How is an Alcubierre drive FTL?

[SargeRho]

It doesn't contract the space in front in the sense that it shrinks the distance you have to travel, that'd be an exessively oversimplified explanation of wormholes.

It produces a "wave" that the ship rides, that wave can move many times the speed of light. You thus move faster than light in relation to your point of departure (or even destination), but don't move within your own frame of refference.

[Accelerando]

I may be thinking about this the wrong way. You don't actually go faster than the speed of light. This is because by compressing space you are shortening the distance that you will have to travel and not increasing your speed. It will only appear to the outside observer that you have travelled faster than light because the distance will appear the same to them. For example, you want to drive a 60 miles to your friends house, but want to do it in 30 minutes without breaking the 60 mph speedlimit. If you take a shortcut that is 30 miles long, then you will get there in 30 mins and it will appear to your friend that you drove 120mph on the way over.

Edit: "Also, this method of travel does not actually involve moving faster than light in a local sense, since a light beam within the bubble would still always move faster than the ship; it is only "faster than light" in the sense that, thanks to the contraction of the space in front of it, the ship could reach its destination faster than a light beam restricted to travelling outside the warp bubble. Thus, the Alcubierre drive does not contradict the conventional claim that relativity forbids a slower-than-light object to accelerate to faster-than-light speeds." from https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Alcubierre_drive.html

this is a comprehension problem. relativity by itself does not strictly forbid moving faster than light, i.e., having a velocity > c relative to an observer outside the warp bubble. it does, as the document points out, prevent a massive object from accelerating faster than light. thus if your warp drive expands and contracts space such that you, in the bubble, stationary relative to that bubble, are apparently moving FTL with respect to observers outside, then that is effectively what you are doing, and that comes with all implications of causality violation, time travel etc.

using the moving cars shortcut analogy, you're folding and stretching the road around you such that your car arrives at the destination point at an apparent velocity of 60mph rather than 30mph by conventionally driving down the same road. thus you are not moving any faster relative to your own little stretch of ground that is not being folded, but relative to everything else, you are still moving 2x faster than via normal road.

[Mazon Del]

SargeRho: Indeed! There are many ways you can solve this problem, such as what you said. You could also always try to overshoot your target and stop just on the other side of them. But I wouldn't recommend this, just in case you stopped too early. Really, as the gamma ray burst is proportional to the distance the ship traveled at warp, the best idea would be to stop at the edge of the solar system to release the pent up rays harmlessly there, then do a series of smaller jumps in to prevent risk to ships/planets.

RedJoker: You have it pretty spot on. This is why the technique does not violate light speed laws, you 'the ship' never actually go FTL you are just moving as fast as you were before you entered warp, but to someone standing outside, you appear to do go FTL.

Dispatcher: Not exactly, while the ship itself never 'moves' locally at FTL speeds, you can still use this method to get around at FTL speeds to someone sitting on Earth. This speed is a useful number, because if your drive takes you at 10c, then it takes you a little over 5 months to travel the 4.3ish light years to Alpha Centauri. Not especially fast given that it is one of our closest neighbors, but luckily we can always make a more powerful engine and go 100c to travel this distance in about half a month. As far as how the drive effects the ship and external observers. The ship itself should be fine and completely untouched by any of the effects. Maybe it sees pitch black till it comes out, maybe it sees some sort of glow, this is about all the ship may experience. There are theories that an outside observer watching a ship go to warp would see a most curious thing indeed! They might see the ship appear to grow in size larger and larger while fading, getting dimmer and dimmer. This has to do with the warp bubble scattering the light from the ship, no actual size differences occur. Basically it is a gravity lense. Additionally, one of the most wonderful effects of a warp drive, is that because the ship itself never locally moves FTL, its time dialation/relativistic motion is the same as it was before the ship left. This means that after our ship spent 5 months to get to Alpha Centauri, their clocks should still match those of Earth!

[KASASpace]

Okay, here is how it works.

Mass immediately changes the dimensions of space-time.

Earth indents that space time.

The theory the Drive works on is the existence of exotic matter. Matter with negative mass.

Negative mass puts a negative indention on space time.

So, you have a super massive object in front, and a super negative massive object behind.

Now, how do you slow down?

Perhaps if we can change how the Higgs Field interacts with specific matter, we can dilate mass and do it very suddenly.

[Taki117]

Here's a video explaining how it works.

[zxczxczbfg]

The Alcubierre drive works sort of like this analogy:

Say you can only walk at four meters per second. It'd be faster (but godawful expensive) to just pick up the chunk of pavement you're standing on with some kind of telekinesis, taking you with it.

[vger]

The Alcubierre drive works sort of like this analogy:

Say you can only walk at four meters per second. It'd be faster (but godawful expensive) to just pick up the chunk of pavement you're standing on with some kind of telekinesis, taking you with it.

In a way, it still feels at least similar to thrust. A radically different method than what we're used to though.

As for the cost, heh, has anyone even tried to estimate how much it would cost to build one of these? Not that it really matters that much. If there's enough funding to build the LHC, there sure as Hell better be enough funding to build an Alcubierre.

Edited March 25, 2014 by vger

[Moon Goddess]

Moon Goddess, what evidence is it you are referring to? I have never seen any evidence to that effect. There are some issues with the warp bubble that are hazardous to people OUTSIDE the bubble, like the massive gamma burst that happens when you turn the engine off. But all that energy is directed 'forwards' away from the ship, so the ship is unaffected. The area where the ship exists would be a flat (or mostly so) area with neither crushing nor expansion happening. Nothing from the outside is getting in to harm the crew, and as far as they are concerned space-wise suddenly all the stars went out (though there is some argument over if the crew would see pitch black, or a uniform glow from the front).

http://arxiv.org/abs/0904.0141 suggest that anything inside the bubble would get a lot of Hawking Radiation, not from out side, from the bubble itself.

Additionally, from what I have read there is relatively strong evidence to support that negative matter exists, or that something that fulfills its purpose for a warp drive does. Your statement of "And even if it did wouldn't make a warp bubble" seems to indicate you are saying that what current physics calculations (some supported by experiment data) say is the key to warp drive would not work "just because it wont". Why is it that you believe it would not work?

Don't have a link on that one. Has to do with stuff i heard about Quantum Gravity getting in the way, I don't understand a lot of quantum stuff so I won't try. But it makes sense to me that something has to get in the way, FTL would allow time traveling, and time traveling would break cause and effect and occam's razor means you'll have to try very hard to make me believe in a universe where that can happen.

The universe doesn't like or dislike anything at all, it is a non-sentient 'thing'. So far scientific observation, calculation, and experimentation indicate it IS possible for a warp drive to work. One heavy part of this is the fact that they have shown from the recent data for the Big Bang, that rapid expansion (at FTL speeds) MUST have occured for our universe to exist the way it is.

Ok, surely you can understand where I'm being serious and where I'm not. When I say the universe doesn't like somethingn I don't mean the universe thinks... duh! I just mean you can't break causality.

[K^2]

I just mean you can't break causality.

You can't break local causality. Global causality is far more flexible.

[Dispatcher]

The universe doesn't like or dislike anything at all, it is a non-sentient 'thing'.

Imagine that a microbe deep within an adult elephant (or for the sake of the following analogy, within an adult human being), was capable of observing or postulating the entirety of the being encompassing it; further, that it could think as we do. Due to its limited lifespan and rapid metabolism (at least for this example), it fails to comprehend that the human host is sentient. I chose an adult solely for the size and mass.

You get where I'm going with this. While I don't tend to attribute feelings or sentience to the known universe, we humans have the limitations of a brief life span (in terms of both individuals and civilizations) and we are limited in our senses and thought capacity. Can we conclude with confidence and certainty that the universe is not an entity; even possibly sentient? While we are not aware of any proof that the universe is sentient (or alive as we might define life), I don't think we are in a position to say that it definitely is not alive or even sentient. Even your sig line alludes to the possibility.

Back on topic, does anyone have any idea just what exotic matter or exotic energy might actually be? Would the use of a particle accelerator be able to detect or generate it?

[K^2]

While I don't tend to attribute feelings or sentience to the known universe, we humans have the limitations of a brief life span (in terms of both individuals and civilizations) and we are limited in our senses and thought capacity. Can we conclude with confidence and certainty that the universe is not an entity; even possibly sentient?

On the scale of observable universe? Yes, we can. Can it be sentient on a larger scale? Yes. But by that point, we don't even care.

Back on topic, does anyone have any idea just what exotic matter or exotic energy might actually be? Would the use of a particle accelerator be able to detect or generate it?

There are a whole bunch of things that we used to classify as exotic matter that is now routinely observed at accelerators. For the rest, it depends on what you mean. There is a list of theoretical and hypothetical stuff that can be classified as exotic matter and at least a couple that can be classified as exotic energy. I'm not at all sure if in this context you are talking about exotic matter and energy as they relate to Alcubierre Drive theory or universe expansion.

[Seret]

Back on topic, does anyone have any idea just what exotic matter or exotic energy might actually be?

"Exotic" is one of those words physicists use when they're talking about a phenomenon they don't yet understand. If they knew the answer to that question, they wouldn't need to slap such a vague label on it.

[Winter Man]

On the scale of observable universe? Yes, we can. Can it be sentient on a larger scale? Yes. But by that point, we don't even care.

That was pretty much the plot of Stargate Universe, taking a ship beyond the observable universe (over a massive timescale) to get a better idea of some hint of sentience they detected.

[K^2]

That was pretty much the plot of Stargate Universe, taking a ship beyond the observable universe (over a massive timescale) to get a better idea of some hint of sentience they detected.

Not so much a plot as a MacGuffin to kick it off, but yeah, I seem to recall something like that as the motivation for the Destiny and the gate seeding ships.

[SargeRho]

The Ancients detected a signal, something they considered a "fingerprint" of creation. To investigate it, they built destinty, and the seed ships. The Destiny's crew only found out about that later in the series though.

[Mazon Del]

Moon Goddess: Thanks for the interesting read! While yes, this does point out that you would get a good bit of Hawking Radiation on the inside, that becomes a matter of sheilding be it physical (lead) or electromagnetic in nature as (while I don't have time this moment, just got in at work, heh, so I will attempt to check this later) my guess is that the source of the hawking radiation would be the areas nearest the warp bubble itself rather than the flat space the ship occupies. If that is the case, then the radiation can be blocked or deflected.

Yeah, I hear you about quantum stuff. There is a relatively (heh) standard joke that when discussing quantum things long enough, one must roll a sanity check before continuing. However, the point about breaking causality is actually false in the instance of the warp drive. This is not your fault as most people tend not to understand why FTL allows time travel. Time dialation is based on several things with relativity, most important to us here is speed. There are some charts in places, but roughly speeking once you get to about 99% the speed of light, you are living at a 1:10 time differential. One second for you is ten to an outside observer. For the equations that determine this, if you plug in FTL speeds you see this get 'worse' 1:15, 1:20, etc. Eventually though, it comes to a peak and starts falling back. At some point way past c you reach 1:1 again, but if you go even faster you get 1:0.5, 1:0.0001, and eventually even 1:0. If you go even slightly faster you now have a time differential of 1:-0.00000001 and are now moving backwards in time. The issue here is that this only applies to local speeds above c. This means it only applies to a ship moving through space. Space moving through space (as in a warp drive) with a flat plane of space in the middle (occupied by the ship) only has the time differential that the ship had before it engaged the warp drive. This is because the ship is traveling through the flat plane of space at whatever speed it had been traveling, but now 'by coincidence' that piece of space just happens to be moving at FTL speeds.

------------

vger: Apologies about not providing this earlier, I thought of it but was a bit lazy. https://www.icarusinterstellar.org/congress-livestream/ That link takes you to the stored videos from the Starship Congress 2013 event where Sonny gave his info. I have not seen the entire thing, but it is absolutely fascinating. Day 1 is current/near term/semi-experimental technologies for space travel, IE solar sails and ion drives. Fun fact, NASA tested a microwave laser assisted solar sail (fire a maser at a solar sail to give it a speed boost) in a lab and acheived 10Gs worth of acceleration! With a cobbled together test assembly on a shoestring budget! Day 2 is slightly further out topics such as the ongoing efforts to rebuild NERVA and the next-gen equivalents to NERVA, discussions on the current UN/treaty bans on certain radioactives in space. Day 3 is where we get into FTL topics. We start if off with Sonny giving us a presentation on the progress his team has made since last year, then we move to other professors and NASA people explaining about wormholes, exotic matter and current/future attempts to create and harness it, as well as the causality physics behind a warp drive, explaining how it is totally possible given our understanding of causality. Somewhere in there is actually the most fascinating news of all. I forget exactly how it was explained, but someone was pointing out methods that may allow us to do away with exotic matter when it comes to making a warp drive or wormhole. I'm not exactly sure what Day 4 is, as I haven't seen that one yet.

As far as why media coverage dropped out, I think its because they assumed that the 'warp tests' were going to involve something akin to the ship in Star Trek: First Contact, rather than the much less visually stimulating experiment involving shooting a laser through a crystal, then a hyper-charged donut, bouncing it off a mirror back at itself, bouncing off a crystal, and into a detector to examine the interference patterns. To be fair, that laser is not just a laser pointer you can buy at any corner drug store (heh), but as far as an outside observer is concerned, it might as well be. All the test is trying to do is to move 1 part per billion photons at FTL over the tiniest of distances, just to prove that it can be done at all. That said, it is thought of to be a "Chicago Pile" moment either as having occured, or in the making.

For those that don't know, the Chicago Pile was the first man-made self-sustaining nuclear chain reaction (http://en.wikipedia.org/wiki/Chicago_Pile). This was acheived by taking bricks of refined uranium and stacking them like blocks while looking at a geiger counter. *place brick...click...click* mhm...*place brick....click...click* mhm....*place brickCLICKCLICKCLICKCLICKCLICKCLICKCLICK* YES! Why the Chicago Pile moment is thought of highly is because before it happened everybody 'knew' nuclear power was impossible. The CPM happened on 2 December 1942, the first commercial nuclear power plant came online June 27, 1954 just 12 years later. We went from an 'impossible' theoretical technology to a practical use device in 12 years because we proved it could be done. While it was just theoretical nobody was interested in funding it. If Sonny (or others) can prove that the warp drive works, and can somehow figure out the exotic matter side of the equation, either through producing some or through that method that nullifies its need, then I can feel safe in guarenteeing that within 10-20 years we'd have our first warp capable probe, and within 10-20 years after that our first manned warp ship.

[Scotius]

LOL, i just realised: Nibb31 hasn't come to this thread yet, and did not burst our bubble with a needle of cold logic. This guy is scarily effective at squashing our foolish ideas

Anyway, i seem to recall reading about Quantum Thruster as one of possible methods of "simulating" negative matter. Effectively producing desired effects without actually creating even one negative molecule.

[Wahgineer]

LOL, i just realised: Nibb31 hasn't come to this thread yet, and did not burst our bubble with a needle of cold logic. This guy is scarily effective at squashing our foolish ideas

Anyway, i seem to recall reading about Quantum Thruster as one of possible methods of "simulating" negative matter. Effectively producing desired effects without actually creating even one negative molecule.

Either:

1) He hasn't seen this thread

2)He saw the thread, read the links in our posts, couldn't think of a reply, said "seems legit", and moved on.

[falofonos]

His paper stated that to acheive 10 times the speed of light with a ship of X kilograms (I forget how many) it would take converting 1,600 lbs (roughly) of matter into energy. That is 'barely' outside our capabilities now (give it 40-50 years).

725.7478 kg * c^2

65.23 exajoules.

Approximately equal to 17 Gigaton detonation. Not sure we'll be harnessing that kind of power anytime soon but who knows?

[K^2]

This is not your fault as most people tend not to understand why FTL allows time travel.

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.

In principle, there exist space-time configurations that allow for not-quite-CTCs which are traversable to a warp ship. (Effectively, making it a CTC from perspective of ship proper.) But I'm not even sure if such geometries exist in nature. At a minimum, we are talking about flying deep under event horizon of a super-massive black hole with extreme amount of angular momentum. If Kerr Metric is a valid interior solution for these, there should be some near-CTCs available, which are accessible to an FTL ship. And even that's a big 'if'.

In short, even if there is some fundamental problem with violating causality, the basic principles of warp drive are not a violation.

That said, as I've pointed out earlier, we only have real restrictions on local causality. There is no fundamental principle in modern physics that says that global causality must be observed. Local causality is sufficient to build an effective field theory for whatever global structure of space-time you happen to have. And thanks to the principle of superposition, there are no history contradictions in such a system even if one happens to traverse a CTC.

[Accelerando]

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.

In principle, there exist space-time configurations that allow for not-quite-CTCs which are traversable to a warp ship. (Effectively, making it a CTC from perspective of ship proper.) But I'm not even sure if such geometries exist in nature. At a minimum, we are talking about flying deep under event horizon of a super-massive black hole with extreme amount of angular momentum. If Kerr Metric is a valid interior solution for these, there should be some near-CTCs available, which are accessible to an FTL ship. And even that's a big 'if'.

In short, even if there is some fundamental problem with violating causality, the basic principles of warp drive are not a violation.

That said, as I've pointed out earlier, we only have real restrictions on local causality. There is no fundamental principle in modern physics that says that global causality must be observed. Local causality is sufficient to build an effective field theory for whatever global structure of space-time you happen to have. And thanks to the principle of superposition, there are no history contradictions in such a system even if one happens to traverse a CTC.

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?

If a warped vessel is able to, for example, near-instantly transport itself from here to α Cen with respect to observers on Earth, why won't it, if it doesn't, appear to be arriving around 4 years in the past relative to Earth? Or is this even the matter at hand?

[SargeRho]

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.

[Accelerando]

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.

Ah - yes, that makes sense.

[Mazon Del]

K^2. You appear to be agreeing with the rest of my statement. ^^ I should have perhaps been a bit more specific in what I wrote though. "This is not your fault as most people tend not to understand why FTL speeds in normal space result in time travel." would probably be a better way of writing it. That sentence you quoted of mine was the first part where I explained where the notion of FTL = time travel comes from. And as I said (and you restated) Alcubierre drive systems do not provide for time travel during normal operation. Doing something interesting like slingshotting around a star (lots of interesting debate on IF a warp drive ship even CAN slingshot while in operation) changes things up. I am just talking about any random normal A->B travel.

Additionally, for those not in on terminology in K^2's statement he refers to CTCs. This means Closed Timelike Curve, and is generally something referred to when speaking about time travel. I am myself a little less familiar with them in practice than an alcubierre drive.

Also, to give a quick summary of that part from the Starship Congress where the guy talks about how causality is fine with a warp drive. There are these things called "light cones" which are used to describe all possible futures for an object at X time. "All possible futures" refers to the fact that since the object cannot move faster than light, it could not have a future that includes an object further away than light could reach given some additional Y time, but anything else the object can reach, it could theoretically interact with. There are also a lot of deeper meanings more backed up by physics than this description provides. Imagine you have a cone that points up, this represents YOUR light cone. Several feet to the left of you is another light cone. Both cones expand upwards (and therefor outwards) to infinity, because of this eventually your cones intersect somewhere. This means that after some amount of time you and that object could interact, but not before. Changing your own location (spaceship!) towards the second cone "moves" yours closer, but because you can only move so fast (as time moves, you also climb in height) you can never get closer then the boundary of the light cone at your original location at the original time. Traditional movement (up/down, left/right, forward/back, pitch, roll, yaw) only moves your cone left/right, forward/back, but always up as well (because time flows forwards). So your cone is ALWAYS pointed straight up. However, what a warp drive does because of its bent space, is that it takes this always up cone, and tilts it to one side (the direction you are pointed in) and now the cone also shifts shape. Your 'new' lightcone is pointed where you are moving, and the angle of the cone and the rate at which the cone spreads out is determined by how fast your new top speed is (if your engine was only designed to move at 100c, then that is your top speed).

In super simple (and therefor inaccurate in the literal sense) terms, you are a dog and your own causality/timeline is a leash made of infinitely strong material that cannot stretch and it is tied to a tree at one end and you at the other. A warp drive is you being a particularly resourceful dog using your owners backhoe to pick up the damn tree and drive it with you where you want to go. Since the distance between you and the tree remains within the limit of your oppressive leash, physics is fine.

[Scotius]

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?