(05-04-2017, 04:30 PM)AmrlKJaneway Wrote: There are papers that show that two Krasnikov Tubes arranged in a certain way can lead to time travel paradoxes.

Myself, I prefer to go with no FTL at all. With relativity, longer lifespans, multiple copies of individuals, and deep sleeps, most of the plot reasons that require FTL can be worked around when you consider future society.

But if you HAVE to use it, then OA's wormholes are probably the best solution. The idea that they destabilize if accelerated to relativistic velocities means they cannot be arranged as time machines, so you get all the benefits of zipping around the galaxy, without the headache of time travel.

(05-04-2017, 06:34 PM)Alphadon Wrote: Small nitpick: You can accelerate a wormhole mouth to relativistic speeds, but if you put them close enough together to create a time machine, then they destabilize.
Come to think of it, such a concept of chronology protection would probably destroy Krasnikov tubes only if they are in a time-travel configuration, making them possible.

(05-04-2017, 07:04 PM)AmrlKJaneway Wrote: So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

edit; my bad on the mistake. I was positive I read somewhere that if high enough relativistic velocities were achieved, Lorentz contractions would alter the size of the wormhole mouth and destabilize it...

edit 2; this is what I'm thinking of - "2) Linear instabilities: Wormholes are typically subjected to linear instabilities during the deployment phase, right after the wormhole has been inflated from the quantum regime, but before it is inflated to traversable size. These instabilities come from Lorentz contraction during the transport of the wormhole mouths to their final destinations and are typically limited to perturbations of less than 50% of the wormhole rest mass. For this reason, wormhole transport velocities are constrained to less than .74c." Although it goes on to say that this restriction doesn't matter if the mouth is transported within a void bubble.

Wikipedia Wrote:While one Krasnikov tube can be seen to present no problems with causality, it was proposed by Allen E. Everett and Thomas A. Roman of Tufts University that two Krasnikov tubes going in opposite directions can create a closed timelike curve, which would violate causality.[3]

For example, suppose that a tube is built connecting Earth to a star 3000 light years away. The astronauts are travelling at relativistic velocities, so that the journey through this Tube I only takes 1.5 years from their perspective. Then the astronauts lay down tube II rather than travelling back in tube I, the first tube they produced. In another 1.5 years of ship time they will arrive back on Earth, but at a time 6000 years in the future of their departure. But now that two Krasnikov tubes are in place, astronauts from the future can travel to the star in tube II, then to Earth in tube I and will arrive 6000 years earlier than their departure. The Krasnikov tube system has become a time machine.

In 1993, Matt Visser argued that the two mouths of a wormhole with an induced clock difference could not be brought together without causing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.[4] (See Time travel using wormholes and the Chronology protection conjecture.) It has been suggested that a similar mechanism would destroy time-machine Krasnikov tubes. That is, vacuum fluctuation would grow exponentially, eventually destroying the second Krasnikov tube as it approaches the timelike loop limit, in which causality is violated.

(05-04-2017, 07:04 PM)AmrlKJaneway Wrote: So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

(05-05-2017, 03:05 AM)stevebowers Wrote:

(05-04-2017, 07:04 PM)AmrlKJaneway Wrote: So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

Alcubierre metrics travelling faster than light would run into causality problems under a wide set of circumstances, since they could be used to send messages back in time, especially between observers who are travelling relativistically with respect to one another. I don't know if they would collapse, but in OA void bubbles (which are based on the Alcubierre metric) are limited to subliminal speeds so the problem doesn't arise.