Warp Drive May Be More Feasible Than Thought, Scientists Say

http://news.yahoo.com/warp-drive-may-more-feasible-thought-scientists-161301109.html

by mtgx • 14 years ago

239 179 14 years ago

news.yahoo.com

cryptoz• 14 years ago

> The only problem is, previous studies estimated the warp drive would require a minimum amount of energy about equal to the mass-energy of the planet Jupiter.

Uh, no. The "only problem" is that the concept relies on exotic matter, a hypothetical matter with 0 evidence for its existence (so far). See http://en.wikipedia.org/wiki/Exotic_matter

randallsquared• 14 years ago

Well, the Casimir effect generates a tiny amount of negative energy, does it not? Maybe that's how they think they can test it in the lab.

sp3322• 14 years ago

Actually the Casimir effect just reduces the "ambient" pressure, it doesn't create negative pressure. http://en.wikipedia.org/wiki/Casimir_effect

Edit: Whoops, looks like it might work anyway http://en.wikipedia.org/wiki/Casimir_effect#Wormholes

Edit2: Actually I think that contradicts this http://en.wikipedia.org/wiki/Wormhole#Raychaudhuri.27s_theor... which makes more sense to me. Unless you have already curved spacetime, a Casimir effect is not enough to stabilize a wormhole.

jerf2• 14 years ago

There are places in physics where "negative mass" can briefly appear, but as I understand it, the quantity of such negative mass times the duration of the negative mass is a constant... and a very, very small constant to boot. It's a balancing term in QM interactions, not something you're ever going to hold in your hands.

As I like to say, assume one impossibility in physics, and it's not surprising that you get other impossibilities as a result. Unfortunately, in practical terms, requiring Jupiter sized amounts of stable negative mass and requiring "Voyager" sized amounts of stable negative mass do not appear to be appreciably different in difficulty, both being totally impossible.

I'd also point out that this article isn't even remotely unique. People have been tweaking the Alcubierre equations ever since they came out. Nobody has yet squeezed the "impossible" out of it, but each and every one of them has been accompanied by, well, basically this exact article. I support people trying to tweak them as a scientific Hail Mary, but I wish credulous science writers would stop writing these stories as if moving from "impossible" to "impossible" was some sort of huge step forward in practical terms.

sp3322• 14 years ago

That was my main problem with this article. http://en.wikipedia.org/wiki/Exotic_matter You could simulate the effect using metamaterials probably, but you couldn't break the real speed of light with it because you don't have real negative mass.

hooo• 14 years ago

> Uh, no. The "only problem" is that the concept relies on exotic matter, a hypothetical matter with 0 evidence for its existence (so far). See http://en.wikipedia.org/wiki/Exotic_matter

There are multiple routes here though. They could also find an "exotic spaceship" that already has a warp drive.

MBlume• 14 years ago

Find one with a halting oracle on board and you're set.

nostromo• 14 years ago

This appears to be the original paper: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2011001... and follow up: http://www.icarusinterstellar.org/daydreaming-beyond-the-sol...

pixie_• 14 years ago

Fine links indeed. I tip my hat to you.

mvzink• 14 years ago

Another reminder of the internet's awesomeness: browse through the comments on the followup to see the original author answering reader questions.

jstalin• 14 years ago

Engage.

heyrhett2• 14 years ago

Warp speed is only 1/10th the speed of light.

As far as some news about faster than light travel, refer to: http://xkcd.com/955/

ChuckMcM2• 14 years ago

Well if you read the paper the effective speed is 10c, not .1c. They postulate 'boosting' the effective speed by contorting space-time around the craft, see this quote in the paper:

"assume the spacecraft heads out towards Alpha Centauri and has a conventional propulsion system capable of reaching 0.1c. The spacecraft initiates a boost field with a value of 100 which acts on the initial velocity resulting in an apparent speed of 10c. The spacecraft will make it to Alpha Centauri in 0.43 years as measured by an earth observer and an observer in the flat space-time volume encapsulated by the warp bubble. "

rickmode• 14 years ago

So if I understand right, the warping does not affect the "internal" speed of the spaceship in relation of the universe outside of the warp bubble.

Maybe someone much better with maths and physics can answer this: will the 0.1c speed have a noticable relativistic age-slowing effect for the travelers compared to their Earth-bound friends?

rewind• 14 years ago

The article says 10x, not 1/10x.

heyrhett2• 14 years ago

Looks like I was wrong about the definition of warp speed. Oh well, sorry, I can't delete it now.

CWIZO• 14 years ago

As much as it pains me to say this, but I think we have to invent some more advanced weapons before we start poking our heads out of our galaxy. Don't get me wrong, this is fantastic and I'm all tingly from excitement (this is the first time I've read about warp drives in the context of real science) ... but like I said, taking a stroll trough the galaxy with our bombs and finding a hostile alien world. Well we'd be screwed.

Just food for though here; this is from the Halo universe, where the timeline goes something like this: humans invent "warp drives", they colonize other worlds, soon this colonies start to rebel which leads to a massive space civil war. Later the aliens arrive and set to destroy all of humanity. Now, if it waren't for the civil war, humans would have no experience in space combat, and they wouldn't have developed more advance weapons which would later allowed them to defeat the alien threat.

aeturnum• 14 years ago

I think that developing "advanced" weapons in isolation would be an exercise in futility. If there's a race that's experienced in interstellar war out there, their weapons would be much more refined than ours. You can't create any product independent of its context.

If we were to meet a hostile alien race, unless they were at a similar developmental stage (which seems unlikely), we'd be at their mercy either way due to the extent of the other side's settlements.

Ygg22• 14 years ago

Seeing how if we use our warp drives to 'colonize' space, other creatures would have far more to fear from us than we from them (think Independence day with roles reversed). If a space faring species encounters non-space faring species, the one with warp is probably (not necessary) in the better position. Even without the weapons, FTL drive implies a magnitudes greater technology than anything a non-FTL race would have. In fact atm we might be the most dangerous creature out there, so in future (if we get that far) I probably see human species branching out into several subspecies and then waging endless war against each other.

Halo is a work of fiction, not a real life based drama. We'll probably be lucky if we find anything more interesting than a microbe. First encounter will probably be more like Solaris than Halo. We'll most likely find an organism that we can't even perceive as one.

So no, we don't need guns in space. Best weapon in space is Newton's second law (i.e. Kinetic bombardment). Cheap and easy to set up, hard to defend if your target is in a gravitation well.

bicknergseng• 14 years ago

Makes me think of Sphere (the novel, not the mediocre movie).

andrewflnr• 14 years ago

You're assuming the other species is non-spacegoing. It seems more likely to me, and at least hypothetically possible, that they've been in space as long or longer than us. And probably if you can bend space enough to make a warp drive, deflecting an incoming piece of matter isn't that hard.

But I agree we're more likely to end up at war with each other.

debacle• 14 years ago

It's very hard to aim and fire from a far distance, though.

rbanffy• 14 years ago

> a space faring species encounters non-space faring species

Any non space faring species either lives on a very boring neighborhood (one where rocks never fall from the sky and other kinds mass-extinction events never happen) or is extinct and we may find their fossil remains.

nullc• 14 years ago

> If a space faring species encounters non-space faring species, the one with warp is probably (not necessary) in the better position.

I read a fun science fiction novella once about a interstellar space faring species that invented their warpdrive whatchamacallit before their industrial revolution, no electricity, machines, etc. Flying medieval space galleons. And they were going about successfully conquering a bunch of similar or more primitive species. Then they encountered 20th century earth...

Wish I could remember the darn name.

ImprovedSilence• 14 years ago

Interesting thoughts you bring up there, and if we ever did colonize other planets, I've absolutely no doubt they would all start warring and civil warring and fighting each other. All one needs for evidence of this fact is a quick look at how we behave on planet Earth, and what we do with just one planet...

anonymouz2• 14 years ago

There are a lot of challenges we have to overcome to travel to the stars, but this is certainly not one of them. What weapon system would be "good enough" anyway? A potential other civilization could be completely primitive, or it could have faster-than-light travel for millions of years by now.

To me it seems entirely absurd to consider this as one of the major problems.

chernevik• 14 years ago

Maybe not weapons development. But doesn't the whole problem bear some thought?

The strategic problem postulated here would be defending planets, presumably from collisions with masses moving faster than light. Warheads aren't more than an afterthought, right? I don't see how anyone spots the inbound attack, never mind puts enough energy on it to change its vector.

Which suggests that the only balance comes from deterrence. But how would any actor know who originated such an attack? Or where they come from?

If that's the battlespace, then "here I am!" might not be a great opening move. And given the importance of information in this battlespace, deliberately avoiding knowledge of who lives where might be the only way of signalling you won't be a problem.

It's all speculation but it's one plausible scenario. Maybe we should have this figured out before we start wandering around.

csense2• 14 years ago

1000 years is the blink of an eye compared to the lifetime of our galaxy or the amount of time it took life to evolve intelligence on Earth. There's no good reason to expect that humanity's age will be within 1000 years of an alien race's. Consider how little chance an army behind 1000 years technologically would stand against a modern army.

Computer strategy games (I'm specifically thinking of Master of Orion and Galactic Civilizations) often make the assumption that all the races that are going to come into contact with each other have the same technological starting point. This is for the good and simple reason of game balance: Nobody would want to play the game if one race started with a game-winningly huge technological advantage due to random luck.

Many other games and science fiction settings make a similar assumption: That alien and Terran technology are on roughly equal footing, because a situation where everyone has a fighting chance simply makes a better narrative.

soup10• 14 years ago

It's absurd anyway. Advanced tech will(and already has, to an extent) put extremely destructive power in the hands of normal people. If we haven't learned to get along by then, we will certainly kill ourselves off.

maxerickson• 14 years ago

If you have the engineering to handle the energy required to travel between stars, you can apply it to making weapons.

wheels• 14 years ago

Such an investment would almost certainly skew the probabilities in the other direction. So far we know of exactly one species that has both taken the initiative to develop weapons that could exterminate humanity and has made threatening gestures with said weapons: us.

If the human race gets destroyed by mega-weapons, it'll almost certainly have been a human that pulled the trigger.

nessus4237• 14 years ago

Fear not. Despite the science behind the OP, faster than light travel will not turn out to feasible. Those who think it will be, haven't really thought through the ramifications of what a world in which causality is routinely violated would be like.

adastra2• 14 years ago

For anyone interested in the (just for fun) topic of technological advancement, weaponry, and encounters with other civilizations, you should check out Vernor Vinge's Zone of Thought novels, a Fire Upon the Deep and A Deepness in the Sky. I can't recommend them enough.

In particular, he assumes that all civilizations will follow roughly the same arc of technological discovery, and that once they hit their information age, it's all about computational power and software. Which, of course, expand in power exponentially with time.

As for us explicitly preparing for that kind of encounter, sorry but if we run into hostiles we're screwed. Take any tech of your choosing and extrapolate. For example, a civilization a few hundred years more advanced than us could easily hit Earth with a custom-made virus or nanobots and wipe out humanity in a day. And if they were a few hundred thousand years more advanced? Impossible to say, but I think our only hope is think that we're too low-level for them to bother messing with.

pyoung2• 14 years ago

Was AI accounted for at all? If super-intelligence can be achieved then technological advancement becomes more or less an issue of logistics, not time.

olalonde2• 14 years ago

This is a fun intellectual exercise but I think it would be fair to dismiss it as a "Maserati problem" for now. http://www.quora.com/Startups/Whats-a-Maserati-Problem

amorphid• 14 years ago

I have no doubt that research into warp will also inspire idea on how to blow stuff up.

johnnymonster• 14 years ago

How about we first build a space ship that travels anywhere near the speed of light?

jerf2• 14 years ago

This is all but a necessary step. If you work out the conventional rocket equations assuming a perfect translation of mass into momentum, to get to appreciable fractions of lightspeed still requires absurd amounts of propellants. At the moment we haven't even got a faint idea as to how we might obtain such total conversion. That's why there's so much interest in designs where you don't carry your fuel with you, like laser-driven solar sails and such.

goldfeld• 14 years ago

I assume travelling near speed of light with conventional physics is impossible, since the ship's mass would crush itself as it increased infinitely.

Warp drives, on the other hand, seem to work by moving space-time, not the ship. So in theory the ship is left intact, without the people inside feeling any acceleration whatsoever.

palmtree3000• 14 years ago

I'm fairly certain that wouldn't be an issue. Relativity means that any (let's say non-accelerating, to keep us within the context of special relativity) reference frame is just as good as another. In the spacecraft's reference frame, it's chilling out, and a bunch of stars off a ways are moving really fast, but they don't really affect it.

It is an interesting problem though; I'm not really sure why it can't crush itself...

acqq• 14 years ago

Infinite Improbability Drive

http://hitchhikers.wikia.com/wiki/Infinite_Improbability_Dri...

Rickasaurus• 14 years ago

Why, I always get my science from Yahoo news.

nessus4237• 14 years ago

You can't have faster than light travel without violations of causality. And while the equations of General Relatively allow for universes where causality is routinely violated, the probability that we are in such a universe seems vanishingly small. E.g., just because you can violate causality does not mean that you can go back in time and kill your grandfather. If you were to try, something would stop you, as a universe in which you kill your own grandfather is not consistent with General Relativity.

A universe that gives you the means to go back in time and try to kill your grandfather, but then somehow always thwarts you, would be a very strange universe indeed.

Edit: To whoever down-voted me, have you taken any classes on General Relativity? I have.

In any case this is from Wikipedia:

Causality is not required by special or general relativity[citation needed], but is nonetheless generally considered a basic property of the universe that cannot be sensibly dispensed with. Because of this, most physicists[who?] expect that quantum gravity effects will preclude this option.[citation needed] An alternative is to conjecture that, while time travel is possible, it never leads to paradoxes; this is the Novikov self-consistency principle.

The "Novikov self-consistency principle" is the the one that states that you could try to create a time paradox, but that you would never be able to succeed. I.e., the universe would be a very strange place!

geuis• 14 years ago

We aren't talking about time travel. So many people confuse the idea of FTL with time travel. General Relativity talks about frames of reference.

Suppose you had a base on Mars and sent a radio signal to it from Earth. It would take between 4.3 to 21 minutes to reach Mars, depending on its distance from Earth at the time.

If you simultaneously jumped in your 10x warp drive and set out, you would arrive at Mars in 25.8 seconds to 1min 26 seconds, again depending on distance.

In this scenario, there has been no violation of causality. You aren't arriving before you sent your radio signal. All that is happening is that you're arriving before your radio signal does. From the perspective of a telescope or radio antenna at the Mars base, you would view Earth in its state 4.3 to 21 minutes before, because it simply takes that long for the electromagnetic waves to get to you.

So yes, you can travel faster than the speed of light without violating causality. If you travel in a relativistic way, i.e. speed all the way up to 99.99999% of light speed, you will most certainly run into time slowing down in your local frame, while it continues at a normal frame back at your departure point.

But with a warp drive that works as described in the article, there is no changing of reference frames. Since you're warping space itself, the spacecraft isn't actually undergoing acceleration in the relativistic way of a rocket. For passengers on board, the clocks are going to run at the same rates as back home.

nessus4237• 14 years ago

You are not correct. If anything with information travels faster than the speed of light, then there are frames of reference in which that information arrived at the destination before it left the origin.

Furthermore, unless your FTL drive is limited in ways that would make it utterly useless, any FTL drive can be used to make "closed time-like loops", which would allow travelers to meet up with their own past selves.

politician• 14 years ago

A pair of FTL communication devices (jargon: ansible) can be arranged in a configuration which would allow the future to speak to the past, violating causality.

stan_rogers• 14 years ago

Causality, though, is a postulate in the system. If you start with the assumption that faster-than-light is impossible and build a system around that, the system you build around that assumption will always prove that it's impossible -- until something else proves otherwise.

nessus4237• 14 years ago

Causality it NOT a postulate of the system. General Relativity fully allows for universes in which causality does not hold. Such universes would either turn out to be either very strange, or actual use of FTL drives would have to be extremely rare.

nlh• 14 years ago

I'm not the down-voter, but I think you were likely down-voted because this article/discussion is not about time travel. And your first statement is that "you can't have faster-than-light travel..." is precisely what this paper discusses -- that with certain loopholes in physics (and consistent with General Relativity), faster-than-light travel is at least theoretically possible.

nessus4237• 14 years ago

Faster-than-light travel allows you to travel backwards in time. This is an essential part of Special and General Relativity. Those who don't understand this and who are not ready to address its consequences, should refrain from speculating on what is and isn't possible.

dhughes2• 14 years ago

I think the "faster than light" part wording seems a bit off since from what I gather the point is not to go fast to get to a location but to get there through warped space-time.

I imagine a 100 km long and 1km high brick wall, you're in the middle but want to get to the other side of the wall.

Option #1 is to travel as fast as you can 50km to one end and then 50km back to the spot on the other side.

Option #2 tunnel through the wall.

Option #1 is fast but the option #2 is faster although you didn't exceed the speed of option #1.

rootbear• 14 years ago

I'm glad you brought this up. I got into this same argument on a Star Trek fan site and my opinions on the feasibility of FTL travel were not popular with that crowd. I would like to know what Alcubierre himself thinks about the causality issue. You simply can't wish it away.

jonmrodriguez2• 14 years ago

Since QED and relativity aren't consistent, and QED says that can photons travel at speeds less than and greater than c, isn't it possible that relativity is just probabilistically true but not "enforced" to be true?

nessus4237• 14 years ago

Since I don't know anything about QED, I'm not equipped to answer this question. What I can say, however, is that much of what we consider to be "causality" is just a matter of probability. There's nothing in the fundamental laws of physics that says that a flower pot that fell off the window sill and broke into a million pieces an hour ago, can't now suddenly put itself back together and then fly back onto the sill, just as if nothing had ever gone awry. The fact that flower pots don't typically tend to do so, is that this turn of events is extremely unlikely.

In fact, as far as fundamental physics goes, the forward arrow of time and the backward arrow of time are indistinguishable. I.e., if you were to look at a movie of subatomic particles interacting, you wouldn't be able to tell, if you didn't already know, if the film were being played forwards or being played backwards. (As a slight caveat to this--a caveat that does not detract from what I am saying, but should be mentioned for the sake of completeness--there is one tiny aspect of fundamental particle physics that breaks this time symmetry, but it is a rather subtle one. The physicists who discovered this won a Nobel prize, and as I understand it, why this symmetry should be broken--no matter how subtle the effect--is still a deep mystery.)

What we consider to be the forward arrow of time comes completely from thermodynamics, not from fundamental law. E.g., if the Big Bang occurred at the end of time, rather than the beginning of time, we wouldn't be able to tell the difference, because from our point of view, that would end up being the beginning of time anyway.

gus_massa• 14 years ago

QED is consistent with Special Relativity. The standard introduction to QED starts with the successful attempt of Dirac to create a "covariant" version of the Quantum Mechanics that is consistent with Special Relativity (in that process he explained the particle spin and predicted the positrons).

Then it's necessary to make the theory more complicated to explain some experiments, but the final QED theory is consistent with Special Relativity (or "covariant" in the jargon). The extensions that involve the weak and strong forces are also covariant and they form the Standard Model (with the slightly hypothetical Higgs boson).

Any reasonable extension of the Standard Model is "covariant". And it would be very difficult to make the physic community consider seriously a no "covariant" theory (nothing is impossible).

Nobody knows the quantum version of General Relativity, so perhaps there is a very tiny loophole, but I would not be very hopeful about it.

jxcole• 14 years ago

A lot of people responding to this thread do not understand relativity. The OP is correct, the fundamental problem is causality, relativity and FTL do not mix. The common way of saying it is that you can have two of the three: FTL, causality, or relativity. You cannot have all three. Here is a very nice explanation of why:

http://thebestforumever.com/community/threads/tachyon-pistol...

Also, I've never understood how the Novikov self-consistency principle would work in reality. Causality violations can be a lot simpler than killing your grandfather. Say, for example, you set up a pool table with a pool ball and hit it through a time machine such that it went back in time and hit itself, making it miss the time machine? Would the pool ball be suddenly acted on by some invisible force? Would it explode? Would the universe just end? The whole scenario seems so implausible that addressing this theoretical concern is a lot more relevant than worrying about the energy levels required to manipulate space time.

gizmo6863• 14 years ago

I remember reading a paper that dealt with this issue. The setup was that a spherical mass hit another one into a wormhole that sent it back in time. The velocity was such that it would deflect the first mass before the initial collision, creating a casual paradox. If I remember correctly (and it has been years since I read the paper), they found that the second mass was garaunteed to come out of the wormhole with a velocity that would cause the first mass to hit the second in a way that would result in the new velocity, leading to a consitent system; however the final velocities were probabalistic, not deterministic, of the initial conditions.

Again, I have not seen that paper in years, so my memory might have just made half of that up.

nessus4237• 14 years ago

> Say, for example, you set up a pool table with a pool ball and hit it through a time machine such that it went back in time and hit itself, making it miss the time machine? Would the pool ball be suddenly acted on by some invisible force? Would it explode? Would the universe just end?

No, none of these. Well, something would prevent this from happening, but whatever happened would have a perfectly reasonable explanation at the time.

You have to understand, firstly, that in GR, the universe doesn't start at the beginning of time and slowly evolve until it reaches the end of time. Instead, all of space and time that ever was or ever will be, exist now and forever. And this one big constant eternal space-time is one of many possible solutions to the equations that constrain the universe.

If a potential universe does not satisfy those equations, then it can't exist and therefore it won't exist.

A universe in which your pool ball goes back in time and deflects itself, is not a solution to those equations, and therefore, that will never happen in any GR compliant universe.

So, what might happen instead? Well, you might get bored with the experiment and give up before you get it working. Your girlfriend might come along and tell you that you are neglecting her. Burglars might steal your time machine. Or most likely, you might never invent the time machine to begin with.

It's quite conceivable that in some sense a time machine is theoretically buildable, but the reason our universe is one of the solutions to GR is that no one ever actually builds one in the history of the universe.

Why not, you ask? Because if they did, that universe is much less likely to be a solution to the equations, so we just so happen to find ourselves in one of the more likely universes.

Another way to think about this is that perhaps all possible universes actually exist. In that case, we have to find ourselves in one of the existing universes. We can't find ourselves in a non-existing universe, since non-existing universes don't exist.

bluekeybox2• 14 years ago

The link had a really interesting but unsatisfactory explanation, IMO. The way I am understanding it is as follows: Man A and Man B are traveling (with combined speed of 0.866c) away from each other. Because of time dilation, to A it appears that B's clock is twice slower (1/sqrt(1 - 0.866^2) == 2). Then A shoots from some sort of weird space gun whose bullet is instantaneous. The bullet passes B at 4 seconds (relative to B). B notices that and shoots back, and B's bullet, due to equivalent effect hits A at 6 seconds causing a dead grandfather paradox.

However what I don't understand is this -- to say that the bullet shot by A passes B at 4 seconds presumes that we are talking about A's frame of reference (because it is from A's point of view that B is 4 seconds "behind" -- and likewise A is 4 seconds behind from B's perspective). However, what if, from B's frame of reference, A's bullet actually passes B at 0 seconds?

cygx3• 14 years ago

Tachyons do not necessarily violate causality if you actually take the time to derive the laws of tachyonic kinematics consistent with special relativity.

Sure, tachyons will be strange (different observes won't agree on direction of travel or charge of a tachyonic particle - if we are even able to localize the particle at all; there will be frames of reference where a tachyon won't have energy but will still have momentum, ...), but not really any stranger that what we deal with in quantum mechanics and field theory, and the laws of physics will still hold in all frames of reference.

stryker4• 14 years ago

I feel like a lot of people here misunderstand the speed of light as a cosmic speed limit. The speed of light as a speed limit is a local constraint. On a larger scale there is no such speed limit. Just because you travel faster than light on a non-local scale does not mean you break causality.

For example, light "travels" more slowly through water than vacuum. Well, actually, photons travel at c always -- it's just that light gets bounced around by water molecules, getting absorbed and reabsorbed many many times before arriving at the destination. It's like driving to a destination as opposed to running to it. If the roads are curvy enough, the runner will win because he is not constrained by roads. Could it be that the Alcubierre drive gets past the roads of empty space in a way light cannot?

For a better example, consider the expansion of the universe. Link: http://curious.astro.cornell.edu/question.php?number=575. I quote:

The fact that galaxies we see now are moving away from us faster than the speed of light has some bleak consequences, however. Astronomers now have strong evidence that we live in an "accelerating universe," which means that the speed of each individual galaxy with respect to us will increase as time goes on. If we assume that this acceleration continues indefinitely, then galaxies which are currently moving away from us faster than the speed of light will always be moving away from us faster than the speed of light and will eventually reach a point where the space between us and them is stretching so rapidly that any light they emit after that point will never be able to reach us.

In other words, vacuum fills in between us and far away galaxies faster than light can traverse it. Again, locally, the derivative of position with respect to time is c, but the universe is able to "cheat" by messing around with the definition of position. If the Alcubierre drive messes with the fabric of the universe in a similar way, I don't see any reason why it cannot succeed.

Here's a related link: http://curious.astro.cornell.edu/question.php?number=56. I quote:

Technically speaking, the speed of light limit only applies when you are in an "inertial frame" -- that is, sitting where you are, without any forces acting on you, and measuring the speed of an object that moves past a ruler and clock that you are holding in your hand. Across the large distances in the universe, however, we have a very different set of circumstances. No one is in an inertial frame, because everyone is being accelerated with respect to everyone else, due to the universe's gravitational field and the fact that the universe is expanding. In effect, the universe's expansion isn't really due to galaxies moving "through space" away from each other, but rather due to the stretching of space itself, which isn't governed by the same limits that we are.

I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum.

nessus4237• 14 years ago

I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum.

The Alcubierre drive most certainly does violate causality, and this follows from just the most basic amount of Special Relativity that you might learn if only you'd spend your time learning real things, rather than spreading disinformation.

pessimist• 14 years ago

In General Relativity, Faster-Than-Light travel is only prohibited locally. Its possible for space-time itself to expand and shrink faster than light. This is why distant-enough galaxies are receding from us at faster than the speed of light.

Mathematically, the structure of the proposed warp-drive is such that the spaceship is moving at << c locally, so no causality is violated. See the author's answer here:

http://www.icarusinterstellar.org/daydreaming-beyond-the-sol...

nessus4237• 14 years ago

Nowhere does he state that causality is never violated.

Steuard4• 14 years ago

[Background note: I'm a physics professor with a background in string theory and relativity.]

You're raising a sensible point, and most of the objections raised here are invalid. But it nevertheless turns out (surprisingly, to me!) that your argument doesn't work. This topic came up a lot during the short-lived superluminal neutrino excitement last year, and one good discussion of it by Sean Carroll can be found at:

http://blogs.discovermagazine.com/cosmicvariance/2011/09/24/...

That post includes a link to a paper by Bob Geroch that goes into the topic in detail. The very short summary is that there is no mathematical necessity for "causality" to be defined just by relativity: the union of the relativistic definition and some other well-defined causal theory can work just as well, even if that theory involved faster-than-light travel.

So thank you for raising a very good point about this topic, even if it wasn't quite accurate. And to those who've objected here: unless your objection explicitly quoted Geroch, odds are very good that they're wrong.

Steuard4• 14 years ago

That being said, I'm not certain whether there's any way to avoid causality violation if the hypothetical FTL travel results entirely from within relativity itself. (Geroch's argument, as I understand it, is based entirely on combining two different theoretical structures.)

nessus4237• 14 years ago

I would certainly defer to your much superior knowledge of physics. Mine is only that of an MIT student who was thoroughly indoctrinated with Special Relativity as part of MIT's general science education, and who sat in on an undergrad course on GR taught by cosmologist Max Tegmark.

If I understand Bob Geroch properly (much of his paper is Sanskrit to me, but I think I understand the parts that are in English), he acknowledges that FTL particles would seem to allow you to set up "closed causal curves" (i.e., violate causality in a way that would allow you to try to generate a time paradox), but he asserts that you would never actually be able to achieve this goal due to something like the "Novikov self-consistency principle", although he doesn't mention this principle by name.

This is similar to the position that many physicist think about the existence of closed time-like loops in GR. Sure, in some sense they are possible, since GR shows you how to set them up, but something like the Novikov principle would either prevent you from setting one up or from doing anything nefarious with one.

So, if I understand things correctly, Geroch's claim doesn't really different too much from assertions that I made elsewhere in this thread, that sure you might be able to build an FTL drive, but if the universe lets you do this, it's not going to let you make good use of one, like, for instance, creating an FTL space-liner fleet, because if it did, you'd generate a time paradox soon enough. Or if the universe were so kind as to let us do that, we'd end up finding ourselves in a very strange world where Novikov's principle was mucking with our daily affairs in a very creepy manner.

michael_nielsen• 14 years ago

The issue is addressed explicitly in Alcubierre's paper where he mentions (at least twice) that there are no closed causal loops in his metric. So there's no violation of causality, and the standard intuitions from special relativity don't apply. Pretty remarkable! The lack of closed causal loops apparently follows immediately from standard results about the sort of metric construction he uses. There's no need to invoke Geroch.

dclowd9901• 14 years ago

It's not that the universe might "literally" prevent you from performing an action. It's just that our universe doesn't allow it in the first place. The coherence of the universe is a biproduct of its inherent order.

I guess what I mean, without sounding too pseudo-scientific, is I believe the way it works is that, if, say, you go back in time and kill your grandfather, then what is actually happening is you are killing the person you thought was your grandfather, but they were not, in fact, your grandfather to begin with.

In fact, all you would know going back is that your grandfather was murdered by someone. Let's say you believe it was you, and you have a change of heart. Decide it's not in you to kill someone, even to prove a scientific point. That doesn't mean he doesn't get killed. Nothing's changed. It's simply that someone else killed him.

The answer to the question of Schroedinger's cat isn't that the cat is dead or alive, it's that it is.

olalonde2• 14 years ago

If this is really possible, it should increase the probability of having been contacted by advanced extraterrestrial civilizations. Maybe we're really alone after all :( http://en.wikipedia.org/wiki/Fermi_paradox

outworlder• 14 years ago

> An Alcubierre warp drive would involve a football-shape spacecraft attached to a large ring encircling it.

Obligatory Star Trek reference.

This seems close enough: http://www.ex-astris-scientia.org/schematics/surak-timur-niv...

dholowiski• 14 years ago

Exotic matter = Magic?

jayfuerstenberg• 14 years ago

At this point I'd settle for impulse and artificial gravity. That should be most of what is needed to get people to Mars and the moon.

gotrythis• 14 years ago

I skimmed through the responses, which seemed to be all about the execution, with little or no discussion or debate questioning if this is a good thing or not.

Even more than the idea excites me, I hope this fails miserably. We're at the Avatar stage of our evolution, not Star Trek.

I think this says it best: (no affiliation) http://www.facebook.com/ResurrectingtheGoddess/posts/3960760...

powertower• 14 years ago

I don't understand how the warp-bubble would itself travel through space-time faster than C (it's effects are inside of it), only that the space-craft traveling through that warp-bubble would. But the space-craft is stationary / in the center of it?

Warp Drive May Be More Feasible Than Thought, Scientists Say

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