Quantum Setback For Warp Drives 627
KentuckyFC writes "Warp drives were generally considered impossible by mainstream scientists until 1994 when the physicist Michael Alcubierre worked out how to build a faster-than-light drive using the principles of general relativity. His thinking was that while relativity prevents faster-than-light travel relative to the fabric of spacetime, it places no restriction on the speed at which regions of spacetime may move relative to each other. So a small bubble of spacetime containing a spacecraft could travel faster than the speed of light, at least in principle. But one unanswered question was what happens to the bubble when quantum mechanics is taken into account. Now, a team of physicists have worked it out, and it's bad news: the bubble becomes unstable at superluminal speeds, making warp drives impossible (probably)."
Re:Proof! (Score:4, Informative)
THANK YOU. Once upon a time we all knew that the gods made things fall to the ground. Then we knew that things have the falling nature, and the world was flat so things fell "down" no matter where you were. Then we knew that F=MA. Now we know that E=MC^2. What will supersede relativity? (QM is just too wacky, it has been said that if it doesn't confuse you, you don't understand it. I think that means it's a bad model, and we should just abandon particles. But whatever.)
I don't think that means what you think it means.. (Score:4, Informative)
Quantum would be an atomically short distance...
IE: a "Quantum leap" is just an electron jumping to another valence level in an atom... it's not a very large distance =)
Re:There is no fabric! (Score:2, Informative)
It is not at all like the aether that people were thinking of in the 19th century, but it does exist. One way of looking at it is that the vacuum is filled with particles that are constantly popping in and out of existence. Another way is to look as the vacuum as having a "zero point" energy. Either way, it is not truly "empty".
Re:Causality (Score:3, Informative)
You bring as proof of a scientific statement an article that demonstrate it using an item (ansible) found in SciFi books?
Um, yes. To show how FTL communication causes paradoxes requires an FTL communication device. None exist in reality, and thus a fictional one must be posited. Ansibles already exist in fiction, so the author lifted that just to make use of the word.
Re:improbability drive (Score:1, Informative)
1 is only "almost" certainly? And 0 is "nigh" impossible? What values then for absolute certainly and definitely impossible?
Re:Causality (Score:2, Informative)
Faster-than-light travel always causes causality paradoxes [orionsarm.com], so a priori, FTL drives are impossible unless special relativity is wrong. (That's is a bit like saying that perpetual motion machines are impossible unless thermodynamics is wrong.) The proposed mechanism behind the FTL drive doesn't matter -- it'll still cause a time paradox.
Just like we know any proposed perpetual motion machine must have a flaw, any proposed FTL drive must also have a flaw. They belong to the same class of impossible device, and deserve the same degree of consideration.
The size of the universe has expanded faster than the speed of light due to the expansion of space. http://en.wikipedia.org/wiki/Observable_universe [wikipedia.org]
Re:WARP 10 (Score:3, Informative)
you and Kate Mullgrue transform into a lizard like species and have mad lizard sex then produce offspring on a planet in the delta quadrant?
Re:We already have faster-than-light communication (Score:5, Informative)
That doesn't work [wikipedia.org]. You can't transmit information faster than light; contrary to popular conception, quantum entanglement does not involve classical information transfer.
If you have one of a pair of dice, and the other is a thousand light-years away, one way to think of entanglement is to imagine that whatever number you roll is the number that shows up on the other die the next time it is rolled. Even if the two dice are linked, you can't control which number shows up, so you can't use the dice to communicate information.
Re:We already have faster-than-light communication (Score:4, Informative)
No we don't, and no you couldn't. I suppose you're thinking of the EPR paradox? Very well. Let us say that I have a set of electrons in equal spin superpositions, and you, at some distant location, have their entangled counterparts. What's the protocol for communication?
Well, if I measure the spin of my electron 0 about the x axis, then in doing so I will also establish the spin of your electron 0 about that axis. The superposition on your electron has vanished without you touching it. Terrific, that's communication, right? I collapse your electrons in sequence, this one on the x axis, this one y, this one x, and so on, a binary code?
Well, no, it doesn't work like that. How can you tell if I've done anything at my end? By making measurements of your electrons? No - because that will collapse the superposition too. Let's say I measure electron 0's spin around the x axis to be positive. Immediately and instantaneously, faster than light across the universe, the superposition on your electron 0 collapses and I know it to be positive about the x axis.
But you don't know that. You might pick the y axis to measure, which is still a superposition. Or you might pick the x axis, and certainly you'll get a +, but you might have got that anyway. You can measure each electron only once - you change its state in doing so - so you can't do a series of tests, build up the statistics and find that on the y axis it's a 50/50 shot but on the x axis it's + every time. That's what you'd need to do in order to determine that I'd chosen the x axis. That's what you'd need in order to communicate faster than light. But since you only ever get one measurement, you get no information about what I did at the other end.
Re:Causality (Score:5, Informative)
entropy dictates that that everything loses to heat. This heat is at such a low energy level eventually that it can't cause any increase in energy to anything at all around it. This is how a system winds down, eventually all the energy in the atom will get sapped off this way and then it will start breaking down. Eventually devolving into the quantum soup that makes up the subatomic particles. Eventually, those too will lose energy to the space around them until everything is the same indistinguishable quantum soup.
This is the cold death scenario, and the only thing that can stop it is space itself increasing the density of energy instead of forever decreasing it. It's the expansion of space that continually provides for this loss of energy.
so no, atoms aren't perpetual motion machines. Though, for practical reasons, unless you need the machine to be functioning billions of years from now, you can call it perpetual.
Re:improbability drive (Score:3, Informative)
So THAT'S where the republicans came from...
Downmod INCOMING!!! Hit the deck!
Re:We already have faster-than-light communication (Score:3, Informative)
Sorry, but we already have faster-than-light communication trough quantum entanglement. The change in state happens instantly, without any delay, no matter what the distance is.
You're mis-interpreting quantum mechanics and entanglement. The second sentence in that quote is right, but it doesn't imply the first sentence, which is wrong.
When a quantum entanglement collapses, both of the entangled particles will end up in states that are strongly correlated, even if the two particles are very far apart. So yes you could have two entangled particles that are separated from each other, have them collapse, and there would be "instantaneous" correlations between them.
However this cannot be used for faster-than-light communication. (It makes for cool sci-fi, but isn't correct when you really look into the details of quantum mechanics.) The reason it cannot is because the collapse is (at least to the local observers) random. Neither side can predict nor influence* what random state their end of the entangled pair collapses into. So if we have two streams of entangled particles (one on Earth and one at Alpha Centauri), both sides would read out a random stream of answers (up/down, yes/no, 1/0 ... or whatever). Only once you compare the two streams do you realize that they were correlated more strongly than random chance (and classical mechanics) would allow. A cool experiment, to be sure! But it can't be used to transfer information.
Many people have tried to devise ways around the rules of quantum mechanics, to allow entanglement to be used for FTL communication. But all such proposals (to date) have been found to have mistakes with respect to our current understanding of quantum mechanics. To date, there is no loophole that allows one to circumvent the (local) randomness of quantum mechanics to allow FTL. Of course it's possible that modern relativity and quantum mechanics are both wrong. But at least for now there is no evidence of any information ever being transmitted faster-than-light.
* Actually you can try to influence the answers you get from your stream by selecting a particular measurement method. And while doing so does influence the states of the distant particles, your faraway friend won't know what sequence of measurements you selected. So he will select his own set of measurements, and end up with what seems like a random stream, and no way to correctly interpret what the stream "means" without knowing the sequence of measurements you performed. Of course the two sides can pre-arranged what measurement sequence to use, but that's not FTL communication... that just becomes old-fashioned communication. The point is that "communication" means "transfer of information", and one cannot transfer information via entanglement (which may be FTL), because one has to additionally transfer the measurement information in a sub-luminal way. This may sound "contrived" to prevent FTL communication, but that's just what our best equations currently describe.
Re:Proof! (Score:3, Informative)
Antimatter still has positive mass. It takes 2*.511Mev to make an electron and a positron. If antimatter had negative mass, it would take 0.
Re:improbability drive (Score:4, Informative)
Re:Causality (Score:2, Informative)
Indeed, the difference is between perpetual motion, and a perpetual motion machine.
Perpetual motion is not a violation of conservation of energy - although it's still in practice impossible due to the 2nd law of thermodynamics (I'm not sure your examples would really be 100% perpetual, just very close?)
A perpetual motion machine however implies doing work, suggesting that energy can continually be transformed from one form to another.
Re:There is no fabric! (Score:3, Informative)
Re:improbability drive (Score:5, Informative)
Also, 0 is not "nigh impossible" - it is the definition of impossible.
Not necessarily. It may be that there are an uncountable number of possible outcomes, and each individual outcome has a zero probability, but large sets of them collectively still have positive probability. At least, models exist where this makes sense...
Re:Longer lifetimes is the answer (Score:3, Informative)
Hours?
1g to Alpha Centauri - 3years, 205 days.
Compress that trip to, say, sixty hours...
2575g.
It gets worse fast. 1g only gets us to 95% lightspeed. Higher acceleration pushes us way up into relativistic effects.
Re:I don't think that means what you think it mean (Score:3, Informative)
Yeah it is funny hearing people say they have made a "quantum leap" which would mean "the smallest possible discreet leap". I mean even the show with that name wasn't implying huge leaps, so I don't know how it came to mean that in their heads. Or maybe they mean it's an advancement worthy of Scott Bakula?
I actually liked how the last Bond movie, "Quantum of Solace", used the term correctly. Though this probably confused some people. My roommate thought the name was stupid till I told him what "quantum" really meant and then he thought it was cool. :)
Re:Got a Better Idea? (Score:3, Informative)
As has been pointed out, there are many answers to the problem of interstellar travel that don't involve rewriting the physics of the past 150 years.
Medicine may allow us to live indefinitely, making travel to the stars possible by the sheer power of our lifespan.
Computers may allow us to upload our consciousness into them, leading to an indefinite lifespan.
Medicine may allow us to freeze ourselves and re-thaw when we get to the destination.
Space propulsion may allow us to accelerate at 1g for long periods of time, thanks to relativity you would get just about anywhere in a matter of years (ship time).
The problem with doing a full scientific inquiry and building a prototype is that first and foremost, every theory that allows FTL requires negative mass, which has never been produced or discovered and isn't included in any of the common particle models. Unless there's a stable, negative mass particle that the Standard Model doesn't predict, FTL is impossible. I would go so far as to say that since such a particle makes FTL possible, such a particle cannot exist in our universe; it simply leads to too many contradictions of too many basic physics concepts (causality & thermodynamics especially).
Re:What scientists do not know could fill a univer (Score:3, Informative)
People forget that scientists used to think that it was impossible to break the sound barrier for various reasons.
No they didn't. Some idiot writing for a news paper may have however.
Re:Hiesenberg says.... (Score:5, Informative)
He was slowly recovering, something that seemed impossible at the beginning.
Re:Longer lifetimes is the answer (Score:3, Informative)
Ah, but you could travel to the stars without immortality at FTL speeds - at least from the point of view of a ship's occupants - as long as you choose not to go home again. A constant acceleration drive would enable you to cross the galaxy in a few years of ship time, thanks to time dilation. Take along a large enough group of people to form a stable society, and for all practical purposes those people will be traveling at superluminal speeds. Relativity will make it impossible for them to return home (at least to the home they remember), but as long as the passengers are willing to accept a one-way trip, effective FTL is absolutely attainable.
Re:Longer lifetimes is the answer (Score:3, Informative)
secondly... P=mv
momentum equals mass times velocity....
higher the mass, the higher the momentum, the more force it takes to change velocity (or stop the object)
personally i'd prefer to catch a 40MPH baseball than be on the tracks trying to catch a 40MPH freight train.....
Re:We already have faster-than-light communication (Score:4, Informative)
That doesn't work either because Joe doesn't know if you have rolled the dice or not.
Entangled particles are like dice that are already rolling, and they stop rolling the moment that either particle is observed.
So you and Joe each have a dice that, say, always roll the same number as each other. You look at your dice to cause it to stop rolling, and see that it rolled a 6. Joe can look at his dice too, and will also see a 6, but he doesn't know if he was the one that caused the dice to stop, or whether it was you who stopped it.
You both see a 6, but no actual information was transferred.
Re:We already have faster-than-light communication (Score:2, Informative)
Re:We already have faster-than-light communication (Score:3, Informative)
The entanglement usually occurs to conserve some physical quantity, such as spin or momentum. So for the dice example, let's say that every pair of entangled dice must add up to 7.
The problem is that they can only become entangled while they're still in luminal communication range - so you have to roll all the dice before the ship leaves.
If neither of you looks at your dice, then the number rolled remains undefined. As soon as one of you looks at a given die, both it and it's twin instantly take on their respective values (or you spawn 6 universes identical in every way except for the values rolled, if you like the multi-universe theory.) But when you look at a die, all you see is a number. You know that the number on the previously entangled die must be 7-#, but you have no way to tell whether you looked first or your counterpart on the ship.