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Old 03-12-2011, 08:42 PM   #11
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Default 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.

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Originally Posted by lwcamp View Post
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.
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.

Last edited by acrosome; 03-12-2011 at 09:07 PM.
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Old 03-12-2011, 08:46 PM   #12
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Default Re: Alcubierre warp drive

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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.
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?
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Old 03-12-2011, 08:49 PM   #13
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Default Re: Alcubierre warp drive

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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?
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)

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Old 03-12-2011, 09:06 PM   #14
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Default Re: Alcubierre warp drive

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Originally Posted by acrosome View Post
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...
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.

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

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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? Or would you just set up this curved path once, then it just sits there and anyone can fly into it? 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?
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?

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Old 03-12-2011, 09:47 PM   #15
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Default Re: Alcubierre warp drive

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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?
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

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Old 03-13-2011, 08:22 AM   #16
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Default Re: Alcubierre warp drive

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

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Old 03-13-2011, 08:39 AM   #17
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Default Re: Alcubierre warp drive

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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?
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.

Last edited by acrosome; 03-13-2011 at 08:55 AM. Reason: add comment
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Old 03-13-2011, 11:05 AM   #18
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Default Re: Alcubierre warp drive

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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...
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.

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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?
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.

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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.
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.

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Old 03-13-2011, 11:36 AM   #19
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Default Re: Alcubierre warp drive

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

A "tramline" gets made, and is a permanent structure.
Hopefully permanent, because they sure would cost a lot to make! But yeah, it would probably be intended to last quite a while.

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(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.
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.

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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.
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.

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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.)
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).

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Originally Posted by acrosome View Post
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.
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.

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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.
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
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Old 03-14-2018, 08:47 PM   #20
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Default 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.
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