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matti23
topic
03:45:50 AM May 1st 2013
edited by 69.172.221.6
Clashing Drives

I'm just working on a sci fi story and was wondering if anyone could solve a problem I had. My Sci Fi universe has 2 different types of FTL drive, a Warp Drive and a Wormhole Drive. The longer the wormhole drive is active the longer your jumps. It creates a tunnel that gets longer and longer and the universe of the story is represented as kind of "spherical bubble" so you can imagine the tube being able to reach further and further. The Wormhole generator is carried within the starship and generates a spherical bubble that grows to encompass the spaceship. The warp drive immediately starts moving you in a given direction by distorting space.

I was wondering how you could make it so that you couldn't just leave your wormhole drive on whilst travelling with the warp drive. This has caused a problem because now the heroes should be able to instantaneously wormhole out of a fight as soon as they meet trouble after "saving" up their wormhole drive. I try to give explanations that kind of make intuitive sense in a very general way eg. The warp jammer machine in the story unevenly induces distortion in the space around the enemy ship to prevent them getting the shape of space needed to warp. Hyperspace and subspace as places with different laws of physics do not exist in this story. You can travel deeper and more shallow to the bubble but the laws of physics are consistent everywhere. Psychic powers do not exist. Communication is by quantum entanglement.

The power limitation explanation doesn't work as the story covers many generations of a exponentially growing civ and they go from having 1/100 of the galaxy to 3 galaxy clusters under their control and millions of dyson swarms per galaxy. This is a pretty hard problem and I was wondering if anyone could figure out the solution.
lmdo
02:32:24 PM Feb 2nd 2014
You could claim that the warp drive requires asymptotically flat space in its interior. It's a bit handwavy but defensible. Or you could claim that the wormhole gets disengaged as soon as the warp drive breaks the speed of light, or gets moving period. This is probably the more storytelly of the two options, and also makes some sense physically: traversable wormholes are extremely fragile if you believe the semiclassical Hawking self consistency calculations. Establish any sort of "twin paradox" between them, however short, and they collapse in a feedback loop of virtual particles.
gs
topic
11:00:42 PM Oct 28th 2011
edited by gs
Could someone explain to me why all the causality discussion matters for a fictional FTL system?

The closest I can discern for the real world physics of FTL is that if you take the equation for dealing with time dilation and extrapolate not only would any object with mass need an infinite amount of energy to travel at lightspeed but experience zero time progression while doing so, ergo going faster means negative time value ergo time travel.

HOWEVER since no fictional FTL I'm aware of actually uses infinite energy (unless one counts the infinite improbability drive) why does the effects of impossibly going faster then lightspeed matter at all when you are using some other method to avoid this impossibility. Like wormholes where from a outside perspective you move faster then light, but only actually do so by traveling a much shorter distance while never getting near lightspeed. Its only a local violation of FTL that matters after all.

Since we have a very firm reason we can't get past the first impossibility (infinite energy) why do estimations on if that was somehow achieved and exceeded (what would that be more then infinite energy?) matter at all.

I've yet to see a link or explanation that doesn't treat itself as somehow self evident without ever actually explaining itself. The quote on the trope page seems to be alluding to the "violates causality" as the same thing as seen in various classic time travel problems. Like the Grandfather Paradox. Yet the "real world" explanations of the problem seem to be ignoring that while not quite as common as time travel itself in stories there are at least partial solutions to these kinds of problems out there. Like the stable time loop where the act of time travel is integral to creating the situation that precipitated the time travel. What exactly makes violating causality a problem in practical terms.
girlyboy
04:25:43 PM Feb 21st 2012
edited by girlyboy
Basically, that's not the reason why it implies causality violation. According to special relativity, the very fact of sending a piece of information from one place to another faster than light can violate causality, regardless of the exact method used, or the energy required.

The reason it's hard to get a clear explanation of this is because there isn't one... :P It's impossible to grasp intuitively, and you have to go over some very hard to understand and un-intuitive physics concepts, starting with "what happens if I bounce a ball while riding on a train" and moving all the way through concepts like "frames of reference" and what-not. I sure as heck know that I don't really understand it.

But if I can remember enough of what I've tried reading about it to summarize — as far as I understand, basically, there is no absolute reference point for time. Everyone perceives time differently based on their relative velocities. This by itself creates the potential for causality violations if two people who are moving at different velocities, and thus experience time passing at different rates, can somehow exchange information instantly (with, say, a Subspace Ansible). Their different perceptions of the passage of time allows for situations where you can use a chain of such observers to basically transmit a message from one of them into their own past, thus potentially violating causality. Please don't ask me to explain exactly how, because I don't remember. :P The light speed limit is basically the only thing keeping this from happening, by placing an absolute speed limit on how quickly information can be transferred that is the same for all observers.

... Or something. This probably isn't the best place to ask, though if I remember right there were a couple of tropers frequenting this page in the past who actually knew what they were talking about. >_>
piroca
12:16:08 AM Feb 22nd 2012
Hi girlyboy, I was one of them :P your explanation was actually quite accurate. The part you aren't sure about, the why, isn't really that clear. In effect, the way the the equations that transform between reference frames are set up, as long as you can assume that transfer of information is limited to the speed of light, observers will always agree on the order between events that are causally related — i.e., no observer will see effects happen before their causes. The math isn't really that complicated, I can point out some references if you're interested. Nothing better to understand this than to actually see an example... :)

@gs: well, first and foremost, the whole argument about velocities and energies works in classical mechanics, but the world is known to be quantum mechanical in nature. Nothing's stopping a particle from disappearing from here and appearing over there instantly — except the whole argument of causality that you dislike. Secondly, as addressed by girlyboy, your explanation of why it is often said that FTL = time travel is not correct: the formula for time dilation becomes imaginary for v > c, not negative (making sense of this is left as an exercise to the reader). There's also the issue that the whole business of traveling at the speed of light doesn't really make sense for massive observers: you get infinites everywhere, as you pointed out. So you are quite right in arguing that it doesn't make much sense to talk about accelerating there. But that ignores the possibility of teleportation or tachyons, particles that exist only at speeds above the speed of light. And finally, the type of causal paradox that results from locally violating the speed limit (you are also correct in pointing out that such things do not happen immediately with wormholes and warp drives — but they might appear later, as they have) is much more nonsensical and harmful from a physical point of view than your vanilla time travel. Time travel, fine, you get an observer going to the past, he does stuff, everybody agrees that he went to the past and did stuff, ok. If you violate the speed of light locally, different observers do not agree on whether you time-traveled or not! No partial solution exists to this.
SNDL
11:07:43 AM Feb 22nd 2012
...the formula for time dilation becomes imaginary for v > c, not negative (making sense of this is left as an exercise to the reader).

It's not that hard to see why: The core of the square root becomes negative due to v / c becoming higher than 1, thus giving an imaginary root as a result.

The whole faster-than-light problem only occurs when a mechanical phenomenon is examined classically. Because of all laws being invariant to the referential systems, the time and mass variables would have to lead to infinity in order to allow the body or particle reach the light speed, while its length and the other space variables tend to zero. Only the phase and group velocities of the electromagnetic waves can be higher than c, and it's not much helpful anyway since all means of information, including matter and energy, must be sent at a lower (or, at best, equal) rate compared to c.
gs
12:33:58 PM Feb 26th 2012
Thanks for correcting me on time dilation, its been half a decade since I did any math relating to it.

Its still seems to me that's an even stronger case for the mere notion of the "problems" of FTL being silly. They seem to be creating an arbitrary condition in physics then wondering why it doesn't work. Tachyon's are said to require imanginary mass, what precisely would that mean and is there anything to say the universe even works that way? Yes I'm sure there's math for this, but I suddenly recall that mathematics are approximations of nature. That a concept is mathematically possible is not the same as it actually existing.

I guess I'm still looking for a macro scale example of the sort of problems. Something that actually applies at a practical level. What are the end results. Let me see if I can construct a situation and see if anyone can explain the issue.

I am at Point A and lets say Point B is half a lightyear away. Accounting for time delay we have synchronized our clocks ahead of time using normal means. Point A has a agreed to send Point B a message FTL. So I at Point A send the message lets say at 12:00 on our clocks. Now normally it would be received at 12:00+6 months from now. However I fail to see the problems given by this taking any less time as long as the the message doesn't arrive before 12:00 at Point B I don't see how it can be a causality violation.
SNDL
02:19:02 PM Feb 26th 2012
If it's possible to go faster than light, then it's necessary to both expand the math of relativity (which is already a scary task) and put it in practice (have tachyons been detected yet?). The point is that, if it's ever possible to go FTL, then it hasn't been discovered yet, since the logic and the math of relativity are prohiitive towards that. In fact, in the actual event a body goes FTL, not only will it acquire an imaginary mass. It will somehow be negative, due to how you have to rewrite the imaginary component that is dividing the real part (by default, a real number is technically multiplied by an imaginary element powered up to four; so, i powered by four over i leaves i powered by three, which translates into negative i). That, even for relativity, is a HUGE Mind Screw that has yet to make sense. The only part that would make sense is the fact that time would have an imaginary and negative value as well, indicating that the body is indeed making a regression towards the past (thus time travel).
girlyboy
11:10:25 AM Mar 2nd 2012
edited by girlyboy
^^ That's a fun hypothetical situation. :) I imagine the very first problem you're going to have is synchronizing those clocks. For that to be possible at all, points A and B have to be at rest relative to each other. If one of them is moving relative to the other, relativity tells us that this already means the two points are experiencing the passage of time at different rates. But I think you're right — if the two points are at rest relative to each other, there's no particular reason the scenario you describe should lead to causality violation itself. The problem is that if you have a Subspace Ansible, as you do in this example, there's nothing stopping you from using it in other situations. What if instead of sending the signal from point A, you put your ansible on a rocket ship, and accelerated to 0.5 c relative to point B? Your clocks would not be synchronised any-more. In itself, this probably still wouldn't lead to weirdness, but it opens up the door for it. I'm going to have to track down the website where I read about this, but there is a way to violate causality using this system if you have three people with subspace ansibles communicating with each other via FTL, while moving at different speeds compared to each other. Basically, it's possible to set it up so that from the point of view of one of these people, the messsage would indeed be sent from one place to the other before 12:00, and this observer could then re-transmit the message to point B so that it arrives earlier than it would have been sent from B's point of view...

Also, thank you piroca. An example would be really great. :P

Edit: I think this is the page that I was thinking of. I'm gonna have to re-read it myself, but as I recall it's quite interesting and should help with this discussion. :)
girlyboy
11:49:59 AM Mar 2nd 2012
edited by girlyboy
P.S. Actually, if I understand that website I linked to, things are even worse, and it's even easier to violate causality than I thought. If points A and B are moving relative to each other, you can violate causality right there by sending a second message. You, at A, send a message to B via ansible, saying "call me back right away." You wait for a reply, but don't get one. Then, thinking back, you realise you received an odd message from B a few months ago saying "sure, what did you want me to call you back about?" that you had no idea what to make of at the time. :P If B is moving relative to you, their perception of "now" is different from yours. "Relative simultaneity" means that you and B would not agree on what events are simultaneous. "Now" for B is already in your past, so when they called you back "right away", it seemed to them like they were sending the reply right after getting your message, but from your point of view, their reply was sent long ago. And from their point of view, your message asking them to call you back was sent by you from the future...
piroca
05:48:14 AM Mar 17th 2012
edited by piroca
gs and SNDL, girlyboy pretty much answered your questions already, but I have a few comments to make.

Tachyons do have a physical interpretation that is related for example, to the Higgs boson. The relationship is non trivial (which is why I left it "as an exercise"... :) ) and requires a lot of knowledge on quantum field theory so I'm leaving it out for now. In any case, if you've ever heard of "tachyon condensation"... that's what it is. That is to say that they are not automatically pathological, just weird. As long as causality is preserved (fortunately, that quantum field theories satisfying certain assumptions preserve causality can be proven rigorously so we're safe for now) you can have velocities that are higher than the speed of light. In fact, make no mistake: special relativity makes no assumptions on the velocities of the particles it's describing. To be just a little bit technical, it talks about kinematics, not dynamics *. The issue lies with causality. That's what fails if we're to have FTL travel under special relativity. The link that girlyboy sent explains why.

Now, to show that the situation mentioned by gs is in fact a problem, you can apply a Lorentz transformation and transform all coordinates to a frame moving with a constant velocity with respect to A and B (assuming they're at rest with respect to each other). In special relativity parlance, such a transformation is called a boost. This is nothing but simply calculating what an observer moving with constant velocity with respect to A and B would see — so there's nothing unnatural here. To use Lorentz's transformations we need to be a little more specific with our coordinate system, so I'll say that A is at the origin, x = 0, and sends the signal at t = 0, and that B is at x = 1 ly, and receives the signal at t = 0.5 y (ly stands for light-year and y for year). Now we use the equations at http://en.wikipedia.org/wiki/Lorentz_transformation#Lorentz_transformation_for_frames_in_standard_configuration to calculate the time at which B receives the signal for a frame with constant velocity equal to 0.6 c moving from A toward B. Think of this as a slower-than-light spaceship looking at the events. In this frame, the signal will be received at t = gamma (0.5 y - 0.6 c * 1 ly / c^2) = gamma (0.5 y - 0.6 ly/ (1 ly/y)) = - gamma (0.1 y). For v = 0.6 c, gamma = 1.25, so we have t = - 45 days. That is, in this frame the message is received by B 45 days before it was even sent!

Now, you might feel uncomfortable about this — but I didn't talk about complicated quantum field theory machinery. This is exactly the same math I'd use if A lit a flare at x = 0, t = 0, and B lit another flare at x = 1 ly, t = 0.5 ly — by coincidence or prior agreement. Special relativity doesn't care. The problem is when you demand that there be a causal relationship between these two events. Then, depending on who's looking, either event comes before the other, in particular, an effect might well precede its cause. Causally unrelated flares don't suffer from this. And we've tested things like those flares over and over again. It works out fine, no matter how counter intuitive it might sound.

SNDL: I hope the above example proves that the math of Special Relativity is in no way prohibitive of FTL movement. Causality in conjunction with SR is what causes contradictions. Now, first I have to tell you the there's no such thing as a complex negative number. In fact, the whole concept of saying one number is larger than the other only makes sense for real numbers. Complex numbers have no natural ordering. If you ever see someone saying that a complex number is "positive" or "negative", they'd be probably talking about a number on the real line that is positive or negative. It's true, however, that the imaginary part (which is a real number) of 1/i is negative. But for example, look at a number like 2 - i: is it positive or negative? How about -2 + i? -2 - i? 2 + i? And lastly, it is not clear that the gamma factor could even be extended in this way. Saying that the gamma factor having a negative imaginary part would imply backwards time travel is exactly the kind of non-experimentally motivated extension of SR to FTL speeds that you guys have been criticizing in the first place! The "time travel" aspect (which is really an abuse of language, as I say time and again) comes from the geometrical considerations in girlyboy's link.

Btw, girlyboy, that's exactly one of the references I was going to send you! Since you got ahead of the game, I'll recommend an excellent book on the subject, written by the great Lev Landau: http://www.amazon.com/What-Relativity-L-D-Landau/dp/0486428060 . It's only 7 bucks :)
SNDL
02:05:06 PM Mar 17th 2012
edited by SNDL
"I have to tell you the there's no such thing as a complex negative number. In fact, the whole concept of saying one number is larger than the other only makes sense for real numbers."

Are you sure? Remember what equals number i in the first place:

  • i —-> Square root of -1 (a no-brainer).
  • Squared i —-> Minus one (obviously, since two equal square roots equal what they have within, in this case number -1).
  • Cubed i —-> -i (Because Cubed i is the same as i multiplied by squared i, the latter equaling -1. Then, i by -1 equals -i).
  • i powered to four —-> 1 (yes, real one. Squared i multiplied by squared i is the same as (-1) x (-1) = 1).//
EDIT: Oh, you mean the entire complex number! My bad, that's a good point. I did mean the imaginary number alone. As I had said, if a body manages to travel faster than light, its mass and referential time become negative, due to how the imaginary components operate.
piroca
03:48:48 AM Mar 19th 2012
Really, this is not a good way to think about this. In fact, it is meaningless. If you have a bosonic quantum field, in all equations of motion the mass appears squared, which means that the overall sign is of no consequence. If you have a fermionic field, you can absorb the minus sign in the field itself so everything works as it always would. A similar case may be made for the overall negative sign in the time coordinate. You should think in terms of Lorentz transformations that are well defined, that is, for which v < c. That's all you *can* do.
girlyboy
topic
03:07:34 PM Sep 23rd 2011
edited by girlyboy
Removing following from the bit about the CERN discovery at the end of real-life section: It feels more like discussion, so here it is on the discussion page!

Unknown20Troper
01:54:58 AM Sep 26th 2011
Not sure that this is really that natter-y, feels to me like it has a place in the actual article. Just saying. (Plus, what's really so bad about a little bit of natter, if it makes the trope article more interesting and informative, which I feel this does?)
girlyboy
04:29:23 PM Feb 21st 2012
edited by girlyboy
Ehhh, disliking natter is officially Wiki policy at this point. I don't necessarily agree with it either, but enforcing it unevenly is just asking for conflict and problems.

Besides, in this case I really do think the natter is not good. I don't think "the futility of writing hard sci-fi" is a productive thing to be discussing on this page. Obviously a lot of people like hard sci-fi, and obviously there are important tropological differences between hard and soft sci-fi. Also I think arguing that the possibility of scientific knowledge getting overturned in the future means we should treat all science as just a bunch of guesses kind of misses the point of science altogether.

The second comment there is just wrong. The experiment had nothing to do with the speed of light passing through the air. And to be honest, discussions of individual tropers' personal and un-supported scientific theories isn't really what I, for one, would want this page to be about...

Again, that's what this page is for, ain't it? Also we now have an Analysis page, and that has its own discussion page. ... Though I guess these pages don't see a lot of visits from a lot of people, which may well be a problem.
girlyboy
topic
09:03:56 PM Feb 3rd 2011
I think this article is still too long in general. Part (though not all!) of the reason for this is that it goes into a lot of detail about the real-life science of FTL travel. While this is interesting, and needs some mention, it's probably given too much focus.

Moving this sort of thing to a separate Useful Notes page — which could then be linked from this article — might make it possible to cut a lot of stuff out of this article, letting it focus on the way FTL is used in fiction, rather than on the details of why exactly it isn't (or why it someday might be) possible in Real Life...

... I'm not qualified to write about the Real Life issues too much, though. Of course since I'm suggesting this maybe I should do it, but... I'm lazy. :P Maybe eventually, if no-one else is up to it.
boldingd
03:26:53 PM Feb 21st 2011
I think that's a pretty good idea. You're right that the real-life science behind Faster-than-Light Travel is only partly relevant when talking about its use as a plot device, and I would agree that it's taking up more space on the page than it needs too. I'd also point out that it seems to be generating more editing and acrimony than is really necessary — every time I come back to this page, the two paragraphs on SR/GR have changed slightly.

I'll be more than willing to chip-in to the battle that the Useful Notes entry would be.
girlyboy
10:11:15 AM Jul 21st 2011
The page is even more confused now, I think. And the distinctions between the three listed "flavours" of FTL are starting to degrade; I think the idea initially was that Hyperdrive would go under Warp Drives, but now it's been mixed in with what used to be the Portal Network, even though they usually act in different ways... I think this may eventually need a general re-write, top to bottom, again.
girlyboy
04:31:20 PM Feb 21st 2012
edited by girlyboy
Hooray! Some kind soul (a troper with call-sign "memememememe", judging from the history page) moved a lot of the discussion to an Analysis page, making the first half of this trope much lighter and better, at least. I think this is a good improvement.

... Wow, it's been a year and I never did anything about it. I am a terrible troper. :(
girlyboy
topic
07:58:46 PM Jul 16th 2010
I tried to edit the text of the article to make it flow a little bit better (well... that was what I tried to achieve, anyway :P) and to edit down the lengthy paragraph about General vs. Special relativity described further above.

To be perfectly honest I changed quite a lot and I'm not really certain that it's all clear and good, so feedback would be appreciated. I saved the text of the article as it was before my edits so that it'd be easier to restore large chunks of it if they turn out to be not very good. >_<

More specifically, here's the old paragraph about relativity:

"There is, according to special relativity, a fundamental reason why FTL travel is not possible: because of the non-intuitive way Einstein's theory works, FTL travel would always imply time travel, and potential violations of causality itself. This would hold true regardless of which exact method is being used to get around the light-speed limit. The vast majority of works that involve FTL ignore this problem outright, without even attempting a hand-wave. But some try to make use of it, such as Stephen Baxter's Exultant, describing an interstellar war in which "it wasn't remarkable to have dinged-up ships limping home from an engagement that hadn't happened yet". It should be noted, however, that this objection is only proven to hold true when dealing with flat (Minkowski) spacetimes. In Einstein's general relativity, however, the whole issue of faster-than-light travel gets trickier to define, and in fact, there are observational examples right now of faster than light "movement" which are known not to violate causality. Whether or not it is possible to exploit general relativity to achieve faster than light travel without causality violations is an open question - in particular, Alcubierre's metric itself (mentioned in the item for Warp drives) does not possess closed timelike curves (there are other problems with it, however, so it is not a viable solution). Interestingly, attempts at creating solutions to Einstein's equations permitting FTL travel have proven elusive, and science is currently waiting on a viable theory of quantum gravity which is generally hoped to set some of these questions straight."
BritBllt
08:04:01 PM Jul 16th 2010
edited by BritBllt
Not bad! There might be a way to work the Hot Tip into the main text, I'll see what I can do later. The major problem with science-related tropes like this is that there are a lot of knowledgeable tropers, and scientific Fan Myopia has a way of kicking in and gradually making explanations longer and longer, and more and more technical, as time goes by. Overall, I'd say you did a good job of trimming things back down.
girlyboy
08:12:17 PM Jul 16th 2010
edited by girlyboy
Thank you. :) I tried to find some way to work the (very interesting) hot-tip in, but couldn't think of anything that flowed, and was afraid of making everything even longer than it originally was... :P

Perhaps one option would be to create a Useful Notes article for Faster-Than-Light Travel, and then simply make sure there's a prominent link to it in the text of this trope? I haven't spent much time in the Useful Notes section of the wiki, so I don't really know what the procedures are, however. >_>
boldingd
09:54:26 PM Jan 18th 2011
Am I insane, or has the offending paragraph been munged again?
71.252.195.120
topic
07:53:49 PM Jun 11th 2010
edited by 71.252.195.120
Hello all, I have a question, and a suggestion.

First—to nitpick—why don't the warp drives have their own page? They aren't actually FTL drives, in the technical sense, even if they give the relative impression of the ship traveling faster than light. Since many books/shows/movies go out of the way to distinguish this difference, how come it's lumped in with FTL?

Secondly, I think the entry talking about honest-to-goodness FTL travel ought to have categories for two different types of FTL travel: those where the ships are traveling under their own power, and those where they are limited by nature, technology or regulations to certain routes within the galaxy.

The first type would be a lot like the guys in Star Wars, where the guys essentially point and click to get from place to place. Can sometimes be a feature of softer sci-fi, especially if the characters are doing this in very populated space, but can also be Justified by a proper appreciation of how big space is.

The second type, which Atomic Rocket refers to as a "Railroad Drive," limits the ships to specific routes in space, especially if there is some sort of beacon or transmitter or other physical thing that facilitates FTL travel. Railroad drives are usually different from a No Warping Zone in that the FTL "tracks" are laid out specifically to facilitate FTL speeds, not to avoid places where it's impossible, though some of the examples listed on the page that I haven't personally read/watched seem to overlap the two.

Thanks in advance for the imput.
girlyboy
08:09:33 PM Jul 16th 2010
edited by girlyboy
Well, I have to disagree about warp drive. If the drive lets a starship get from point A to point B faster than a beam of light travelling through a vacuum would cover the distance between those two points, then it's a faster-than-light drive. It's true that, say, warping space around the ship isn't the same as simply claiming the ship can keep accelerating past c, but in fact, modern science gives us no really plausible way to do either.

Also, remember that TV Tropes is concerned primarily with stories and narratives, not with the technical details of real-life science. In practice, Warp Drive fulfils the function of an FTL drive in stories where it is present — it lets The Captain and the Cool Starship get from Earth to the Planet of Hats quickly enough not to lose the audience's interest, and then lets them get back home to be awarded medals by The Federation, all before the episode is over. The technical details are less important to this as a trope than are the implications for story-telling.

As for your second point, about the distinction between ships travelling under their own power and ships travelling along fixed routes, I think we actually already cover this. We have Warp Drive, Jump Drive, and Portal Drives. The first two generally involve the ship travelling under its own power; the Portal Drive involves the ship travelling between fixed transit points, which seems to more or less cover what you are describing.
girlyboy
08:20:58 PM Jul 16th 2010
edited by girlyboy
P.S. I added a slight note under the "Portal Drive" paragraph: "One key difference from Warp and Jump drives is that here the ship usually doesn't achieve FTL travel under its own power, but relies on some external force or device — though there are exceptions (such as where the ship has its own drive, but can only use it at specific points of space)." I hope this is all-right, and doesn't muddle the differences between the different drives too much.
71.252.195.120
04:06:04 PM Sep 27th 2010
No problem, I think that works. I also write sci-fi so sometimes I do get a bit overcritical with things like this.
boldingd
topic
05:48:59 PM Apr 8th 2010
I don't know what discussion preceeded this, and I hope I'm not re-igniting a flame-war, but:

I have a problem with the latter half of the following paragraph:

There is, according to special relativity, a fundamental reason why FTL travel is not possible: because of the non-intuitive way Einstein's theory works, FTL travel would always imply time travel, and potential violations of causality itself. This would hold true regardless of which exact method is being used to get around the light-speed limit. The vast majority of works that involve FTL ignore this problem outright, without even attempting a hand-wave. But some try to make use of it, such as Stephen Baxter's Exultant, describing an interstellar war in which "it wasn't remarkable to have dinged-up ships limping home from an engagement that hadn't happened yet". It should be noted, however, that this objection is only proven to hold true when dealing with flat (Minkowski) spacetimes. In Einstein's general relativity, however, the whole issue of faster-than-light travel gets trickier to define, and in fact, there are observational examples right now of faster than light "movement" which are known not to violate causality. Whether or not it is possible to exploit general relativity to achieve faster than light travel without causality violations is an open question - in particular, Alcubierre's metric itself (mentioned in the item for Warp drives) does not possess closed timelike curves (there are other problems with it, however, so it is not a viable solution). Interestingly, attempts at creating solutions to Einstein's equations permitting FTL travel have proven elusive, and science is currently waiting on a viable theory of quantum gravity which is generally hoped to set some of these questions straight.

To the admitedly-limited extend of my knowledge, this is incorrect. Two points separated by a spacelike interval cannot be causally ordered, end-of-story. Therefore, any time you cross a spacelike interval, completely and totally regardless of how you do it, you have placed yourself beyond the reach of the causal ordering of the place you left.

Let's suppose there are two events, A and B. Both happen in reference-frames at rest with respect to one another — that is, the place where A happens isn't moving with respect to B. A and B are two light-years appart, and B happens one year after A, as measured in A's reference frame. Here's the catch: other inertial observers — that is, observers who are not accelerating — will not agree with this: some of them will see B happening before A. This is true even if no-one is accelerating or traveling faster than light, it is a simple consequence of the fact that A and B are, to use sloppy language, further apart in space than in time.

If you get from A to B — no matter how you do it, just if — then whatever actions you take at B cannot be causally ordered with respect to what you where doing at A before you left. Some observers will see you at B before you left A. And they won't be "wrong": this is not an illusion or product of imprecise measuring, but a fundamental quick or the universe. This is also the point where, if you then travel from B back towards A, using whatever trick you used the first time, you'll quite potentially end up before you left, as the old saw goes.

Just to mess with you guys, it's also possible that you could travel in a conventional, slower-than-light fashion... and still get back to (the past of) A before you left. (Well, maybe not with the exact numbers I used, I'd have to think about it for a minute — but it would be, with certain set-ups, perfectly possible.)

So, yeah, long rant concluded, the latter half of that paragraph both clashes with everything I've ever been taught about relativity... and sounds suspiciously like what you'd get if a laymen read an article by a fringe-nut and declared himself an authority.
girlyboy
10:50:13 PM Apr 8th 2010
Odd — I remember participating in a discussion on this, but I can't find it in the archive now. :( Anywho, from my also very limited knowledge you are correct... except for the part about slower-than-light travel possibly leading to travel to the past, which I have not heard of before! :( It was my understanding the very point of the whole FTL thing is that going faster than the speed of light is the only way to mess with causality; if you travel slower then light... something something, light cones, etc., doesn't work. :P Anyway, as is clear, I didn't then, and don't now, know enough about this to conclusively take a side... and I remember the person who added the long note about general relativity being fairly convincing. :P Bears further study, I guess? >_>

This is what happens when TV Tropes discussions get too far into Real Life... we need an expert. :P It's my fault for adding the first bit of that paragraph in, and opposing its deletion, really... >_<
boldingd
12:26:35 PM Apr 9th 2010
edited by boldingd
Heh, pardon me for being ambiguous. What I was trying to say was, the return trip doesn't have to be faster-then-light to mess with causality. You can travel from A to B faster-than-light, and then return from B to A slower than light, and still wind up in the past. You're right, you can't mess with causality while not traveling faster than light (otherwise, the universe would have a large problem in general right now ;). I didn't mean to imply you could, that was very poor wording on my part. What I was trying to illustrate was that "traveling faster than light destroys causality" is the rule, not the exception. I could have worded it better.

Relativity is fun. I can't really blame most SF producers for not worrying about it's consequences.

Does anyone mind if I edit-out the latter part of that paragraph?
201.82.136.148
05:49:12 PM Apr 19th 2010
Be careful, you guys. You are all dealing with *special* relativity, and as far as that theory goes, you are correct. However, special relativity is known *not* to accurately describe the universe in a cosmological scale. The theory in question is Einstein's *general* relativity, in which even *defining* faster than light travel gets blurry at times. See, in a curved spacetime, there *is not*, in general, a single reference frame for two independent events regardless of their separation. That is to say: you cannot calculate, say, the milky way's and a distant quasar's movement in the same reference frame - the expansion of the space between them (go read something about the FRW metric on wikipedia if you're interested) cannot be disregarded.

What messes up the argument about the spacelike interval is the fact that the light cones are not cones, that is, the problem of parallel transport. For illustration purposes, that particular problem also causes the breakdown of conservation of energy in general relativity. It is a very non intuitive theory. To check if there are causality problems with a proposed solution to Einstein's equations, you cannot, as in special relativity, simply compare intervals. You must determine if in such a solution, a closed timelike curve is a valid solution for any observer's worldline. That, naturally, generalizes the conditions for special relativity. It might be interesting to note that since Alcubierre's solution does *not* allow the existence of closed timelike curves, it does *not* provoke causality violations! This simple fact proves that my earlier half of this post is not mere rhetoric - the curved spacetime *does* complicate the causality violation argument. The problem is *open* for a reason, people - it's not at all a simple matter, or else we would see in bright neon lights that general relativity forbids FTL travel.

Incidentally, here are some 'fringe nuts' that agree with everything I said:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/GR/energy_gr.html

I would like people to consider more adequately the content of posts before outright deleting them. That paragraph said clearly that the realm of the discussion was general relativity, and all previous arguments in this discussion pertain to special relativity. Which was also covered in that paragraph, as a special case.
boldingd
10:53:10 AM Apr 22nd 2010
edited by boldingd
I won't delete the paragraph again, out of a desire to not start an edit war, but I have a problem with this. While I'm not well-enough trained in physics to call bullshit with complete confidence, I have had some physics schoolin', and in all of it I have consistently been taught that the information that I have relayed is completely accurate, and never have I heard any hint or mention that General Relativity does not state just as unambiguously as special relativity that faster-than-light travel always implies causality violations. I have never heard any hint of this problem "being open": everything I've ever heard has been that faster-than-light travel always implies time travel, for exactly the reasons I've specified.

Equally, I've never even heard of Alcubierre's solution (and, again, I have had some physics schoolin'). And, a personal-named solution that contradicts everything I've ever been taught about relativity and seems to hint at allowing something that everyone I've ever talked to says has been known to be impossible for decades... that has all the hall-marks of a crack-pot fringe theory.

Edit: As a note, I did wait for some time after asking to delete that paragraph. I did my due-diligence.

Edit: I'm not trying to be too much of a hostile jackass here. Perhaps a compromise: let's split the paragraph in question into two, one about the SR reasons that faster-than-light travel always implies time-travel, and one about Alcubierre's nifty trick? Then, hopefully, we'll both be happy, and readers can decide what they want to believe. :)
Micah
04:46:04 PM Apr 22nd 2010
The Alcubierre metric is very real, though my sense is that it's far better known to laypeople than it has any right to be based on its actual importance to the field. If you know some of the math behind GR, his original paper is actually pretty readable for theoretical physics, so you can judge for yourself. If not, just note that it's peer-reviewed and scholar.google.com lists something like 152 cites for it. (Also, so far as I can tell he himself never calls it "the Alcubierre metric". Other people do that for him.)

The key point is that FTL travel implies the existence of closed timelike curves unless there's some way of distinguishing between different possible time coordinates. Special relativity doesn't prefer any particular timelike direction, so introducing FTL travel in a special-relativistic setting necessarily means introducing closed timelike curves for all the reasons you've stated. On the other hand, a particular metric in GR certainly can have such a preference (and the Alcubierre metric as given does—the first step in constructing it is to pick out a preferred time coordinate).

I agree with you that that paragraph needs some rewriting, though. In fact I tend to think the entire writeup could use a reworking, not because anything in it is false but because it's poorly organized. Splitting that paragraph looks like a reasonable start...
201.68.124.86
01:28:35 PM Apr 23rd 2010
Yes, I agree. I wrote the latter half of the paragraph knowing it was poorly organized, out of a desire to keep all the information it contained while remarking that things aren't as black and white as it made it look. I tried to write it better, but I'm afraid I'm at the limit of where my broken English allows me to go.

@boldingd: It is important to note that these are not some crackpot physics concepts spread forth by random fringe nuts. I assume you never had any formal training in general relativity, correct? You should read the references I cited. They were written by John Baez (who most definitely is not a crackpot - in fact, he's one of the most important mathematical physicists today), and, like all articles whose title is a question, it delves on how the question is tricky to define in a precise sense and that there is no simple answer.

I cited Alcubierre's metric as a clear-cut example of why superluminal travel *might* not involve causality violations. There are others - for example, even though wormholes (proposed by Kip Thorne - not a fringe nut, right?) *do* allow for closed timelike curves, Hawking (whose reputation is also above reproach) conjectures that some effects of quantum field theory would make the throat of the wormhole collapse before any information could be relayed to the past. Wormholes actually might be doable - for example, it is again conjectured that a pair of Kerr black holes might serve as a bridge between two widely separated points in space. Why do I keep using the word 'conjecture'? Because all of these questions cannot be answered without a workable theory of quantum gravity.

I guess the point I'm trying to make is that we cannot answer that question for sure. Baez didn't even try, and he knows a lot more physics than everyone in this thread combined. I agree that the paragraph is very poorly written, and have no objections against a split, but I would like to convince you that it is at least not false, and not fringe science. Read that article. John Baez makes a far better point than I ever could.
boldingd
11:13:32 AM Apr 26th 2010
edited by boldingd
@Micah:

The key point is that FTL travel implies the existence of closed timelike curves unless there's some way of distinguishing between different possible time coordinates. Special relativity doesn't prefer any particular timelike direction, so introducing FTL travel in a special-relativistic setting necessarily means introducing closed timelike curves for all the reasons you've stated. On the other hand, a particular metric in GR certainly can have such a preference (and the Alcubierre metric as given does—the first step in constructing it is to pick out a preferred time coordinate).

I find this paragraph interesting. As I mention below, I have no G.R. training, so I can't really contradict you. But, sitting in the back of my mind is, essentially, the idea that S.R. is a tested model that is known to be quite accurate for at least the systems it models, within its given constraints (i.e. it doesn't model acceleration, which is kinda significant). To my very limited understanding, it looks like something conspiciously Magical would have to happen to allow superluminal travel without potentially contradicting S.R.'s predictions about cases that it's known to model well and accurately. That makes me highly skeptical.

But I'm forced to admit that I'm out of my depth — by a lot — in a discussion on General Relativity, and I'm probably just digging a whole a lot deeper here. As long as it's retained that S.R. clearly forbids causal superluminal travel, I'm fine. :)

@201.68.124.86:

You are correct, I have not had any training in G.R. I completed a physics minor for my undergrad degree, and took an S.R. course while doing that, for what it's worth.

"but I would like to convince you that it is at least not false, and not fringe science."

Regardless of my own personal skepticism, I must, at this point, concede at least that much.
girlyboy
11:33:28 AM Apr 26th 2010
General Relativity has had some confirmation, btw. Nothing about FTL travel as far as I know... :P But it is known to accurately describe the interaction of objects on truly vast scales and what-not. In fact there was just a news article about two competing models being dis-confirmed in some study involving distant galaxies, X-Rays, and gravity, while General Relativity very accurately predicted the way everything interacted.
boldingd
11:36:11 AM Apr 26th 2010
edited by boldingd
@girlyboy

Oh, yes, I'm not trying to claim that I don't believe in General Relativity, what I'm saying is that I'm thinking that the claim that "G.R. claims causal faster than light travel is possible" is, if nothing else, very premature. Since it's pretty flatly impossible in S.R., and S.R. works adequately well at least some of the time.
187.106.52.49
12:50:15 PM May 3rd 2010
@boldingd: What's important is: what are the conditions for SR to be an acceptable approximation for GR? If you have done any coursework in physics, which I assume you have, you are probably familiar with Taylor expansions, — i.e., approximating a well-behaved function by a line, in the vicinity of a point. In 3 dimensions, you are approximating a plane. In 4-dimensional spacetime, you approximate via a Minkowski space, tangent to the manifold. That is to say: SR is a reasonable approximation as long as you consider a vicinity small enough so that the underlying spacetime is almost flat.

When attempting to build a metric that exploits GR's features to make FTL travel possible, you specifically ask that the metric is curved enough to be useful, and hence why SR ceases to be a valid approximation. That is not to say, however, that GR *allows* FTL travel without closed timelike curves. If you say that you are skeptical and that it's probably not possible, then we are in agreement. The message I want to send is: it's not proven that it's not possible. I feel that it is justified, however, to keep an open mind - given the existence of solutions with FTL travel and no causality violations. Even if the current solutions are non physical (depriving us of a clear-cut positive answer) maybe there are some which *are*.

We can't say that it is possible, but we also can't say it is impossible. That is all I would like to convince you of.
boldingd
02:10:52 PM May 5th 2010
edited by boldingd
>What's important is: what are the conditions for SR to be an acceptable approximation for GR? If you have done any coursework in physics, which I assume you have, you are probably familiar with Taylor expansions, — i.e., approximating a well-behaved function by a line, in the vicinity of a point. In 3 dimensions, you are approximating a plane. In 4-dimensional spacetime, you approximate via a Minkowski space, tangent to the manifold. That is to say: SR is a reasonable approximation as long as you consider a vicinity small enough so that the underlying spacetime is almost flat.

That's actually a very good explanation, well written.

> snip < > We can't say that it is possible, but we also can't say it is impossible. That is all I would like to convince you of.

You've succeeded, quite nicely.

I suppose that sort-of brings us around to the question of if and how we'd like to modify the article?
girlyboy
topic
04:35:44 AM Mar 16th 2010
Please don't leave a job half-done. "Shortcut drive" has been renamed by someone. There are now a dozen or so mentions of "shortcut drives" throughout the article that no longer make sense.
esr
05:00:27 AM Mar 16th 2010
I finished the job. This was a response to the confusion about shortcut drives including ocean-like hyperspaces with exotic properties. Sorry about thre two-stage edit, that was a finger error on my part.

girlyboy
09:06:20 AM Mar 16th 2010
edited by girlyboy
Yay! Sorry I was a bit bitchy above, I just thought "hey, whoever changed this forgot that the rest of the article uses the term too!" I agree the change probably makes things clearer.
coppro
topic
12:34:04 AM Mar 9th 2010
Should hyperspace/subspace - travel in an alternate dimension in which the laws of physics permit FTL travel - be its own category? It's increasingly common and doesn't fit into one of the existing categories very well.
girlyboy
04:56:35 AM Mar 9th 2010
edited by girlyboy
I think those count as "shortcut" drives — ship gets off the "local streets" and gets on the "freeway," an "exotic form of space" where it can go faster. Usually when I hear "shortcut drive" I think "wormhole," but I think hyperspace/subspace is supposed to fit there as well.

Maybe it should be split into two though, since the ways wormholes and subspace/hyperspace are used in fiction are substantially different. Wormholes usually come with fixed entry and exit points, and even when not, their distinguishing characteristic is usually that the ship is on a fixed course while travelling, like in a tunnel. Hyperspace/subspace is usually treated more freely.

... But not always.

... And sometimes wormholes fit better under the "jump" category, anyway.

It's all quite confusing.
esr
05:03:56 AM Mar 16th 2010
OK, this is fixed now. The key distinction is: a jump drive is anywhere to anywhere but you don't have to cross intervening space, a portal drive only works with Portal Network structure, and a warp drive is anywhere to anywhere but you do it in some kind of exotic condition that removes the Einstein limit.

Rgis tis the right set of distinctions because the technobabble matters less than the plot implications, and these three types have different ones.
girlyboy
09:10:16 AM Mar 16th 2010
I agree that the way the methods of travel are used is what matters. This change seems like it makes sense, though I think a bit more editing may be needed for polish.
back to Main/FasterThanLightTravel

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