass (negative mass)! You know most of the problems in Cosmology and GR lies in the shortage of something like exotic matter which is a diamond in the rough and has yet to become we-established among physicists if they keep believing in symmetries of nature without including mass-inversion! But how can one come to a negative mass in Physics? Well, simply you need to find other approaches to solve problems which, in my eyes, is not impossible at all, though it would take a lot of time to see that sticking to old ideas is not going to get us out of messes we are in right now!

AB

 

[Frame Dragger]

I'm very unhappy with what Feynman and Stackelberg give away as to the mechanism of working with systems containing particle-anitparticle interactions and how these systems are invariant under TCP just because arrow of time is something more weird than Bondi mass (negative mass)! You know most of the problems in Cosmology and GR lies in the shortage of something like exotic matter which is a diamond in the rough and has yet to become we-established among physicists if they keep believing in symmetries of nature without including mass-inversion! But how can one come to a negative mass in Physics? Well, simply you need to find other approaches to solve problems which, in my eyes, is not impossible at all, though it would take a lot of time to see that sticking to old ideas is not going to get us out of messes we are in right now!

AB

Ah, perhaps here I can help. A negative mass in physics COULD be achieved as a local imbalance in the ergoregion of a BH. Think of a really REALLY extreme Casimir setup. That said, we're talking about a QM/GR theory around a GR object that may or may not exist and 'live' as described by the math, so who knows?

 

[Altabeh]

Ah, perhaps here I can help. A negative mass in physics COULD be achieved as a local imbalance in the ergoregion of a BH. Think of a really REALLY extreme Casimir setup. That said, we're talking about a QM/GR theory around a GR object that may or may not exist and 'live' as described by the math, so who knows?

Ah, it will be of great help and assistance if you mind naming some article or book regarding that matter! It sounds so catchy to me if negative mass exists in the ergoregion of a BH, say, a Kerr BH, because my own PhD thesis is somehow related to this stuff and interaction of ordinary and non-ordinary matter inside a Kerr BH!

AB

 

[Frame Dragger]

Ah, it will be of great help and assistance if you mind naming some article or book regarding that matter! It sounds so catchy to me if negative mass exists in the ergoregion of a BH, say, a Kerr BH, because my own PhD thesis is somehow related to this stuff and interaction of ordinary and non-ordinary matter inside a Kerr BH!

AB

It is specifically Kerr BHs that this is concerning (since they rotate). I'll do some checking around, but it's been a while since I read about this. I should be able to find some decent citations however.

 

[bcrowell]

Ah, perhaps here I can help. A negative mass in physics COULD be achieved as a local imbalance in the ergoregion of a BH. Think of a really REALLY extreme Casimir setup. That said, we're talking about a QM/GR theory around a GR object that may or may not exist and 'live' as described by the math, so who knows?

I think the Casimir example is noncontroversial, but because it's quantum-mechanical it's kind of hard to interpret. We don't have a theory of quantum gravity, and we don't know whether semiclassical gravity (coupling a classical gravitational field to quantized matter and radiation fields) gives predictions that match up with reality. E.g., semiclassical gravity blows up and needs to be renormalized at the event horizon of a black hole, and even with renormalization it predicts things that most physicists don't believe are true.

As a purely classical example, a non-minimally-coupled scalar field violates every known energy condition, including the averaged null energy condition, which is the weakest that anyone has been working with ( http://arxiv.org/abs/gr-qc/0205066 ). The only question is whether nature has any fundamental scalar fields, e.g., the Higgs.

 

[Frame Dragger]

I think the Casimir example is noncontroversial, but because it's quantum-mechanical it's kind of hard to interpret. We don't have a theory of quantum gravity, and we don't know whether semiclassical gravity (coupling a classical gravitational field to quantized matter and radiation fields) gives predictions that match up with reality. E.g., semiclassical gravity blows up and needs to be renormalized at the event horizon of a black hole, and even with renormalization it predicts things that most physicists don't believe are true.

As a purely classical example, a non-minimally-coupled scalar field violates every known energy condition, including the averaged null energy condition, which is the weakest that anyone has been working with ( http://arxiv.org/abs/gr-qc/0205066 ). The only question is whether nature has any fundamental scalar fields, e.g., the Higgs.

Well... if the Higgs boson isn't found who knows, but if it IS... just think, a few years from now... that question could be answered.

 

[Phrak]

The claim that one can travel only to the future makes sense only if future can be distinguished from the past. However, the theory of relativity alone does not distinguish the future from the past. Instead, one must introduce a time arrow that points from the past to the future, which defines a preferred direction. On the other hand, a preferred direction seems to be in conflict with the theory of relativity.

I'm not sure what your context is. If we keep it in terms of special relativity as titled by the OP then continuous Lorentz transform ensures that t'=kt, where k is a positive value. What we label positive time is different than negative time for transformations about any given spactime event.

But if we are talking about the flat spacetime of special relativity, without curvature, the manifold can still be toroidal, and future events can lead to past events and result in closed timelike curves. I don't think the OP was actually asking about this sort of thing, but you never know.

The result, 't=-kt, can be obtained with a nonorientable manifold can't it?

 

[kg4pae]

Relativity theory allows for the possibility of time travel to the future, according to time dilation as illustrating by the twin paradox.

Wikipedia says, "it is currently unknown whether the laws of physics would allow backwards time travel."

My question is, doesn't relativity theory remove any uncertainty as to the possibility of past-ward time travel? What unknowns in SR give room for its possibility?

In Relativity, this question revolves around the line element

$$- c^{2} {d \tau}^2 = g_{\mu\nu} {d x^\mu}{d x^\nu}$$

For Special Relativity this reduces to

$$- c^{2}{d \tau}^2 = \left(-c^2 + v^2\right) {d t}^2$$

From this we get

$$\gamma \equiv \frac{d t}{d \tau} = \frac{1}{\sqrt{1 - {\left(\frac{v}{c}\right)}^2}}$$

Now for $$\gamma$$ we normally could choose either postive (forward time-travel) or negative (backward time-travel) on the square root. In practice the postive is chosen because there is no experimental evidence for choosing the negative. In that sense Relativity can allow for backward time-travel without it actually being possible in reality. This argument also applies for General Relativity with the same definition for $$\gamma$$.

 

[kg4pae]

In a way, it does disallow it, but SR doesn't describe the universe perfectly.

It's not suppose to: that's what GR is for.

SR is a kinematical theory saying that no object can be accelerated up to or above the speed of light.

That is if it has mass since then if $$v > c$$ then $$\gamma = \frac{1}{\sqrt{1 - {\left(\frac{v}{c}\right)}^2}}$$ is imaginary. In fact then $$E = \gamma m c^2$$ is also imaginary.

However, it doesn't say that no tachyons exist. You can read about tachyons here:

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

I find that URL actually a bit pathetic in explaining tachyons. Since no material particle can go faster than the speed of light in a vacuum (cf above), the only real way to define tachyons is material particles that go faster than light in their medium. Such tachyons have been observed in nuclear reactors.

 

[kg4pae]

That has nothing to do with language and I think you are trying to digress the main discussion! You say

"You could travel into the past in SR (if you ignore all modern thinking on the issue) if you achieved geometries in space-time that would have to contain a negative curvature. While it's possible on paper to imagine such geometries in line with SR, it is hard to imagine that they could exist in nature without the universe being radically different than we observe it to be."

Could you please explain to me (who has no knowledge of GR!);

1- What does "negative (scalar) curvature" have to do with time traveling and why positive curvature can't do this job?

Seeing how the topic of discussion is "Is SR-allowable time travel 'future only'?", we must be talking about the metric $$diag\left(-1,1,1,1\right)$$. Since this metric has no coordinate-values in it, automatically we can say that the Christoffel-symbols are zero, the Ricci-tensors are zero and thus the curvature is zero. If the curvature is anything but zero, we must be talking GR and thus must be careful not to overuse SR where it doesn't apply.

2- How, again, can we assume that SR allows us to travel in time backwards?

Strictly speaking yes, but you have to be careful how you interpret it. Since

$$\gamma\equiv\frac{d t}{d \tau} = \frac{1}{\sqrt{1 - {\left(\frac{v}{c}\right)}^2}}$$

whether $$\gamma$$ (the rate at which observed time changes with respect to proper time) is positive or negative depends on how we choose the square root (in SR). Until now, the justification is that no experiment allows us to choose the negative square root and backward time-travel.

 

[kg4pae]

Ah, perhaps here I can help. A negative mass in physics COULD be achieved as a local imbalance in the ergoregion of a BH. Think of a really REALLY extreme Casimir setup. That said, we're talking about a QM/GR theory around a GR object that may or may not exist and 'live' as described by the math, so who knows?

I was wondering if you could post details about this. I'm interested and have never heard of negative mass existing anywhere.

 

[Frame Dragger]

I was wondering if you could post details about this. I'm interested and have never heard of negative mass existing anywhere.

For a total negative Komar Mass: http://www.iop.org/EJ/article/0264-9381/23/24/L01/cqg6_24_l01.pdf?request-id=b2ceed1b-e7eb-4edb-b801-d0ecd3739f7c

The other issue is the that counterrotating particles in the ergoregion will have negative energy, causing regions of 'imbalance' in line with QM. It's a central feature of rotating BHs. Everything from Wikipedia onward will have info for you. As far as I know, the issue is negative MASS, in the form of negative energy. Kerr BHs are the models for this.

EDIT: You said, "existing anywhere"... Well... to be fair everything I'm talking about is pure theory and conjecture. The ergoregion is math and theory and that's pretty much it right now.

EDIT2: Here's some more postage. http://adsabs.harvard.edu/full/1996MNRAS.282..580Y
Also, if you haven't already do some research on The Penrose Process for some more insight.

EDIT3: For the OP, sorry, I realize that this has 0% to do with SR.

 

[Altabeh]

Seeing how the topic of discussion is "Is SR-allowable time travel 'future only'?", we must be talking about the metric $$diag\left(-1,1,1,1\right)$$. Since this metric has no coordinate-values in it, automatically we can say that the Christoffel-symbols are zero, the Ricci-tensors are zero and thus the curvature is zero. If the curvature is anything but zero, we must be talking GR and thus must be careful not to overuse SR where it doesn't apply.

Pardon me, but what is this all about? Did we say something very strange to you that made you start giving away some fundamental things in the theory of Relativity?

Strictly speaking yes, but you have to be careful how you interpret it. Since

$$\gamma\equiv\frac{d t}{d \tau} = \frac{1}{\sqrt{1 - {\left(\frac{v}{c}\right)}^2}}$$

whether $$\gamma$$ (the rate at which observed time changes with respect to proper time) is positive or negative depends on how we choose the square root (in SR). Until now, the justification is that no experiment allows us to choose the negative square root and backward time-travel.

The mechanism of traveling backwards in time is not like this in SR! We talked about this earlier and said that it only comes from twin paradox and FTL scenario which in turn gives rise to the violation of causality! Not only experiment does not allow us to use

$$\gamma\equiv\frac{d t}{d \tau} = -\frac{1}{\sqrt{1 - {\left(\frac{v}{c}\right)}^2}}$$,

but also the theory itself does not let one use the negative sign because coordinate time and proper time are both either future-directed or past-directed.

[EDIT]: Suppose the time boost in Lorentz transformations. If we take a differential of it,

$$dt'=\gamma(dt-\frac{v}{c^2}dx)$$,

then obviously under a time inversion $$dt\rightarrow -dt$$, we have $$dt'\rightarrow -dt'$$ and thus $$dt dt'>0$$. So the proper time

$$\tau= \int_{t_0}^{t_1} {1/ \gamma} dt$$ changes sign under time inversion because of that dt in the integrand.

AB

 

[Altabeh]

For a total negative Komar Mass: http://www.iop.org/EJ/article/0264-9381/23/24/L01/cqg6_24_l01.pdf?request-id=b2ceed1b-e7eb-4edb-b801-d0ecd3739f7c

Very helpful!

AB

 

[Frame Dragger]

Very helpful!

AB

Glad to hear it. :)