Some Emerging Possibilities For Space Propulsion
nasa.gov
nasa.gov
They spend a lot of time discussing quantum vacuum energy, but the one short note about its "propulsion implications" includes the comment that "Those particular gravity theories are still up for debate." That's perhaps generous: quantum gravity isn't far from my field, and I've never heard of this take on the origin of gravity and inertia.
I'm also rather puzzled by the quote "Although gravity’s effects on electromagnetism and spacetime have been observed, the reverse possibility, of using electromagnetism to affect gravity, inertia, or spacetime is unknown." That's really not at all true. Our models of gravity all include very well-defined effects of electromagnetism on spacetime: the energy density in those fields causes space to curve, just like any other form of mass or energy. Hypothesizing anything else would require fundamental changes to the theory. That's entirely possible! But it's misleading to characterize the current state of affairs as having an unfilled gap like this.
So yeah. I'd love a good spacedrive as much as the next guy. But this article doesn't do much to make me optimistic about it.
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?
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.)
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.)
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.
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...
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.
Wouldn't a spaceship traveling FTL be able to carry information?
http://exvacuo.free.fr/div/Sciences/Dossiers/Time/A%20E%20Ev...
TL/DR: Yes, the Alcubierre drive does violate causality.
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.
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.)
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.
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.
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.
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...
Out of curiosity, what is your background for knowing all this stuff?
I remember reading about it somewhere, but I can never find it again.
But how can you have a current in a single wire without a loop?
That's meatier but I thought it was still light on information.
That's probably because nobody in the field pays any attention to Haisch, Puthoff, and Rueda, who have been publishing papers on "zero point energy" and how they claim it causes gravity and inertia for a couple of decades now.
The thing is, we've already got some awfully good theories about gravity and inertia, and a lot of their predictions are very well tested. Just as one example, if these folks think that what they've been working on is "a possible alternative to the Higgs" (a quote by Haisch himself in 2006 on the Talk page for his own Wikipedia entry), doesn't that mean that the actual recent discovery of the Higgs (and its good agreement with mainstream theory) strongly disfavors their alternative explanation for inertia?
There are too many fringe theories like this for any working physicist to have time to refute them all in detail. (I did my time years ago, trying to refute the nonsense of "Heim Theory" that had a crowd of Wikipedia proponents and a few overly credible mentions in the popular press.) At some point, when people start talking about replacing the foundations of modern physics you have to be skeptical, and raise the bar quite high before they merit serious attention.
http://www.nasa.gov/topics/technology/features/ntrees.html#....
They should just have a page that says: "FTL is impossible, but that's fine, once you get used to the way things are."
Also of note: https://en.wikipedia.org/wiki/Alcubierre_drive
No offense intended to you, but I feel this is a disastrously negative attitude to have. Except where morals dictate otherwise (which are outside the scope of science anyway) nothing is banned from exploration. Suggesting otherwise implies that we know all there is to know about a subject; to me, this is analogous to willful ignorance. Science is dedicated to removing ignorance, not enforcing it.
Nothing is banned, per se... but there's just too much search space to sift through to not be smart about where you look.
As it is now, with our current level of understanding, the best way to move around in space is by throwing tiny things very fast in one direction, to make a big thing move in the opposite direction, then wait a few thousand years for that big thing to get somewhere.
Given that we sometimes have difficulty being alive after waiting a few thousand years, it is understandable that some people have a problem with this, and want to explore other options.
For real?