## Question on the CPC and wormholes

I've been reading into wormholes lately and understand how a wormhole with one mouth moving with a relativistic speed relative to the other mouth would constitute a time machine if the two mouths were brought close enough together. Apparently a physicist called "Matt Visser" calculated that it's probably impossible to bring the mouths close enough together and even if you do succeed the wormhole would become impassable. This supports Hawking's CPC and saves the universe from time travel paradoxes as long as you only have one wormhole.

What I don't understand is why Visser doesn't mention using a second wormhole to make the journey back very quickly and thus travel back in time.

What happens to Visser's conclusions when you add a second wormhole to make the return trip? Would using a second wormhole require additional physical phenomena to not result in time travel or have I overlooked something?

 Quote by Gulli What happens to Visser's conclusions when you add a second wormhole to make the return trip? Would using a second wormhole require additional physical phenomena to not result in time travel or have I overlooked something?
Thomas Roman seems to have been the first to realize that wormholes can be used to create time machines. His argument involved 2 wormholes. This avoids the wormhole-destroying feedback loop that you alluded to.

The trick is that with two wormholes, you can have a time machine without either of the wormholes having one of its mouths being able to influence the past of its other mouth. That is, each wormhole ceases to be a universal time machine (one that observers in all states of motion agree allows time travel), even though it appears to allow a time jump (together with a spatial jump) in the reference frame of the wormholes.

Visser generalized this idea to multiple wormholes, which he called a "Roman Ring".

If I use a Roman ring of wormholes to travel to the future, I enter a mouth on Earth, and exit a mouth in the future (according an Earth observer) on say, Alpha Centauri. Then I continue to circle the galaxy through several wormholes (jumping forward in time with each traversal), until I finally emerge from the mouth of the last wormhole on Earth in the distant future.

Source: The Physics of Stargates -- Parallel Universes, Time Travel and the Enigma of Wormhole Physics by Enrico Rodrigo

 Quote by HarryRool Thomas Roman seems to have been the first to realize that wormholes can be used to create time machines. His argument involved 2 wormholes. This avoids the wormhole-destroying feedback loop that you alluded to. The trick is that with two wormholes, you can have a time machine without either of the wormholes having one of its mouths being able to influence the past of its other mouth. That is, each wormhole ceases to be a universal time machine (one that observers in all states of motion agree allows time travel), even though it appears to allow a time jump (together with a spatial jump) in the reference frame of the wormholes. Visser generalized this idea to multiple wormholes, which he called a "Roman Ring". If I use a Roman ring of wormholes to travel to the future, I enter a mouth on Earth, and exit a mouth in the future (according an Earth observer) on say, Alpha Centauri. Then I continue to circle the galaxy through several wormholes (jumping forward in time with each traversal), until I finally emerge from the mouth of the last wormhole on Earth in the distant future. Source: The Physics of Stargates -- Parallel Universes, Time Travel and the Enigma of Wormhole Physics by Enrico Rodrigo
So, in other words, if you have more than one wormhole at your disposal, Visser's findings are not enough to enforce the CPC (prevent time travel)?

Is it then correct to say that either time travel is possible, or it's not possible because a) wormholes don't exist and cannot be created, or b) wormholes are rigid (the mouths can't move relative to each other) or c) there are still unknown effects that will enforce the CPC even for multiple "flexible" wormholes?

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## Question on the CPC and wormholes

 Quote by Gulli Apparently a physicist called "Matt Visser" calculated that it's probably impossible to bring the mouths close enough together and even if you do succeed the wormhole would become impassable.
Do you have a reference for this?

 Quote by HarryRool Thomas Roman seems to have been the first to realize that wormholes can be used to create time machines. His argument involved 2 wormholes. This avoids the wormhole-destroying feedback loop that you alluded to.
Same question: do you have a reference? (Other than the book you mention, which is a popular book, not a scientific reference. If the book is talking about actual science, its bibliography should include scientific references.)

 Quote by HarryRool The trick is that with two wormholes, you can have a time machine without either of the wormholes having one of its mouths being able to influence the past of its other mouth.
If this is true, then it seems to me that a series of such wormholes would not allow you to travel backwards in time; it would only allow you to travel "forwards" between two points, possibly faster than you could "directly". More precisely, it seems that the wormholes would not produce any closed timelike curves (CTCs), which are what Hawking's Chronology Protection Conjecture says are impossible. The "Roman Ring" scenario you describe seems to involve travel to the future only.
 @Peter Donnis This is my reference: http://arxiv.org/abs/hep-th/9202090 Also, I'm pretty sure a "Roman Ring" can be used to travel back in time (up to the moment the Roman Ring was first constructed) as well. If I understand correctly wormholes become time machines when the mouths are moving relative to each other, so a "rigid" wormhole would allow FTL without time travel. Here's to hoping there are lots of rigid wormholes out there!

 Quote by PeterDonis Do you have a reference for this?.
Check out section 23.3 in Visser's Lorentzian Wormholes -- From Einstein to Hawking .

 Quote by PeterDonis Same question: do you have a reference? (Other than the book you mention, which is a popular book, not a scientific reference. If the book is talking about actual science, its bibliography should include scientific references.)?.
The reference in the scientific literarature (for Thomas Roman being the first to realize that wormholes permit time travel) actually points to a popular reference by Kip Thorne. In "Traversable Wormholes: The Roman Ring", Matt Visser states parenthetically that

"The realization that wormholes generically seem to imply time travel can be traced back to an observation by Tom Roman".

Visser's reference for this is Black Holes and Time Warps -- Einstein's Outrageous Legacy by Kip Thorne. Thorne mentions that Roman pointed out this feature of wormholes in a conversation he had with him at a physics conference.

 Quote by PeterDonis ...it seems to me that a series of such wormholes would not allow you to travel backwards in time; it would only allow you to travel "forwards" between two points, possibly faster than you could "directly".
No. A traveler can go back in time by reversing his path through the Roman Ring. The whole point of the Roman ring is that it can make the wormhole destroying feedback loop of virtually particles arbitarily small. This results from no individual wormhole in the ring working as a true time machine, even though all of the wormholes taken togther do.

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 Quote by Gulli @Peter Donnis This is my reference: http://arxiv.org/abs/hep-th/9202090
 Quote by HarryRool Check out section 23.3 in Visser's Lorentzian Wormholes -- From Einstein to Hawking .
I'll take a look; I hadn't seen any of these papers by Visser before, or the book. Another item on my Amazon wish list.

 Quote by HarryRool Also, I'm pretty sure a "Roman Ring" can be used to travel back in time (up to the moment the Roman Ring was first constructed) as well.
If it works like a series of Thorne-style wormholes, as referenced below, then yes, this is how I would expect it to work.

 Quote by HarryRool The reference in the scientific literarature (for Thomas Roman being the first to realize that wormholes permit time travel) actually points to a popular reference by Kip Thorne.
I was pretty sure that book was going to come up. I've read it; I don't remember it talking about "Roman Rings", but I certainly do remember it talking about wormholes and time machines. However, the idea of the "Roman Ring" is quite clear once you admit the possibility of single traversable wormholes.

 Quote by HarryRool No. A traveler can go back in time by reversing his path through the Roman Ring.
Yes, I see that now. I'll look at the Visser papers in more detail when I get a chance.

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 Quote by HarryRool The whole point of the Roman ring is that it can make the wormhole destroying feedback loop of virtually particles arbitarily small.
Having read the Visser paper on the Roman Ring, I'm not sure its conclusion is actually that broad. He talks only about the gravitational back-reaction, which is only one of several "protection mechanisms" he discusses in his other paper on what keeps single wormholes from becoming time machines. He doesn't talk about how the other "protection mechanisms" behave in a Roman Ring, unless I'm missing something.

Also, he does not show (and does not claim to show) that the gravitational back-reaction actually *is* arbitrarily small for a Roman Ring. He only shows that it appears to be possible to make it arbitrarily small, if it is calculated using the semi-classical approximation. But we know that approximation is wrong, and it's not always clear that it's even close; it may be *very* wrong in some cases, such as this one. His actual conclusion is this, from the end of the paper:

"I view this not as a vindication for time travel enthusiasts but rather as an indication that resolving issues of chronology protection requires a fully developed theory of quantum gravity"

In other words, basically the semi-classical approximation isn't reliable enough for this problem.

 Quote by PeterDonis In other words, basically the semi-classical approximation isn't reliable enough for this problem.
Yes. A semiclassical calculation is of course understood to be unreliable at the Planck scale. However, to whatever extent and in whatever regime it might be reliable, the calculation's result that seems to indicate the destruction or non-traversability of time-machine configured wormholes (and therefore bolsters the Chronology Protection Conjecture) can made arbitrary small in a Roman ring configuration. In the same paper Visser writes,

"Thus with enough wormholes, we can arrange the gravitational vacuum polarization, and therefore the backreaction, to be arbitrarily small all the way down to the reliability horizon."

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