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Science

Warp Drive Breakthrough 264

NIck Porcino writes "Warp drive one step closer to reality! From the abstract: A spacetime is presented for which the total negative mass needed is only the order of grams, accompanied by a negligible amoung of positive energy. This constitutes a reduction of the absolute value of the energy by 65 orders of magnitude. The new geometry satisfies the quantum inequality concerning WEC [Weak Energy Condition] violations and exhibits the same advantages as the original Alcubierre spacetime. Read it here. The two big problems to be resolved are 1) how do you get an object inside a warp bubble? 2) What happens to the object when the warp bubble collapses? "
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Warp Drive Breakthrough

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  • by Anonymous Coward
    Being a physics major there seems to be major problems with this idea as a few of you have already pointed out, with the idea of negative mass and negative energy. These things from my understanding can exsist only in two places both which are very hard to find or create those are a black hole and a wormhole. Negative mass some would relate to the idea of hyperspace or extra dimensions. I still believe though that warp drives are a possibility but not beneficial, I think we need to spend more time working with ID jump drives which might not be as far off as some would say.
  • >energy is abundant at this time in the universe

    There is no net energy in the universe. Consider:

    (1) Objects far apart have more energy than objects close together (because it takes energy to separate them)
    (2) The universe is expanding.

    Now there's a theory that the expansion of the universe consumes exactly offsets the energy necessary to separate all the galaxies (and all other matter) from one another. This neatly solves the problem of "When the universe contracts, won't things get hotter and thus violate the law of thermodynamics?" No, because a contracting universe loses energy bacause objects get closer together and this again exactly offsets supposed increased energy as things get crushed together. So the total energy of the universe is exactly zero. Everything's pointless!
  • by Anonymous Coward
    The effect was predicted by the Dutch physicist Hendrick Casimir in 1948

    Casimr effect [iastate.edu]
  • by Anonymous Coward
    It seems to me that charging a capacitor makes it more massive for the same reason that lifting an object against a gravitational field makes it more massive (though not necessarily heavier, in a nonuniform field). In both cases, you're doing work and storing energy. E = mc^2, so m must increase.

    Vanishingly small is the right way to describe the effect. It would take a staggering amount of energy to increase the capacitor's mass by any measurable amount.

    Also, the capacitor's mass can only increase from its rest mass. I don't see how you could make it less massive (let alone negatively massive!) by charging and discharging it than it would be just sitting uncharged.

    AC
  • by Anonymous Coward
    Just a thought here, try not to think of "negative" as the opposite as in the asteroid example, think of it as more a "lack of", where negative mass is a lack of mass where there should be mass. Perhaps an asteroid of negative mass which struck the oceans wouldn;t even notice. Perhaps we are bombarded by negative masses all the time, but dont notice them? Perhaps negative mass is something not-so exotic? Perhaps all this physics talk is making me insane? Probably, or maybe I just channeled Einstein and he wanted to play a creul trick on me. No more Physics today, please.
  • by Anonymous Coward
    > Secondly, FTL travel *doesn't* neccessarily lead to time travel.

    I thought that one of the outcomes of the "Chronology Protection Conjecture" and related works was that path lengths through the wormhole and conventional space would have to be of roughly equal length to prevent the collapse, thus preventing both time- and FTL travel?
  • by Anonymous Coward
    The universe seems pretty complex. Is there anything about it that constitutes evidence that the physical laws of our universe were or were not deliberately designed by some outside entity?
  • by Anonymous Coward
    BAsed on what I have learned so far, I thought that energy, as measured, was always relative, because it is so hard to find a total absoulte ground state. Note the energy is made up of enthalpy and entropy, of which entropy is only definable (and difficult at that since absolute 0 cannot be attined experimentally).

    Therefore, when we witness the Casimir effect, are we really seeing "negative" energy or just a lower energy state (closer to zero or at zero without being under zero) than has previously existed, or that exists naturally. Thus, on these grounds can we define an absolute zero now? Does this mean the average energy of the universe exists above zero? Is negative energy actaully negative in this regard or just an energy that is below the common state at ehich the universe exists, which would now have to be > 0.

    Something for all you guys to chew on and lose sleep over :)
  • by Anonymous Coward
    I recomend that you check out http://www.cco.caltech.edu/~qoptics/teleport.html if you want to find out information on FTL travel. I beleive there was an article on slashdot a while back on this.
  • by Anonymous Coward
    Consider three ships stationary relative to each other, in a very dull universe (no other objects besides the three ships). They each synchronize their clocks (ie. they all 'tick' at the same rate.) They then set their computers to fire the engines according to a special program while the crew goes into hibernation. The program for the engines goes something like this: fire for a random time, finally winding up with A and B moving in opposite directions relative to C but at the same rate. Then shut down the engines, and awaken the crew. (For each crew, it appears as if it's ship is just as stationary as it was when the three ships were together.)

    Crew A sees itself as stationary, and ship B as moving away, and observes that ship B's clock is ticking at a slower rate than ship A's. They conclude that time is passing slower for B's crew.

    Most people claim this time dilation as the paradox of the twins. How can time be one rate for A and another rate for B?
    BUT... the REAL paradox comes in when looking at B's point of view.

    Crew B sees itself as stationary, and ship A moving away, and observes that ship A's clock is ticking at a slower rate than ship B's. They conclude that time is passing slower for A's crew.

    So, A sees (meaning measures) B's clock as slow, AND B sees A's clock as slow.

    Here is the real paradox. How can each ship measure the other as being slower than itself?

    Now let's consider things from C's point of view:

    Crew C sees itself as stationary, and ships A and B as moving away in equal, but opposite directions. Crew C observes that ship B's clock is ticking at EXACTLY THE SAME RATE as ship A's clock. They conclude that time is passing equally for A's crew and for B's crew.

    It's like a railroad track. One can observe that the rails get closer together the farther away you look, until they converge at infinity. The reality is that the tracks do not converge. It is just perspective.
  • by Anonymous Coward
    Hmmm... if you change the spin state of one particle, how will you know that the spin state of the one on the other side of the universe changed? You'll find out eventually... at the speed of light.
    There are some known incompatibilities between Einstein's relativity, and the current model of quantum mechanics. Relativity requires that space be "smooth" but QM shows it's quite broken (on small scales, where Relativity breaks down).
    Now there's superstring theory, which isn't quite done yet, but it makes great strides in resolving the conflict. But it requires that the universe have at least TEN dimensions (nine spacial, one time).
    There are the three "extended" dimensions we can see, and the other 6 spacial dimensions are curled up and very tiny, according to the theory.

    Thud (still an Anonymous Coward)
    http://j.bruce.home.mindspring.com


  • by Anonymous Coward
    No. The process he's describing is taking two clocks and setting the alarms then separating them. Then after separation you speed up one clock and the other clock is sped up the same amount instantaneously. The only problem is the clocks are both already randomly chanding speed on their own and the only way to find out which speed change was yours is to already know exactly when you changed the speed and by how much.

    -Zane
    Zane@SPAMNO!.supernova.org
  • by Anonymous Coward
    >1. Piezoelectric crystal can be made to expand
    >and contract with another alternating current.
    >The idea is, push on the capacitors when they
    >have low mass, and pull on them when they have
    >high mass. The resulting machine should be able
    >to float in mid-air or even accelerate, if the
    >amplitude of the effect can be made high enough.

    I've actually seen a Discovery channel show about this. They were talking about some alternate propulsion schemes the military was looking at. They showed a film of this actually happenning. It was actually on a decent show too, not one of those "UFOs are real" type of shows.
  • by Anonymous Coward
    1. Warp 1000 light years away, simultaneous with the frame of reference of the solar system.
    2. Accelerate to near light speed.
    3. Warp back to earth, but 999 years before it left.

    OK, this is kind of bogus. Assume your frame of reference is indeed the solar system, and we measure time by, say, a radio broadcast giving the current time from earth. Warp 1000 light years away. Poof. We appear (by the ancient radio clock broadcasts, which are only just reaching our current position) to have already travelled back in time 1000 years. Whoosh, we rush back at near c. Doppler makes time in transit appear to speed up, with 1000 years elapsing in 'radio time' as we travel, even if our wristwatches tell us it took fifteen minutes. No time travel. (or, as a fop to those who use relativity to define time, "no time travel at the same spacial coordinates".)

    If you're still puzzled, try thinking about putting the radio clock at the destination instead of the origin...

    smoog

  • by Anonymous Coward on Thursday May 27, 1999 @10:35AM (#1877229)
    Her's how the Casimir effect works:

    • Metal conducts electricity. Therefore, electromagnetic radiation at the surface of a piece of metal has zero electric field; otherwise the electrons in the metal would flow. (Of course if the frequency is high enough, the electrons can't keep up, and the metal becomes "transparent" to those fields. In other words, metal blocks radio waves, but is semi-transparent/semi-opaque to x-rays, and very transparent to gamma rays.
    • A pair of parallel metal plates can have a standing wave electromagnetic field between them, much like pipe organs have standing air waves, or plucked guitar strings are a standing wave, and have a node at the bridge and the nut. When done with radio waves, you have what is commonly called "radar cavities", and indeed all of microwave & radar is based on this principle.
    • Quantum space is filled with "zero-point" fluctuations in the electromagnetic field. These are basically electromagnetic waves that zip into existance for brief periods of time, and then cancel each other out again. When they occur between a pair of metal plates, these "unreal" quantum fluctions *must* have zero electric field on the metal plates, else the electrons would move.
    • The pair of metal plates thus limit the types of virtual fields between them, resulting in a very weak attraction proportional to the distance between them taken to the fifth power. As someone mentioned, this effect was actually measured in th 1920's (?) and is almost simple enough to use a undergrad/grad school lab experiment.
    • The theoretical interpretation is that the field between the plates has less than the average, i.e. less than zero amount of energy between them; i.e. since work=force x distance, and the force is attractive, the work (energy) must be negative.
    So what do we have? We need to find a pair of very good conductors, made out of something much much smaller than atoms, and hold them a few hundred planck lengths apart from each other. The negative energy in between them should be enough to satisfy that needed in the theoretical paper.

    BTW, a planck length is *very* small -- 10^-43 meters if I remeber right. Like a few *zillion* times smaller than an atom.

  • by Anonymous Coward on Thursday May 27, 1999 @09:50AM (#1877230)
    I read something in Analog a few years back about
    how running high frequency alternating current
    through large capacitors causes their mass to
    vary sinusoidally at about twice the frequency
    of the current (and the amplitude of the
    variations is proportional to the frequency).
    (And I thought if you did this, the capacitor
    would just get hot.)
    I don't remember what this was called or who
    discovered it; it has
    something to do with Mach's Law. The effect
    was supposed to have been observed, and
    vanishingly small. But

    1. Piezoelectric crystal can be made to expand
    and contract with another alternating current.
    The idea is, push on the capacitors when they
    have low mass, and pull on them when they have
    high mass. The resulting machine should be able
    to float in mid-air or even accelerate, if the
    amplitude of the effect can be made high enough.

    2. The amplitude of the variation can (maybe) be
    greater than the total mass of the capacitors,
    leading to brief repeating periods of negative
    mass. Don't think they last long enough to make
    a very good warp drive, though...

    Anybody care to burn up some capacitors and
    test this?

    --edkiser
  • by Anonymous Coward on Thursday May 27, 1999 @11:45AM (#1877231)

    First thing: If you want to read how traveling back in time to meet your former self might actually work, you MUST, *MUST* read "The Fabric of Reality" by by David Deutch.

    He explains for the first time in any place, in a simple thought experiment, how such contradictions get resolved. He uses the language of recursive computing and turing machines, not physics, so you will be quite at home if you're are a CS major.


    Secondly, FTL travel *doesn't* neccessarily lead to time travel.

    As an example, I give you wormholes. Now, most people understand that all you have to do to cause wormholes to become time machines is accelerate one of the mouths until it has a different clock.

    So for instance, let's say the entry point has a clock at 2pm, and the exit point has a clock of 1pm (because it is moving close to the speed of light and has a slower clock)

    Now, if you bring those mouths of the wormhole within 30 minutes traveling distance of each other, you could enter the entrance at 2pm, arrive at the exit at 1pm, and fly back to the entrance and arrive at 1:30pm which is 30 minutes before you left. Then, you could stop yourself from going in.

    However, there is a fundamental flaw in this argument. Matt Visser used relativistic quantum mechanics to prove that if you bring the mouths of the two wormholes close enough, virtual particles will form closed-timelike-loops *first*, the energy density of the space between the mouths will quickly diverge, and the wormholes will *close*

    In other words, as soon as a closed-time-loop is close to being realized, radiation in the space diverges toward infinity rapidly, and the whole jumpgate collapses. :)

    So, if we were talking about Babylon5 or StarTrek, there would be a fundamental law of the universe, which is wormholes can only be moved so close together. If you move them closer such that the travel time for light is less than the difference between their clocks, the wormholes will collapse.


    Incidently, Hawking also proposed this, he called it the "Chronology Protection Conjecture", that the universe won't allow time loops to exist, and there would be an infinite radiation wall to travel through in any such loop.

    There are many such theorems in physics, such as the "naked singularity" rule for black holes. Can a singularity exist without an event horizon, such that the rest of the universe could view it?
    Probably not.
  • imagine it, a 9600bps link, with -5sec latency!

    =)
  • by Yarn ( 75 ) on Thursday May 27, 1999 @09:59AM (#1877234) Homepage
    In quantum tunnelling, the tunnelling particle has been considered to have negative energy, as it enters a region it doesnt have sufficent energy to enter (classically)

    Some theorists postulate that it has "borrowed" energy from its surroundings, and has an energy debt, so has negative energy.

    Putting negative energy into e=mc**2 gives interesting results for the mass, obviously.

    The article was very interesting, but I dont think its been refereed yet, so I wouldnt get excited yet. It seems to rely on "Alcubierre Space" (which I've not seen defined) being either wrong, or adjustable. As its raining, and I should be revising for an exam tomorrow, I dont want to chase up references!

    If, as is stated in the paper, microscopic warp bubbles are possible, would it not be possible to have a "warp foam", ie: a huge number of warp bubbles, and have them move together, that would avoid the size problem.
  • Anonymous Coward wrote:

    ...use one of these guys to put an olympic sized swimming pool in my closet. The author seems to indicate that the warp bubble is larger on the inside than on the outside.

    Sure, it would just take many years of research to engineer the closet, and millions of dollars to build the closet. Oh, and if you try to enter the closet to use the pool, your body will be destroyed by tidal forces.


    ...implement that inertial damping drive system I've been kicking around.

    You don't need inertial damping with a warp drive. The warp bubble moves, the vehicle stands still.


    ...avoid speeding tickets on I75 by travelling in a microscopically small bubble of space.

    I don't think the highway would be all that happy with a Warp Bubble travelling down it, but I don't think the state troopers have warp detectors yet.
  • by Gleef ( 86 ) on Thursday May 27, 1999 @09:59AM (#1877236) Homepage
    This paper is part of the ongoing abstract research into the possibility of travelling faster than light without breaking the laws of relativity. There are two leading proposals, usually referred in laymans terms as "Warp Drives" and "Wormholes".

    The Warp Drive idea was first formalized (i.e. given all the math to show it should work, given sufficient engineering prowess) by M. Alcubierre, so it is sometimes called the Alcubierre Warp Drive. It has three big drawbacks: it requires an absurd amount of exotic energy and matter (some of which we don't yet know how to make), you can't see anything while Warping, and there is no theory on how to stop. This paper addresses the first problem, with the equations given, you need far less exotic energy and matter.

    For some excellent laymans info on Faster Than Light issues, check out NASA's Warp Drive, When? [nasa.gov] site.
  • I wonder if BableFish will translate this one?

    :)

    Wayne
  • Actually, it's a good bet that the "Physics of Star Trek" is never true. :)

    Also, I don't claim to understand the math exactly, but from the paper it looks like the researcher found a way to reduce the amount of energy you need to power a "warp bubble" from astronomical to merly enormous.
  • Despite what the blurb implies, this is all still very theoretical stuff. Don't expect to see warp drives in actual use in your lifetime.

    Reading through the paper (which it WAY over my head) it sounds like the researcher found a much more efficent way to travel once we figure out how to fold space-time (with negative energy I guess).

    Of course only a few years ago (like 70 or so) I'd be saying the same thing about general purpose personal computers...
  • Time And Relative Dimentions In Space

    Yup, yet another Dr Who fan (though I haven't seen it for many years *sob*).

  • You are saying approximatly (words, not gist (your gist and mine pretty much match up)).

    TWIUI, you do not travel forwards (any faster than normal) or backwards through time when you go FTL. Instead, you out-race the information of where you've been, and so it appears (to the observer at your destination) you arrived before you left. In fact, if you could come in visibly, the observer would first see you suddenly show up out of nowhere (assume instant accelleration, eg stutterwarp (2300AD, GWD(rip))), and a copy of you leave your destination along the path you came in on. In fact, you should be able to see this yourself if you look backwards (note the assumption of visibility).

    Things get even more interesting if the observer is to the side: he sees one ship suddently show up in the middle of is FOV, splitting into two, one retreating along your path (your past image) and one trailing your actual progress. Note: I beleive all these images will travel at the speed of light (or slower?). ie if the observer measures the speed of your travel, it will look like you're going =c.

  • (Disclaimer: I read this in a sci-fi thriller-type book, so take it with a grain of salt).

    Somebody opened up a container with anti-matter in it (approximately the volume of a pea). Result? BIG explosion (crater with a diameter of 10 miles if I recall correctly), and a major electromagnetic pulse for thousands of miles around.
  • Posted by My_Favorite_Anonymous_Coward:

    ... my money would still be on it being impossible (Due to killed grandfather paradox, lots of bad episodes of star trek, etc).....

    That's why I like Dragon Ball so much (One of the greatest japanese manga even written in the golden age (80's-early 90's)) Its explaination is that everytime you use the time machine, you "create" an alternate universe which has the same "you" and same grand dad with the same names, except that the grand dad you save ain't the grandpa who was killed in "your" universe.

    James Cameron's original ternimater 2 script has the same alternative universes idea which had old linda hamulton sitting on the park watching children playing game happily ever after (remember the park?) Unfortunately it was replaced by the heroic/cheesy/simpler suicide scene.

  • Posted by BrainMold:

    I'm no physicist and may be totally wrong, but here goes...

    Assume Time Travel is possible. You start here on Earth and somehow travel back 4 months without traveling around outer space at lightspeed. If you did so, taking into consideration that the Earth revolves around the sun, wouldn't you end up in the middle of outer space because the earth would be in an entirely different position than when you left? This is assuming you don't change your position in space in the process, only time.

    This, of course, would only matter if you could stay in a stationary location to travel through time and you don't move from that position in the process.

    Would it even be possible when stationary?

    [please excuse errors in spelling or grammer, I am in a hury]

  • by Ping ( 733 )
    Sorry, but I haven't taken a class in quantum physics yet. I wish I could understand what this means, particularly without all the star trek refrences. I like star trek, but I've accepted that it's fiction (and rather good fiction, at that) so I try to think of real physics separately from star trek.

    Could somebody explain this bit of science news in a way I could understand? Thanks!

  • Take two individuals A and B. Pick the earth as a frame of reference. Assume two identical vehicles which are capable of travel a light speed. Person A accellerates to light speed. Person B remains motionless. For each, time is moving forward. Now, when A decides to return from light speed, he has been gone for X years (a short time). B has been stationary for Y years ( a long time). Now, the reference point has also had a passage of time (Y years). Although A has been gone for an absolute number of years fewer than have elapsed at the reference point, (or for B), it still remains that time has moved forward and that A returned to the reference point after Y years relative to the reference point. Thus, it may seem like the shorter period of "time" allows jumping into the past but this is an illusion.

    may be slightly unclear...please patch it up if you know how to express it better


    Who am I?
    Why am here?
    Where is the chocolate?
  • Could someone summarize the article in laymans' terms, please? :)


    Who am I?
    Why am here?
    Where is the chocolate?
  • Is a logical contradiction.

    The simple fact that travelling backwards in time would allow me to kill my former self, thus preventing me from ever having gone back in time to do so, is a complete logical contradiction, causes me to not care even in the slightest about this or that new theory which suggest FTL travel.

    It just ain't possible.
  • Calling light a wave is a convenient model for describing some of its behavior. Calling light a particle is likewise a model which describes some of its behavior.

    I don't think there's really any contradiction there because we're not saying that light IS a wave and that it IS a particle - that would be a contradiction. What we say is that light exhibits behavior of both a wave and a particle, which isn't a contradiction - it's just evidence of how incomplete either the wave or the particle model is for describing the behavior of light.
  • I could read every book in the world.

    But it still wouldn't make logical contradictions possible ...
  • Going back in time implicitly creates a logical contradiction. The space occupied by my body when it appears in 1969 to witness the birth of Unix would have been, in 1969, either there, or not there. If it was there, then my course of actions are already decided -- I can do nothing except follow exactly the course of events which would have put me in the time machine on my way back to 1969. In which case, I can't cause the future as you said.

    Or, my body was originally not there in 1969, but when I warp back, I change history such that my body was there. That is a logical contradiction -- I can't do that, because it would imply that at the same moment in time, my body was there and it was not there. That is as close to a pure logical contradiction as you can get.

    So it must be the first case, in which the universe is deterministic, everything is going to happen according to preordained rules, we have no free will, and furthermore, the rules and properties and actual events of the universe just happen to be structured in such a way as to not actually cause a logical contradiction.

    I don't find this particularly compelling ... do you? :)


  • These are interesting possibilities, and I have considered both before ...

    There is also the possibility, equally as compelling and as likely as either, that we are all completely crazy, and that every moment of our perceived life is a hallucination that doesn't have to follow any rules or satisfy any logical constraints. In which case, onwards time travel!
  • Well thanks for the suggestion, but I have given this quite a bit of thought. Unfortunately, my world view is one which demands logical consistency and the box you speak of is in fact defined completely by the rules of logic.

    I will think outside of any box you like, just not logic. Sorry.
  • Out of curiosity, does anyone know of any True statement, of any sufficiently powerful logical system, which is unprovable?

    The only one I am familiar with is the "This statement is false" self-reference statement (which is basically the statement that Godel used to prove his Theorem, from what I understand). I read recently (in Scientific American) that Godel's Incompleteness Theorem, while a great shock to all scientists who thought they were going to be able to prove everything, has actually not been very fruitful.
  • by questor ( 960 ) on Thursday May 27, 1999 @05:42AM (#1877256)
    Have I missed something, or is negative mass and negative energy still in the "pure speculation" state -- the equations allow for it, and from a symmetry point of view it seems plausible, but last I heard we have no evidence for its existance, no theoretical way to produce any, etc.?

    (Note: Negative mass is not anti-matter. Anti-matter has positive mass, and opposite charge/spin/other properties of "normal" matter. Matter plus anti-matter equals energy (which manifests as other postive-mass particles). Matter plus negative matter should equal nothing, zero, zip.)
  • by sjames ( 1099 ) on Thursday May 27, 1999 @02:08PM (#1877260) Homepage Journal

    The problem is the mass increase. As you accelerate to the speed of light, you gain kinetic energy (naturally). We know that energy has mass. Solving the system of equasions (left as an exercise), we find that as v approaches c, mass approaches infinity. Thus, it takes an infinite amount of energy to actually reach lightspeed for ordinary matter (tachyons are another thing entirly).

    So far, experimental evidence has matched the predicted values very nicely.

  • by C.Lee ( 1190 )

    >> 2) What happens to the object when the warp bubble collapses?

    If it relies on a Microsoft-based technology, the object goes ***BOOM**** when the warp bubble collapses...
  • you wouldn't have to pay taxes anymore, but you'll definately have a problem collecting social security if you were never born.



    "The number of suckers born each minute doubles every 18 months."
    -jafac's law
  • . . . and if Microsoft bought the rights to this machine, and included one free with every copy of Windows95 to destroy the competition, then their corporate slogan would be:

    When do you want to go to yesterday?



    "The number of suckers born each minute doubles every 18 months."
    -jafac's law
  • Professor Steven Hawking disagrees. And when Professor Hawking says something, people generally listen. If you take two regions of connected space-time, and accelerate one to relativistic velocities, then traverse that connection, you WILL go back in time.

    (No further than the construction of the system, but that is irrelevent. You WILL have travelled back in time. God, Star Trek, et al, have NOTHING to do with it. If anyone's stupid, it's those who assume everything and know nothing.)

  • I think that basically anything that has to be assumed falls in this catagory--eg, the axioms of Euclidian geometry. You can 'prove' some of them but IIRC to do so you have to assume others which were previously proven.

    Non-Euclidian geometry is an example of what happens when you change the unprovable premises. :-)

    Daniel
  • The paradox does not prove that FTL travel is impossible. It merely demonstrates that some of it's consequences might be a little...odd.

    Hey, does anyone remember the 'Future Echoes' episode of Red Dwarf?

    Nick

  • by nstrug ( 1741 ) on Thursday May 27, 1999 @11:01AM (#1877267) Homepage
    Well, a class in quantum physics ain't going to help you. A class in general relativity might though.

    The basic deal is that in the presence of extremely strong gravitational field movement in space along a certain path results in a movement in time. This is what is meant by the sentence: 'The idea is to start with flat spacetime, choose an arbitrary curve, and then deform spacetime in the immediate vicinity in such a way that the curve becomes a timelike geodisic, at the same time keeping most of spacetime Minkowskian'.

    I remember (back as a physics undergraduate) learning a pretty cool visual explanation of this with little spacetime cones (the 45 degree edge of the cone representing light speed). The idea was to tip the cones over. Or something.

    Anyway, Alcubierre's initial formulation of this space time can only be created by a ridiculously strong spinning gravitational field. I seem to recall something about a bloody huge sphere of material the density of a neutron star spinning so it's surface is going at 0.999c. Massive it was.

    There are other problems with the Alcubierre geometry, namely basic things like electromagnetism gets buggered up, it requires lots of negative energy (think of negative pressure rather than straightforward energy) and anyway as soon as you start going faster than the speed of light it becomes physically impossible (with this geometry) to go faster than the speed of light. Go figure.

    So this Belgian bloke has come up with a absolute wheeze: keep the surface area of the warp bubble you create really small but expand its volume to something you could reasonably fit a Volkswagen Polo into. Try this at home and you may run into the slight problem that volume usually increases to the 3/2 with area. Not a problem because this dude's a cosmologist, he's got all the paper and pencils he needs, and it always rains in Leuven so he's got nothing better to do. Check out eqn (4), it's a beauty.

    Working through the maths he comes up with some numbers and - suprise - his new warp geometry requires much less negative energy than the Alcubierre geometry (which required rather more negative energy than the total positive energy in the universe, just to produce a warp bubble that could have been described as 'cramped and bit stuffy' by a vole). In fact using some sensible (i.e. off the top of his head) figures, he finds that you only need 3.4 grammes of negative energy.

    The problems of where do you get the negative energy from, the massive densities involved and how do you get a macroscopic object into a bubble with infinitesimal surface area of course remain.

    Your tax Belgian Francs at work people.

    Hope this helps, Nick

  • by rokhed ( 1896 )
    Your problem is in trying to define a "REAL" velocity. What frame of reference are you using to define your "REAL velocity"? To measure a velocity you need a reference point. You then measure the velocity of your subject *relative* to your frame of reference. Part of the point of relativity is that ALL frames of reference are equally valid. Basically observer-based velocity is the ONLY kind of velocity there is.

  • And we thought that a Nuclear Power Plant melt down is bad. I woudn't want to be anywhere near an anti-matter storage tank if the containment mechanism broke.

    Ex-Nt-User

  • Mabey you can't go faster then the speed of light. (But that's assuming we understand that part of physics.. which is still in much debate) But by the same token you can "FOLD" space time. (IE wormhole). This would alow you to travel large distances in very little time. Your velocity would be well below lightspeed.. but the distance would have "folded" from light-years.. to light-minutes.

    Now if someone came up with a device that could fold space directly infront of a space ship.. you could effectively travel faster then light..

  • If God had meant men to fly he would have given us wings! It's your type of radical thinking that will be the death of us.

    ...or He/She/It (whatever - it's still undefined) would have given us brains, creativity, and curiosity to invent wings and other stuff.

    Those who believe in an omnipotent and omniscient supernatural being shouldn't claim we could be dangerous to it or its plans.

    Man (not just men!!) is gifted (or cursed?!) to reconsider, evaluate, and think about the world we live in. That's human nature. Superstition as well as FUD don't help anyone, we have to be open for discussion, ready to learn and change our opinion whenever it's necessary.
  • >Witness the progress of AntiMatter. Until recently it was just theory, then some scientists managed to get a couple anti-hydrogen (I think) atoms happening.

    Antimatter did start off as theory, but experimental confirmation came a long time ago (positively-charged electrons were discovered in cosmic rays). Antiparticles have been a routine part of experimental physics and medical imaging for decades (a "PET" brain-scan uses a positron-emitting isotope). Getting an antiproton and an antielectron to slow down enough to form an atom was a tricky engineering problem, but it wasn't anything fundamental.

    The general path of most discoveries (for example, a nuclear reactor) seems to be:
    1. It's possible in theory
    2. We can do it experimentally
    3. Nature already did it

    It'll be interesting to see what happens with this warp-drive idea. BTW, for a good layman's intro to wormholes and time travel (including a new way of looking at 'causality'), look at:

    "Black Holes and Time Warps : Einstein's Outrageous Legacy" by Kip Thorne (ISBN: 0393312763, $13.56 at amazon.com)

  • [somewhat offtopic; sorry]

    >how come their isn't an explosion when the positrons hit the electrons in the atoms of body tissues? or don't they colide?

    They do collide and "explode" into a pair of gamma-ray photons moving in opposite directions. These two photons travel to a ring of detectors around the patient's head, and the difference in arrival times tells you how far along the line between detectors the positron was when it hit the electron. With some serious acquisition bandwidth and number-crunching, you can transform this set of detector events into a pretty color picture of the inside of someone's head (a map of the rate at which the tagged chemical is being consumed at each part of the brain).

    The energy of each photon is 511 keV, equal to the mass of an electron or positron (in a system of units where "c" is defined as 1).
  • Wow, someone watched Event Horizon one too many times =)
  • This is assuming that existence is by definition
    linear. That's the way we experience time and life, but I have my doubts that that's the way
    it really is.
  • Can't you follow the link? The site provides Postscript and PDF file formats for this paper (along with a few other odd ones...)
  • It would probably be better to not publicize something like this until it has been read be many specialists

    yeah, cause otherwise a bunch of slashdotters are going to go try build one! ;)

    While it's not been peer-reviewed, it's still a lot of fun to think about. I'm glad it was posted.

  • What happened to the other problems with FTL travel?

    I'm thinking specifically here about the more slippery definition of "simultaneous" in special relativity, and it's consequences. Assume a ship that can:

    a. Accelerate to any sublight speed (and thus any "normal" reference frame) arbitrarily fast.
    b. Use "Warp Drive" to move between two points in space time faster than light.

    That's all you need, and assumption a. is perfectly in accordance with physical laws (and technically plausible with some externally powered propulsion system).

    Throw in the cute fact that for any two points in spacetime that are not in each other's light cones, there is an inertial frame of reference where those two points are simultaneous, and your ship can:

    1. Warp 1000 light years away, simultaneous with the frame of reference of the solar system.
    2. Accelerate to near light speed.
    3. Warp back to earth, but 999 years before it left.

    I wish I could link in a diagram of this... but go look in a physics textbook; it's a classic paradox meant to show why FTL travel is impossible.

    I've never heard a good explanation of how Alcubierre's theory (not to mention whatever new concept has come up) deals with this.
  • I'm reminded of the book by Robert Forward where they find that critter made from negitive matter

    The book you're thinking of is Timemaster. In it, a space company finds exotic creatures (called Silverhairs in the book) composed of negative matter, and containing wormhole entrances. Using the negative matter, they construct a relativistic starship to travel to the stars, leaving the wormhole intact after the trip for instant travel. After troubles from a competitor, the company's president makes a round trip via several stars to create a working time gate, thus bringing up the consistency problems of time travel.

    Any Slashdot reader interested in warp drives or time travel should check it out. Amusingly enough, the book itself was a reference for a real-world scientific paper on the consistency principle for time travel. Robert Forward's other science fiction books are also highly recommended.

    Colin

  • I am not a physicist. Nevertheless, I've had an idea, although it may be wrong-headed.

    The problem here is distorting spacetime, as I understand it. Negative energy is just the means to that end.

    It's my understanding that gravity waves are produced by moving objects, and that these have both a "pull" and a "push" component - and that the trouble here is to get the "push" component of the warp geometry.

    My idea is: perhaps, if objects could be moved in a very synchronized way, the gravity waves could be made to interfere additively such that there are "push" and "pull" distortions of the right shape in the right place.
  • Yeah, that was the days of good tv shows. I will remember that phone booth till my dieing days :)
  • I would have to look at it again, but one of the arguements against warp drive from a "physics as we understand it" perspective is that it requires so much energy that we have to drag around huge amounts of anti-matter to get enough energy. The Physics of Star Trek was where I got that from, but I am glad to see that that may no longer be true.

    -Chris
  • why, exactly, would the existence of a logical contradiction make a time travel scenario impossible?

    consider, for a second, the possibility that the paradox does not disallow time travel, it simply means that the paradox will have to resolve itself in ways that we may or may not be able to predict. given the highly theoretical nature of any discussion on time travel, i believe that the unpredictable resolution of the paradox is just as likely the case as the paradox disallowing time travel altogether.

    of course, if this theory is the correct one, time travel would then be very, very dangerous.
  • Yes, causality would be preserved using a warp drive since nothing is travelling faster than light. Warp drive does not increase your velocity, it decreases the distance you must travel.

  • A better way to appreciate things is to look back 100 yrs. At the last turn of the century, the physics community was astounded by some amazing new discoveries. What were these mysterious X-rays? And did you read about radioactivity? I wonder if something besides uranium was radioactive? And electrons! If these things had a negative charge, there just has to be something in the atom with a positive charge. Finally, I just know that someday, man will truly fly through the air.

    Moral: Who knows what the future may bring.
  • An interesting variation on FTL information transfer (not to mention a serious slam of the Copenhagen Interpretation) can be found here [washington.edu].
  • First off, it's a microscopic bubble. He says that the walls can initially be only a few hundred Planck's lengths thick at the start

    True, but if I'm reading this right the area inside the walls can be significantly larger. Region I in the diagram is a 'pocket which has a large inner metric diameter.' he also states in the abstract that his solution supports the 'transport of macroscopic objects.'

    In the ever popular rubber sheet model, it seems he has a large inflated balloon where the lips comprise the warp bubble. Except that the space inside the bubble is also locally flat.

    In other words, what he has is not just a warp drive, but also a TARDIS....

  • I forget who told me this, once upon a time... "once its been proven on paper, it becomes merely an engineering problem".

    More corporations need to get together and start investing millions into independent research labs, where they're NOT expected to produce results immediately. Raw science, more research into highly advanced projects. Wonderful stuff can happen. We could see a working warp drive in our lifetimes, given the time and resources.

    Of course, I'm taking the "how things should be" view again, rather than "how things are"... keeps me from getting depressed, though.
  • The article was very interesting, but I dont think its been refereed yet, so I wouldnt get excited yet. It seems to rely on "Alcubierre Space" (which I've not seen defined) being either wrong, or adjustable.


    It's adjustable. If I understand correctly, "Alcubierre Space" is just one possible (and geometrically simple) configuration of spacetime that allows an object to (for practical purposes) "move" faster than light with respect to its surroundings. This is an extension and modification of those results that uses a more complicated geometry but has fewer practical problems.

  • it is possible and even probable that the entire universe is composed of a single electron in a parallel time frame for each instance (near infinite).


    Actually, no. If only electrons and positrons and photons existed, then you might be able to draw a Feynman diagram with one line representing all electrons and protons, but beta decay takes care of that. A quark can decay into another quark, an electron, and an antineutrino. This provides the start of a new electron line. Similarly, a quark can decay into a positron, another quark, and a neutrino. This provides the end of an electron line. In principle, the reverse reactions could occur as well.

  • This neatly solves the problem of "When the universe contracts, won't things get hotter and thus violate the law of thermodynamics?" No, because a contracting universe loses energy bacause objects get closer together and this again exactly offsets supposed increased energy as things get crushed together.


    Actually, that's not accurate, if I understand correctly. As the universe contracts, GPE will certainly become more negative, but as a result of this kinetic energy goes _up_ for everything that's contracting. Thus, the universe gets hotter again.


    (1) Objects far apart have more energy than objects close together (because it takes energy to separate them)

    (2) The universe is expanding.


    This just is just a statement that the total kinetic and rest energy of the objects in the universe has the same magnitude as the total gravitational potential energy of all objects in the universe. This is actually what _causes_ the universe to heat up as it contracts (and cool as it expands; the microwave background is a good example of this).


    As an aside, the jury is still out as to whether the universe is really flat or not (i.e. whether or not this relation holds). We usually assume so because it answers a lot of questions, but we're having a hard time proving it.

  • 1. Warp 1000 light years away, simultaneous with the frame of reference of the solar system.

    2. Accelerate to near light speed.

    3. Warp back to earth, but 999 years before it left.


    You could indeed do this with a faster-than-light drive. Where's the problem :)?


    The question then becomes, what are the physical consequences of time travel, and does this always result in contradictions? This will either rule out or severely limit what can be done with time travel, and by extension FTL drives.


    You run into similar questions with tachyons.

  • by Christopher Thomas ( 11717 ) on Thursday May 27, 1999 @11:00AM (#1877308)
    The simple fact that travelling backwards in time would allow me to kill my former self, thus preventing me from ever having gone back in time to do so, is a complete logical contradiction, causes me to not care even in the slightest about this or that new theory which suggest FTL travel.


    It just ain't possible.


    Not strictly true. All that your example shows is that you would not be able to kill your former self. Two of the several solutions that I've heard postulated are:

    • Actions of time travellers must be consistent with observations.

      Under this system, you would certainly not be able to kill yourself, becuase you didn't (you survived to travel backwards in time, didn't you?). This physically corresponds to limiting (drastically) the number of possible events that can occur within a loop of spacetime that folds back on itself in the time direction. This in turn means that such loops are entropically very unfavourable, but they could still in principle occur if a greater increase in entropy happened elsewhere.

    • Time travel is actually travel between multiple histories.

      This refers to the "multiple histories" interpretation of quantum dynamics. Under this system, when you flip a coin, it lands on both sides - just in different universes. What actually happens is that all possible ways for a probability waveform to collapse happen, in different universes. If you travel back in time, you arrive in another universe, that looks a lot like the one you remember from that time - but in which a time traveller spontaneously appeared and killed the person who would have been you in your universe. This system doesn't impose entropic limits, but how exactly you travel between parallel universes is left as an exercise.



    Both of these systems avoid the paradox that you menion.

  • Way back when I took Physics:

    Recall the 'right hand rule' of forces and magnetism WRT the direction of current flow.
    If current flows in the direction of the thumb of the right hand, the magnetic field lines wrap around the wire in the direction of the fingers.

    So given two wires there is an attractive force between them if the currents are co-directional, and a repulsive one if the currents are opposite.

    It's been a long time, so I've forgotten what goes on with regular AC, but as I remember - this principle, with extremely high currents, extremely close proximity of wires, and alternation in currents at a quarter-phase, would generate unidirectional forces == motion of the whole assembly.

    Not quite as sexy as folding space-time and blowing warp-bubbles, but not your vanilla friction or exhaust driven method either.

    Kirk-Star, you out there to clarify this?
  • Everyone who is concerned with creating a paradox by going back in time to kill themselves thus preventing themselves from doing so and creating a paradox is forgetting one little detail, our universe has an infinite number of facets. Quantum physisists have found that this is indeed true, the only way to determine the characteristics of a particle is to look at it. While this sounds like something you might take for greanted it isn't. Shrodingers (sp?) cat is the most often used example. The properties anything are undeterminable until viewed from an outside point of reference. This can apply not only to particle physics but to dimensional physics. For every moment there is an infinite number of possibilities for that moment for every point in the universe, and you only see one set of facets every moment. imagine points in the universe like coins. Spin them on their sides and let them all land, when they all land, face up or down that is a single moment, now repeat this process with an infinite number of coins with an infinite number of sides to understand what I'm getting at. Therefore you can only create a paradox in so many parallel instances. And therefore not create a true paradox, only one thats limited by the number of instances you actually go back in time and kill yourself.

    As to faster than light travel, you would not go back in time unless you actually accelerated near the speed of light, the whole theory of FTL travel is never actually reaching the speed of light, just sidestepping part of the distance between your point of origin and your destination. Which needs no "inertia dampeners" or the like, just a warp feild generator (I say that like you can pick one up at Pep Boys).

  • > 1) how do you get an object inside a warp bubble?

    Two words: Klein Bottle.

    Now If I could only find the fine Irish Whisky I poured in there.
  • Well, there's Woodward's work. Basically, as a result of Mach's principle, it predicts a transient mass fluctuation in an LRC circuit.
    It could be the "impulse engine" of ST fame, or the hover cars off the Jetsons...

    (Just maybe, it could also provide a large enough mass fluctuation for more exotic uses, like temporary wormhole stabilisation...)

    NB. IANAP (I am not a Physicist (...and boy, does it show...))

    Woodward carried out a test, which seemed to confirm the theory. However, an unforseen non-linear response in some of the experimental equipment casts doubt on the first results. Even if the fluctuations were a couple of orders of magnitude smaller than observed, it would still be a major breakthrough. NASA seems to think so too, and are, AFAIK, quietly working on a repeat test in their breakthrough propulsion labs...

    Anyway, the theory makes interesting reading.

    Here's the relevant links:

    chaos.fullerton.edu/Woodward.html [fullerton.edu]

    www.inetarena.com/~noetic/pls/wo odward.html [inetarena.com]
  • by Shadowlion ( 18254 ) on Thursday May 27, 1999 @09:56AM (#1877333) Homepage
    Actually, you're wrong on both counts.

    1) You _can_ go faster than the speed of light, just not directly. You can't just turn the engines of a spaceship on "high," you have to skip over some of the space you're travelling through, and your _effective_ speed jumps past the speed of light. Both wormholes and "spacefolding" technologies - both of which are theoretically proven - would enable one to do this.

    2) Yes, you can. In fact, we know how to do it today. The problem is, it takes an astronomically large amount of energy. It requires a wormhole in order to achieve it. You take one end of the wormhole and take it on a tour of the solar system at _relativistic_ speeds. You keep the other one stationary. Due to the time dilation, time passes for one a lot slower than it does for the other. Since one end of the wormhole has experienced less passage of time than the other, one end resides in the past, and one resides in the future. Then you can freely travel backwards and fowards through time via the wormhole.

    There are other ways to travel backwards and forwards through time, but most are terribly implausible (e.g., infinitely long rotating cylinders spinning at relativistic speeds - I don't understand the full complexities of that particular process). And all have been discovered as parts of solutions to Einstein's equations, which have very little to do with quantum theory. So even assuming only relativity applies, you're still hosed.
  • by Shadowlion ( 18254 ) on Thursday May 27, 1999 @10:02AM (#1877334) Homepage
    It's been demonstrated - the article even mentions the demonstration.

    It's called the Casimir effect, and is based on the idea of virtual particles. Spacetime isn't flat - it's teeming with zillions of virtual particles that pop in and out almost instantaneously. Their existence is so short that it doesn't really matter much. However, an Italian scientist named Casimir theorized that if you took two uncharged metal plates and put them close together, they would attract each other via the net force of all these virtual particles.

    A few years later (IIRC, mid 1920s), when somebody actually _tried_ this, sure enough they detected a net attractive force. As science progressed, it was determined that this attractive force is based on negative energy. Negative energy is one of the driving forces behind wormholes. In order to make a stable wormhole, you need "exotic matter" - a form of matter that has an average negative energy density. Nobody's ever seen exotic matter, but no equation or physical process has been seen or discovered yet that would rule out its existence.

    Negative energy/virtual particles are also the process by which black holes give off radiation (yes, THEY DO RADIATE). If a black hole gives off more matter than they consume, they will shrink and eventually explode.
  • I think that I, along with anybody else of the personality type who would read Slashdot, am damn neard giddy about the idea of warp drive.

    The method, as the author himself points out, does have quite a few problems, though. First off, it's a microscopic bubble. He says that the walls can initially be only a few hundred Planck's lengths thick at the start. And as you slow it down, the "warp bubble" will expand, thus making the walls thinner until they actually shrink to lenghts less than Planck's constant, which would cause unpredictable results to say the least.

    Also, the authors says that though this is orders of magnitudes less energy, it still requires "unreasonably large energy densities."

    And, as another comment said, negative energy isn't exactly something that you can just buy at Amoco, and until a method to easily create it is developed, none of this is realistic.

    Still, this is damn cool to say the least. I can't wait to get to work in seconds. And this could possibly do wonders for communication. A superluminal link would be just a little bit faster than my Ethernet connection, I'd guess :)

  • If I remember correctly, doesn't the Casimir Effect [ucr.edu] result in negative energy?

    The Casimir Effect, basically (and don't flame me if I'm wrong! Just post a correction.. ;) consists of two electrically neutral plates placed VERY closely together. So closely together that the distance between them is smaller than the wavelength of some virtual particles [ucr.edu]. Because there are fewer virtual particles between the plates than outside, the energy density is LESS than the vacuum energy density -- negative energy? Not in large amounts, but a clue as to where to find the stuff, perhaps...

    -g.

  • Perhaps that is a "law" of backward time travel..if I go back and attampt to kill myself, I won't be able to, since that will prevent my trip in the first place, which means I won't die, which means I'll live to travel back in time. This only means I can't cause my own death somehow since that would violate causality, not that if it were physically possible that I couldn't travel back in time(maybe killing myself would automatically "self correct" and the knife I stabbed my younger self with would have no effect).
    A more likely scenario is that, as may be postulated by Hiesenberg followers, that every possible past and future state exists in parallel. Going back in time and killing an earlier me only disrupts one possible time line, not the one from which I originated (Sliders!). This fits very well with Quantum theory and violates no tenet of relativity.

    Have some imagination. Orville Wright once said that man (sic) would not fly in his lifetime. Try to think outside the box instead of dismissing ideas outright because they don't agree with your world view.

  • Mind you, my physics and cosmology is ~15 years out of date, but as I recall, the net energy of the universe as a whole is a positive number, which (theoretically) can be tapped. As a practice, the number was re-set to zero, but if we're already warping space-time to produce a bubble that moves across it at speeds faster than that which matter can transit, the stressed space-time would likely make it easier to tap the so-called Vacuum Energy, and thus maintain and propagate the bubble. . . .

    If my physics IS outdated, then ignore this, but I recall Dr. Charles Sheffield, NASA physicist and SF author discussing tapping vacuum energy as little as 2 years ago, so this MAY be a viable approach, assuming energy density is sufficient for the application.

    Getting the negative mass is, alas, an area where my ignorance is sufficient to even guestimate an answer. . .
  • by mwood ( 25379 )
    >The two big problems to be resolved are

    Already well understood:

    > 1) how do you get an object inside a warp bubble?

    Wander into Main Engineering while your genius son is tinkering with the engines.

    > 2) What happens to the object when the warp bubble collapses?

    People start disappearing and nobody remembers that they ever existed.

    (For the humor-impaired: ST:TNG "Remember Me")
  • by debrain ( 29228 ) on Thursday May 27, 1999 @09:35AM (#1877363) Journal
    The energy that would be required for warp travel is irrelevent; energy is abundant at this time in the universe. (And worse come to worst, we sacrifice some matter.) Much of the physics we are dealing with is not conventional, and we will soon see a new relevation to change the entire perspective on the reality surrounding us. Warp drives, so cliche as that may be, can exist. Indeed, it is likely that they do. Just to throw you for a loop, it is possible and even probable that the entire universe is composed of a single electron in a parallel time frame for each instance (near infinite). Interesting, no? I say this because a positron, the electron's positive energitic counterpart, is mathematically described as an electron travelling back in time (i.e. negative time). Still, much work is to be done on this yet. The reality constraining us is not what it used to be. We should not forget what we have overcome, lest we fail to overcome the barriers before us now.
  • by adimarco ( 30853 ) on Thursday May 27, 1999 @10:47AM (#1877365) Homepage
    Not only is faster than light travel possible, but I believe it has been demonstrated.

    Information travels faster than light. (because information doesn't actually travel, for a normal definition of that word) Would someone with a heavier physics background please correct me here? I'm pulling this from memory:

    Two particles can be bonded such that their collective spin state is always 0. knowledge (or lack thereof) of a particle's spin state is information. once these particles are bonded, the space seperating them is irrelevant. they can be next to each other or on opposite ends of the universe, and as soon as you change (observe) the spin state of one, the spin state of the other *instantly* becomes defined. the information travels without actually entering the space seperating the particles, which means that it isn't subject to the usual limitations placed on physical objects (namely a ~186,202 miles/sec speed limit).

    i'm not sure how modern physics handles this. i believe einstein rejected it, but i think it's actually been experimentally confirmed in the past several years. you could incorporate yet another dimension into your conception of existance (5? 6? n?).

    the curious part is how closely this resembles what mystics have been saying for millenia: space is an illusion.

    another curious side effect of this is that the speed of light (in very much simplified laymans terms) seperates the "past" from the "future" and keeps them both out of the ever-present "present" (be here now). if you allow for faster than light travel, the chain of causality as we know it must be abandoned. the past and the future get all mixed up, and you have cause following effect etc.

    i will cease my rambling now and get back to work :)
  • by remande ( 31154 ) <remande.bigfoot@com> on Thursday May 27, 1999 @11:54AM (#1877367) Homepage
    Sometimes a paradox means that you don't understand all the facts. Other times, it means that you are simply not on enough hallucinagens. I suggest that the latter is the case here. The paradox only exists because we want to use our concept of time in a realm where we break the laws of our concept of time.

    If I enter a time machine in 1999, go back to 1979, then kill the "other me" in 1989, then the "paradox" is that I cannot exist in time 1999 to do these things.

    To an observer alive during all this time, they will see one of me until 1979, then sees my evil twin materialize for some unknown reason in 1979. Between 1979 and 1989, the observer sees both myself and "my evil twin" (who is actually myself, but may as well not be). After time 1989, the non-time-traveling me is in a pine box, while the evil twin is walking around.

    From this observer's perspective, nothing of particular importance occurs in 1999. Nobody enters the time machine. The fact that my evil twin remembers a particular event in 1999 is irrelevant. It is irrelevant because 1999 is no longer what we think.

    To those of us unfamiliar with time travel (I'll assume that's all of us, save the Gallifreyan contingent), 1999 is a fixed series of events. Or at least, the first five months of it is a fixed series of events--we don't remember the other seven months, because "they haven't happened yet". We have a one-to-one correlation between personal time and wall time. That is, we've already experienced "April 1999", and never expect to experience it again.

    To my "evil twin", what we call "April 1999" has a many-to-one correlation with his own memory. He can go through April as many times as you can walk through a revolving door.

    To the third-party observer, time travel didn't happen. Somebody shows up out of nowhere in 1979 (surely weird, but no paradox). He kills someone who looks like him in 1989, and lives past 1999 and well into the next century. No time travel, no paradox.

    So who sees the paradox? The time traveler sees no paradox. Non-time-travelers see no paradox. The only way to see a paradox here is to exist outside of time. The only one I know like that is God Himself, and I don't think that He will get thrown by somebody dinking with a knife and a time machine.

    Remember the Bart Simpson correlary to Shrodinger's Cat: "I didn't do it, nobody saw me do it, you can't prove anything". Since nobody can get both the precice position and the precice velocity of a particle, it is arguable that they do not exist. If no observer can record the phenomenon, it didn't happen. Since nobody can witness the paradox, it doesn't exist.

    Note: I was kidding about the hallucinogens. If you need to stretch your mind in those sorts of directions, just stay away for four days straight. It works for me ;^>

  • Well then, it's a logical impossibility for you to go back and create a logical contradiction. Let's say I go back in time and try to kill off a previous self. The simplest break to this paradox is for me to fail. (Possibly through bizzare coincidence leading to the discovery of a way to time travel, if you're feeling ironic.)

    Basically, anything you go back in time and do, has already been done. You can't edit the future, but you can certainly cause it.

    At least, that's how I view it. This is leaning towards Philosophy, not Physics, and only one of those is my major.
  • Uh... that was not a troll. I was quite serious, actually. I guess admitting a lack of space/time theory knowledge isn't popular? Oh well. No biggie...

    DDuck
  • So this Belgian bloke has come up with a absolute wheeze: keep the surface area of the warp bubble you create really small but expand its volume to something you could reasonably fit a Volkswagen Polo into. Try this at home and you may run into the slight problem that volume usually increases to the 3/2 with area.

    Actually, this really is not a problem, relative to the other stuff he's proposing. The only reason that volume typically increases with surface area is that we typically live in a (fairly) flat spacetime. The whole point of the warp drive is to theorize a nonflat spacetime. Actually creating such a spacetime is of course not so easy. :)

  • by David Roundy ( 34889 ) on Thursday May 27, 1999 @02:39PM (#1877384) Homepage
    I'm afraid you seem to have misunderstood the principle of relativity. There is no "REAL velocity". The whole point of relativity (at least, where it got its name from) was that it doesn't matter at all which observer you consider.

    Of course, your speed will depend on the observer, just because it is defined relative to the observer. But the physics (i.e. what actually happens) is independent of the observer. In your example of a tree and an observer falling in the woods, the observer would certainly see that the tree is stationary, but the ground would seem to be travelling at a very disconcerting speed!

  • *sigh* Every time anyone mentions time travel some well-meaning idiot feels the need to mention the old "What if I went back in time and killed my former self?" cliche. Since you were able to type this stupid, overworked, meaningless waste of bandwidth, I will personally assure you that, should you go back in time and kill your former self at some point in the future, you will elect to perform this service to humanity at such a time as to preserve this winning example of the journalist's art.

    DoktorMel@yahoo.com
  • The problem is that the two states are highly *correlated*, but you can't actually get any data without communication between the points (i.e. each side will get a random stream of data, but if they compare their data, they can see when it was correlated, and therefore whether spin coupling had happened at that point). Since you need to transmit some data in order to tell, this doesn't let you transfer information faster than c.
  • by Dreamweaver ( 36364 ) on Thursday May 27, 1999 @01:36PM (#1877390)
    Okay, i'm no physics major.. but i try to keep current and have read enough to consider myself at the very least a knowledgable layman (yeah, i know the whole thing about a little knowledge being dangerous) and i cant say that i understand why exactly FTL travel is so impossible. I mean, why is 300,000km/s such a fundamental barrier? Okay, objects travelling anywhere near that velocity do behave oddly as we view them.. but who says that's so important? so the math says objects shouldnt exceed that speed without doing x y and z.. 100 years ago the math said a whole lot of things we know are patently false. *cough cough ether cough cough*

    Now here's my take on it: relativity says that we cant exceed X m/s without having the rate we move through time change. So then in order to calculate exactly how fast you're moving, you have to do one of two things: calculate your base movement rate from an exact zero state an include your movement across the planetary surface, the revolutionary speed of the planet, speed of planet around the sun, speed of sun around galactic core, speed of galaxy in direction X (not to mention possible rotation of galaxy around unknown object(s) etc) or, the approach normally taken: ignore it. So far as i can tell from what i've read, relativity uses observer-based velocity. If your REAL velocity is 290,000km/s but the guy watching you sees you travel at 60 km/h.. according to relativity, you're doing 60 km/h. Now perhaps it's just me.. but this seems just a little silly. Why should who's watching alter everything? It's like the old 'if a tree falls in the woods and nobody's around, does it make a sound?' the obvious answer being 'yes'. Afterall, falling trees always make noise.. why would they stop? So then according to relativity, if a tree falls in the woods, and the only person around is falling too, it doesnt make a sound because, at the perspective of the observer, the tree never fell over. *shrug* i can understand collapsing waveforms and the uncertainty principle, but observer-based math just doesnt make any sense to me.
    Dreamweaver
  • by johnynek ( 36948 ) <boykin@pobox.com> on Thursday May 27, 1999 @09:38AM (#1877391) Homepage
    Keep in mind that xxx.lanl.gov (where the paper is posted) is not a refereed journal. This paper
    may not have been subjected to any peer review, so it's contents should be taken accordingly. Many times serious corrections or withdrawals are made to this pre-print archive. It would probably be better to not publicize something like this until it has been read be many specialists.
  • The current issue of Scientific American has an article on Kurt Godel. Although most of his important contributions were in mathematical logic and theorem proving, he also did some work on relativity and spent a lot of time talking to Einstein. One of his contributions to relativity was a mathematical proof that time travel is not inconsistent with Einstein's theories of relativity. It might be inconsistent with something else we don't know about (or perhaps with something we've learned since then?), but it seems like we should keep our minds open.
  • Besides the points that other posters have brought up below, I think there is a specter lurking in the background. Logic and causality are really human constructions with deep roots in the mind. There's no conclusive proof that the universe itself fully obeys the laws of logic and causality we have inferred from local and limited observation (this is especially true at very large and very small scales), nor can there be. Our whole system of logical and causal thinking is local and essentially axiomatic, and may not ultimately have little to do with how things truly work. What we know about physics at this point certainly challenges even the most basic assumptions about causality, if not logic itself.

    What's more, even if the universe is somehow (from what little I know about quantum, I just don't see how) completely causal and logical, that doesn't necessarily make it any less mysterious. Formal logic has been shown to have some startling limitations. Kurt Godel's Incompleteness Theorem demonstrates beyond any doubt that there are True statements, in any sufficiently powerful logical system, which are unprovable. This essentially means that logic can be "undecidable" in some cases. There's nothing that says the universe itself can't have logic/consistency/decidability problems itself.

    BTW, I'm not just trying to be argumentative here. Locality of causality and logic is all a very real possibility. Just ask a serious physicist.

  • Would we have to sacrifice relativity for a fixed frame to measure FTL? One could certainly model an infinite n-dimensional system, whereby within any given dimension all measurements were relative to its cohabitants. A unified relative constant could exist for each frame and only become non-essential based upon the knowledge or a more precise fixed frame (the next layer out).

    Thus, any set of relative measures would still apply as long as you remained within a single dimension. The knowledge of the (next) absolute frame of reference would only make it Easier to develop physical law. It would not eliminate the system based upon relative physical law.

    Maybe that's a little more mathematics oriented than physics; however, I would be interested in any thoughts.

In Nature there are neither rewards nor punishments, there are consequences. -- R.G. Ingersoll

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