How is an Alcubierre drive FTL?

[vger]

Thank you very much for that link, Mazon. That will keep me busy for a while.

It won't appear to arrive 4.7 years in the past. It will appear to arrive 4.7 years after its departure, when looking at it from Earth, since that's how long the light from the ship would take to get to Earth.

Well, won't that make life interesting if humanity actually achieves FTL travel. The astronauts could fly to Neptune, come back, and then tell us when and where to point our telescopes so we could see the ship in a couple of hours. *mind boggling*

[K^2]

What marks the difference between local and global causality? For what observer is local defined here, or is this even a matter of observer, and what consequences does violating global causality have?

Local causality is consequence of more general local properties of space-time, but what it really means is that no matter how weird things get, around any point, you can find at least a small, finite neighborhood where causality is preserved. This is equivalent to saying that two particles can't pass through the same point at the same time moving faster than speed of light with respect to each other. It doesn't sound like a significant limitation until you consider the fact that all real particles are delocalized, and their probability amplitudes overlap quite frequently.

Local causality violations would break the entire field theory. It's total anarchy, and you can't build a self-consistent theory, at least anything like what we have, without causality working at least locally.

Global causality violation is future events causing past events. The most extreme case is time travel, and the reason why people don't like it is that it causes things like Grandfather Paradox. Which would be a problem in classical setting, but it isn't a problem with field theory. It doesn't mean that it's necessarily possible, but if there is some grand cosmic reason why it can't be so, we don't know it yet.

I wonder if some of short gamma ray bursts (of which origin remains unknown) could be created by an alien starship dropping out of warp after a long voyage, during which it accumulated massive amount of energy in warp bubble?

That'd make for an interesting paper. If spectra and duration of these bursts is available, it'd be possible to compare to dropping out of Alcubierre warp. I know people have written some general analysis papers on what happens to matter trapped in the warp bubble at the exit.

[ThirdHorseman]

I have to stop you right here. Alcubierre Drive does not allow for time travel in flat space-time. While it's true in general that a pair of FTL ships make up a time machine, it requires communication between them mid-voyage. This is possible, in general, because we picture the two FTL ships pass arbitrarily close to each other. For Alcubierre Drive, it requires the two ships to exchange information mid-warp, while heading on courses that prevent direct communication with each other without warp bubbles overlapping. And later is impossible to achieve without disrupting the warp.

I haven't read anything that said there could be no interaction while moving with an Alcubierre Drive. Is that true?

[Mazon Del]

ThirdHorseman: While it is not outright impossible, it is effectively so given that the gravity shell around the ship is going to warp space so radically that matter would be torn apart at the subatomic level (part of why the gamma ray bursts are a thing) and signals would get so jumbled that they become just static noise.

A fascinating, and somewhat scary, question for warp drive which only a few sci-fi authors ever get into is: What happens if a ship with a warp drive runs into a planet at warp? Or worse. What happens if it runs into a STAR at warp?

Theoretically the ship would be fine, the planet? Oh no. nononono...the star? Honestly...nobody has any real idea. Some think it could result in an effect large enough to destablize the fusion and cause a semi-nova explosion to occur. But again, nobody is really sure.

[K^2]

I haven't read anything that said there could be no interaction while moving with an Alcubierre Drive. Is that true?

It's not outright impossible. In principle, a ship under warp can still send and receive messages. However, since it is moving FTL, it cannot send messages forward, and it cannot receive messages from behind. That should be pretty self-evident, but there are some papers that derive that rigorously for an Alcubierre Drive, in case anyone has any doubts. And the limitation here is precisely in that the ships have to be heading roughly head-on to break causality, which is exactly the direction the ships can't communicate in.

In contrast, there is a specific direction in which messages can be sent without significant blue/red shifts, so a ship under warp can, in fact, briefly communicate with stations, outposts, or other places of interest as it passes by.

[KASASpace]

ThirdHorseman: While it is not outright impossible, it is effectively so given that the gravity shell around the ship is going to warp space so radically that matter would be torn apart at the subatomic level (part of why the gamma ray bursts are a thing) and signals would get so jumbled that they become just static noise.

A fascinating, and somewhat scary, question for warp drive which only a few sci-fi authors ever get into is: What happens if a ship with a warp drive runs into a planet at warp? Or worse. What happens if it runs into a STAR at warp?

Theoretically the ship would be fine, the planet? Oh no. nononono...the star? Honestly...nobody has any real idea. Some think it could result in an effect large enough to destablize the fusion and cause a semi-nova explosion to occur. But again, nobody is really sure.

If the semi-nova can happen, then that would be kind of cool for a weapon against an enemy in a Sci-Fi book........

although it reminds me of mutually assured destruction here on Earth.....

[vger]

ThirdHorseman: A fascinating, and somewhat scary, question for warp drive which only a few sci-fi authors ever get into is: What happens if a ship with a warp drive runs into a planet at warp? Or worse. What happens if it runs into a STAR at warp?

Given the number of calculations that go into planning a 'normal' trip to another planet, if something like that were to happen, it's either a joint effort between the U.S. and the U.K. (where someone failed to convert English to Metric) or you're Han Solo.

I think accidentally hitting a massive body is the least of our concerns with getting a functional warp ship.

Though that makes me wonder... given what would be necessary for a successful warp ship test, would we even bother trying to drop out of warp near a planet? I think simply being X-distance from Earth would be sufficient, regardless of whether or not any significant objects were nearby. Worry about trying to rendezvous with a planet after we're sure it works the way it should.

Edited March 26, 2014 by vger

[Scotius]

Have you tried KSP Interstellar mod? It does have working Warp Drive, as realistic as it can be made in this game. Even on interplanetary distances navigation is far from trivial, and you still need powerful conventional engine and a lot of dV stored. Ship dropping out of warp still moves with the same velocity it had in Kerbin's orbit. If your target is Duna, you are moving faster than the planet, and must decelerate to be captured into orbit. If you are going to Moho, in its SoI you are moving very slowly compared to the planet. Believe me, if you pick sub-optimal transfer trajectory dV requirements at your destination can be brutal - 6000+ m/s dV is not unusual.

I was very happy to read that forward half of Alcubierre drive can be used as a very powerful STL drive. Trying to cancel relative velocity in real world using chemical, ion or even low-tech nuclear propulsion would be a nightmare. I'm afraid we would need at least fusion drive (or even better - incredibly powerful Quantum Thruster) to effectively travel across our Solar system with Warp drive.

[K^2]

That really shouldn't be how a warp drive operates. I mean, it doesn't cancel conservation of energy. If your potential energy has changed, something has to give. There is a very "natural" sort of trajectory that an Alcubierre Drive can follow in a solar system to conserve energy and momentum, but it's nothing like what you're describing.

I've been meaning to work out the actual math behind this for a longest time, but I never get far in it before I'm starting to develop a headache. I don't think I'm built for tensor calculus. I don't mind the abstract notions of it, but dealing with all of the coupled equations you get is just blargh.

[Mazon Del]

K^2: From all the indications that I've seen the way Scotius describes the warp drive in that mod is actually pretty spot on. The warp drive itself does not influence your real-space speed, it only provides the warp speed boost, once you drop the warp bubble you still have your initial velocity vector (not just your speed, but also the direction).

In the recent Star Trek movie (spoilers ahead) I was quite happy with something they did towards the end. In the ST universe their warp engines are so trivially used that they often just use them on very lower power mode to hold an orbit that is impossible with a standard ballistic model (think being in geosync orbit, except you are mainting it only 100 miles above the Earth, ballistically this is impossible without an engine working). For convenience this usually ends up having them cancel out most of their real-space velocity so they can drop out of warp next to something without needing to immediately adjust their vector. So, later in the movie when the ships engines go offline, the low-level warp bubble is no longer holding the ship up in orbit around the moon, so the ship starts dropping straight down like it should.

vger: Actually, the scary point I was bringing up was not what happens if someone does this by accident, but what happens if they try it on purpose?

KASASpace: I found out about this possibility through a sci-fi series (I forget exactly which, but it is one of the ones written by the author of Blood Star, about a corpsman (medic) in the space marines, pretty good) and it seems to jive with what is known/theorized about warp tech.

[andrewas]

So, later in the movie when the ships engines go offline, the low-level warp bubble is no longer holding the ship up in orbit around the moon, so the ship starts dropping straight down like it should.

Except that, before engines went offline, they were stationary with respect to the cloud of debris. I believe you actually see it accelerating WRT that debris when the engines go online, but I can't find the relevant scene on youtube and my DVD is way over there. Also, falling from that altitude should take days, and they arrive in Earth's atmosphere way to slowly for something that fell most of the way from the moon in a couple of minutes.

[Mazon Del]

I don't have any data on just how far the warp fields extend out from the ships, so all I can assume was that they were caught up in the warp bubbles. Not sure what WRT stands for though.

It is true that falling from that altitude would take days, and their speed was low, it is also true that given where they were in orbit they probably should have fallen towards the moon rather than Earth. But the point is that they still showed the correct effect, if not the correct scale.

[Scotius]

Wow. I've watched Dr. White's talk during SpaceVisions. What he said about Quantum Thrusters is simply mind-blowing. With 4N engine and 2 MW power source he predicts missions that could take under 200 days to freaking Pluto! And (if i undertood him right) they already have test setups producing 7 times the thrust available from well known Hull thrusters. Now we need 2 MW power source

[Dust]

The fact that we have a cosmological horizon (the edge of the visible universe) shows that one area of space can move faster than the speed of light relative to another, as it's the point at which objects are travelling away so fast as that their light will never reach us.

The cosmological horizon is not the edge of the expanding bubble of things, it's the distance at which the things we are looking at are so far away that we are looking into the birth of the universe. If you looked out to the "edge" and were to magically place yourself there, you would - as I understand - see the same thing we see here (not half the sky full of stars and the other half void of matter and energy, as you suggest). It begs the question, though: If you had the ultimate FTL ship and continued traveling in a straight line away from the Milky Way, would you - at some point - find the Milky Way in front of you?

[Mazon Del]

Scotius: Oh yes! It is quite amazing what that device is capable of. And all you need is electricity, your mass never changes (unless you vent waste or something), I believe (could be wrong) that I had heard they got funding to make a test system for possible launch and test. It would be solar powered and quite wimpy, but capable at all. Quite exciting!

[K^2]

K^2: From all the indications that I've seen the way Scotius describes the warp drive in that mod is actually pretty spot on. The warp drive itself does not influence your real-space speed, it only provides the warp speed boost, once you drop the warp bubble you still have your initial velocity vector (not just your speed, but also the direction).

Question of "original direction" in curved space-time is not a trivial one. Would you say that an object in orbit ever changes its speed and direction? Not so, according to General Relativity. It continues "in a straight line", because the covariant derivative is precisely zero along a geodesic.

You are going to get precisely the same effect with a warp bubble. The difference is that you are going to go along a spacelike geodesic, allowing you FTL travel. When you arrive, your velocity relative to the bubble will be exactly the same, but bubble's own trajectory is going to change because you've been moving in curved space-time. As a result, when you exit, your velocity, in Newtonian sense, is going to be very different. Different how? Well, that's a more complicated question. You'd have to work out what the spacelike geodesics actually are going to be. I suspect that patched Schwarzschild solutions are going to be a good enough approximation, just like patched conics are a good enough approximation for KSP to use. But I'm not familiar enough with Schwarzschild time-like geodesics to say what it's going to be like right off the bat.

What he said about Quantum Thrusters is simply mind-blowing. With 4N engine and 2 MW power source he predicts missions that could take under 200 days to freaking Pluto!

Except that you can't get 4N of thrust with a 2MW supply. You need about 1.2GW for 4N of thrust with quantum thrusters. The early experiments in the field have had errors. Theoretical analysis of quantum thrusters suggests that it cannot exceed efficiency of a perfect photon drive.

Edited March 27, 2014 by K^2

[noobsrtoast]

heres how it works from my knowledge: the ship which the warp drive is mounted does not actually move at all and will arrive at its destination with the same velocity as when it left e.g if your ship is moving 2km/s when you leave it will be moving 2km/s when you arrive. basically the concept works around harnessing the power of the expanding universe from what I know the universe is expanding at a rate of about I believe 30 billion billion times the speed of light or something like that it used to be slower but a mysterious force called dark energy stepped in and accelerated the expansion, basically this means that galaxies can also move faster than the speed of light when dark energy forces the galaxies to expand outward faster than gravity can pull them back in so they end up picking up some of that dark energy and moving outward at many times the speed of light because for a reason we haven't quite figured out yet dark energy seems to sort of defy the law that you can only go 186000miles/s. we want to find a way to utilize that concept on a spacecraft whereby the drive would expand space behind the ship and contract space in front of it moving the ship across space at many times the speed of light up to 10 times the speed of light by current calculations. right now I believe its just a matter of gathering enough energy to do so cause that seems to be how our own universe does it take a look at black holes to get a better understanding of that you may have seen Stephen hawkings video where he explains how you could travel forward in time by orbiting around a black hole. the reason for that is because if you get an orbit just above the event horizon your travelling at velocities near the speed of light and as we know when you start to get near the speed of light time slows down to prevent you from passing the limit. also the black hole has such powerful gravity that it has the ability to contract space which is definitely one of the steps involved in making a warp drive work it also means that when your orbiting a black hole technically your travelling through more space at a time its just more tightly packed. rant complete

[K^2]

heres how it works from my knowledge: the ship which the warp drive is mounted does not actually move at all and will arrive at its destination with the same velocity as when it left e.g if your ship is moving 2km/s when you leave it will be moving 2km/s when you arrive.

In flat space-time. Space around star/planet is not flat. That's why we have gravity.

In curved space-time that's sufficiently flat locally for Alcubierre Drive to operate, this is going to work differently. This is basic GR.

[noobsrtoast]

I never said space was flat nor did I even mention that the alcubierre drive would work around a mass with gravity because space in that local area would be more concentrated I was merely explaining how it would work assuming space is flat is for people of medieval times

[K^2]

The discussion we have is on how much the velocity of the ship relative to star changes if the Alcubierre Drive is used within the system. Unlike flat space-time, there is a change in velocity and heading. It is a consequence of the conservation laws. While extra energy might come from warp drive itself, there is nothing you can do about momentum conservation. If you warp from 1AU to 10AU and your velocity does not change, you suddenly have 10x as much angular momentum.

I did find geodesic equations for Schwarzschild metric in a nice form that can be taken to Newtonian limit (r_s << r) for the most general case, including space-like geodesics that the FTL ship will follow. (And in v << c case, these do match classical trajectories.) I should be able to use these to work out precise trajectories for warp ship under different warp factors and the resulting change in velocity.

From conservation laws alone, I would say that, most likely, you will need to perform a burn to Hohmann transfer as if you are planning a normal trip, then engage warp (In what direction? Or can warp drive simply follow Hohmann?) And if you drop out at apoapsis altitude, you should have the same velocity as you would if you followed classical trajectory without warping. Depending on trajectory of the warp bubble, you might end up with a completely different argument of the periapsis, however.

As you can see, I still have some questions, but I'm getting close to working it all out.

[Mazon Del]

K^2: You bring up a very good point. I had always assumed that given the space inside the warp bubble should be as flat as your drive can make it (efficiency, safety, etc) that as a result you would just sort of hold onto the vector you held before relative to the 'front/back' of your bubble.

A thought I just had about the drive itself. As I had stated before, even if we don't get the expansion part working, the front end itself (contracting space) should work and can be a pretty great STL engine. Keeping that in mind, I'd be curious to see if it would be possible (without ripping the ship apart) to set up your warp drive, such that when you deactivated it instead of allowing the contraction/expansion sections to fall at a 1:1 rate (they turn off at the same time), you ease off the expansion portion of the engine faster so that you can more efficiently transition into the forward-only STL engine mode to quickly correct your speed/vector to what you want.

Let us know what you find out!

[K^2]

Where would the momentum go? You can send a gravitational wave and get recoil from that, but that would carry away same amount of energy that an EM wave would. In other words, you are back to photon drive, and you might as well just hook up a flashlight to your power plant. And we have neither the flashlights nor power plants with sufficient power to make it worthwhile.

No, even in the STL mode, this is not going to propel you. It can move you from one location in the system to another, but your ship will still obey energy and momentum conservation, and you'll have to use conventional fuel to adjust your orbit. Still, a drive capable of even 10% of speed of light would change the way we see space travel within our system even if we have to burn conventional rockets as well.

[Dispatcher]

The Star Trek analog for the chemical propulsion is impulse power.

[K^2]

What does Star Trek have anything to do with it?

[Mazon Del]

As a note, in Star Trek their impulse engines were suped up ion engines.

K^2: As has been described to me, utilizing just the front half of a warp drive should (as far as the ship is concerned) be analogous to 'falling' towards a gravity well equal to the effect you are creating.