wormhole physics

HarryRool

Yes it's speculative alright, but only because the entire topic of wormholes is speculative, as was the topic of black holes for most of the 20th century.

The objection that you raise involving excess mass seems to be totally abatable, though. I think that you could compensate for the extra mass by throwing in more exotic matter. And you must assume that that exists, if you have already assumed that it's possible to make a single traversable wormhole.

As to your second point about causality, the same FAQ seems to have an answer for that:

Don’t quantum effects prevent wormholes from being turned into time machines?
No. It’s true that calculations seem to show that a wormhole-destroying feedback loop of virtual particles appears whenever a wormhole is configured as a time machine. But these calculations don’t assume the existence of the parallel universes that would have prevented the feedback. Even without this assumption, it’s possible to make the feedback arbitrarily small by replacing the single wormhole with a Roman ring of wormholes.

What is a Roman ring?
An arrangement of several wormholes that functions collectively as a time machine, even though no subset of the wormholes functions as one.
----

This FAQ is an excerpt from a book that explains this in detail and cites the corresponding references in the physics literature.

Ryan_m_b

Throwing more exotic matter might help or you could just send a mass the other way once travelers have gone through i.e. have a series of 1 tonne probes orbiting the wormhole and once a ship has gone through send a series of probes through the other way to cancel out the mass change.

I really don't get the parallel universe thing and how that would help. If the wormhole connects to another universe then time travel won't apply. If quantum fluctuations or even real particles would cause a wormhole to collapse then a roman ring wouldn't help, no matter how you do it when the QFs or real particles enter their own past light cone the wormholes will collapse.

If CPCs don't exist though the Novikov self-consistency principle might prevent any possibility of changing the past.

JesseM

This is based on calculations in semiclassical gravity though, and many physicists would argue they just suggest that semiclassical gravity is probably no longer a good approximation to quantum gravity in situations involving wormholes. For example, see this paper on Roman rings by wormhole expert Matt Visser (author of the textbook Lorentzian Wormholes: From Einstein to Hawking), where in the conclusion on p. 4 he writes: "My interpretation is perhaps a little different: 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 [5]."

HarryRool

Yeah, there's a FAQ (answer) for that:

Isn’t time travel by wormhole or any other means impossible due to the paradoxes that it implies?
Not necessarily. Dealing with time travel paradoxes by conjecturing the impossibility of time travel is only one of three ways of resolving the issue. The other ways are:
1) Impose self consistency on classical physics: A time traveler cannot change the past because he was always part of it. When he attempts to change the past, his efforts will be thwarted by an apparent conspiracy of events. 2) Impose self consistency on quantum physics: A time traveler cannot change the past because all possible pasts have already occurred in parallel universes. When he attempts to change the past, his efforts will not seem to him to be thwarted. This is because he will have entered the past of a preexisting parallel universe in which he has already made the changes that he seeks to effect.


Matt Visser has shown that the semi-classical calculation of the wormhole destroying feedback loop of virtual particles is unrealiable, because semi-classical calculations also show that these effects can be made arbitrarily small with a Roman ring. Here's the abstract for his 1997 paper entitled, "Traversable wormholes: the Roman ring":

"In this brief report I introduce a yet another class of geometries for which semi-classical chronology protection theorems are of dubious physical reliability. I consider a ``Roman ring'' of traversable wormholes, wherein a number of wormholes are arranged in a ring in such a manner that no subset of wormholes is near to chronology violation, though the combined system can be arbitrarily close to chronology violation. I show that (with enough wormholes in the ring) the gravitational vacuum polarization (the expectation value of the quantum stress-energy tensor) can be made arbitrarily small. In particular the back-reaction can be kept arbitrarily small all the way to the ``reliability horizon''---so that semi-classical quantum gravity becomes unreliable before the gravitational back reaction becomes large."

Ryan_m_b

Yeah I got that my point was that it's not really time travel if you are travelling to another universe because you haven't arrived in you're past light cone, you've arrived in a universe which is exactly the same as conditions at the time you were aiming for in your past light cone.


I'll admit that the maths here is beyond me so maybe that's why I can't understand why a roman ring would avoid the problem. If the quantum fluctuations get back inside their own past light cone via any mechanism surely that should lead to collapse? However I feel I should conclude the same way Visser has