Slashdot Mirror
Warp Drive Breakthrough
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?
"
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.
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? site.
----
Open mind, insert foot.
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
-- "It's a sad day for American capitalism when a man can't fly a midget on a kite over Central Park" - Jim Moran
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:
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.
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.
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.
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
"I think any time you expose vulnerabilities it's a good thing." -Attorney General Janet Reno
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.
jabber: johnynek@jabber.org