6

Some setup is necessary before I can get to asking my question.

In Ken MacLeod's Newton's Wake, FTL is possible. There is a form of chronology protection:

Say ye try tae send a signal intae the past, or your ship’s course mucks about too much wi the light-cone consistency conditions. Ye’ll find the transmitter disnae work, or your course takes longer or goes a different way than you plotted it, or – as the saying goes – she was never your grandmother in the first place.

Chronology protection is leveraged in a combat scene to win a battle:

The other two friendly ships, having diverged, were again converging. Another of the enemy ships fittled [went FTL]. The Stanley Blade’s and the Small Arrangement’s trajectories instantly halted. The enemy appeared, as it seemed on this scale, right beside them, and as rapidly was destroyed.
‘Ya beauty!’ yelled Lucinda.
‘What happened there?’ Armand asked.
‘Chronology Protection trap. It came out of the jump just too far away to hit them, and it couldnae fittle the remaining distance without going outside its own light-cone or back in time. They had a moment while it waited tae catch up wi itself, fired off a nuke, and—’ She clapped her hands.

So on to my questions: when she says "couldnae fittle ... without going outside its own light-cone or back in time," I'm confused because any time you go FTL, you leave your light cone, and there exists some frame in which you appear to travel backward in time. But this character is talking like sometimes that isn't true. So what am I missing here?

My other question is, if the author's treatment of FTL travel makes sense here, what does it mean for the ship to "catch up wi itself"? It has something to do with waiting until causality constraints pass, but how exactly does it work? I can't quite wrap my head around it.

  • Trying to find a scientific explanation for a pseudo-scientific phenomena in fiction... – HorusKol Feb 20 '15 at 4:21
  • I think he's just trying to understand the way FTL works inside that universe. – Deleteman Feb 20 '15 at 7:21
  • Deleteman has it. – MackTuesday Feb 20 '15 at 20:53
2

To actually get causality violations in which an event is influenced by its own future, you need multiple FTL signals going in different directions, as in the tachyonic antitelephone; a single unidirectional signal may be received before it was sent in certain frames, but this alone does not qualify as a causality violation if there is no actual causal loop. So without having read the book, I presume the idea here is something like this. Suppose I send an FTL signal or make an FTL jump from event A to event B (with A and B being points in spacetime, not locations in space). And suppose there is another observer whose worldline passes through B, who immediately after that tries to send a signal or make a jump back in the direction of whatever spatial coordinate they would assign to A in their reference frame. Then they could find that FTL is strangely limited in that direction, in such a way that the return jump/signal always ends up in the future light cone of A. This would be the case even if they were in a reference frame whose definition of simultaneity assigns A a time-coordinate somewhat in the future of B, so that if FTL were completely unlimited in that direction, they would be able to arrive at the location of A before the event A actually happens. As for "catching up with itself", it could just mean a single ship making a pair of FTL jumps in opposite directions such that at the end of the second jump it ends up in the past light cone of the start of the first jump, which again would have to be forbidden if you want to preserve causality.

A similar idea, in which traversable wormholes allow for trips that look FTL from the perspective of space outside the wormhole, but where the wormholes blow up if you try to arrange them in a pattern that would allow for causality violations (inspired by real models of how chronology protection might work), is discussed on this page. Depending on how vague the book is about how "fiddling" works, perhaps one could imagine that it works by creating temporary wormholes, and that they obey this sort of chronology protection rule.

  • Awesome answer! Does this make you the paradoctor? – rosesunhill Mar 25 '16 at 22:08
  • Hmm, OK, so it must be based on different observers in different frames. – MackTuesday Mar 26 '16 at 4:13
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It is all about Causality

Relativity proves that FTL travel is identical to Time travel.

Time Travel can destroy causality if it is used to make a temporal paradox.

So the only way to have FTL travel and not destroy physics is for there to be some rule or law of physics which prevents using time travel to create a temporal paradox. This is called Chronology protection.

Party-pooper Dr. Stephen Hawking is a proponent of the Chronology protection conjecture which basically says FTL travel is impossible. Which is no fun at all.

Jason W. Hinson shows there are four ways of enforcing a "no-paradox" rule for time travel and thus allowing FTL travel:

  1. Parallel Universes
  2. Consistency Protection
  3. Restricted Space-Time Areas
  4. Special Frames

In some ways Special Frames is the best, though it directly contradicts part of Relativity (the first postulate of special relativity is that there are no special frames, "no privileged inertial frames of reference").

The latter three are examples of the Novikov self-consistency principle.

  • Yeah, these concepts are covered in my post. So my first question, stated a bit differently, is that she says, "it couldnae fittle the remaining distance without going outside its own light-cone or back in time," but how can you ever fittle without going outside your own light-cone or back in time? She's talking like that's possible when it clearly isn't. In my second question I ask about "catching up [with] itself". Maybe that means it needs to wait until it re-enters its previous light cone, but why would it need to wait if it were able to exit its light cone in the first place? – MackTuesday Feb 24 '16 at 22:47
  • 1
    Most of your comment looks correct (though I don't think it really answers the question), but I'd quibble with your last line--the Novikov self-consistency principle is specifically about what happens if time travel is possible and causality can be violated, it basically says that all causal loops are constrained to be self-consistent. So it would be what is meant by #2, "consistency protection", but since #3 and #4 remove the possibility of causal loops altogether, they aren't really forms of the Novikov self-consistency principle. – Hypnosifl Mar 25 '16 at 21:04

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