Alcubierre warp drive
 

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.

Re: Alcubierre warp drive
 

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.

----------------------------------------------------
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.

Re: Alcubierre warp drive
 

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.

Re: Alcubierre warp drive
 

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

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.

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.

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

Re: Alcubierre warp drive
 

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.

Re: Alcubierre warp drive
 

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.

Re: Alcubierre warp drive
 

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

Re: Alcubierre warp drive
 

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...

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

Re: Alcubierre warp drive
 

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...

Re: Alcubierre warp drive
 

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

Re: Alcubierre warp drive
 

Huh. The Alcubierre thing is sounding much different than I had thought it was. I had heard stuff along the lines of it working like whatever drive Star Trek uses, in that you can still interact with the universe, but move at arbitrary pseudovelocities. I guess that's yet another example of why one shouldn't listen to the press, even the science press...

Frankly, the whole thing sounds more like what GURPS calls a Jump drive rather than a Warp drive.

TRAMLINES! A-la The Mote in God's Eye!

I think this sounds the most workable, eh? You'd have defined "routes", which means that you have a reason to have a starmap rather than just a chart of the locations of starts (which is rather, er, unromantic..). I actually really like the idea of tramlines, from a campaign viewpoint, because it allows for strategic systems where multiple tramlines converge. But I couldn't come up with decent technobabble to explain them, so I was pursuing the Alcubierre thing.

But you're saying that a ship couldn't carry it's own drive? In essence, there must be a "stargate" or "jump-station" or whatever? In which case, would you need a jump station at both termini, or only one? Or would you just set up this curved path once, then it just sits there and anyone can fly into it, working more like a stable wormhole than a jump-station? If so, could you "build" a pathway from one terminus, or would you need to send a ship in real space (sublight) to the other terminus?

Would the termini orbit? Or would they just sit there, ignoring the gravitational gradients around them? If the latter then the delta-v to get to them would be inconveniently high, right? (If there is a station involved then I suppose it must orbit, but what if there is no station?)

I'm ruminating on a campaign that is very hard science fiction except of course for the existence of a means of FTL travel. Sort of in the same vein as 2300AD.

Re: Alcubierre warp drive
 

So, to do this, at some point you'd need the ability to acquire an immense real velocity? In other words, just what sort of reference frame would allow you to observe that exiting warp occurred before entering warp?

Re: Alcubierre warp drive
 

Funny, because I was going to name my drive the "Alphabet Drive", for ABC, or Alcubierre-Broek-Casimir...

Oddly enough, I think I get the negative energy concept.

And I got the joke. Very droll. :o)

Re: Alcubierre warp drive
 

To be fair, Miguel Alcubierre's original idea did act a lot like the Star Trek warp drive, and the press picked up on this. Later, when other physicists looked deeper and exposed more of its limitations the press didn't pay much attention.

Other proposed FTL methods, such as wormholes or Krasnikov tubes, also have similar pre-defined routes that give you a star map.

A spacecraft very well might require some sort of "key" to enter a warp terminal. Perhaps it needs the key to put itself into a basement universe so that the warp line can warp the wormhole entrance to the basement universe. Otherwise the region being warped is less than the diameter of a proton - difficult for a spacecraft to fit into something that small. Or perhaps just anyone can go into a warp terminal and be warped to the other end. Since we don't know, what would you like for your setting?

Luke

Re: Alcubierre warp drive
 

That depends on how long the warp seems to take and how far you warp. Suppose we warped from Earth to a star 100 light years away. Suppose the time it takes in years is 1/10000th the distance in light years, so our journey takes 3.65 days. Since I don't know the answer yet, I'm going to work out the math. Let our departure event occur at position x0 = -100 ly and time t0 = -0.01 y and our arrival event is x1=0, t1 = 0.

Now assume that we can change our velocity in essentially zero time. I will use primes (') to denote the coordinates measured in the reference frame with a changed velocity. This gives us
x1' = 0
t1' = 0
and
x0' = cosh(eta) x0 - sinh(eta) t0
t0' = cosh(eta) t0 - sinh(eta) x0
The parameter eta is called the rapidity, and corresponds to a speed v = tanh(eta) * c. The function cosh is the hyperbolic cosine
cosh(x) = (exp(x)+exp(-x))/2
sinh is the hyperbolic sine
sinh(x) = (exp(x)-exp(-x))/2
and tanh is the hyperbolic tangent
tanh(x) = sinh(x)/cosh(x)
exp(x) means raise the base of the natural logarithm (often denoted e) to the power x (or e^x).

Since I don't feel like doing algebra right now, I wrote a quick program in c++ to run the calculations. For a speed of 0.0009999997 c (eta = 0.001, v ~ 300 km/s) I find that x0' = -100.00004 ly and t0' = 0.09000001 y. Since it still takes pretty near 0.01 year to get back, we would arrive at a time coordinate of t2'=0.01 y (and, of course, a space coordinate of x2' = x0' to a very good approximation, although to be rigorous we would include the change in position due to our relative motion). This is 0.08 y before we left, or about a month. So in this example you get back a month before you left (rigorously, we would transform from x2',t2' coordinates back to x2,t2 coordinates if you wanted to find out how long you had been waiting for your former self to jump in the frame of reference of the earlier you, but for these small velocities it doesn't make much of a difference, you still get back a month before you left in both reference frames).

In fact, in this example a change in velocity of little more than 60 km/s gets you back just before you left.

Luke

Re: Alcubierre warp drive
 

So on any rational scale you'd either need instantaneous acceleration or really long and fast tramlines to do this? Well, heck, THAT ain't gonna happen in my campaign, so I guess I won't have to worry about players screwing with causality...

If I'm willing to assume that the exotic matter problem has somehow been solved I assume that stable wormholes are also possible. And after all, I'd have to assume that for the Alcubierre drive, anyway. So, how would they differ, in practice?

I'll have to google Krasnikov tubes...

EDIT- I looked up Krasnikov tubes. Both they and wormholes seem to have immense potential for screwing with causality, don't they? The Alcubierre drive as you explained it, assuming that I understand it correctly, seems to be a bit more difficult to (mis)use in that manner.

Re: Alcubierre warp drive
 

Ok. To be sure that I understand this, in Alcubierre Drives for Dummies style:

A "tramline" gets made, and is a permanent structure.

(I'm still interested if this would most likely be a "jump-point" or if it would require a "jump-station." I'd prefer the former, but I would want it to orbit and can't think of a reason why they would. And if they don't orbit, what reference frame are they attached to?!? Would a given solar system drift away from them? And I suppose that it would be too much to ask for if these could be naturally-occurring structures. If they are man made, presumably at great expense and over the course of centuries as sublight ships make that first run, this leads to tramlines radiating out from Sol like spokes with no cross-links and precluding strategic systems (other than Sol) which is boring.)

The Alcubierre drive puts the ship into a pocket universe which is connected to our universe by a proton-sized "mouth", akin to a wormhole. Presumably, this is done by temporarily dilating this mouth open via some mechanism. It is this mouth that gets transported along the tramline at ludicrous speed, and when it arrives at the destination the mouth is again dilated and the ship emerges. Thus unlike a wormhole the transit time is something greater than zero.

Is this making "sense"?

EDIT- If the tramline termini are natural structures and they orbit a star, this is sounding oddly similar to the Ten Worlds setting:

http://www.adastragames.com/universe.html

But you'll have to dig around quite a bit to find the info on the setting- it is ratehr scattered at the moment. You might have better luck searching for the game, Attack Vector: Tactical. It's the most "realistic" space combat game I know of, and the Atomic Rockets website agrees.

Re: Alcubierre warp drive
 

Not so much instantaneous acceleration as the acceleration time is included in the travel time use for the example (such as 2.65 days warping and 1 day accelerating before warping back).

You can always say that the same thing that prevents wormholes from forming time machines works for warping as well - as soon as you are in a configuration in which a time machine is just barely possible, you get a sort of perfect resonator for quantum fluctuations that follow the time travel path and meet themselves to build up and amplify themselves to destructive levels. This not only prevents time machines but discourages people from trying.

In practice, the main difference would be that a wormhole wouldn't leave a physical artifact threading through the usual space we are used to between stars - the connection would occur through a bridge space-time that is outside the space-time that we would normally travel through. Thus, you wouldn't have to worry about an orbiting solar power station or something striking the tramline.

Also, think about the implications of a spacecraft being able to bottle itself up in a basement universe, connected to this universe only via an umbilicus the diameter of a proton. It would seem to be a near-perfect defense, for example. You could save a lot of propellant by not decelerating at your destination planet - just turtle up and plow into it, then dig yourself out (although at proton size you are likely to zip through the entire planet, so maybe this isn't the best example).

Warp tramlines likely operate on schedules - to get around the issues of information not being able to propagate from the inside of the warp bubble to the space-time in front of it (resulting in a metric pileup and singular surface, which gives you the nasty radiation problems) what you do is have the equipment on the tramline synchronized to pulse at the right time to give the effect of a superluminal disturbance traveling down the line without actually having to transmit the information superluminally. So if you have arranged for this to happen once per day, for example, you can only take the warp line once per day. With a wormhole you can go through any old time.

When forging new routes to distant unexplored stars, wormholes can potentially lead to wait times much shorter than what you would expect from the difference in coordinate time of sending a light speed signal and the signal arriving at the new star (you do this by exploiting the time dilation of the projected end of the wormhole, and the fact that the other end, the end left at home, is at rest with respect to the projected end when viewed through the wormhole). If you can do the same trick with a warp tramline, it is not so obvious.

As far as I can see, they all have the same potential for messing with causality - the method involved is the same in all cases and so the math tends to work out the same. As mentioned before, you can prevent this by invoking the causality protection postulate that destroys potential time machines before they form.

As an aside, I am getting a significant amount of deja vu from this discussion - Rick Robinson't Rocketpunk Manifesto blog recently features an entry on the limits of realistic FTL and I participated in the extensive discussion in the comment section. You might find reading through the comments interesting - we cover a lot of the ground you have been asking about, and you might get additional insights into how to structure your setting
http://www.rocketpunk-manifesto.com/...est-cheat.html
You might want to just skip through the parts where people digress to talking about the metaphysics of duplication, Herbert's kludge of shields in Dune, and other side-tracks, of which there are many. For example, you could start at this comment
http://www.rocketpunk-manifesto.com/...13686130748261
and not miss much.

Luke

Re: Alcubierre warp drive
 

Hopefully permanent, because they sure would cost a lot to make! But yeah, it would probably be intended to last quite a while.

I am not entirely sure what you see the difference as? Since this is an artificial structure with a definite entrance/exit point, a station seems a natural description.

The stuff that makes up the physical structure of the tramway will be following its normal geodesic path through spacetime (neglecting the forces that the stuff exerts on itself - if you need self forces to keep the tramway intact the tension built up over light years would break it). These geodesic paths are called orbits when the occur around stars, planets, and other things that can be approximated as a localized bit of mass. However, the tramway itself might be very massive - we already assume that it has a significant effect on space-time locally - which could affect the paths of things around it. A massive tramline of this sort might need to be located very far away from a star to keep from messing up the planetary system. Of course, you would also need to find enough mass to make the tramline in the first place. Maybe it is best to assume the tramlines are not that massive.

While not particularly real life plausible, you could always say that cosmic strings just happen to be natural tramlines. Then you just need to find some cosmic strings. For a fiction setting, I would swallow this.

I don't see why you wouldn't have cross-links. If you are 10,000 light years from Sol and want to get to another system that is only 5 light years away from you but on another trunk-level branch of the tree that is rooted at Sol, it would be a lot more convenient to go directly there (via a tramway that you convince people to build) rather than going through all those solar systems between you and Sol for 10,000 light years and then back up the other way. Since on a warp tramway you control the reference frames of things that enter warp, you can avoid those annoying time lags that wormhole networks bring (there are ways to deal with wormhole time lags and ways to connect up wormhole networks, too, but they involve different solutions from what you would use in warp networks. There may not be much of a practical difference, though).

For a tramline the transit time is greater than zero, yes. Wormhole transit times are also going to be something greater than zero (unless you are dealing with Visser wormholes, which have no thickness), although depending on the geometry the wormhole transit time could be quite short. Of course, depending on how you arrange your superluminal pulses on the tramline, the warp transit times may also be quite short. There is enough that is not known about the engineering of either of these modes of transit that for a fictional setting you could choose whatever you think is most interesting. Do you want warp travel, but with warp taking only a second? It is possible. Do you want wormhole travel but with 100,000 km wormhole tunnels that you need to go through (and enforced low entrance and exit speeds due to tidal effects)? Also possible.

Heh, yeah. I'm somewhat familiar with the Ten Worlds setting - I participate in the SFConsim group on Yahoo, where a lot of the Ten Worlds developers also participate. I've also got a copy of Attack Vector: Tactical on my shelf behind me as I type, although I never got around to actually playing the game.

Luke

Re: Alcubierre warp drive
 

How could a spaceship go through a Wormhole smaller than the width of a proton without being destroyed? I don't get it. It doesn't make sense. The link above doesn't work anymore.

Re: Alcubierre warp drive
 

Not sure if this helps you at all with understanding Alcubierre's idea, but here's his actual paper on it.

Re: Alcubierre warp drive
 

I think this covers it:

Re: Alcubierre warp drive
 

I read the paper. I understood most of the words, but the math means nothing to me. It might as be written in Martian for all it means to me. Please explain to me what it means in words. Why can't people just use words? I still don't understand how a spaceship could go into a wormhole without being crushed by gravity and everybody getting killed. I had to look up some words that I didn't understand.

Re: Alcubierre warp drive
 

Not that I understand the math myself, either, but I believe the suggestion was to design a drive system that uses a wormhole that you can make larger or smaller.

So:
(1) Create your pocket universe (in this context, I'd really rather call it "subspace" for obvious reasons :) with a big wormhole to normal space;
(2) Move the ship into subspace through the big wormhole;
(3) Shrink the wormhole to tiny size;
(4) Warp somewhere, enjoying the benefits of your not-quite-entire-bubble with tiny connection;
(5) At your destination, expand the wormhole again to get the ship out of subspace and back into normal space.

Re: Alcubierre warp drive
 

In Words

Because math is the language of science.


You are creating a bubble of compressed spacetime. The best way I can think to explain this is to think of a number line:

|..........|..........|..........|..........|..... .....|..........|..........|

Each tick is a gradation on the number line. You might have something that has a dimension of 3 ticks:
.......................___________________
|..........|..........|..........|..........|..... .....|..........|..........|

Now you might use some exotic material or some such to compress spacetime locally, decreasing the space between ticks, but retaining the length of in number of ticks:

......................____________
|..........|.........|......|......|......|....... ..|..........|

The line is still 3 ticks long. It is still in the same location: starting at the third tick. The space it exists within is simply curved so that, relative to the rest of the space, the line is shorter without actually changing any of the line's properties - it remains 3 ticks long.

Compress your bubble small enough, and it just might fit through your wormhole.

Incidentally, this is the way in which spacetime is already curved by gravity. This probably isn't the best analogy, but it's better than just throwing out a trite "compress spacetime" as an answer. I hope it helps.

Re: Alcubierre warp drive
 

Also because it often takes a *lot* longer to say with words. Compare "Energy equals mass times the square of the speed of light" to "e=mc^2"; that's a very simple bit of math to put into words, and it still takes almost eight times as many characters.

Re: Alcubierre warp drive
 

I did understand it in words. I don't care that math is the language of science, it's extremely elitist. Alan Alda is right when he says that scientists can't communicate with average people. But I still don't know how the warp bubble can shrink and fit into a wormhole. The paper written by Alcubiere doesn't mention how to shrink it.

Re: Alcubierre warp drive
 

The problem is that in order to properly do science you very often need the math. It's a necessary part of both testing hypotheses and developing speculative models. It also cuts across international barriers; everyone who understands the math can understand an equation, regardless of the language they speak.

Imagine talking about GURPS without using any GURPS terms. Possible in some cases, yes, and even necessary starting out. But if you're getting into a discussion of, say, whether modifiers for Appearance apply to Fast Talk rolls, it would certainly expend a lot of time and effort and would probably not be possible. I don't see how that's "elitist."

Not that I disagree (and his annual contest to explain a scientific concept in layman's terms is a spectacular idea), but I don't think the average person is very good at talking about their job or hobby to outsiders.

That wasn't in the paper. That was something come up with later (possibly by lwcamp in this very thread) to make the revised version of Alcubierre's hypothesis workable as a means of transport.

Re: Alcubierre warp drive
 

It's because mathematical expressions are designed to denote the sorts of relationships among their elements that turn out to be important in physics (and some other sciences), whereas sentences, being linear, are not. That makes it much, much easier to describe those sorts of relationships in explicit detail, unambiguously, and compactly if you use an mathematical expression.

Re: Alcubierre warp drive
 

It's hardly elitist to use the tools of your trade to practice your trade. That paper is one scientist speaking to other scientists about their trade. Would you expect your doctor to converse with your surgeon about your operation using phrases like "the vein is the one on the inside of his forearm, but not the big one. The little one that tucks under the inside of his elbow and branches three times before reaching his wrist. You're going to have to put the balloon thingy in there to break the wall of blood, but make sure no bits get in his noggin because then he might get brain-bleeding and die."

As for physics using mathematics to explain things, it's because math lets you lay out definite and concise relationships and, more importantly, make predictions that can later be tested. Words don't do this.

For example:

"Gravity only acts downward"
"An object in motion stays in motion until otherwise acted upon by a force"
"Acceleration is the time-rate change of velocity"
"Velocity is the time-rate change of position"

That's four bits of rudimentary high school physics in words. Now tell me how far downrange a shell fired with a muzzle velocity of 800 m/s at an elevation angle of 23 degrees while sitting on a 12-foot-high hill above a plain, neglecting friction. You can't without using math:

d = [v^2/(2g)]*[1+sqrt(1+(2gh)/(v^2*sin^2(theta)))].

And you can't get that formula, if you don't first describe gravity mathematically: g = -gj = [dv/dt]*j, velocity mathematically: v = x_doti + y_dotj = ds/dt, and finding useful relationships between them:

v_f^2 = v_i^2+2as
s_f = s_i + vt + 0.5at^2
v_f = v_i + at

Now you can actually start to dig into science. And if this process seems difficult, all I can say is that there's probably a reason some jobs require more schooling than others. If you put in the time to learn some basic math, you can follow a great deal of physics, and the bits that require stuff like set theory, tensor calculus, and differential geometry aren't readily explained in anything less than broad brush strokes.

His paper isn't about shrinking things. It's about a means of superluminal travel.

Re: Alcubierre warp drive
 

Then why was shrinking the ship and going into it mentioned?

Re: Alcubierre warp drive
 

Re: Alcubierre warp drive
 

Because that's one approach to envisioning the effects of the math in a later paper -- not Alcubierre's original one, so Alcubierre wouldn't have mentioned it, but follow-up work done by others*, inspired by Alcubierre's result. That paper showed a dramatic decrease in the amount of energy (or mass, if you prefer) required to form a warp by using a shape of spacetime that has that wormhole mentioned earlier in the thread. A tiny wormhole works better than a larger one for the purpose of warping, apparently the tinier the better as long as it still exists. But that leaves you with the problem of how to get the ship through that tiny wormhole.


--
* Chris van der Broeck, "A `warp drive' with more reasonable total energy requirements". Sergui Krasnikov followed up that paper with a modification that reduced the requirement to a few milligrams. The infamous Harold White also had a paper that showed a reduction in matter requirements down to about 1000 kg.

Re: Alcubierre warp drive
 

Harold White is infamous? Why? I've read about him.

Re: Alcubierre warp drive
 

His EM Drive would be a dramatic game changer if it works, but very few in the field seem to believe it works.

Re: Alcubierre warp drive
 

Infamous means being well known for something bad (in this case, controversial claims of creating a reactionless drive), it doesn’t mean “not famous.”

And this is an interesting thread, going to reread Luke’s post to see if it finally helps me understand the whole “FTL = time travel” idea.

Re: Alcubierre warp drive
 

Near as I can tell (I'm not a physicist; my sister has a double masters in Math and Physics, though), the whole "FTL = time travel" idea comes from Einsteinian physics and time dilation where as you approach the speed of light in a vacuum (c), observed time (t)slows down. Exceeding c (such as with the hypothetical tachyon particles) means that t is a negative. How that meshes with quantum mechanics or the proposed Alcubierre drive, I have no clue.

And there's also the idea that plotting a wormhole means that you're creating a pathway between points A and B means putting B where you can observe it, not where you calculate it to really be. For example, calculating a wormhole between Earth and Alpha Centauri means making a 4.3 lightyear trip, which at c takes 4.3 years, but puts you where Alpha Centauri is in the sky, or where it was 4.3 years ago, and hence putting you 4.3 years in the past.

Of course, some settings can (and should!) ignore this by saying that computations of the destination include taking proper motion and radial velocity of the target location into account, so that when you reach Alpha Centauri you are roughly where it really is when or after you left, or will be when the trip ends (if said trip is to take 3 years, where it will be three years from departure).

Re: Alcubierre warp drive
 

Not quite. It comes from the offset term in the Lorenz transformation for t, not from the time dilatation term. It's easiest to explain with simple algebra or a space-time diagram, and you have to familiarise yourself with the Lorenz transformations first.

It's best not to try to guess what the physics is here. With all due respect, if you haven't studied it, please don't try to explain it.

Re: Alcubierre warp drive
 

Give this page a try. It has some space-time diagrams that show how to construct a causality-violating loop given FTL comms.

Re: Alcubierre warp drive
 

It's basically a side effect of frame invariance. In relativity, if two events occur at different locations, two different inertial reference frames can disagree about which event came first. Thus, if you have a means of changing from one reference frame to the other, and you can travel between those two locations in less time than the disagreement, you can travel backwards in time. The disagreement, if you pick your frames correctly, can be any value less than the time it takes for light to travel between the two locations.

Re: Alcubierre warp drive
 

I think this page gives a fairly clean and understandable explanation for why FTL travel/communications violates causality. At least, it's the one that finally helped me get my head around the idea.