Time Travel Revisited

[joigus]

 

Scientists from the University of Queensland claim to have found a possible mechanism for time travel.

Laypeople-level account:

https://www.uq.edu.au/news/article/2020/09/young-physicist-squares-numbers’-time-travel

The paper:

https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc/pdf

The abstract:

Quote

The theory of general relativity predicts the existence of closed time-like curves(CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a framework for deterministic, reversible, dynamics compatible with non-trivial time travel,where observers in distinct regions of spacetime can perform arbitrary local operations with no contradiction arising. However, only scenarios with up to three regions have been fully characterised, revealing only one type of process where the observers can verify to both be in the past and future of each other.Here we extend this characterisation to an arbitrary number of regions and find that there exist several inequivalent processes that can only arise due to non-trivial time travel. This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.

Keywords: closed time-like curves, causality, time

The paper is highly mathematical, and I haven't found the time to take a more detailed look at this topic. I've just learnt about it. It seems that the key idea is to find plausible trajectories in phase-space for particles in the background geometry.

That wasn't very informative. Sorry I can't say anything else significant right now. Any comments welcome.

Edit: Whenever I think of these mathematical solutions, I can't help picturing the quite terrifying accretion disks of black holes... You know what I mean.

Edited October 9, 2020 by joigus

[MigL]

CTCs have 'provided a mechanism for time travel for quite some time, but they provide no answers for the causality breaking paradoxes that arise.
I haven't looked at the paper, and probably wouldn't begin to understand it, but I assume it attempts to provide those answers.

And saying CTCs provide a mechanism for time travel is being generous, there is still the problem of space-like translation via wormhole ( or some such device ) to achieve time travel. CTCs just move the goalposts of the problem from one ( current ) impossibility, to another ( current ) impossibility.

[Kartazion]

8 hours ago, joigus said:

It seems that the key idea is to find plausible trajectories in phase-space for particles in the background geometry.

In the same genre: Spacetime from bits
 

Quote

Spacetime, reconstructed
Theories of holographic duality feature a correspondence between a gravitational system and a strongly interacting conformal field theory (CFT) living on the system's boundary. Through this correspondence, the CFT encodes the geometry of spacetime in the gravitational system. Van Raamsdonk analyzed the role of entanglement in this theoretical framework. Instead of considering a single CFT, the author's starting point was a collection of CFT “bits” that are mutually entangled but do not interact with one another. The spacetime that these bits collectively encode was then shown to be arbitrarily close to the one encoded by the original CFT, suggesting that entanglement plays a crucial role in the emergence of spacetime.
 

 

Quote

In the anti–de Sitter/conformal field theory approach to quantum gravity, the spacetime geometry and gravitational physics of states in some quantum theory of gravity are encoded in the quantum states of an ordinary nongravitational system. Here, I demonstrate that this nongravitational system can be replaced with an arbitrarily large collection of noninteracting systems (“bits”) placed in a highly entangled state. This construction makes manifest the idea that spacetime geometry emerges from entanglement between the fundamental degrees of freedom of quantum gravity and that removing this entanglement is tantamount to disintegrating spacetime. This setup also reveals that the entangled states encoding spacetimes may be well represented by a certain type of tensor network in which the individual tensors are associated with states of small numbers of bits.

 

Time travel and time dilation use the same mechanism. Is not it?

Or how to qualify this difference?

[joigus]

7 hours ago, MigL said:

CTCs have 'provided a mechanism for time travel for quite some time, but they provide no answers for the causality breaking paradoxes that arise.
I haven't looked at the paper, and probably wouldn't begin to understand it, but I assume it attempts to provide those answers.

And saying CTCs provide a mechanism for time travel is being generous, there is still the problem of space-like translation via wormhole ( or some such device ) to achieve time travel. CTCs just move the goalposts of the problem from one ( current ) impossibility, to another ( current ) impossibility.

Generally, I agree with you. In fact, I don't think time travel will ever be possible.

My interpretation was that they claim to have provided a possible mechanism. Here's what I interpreted as the claim:

Quote

This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.

So I was not being that generous, if you think about it.

1 hour ago, Kartazion said:

Time travel and time dilation use the same mechanism. Is not it?

Not really. Hypothetical time travel uses curvature, and time dilation is a different thing altogether and does not require curvature. Perhaps someone can provide a more complete explanation.

1 hour ago, Kartazion said:

In the same genre: Spacetime from bits

Well, spacetime from bits requires quantum mechanics and the holographic principle. Conjectural time travel is based on classical GR.

Edited October 9, 2020 by joigus

[MigL]

I think they are claiming that time travel is self-consistent, and paradoxes do not necessarily occur.
And have come to this conclusion by looking at the 'general' case, not specific cases.

And sorry if I gave the impression that it was your claim of a viable mechanism.
I intended their claim was 'generous'.
 

[Kartazion]

11 hours ago, joigus said:

Not really. Hypothetical time travel uses curvature, and time dilation is a different thing altogether and does not require curvature. Perhaps someone can provide a more complete explanation.

Yes. It's because I read for an acceleration: Whether it is time dilation or time travel, the common responsible constant is the speed of light.

Here is an excerpt from wikipedia of time travel which implies a link with time dilation in relation to c 

Quote

https://en.wikipedia.org/wiki/Time_travel#Time_dilation

The theory of relativity states that the speed of light is invariant for all observers in any frame of reference; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light. Time dilation may be regarded in a limited sense as "time travel into the future": a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at relativistic speeds or through the effects of gravity.

But I understand that there is a difference between the Quantum Spacetime and Spacetime.

[Markus Hanke]

22 hours ago, joigus said:

The theory of general relativity predicts the existence of closed time-like curves(CTCs), which theoretically would allow an observer to travel back in time and interact with their past self.

I have several issues with this statement, and other careless statements like this.
First and foremost, GR does not predict the existence of CTCs; it's rather the other way around, in that CTCs are consistent with the laws of GR, in the sense that such spacetimes are valid solutions to the field equations. The problem here is that a) not every solution to the field equations is necessarily physically realisable, and b) spacetimes containing CTCs are known to be highly unstable under even miniscule perturbations of initial conditions - and those conditions are only approximately true in the real world to begin with. Personally I would be very surprised if CTCs existed in the real world, and they are certainly not "predicted" by GR.

Secondly, in what sense would a CTC be a "time machine"? A topological construct like this would connect an event to itself via a world line of non-zero length; not only would you not be able to travel forward or backward in time with this in a global sense, you would in effect be doomed to just 'loop through' that same event over and over again, without any means of ever escaping (neither spatially nor temporally). To me, this is actually the opposite of a time machine - it's like a Groundhog machine, if that reference makes sense. In ordinary spacetime, we age forward in time, so at least we travel through time in that (very limited) sense; when trapped in a CTC, you can't even do that much, you can only repeat the same cycle again and again.

Thirdly, they seem to forget to mention that CTCs can exist only in vacuum; so even if you could physically realise such a spacetime (a possibility at least in principle), it would be impossible to introduce any kind of test particle without collapsing the spacetime into some more conventional geometry.

[ahmet]

23 hours ago, joigus said:

The paper is highly mathematical

   

maybe it would be good to wait @HallsofIvy 's assessment, but to me ; hearing "highly" word only reminiscents my romantism or colors rather than mathematics.

surely,it is not a mathematical expression  I am highly romantic hahah ha  

Edited October 10, 2020 by ahmet

[MigL]

4 hours ago, Markus Hanke said:

Secondly, in what sense would a CTC be a "time machine"?

This is the example I've always used...

Bart and Lisa Simpson each have a wormhole generator, such that they can look through the wormhole and 'see' ( world line of zero length ) each other. Bart takes his wormhole generator and goes on a relativistic journey to another star, and returns, all the while looking at Lisa through the wormhole. His journey takes ten years, but when he gets back to Earth, 100 years have elapsed, and Lisa is long dead.
So Bart steps through the wormhole and rejoins Lisa, whom he can still see, 90 years in the past.

A lot of 'what ifs' and assumptions are involved, such as stable and predictable wormholes that can be 'linked', Relativistic ( close to c travel ), etc., but, current impossibilities aside, the 'mechanism' is there. As to whether it will ever be realizable, I doubt it very much also.

Edited October 10, 2020 by MigL

[joigus]

4 hours ago, Markus Hanke said:

On 10/9/2020 at 2:02 PM, joigus said:

The theory of general relativity predicts the existence of closed time-like curves(CTCs), which theoretically would allow an observer to travel back in time and interact with their past self.

I have several issues with this statement, and other careless statements like this.

Just to clarify, because the quoting function makes it look as if you said something, which you didn't. Again: I didn't say this. It's on the abstract.

But thanks a lot for your reply, Markus.

16 hours ago, MigL said:

And sorry if I gave the impression that it was your claim of a viable mechanism.
I intended their claim was 'generous'.

No ofence taken, @MigL. I know. Thanks for clarification.

4 hours ago, Markus Hanke said:

First and foremost, [...]

Secondly, in what sense [...]

Thirdly, they seem to forget [...]

Good points here. I'll go over them more deeply as soon as I have the time.

The moment you introduce matter travelling on background geometry you change the metric is a deep observation.

Then, I also find an almost insurmountable amount of practical objections. Like accretion disks I mentioned.

Perhaps also, the rigorous solution should include quantum mechanics, as @Kartazion suggests.

From what I've read (in a hurry) by Markus, the only classically consistent solutions to me would be cyclic, so as not to have problems with causal paradoxes. But what about quantum mechanics then?

Sorry I'm being so sketchy. Not much time.

[joigus]

5 hours ago, ahmet said:

hearing "highly" word only reminiscents my romantism or colors rather than mathematics.

surely,it is not a mathematical expression  I am highly romantic hahah ha  

?

(My emphasis.)

By highly mathematical I meant something like one of my old teacher's book on exact solutions in GR. It started with "the universe is a C-infinity differentiable manifold, dense, simply connected and boundary-less" --something like that.

[ahmet]

17 minutes ago, joigus said:

It started with "the universe is a C-infinity differentiable manifold, dense, simply connected and boundary-less" --something like that.

yes this expression contains mathematcal keywords.

but to me, very mixed. What do you mean by "universe" ?

I could not understood well C-infinity?

dense set is any set when that set's closure is equal to itself as I remember.

I think I have not currently improved my geometry skills yet,so not commenting on manifolds.

but differantiability is simply multidimensional derivation (has its fromula and criteria) (e.g. all of partial derivations should exist and should be continuous)

mmm,I am not sure on wheher the rest of forums will reflect to my post negatively about an idea,thus I prefer to be silent. (It is not about you, a general manner across science and almost huge amount of scientists)

but I shall try to create good projects.

anyway,I have not read the paper but...I have a question: as a phsicist or phsicists "do you believe that time was really interferrable?" 

I ask this because  I know and saw in the past and still there have been many debates, unfortunately (to me) big amount of those debates have been  void. (>95% roughly)

 

Edited October 10, 2020 by ahmet

[joigus]

13 minutes ago, ahmet said:

I could not understood well C-infinity?

Infinitely many times differentiable.

13 minutes ago, ahmet said:

dense set is any set when that set's closure is equal to itself as I remember.

Exactly. It's a topological concept. Its adherence (the set of all its accumulation points = points that can be reached by limits) is contained in it.

13 minutes ago, ahmet said:

anyway,I have not read the paper but...I have a question: as a phsicist or phsicists "do you believe that time was really interferrable?" 

Sorry, I don't understand. Interferrable?

Edited October 10, 2020 by joigus

[ahmet]

31 minutes ago, joigus said:

Sorry, I don't understand. Interferrable?

some physicists state or divide the time roughly to three parts.

its surface...past and future (or I understood so). some other physicists describe more complex things on the issue.

but nothing happens as all I can see.

I am almost sure about these: 

1) travelling past times (leaving the existing time) is not possible. 

2) mentioning "time travel machine" is not only far away,but is also impossible. it is utopic.

3) all in all some sources that we could have consist of only belief about thats and state that travelling future was not as same as travelling to past times and was possible.

but if you ask my own idea: I don't believe the possibility of such things.

.........

(i.e. there are many void or valueless papers!)

 

 

 

 

 

 

 

Edited October 10, 2020 by ahmet

[koti]

9 hours ago, Markus Hanke said:

...The problem here is that a) not every solution to the field equations is necessarily physically realisable, and b) spacetimes containing CTCs are known to be highly unstable under even miniscule perturbations of initial conditions - and those conditions are only approximately true in the real world to begin with. Personally I would be very surprised if CTCs existed in the real world, and they are certainly not "predicted" by GR...

The are actually saying very strongly what youre pointing out in their own paper right after the 2 proofs on page 11:

"4. Examples

The above characterisation of process functions allows us to consider specific examples that cannot occur in an ordinary, causally ordered spacetime"

is my layman understanding of this correct?: We can very much time travel without the risks of getting into the grandfather paradox with just a small exception - we can’t do it in our universe? 
 

 

Edited October 10, 2020 by koti

[Markus Hanke]

18 hours ago, MigL said:

Bart and Lisa Simpson each have a wormhole generator

A wormhole and a closed time-like curve are very different topological constructs - though you could conceivably use wormholes to ‘build’ something that behaves like a CTC, at least in principle.

13 hours ago, koti said:

We can very much time travel without the risks of getting into the grandfather paradox with just a small exception - we can’t do it in our universe? 

I don’t know how to answer this - I cannot think of any way to time travel without creating a paradox. Even the mere presence of a time traveler would already create all manner of problems, never even mind accidental or intentional interference in events.

[MigL]

5 hours ago, Markus Hanke said:

you could conceivably use wormholes to ‘build’ something that behaves like a CTC

As far as I know, every CTC has to have some means of spacelike translation, and wormholes ( even if impossible to realise ) are one such means.
( otherwise, how could an object return to the same co-ordinates in space-time previously occupied ? )

Are there other ways to 'construct' a CTC ?

Edited October 11, 2020 by MigL

[Kartazion]

I had already made this example of CTC. It comes from an oscillator

[MigL]

2 hours ago, Kartazion said:

I had already made this example of CTC. It comes from an oscillator

I don't understand.
How exactly does your oscillator's worldline loop back to a previous co-ordinate ?

Edited October 11, 2020 by MigL

[Kartazion]

31 minutes ago, MigL said:

How exactly does your oscillator's worldline get back to a previous co-ordinate ?

Moderation will have to split.

Position A or 0 does not move. Only the position B can be different. The particle therefore oscillates between A and B, and then between A and B'
To be exact the cycle of oscillation is  A → B → A → B'→ A

I don't know if you understand what I'm trying to explain. But give me some time to write something more explicit
 

There is also this article https://physics.aps.org/articles/v13/99

Quote

A new theory proposes that time is a fundamental property of the Universe governed by an oscillator that interacts with all matter and energy.

 

[MigL]

That is speculative.
And not at all consistent with the accepted definition of a Closed Timelike Curve.

https://en.wikipedia.org/wiki/Closed_timelike_curve

[Kartazion]

I thought that the worldline is the flow of the particle between its two positions of A and B.

[joigus]

As @MigL says, those are periodic trajectories in phase space, not CTCs. CTCs involve time as a dimension too.

[Kartazion]

23 minutes ago, joigus said:

CTCs involve time as a dimension too.

As I will go into speculation I can already give a draft. But give me some day to better explain it.

 

 

Time dilation:

 

Here is an example of a triangle-shaped object of a spherical structure:

Legend of the distribution of time granted from the effective position:
po = origin point = 0,0001ns (position A)
→ = flow = ~0s (distance between A et B)
point(n) = dot matrix = 1ns (position B)

(po–>point1–>po–>point2–>po–>point3–>) = line1 = ~3,0003ns
(po–>point4–>po–>point5–>po–>point5–>po–>point6–>po–>point6–>po–>point7–>) = line2 = ~6,0006ns
(po–>point8–>po–>point9–>po–>point10–>po–>point10–>po–>point11–>po–>point12–>) = line3 = ~6,0006ns
(po–>point13–>po–>point14–>po–>point15–>po–>point16–>po–>point17–>po–>point18–>) = line4 = ~6,0006ns

(line1 + line2 + line3 + line4) = cycle

cycle x frequency

 

From the first to the last point of the complete object, will be formed in about 9ns (~ 9,0009ns) for one cycle. The total refresh of the structure ends in about 21ns (~ 21,0021ns) for one cycle.

Here if we continue this same structure with additional lines to form a second object in the form of an identical triangle called object2, we have:

 

(po–>point19–>po–>point20–>po–>point21–>po–>point22–>po–>point23–>po–>point24–>po–>point25–>) = line5 = ~7,0007ns
(po–>point26–>po–>point27–>po–>point28–>po–>point29–>po–>point29–>po–>point30–>po–>point30–>po–>point31–>po–>point32–>po–>point33–>) = line6 = ~10,0010ns
(po–>point34–>po–>point35–>po–>point36–>po–>point37–>po–>point38–>po–>point38–>po–>point39–>po–>point40–>po–>point41–>po–>point42–>) = line7 = ~10,0010ns
(po–>point43–>po–>point44–>po–>point45–>po–>point46–>po–>point47–>po–>point48–>po–>point49–>po–>point50–>po–>point51–>po–>point52–>) = line8 = ~10,0010ns

(line1 + line2 + line3 + line4 + line5 + line6 + line7 + line8) = cycle

cycle x frequency

The object2 is formed around 13ns (~ 13,0013ns). Either the same object is about 4ns longer on the high lines of the structure, than its double in ~ 9,0009ns on the low lines. The object2 on the upper part of the structure is about 4ns older than its brother on the lower part. Because the effective position sweeps all the free positions end to end, it is logical to find in this case the extension of time when forming entirely identical objects. Subsequently we can understand that the movement of the object2 annihilates the scanning of the effective position, and thus the object2 is formed faster than if it remains motionless.

Edited October 11, 2020 by Kartazion

[Markus Hanke]

5 hours ago, Kartazion said:

I thought that the worldline is the flow of the particle between its two positions of A and B.

It's a world line of non-zero length connecting the same event. This means the world line will return to its original position in space at the same instant in time.