How accurate is the KSP interstellar Alcubierre Drive?

[Technical Ben]

Causality is only locally relevant in General Relativity. FTL is not a violation by any measure. Otherwise, Universe wouldn't be expanding at FTL speeds.

The biggest inaccuracy of KSP's Alcubierre Drive is that it doesn't properly take into account curvature due to gravity. You drop out of warp traveling at the same speed that you went into the warp at, but the frame of reference is accelerated. To account for this properly, author should have carried out parallel transport of the velocity vector in a curved space-time. Naturally, since gravity in KSP is a patched-conics approximation, it would be entirely acceptable to use Schwarzschild Metric for parallel transport as well, patching at SOI boundaries as necessary.

There are a few minor issues as well. Energy consumption, visual effect of the warp, etc. But these could be chalked up to sacrifices in the name of gameplay and limitations of the rendering engine.

When I write up my "Space rescue team in space" themed sci-fi with FTL... can you do my Hyperspace jump physics? It would be wonderful to know the technical limitations of FTL if required to carry over acceleration and relative velocities etc. One aspect I'd consider interesting is FTL travel requiring carefully planned orbits of intermediary destinations, to allow gravity assists to help change direction and velocity efficiently before arriving at the final destination.

The main question is. Why does FTL reverse time travel, instead of reducing travel time?... It makes my head hurt too (though I accept the maths and observations up to light speed are all 100% correct, I still need time to digest the maths).

Edited November 21, 2014 by Technical Ben

[cicatrix]

Back to the game plugin:

Here's what's wrong (IMO):

Exotic matter accumulates and just stays there like any other resource. It should be produced when the warp drive is activated and the exact amount produced should determine how far the jump will be made.

You cannot stop at any time during warp transition. Your exit point should be set BEFORE you activate the drive and you can't change it during the transit. So, in order to travel somewhere you should somehow enter the destination coordinates that you won't be able to change once you've activated the drive. It should take some time to produce the necessary amount of exotic matter and if you changed your mind and deactivate the drive it should be dissipated slowly (alternatively, in order to make it less cheaty it should just destroy your ship).

The amount of energy needed to produce exotic matter is laughingly small. Even if you use antimatter power it should take much longer time, I think. The fact that you move during the time the necessary amount of exotic matter is produced should make the exit point change its coordinates as well (the more the distance is the more error is).

[Hary R]

Well i know that all is very speculative but i don't understand how someone is tempted to assume in such scenarios that you will be doin an absurd time travell back into your time where you can kill your grandfather or whatever and not trying to think of a more rational sollution?

I think it just a statement of the theory as we know it right now, the time traveling thing is a big "IF" as in reality no experiment has shown what will really happen in such a case or if such case is really impossible. But As for our current understanding of relativity, FTL induce time travel to some degree.

I don't know about the scientist one but as for me, a non scientist, I will be happy to see that we were wrong, and see that FTL travel don't cause any problem in the continuity of space time. That will be a great achievement.

[Z-Man]

Actually, scientific approach is quite the reverse: until you can't prove they CAN be created, OR suggest an experimental way to prove they can't it's not science, it's just speculation. (see Falsifability)

Yes. Note that I did not claim it can be created After all, it relies on the existence of speculative states of matter and our ability to suitably control them. But without good reason, you can't claim something is impossible, either. I can, for example, state that it is impossible to divide an arbitrary angle into three parts using only ruler and circle. We have proof for that. But we don't have any theorems regarding the impossibility of creation of closed timelike curves, not even under the strictest conditions on matter fields. So you don't get to use anything like that as a serious argument.

[rtxoff]

Isn't the resulting creation of paradoxes reason enough to highly doubt the possibility of closed timelike curves?

[KerikBalm]

Isn't the resulting creation of paradoxes reason enough to highly doubt the possibility of closed timelike curves?

And thus reason to highly doubt the possibility of FTL, because unless relativity is false, FTL would make such curves possible.

[Rakaydos]

Isn't the resulting creation of paradoxes reason enough to highly doubt the possibility of closed timelike curves?

There are three schools of thought, of which "it's impossible" is just 1.

"Many worlds" says you can change the past, and nothing that happened before you went back in time is certian- Dont try playing the stock market with tomorrows newspaper, you'll lose.

The other school of thought, I dont remember the name but you can think of it as destiny. You can go back in time, just as you always have gone back in time. yo cannot change the past, and if you tried, you must have failed, or the past wouldnt have happened in the first place.

For instance, lets imagine the first FTL explorers returned to earth... and found themselves in the mid 1900s. After a close call over Roswell, they decide against interfering with WW2/the cold war, and are forced to resort to cattle abduction to sustain their supplies.

[Mazon Del]

For instance, lets imagine the first FTL explorers returned to earth... and found themselves in the mid 1900s. After a close call over Roswell, they decide against interfering with WW2/the cold war, and are forced to resort to cattle abduction to sustain their supplies.

Hah! That would make a pretty decent plot for a scifi book. I suppose they harvest crops by making crop circles since this ends up being less 'conspicuous' as far as someone mysteriously stealing crops is concerned. A guy complaining his crops were stolen in the formation of a crop circle ends up looking like a crackpot rather than a guy with a legit claim of theft.

[Z-Man]

Isn't the resulting creation of paradoxes reason enough to highly doubt the possibility of closed timelike curves?

No, not really. Undesirable consequences, no matter how nasty they may be, are no reason to assume something is impossible. Chances are that nature has a way of dealing with them either shortly before they are created (and yes, there is a well defined shortly before) or when they are created. We just don't know what it is yet. It may even be harmless.

[rtxoff]

And thus reason to highly doubt the possibility of FTL, because unless relativity is false, FTL would make such curves possible.

Yes something traveling FTL would make such a curve possible indeed. The thing is, nothing is traveling FTL with an Alcubierre drive, also nothing is traveling FTL when you are traversing a wormhole. When the space is curved to such an extreme you can be literally going with peasant speed and you will arrive at your destination that is lightyears away in minutes. The Alcubierre drive might work with a different mechanic then the wormhole but the results should be the same.

[Hary R]

The thing is, nothing is traveling FTL with an Alcubierre drive, also nothing is traveling FTL when you are traversing a wormhole.

Well the information that the drive is carrying with it it traveling at FTL speed...

[N_las]

Nobody is denying that an alcubierre drive would break global causality the problem is people claim it would break also the local causality. They are claiming you could travel back in time before you are born and kill your uncle and such things with it. Just think about how absurd that sounds.

That isn't a violation of local causality. Please learn the meaning of words before you use them.

Imagine Causality to be a law like: "All people are only allowed to travel from west to east, not the other way around."

Local causality would be only violated if you find a place on earth were somewere is traveling against the stream.

But if someone travels always to the east, and he finds himself at a city WEST of his starting point (because the earth may be round), then only global causality is broken. Local causality still isn't violated, because he never took a step in westward direction.

They seem not to understand that an returning travel with an FTL drive will bring you back to your starting frame of reference and to your starting time plus the time the travel took. Basically a warp drive does nothing else then a wormhole (Einstein-Rosen bridge) would do, a shortcut through time and space. A travel by such will be a time travel but the return will be travel back into to your starting time/space and not into your past where you can kill your uncle.

IF you look at my examples (http://imgur.com/a/9x5pV), you will see that you are basically right. If the people that jumped after the rocket in their alcubierre-ship would simply jump back, they wouldn't be in their past. It would just be like you claimed.

But thats the reason my example contains the rocket with a second alcubierre-drive. Because the earth is travelling away from the rocket at 0.5c, when they use the second alcubierre-drive, they will land in earths past.

Please find an actual flaw in the explanations given in the diagramms. They clearly demonstrate that time travel with alcubierre-drives is possible.

What bothers me most here is that people here try to explain something they don't understand themself and they refuse to see the matter from a different point of view.

Cognitive bias is strong in this one^^

They only are able to talk about relativity and how it is the basic rule for everthing in the universe totally ignoring the fact that Einstein himself came up with solutions where there are ways to overcome the limitations of relativity.

Just like the alcubierre-drive, Einstein-Rosen bridges can also be used to travel through time. The mechanism described on wikipedia may be differnent than my example, but that doesn't matter.

I am not ignoring that "Einstein himself came up with solutions where there are ways to overcome the limitations of relativity". I am simply pointing out, that this solution will enable time travel. And everyone who has a basic understanding of physics can see that.

Yes something traveling FTL would make such a curve possible indeed. The thing is, nothing is traveling FTL with an Alcubierre drive, also nothing is traveling FTL when you are traversing a wormhole. When the space is curved to such an extreme you can be literally going with peasant speed and you will arrive at your destination that is lightyears away in minutes. The Alcubierre drive might work with a different mechanic then the wormhole but the results should be the same.

You are still using the argument....

Please, Please, PLEASE look at my diagramms: (http://imgur.com/a/9x5pV) look at them hard and try to understand them. JUST LOOK AT THEM. Now tell me: Where does the "movement" or "not movement" of the ship during its FTL "travel" matter? Please point it out! WHERE, WHERE?

Nowhere!!! The argument of "no actual movement" is just weak and shows that you havn't understood anything. The only FTL related thing that matters in my example are the starting points and the end points of FTL "jumps".

[rtxoff]

Do you really expect that someone concludes something serious about 4D space through an handdrawn 2D diagramm? If you can't form the picture in your mind and you need a drawing for it that would not be sufficient to understand anything about the relativistic universe.

I really suggest that you stop ridiculing yourself any longer and stop acting like you understand anything about what Einstein had in mind because you clearly do not. You made so many logical errors by now like the bomb example i would be ashamed and shut up before talking any further here.

[Z-Man]

Do you really expect that someone concludes something serious about 4D space through an handdrawn 2D diagramm?

Err. Yes. It's a thing called 'projection'. You project away, ignore, the two spatial dimensions that are not relevant to the situation at hand. 2D space/time drawings are the perfect tool for explaining what Lorentz transformations do. If you think your imagination is superior to them in this case, sorry, chances are your imagination is wrong.

[K^2]

The diagrams look like it's been fixed, but original example had time shifts backwards. If you are flying towards an event, it appears to have happened earlier. So both rockets need to be departing away from Earth. It still causes a "paradox", but honestly, time travel isn't an issue with real physics. The fact that you prevent event's cause using a CTC resolves itself trivially in field theory. Like many things in QM, it's easiest to understand from MWI perspective. All you did was create another branch in the many worlds. One that is, admittedly, a bit odd for anyone involved. But there are no fundamental contradictions in logic anywhere.

[N_las]

Do you really expect that someone concludes something serious about 4D space through an handdrawn 2D diagramm? If you can't form the picture in your mind and you need a drawing for it that would not be sufficient to understand anything about the relativistic universe. I really suggest that you stop ridiculing yourself any longer and stop acting like you understand anything about what Einstein had in mind because you clearly do not. You made so many logical errors by now like the bomb example i would be ashamed and shut up before talking any further here.

Are you for real? Of course we can use 2D diagramms for space-time, as long as only one spacial dimension is relevant. I am not ridiculing myself, I understand "what Einstein had in mind" to a reasonable degree. If you think I "clearly do not", then point to a single error I made. I made "so many logical errors"... I don't see any.

The only error I made with the bomb example is the direction the hostile bomb had to travel. It renders the story around the example meaningless, but it is a irrelevant detail for the idea behind the process. I corrected that in my diagramms.

The diagrams look like it's been fixed, but original example had time shifts backwards. If you are flying towards an event, it appears to have happened earlier. So both rockets need to be departing away from Earth. It still causes a "paradox", but honestly, time travel isn't an issue with real physics. The fact that you prevent event's cause using a CTC resolves itself trivially in field theory. Like many things in QM, it's easiest to understand from MWI perspective. All you did was create another branch in the many worlds. One that is, admittedly, a bit odd for anyone involved. But there are no fundamental contradictions in logic anywhere.

Yes, i am not trying to argue that the paradox would be reason to belive that FTL must be impossible because it would "break" physics. I just want for Gpysic to finally understand that time travel and FTL has to go hand in hand.

Edited November 22, 2014 by N_las

[K^2]

You have my seal of approval on that, for what it's worth. 2D diagrams are entirely legitimate for working out almost everything in Special Relativity^*, and great deal of General Relativity. Even gravity can be demonstrated to arise in a 1D + time world. And warp drives, other than the actual warp bubble, can be entirely worked out in SR.

^* There are SR topics, like acceleration and scattering, that require at least two spacial dimensions.

[shynung]

"You must spread some Reputation around before giving it to N_las again."

[Rickenbacker]

We don't need a scout for this scenario. And why are you talking about the counter-bomb to "arrive earlier than #1"? What does the start of the hostile bomb has to do with anything?

If event A is the start of the counter bomb, and event B is the meeting of the bombs, from earths inertal reference frame, then event B happens after event A. But because the counter bomb reached event B earlier than a light beam from event A would reach it (a light beam could not travel from event A to event , event B happens outside of the light cone of event A. Because Event B is outside of event As light cone, from the hostile bombs inertial reference frame (it travels near light speed) event B can happen before event A.

Nope. Event B happens after event A in any case, it's just that the enemy bomb gets no warning of it. Even if the light from the launch (A) hasn't reached the enemy bomb by the time B happens, some time has still passed since the launch. No causality violations there, as the enemy bomb doesn't have to KNOW that A has happened, it happens anyway.

[N_las]

Nope. Event B happens after event A in any case, it's just that the enemy bomb gets no warning of it. Even if the light from the launch (A) hasn't reached the enemy bomb by the time B happens, some time has still passed since the launch. No causality violations there, as the enemy bomb doesn't have to KNOW that A has happened, it happens anyway.

Relativity isn't just signal delay. There is a lot more to it. And no, event B doesn't happen after event A in ANY case. Just look at this: http://en.wikipedia.org/wiki/Relativity_of_simultaneity

It doesn't matter which two events A and B you choose in the universe, there will be always frames of reference were the order is A-B and others were the order is B-A. This is not due to signal delays.

[rtxoff]

Yes, i am not trying to argue that the paradox would be reason to belive that FTL must be impossible because it would "break" physics. I just want for Gpysic to finally understand that time travel and FTL has to go hand in hand.

Now where did i say that time travel is impossible? I am only saying it's not possible in scenarios like doing some warp drive roundtrip ending up in your own past and place. Or scenarios where you would somehow magically warn of someone what is going to happen. Time travel is for sure possible with the warp drive but traveling into your own past is not at least not while not moving away from it. Time and place are linked together one simply cannot go to the past without moving away from that place, the thing is the roundtrip back reverses the effect and you will be back in your time and your place. I really do not understand why that's so hard to understand.

[Z-Man]

I really do not understand why that's so hard to understand.

Because it is wrong. You have still not pointed out a concrete error in N_las's pictures.

... at least not while not moving away from it.

Well, yes. Precisely. And kind of obvious, you can't get back to Earth if you move away from it. That's why you (let the information) switch warp ships or do some manoeuvres to turn around while continuing to move backwards in time according to Earth's rest frame. Edited November 22, 2014 by Z-Man

[rtxoff]

Because it is wrong. You have still not pointed out a concrete error in N_las's pictures.

Well what else should i have to say about that. He is trying to explain 4D space by drawing it to 2D space, even if you project a 3rd dimension onto 2D you are still lacking one dimension.

While such diagrams may be useful to solve some specific problems they are totally useless in this specific case where we are working with bended spacetime in a very unusual way. If he manages to bring down the maths of a wormhole down onto a sheet of paper in a 2D diagramm i will bow before him, unfortunately it's something that's impossible. The only way someone could reasonable do it by writing some sort of simulation that would be projected in 3D on some screen where you have the freedom to move around and watch how things are related to each other while moving through simulated spacetime. I do not know of anyone made such a program but it would surely help some folks to understand more.

Well, yes. Precisely. And kind of obvious, you can't get back to Earth if you move away from it. That's why you (let the information) switch warp ships or do some manoeuvres to turn around while continuing to move backwards in time according to Earth's rest frame.

Well that's not going to happen with a warp drive, once you are in the bubble there is no way to switch warp ships exchange information or whatever with the rest of the universe. You will be captured in the bubble until it collapses by running out of whatever energy was needed to create it. That is also a important point preventing paradoxes. A wormhole is no different in that, you might be able to exchange information with someone at one of the 2 ends while you are inside but not with someone standing near by at the middle of it. At least not without taking the long way out of one of the two ends. Causality is in all cases preserved.

On the other hand effects like seeing yourself departing from Earth while already arrived at Mars for example are very possible and nothing to be concerned of, there would be no way to prevent your launch from happening even if you see yourself departing from Earth.

Edit: Well i certainly have enough from this thread and i am not going to argue any longer about that matter especially not with the unthankful attitude of some folks trying to teach me stuff they don't really understand and such statements that i am wrong without having anything reasonable to say to that matter.

Edited November 22, 2014 by gpisic

[lincourtl]

Do you really expect that someone concludes something serious about 4D space through an handdrawn 2D diagramm?

Descartes, Fermat, Feynman, Dirac... pikers all.

[N_las]

Now where did i say that time travel is impossible? I am only saying it's not possible in scenarios like doing some warp drive roundtrip ending up in your own past and place. Or scenarios where you would somehow magically warn of someone what is going to happen.

(...)

Well that's not going to happen with a warp drive, once you are in the bubble there is no way to switch warp ships exchange information or whatever with the rest of the universe. You will be captured in the bubble until it collapses by running out of whatever energy was needed to create it. That is also a important point preventing paradoxes. A wormhole is no different in that, you might be able to exchange information with someone at one of the 2 ends while you are inside but not with someone standing near by at the middle of it. At least not without taking the long way out of one of the two ends. Causality is in all cases preserved.

But that is the only thing they do in my scenario. The two ships can only communicate with each other while both are outside of any warp bubbles. In the example given, the A-drive-ship they are using to follow the rocket has the lottery numbers painted on its hull. They leave the warp bubble at a point near the rocket. The rocket is moving by that point at 0.5 c, but there is nothing there to prevent them to see the lottery numbers. Then the rocket engages it's own alcubierre-drive and jumps back to earth. Nowhere is any communication while anyone is inside a bubble. And as you can see in my diagramms, the rocket ends up at earth BEFORE the alcubierre ship is leaving, so at least information could make a roundtrip.

Well what else should i have to say about that. He is trying to explain 4D space by drawing it to 2D space, even if you project a 3rd dimension onto 2D you are still lacking one dimension.

You can collapse 3-dimesions into one, if you are only using one direction of travel. You always move one way, the other two directions are irrelevant. And even if they were relevant: Then show me in my diagramms! Where is the error in the diagramm due to "not-enough-dimensions"?

While such diagrams may be useful to solve some specific problems they are totally useless in this specific case where we are working with bended spacetime in a very unusual way. If he manages to bring down the maths of a wormhole down onto a sheet of paper in a 2D diagramm i will bow before him, unfortunately it's something that's impossible. The only way someone could reasonable do it by writing some sort of simulation that would be projected in 3D on some screen where you have the freedom to move around and watch how things are related to each other while moving through simulated spacetime. I do not know of anyone made such a program but it would surely help some folks to understand more.

I have the suspicion those 2D-diagramms are just over your head, and you simply don't understand them. Even if I would make such a simulation, you would still claim that it is wrong. Anyone who understands my diagramm knows that the other two spacial dimensions are just empty. All the lines simply fall on one "2D-slice" in the 4D-diagramm, so a 2D diagramm works perfectly well.

Edit: Well i certainly have enough from this thread and i am not going to argue any longer about that matter especially not with the unthankful attitude of some folks trying to teach me stuff they don't really understand and such statements that i am wrong without having anything reasonable to say to that matter.

Yes, and in a week or two you will yet again claim in another thread that "the alcubierre drive" isn't actually moving and thus can't be used for time travel. I know that it is impossible to teach you anything. But by now, every reader of this thread will see how strong the Dunning-Kruger-effect is with you, how strong your cognitive bias is. I simply have to link to this thread in the future, and at least other forum users, who actually wan't to learn something new and are open to information, will be educated against your nonsense.