"Viable" FTL tech?

[Tolya]

I've been researching the "viable" ways of attaining FTL, from a space opera point of view, when a plot that requires getting to superluminal speeds, so you can't use the normal subluminal methods of travel.

1) Jump gates are perhaps the most easy for suspending disbelief, since they operate on "we don't know therefore we don't have to explain basis". Putting a jumpgate somewhere however does usually require for network constructor's to travel there. Then, they have to be sufficiently advanced to be apple to put them together (like WH40k does). Alternatively, you can put that burden over to some long lost (or not) superadvanced civilization (like Stargate does).

2) Hyperspace/subspace. So, you're not getting superluminal speeds here, but supposedly there is an alternate dimension that you can go to where the traditional relativity does not apply, therefore you can get superluminal speeds easily. Like Starwars does. Or Descent Freespace.

3) Warp drives. These were perhaps the most technobabbled methods of transportation in history, since they rely on ideas like bending the space, making two points closer to each other, thus eliminating the need for actual superluminal speeds. You just make the trip on normal, subluminal speeds over a smaller, distorted distance. This is Star Trek.

4) Quantum drives. Now, people are getting excited over the quantum entanglement phenomena. Right now, you can't send any information through quantum entanglement, therefore it also eliminates the possibility of superluminal speeds, like instant teleportation - if you can send information, you could also send mass. Getting anything out of it with the current state of science is equal to a warp drive technobabble or any other option.

I am looking for the least silly option of attaining FTL speeds, without using option numer 1. The actual speed here is not important, I just want a viable method (taking weeks) of travelling between star systems. Options 2 and 3 from scifi standpoint are not bad, but as I understand, they require enormous energy levels, especially 3.

Any opinions? I am looking for something that does not, at least at first glance, turn on "the bullshit alarm". Is that even possible, considering Einstein still stands undefeated?

[Borgholio]

Jumpgates are probably the most realistic form of FTL you will find. Two locations generating a wormhole between them for ships to travel through, as opposed to warping space. The problem of needing jumpgates on both ends of the wormhole can be solved in one of three ways. First, just require a construction ship to slow-boat it to the next star system and build a jumpgate. The ship could be fully automated or have a crew in hypersleep. Travel to a new system at nearly light speed, build a jumpgate, head back to base for more parts / crew rotation, jump back to the newly connected system then head from there to the next closest system. Rinse and repeat. Second, allow temporary wormholes to be generated from one point to another, allowing the construction ship to get there faster but still needing to finish construction before returning. Finally, do it like Babylon 5 where any large ship can make it's own jump point, but special ships like the Explorer class are used to build jump gates for smaller ships to use.

I'd say go with the first option. It makes more sense as far as realism goes, it doesn't break as many laws of physics, and it allows plenty of plot devices revolving around the gates themselves or the construction ships arriving in a brand new system and finding something unexpected there. It's also the least cliche.

I think you'll find that the idea of creating a wormhole between two locations is the most scientifically viable of all the methods you listed. If you insist on not using jumpgates or wormholes, then I'd say do something unique...such as mass lightening. The trouble with going to lightspeed is that your mass increases exponentially. What if it didn't though? You'd be able to exceed the speed of light if you could control your mass. Invent something along those lines and you'll have a non-cliche method of FTL travel.

[Channel72]

Warp drives are probably the most "realistic" (for a certain minimal value of "realistic"), since they are the only fictional FTL technology for which there exists a speculative mathematical model which doesn't violate general relativity. Of course, fictional warp drive (at least the Star Trek variety) requires the existence of subspace, whereas the actual model for an Alcubierre warp drive relies on generating enough energy to contract and expand spacetime around the spacecraft. But the general idea of creating a "warp bubble" is really the only FTL technology which is even remotely grounded in any actual mathematical model. You can then use suspension of disbelief to handwave away the absurd energy requirements and other engineering difficulties.

[Irbis]

Warp drives are probably the most "realistic" (for a certain minimal value of "realistic"), since they are the only fictional FTL technology for which there exists a speculative mathematical model which doesn't violate general relativity.

I was under impression Georg Riemann (mathematician) also proposed relativity-free problem model for his sort-of wormhole hypothetical way of travel, Riemann Cuts. Sadly, don't have book that discussed this on hand, so can't provide references, though it looked very solid.

[Blayne]

Before Mass Effect came out I had thought of an idea of where the ship manipulates its gravity to travel along the curvature of space time with its speed relative to the gravitational bodies "near" it, so in deep space between suns you're traveling several times the speed of light, but within a star system you slow down something more along Star Trek's impulse speeds, since the drop off is exponential you can't use it to crash into planets as a hyperrelativistic weapon.

You could also ditch the difficulty of justifying interstar travel by recalibrating the story to be interplanetary instead, as there's already some good theoretical systems out there that are high impulse/high thrust that if we had a few generations of the technology could allow a 5 day round trip to and from Mars; just make up a star system with 100 planets like Firefly and weird solar orbits and your set, heck you can even simulate it using that universe simulator available on steam.

[Simon_Jester]

I think you'll find that the idea of creating a wormhole between two locations is the most scientifically viable of all the methods you listed. If you insist on not using jumpgates or wormholes, then I'd say do something unique...such as mass lightening. The trouble with going to lightspeed is that your mass increases exponentially. What if it didn't though? You'd be able to exceed the speed of light if you could control your mass...

Only as a physics-for-poets thing. It's more complicated than this if you try to do it seriously. What you really need isn't "reduce your mass," it's "press this button to turn off relativity." Just reducing mass isn't going to cut it; photons have zero rest mass and travel at exactly the speed of light.

Jumpgates of various kinds are the way to go if you want to preserve physical plausibility, because they don't create a causality problem. All objects passing through jumpgates remain in the same inertial frame, plus or minus very small factors.

If you're dropping jumpgates, then your best bet is to simply ignore that whole "relativity, causality, FTL, pick any two" rule and do whatever you please.

NOTE:

You COULD play causality games with jumpgates, but it's easy enough in fiction to avoid creating a situation where this would arise. The problem doesn't become conspicuous.

Whereas when someone like David Weber has fleets approaching each other at something like 30% of light speed, interacting via FTL signals that propagate at 64c... that SHOULD create causality paradoxes pretty easily]

[Borgholio]

You COULD play causality games with jumpgates, but it's easy enough in fiction to avoid creating a situation where this would arise. The problem doesn't become conspicuous.

In Contact, Ellie was away for a total of 18 hours according to her instruments, but in fact she was out of contact with Earth for a fraction of a second. They played that really well in the movie.

[Baffalo]

If you want to justify using a gate without an anchor on the other end, just use the new Lost in Space movie as a template: hyperdrive works assuming you know exactly where you're going. If you anchor one end of your wormhole and let the other end 'drift' in the general direction you want to go, if you wait until it's relatively close to the target location, you could shave off quite a bit of time off the trip. It isn't perfect because you still have to justify spending time in normal space, but your distance is now close enough that instead of spending a few years of travel between stars, you just have a few months or maybe even a few days. If the ship doing the traveling has a second wormhole anchor in pieces ready to be thrown together, you could open up several stars a year with relative ease.

From what I remember, Wing Commander had a FTL system in which jump points were calculated between stars based on navigational beacons instead of actual jump gates. If you were at the jump point and knew where you were going, you could jump even with strike craft. You would still need to accurately plot the data, but given that you're working with two fixed points of space, you could make the calculations infinitely easier. Not perfect, but it's a start.

[Stark]

If you're worried about people getting cross at what name you give your plot magic, you've got bigger problems than the difference between hyperdrive and shunts.

[someone_else]

I'll link to a guy that seems to have a PhD that talks about this stuff and some interesting tricks and side effects. Luke Campbell, in this (and a few following) comments in a blog post about torchships.

For your convenience (and in case the source dies for some reason) I'm posting here the relevant comments. It's longish, but a cool reading imho. Feel free to point out mistakes if you can.

Rick: I'll haul out my PhD in physics and the work I've done in general relativity to mention that wormholes, warp drives, and Krasnikov tubes are viable solutions of Einstein's equations of general relativity. They require some rather odd conditions, namely regions of space-time with negative energy densities. We know this is not unphysical, since there are odd cases we know or strongly suspect exist with negative energy densities (black hole event horizons, the Casimir effect between nearby conducting surfaces). The fun stuff tends to require an awful lot of negative energy, but the amount needed tends to keep getting smaller with more research.

A few highlights of the various space-warping methods:

Wormholes are shortcuts through space-time. One end of a wormhole connects on another end, and going through takes you somewhere else in space and time. Wormholes are two way - you can go back again, and going through a wormhole may (or may not) involve strong tides but is otherwise just like traveling through any other region of space (none of this shimmery barrier like you see in StarGate). It is strongly suspected, but not yet proven, that a wormhole cannot take you farther back in time than it would take for a light signal to propagate from where you are going to where you left - in otherwords, wormholes can be used for FTL but not time travel (in relativity jargon, they only connect space-like intervals). Trying to move a wormhole around so as to make a time machine is thought to result in the destruction of the wormhole (or possibly just large forces that prevent the wormhole from entering into configurations that let you travel into your own past). All conserved quantities are conserved locally at wormholes - if a wormhole end has a given mass, pushing something with extra mass through the wormhole from that end will add its mass to the wormhole end, while if something comes out of that end, its mass will be subtracted from that end of the wormhole. The same goes for electric charge and (in a vector sense) momentum. If wormholes cannot have negative mass, this limits the amount of stuff you can send one-way through a wormhole before needing to send more mass back the other way. Many Sci Fi authors posit wormholes orbiting around stars in the vacuum of space, but there is no real reason I can think of not to have them located some place more convenient, such as in the aforementioned Spokane, WA. You would probably want to put them in an airlock to keep all the air from whooshing through from high pressure to low, and if you have more than one wormhole you will need to be careful that that there are no round trips you can take that bring you back into your own past (because if there was, some wormhole leg of that trip would collapse to prevent this).

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Continued...

Warp drives let you take a spacecraft and warp space-time around it so that a bubble of space-time around the spacecraft surfs through space-time at an apparent superluminal rate. The spacecraft, however, is at rest inside its bubble and is not actually moving. The most plausible form yet devised is the Alcubierre/Van-der-Broek geometry, which pinches the spacecraft off into a pocket universe connected by a microscopic wormhole to our universe through a region smaller in volume than a proton. Then you warp the microscopic wormhole end rather than the huge volume of the entire spacecraft. Clearly, the spacecraft would be blind while warping. There are unresolved issues with a warp drive - when moving at super-luminal speeds you get a singularity "bow shock wave" at the front of the bubble, which may not be physical (we are not sure yet). Also, when going super-luminal, the spacecraft is causally disconnected from the rest of the universe, so it could not maneuver while warping, only travel on a pre-planned course. These last two limitations go away if you only use the warp drive for sub-luminal journeys (making a warp drive a sort of reactionless drive). The conservation laws still hold - if you warp close to a planet, the planet's gravity will pull on the warping craft and change its velocity, building up momentum toward the planet.

Krasnikov tubes are not well researched yet, but they seem to work. You prepare a path through space-time along which material objects can move back and forth at apparent super-luminal speeds. This is sort of like an interstellar rail line.

Note that none of these tricks allow local faster than light motion through space-time - you only seem to move faster than light to distant observers.

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VonMalcom: I will mention that we already know of at least two cases which are experimentally verified as having negative energy density - the Casimir vacuum between conductive surfaces and so called "squeezed states". If black holes exist, then the event horizon of a black hole will also have a negative energy density.

One nice thing about wormholes is that they let you adventure in a universe filled with interesting aliens that are naturally neither so god-like in their technology that they completely out-class you nor mere stone-age primitives.

Consider - suppose we humans invent a way to split off a pair of connected wormhole mouths from the vacuum and keep them open. We can use them for interstellar transport by charging up one of the mouths and putting it in a particle accelerator to shoot it out toward an interesting looking star at ultrarelativistic speeds (make sure to discharge it in flight, or it may be deflected by interstellar magnetic fields). When it reaches the destination star, slow it down by shining an intense laser through it and using the light beam as a photon rocket. Once you stop, gobble up some mass so you can send things through.

Now, the thing about wormholes is they do not connect points in space, they connect events in space-time. That ultrarelativistic wormhole you shot out will have a very high time dilation while it is in motion. From the point of view of the wormhole mouth in motion, it might only take a month to make a 100 light year journey due to time dilation. Since the wormhole mouth back home is connected to the wormhole mouth in transit both in space and time, the people back home only need to wait one month before they can look through the wormhole and see the virgin star system, ripe for colonization. We'll call our new conquest Terra Nova.

Of course, in our reference frame that is not looking through the wormhole, it takes somewhat over 100 years for the wormhole mouth to travel those 100 light years (for the listed time dilation, it takes 100 years, 18 minutes). This means the wormhole is a time machine that takes you (roughly) 99 years, 11 months into the future if you go from Earth to Terra Nova, or 99 years, 11 months into the past if you go from Terra Nova back to Earth.

Now there are certain details we will need to follow if we have wormholes to many star systems, to prevent the creation of time machines (which will probably break the wormholes involved before we can make the time machines). The main idea, though, is that an expansion front of earth civilization sweeps through space at almost the speed of light - and due to time dilation, as the expansion front overtakes regions of space, they are linked back to human civilization at a time (and thus level of technological advancement) not too far beyond what is needed to make wormholes.

Now, suppose there is another technological civilization in a distant galaxy. Maybe they have not even evolved by the time we start sending out wormholes (in some galaxy centered reference frame). Maybe (in that galaxy centered reference frame) they were ancient long before our distant ape-like ancestors came out of the jungles to gaze across the African savanna. Nevertheless, due to time dilation effects of wormhole transport, when our expansion front meets their expansion front, we will both have only recently invented wormholes (well, maybe within a few hundreds of years - but not millions of years).

Luke

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Luke said...
Jean Remy: Perhaps a timeline would help. I will use GMT to refer to the Greenwich Mean Time coordinate frame. Keep in mind that the actual time coordinate depends on your frame of reference.

Jan 1, 00:00:00.00 2050 AD GMT
Mankind launches a wormhole mouth toward Nova Terra. The other mouth remains on earth. Nova Terra is 100 light years distant from earth. The launched wormhole mouth has a time dilation factor of 1200 - for every second of proper time experienced by the mouth, 1200 seconds pass in the GMT coordinate frame. To make this explicit, a motor is placed inside the wormhole. The motor turns a drive shaft that connects to an analog clock face on each side of the wormhole. Since the shaft turns at the same rate for both clock faces, anyone looking through the wormhole sees the same time on both the clock face on Earth and the clock face on the other side of the wormhole. The clock drives the shaft at a rate such that the clock faces turn at one second mark per second of proper time. A time dilation factor of 1200 corresponds to a speed of 0.999999653 c.

Jan 1, 00:18:15.75 2150 AD GMT
The wormhole mouth arrives at Terra Nova. 100 years, 18 minutes and 15.75 seconds have passed in the reference frame at rest with respect to Earth. This is 3,155,761,095.75 seconds. Due to time dilation, the projected wormhole mouth experiences only 1/1200 of this of its own proper time (equivalently, time in its own inertial coordinate frame). This means the proper time of the projected wormhole mouth is 2,629,800.91 seconds, or 30 days, 10 hours, 30 minutes, and 0.91 seconds. Anyone who had been drifting along with the wormhole mouth would have experienced a passage of time of 30 d, 10 h, 30 m, 0.91 s. If she were watching the clock, she would have seen it tick off that amount of time. Since the clocks on both sides of the wormhole are ticking along at the same rate from the point of view of someone looking through the wormhole, anyone sitting back on earth watching the clock would have seen it tick off 30 d, 10 h, 30 m, 0.91 s. This means that 30 d etc after launching the wormhole, people on earth experience the wormhole's arrival as viewed through the wormhole. This then means -

Jan 30, 10:30:00.91 2050 AD GMT
People on Earth experience the arrival of the Terra Nova wormhole. They can start sending explorers and colonists through.

Of course, our time-line is a bit out of order. Putting it in order, we have

Jan 1, 00:00:00.00 2050 AD GMT - wormhole launched

Jan 30, 10:30:00.91 2050 AD GMT -
Earth wormhole mouth experiences arrival of Terra Nova mouth.

Jan 1, 00:18:15.75 2150 AD GMT - Terra Nova mouth arrives.

An explorer going through the wormhole the moment it arrives would go from a time coordinate of Jan 30, 10:30:00.91 2050 AD GMT to a time coordinate of Jan 1, 00:18:15.75 2150 AD GMT. This is a jump forward in time of 99 y, 334 d, 19 h, 48 m, 14.84 s. If one of the little green native inhabitants of Terra Nova were to jump through the wormhole the moment it arrives, he would go from a time coordinate of Jan 1, 00:18:15.75 2150 AD GMT to a time coordinate of Jan 30, 10:30:00.91 2050 AD GMT, a jump backwards in the time coordinate of 99 y, 334 d, 19 h, 48 m, 14.84 s.

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Jean Remy: Ah, I see a bit clearer now. Let me see if I can address your points.

First, keep in mind that a wormhole is, by its nature, a general relativistic object. The reference frames in flat spacetime from special relativity should not be expected to hold in the highly curved spacetime of a wormhole. I've tried, as much as possible, to avoid the curvature of the wormhole and use only observers located in spacetime that is mostly flat (i.e., on one side of the wormhole or the other) so as to be able to use special relativity to analyze the motion. However, you do need a coordinate patch at the wormhole - although spacetime across the wormhole is continuous, the specific coordinates that you use in flat spacetime will become discontinuous across the wormhole (alternately, you can choose continuous coordinates across the wormhole, but then you need to patch your coordinates together someplace else, creating a discontinuity in the coordinate representation between Earth and Terra Nova.

The key point is that the wormhole mouth en route to Terra Nova is both at rest with respect to Earth (through the wormhole) AND moving at relativistic speeds with respect to Earth (through flat spacetime). Likewise Earth is both at rest with respect to Terra Nova (through flat spacetime) AND moving at relativistic speeds with respect to Terra Nova (through the wormhole). The perceived speed is path dependent in this particular spacetime geometry.

Note that for just one wormhole causality is not broken. At Terra Nova, you can go back in time by 99 years, 11 months by going through the wormhole to Earth. However, you can never get back to Terra Nova before you started. If you go back through the wormhole, you will go forward in time by 99 years, 11 months, so when you add in however long you spent on Earth, you get back after you left. If you try to go back to Terra Nova the long way through flat spacetime, it will take at least 100 years since Terra Nova is 100 light years away - even if you sent yourself a lasercom signal to Terra Nova as soon as you got to earth, the message would not arrive until a month after you left. We maintain time ordering, and causes always precede their effects.

Out of convenience, it is often useful to consider a specific kind of wormhole called a Visser wormhole (after its inventor, Matt Visser). A Visser wormhole is essentially supported by a "cage" or "circle" of negative energy stuff, and paths through the wormhole that do not touch the cage only go through flat spacetime. Thus, any trip through a Visser wormhole is no different from traveling through flat spacetime. Visser wormholes are valid solutions of Einstein's equation for the geometry of spacetime in general relativity. This makes them convenient for analyzing cases like this - the flat spacetime through the wormhole no more impedes the flow of matter or information than any other region of flat spacetime, like the spacetime between my library and my living room.

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Citizen Joe: The ends of wormholes follow the same paths that any object would. They have mass, and if you exert a force on them they accelerate in accordance with Newton's second law. If you have one in a star system, it will follow a Keplerian orbit around that star just as would any bit of inert matter. If you keep your wormhole on a planet, you will need to support it against gravity (perhaps just resting on the ground will do this, we do not know). Each end moves independently on its own trajectory, irregardless of what the other end is doing. The main complication is that a wormhole absorbs the momentum as well as the mass of anything going through, and gives up the momentum as well as mass of anything coming out. Thus, traffic through a wormhole will generate forces that can alter its trajectory.

All of the wormhole geometries I am familiar with don't have the ends moving with respect to each other through the wormhole, as much as they might move with respect to each other through flat space-time. That is, look through the wormhole and the other end is a constant distance away, always. Look at the other end through flat space-time through a telescope and you might see the other end moving quite a bit.

You can see how a wormhole is useful for travel by considering our previous example - one end on Earth and one on Terra Nova. I am on Earth and I want to visit Terra Nova. I step into the wormhole end on Earth, jump across the wormhole tunnel (we'll make this one have a short tunnel, just because we want to, but you can have a long tunnel, or just a vanishingly thin portal if you prefer), and you will be on Terra Nova, 100 light years away. When you get bored of life on the frontier, you can go back to the wormhole, jump through, and be back on Earth. So long as the wormhole does not take you further backward or forward in time than 100 years, it is impossible to violate causality (we say that they have a space-like separation). So long as the separation is space-like, it is thought that the wormhole mouths exert no forces on each other, and the wormhole is stable.

However, what happens if Terra Nova orbits a heavier star than Earth, so it is orbiting faster and deeper in a gravity well. It is also farther into the galaxy's gravity well. Uh oh! The Terra Nova end of the wormhole is continuing to experience extra time dilation not felt by the Earth end. Eventually, more than 100 years of time lag will build up. Perhaps Terra Nova's sun (and thus Terra Nova itself) is drifting toward Earth, so the distance is getting closer. As soon as the time lag (in years) is more than the distance (in light years), you can use the wormhole to go back in time and then send a lasercom signal to yourself before you left. (Terminology: when the time lag is exactly equal to the distance, we say the separation is light-like. When the time lag is more than the distance, we say the separation is space-like.) It is thought that as soon as you get a light-like separation, the path back in time through the wormhole and then returning through flat-space forms a perfect amplifier for radio, light, and any other electromagnetic signal (not to mention gravitational waves). Fluctuations in these waves spontaneously appear and build up to such huge amplitudes that they either destroy your wormhole or exert a force that pushes the wormhole ends apart so as to keep them from forming a time machine.

Fortunately, there is a way to prevent this. Charge up your wormhole, shrink it back down to what it was when it was traveling, and put the Earth end in a cyclotron. Spin it up to ultrarelativistic speeds. The time dilation on the Earth end decreases your time lag across the wormhole. Stop spinning the earth end when the time lag gets small enough, discharge the wormhole, inflate it back up to usable dimensions again, and open it back up for travelers.

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Oops - when the time lag is more than the distance, we say the separation is time-like, not space-like.

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Citizen Joe: There is no conservation of momentum issue. Momentum is automatically conserved locally. Here's an example:

Suppose we have a stationary wormhole mouth with mass M. It has a maglev train track going through it. A maglev trolley with mass m and velocity v floats along the track and through the wormhole. Before the trolley goes through, the total momentum of the system is
M * 0 + m * v = m * v.
After the trolley goes through, the wormhole mouth has a mass of
M + m
and a velocity of
v * m / (M + m),
drifting along the track.
The total momentum of the system is
(M + m) * v * m / (M + m) = m * v,
the same as before. Momentum and mass (energy, actually, and also angular momentum and electric charge) are conserved locally, with no reference at all to what is going on at the other end. (In practice, the wormhole end will probably be braced if it is on a planet's surface, not free floating along the track. In this case the wormhole exerts a force on the braces, which in turn push back on the wormhole via Newton's third law of motion. This transfers the momentum between the planet and the wormhole as the trolley goes through which keeps the wormhole stationary with respect to the planet).

But let's look at the other end for a moment. This end has a mouth with a mass M', also initially at rest. The initial momentum of the system is
M' * 0 = 0.
When the trolley comes out of the mouth at velocity v, the mass of the mouth decreases to
M' - m
and it acquires a velocity of
- v * m / (M' -m)
backwards along the track such that the total momentum is still
[m * v] + [(M' - m) * (- v * m / (M' - m))] = 0.
Again, momentum and mass are conserved locally. There is no dependence on the dynamics of the other end of the wormhole.

However, now we have an interesting question. What if the mass of the trolley is larger than the mass of the wormhole mouth that the trolley comes out of? The conservation of mass tells us that the wormhole mouth ends up with a negative mass! Negative mass is weird - if you push on it, it comes toward you! It seems unphysical. Perhaps it is - some relations in quantum mechanics indicate that regions with negative energy (mass) density must be bounded with regions of positive energy (mass) density and with more positive energy (mass) than negative energy (mass). If this holds, a wormhole will never acquire negative mass. Perhaps it collapses before this can happen (shearing off anything inside of it that is about to give one end negative mass). Perhaps some sort of force develops which bounces back anything in it that is about to give one end negative mass. Or maybe you really can have negative mass general relativistic (as opposed to quantum mechanical) objects. We do not know.

Personally, I think it is more interesting if you have to keep the mass of both ends positive. Now you need to be careful to balance the mass going through, which adds an interesting and novel constraint on our wormholes that is not generally seen in FTL used in fiction. But my preference is not certain, you can write stories with negative mass wormholes in them and still have them be hard science fiction if that is what you prefer.

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Francesco: Exactly right. Well, not quite inside the future light con - if you send the wormholes slowly so that they only built up a time lag of, say, 6 months, you could have a wormhole from Earth to Terra Nova, and another from Terra Nova to anywhere further than a light year of Earth.

There is a way around this. I mentioned taking the Earth end of the wormhole, putting it in a particle accelerator, and letting it go around in circles at ultrarelativistic speeds to reduce the time lag. If you do this for long enough, you can completely get rid of the time lag, or even reverse it. For the Earth - Terra Nova wormhole, it will require the wormhole to go around and around in the accelerator for at least 100 years, although you could always stop it every so often to let people and equipment through. Note that on Terra Nova it will seem to be much less than 100 years, since the wormhole end on earth is undergoing time dilation. This trick would allow you to build round trip wormhole networks, but you will need to be careful to keep them all synchronized to prevent time machines.

Also, the powers on Earth might not want this. Suppose we Earthlings send a wormhole to Tera Nova. And then we send another to New Carolina, 100 light years away in another direction. And maybe other wormholes to Homestead, and Johnsworld, and Zemynia, and perhaps a few other colonies. In order to trade with each other, these colonies must route their traffic through Earth, since they cannot send wormholes to each other without making a time machine. The colonies can extend their wormhole networks away from earth, but you end up with a branching tree-like network in which Earth is at the nexus, the root node, and thus all trade between major branches will come through Earth. You can see how there would be those on Earth who would be making a lot of money off of this.

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Nick P. : One minor detail - remember that mass must be conserved locally (well, energy must be conserved locally, but to our approximation it would be mass). So if uncle Ernie wants to put his super-heavy home made ship into orbit (and assuming net negative masses are impossible), he will need to find an equal mass of stuff in orbit to bring back. The sequence might go something like this:
(1) Ernie launches a 10 nanogram wormhole mouth up into orbit. The corresponding mouth stays at home with him (also 10 nanograms).
(2) The orbiting wormhole mouth finds a 1,000,000 ton asteroid up there, and "eats" it. The asteroid is now inside the wormhole. The orbiting wormhole mouth now has a mass of 1,000,000 tons (plus ten nanograms, but I'll ignore that for now).
(3) Uncle Ernie puts his 400,000 ton Ernietopia habitat through the wormhole. The wormhole end back at home has a mass of 400,000 tons and the orbiting end has a mass of 600,000 tons.
(4) Ernie still has 1,000,000 tons of stuff inside his wormhole.

It's this minor detail that makes getting to empty space difficult, but it certainly makes getting to other planets easier.

and

Jim Baerg: That is certainly an option, but it must be done while one end of the wormhole is en route. For example, let's consider the case of Earth, Terra Nova, and Johnsworld, all 100 light years from each other. Earth already has a wormhole to Terra Nova and Johnsworld, with a 99 year, 11 month lime lag (that is, going from Earth to Terra Nova or Johnsworld takes you 99 years, 11 months into the future, while going from Terra Nova or Johnsworld to Earth takes you 99 years, 11 months back in time). Now the merchants at Terra Nova want to cut out the middleman and get around the crippling tariffs at Earth, so they plan to establish a wormhole route to Johnsworld. They launch a wormhole mouth at the time dilation factor of 1200 we've been using before. As soon as the wormhole mouth gets roughly half-way there, light can go from Terra Nova back in time 99 years 11 months to Earth, forward in time 99 years 11 months to Johnsworld, spend 50 light years going through flat-space from Johnsworld to the wormhole, and then go back in time 50 years to Terra Nova to arrive as soon as it left. At this point, the weakest link the the wormhole network breaks to prevent a time machine from occurring - probably the Tera Nova to Johnsworld wormhole since it is still small and weak and not reinforced like the wormholes to earth are. Thus, the merchants at Terra Nova have to put their end in a cyclotron and spin it around at a time dilation factor of 1200. Now both wormhole ends have the same time dilation, and no time lag builds up. The major drawback is that the Terra Nova merchants have to wait 100 years until they can trade with Johnsworld, rather than 1 month if they can let time dilation do its thing.

Now, if you can propel a wormhole that is strong and heavily reinforced, even though it is low mass for travel, then you can get wormhole wars! The Terra Nova merchants can try to bully through with their extra strong wormhole to Johnsworld, hoping one of the Earth wormholes breaks first. Probably, they want the Johnsworld-Earth wormhole to break, leaving them at a trade nexus. At this point, everyone starts beefing up their wormholes so that theirs is not the weak link in the chain.

and

Jean Remy: It's not so much a light cone (which after all, is defined for an event, and if you wait long enough for any event on Terra Nova, say, the world line of Earth and of Johnsworld will eventually enter its light cone). It is that if you are on Terra Nova and you go to Earth (going back 99 years, 11 months) and then go to Johnsworld (going forward 99 years, 11 months), you end up at the same time coordinate as you left. Now, to get back to your original position, you need to spend at least 100 years crossing that 100 light years between Johnsworld and Terra Nova. You can't possibly get back before you left, and you are perfectly safe. The same argument works no matter how far apart Terra Nova and Johnsworld are, so long as the same amount of time is lost and gained on the trip to Earth.

Just to shake things up, suppose the people on Earth decide to put the Johnsworld wormhole in the cyclotron and let it sit there for a bit over 100 years, such that going from earth to Johnsworld takes you back in time by one month. Now things are dangerous - from Terra Nova to earth you go back in time by 99 years, 11 months, from Earth to Johnsworld you go back by another month, taking you back by a total time coordinate of 100 years. Since it takes 100 years for light to get from Johnsworld to Terra Nova, you can send a signal to yourself that arrives just as soon as you leave - you are on the verge of making a time machine, and this is the condition for the perfect resonator that is thought to blow up any wormhole network where it occurs (so far it has only been shown for single wormholes and two wormhole networks, but is suspected to work for any number of wormholes).

I find it useful to make a simple diagram of the paths - say put three dots as vertices of a triangle and label the vertices as Earth, Terra Nova, and Johnsworld. Now you can draw in the change in time coordinate for travel along each of the legs of the triangle (note that this depends on which way you go, if there is a wormhole on that path). If any round trip journey can get you back before you left, you have a time machine and your wormhole network would be destroyed before it could be made (according to our best guess of how wormholes work).

and

Franceco: Kip Thorne, Sung-Won Kim, and Stephen Hawking have shown that for a single wormhole, electromagnetic vacuum fluctuations will be focused to a strong pulse just at the moment the wormhole becomes a time machine. They concluded that this may be intense enough to destroy the wormhole (Kim and Thorne, Physical Review D, vol 43, article 3939, 1991). Hawking strongly believes this to be the case, and advanced the chronology protection postulate that vacuum fluctuations will always destroy a time machine before it forms. Thorne suspects he is right. Matt Visser later showed that a single wormhole will almost certainly be either destroyed or subject to such intense forces that it will "bounce away" before becoming a time machine, and that any configuration of two wormholes will probably also not be able to become a time machine (Visser, Lorentzian Wormholes: From Einstein to Hawking, AIP press, 1996). As far as I am aware, there is still no general proof that any configuration of wormholes will be prevented by vacuum fluctuations from becoming a time machine.

[Chirios]

So, if I understand the above exchanges correctly, the best form of FTL from a current scientific understanding is a naturally existing wormhole that just happens to connect two viable planets?