Alcubierre warp drive

[acrosome]

Can anyone here speak intelligently about the Alcubierre drive, in game terms? Assuming, that is, that we ignore the energy requirements, zero mass, and such?

What would space look like as viewed from an Alcubierre driven ship at subluminal and supraluminal pseudovelocities? If you can see out of the warp field, can you fire lasers out of it? Can it send and receive radio transmissions?

What happens if you steer one into a planet or other large mass? For that matter, what happens to all of the particles in the space in front of it? Do they decay into various forms of radiation as the space is collapsed?

Can they turn arbitrarily fast, or is the drive polarized to unidirectional travel?

Would such a ship have the same ability to acquire an enormous true velocity by warping just enough to hold steady over a planet, then destroy the planet by turning off the drive and impacting it at an arbitrarily large fraction of c?

And what's this stuff about using it as a time machine?

I'm intensely curious about this drive. A forum search for "Alcubierre" had a few hits, but not the information I'm interested in. lwcamp gave some hints once, but not a lot.

[robertsconley]

The view around the ship would be distorted as light passes through the curved time-space around the ship. Essentially the ship is flying blind unless a means of compensating for the distortion of incoming radiation to produce clear sensor images. There also an issues of steering the ship.


There is a modification of the Alcubierre warp drive, called micro-warp, that proposes to fold space-time around the mass in such a way that the bubble of normal space around the ship is only connected by a small tendril. This makes the energies needed to move a mass through the technique several order of magnitude less. As you only need to form the warp around the opening.

http://www.npl.washington.edu/AV/altvw99.html

I used this to come up with a theory of Traveller Jump Drive.

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Kaukji Hyperspace Shunt or KHS Drive is an outgrowth of continuing research into control of gravity. The manipulation and control of gravity became possible in 2071. When this occurred researchers realized that several avenues of achieving Faster Than Light (FTL) speeds became possible. The late twentieth and early twenty-first century saw the emergence of several FTL theories that remain consistent with Einstein's Theory of Relativity. All of them had massive engineering problems to overcome or required the generation and control of massive gravity fields.

The KHS drive consistent of a gravity generator capable of generating intense gravity fields. To produce these fields the generator needs to be coupled with a special fusion plant design that required to output massive amount of power in a short amount of time.

In addition raw mass was needed to maintain the real space bubble that the ship rides in while in hyperspace. This was found to equal an amount of hydrogen equal to 60% of the desired volume. As the control hardware and software improved this was later reduced down to only 20% of volume. In addition a minimum volume of 100 displacement tons of hydrogen was found to be the lower limit of a stable real space bubble. Smaller bubbles tended to destabilize and collapse.
----------------------------------------------------

Basically there were stable configurations that could be created by folding spaces around a mass and that they would decay in about 152 hours more or less. When the field is created the mass would begin moving in the preset direction faster than light.

There were 36 levels that produced travel distances of to 36 parsecs. Of the 36 only the first 6 were naturally stable. The energy profile need to create the Alcubierre fields for the first six were like a trough, slight variations and errors will cause the field to "roll downhill" to the stable configuration. Above 6 the energy profile is like a hill where slight variation and errors would roll off the stable point producing gross misconfiguration resulting in a misjump.

Each level has a smaller and tighter "opening" allowing the energy of the fusion reactor in the jump to create a more extreme warp causing the mass to travel farther.

One reason I adopted this for traveller that it makes jump drive the result of grav technology which seems more elegant to me. Rather than the "alternate hyperspace" of canon.

[David Johnston2]

Originally Posted by acrosome

Can anyone here speak intelligently about the Alcubierre drive, in game terms? Assuming, that is, that we ignore the energy requirements, zero mass, and such?

What would space look like as viewed from an Alcubierre driven ship at subluminal and supraluminal pseudovelocities? If you can see out of the warp field, can you fire lasers out of it? Can it send and receive radio transmissions?

What happens if you steer one into a planet or other large mass? For that matter, what happens to all of the particles in the space in front of it? Do they decay into various forms of radiation as the space is collapsed?

Can they turn arbitrarily fast, or is the drive polarized to unidirectional travel?

Would such a ship have the same ability to acquire an enormous true velocity by warping just enough to hold steady over a planet, then destroy the planet by turning off the drive and impacting it at an arbitrarily large fraction of c?

And what's this stuff about using it as a time machine?

I'm intensely curious about this drive. A forum search for "Alcubierre" had a few hits, but not the information I'm interested in. lwcamp gave some hints once, but not a lot.

What I have heard is that the occupants of the warp bubble are causally disconnected from it and the universe outside. Therefore, no steering, no shooting weapons, and probably no colliding with things either. As for "acquiring an enormous true velocity", one of the issues with Alcubierre drive is that it ain't cheap. Realistically, it's prohibitively power hungry, so much so that this problem would have be hand-waved in gaming because it's impossible that it would ever actually be used. Since that's no fun we can cheat on that, but even then, Alcubierre Warp Drive still requires the consumption of "exotic matter" a substance which arbitrarily has the right characteristics to make the idea work. Exotic matter isn't easy to find, seeing as how it's imaginary. Even if it could be made to exist, it wouldn't be cheap, so there would probably be cheaper ways to devastate planets than using it to build up large amounts of real velocity for a kamikaze attack.

[lwcamp]

Originally Posted by acrosome

Can anyone here speak intelligently about the Alcubierre drive, in game terms? Assuming, that is, that we ignore the energy requirements, zero mass, and such?

How can we talk intelligently about it if we ignore its properties?

Originally Posted by acrosome

What would space look like as viewed from an Alcubierre driven ship at subluminal and supraluminal pseudovelocities? If you can see out of the warp field, can you fire lasers out of it? Can it send and receive radio transmissions?

The classic Alcubierre space-time geometry would allow light and radio to pass in and out. Unfortunately, this geometry required a mass greater than that of the observable universe. Van der Broek and others have refined the warp drive by only warping the microscopic mouth of a wormhole that connects to a "basement universe", which can get by with much less mass (since you presumably want to warp a spacecraft, you put the spacecraft in the sub-universe and use the wormhole to get in and out - presumably you have some way of expanding the wormhole at your trip origin and destination). However, since the thing being warped is smaller than the diameter of a proton, light and radio would have a hard time getting in or out.

Originally Posted by acrosome

What happens if you steer one into a planet or other large mass? For that matter, what happens to all of the particles in the space in front of it? Do they decay into various forms of radiation as the space is collapsed?

In the classic Alcubierre version, the world-lines of particles could pass through the warp metric - although as far as I know the effects of the strong gravity of the warp bubble on matter passing through it has not been worked out. In the more practical extensions of warp drives, the sub-proton size entrance to the warp pocket would presumably present a very small target for matter to get into.

Originally Posted by acrosome

Can they turn arbitrarily fast, or is the drive polarized to unidirectional travel?

More and more, it is looking like warp drives can only warp along prepared "railways to the stars", and likely these railways only warp on a certain schedule. Thus, you would take your spacecraft to the "station", enter a warp chamber, and then at a pre-arranged time space-time folds around you and a superluminal wave of space-time distortion shoots you to the exit station.

What has been determined is that a superluminal warp drive is causally disconnected from the rest of the universe (EDIT: except along the pre-prepared, pre-scheduled railway mentioned above). You can't steer and you can't turn it off from the inside. Perhaps you could arrange for it to turn off upon warping for a certain distance. But you run into another problem - the front surface will produce radiation of remarkable intensity that is impinging on your spacecraft. As a result, classic Alcubierre geometries likely result in your spacecraft getting vaporized in short order unless you only go at sub-light speeds.

There is another issue - local conservation laws - which I will discuss in another reply due to length constraints on posts

[lwcamp]

Originally Posted by acrosome

Would such a ship have the same ability to acquire an enormous true velocity by warping just enough to hold steady over a planet, then destroy the planet by turning off the drive and impacting it at an arbitrarily large fraction of c?

I am going to have to get technical here, and lay out some terms, some theoretical results of general relativity, and the consequences that follow from them. Bear with me - understanding these will help you understand the restrictions on warp drives.

In general relativity, energy and momentum and angular momentum are well defined and local in the weak field limit. The weak field limit is basically Newtonian gravity (with a few extra bells and whistles, such as gravitational waves). So when you are in a region where gravity is approximately Newtonian, then general relativity predicts that energy, momentum, and angular momentum are not only conserved, but conserved locally. That is, if you take any arbitrary closed surface, the energy & etc. inside that surface only changes by the amount of energy & etc. that goes in or out of that surface.

But what about warp drives? Warp drives are definitely not in the weak field limit. They distort space-time so much that you are far beyond Newtonian gravity. Do you still have to worry about the conservation laws? Well, the weak field result still holds if you can surround the strong field region with a closed surface where everywhere on the surface remains in the weak field limit (even if the surface is very far away). Inside that surface, energy & etc. might not be localized but it will not change without passing through the surface. So lets look in sequence at what this means for energy, momentum, and angular momentum (with increasingly depressing results as we go on).

Energy first. Usually when we think about a warp drive, we envision a spacecraft that is not yet warping sitting in a region of normal space-time (say, around a planet or star, or even around a black hole or neutron star since if you get far enough away you can enclose the black hole or neutron star with a surface that is everywhere in the weak field limit). Then the spacecraft does its warping thing, moves someplace else, and turns off its warp to again become a normal, non-warping body in a region of normal space-time. Now, if we enclose the entire path taken by the spacecraft in one of our imaginary surfaces, far enough away that any warping effects have damped away to the weak field limit, then we can see that the energy of the spacecraft is unchanged minus any radiation it produced in the operation of the warp drive. This is probably no surprise. By playing around with these surfaces (such as a co-moving surface surrounding the warp bubble but out in the weak field limit) you can probably convince yourself that the energy travels with the warp bubble. Since mass is equivalent to energy that isn't associated with motion, you can think of the warp bubble as having mass.

Next, momentum. Momentum has a direction as well as magnitude, and that direction is also conserved (mathematically, we say that it is a conserved vector). A force is a rate of change of momentum, and corresponds to an exchange of momentum between two things. Classically, an object's momentum is its mass times its velocity. So let's go back to warping from empty space to empty space. There are no forces, so since momentum is conserved the spacecraft ends up with the same momentum before and after the warp. Since the mass is unchanged by the above argument (neglecting radiation) then the spacecraft ends up with the same velocity as before. So now we can answer your question about hovering over a planet. There are two ways of looking at this - the Newtonian viewpoint of the planet exerting a force on the warp bubble's mass (increasing the warp bubbles momentum toward the planet) or the Einsteinian version of the spacecraft being in free fall so that it keeps warping forward in such a way to keep its distance from the planet constant. In both cases, the spacecraft's momentum, and hence its velocity when it finally turns off its warp drive, are constantly increasing in the direction of the planet. If this continues the spacecraft can build up arbitrarily large velocities. Note, though, that since the spacecraft's kinetic energy is also constantly increasing and energy is conserved, this energy has to come from somewhere such as the fuel to run a nuclear reactor. Once the energy in that fuel is exhausted the spacecraft cannot continue to hover since doing so would increase its energy further and there is no energy left. This ultimately limits the available velocity - although with nuclear fuels available you can still get nuclear bomb size explosions. Note also that this means when you go away from a planet you must use power and when you warp toward a planet you gain power from your warp drive.

Now for the really depressing one - angular momentum. Angular momentum is also a vector (classically, anyway - relativity adds some additional components that are hard to visualize, but I'll just stick with the classical 3-vector version). Angular momentum is defined as the magnitude of the momentum times the perpendicular distance from the observer to the object (in a direction mutually perpendicular to the momentum vector and the direction from the observer to the object). If something isn't moving, it has no angular momentum (assuming no internal motions - a rotating object has angular momentum but not everything with angular momentum needs to be rotating). If something is moving but is headed straight toward you or straight away from you, it also has no angular momentum. The further its distance of closest approach becomes (assuming a straight line trajectory) and the faster it is moving and the more massive it is, the greater its angular momentum. So, consider an observer moving toward the spacecraft, and then have the spacecraft warp away in a direction perpendicular to the direction to the observer. From the observer's point of view the spacecraft starts off with non-zero momentum (it is moving in his reference frame) but zero angular momentum (it is going straight toward him). After the warp we know that it must be going the same speed and direction from conservation of momentum, but now it is not headed straight toward the observer. Consequently it has non-zero angular momentum. Since you can always find a frame of reference that does this, free-form warping violates the conservation of angular momentum and is thus disallowed in general relativity.

How do we get around this? I can think of three ways. First, the spacecraft can dump all of its energy when it turns on its warp drive. Without energy it has no mass and thus neither momentum or angular momentum. It leaves the energy behind in some form - matter or radiation (if it is in the form of radiation, you get the release of 20,000 megatons of energy per ton of spacecraft mass). Then, when it gets to its destination it absorbs as much energy as needed to precipitate the spacecraft out of warp (that is, a 100 ton spacecraft would need to have its warp bubble gobble up 100 tons of mass at its destination in order to release the spacecraft). The spacecraft will end up with the velocity of the mass the warp field gobbled up. Until the spacecraft precipitates out of warp it cannot exchange energy with the rest of the universe (doing so would add or subtract mass to the warp bubble, which would then violate angular momentum conservation). This probably means the spacecraft is flying blind and will have a difficult time finding the mass it needs to exit warp.

Second, you can have a prepared pathway of highly curved spacetime, and all warp travel is along this path. Since the spacetime geometry of the pathway is highly curved, we are outside of the Newtonian limit and angular momentum (and energy and so on) is not localized along the path. This lets you go from one point on the path to another, but you can't warp to places off the path.

Third is that the warp drive emits some sort of radiation which never decays away to the weak field limit. This last possibility is not very good for adventure fiction since strong field gravitational radiation smashing through the universe out to arbitrary distances is rather hard on the setting.

[lwcamp]

Originally Posted by acrosome

And what's this stuff about using it as a time machine?

Jargon time: In relativity, an "event" is a spatial location at a given time. Two events are said to have a space-like separation if light does not have time to travel between them (meaning entering and exiting FTL, such as via a warp bubble, always has a space-like separation). If light has just enough time to get between them, the separation, or "interval" is said to be light-like, and if light has more than enough time to get between them the interval is said to be time-like.

Now the details. All observers can agree that for events with a time-like separation, that one event occurred before the other. This gives you an unambiguous past and future. However, the same is not true for a space-like separation. If one observer thinks that in his reference frame one event happened before the other, another observer will think they happened in the opposite order and a third observer will think they happened simultaneously. By choosing your frame of reference you can choose what order space-like events happen in your coordinate system.

So, start at Sol and warp to Sirius. Entering and exiting warp are events with a space-like separation. In your frame of reference exiting warp is either simultaneous or after entering warp - not a problem yet. Now when you are at Sirius, change your velocity by a lot so you are in a new reference frame (perhaps using the hovering method described earlier). Get into a reference frame where exiting warp happened a lot earlier than entering warp. Now warp back to Sol. You arrive before you left. If you want to be contrary you could try to cause a paradox, such as trying to ambush yourself and blow your other-self up before he leaves so you could never get back to blow yourself up.

[lwcamp]

Originally Posted by David Johnston2

Alcubierre Warp Drive still requires the consumption of "exotic matter" a substance which arbitrarily has the right characteristics to make the idea work. Exotic matter isn't easy to find, seeing as how it's imaginary.

Not exactly. Warp drives require regions of space-time with negative energy densities (according to some observers. Due to the way properties transform according to the observer, in some frames of reference this just means that the tension is greater than the energy density). If you have "exotic matter" with negative energy it will work. However there are ways to manipulate fields such that you get the desired property, you don't need "matter". The most famous of these ways is the Casimir effect, which has a negative energy in the vacuum between two parallel conducting plates. The event horizon of a black hole also has a negative energy density. So we know of two ways to get the type of effect we need, although of a magnitude far too low to get a warp drive to work. Perhaps there are other ways as well?

Nit - exotic matter is negative, not imaginary. Imaginary is what you get when you take the square root of a negative. ;-)

(For the humor impaired, the last line was a joke.)


Luke

[Johnny1A.2]

Thanks to Luke for a fascinating set of posts! :thumbs up:

That said, the twisted part of my brain can't help but read this one bit here...

Originally Posted by lwcamp

Van der Broek and others have refined the warp drive by only warping the microscopic mouth of a wormhole that connects to a "basement universe", which can get by with much less mass (since you presumably want to warp a spacecraft, you put the spacecraft in the sub-universe and use the wormhole to get in and out - presumably you have some way of expanding the wormhole at your trip origin and destination). However, since the thing being warped is smaller than the diameter of a proton, light and radio would have a hard time getting in or out.

...and have this sudden image of Rasilon on Gallifrey reading it from a distance and and thinking: Hmm...they're starting to 'get' it!

[acrosome]

Originally Posted by lwcamp

How can we talk intelligently about it if we ignore its properties?

OK, oh semantic one. I meant something along the lines of "If I could hand-wave away the qualities that make it potentially impossible..." Any time you're talking FTL you've got to handwave something.

Great stuff, though...

[lwcamp]

Originally Posted by Johnny1A.2

and have this sudden image of Rasilon on Gallifrey reading it from a distance and and thinking: Hmm...they're starting to 'get' it!

Hmmm, bigger on the inside than out, capable of traveling across space and, by traveling across space, through time as well. Now if only general relativity limited them to looking like blue police boxes ...

Luke