[mrfusion]

How do these "legitimate" ideas for FTL deal with causality? I've always wondered that.

People always use causality to argue against FTL, but then readily acknowledge that wormholes or Albicurie might allow it. I'm not getting that disconnect?

[tinco]

I think, definitely not an expert, that when thinking about ways to travel faster than light would not actually involve an object moving through space in the classical sense.

You could look instead of at the means (i.e. moving through space) at the goal: Mass first being somewhere, and then being somewhere else in a relatively short amount of time.

So the solution would be to cross distance at a low velocity, so you don't have all the downsides like getting heavier, requiring insane amounts of energy, but still arriving at a location sooner than a conventional ray of light would.

The concept of a worm hole would be that there would be a path between two points in space, over which matter can travel, that has a shorter physical length than the shortest paths we can come up with now.

So faster than light travel might simply be 'regular' velocity travel over undiscovered paths.

(Note that in a mathematical sense, under the rules we learn in school, there can not be a path between two points that is shorter than a straight line. What we know however that the universe around us does not conform to these rules precisely, we already know for example that space has curves in it.

The key to knowing whether there are paths or if we can create those paths, that are shorter than the ones we experience now lies in understanding the structure of the universe better.)

[Strilanc]

Whether or not you drift there or instantaneously teleport there, there are still causality issues. You can make it into your own past by jumping once, changing your speed to tilt the frame of reference you're moving "instantaneously" in, then jumping again.

http://casa.colorado.edu/~ajsh/sr/simultaneous.html

[logfromblammo]

I suggest that encountering yourself in your own past would be on the same order of difficulty as plotting the course of a photon across an entire galaxy, to hit a "mirror", precisely positioned to return that photon back across that same galaxy, this time apparently as an antiphoton, to another "mirror", that would bounce the photon back towards itself, prior to the time at which you redirected it across the galaxy. The precision required makes it effectively impossible.

But, given the nature of probabilities, if enough trials occurred, eventually one would succeed in causing something to interact with its past self. How would you even detect if that occurs? The particles that have the highest likelihood of doing this by chance all look pretty much the same anyway.

[Someone]

That argument assumes that our understanding of physics is correct and complete. If FTL is possible, that is an incorrect assumption. But yes, any theory including FTL travel must explain how nature behaves if this happens.

[jerf]

No, it's much weaker than that... that assumes that our understanding of relativity is even remotely correct. You can obtain the FTL/causality problem from even a brutally simplified version of general relativity, such that one can literally draw the problem on a piece of paper. For FTL not to break causality essentially requires that A: you can only FTL jump once and only once in the entire history of the universe as long as you don't leap into your own past light cone, B: FTL is impossible or C: Causality is not absolute.

I've often thought that the announcement that we've developed FTL would actually be a very bad thing, as fun as it sounds at first. Sort of like this story, where it turns out a proof of P = NP is just about the worst possible thing: http://www.antipope.org/charlie/blog-static/fiction/toast/to...

[Steuard]

Here's a link to a good discussion of some related issues by cosmologist Sean Carroll:

http://www.preposterousuniverse.com/blog/2011/09/24/can-neut...

(It's in the context of that mistaken "neutrinos move faster than light" claim from a few years back, but the arguments are general. Bob Geroch's article mentioned near the end is pretty much the gold standard for dealing with these issues as far as I know.)

I really need to sit down at some point and figure out how Alcubierre "warp" spacetimes would relate to causality. It may be a moot point: I've heard people claim that one feature of Alcubierre's solutions in General Relativity is that they are "eternal", in the sense that there's no obvious way to "turn on" a warp bubble around some given region or to "turn off" the warp once you reach some destination. If that's true, it wouldn't be useful for sending information, which might be all that's needed to preserve causality.

As for wormholes or similarly strange spacetime topologies, those really do open a can of worms for causality. There's some interesting physics/philosophy involved, and I'm not sure that we've got any solid answers. (I have my own strong suspicions about what the answer looks like, but I won't try to explain them here.) But that may also be moot: every known wormhole solution in relativity requires matter with properties entirely different than anything ever observed in the universe, so we won't be testing it out any time soon.

[pdonis]

The causality question was addressed years ago in this paper:

http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev...

TL/DR: Yes, the Alcubierre drive does violate causality.

[Steuard]

Thanks! I knew someone must have analyzed this before.

[mrfusion]

Thanks! Could you explain a bit more about how Alcubierre can't be used to send information?

Wouldn't a spaceship traveling FTL be able to carry information?

[Steuard]

You know what? I take that back. At some point, I'd had the impression that Alcubierre's solution required a fixed velocity (so you couldn't start or stop the spaceship), but evidently that's not the case. It gets to be problematic after all! :)

[ethbro]

I believe his point was "only if they can stop traveling at FTL".

[trhway]

>People always use causality to argue against FTL,

for some definitions of causality. Anyway, far-far galaxies are running away from us FTL using part of the same machinery as Albicurie drive - ie. space expansion and no causality violation so far.

Anyway before FTL, we need to get basic aneutronic fusion drive working - emitting 0.05c alphas - so that we can comfortably travel inside Solar system and send first near-by star systems probes. Unfortunately continuing work on non-starter schemas like Tokamak or laser-driven (better than Tokamak, yet laser driver is the worst one) neutronic fusion wastes so many resources and creates public distrust to the fusion as a technology.

[marvin]

What would be a better approach for fusion research?

[trhway]

Tokamaks are fine, even great, for "research". It is the "development" where they are non-starters. For the last 20+ years it has been pretty obvious that inertial confinement is a way to go to build a working fusion device.

For fusion development where are 2 main factors in play:

1. in the second half of 199x it was decided that fusion plant produced electricity can't be cheaper than the current then $0.04/kwh of coal electricity, thus focus on "research" (especially to support modelling instead of real testing of conventional nukes) not on "development" as energy source

2. non-proliferation. A working inertial confinement device may probably be miniaturized to the military acceptable size of say a large truck/container. Without all the fuss of gigantic multi-year enrichment programs of fissile materials.

Laser driven is an inertial confinement which is well resistable to miniaturization/weaponization due to its complexity and low efficiency [though even that may start to change slowly with semiconductor lasers]. That makes it fine for "research", and there is no need for "development" due to the above mentioned factor #1.

Even 15 years ago it was obvious that somebody need to put DT target inside the wire spool of the Sandia Z-machine discharge target. They probably even did, un-publicly, put it there back then :) They upgraded the machine since then, slowly, and sometimes later (~2010) publicly announced shooting into DT target. Yet it is pretty obvious that they are not in a rush, very very not in a rush... The Z machine schema has much higher efficiency and much lower level tech (i.e. higher theoretical proliferation potential) than femto-lasers.

There is even slower movement, pretty much no movement at all in areas of DPF or fusor-descendant schemas (except a bit for polywell) - the schemas which were beating Tokamaks since the dawn of times, and one can only speculate how things would be if they got even small share of funding that Tokamaks got...

[marvin]

Thanks for your comprehensive comment. It's incredible what level of damage the Cold War has done to nuclear research. Although I'm not 100% convinced it is only bad; we don't want an AK-47 of nuclear bombs until the world is a much more peaceful place than today.

Out of curiosity, what is your background for knowing all this stuff?

[trhway]

thanks, at the end of 199x i just asked myself "where is my fusion drive?" :)

[logfromblammo]

I have a hypothesis, unsupported of course, that everything has a fixed spacetime velocity. As beings with mass, we perceive the universe as having three spatial dimensions and one temporal dimension. We move relatively slowly in the spatial dimensions, and we find it easy to change direction. As a result, we are hurtling incredibly fast in the temporal dimension.

A photon, on the other hand, moves incredibly fast in the spatial dimensions, and finds it difficult to change directions in them. From its perspective, the spatial dimensions might as well be one dimension, represented by its vector of movement in space. It moves relatively slowly in our temporal dimension, however. And that leads to the question.

What if we only perceive time as a single temporal dimension because we are moving so fast through time that we have so far been unable to perceive any other temporal dimensions or change temporal direction?

It may be that if a human were to accelerate close to the speed of light in the known spatial dimensions (thus decelerating in the temporal dimension) it would become possible to perceive additional temporal dimensions, and maybe alter one's course through time. Our current understanding of causality assumes that one's movement through time does not change direction.

If time does have additional dimensions, a massless particle could execute a u-turn in the temporal dimensions, and it would appear to us as though it turned itself into its antiparticle. If the u-turn was in any way imperfect, the particle probably could never intersect any point in spacetime that it has ever previously passed through and interact with itself in a way that could create a paradox. Or maybe all electrons and positrons really are the same particle, endlessly interacting with itself as it bounces around in spacetime, and all paradoxes become self correcting, since causality flows in multiple directions.

TL;DR: Our assumptions about causality may be wrong.

[jerf]

"We move relatively slowly in the spatial dimensions, and we find it easy to change direction. As a result, we are hurtling incredibly fast in the temporal dimension."

There exists an invariant such that you can look at anything and sum its spatial and temporal velocities in a certain manner to obtain c, but it's not a very useful observation because you end up with only two categories: Things moving at c, and things not moving at c. You don't have "slow" or "fast" things because that all depends on your frame of reference. For instance, from an object's frame of reference, it is always at rest.

(This is also why old sci-fi about a drive that can push you to FTL if you get "close" to the speed of light is gibberish. Your velocity is always zero relative to yourself. You can never get close to c because you can't even get closer to c. It is always and forever c beyond you.)

[logfromblammo]

That seems like a form of solipsism for intergalactic travelers. Wherever you go, there you are.

If you have a point of origin or a destination, you can define your speed relative to either of those points. And if you are at rest with respect to another object, you will both have the same velocity vector for (x,y,z,t) = (0,0,0,c) or maybe (0,0,0,-c). You both move at the same velocity through time, so you age at the same rate.

Now, if you throw that object away from you, in your frame of reference, you are at (0,0,0,c) and the object is satisfying something like sqrt(x^2+y^2+z^2-t^2)=c (real physics is more complicated, naturally). You age normally, and the object ages more slowly. From the object's frame of reference, it is aging normally, and you age more slowly, because you're the one moving fast.

Now as that spatial component approaches c, it becomes more and more difficult to hang a left at Betelgeuse. So you can usually treat all the dimensions that square to a positive number as the magnitude of their combined vector (let's call it s). The dimension that squares to a negative number, t, could be the magnitude of a vector in multiple dimensions that all square to a negative number (let's call them u, v, and w).

So a photon might prefer to see its spacetime vector as (s,u,v,w) = (c,0,0,0) or maybe (-c,0,0,0), provided that x^2+y^2+z^2=s^2 and u^2+v^2+w^2=t^2 (or similar equivalent constraints that change the signs around). And whatever happens that might appear to us as though the photon is traveling at less than c is actually the photon stepping sideways through temporal dimensions orthogonal to our own hard-to-divert movements through time.

If you look at dual quaternions or geometric algebra, the math for systems with dimensional signatures (+,+,+,-) and (+,-,-,-) are often nearly identical, with a few sign flips here and there. And it would allow for odd things like imaginary mass and imaginary energy. We simply might not notice the additional temporal dimensions, for the same reason that it is difficult for us to change the direction of a massless, chargeless photon without touching it.

[ceejayoz]

If it's unsupported it's a hypothesis, not a theory. Conflating the two is what gives us the "but it's just a theory!" statement people try to make.

[pdonis]

> People always use causality to argue against FTL, but then readily acknowledge that wormholes or Albicurie might allow it

Wormholes and Alcubierre drives do lead to causality violations. That's why most physicists view them as not physically possible. They're just mathematical solutions to the Einstein Field Equations that can't be physically realized. There are plenty of other such solutions; nobody has ever claimed that every mathematical solution of the equations has to correspond to something that's physically possible.

[TheLoneWolfling]

http://arxiv.org/abs/1005.1614

[jordan0day]

That was an interesting paper, but I'm having trouble with the concluding paragraphs. Is the argument against the grandfather paradox essentially "You can set up the mechanism to create the grandfather paradox, but it won't actually work the way you think it will?"

[Steuard]

Bob's argument is (more or less) that it's easy to set up a thought experiment in which you imagine setting up closed causal curves, but that the devil is in the details.

Earlier in the paper, he argued that the new physics will still have its own causal structure, in a well-defined sense involving initial conditions and differential equations. If you start with complete and well-behaved initial data, his argument is essentially that the state of the system will always evolve forward in time in a consistent way (and in particular, there's no way for closed causal curves to arise out of nowhere).

So those last paragraphs that you're asking about attempt to interpret that conclusion in the context of a thought experiment aimed at creating a grandfather paradox. Geroch's answer is to say "the details will depend on your precise model, but the math makes it unavoidable that something about the model will make your plan impossible". Maybe the experimenter's normal-matter hands just can't ever move fast enough to push the special fluid past light speed. Maybe FTL-fluid is explosive when it comes in contact with normal matter. But the ultimate mathematical conclusion must hold.

(I won't swear to have worked through every step of these arguments in detail myself, but Bob Geroch is a world-class expert on this sort of thing. The one caveat that I'd add is that I think his arguments here are classical, and that bringing quantum mechanics into the mix might change things. It's possible that it would only change things in topologically disjoint sectors within a general-relativistic spacetime, but something like that might conceivably escape his premises.)

[jordan0day]

Thanks for the thorough response. My original paraphrasing was probably a little too simplistic, but I guess I got the gist of the general argument.

[TheLoneWolfling]

Pretty much.