How accurate is the KSP interstellar Alcubierre Drive?

[cicatrix]

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.

So, what's the problem with that?

Wiki article: Relativity of simultaneity

Where do we observe a causality paradox?

[N_las]

So, what's the problem with that?

Wiki article: Relativity of simultaneity

Where do we observe a causality paradox?

When from the hostile bombs inertial reference frame event B can happen before event A, the hostile bomb can react to B by engaging its own warp-drive and reach earth before A happens.

So the earth can be destroyed by the hostile bomb BEFORE we sent the counter-measure.

If we don't send the counter measure, then the earth is destroyed by 2080. But because we send the countermeasure in 2040, the earth is destroyed in 2039. But if the earth is destroyed in 2039, we couldn't have sent the countermeasure in 2040. But if we couldn't have sent the counter-bomb, the earth should have been destroyed in 2080. But then we CAN launch the countermeasure in 2040 ....

The reason why we CAN'T launch the countermeasure is that we WILL sent the countermeasure. Global causality is broken.

Edited November 20, 2014 by N_las

[cicatrix]

If we don't send the counter measure, then the earth is destroyed by 2080. But because we send the countermeasure in 2040, the earth is destroyed in 2039. But if the earth is destroyed in 2039, we couldn't have sent the countermeasure in 2040. But if we couldn't have sent the counter-bomb, the earth should have been destroyed in 2080. But then we CAN launch the countermeasure in 2040 ....

The reason why we CAN'T launch the countermeasure is that we WILL sent the countermeasure. Global causality is broken.

Please, explain this:

How the enemy warp-drive can happen to work back in time and arrive to Earth in 2039?

[*Aqua*]

Please explain it to me, too. I still don't understand this causality problem.

How can the bomb execute an evasive maneuver before the counter-bomb started? The bomb can only know of the existence of the counter-bomb when the later one pops out of warp near it.

Or is the fact that the bomb reacts to the counter-bomb before it receives the information of the start of the counter-bomb a break of causality? I would just call it lag.

[N_las]

I already explained it several times:

Imagine a Bomb travelling with 99.999% light speed towards earth. It was set up by a hostile civilization. A Countermeasure-bomb is started from Earth to explode in the vicinity of the hostile bomb in the hope to destroy it. To have this counteraction take place as far from earth as possible, the Counter-Bomb is fittet with an alcubierre drive. It travels with greater than lightspeed towards the hostile bomb.

Event A is the start of the counter-bomb from Earth, and event B is the meeting of the counter-bomb with the hostile bomb. You will agree, that it is possible for event B to happen before event A in the inertial reference frame of the hostile bomb.

As soon as the counter-bomb reaches the vicinity of the hostile bomb, it explodes. I am not sure what should happen with the warp bubble. I assume the bomb has to exit it before or during the explosion. Now imagine the hostile bomb is also fittet with an alcubierre drive. It withstands the explosion (maybe the counter bomb exploded to far away). As some kind of counter-countermeasure, after detecting the counter-bombs explosion in its vicinity, it starts its alcubierre drive immediately. From it's point of view, event A hasn't even happen yet. If it travels at greater than light speed towards earth, it could reach it before event A. So it will destroy earth earlier BECAUSE we sent a counter-measure but BEFORE we sent the counter-measure.

Clearly, the information that we sent the countermeasure travelled into the past. And cleary, global causality is violated (if earth is destroyed because we sent the counter-bomb but before we sent the counter-bomb, then we couldn't sent the counter-bomb in the first place.)

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.

If the hostile bomb would travel at lower than light speeds, it couldn't reach the earth before event A, but if it itself has a warp-drive, it can reach Earth before event A.

There is another explanation on wikipedia: http://en.wikipedia.org/wiki/Time_travel#Via_faster-than-light_.28FTL.29_travel

And here is another explanation: http://www.theculture.org/rich/sharpblue/archives/000089.html

Please explain it to me, too. I still don't understand this causality problem.

How can the bomb execute an evasive maneuver before the counter-bomb started? The bomb can only know of the existence of the counter-bomb when the later one pops out of warp near it.

Or is the fact that the bomb reacts to the counter-bomb before it receives the information of the start of the counter-bomb a break of causality? I would just call it lag.

The hostile bomb can execute an evasive maneuver before the counter-bomb is started. That is, because from the hostile bombs point of view (remember, it travels at 99.999% the speed of light) the order of events is reversed: FIRST: The counter-bomb reaches the hostile bomb and executes the countermeasure. SECOND: The counter-bomb is started at earth.

This is only possible because the hostile bomb uses a warp drive. see the above Explanations.

Edited November 20, 2014 by N_las

[cicatrix]

Those are explanations which assume that those who travel faster than light are subject to apparent time reversal due to their actual movement in normal space. That's the difference. Alcubierre's Drive's not moving in normal space and does not experience time reversal at superluminal speeds.

I tried very hard to think of a scenario when using warp drives CAN actually make a causality paradox, but failed. Yes, in some cases, some event may happen earlier or later in different reference frames, but since the ship with a warp drive is not subject to time dilation, I cannot see any way I could make the consequence happen before its cause.

Edited November 20, 2014 by cicatrix

[N_las]

Those are explanations which assume that those who travel faster than light are subject to apparent time reversal due to their actual movement in normal space. That's the difference. Alcubierre's Drive's not moving in normal space and does not experience time reversal at superluminal speeds.

No, you havent understood the explanations then. It doesn't have to do with the "movements" during the FTL travel. The only relevant "movement" is the hostile bombs movement at nearly the speed of light, which doesn't need any FTL technology.

The Warp drive is only used to bring some information from event A to event B (while B is outside of As light cone). It doesn't matter how the information got there, it doesn't matter it the information carrier moved through space or simply teleported by magic.

I tried very hard to think of a scenario when using warp drives CAN actually make a causality paradox, but failed. Yes, in some cases, some event may happen earlier or later in different reference frames, but since the ship with a warp drive is not subject to time dilation, I cannot see any way I could make the consequence happen before its cause.

Please read the infamous "FTL communication" thread: http://forum.kerbalspaceprogram.com/threads/93377-FTL-communication

It gets interesting at post #96 (on page 10)

Edited November 20, 2014 by N_las

[rtxoff]

Those are explanations which assume that those who travel faster than light are subject to apparent time reversal due to their actual movement in normal space. That's the difference. Alcubierre's Drive's not moving in normal space and does not experience time reversal at superluminal speeds.

I tried very hard to think of a scenario when using warp drives CAN actually make a causality paradox, but failed. Yes, in some cases, some event may happen earlier or later in different reference frames, but since the ship with a warp drive is not subject to time dilation, I cannot see any way I could make the consequence happen before its cause.

I fear you are up against a windmill here , don't worry you are not the first one, some people just don't want to understand. I gave up that confrontation already.

[*Aqua*]

I already explained it several times

I didn't understood the explanations. (English isn't my native tonge. I can be quite difficult to get the correct meaning of scientific terms, for example in the linked wiki article.)

Or is the fact that the bomb reacts to the counter-bomb before it receives the information of the start of the counter-bomb a break of causality? I would just call it lag.

The hostile bomb can execute an evasive maneuver before the counter-bomb is started. That is, because from the hostile bombs point of view (remember, it travels at 99.999% the speed of light) the order of events is reversed: FIRST: The counter-bomb reaches the hostile bomb and executes the countermeasure. SECOND: The counter-bomb is started at earth.

This is only possible because the hostile bomb uses a warp drive.

So the lag triggers the break of causality. Ok... I still don't know why it's a problem but at least I got to know where you say the problem is.

When the hostile bomb reaches earth the information of the start of the counter-bombe (more or less) instantly reaches the hostile bomb. That's why I don't see a problem.

[N_las]

This diagram really does explains it well:

http://www.theculture.org/rich/sharpblue/images/causalityviolation.png

There are four people involved: Alice, Bob, Dave and Carol

The white lines describe the frame of reference for Alice and Bob (stationary to each other).

The blue lines describe the frame of reference for Dave and Carol (statinary to each other, but no to Alice and Bob)

Ignore the red lines for the moment. There are three events depictet in the diagramm. Event P, event Q and event R.

The yellow cones are the light cones of event P and event Q. A diagonal of 45° is a light beam that travels with c.

From Alices and Bobs point of view, event P and event Q happens at the same time and event R happens before them. From Dave and Carols point of view, event R and event Q happens at the same time, and event P happens after them. This has nothing to do with any FTL travel effects, it is just basic relativity and stems from the fact that Dave and Carol are moving in respect to Alice and Bob.

This wiki-page explains it in more detail: http://en.wikipedia.org/wiki/Relativity_of_simultaneity

Normally, event P couldn't influence event Q in any way, because Q is outside of Ps light cone. And event Q couldn't influence event P or event R. Only event R can influence event P, because P is in Rs light cone.

The following explanation assumes instant communication, but any communication faster then light will allow for a similar situation:

At event P Alice makes a instant transmission to Bob (event Q). Carol can be influenced by event Q. Carol can send an instant transmission to Dave at event R. Event R can influence event P.

Notice, that from Alice and Bobs reference frame (the white lines that are drawn at right angles), the commnucation between Q and R goes backwarts in time. From Dave and Carols reference frame (the blue angled lines) the communication between event P and event Q goes backwarts in time.

Furthermore, one could draw the same situation with the blue lines for Dave and Carols reference frame beeing at right angles, and the white lines for Alice and Bobs frame beeing angled. Such a diagramm would be just as correct.

Notice, that it doesn't matter how the communication between P and Q or Q and R takes place. The claim from gpisic and cicatrix, that the effect would come from the actual "movement" of the infromation carrier is simply false. The only relevant movement is the one of Dave and Carol at slower than light speeds. The information can be magically transfered, teleported, Uri-Gellered or transfered via Warp drives. As long as Event P can somehow influence any event outside of its future light cone, it will allow for this sort of time travel.

[Tw1]

I too never quite got that part of causality.

In the example, the counter bomb reaches the first bomb and explodes. Though the first bomb hasn't seen the launch yet, due to speed of light delay, the launch still happened. It's not going to meet a past version of the counter bomb on it's way to earth.

No matter how fast the bomb travels, it's not going to arrive before the counter bomb's explosion.

If, at this exact second, I hear the sound from a lightning strike several kilometres away, that doesn't mean that that event (the lightning strike) is happening now, it happened a few seconds back.

[N_las]

I too never quite got that part of causality.

Though the first bomb hasn't seen the launch yet, due to speed of light delay, the launch still happened.

(...)

If, at this exact second, I hear the sound from a lightning strike several kilometres away, that doesn't mean that that event (the lightning strike) is happening now, it happened a few seconds back.

The effect has nothing to do with any singal delays due to the speed of light. I made a detailed explanation with diagramms. It isn't the previously described bomb scenario, but a different one. I hope everyone can understand it. The explanation is in the image descriptions: http://imgur.com/a/9x5pV

I think the previously described bomb scenario doesn't work exactly as described earlier, because the hostile bomb actually has to travel away from earth, not towards it. But the principle behind the idea is still correct (just the story about an insterstellar war has to be changed to something different).

If anyone has more questions about the diagramms, please ask. I want this out of the world. This is the third or fourth time this exact topic has come up, and there are always people who are immune to understanding. I want to improve this example, the diagramms and the explanation up until the point were it is optimal. Then in the future hundrets of discussion and questions, we simply can link to this post.

Edited November 20, 2014 by N_las

[Hary R]

I think the part people can't get is this one:

one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backwards in time in any one frame, it must be possible in all frames

in other word, what happen for one observer is true for every observer. following that postulate: If The bomb see the counter bomb before it was launch then its true for everyone. So for everyone, the counter bomb arrived before it was launch, but that can't be true... so it violate the law of causality as we understand it now.

Unless I get it wrong.

[N_las]

(...) in other word, what happen for one observer is true for every observer.

Thats not what it said. The observers can disagree about the order of events (as long as the events happen at different places).

please look at the diagramms I made: http://imgur.com/a/9x5pV

There, I explained it clear and without ambiguity.

[*Aqua*]

there are always people who are immune to understanding.

I'm sorry but I still don't understand it. But I appreciate your effort. *thumbs-up*

[Hary R]

Thats not what it said. The observers can disagree about the order of events (as long as the events happen at different places).

please look at the diagramms I made: http://imgur.com/a/9x5pV

There, I explained it clear and without ambiguity.

I get it, but explain this:

But from the rockets frame, event #3 happens earlier than event #2. This isn't just some kind of information delay due to the speed of light limit. Event #3 is actually happening earlier in this frame.

And everyone will understand where the paradox of FTL come from.

Well bottom line is from our understanding of physics, FTL is just a fantasy as it will create paradoxes (causality violation).

[N_las]

I get it, but explain this (...), and everyone will understand where the paradox of FTL come from.

That is just relativity. As long as one doesn't understand that, one is still thinking like Galileo Galilei about space and time (two seperate entities).

Here is an exhaustive explanation: http://en.wikipedia.org/wiki/Relativity_of_simultaneity

[Rakaydos]

Relativity says time travel is possible, which says global causality is false.

the Abercrombi Drive is just the most energy efficent method of doing so, as it doesnt require relativistically spinning black holes or other shenanagans.

[rtxoff]

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. 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. The only exception to this is when one side of the wormhole is moving with relativistic speeds but that's obviously not the case with the warp drive.

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. 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.

[KerikBalm]

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. The only exception to this is when one side of the wormhole is moving with relativistic speeds but that's obviously not the case with the warp drive.

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.

How ironic....

With any form of FTL coupled with relativistic velocities, it can return to before you left, and there is nothing preventing you from going arbitrarily back in time, to the point that, yes, you could kill your "uncle" (I believe father/ grand^n father are more apt examples for causality violations here, but whatever).

The alcubiere drive currently has no proposed mechanism for expanding and contraction space time. Its sort of like imagining an interstellar spaceship before you've invented a rocket or a gun that can acheive your velocities. Sure, you can calculate that if you could accelerate at X gs for Y seconds, the journey would take Z time... but you haven't actually designed a drive capable of doing that.

The alcubierre "drive" is analagous to that... if you could contract and expand space time, then you could do that, but we have no idea how to contract and expand space time, and we don't even know if its possible to manipulate... much like the "flow" of time. We can imagine all sorts of time machines, but lack any mechanism to accomplish what we want.

FTL is time travel. Its time travel beyond the case of a wormhole with one opening going at relativistic velocity towards another...

It is most likely impossible for us. Of course, if the universe is a simulation, then those running the simulator could do whatever they want.... so I won't say anything is impossible, as its not impossible that we're living in a simulation.

But I'm not going to go around pretending that we're on the cusp of making an FTL drive.

[Z-Man]

They are claiming you could travel back in time before you are born

The thing is, you can. Accelerate your craft normally to .5 c (relative to Earth) in the negative x direction. Engage warp at 10c relative to your current rest frame into the positive x direction. Congratulations, you're already moving backwards in time relative to Earth's reference frame. See N_las's pictures. Now you disengage warp, turn your ship around to .5 c into the positive x direction, engage 10c warp into the negative x direction, disengage and break down to 0 relative to Earth. Make the warp phases long enough, time it right and you are now back at earth at an earlier time. You're still only breaking global causality, though.

The important bit is the bolded one. You have to assume the warp drive is capable of getting you to superluminar speeds relative to whatever is your current rest frame. The Alcubierre drive would do precisely that. And what's superluminar in one reference frame can go backward in coordinate time for another.

Still, you can't use that argument to declare such a drive is impossible. Global causality violations are a bit like spacetime singularities. They are both very annoying, but if you can't rigorously prove they can't be created, you have to deal with them. You can't claim stellar collapses don't happen because that would create a singularity, you can't claim any kind of FTL can't happen because that would cause causality problems. You can use it as a motivation for further research in that direction, sure, but you need to find a more fundamental reason. "I don't like B, it sounds horrible. A would imply B, so A can't be true" does not cut it.

Back on topic, I did the parallel transport calculations for arbitrary constant weak gravity fields. Nothing unusual happens. Your velocity at the end of warp has three contributions:

1. The original velocity before warp

2. The acceleration you would have picked up if you were a regular Newtonian particle moving along the warp path in Newtonian gravity

3. A tiny term due to the spatial part of the curvature. Zero if your initial velocity is zero. If your warp direction is identical to the classical ingoing velocity, this bends your velocity by half the value it bends the direction of a photon going down your warp path.

Unless both your warp factor and your initial velocity are huge, the third term can be safely ignored. For large warp factors, the second term can also be ignored.

Dunno. How does KSPI's warp drive operate? If it's "velocity relative to the sun stays constant during the warp jump", it's probably accurate enough.

[KerikBalm]

I found this to be the clearest explanation:

https://web.archive.org/web/20100212103404/http://sheol.org/throopw/tachyon-pistols.html

We can describe this effect by idealizing FTL to be "instantaneous", and describing how the more familar time dilation implies this effect. But remember, the same points apply to any FTL speed, you just have more messy arithmetic to grind through.

Consider a duel with tachyon pistols. Two duelists, A and B, are to stand back to back, then start out at 0.866 lightspeed for 8 seconds, turn, and fire. Tachyon pistol rounds move so fast, they are instantaneous for all practical purposes.

So, the duelists both set out --- at 0.866 lightspeed each relative to the other, so that the time dilation factor is 2 between them. Duelist A counts off 8 lightseconds, turns, and fires. Now, according to A (since in relativity all inertial frames are equally valid) B's the one who's moving, so B's clock is ticking at half-speed. Thus, the tachyon round hits B in the back as B's clock ticks 4 seconds.

Now B (according to relativity) has every right to consider A as moving, and thus, A is the one with the slowed clock. So, as B is hit in the back at tick 4, in outrage at A's firing before 8 seconds are up, B manages to turn and fire before being overcome by his fatal wound. And since in B's frame of reference it's A's clock that ticks slow, B's round hits A, striking A dead instantly, at A's second tick; a full six seconds before A fired the original round. A classic grandfather paradox.

Note, this is NOT a matter of when light gets to an observer, it is NOT an optical illusion. It is due to the fact that, in SR, the question of what occurs at the "same time as" something else is observer dependent.

As A fired that first show at tick 8, the bullet effectively teleported from A's gun to B's back instantly --- instantly according to A. But for B, who was moving at 0.866 lightspeed WRT A, B was hit in the back by the bullet 4 seconds BEFORE the bullet was fired. And again note, this is NOT due to the optical illusion of lightspeed delay in viewing A's turn-and-shoot; the light form that event wouldn't reach B until MUCH later, not tick 4.

Here's a spacetime diagram of a referee (O3) and two duelists (O1 and O2). Space is up/down, time is left/right, in the diagram.

mutualdilation.gif

The paths O1 and O2 take through spacetime are the colorcoded arrowed lines. The events in spacetime that each considers "simultaneous" and "8 seconds after the start" are along the thinner, non-arrowed colorcoded lines. So we see that, each of the three observers thinks the other two have slow clocks, and that if we are allowed to move faster than a lightcone, we'll end up going "pastward" in somebody's reckoning.

We can trace out the shots on this diagram, also. Step through it one more time: if green shoots at 8 seconds out, the shot will go along the green "line of simultaneity", and hit blue at 4 seconds elapsed. If blue returns fire from there, it will return along a line paralel to the blue "line of simultaneity", and catch green napping at 2 seconds elapsed.

You can see more about spacetime diagrams, and more about the notion of "lines of simultaneity" at

sr-ticks-n-bricks.html sr-twin-01.html

Finally, FTL still can can bite you in non-instantaneous cases; where we're only going a "little bit" faster than light.

If you warp out, go to Tau Ceti, then with normal reaction engines accelerate away from earth, warp out again to go back to earth, you will indeed get back before you left. (Presuming that the real-space delta-v before the warp/hyperdrive/tachyon-watziz trips was "large enough"... there are formulas for such things in the textbooks).

[rtxoff]

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 never mention that one before but now i will: Occam's razor

It's not a proof for anything but if it comes to what i will believe might be true i will hold on to it before accepting some far fetched paradox creating mumbo jumbo.

[KerikBalm]

The far fetched thing is the FTL drive in the first place.

The rest is just derivation.

The only problem I have with what I quoted, is that the example had significant time dialation before any use of FTL, whereas the bolded part of the quote "if you warp out, go to Tau Ceti, then with normal reaction engines accelerate away from earth, warp out again to go back to earth, you will indeed get back before you left. (Presuming that the real-space delta-v before the warp/hyperdrive/tachyon-watziz trips was "large enough"... there are formulas for such things in the textbooks). "

has the relativistic effect taking place after the first use of FTL....

Unless "getting back before you left" implies leaving tau ceti, and not earth... but....

Bah... it makes my head hurt.

I admit I don't fully understand it

In the first example, it seems that the FTL round travels back in time equal to:

(Time there has been relativistic velocity between objects)/ (1- 1/Time dialation factor)

8 seconds of a time dialation factor of 2 = 4 second time travel

20 seconds of a time dialation factor of 2 = 10 second time travel

20 seconds of a time dialation factor of 4 = 15 second time travel

20 seconds of a time dialation factor of 1 = 0 second time travel

20 seconds of a time dialation factor of 999999 = 20 second (rounded) time travel

In this case... you can never get back to before the relativistic motion between the objects.

So I guess it is like the wormhole with one end sent off at the speed of light.

In the warping example, I guess if you fired up your reaction engines, warped out, and then warped back, you could get back before you warped out, but not before you fired up your reaction engines... if I am understanding correctly....

[cicatrix]

Global causality violations are a bit like spacetime singularities. They are both very annoying, but if you can't rigorously prove they can't be created, you have to deal with them.

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)