• 

• 

      I always dim the lights before the navigator starts the final jump sequence. We don't have to do it, of course, but there's a long tradition by Vilani pilots to do it. In the early days (about 10,000 years ago), the jump systems weren't what they are today, and a lot of captains found out that they needed every watt of power to make a safe jump. So they cut most of the lights on the vessel just to make sure.

     I don't mind admitting that I'm superstitious, and I was trained by Vilani pilots: laugh it you want to, but I always dim ‘em.

     That doesn't mean that I go overboard. My ship's jump drive doesn't have a set of gold coyns underneath its floor bolts, the way a Droyne would do, and I don't beat the first crew member who comes aboard, like the Vargr. Superstition isn't what brings luck: skill is what brings luck. If you've got the crew and the ship, you've got all the luck you need. (But I still dim the lights.)

     There's a few captains, they've got the attitude that just because they turn up the drive only once every two weeks that that's all the attention they have to pay it. Those jokers are asking for trouble. You've got to treat your jump drive the way you would treat your lover: nothing’s too good for it. You think that because you see your girl only twice a month that you can ignore her the rest of the time? Just try it.

     Seriously, a misjump might be better than what you'd get from your girl. I learned this from experience. I won‘t tell you about the womenfolk, but I've been in two misjumps, and I'm not looking forward to the next one.

     Once, in the Fourth Frontier War, we came out of jump and the rest ot the fleet was missing. We figured something was wrong because about half the crew had spent the week coughing up their guts in the freshers. (You don't need to know whether I was among them.)

     Anyway, the navigator spotted right off where we were, and after checking systems over for two days, we headed back for the rendezvous point. You should have seen the look on those Zhos' faces when we showed up. They thought we were part of a second armada held in reserve, and it wasn't 10 minutes that their whole fleet had jumped out. Our captain would have been knighted if he'd done it on purpose.

     The other time was with my own ship, and we lost two crew because of it. The engineer was on the final countdown when the grid display showed a rupture. Tore a hole about half a meter across in the aft bulkhead, and my medic was sucked out of her bunk and into that black inkwell outside the ship.

     There wasn't a damn thing that I could do about it at the time: the jump field closed up all right, and we were gone before any of us even realized Doc was missing. It was the steward that went looking for her, wearing his vacc suit of course, but the field deformation extended too far into the room, and he died a few days later of jump sickness. If Doc had been around we might have saved him.

     Anyway, nine days later we were still in jump, and I was having the devil's time not letting on to the crew that I was as worried as they were. Discipline's a fragile thing, and I had to keep it or none of us would make it. You've heard of people who are "jump crazy“? It's the fear of it that gets ‘em — they have to be pumped up with tranquilizers to take a star trip. Imagine having a whole crew like that.

     The navigator acted busy the whole time, bless his soul, and the rest ot the crew thought he knew what he was doing, but it was all just a show. There wasn't a digit he or his computer could gnaw on to tell us anything about where we were or where we were headed.

     Two days later, we came out. I flagged an "all systems“, and the crew got to work. If the navigator hadn't been sound asleep, he could have told us where we were in 20 seconds, but commo might have beat him to it. With our transponder blaring out, he picked up an auto-signal welcoming us to "Jewell, capital of the subsector". Trouble was, we weren't headed for Jewell. We'd come out 11 parsecs from where we'd gone in.

     We pulled into orbital and went over the systems with a fine-toothed comb. One of the Zuchai crystals had a hairline fracture in it. Couldn't see it without a ‘scope, but I lost two good friends and two months profit because of it.

     One thing about jumping that l always enjoy is the sheer beauty of it — not inside the ship, of course — all you can see from there is so much gray. But from outside, watching a ship go into jump just takes my breath away every time. The whole thing takes maybe 20 seconds, but it you're lucky enough to see it you'll remember it for a lot longer.

     The lanthanum network spread throughout the hull turns blue, or rather the ship does all along its lines. The thing looks like one of those computer drawings that naval architects use. It doesn't light up all at once, either, but extends from one point to another, however the navigator set it up for whatever jump vector he's trying to hit.

     So once this grid shape is formed, the whole area around the hull starts glowing this same shade of blue. A couple seconds later and the ship's gone, and this blue patch just shrinks down into a point before fading completely.

     When a ship comes out of jump, the whole sequence is reversed: first there's the blue glow, then the ship appears out of nowhere, and the glow fades a little so you can see the grid lines, and then they're erased, one by one.

     It's harder to see a ship come out of jump, of course, because you don‘t know where to look. I remember growing up, when my sister and I used to watch the ships moving away from the orbital port. They had ‘scopes set up in the view areas, so you could watch them for hours, and when they got far enough away, then zip zip with the lines, then that blue glow, and then nothing to see at all.

     We especially liked to watch the alien ships, because their grid patterns were always different than what we were used to. Droyne, I guess, always used a hexagon shape, and Aslan ships had whorls and spirals all over them. In the war, I watched a few Zho get away: triangles is what they used. Never saw a Hiver or K‘kree ship, so I can't say what they look like. Vargr, though, look like a hodgepodge, and I can't tell you why. Must be hell for their navs to plot the right sequence.

(ed note: interesting description of the physical basis of the Alderson drive omitted because it is a cause, not an effect)

Travel by Alderson Drive consists of getting to the proper Alderson Point and turning on the Drive. Energy is used. You vanish, to reappear in an immeasurably short time at the Alderson Point in another star system some several light-years away. If you haven't done everything right, or aren't at the Alderson Point, you turn on your drive and a lot of energy vanishes. You don't move. (In fact you do move, but you instantaneously reappear in the spot where you started.)

That's all there is to the Drive, but it dictates the structure of an interstellar civilization.

To begin with, the Drive works only from point to point across interstellar distances. Once in a star system you must rely on reaction drives to get around. There's no magic way from, say, Saturn to Earth: you've got to slog across.

Thus space battles are possible, and you can't escape battle by vanishing into hyperspace, as you could in future history series such as Beam Piper's and Gordon Dickson's. To reach a given planet you must travel across its stellar system, and you must enter that system at one of the Alderson Points. There won't be more than five or six possible points of entry, and there may only be one.

Star systems and planets can be thought of as continents and islands, then, and Alderson Points as narrow sea gates such as Suez, Gibraltar, Panama, Malay Straits, etc. To carry the analogy further, there's telegraph but no radio: the fastest message between star systems is one carried by a ship, but within star systems messages go much faster than the ships.

Hmm. This sounds a bit like the early days of steam. Not sail; the ships require fuel and sophisticated repair facilities. They won't pull into some deserted star system and rebuild themselves unless they've carried the spare parts along. However, if you think of naval actions in the period between the Crimean War and World War One, you'll have a fair picture of conditions as implied by the Alderson Drive.

If the Drive allowed ships to sneak up on planets, materializing without warning out of hyperspace, then there could be no Empire even with the Field. There'd be no Empire because belonging to the empire wouldn't protect you. Instead there might be populations of planet-bound serfs ruled at random by successive hordes of of space pirates. Upward mobility would consist of getting your own ship and turning pirate.

• 

      In the CoDominium/Empire of Man universe there are two drives needed to travel between the stars. We’ll discuss the Alderson Drive here and come back to the fusion drive later. As mentioned in Pournelle’s future history, both are invented early in the CoDominium period. The Alderson Drive allows spaceships to travel from star system to star system instantaneously. However, there are limitations on what the Drive can do. And these limitations mean that spaceships cannot sneak up on a planet and attack it. This is critical for the formation of interstellar organizations.

     Here are some of the characteristics of the Alderson Drive. Alderson points (aka Jump points) are found outside the habitable zone in solar systems. For instance, a G2 star like the sun has Alderson points between 2.2 and 30 AU (astronomical units) from Sol. This is the distance from the sun to the inner edge of the asteroid belt and Neptune, respectively. This means that spaceships do not travel through interstellar space in our universe though they do Jump through interstellar space from star system to star system in hyperspace. This, in turn, means that spaceships must travel through normal space to reach an Alderson point, the place where you can make a Jump to another star system. Fusion drives or a variation of them serve to propel spaceships through normal space to, from and between these points. The normal space distances are still immense and, as a result, take a lot of time (weeks or months) to cross (remember the physics we know still applies).

     A single-star system with Alderson points can have anywhere from one to six of them. Multiple-star systems may (but usually don’t) have more than six Alderson points. The number of additional Jump points in this case would still be small. Because systems with no Alderson points are not visited, and systems with only one point are dead ends, the average number of Alderson points in any system in the network is likely higher than three.

     Each Alderson point connects to one Alderson point in another star system and only one. These connections are called tramlines. Tramlines always connect to that same point in that same star system. If you draw a line between the two stars the tramline would be on that line.

     The lengths of Jump distances vary. In the published literature Jumps of anywhere from 1/3 to 35 light-years have been described. It is possible, in rare cases, for Jumps to be longer. However, most Jumps are between 5 and 15 light-years in length.

     To get from one star system with a habitable planet to another star system with a habitable planet in most cases requires multiple Jumps (i.e., a series of Jumps). Possibly a lot of Jumps. There is a tradeoff between making a lot of little Jumps between low mass stars and one Jump to a bigger star. As a simple example, it is faster to make seven 5 light-year Jumps through M-class star systems than one 35 light-year Jump to an F-class star. This is because the average normal space transit distance through the M-class stars to reach their Alderson points adds up to less than the normal space distance through an F-type star system.

     Alderson points are fixed locations in space. That is, they remain a fixed distance from their star and do not move relative to the background stars. Planets move in orbits around their sun. Alderson points do not. Therefore, the distance between planets and Alderson points continually change in single-star systems but in a repetitive manner except if the point is directly above the star system’s planetary plane. Alderson points can shift some under certain circumstances but they do not move far. There is a major exception to this rule. Multiple-star systems may have Alderson points that are in fixed locations in space or they may have Alderson points that shift position some as the stars in the system orbit each other (or there may be some combination of fixed and shifting). Also, if one of the stars has a variable energy output the location of the Alderson points can shift.

     Jumps take no measurable time. The travel is through the continuum universe (aka hyperspace). So, the Alderson Drive can be used to cross light-years instantaneously. But once you arrive at an Alderson point, interplanetary distances must be crossed using normal space drives (e.g., fusion drives) to either get to the next Alderson point, to a planet in that system or to a refueling station. And this travel takes time. Depending on spaceship velocity, fuel levels and system geometries it can take a long time (weeks or months). This is fundamental in determining what kind of interstellar political entities can form (i.e., the CoDominium, empires, etc.). The reason for this is that if a spaceship could materialize right next to a habitable planet then an interstellar organization like an empire couldn’t protect it from raiders. There would be no reason for a planet to join an empire.

• 

      Five years. Five years ago Barbara Jean Ramsey and their son Harold were due back from Meiji. Superstitiously, Bart had waited for them before accepting his promotion... Barbara Jean had never come home from Meiji. Her ship had taken a new direct route along an Alderson path just discovered. It never came out into normal space. A scoutcraft was sent to search for the liner, and Senator Grant (Barbara's father) had enough influence to send a frigate after that. Both vanished, and there weren’t any more ships to send.

---

     “You got five forces in this universe we know about, ja? Only one of them maybe really isn’t in this universe; we do not quibble about that, let the cosmologists worry. Now we look at two of those forces, we can forget the atomics and electromagnetics. Gravity and the Alderson force, these we look at. Now you think about the universe as flat like this table, eh?” He swept a pudgy hand across the roseteak surface. “And wherever you got a star, you got a hill that rises slowly, gets all the time steeper until you get near the star when it’s so steep you got a cliff. And you think of your ships like roller coasters. You get up on the hill, aim where you want to go, and pop on the hyperspace drivers. Bang, you are in a universe where the Alderson effect acts like gravity. You are rolling downhill, across the table, and up the side of the next hill, not using up much potential energy, so you are ready to go again somewhere else if you can get lined up right, O.K.?”
     Ramsey frowned. “It’s not quite what we learned as middies—you’ve got ships repelled from a star rather than—”
     “Ja, ja, plenty of quibble we can make if we want to. Now, Captain, how is it you get out of hyperspace when you want to?”
     “We don’t,” Ramsey said. “When we get close enough to a gravity source, the ship comes out into normal space whether we want it to or not.”
     Stirner nodded. “Ja. And you use your photon drivers to tun around in normal space where the stars are like wells, not hills, at least thinking about gravities. Now, suppose you try to shoot past one star to another, all in one jump?”
     “It doesn’t work,” Ramsey said. “You’d get caught in the gravity field of the in-between star. Besides, the Alderson paths don’t cross each other. They’re generated by stellar nuclear activities, and you can only travel along lines of equal flux. In practice that means almost line of sight with range limits, but they aren’t really straight lines. . . .”
     “Ja. O.K. That’s what I think is happening to them. I think there is a star between A-7820 and 82 Eridani, which is the improbable name Meiji’s sun is stuck with.”
     “Now wait a minute,” Admiral Torrin protested. “There can’t be a star there, Professor. There’s no question of missing it, not with our observations. Man, do you think the Navy didn’t look for it? A liner and an explorer class frigate vanished on that route. We looked, first thing we thought of.”
     “Suppose there is a star there but you are not seeing it?”
     “How could that be?” Torrin asked.
     “A Black Hole, Admiral. Ja,” Stirner continued triumphantly. “I think Senator Grant fell into a Black Hole.”

---

     “Then how would Black Holes interact with—oh,” Rap Torrin said, “gravity. It still has that.”
     Stirner’s round face bobbed in agreement. “Ja, ja, which is how we know is no black galaxy out there. Would be too much gravity. But there is plenty of room for a star. Now one thing I do not understand though, why the survey ship gets through, others do not. Maybe gravity changes for one of those things, ja?”
     “No, look, the Alderson path really isn’t a line of sight, it can shift slightly—maybe just enough!” Torrin spoke rapidly. “If the geometry were just right, then sometimes the Hole wouldn’t be in the way. . . .”
     “O.K.,” Stirner said. “I leave that up to you Navy boys. But you see what happens, the ship is taking sights or whatever you do when you are making a jump, the captain pushes the button, and maybe you come out in normal space near this Black Hole. Nothing to see anywhere around you. And no way to gets back home.”
     “Of course.” Ramsey stood, twisted his fingers excitedly. “The Alderson effect is generated by nuclear reactions. And the dark holes—”
     “Either got none of those, or the Alderson force stuff is caught inside the Black Hole like light and everything else. So you are coming home in normal space or you don’t come home at all.”
     “Which is light-years. You’d never make it.” Ramsey found himself near the bar. Absently he poured a drink. “But in that case—the ships can sustain themselves a long time on their fuel!”
     “Yes.” Lermontov said it carefully. “It is at least possible that Senator Grant is alive. If his frigate dropped into normal space at a sufficient distance from the Black Hole so that it did not vanish down.”
     “Not only Martin,” Bart Ramsey said wonderingly. His heart pounded. “Barbara Jean. And Harold. They were on a Norden Lines luxury cruiser, only half the passenger berths taken. There should have been enough supplies and hydrogen to keep them going five years, Sergei. More than enough!”

(ed note: Spoiler: a rescue mission is sent. The only way to get back is to somehow generate a large nuclear reaction to create the Alderson effect. Crashing one of the trapped starships on the Black Hole will work. Unfortunately this is even more difficult that crashing a ship onto the Sun: the intense gravity will make the ship miss if you are a fraction of a degree off. Which means some brave volunteer will have to sacrifice themself to save the others, manually piloting the ship into a collision while the rescue ship is poised to jump. The rescue ship waits with their Alderson drive turned on, when the crashing ship creates the Alderson effect the drive will have something to make the ship jump.)

Overview

Faster Than Light travel in Outsider is via "jump drive", which is a form of point-to-point hyperspace travel. A starship activates its jump field generator while on a vector from one star to another, and the ship is propelled into hyperspace, through which it travels (nearly) instantaneously on a ballistic trajectory and re-enters realspace within the gravity well of the destination star. There are no "gates", but the jumping starship must be within the proper outbound zone and have the correct velocity vector to escape from the originating star and to arrive safely at the destination star.

Optimal jump points tend to be located at significant distance from the system primary, so after jumping, the ship must travel through the normal space of the solar system (using conventional drives) before it can reach the next jump point and jump again to the next star. Jump drive only works between adjacent stars because the gravity wells are needed to govern the "pitch and catch" of the hyperspace transit. Other stars' gravity will interfere with this ballistic hyperspace trajectory, so it's usually not possible to jump "past" a nearby star to a more distant star. This effectively limits safe jump range to roughly 6-10 light years, depending upon the density and mass of stars in the area.

• The energy required for jump is significant, and must usually be built up for several minutes before jump.
• The energy cost to jump is up-front, and the ship is ballistic while in hyperspace. It's like a cannon-shot.
• The energy cost of a hyperspace jump is proportional to the mass of the ship.
• The ship must have some kind of inertial damping system to prevent being torn apart by the transition to hyperspace.
• Both entry into and return from hyperspace cause a bright flash of light that is very detectable at long ranges.
• The jump is nearly instantaneous, so there is not much you can do while in hyperspace.
• Since it is moving faster than light, the ship is blind while in hyperspace.
• Realspace momentum is preserved; you have the same velocity after the jump as you did before you jumped.
• Hyperspace transit has different effects on different species. Many find it unpleasant and disorienting.
• Two masses are required at the start and end points of the jump. You can't jump to or from deep empty space.
• It is generally not possible to do short-range jumps from within the same star system.
• Hyperspace is chaotic and cannot be directly observed, so accuracy of jumps can never be perfect.
• Optimal jump distance (both entry and exit) from a Sun-type star is at about 4-5 AU from the star (Jupiter orbit distance).
• By varying pre-jump velocity and position, a ship can exit shallower or deeper into the target well, but at added risk.
• A hyperspace "miss" usually means that the ship is never seen again.
• The length of a long trip is measured in the time required to travel in normal space from jump point to jump point.
• If a jumping mass returns to normal space where another mass already exists, the result is a high-energy collision.

Jump Mechanics

A jump zone is a conical volume centered on the vector connecting two masses (Fig. I). The outbound jump zone is very wide, and extends some distance out into interstellar space; as long as your initial vector will carry you close enough to the destination star for its gravity well to pull you back out of hyperspace (and as long as you are far enough out / have enough velocity from the departure star to escape its own well), then you don't have to be exactly on the line (the “jump vector”). The inbound jump zone is much narrower; a ship coming out of hyperspace will appear fairly close to this jump vector. How far from the destination star it appears will depend on the ship's hyperspace momentum, which is increased by departure velocity and decreased by jumping from deeper within the departure star’s gravity well.

As described in (Fig. II), gravity wells are necessary at the start and end points of the hyperspace jump to achieve proper entry and exit angles into hyperspace. The vessel's starting space-time velocity is added to the +hyperspace momentum provided by the jump drive to give the transiting vessel a ballistic trajectory through hyperspace. Gravity from the stars in realspace still acts on the ship in hyperspace, pulling it laterally between the stars but also "down" in the -hyperspace direction back towards realspace. If the trajectory of the transiting ship again intersects space-time at the proper angle, it will re-embed itself and return to normal space.

The more hyperspace momentum you have, the "deeper" into the well you travel and the closer you will appear to the arrival star. If you have too much momentum it’s possible to exit hyperspace too close to or even inside the star, or to overshoot it entirely causing a hyperspace “miss.” If you don’t have enough momentum to escape the departure star’s gravity well, you’ll be pulled back in, either exiting hyperspace inside the star or popping out the other side still in hyperspace, again causing a miss. If you intersect space-time at an improper angle, you may bounce off or even punch through to the other side.

Safety Issues and Failed Jumps

Jumps and exit points can't be calculated with great accuracy, because the exact geometry of the hyperspace-time "curve" you'll be traveling on can't be directly measured. The n-dimensional curvature of hyperspace is chaotic and is affected by many sources, from the gravitation of nearby stars, planets and interstellar gas and dust, to the rotation of the stellar masses and their electromagnetic fields, not all of which you can measure accurately, so there is always an uncertainty factor to account for in your calculations. Therefore, a jumping ship must whenever possible allow for the largest safety margin that it can: it must endeavor to be as close on the vector between the stars as possible, be moving at the optimal escape velocity, and jump at the optimal slope in the departure star's gravity well.

If you jump close on the jump vector, you limit the perturbing influence of your departure star's gravity well to a linear quantity, meaning that it might only affect how deep into the destination star system you arrive. If you jump from a tangential point (Fig. III), then the departure star is pulling you laterally rather than directly back, increasing the chance that you might miss the target altogether. In theory, if your calculations are correct you can jump from a tangent point as illustrated above, but in practice it's extremely dangerous. Maximum arrival distance from the destination varies with the mass of the star, but a successful "short-jump" can often bring you in at the edge of system, outside the orbits of most of the planets. The deeper your jump starts in the departure gravity well, the shallower the exit point is likely to be (Fig. V). Greater starting velocity will also cause the vessel to exit deeper into the destination well.

Hyperspace jumps can be compared to putting a golf ball. In theory, if you hit the ball hard enough on the right trajectory, you should be able to get the ball in the (gravity well) hole from any distance... but in practice, the irregularity of the putting surface makes an accurate putt exponentially more difficult the farther you get away from the hole.

In most cases, the maximum jump distance between stars is about 10 light years, and preferable safe distance is about 6 light years or less. The limitation on jump ranges is based both on limited ability to calculate trajectories past a certain distance (the chaotic element causes the effect of tiny errors to increase geometrically with distance), but also on the interference of nearby stars. The farther you try to jump, the more likely that other stars are going to perturb your trajectory. Higher density of stars will reduce safe jump distance; lower density will increase it.

In a safe jump, the transiting ship reconnects with the space-time curve at the appropriate angle and successfully re-embeds into space-time, usually appearing 4-5 AU from the target star. In a "short jump," the vessel has less than optimal velocity, and so reenters at a more shallow point in the well, and appears farther from the star (often 6-10 AU). Short jumping risks reconnecting with the space-time curve at too steep an angle, causing the vessel to "skip" back into hyperspace. In a "deep jump," the vessel has more than optimal velocity, and so reenters deeper in the well and closer to the star (3 AU or less). Deep jumping risks being pulled directly into the star itself.

Jumping vessels that "miss" the target are rarely seen again in this universe. The various conditions of a failure on reentry into realspace illustrated in (Fig. IV) include:

Overshoot. If either the linear realspace velocity is too great, or the +hyperspace momentum is too great, the ship may miss the target well entirely ("whiff"). If the ship has achieved escape velocity in the +hyperspace direction, it may never return to realspace. Otherwise, gravity from realspace will eventually pull it back toward realspace, at which time one of the results below will occur.

Failure to re-embed into realspace because of angle of entry. This can result in the ship rebounding back into hyperspace ("doink"), or in rare cases punching through realspace altogether and being "liberated" into negative hyperspace. The result of a rebound is usually a series of subsequent further skips until the vessel happens along another gravity well, at which point it will have a chance to re-embed, but will most likely do so in an unsafe manner (see: Collision below). Negative hyperspace is an unknown quantity; objects that enter have never returned.

Collision. Objects in realspace do not physically interact with those in hyperspace (except gravitationally), but if the transiting object reenters realspace at the same location as another mass, the result is a high-energy collision. Matter returning from hyperspace does not "materialize," but rather pushes its way through an extra-dimensional portal. The transiting object may collide from "inside" the obstructing object (particularly if it is a star or planet), but it does not technically occupy the same space as the obstruction, but it treated as a normal kinetic impact. Since this entry is very rapid, and the preserved realspace momentum of the transiting ship is usually quite significant, the kinetic energy of any such collision is considerable and usually catastrophic. The most common collision is with the target star itself. Collisions with planets are rare, because inbound jump zones are seldom in the same plane as the planets' orbits, and if it is, then that jump link is probably too dangerous to be used for safe travel. Collisions with smaller objects are very unlikely; the volume of space is very large compared to the size of ships and debris, even in the restricted area of a jump zone.

Hazards Posed by Very Massive Objects

Very massive objects present a hazard to navigation because their mass can pull a ship off course in hyperspace. This can happen with any star, but a very massive star affects a larger area. In addition to making nearby stars more dangerous to hit, very massive star systems can be difficult to jump directly into, because the gravity well becomes so steep that it's hard to hit the target slope without being pulled all the way into the star. This is why the star-forming regions with star clusters and short-lived massive stars (such as the Gould belt surrounding the local bubble) form natural boundaries to safe jump travel.

Stellar remnants (black holes, pulsars, neutron stars) of very massive stars pose additional hazards to hyperspace travel; because they form through the collapse of a star, they usually have an incredibly high rate of spin, which causes gravitational waves. These waves propagate into hyperspace and have an unpredictable effect on the trajectory of objects transiting through nearby hyperspace, kind of like trying to putt a golf ball on an undulating surface.

Power and Scope of Jump Fields

Because of the high power requirements of the jump field, the field generator must usually be coupled with an array of capacitors (or "accumulators") that can build up the necessary charge over a period of time, usually several minutes. Combined with the requirement of an inertial damping system to protect the ship and crew from the extreme forces experienced when leaving and reentering space-time, this usually means that a jump-capable vessel can't be very much smaller (given Loroi or Umiak technology) than a ~100m gunboat-sized vessel. The smallest jump-capable scouts and couriers tend to be between 100-150m. There is no theoretical upper limit to the size of a starship, but the power required to jump increases with the mass of the vessel.

In order for an object to be successfully propelled into hyperspace, a jump field must be generated that encloses the object and is of sufficient intensity according to the object's mass that it overcomes the inertia that holds the object in realspace (which I suppose could be thought of as a kind of "surface tension" of space-time). If the field is not strong enough, nothing happens. If the field is strong enough to breach space-time but does not cover the entire object, then the forces acting on the part of the object covered by the jump field will attempt to rip it away from the rest of the object. If the object is not strong enough to withstand this tensile stress, then the object will be ripped apart, and the portion within the field will be pulled into hyperspace while the rest stays in realspace (though it is very likely that the retarding force of the object's structural failure may fatally reduce the jumping portion's hyperspace momentum). If the object can withstand this tensile stress (or if there is an inertial damping field in effect around the mass, as is likely in the case of a starship), then the field will try to push the whole object through the portal it has created, but if the energy of the field is not sufficient to propel the whole object through the portal, then the jump attempt will fail, and no part of the object will enter hyperspace.

Any jump-capable tug must therefore usually have jump field generators powerful enough for the total mass of both itself and any towed ship, and able to project the field to cover both ships.

Effects of Hyperspace on Biology

The experience of hyperspace transit has differing impact on various species. Humans are typical in this regard and experience transitory "jump sickness" which may include: vertigo, nausea, headache, disorientation, visual and auditory hallucinations, waking dreams, and nightmares (for those already asleep). These symptoms usually pass after several minutes. Some humans (especially civilian passengers) may resort to various drugs to help lessen the effect of these reactions.

Umiak can experience more severe reactions, including unconsciousness and sometimes mania, and so most Umiak must use drugs to mitigate these effects. Because of this, an Umiak crew will often be at reduced effectiveness for up to an hour after hyperspace transit. 

Soia-Liron species (Loroi, Barsam, Neridi) have very little reaction to hyperspace transit.

Q & A

it should be possible to go around the front lines, yes? there are plenty of stars around after all....and everyone of 'em are a potential jumpgate..... which effectively makes the jumpzone a 360x360 degree sphere..... yes?

Not really. Only nearby stars have workable jump links (max 10 light years, and preferable safe distance is about 6 light years or less). Earth has 7 possible jump points (Alpha Centauri, Sirius, Barnard's Star, Ross 154, Lalande 21185, Wolf 359 and Luyten 726-8, if you want to know), but only the two shortest safe for use by most shipping (Alpha Centauri and Barnard's). Most systems will have fewer points. All the entry points must be accounted for in a defense scheme; raiding aside, if an enemy can get a significant force past your front lines into undefended territory, the war is over. All borders must be guarded. That said, finding a new "back door" into enemy territory is the Holy Grail of a frustrated combatant. New systems that might offer a new route to enemy territory are always being sought -- hence the plight of the Humans and other would-be neutral entities.

It's always possible to take the way-long way around and try to come into enemy territory from the rear, but it can take a long time (each system transit can take several days to a week), and supply can become an issue. Such missions will also have a poor survivability rate; if you go the long way around, you have to return the same way, which may take many months. If you were damaged in the raid, or if you should happen to run into enemy forces on the way in or back...

Remember also that the Loroi are not easily taken unawares; thanks to their telepathically amplified farseers, they can often tell when the attacks are coming, and can arrange for a fleet to meet the raiders. Loroi don't spread their forces across the front; they concentrate them at the point of attack.

Did humanity develop any slower-than-light travel methods before they got Hyperspace? Say, for instance, Bussard ramjets?

Unlikely, but even if they had, they would have been overtaken by the FTL ships that were developed soon after.

If I’m in a jump zone, how easily and how accurately can I jump to somewhere else in the same zone? If it's a field, can you 'jump' photons, or other really fast particles?

Generally, you can't jump to somewhere else in the same system. To escape being pulled into the primary star, you usually need to have escape velocity out of the star's gravity well, on a vector for another star. Entering "hyperspace" you're hurled toward the other star, the gravity of which rips you back out into normal space. If you try to jump say, from Jupiter to Saturn, chances are you will either be pulled back into the Sun, or you will overjump Saturn and end up who knows where.

How long does it take, in hyperspace, to go 1 ly? And for in-system purposes, what's the max speed for most ships?

The jump is almost instantaneous, but since your jump range is limited to about 10 LY, traveling a long way means making a lot of jumps, and traveling in-system from one jump point to another. There's no maximum in-system speed, but ships will very rarely go more than 10% lightspeed (because it would take too long to stop, otherwise) and even that is extreme; at 30g it would take 28 hours to reach 10% c, and the ship would displace some 10 AU during that time. A more reasonable in-system speed is something closer to 1% lightspeed (3,000 km/s). So, It will generally take several days to a week to transit each system. It took the Bellarmine nearly two months to reach Loroi space from 82 Eridani, a distance of some 200 LY.

So, if you miss, you may never drop out of FTL?

Those ships that have overjumped have never been seen again... so it's hard to say for certain what happened to them. It is assumed that most eventually dropped out of hyperspace far away, likely ending up in the center of a star somewhere. Some might never have left hyperspace. Some might have ended up in the same extradimensional place that the Event Horizon went. Libera te tutamet ex inferis!

IF you missed everything, then head in the only direction that is truly up (up being opposite of down; down being towards matter)

The problem there is that the jump is nearly instantaneous, and for the fragmentary moment you're in hyperspace, you're ballistic. Either you hit the target, or you go bye-bye to goodness knows where. Don't burn too many neurons over this... it's completely inconsequential to the story.

Could something really weird like this [(Fig.VI), at right] happen? I know it's extremely unlikely, I'm just curious if it would be possible to "glimpse the beyond".

Anything is possible, though this seems unlikely. But since the ship is completely blind during hyperspace transit, there's nothing to see, and it would be hard to know whether this really happened or not (though I suppose it might be a clue if the crew starts to gouge their eyes out and vivisect each other). But I think a more likely result of this scenario is illustrated in (Fig.VII) at right:

Would the above be an example of entering negative-hyperspace as discussed?

Yes, hence the screaming.

Also; why is your jump range limited to 10 ly? If only a massive gravity well can pull you out, then isn't it only limited by how much risk you're willing to take?

That's right. Because stars are so densely packed, a "safe jump" is usually 10 LY or less. If you were trying to jump somewhere outside dense galactic space, your safe range might be much longer, but that's outside the scope of the story.

if coming out of hyperspace makes a lot of light (right now I'm assuming all wavelengths), then with a big enough mass coming out of hyperspace, couldn't you fry a lot of things? And would it also create an EM shockwave?

Not quite that much light. If you tossed a planet through hyperspace at them I'm sure you'd cause a great deal of havoc, but none of the combatants has access to quite that much energy.

Can you use one ship to throw another into hyperspace without needing to actually follow? 

It's not possible for one ship to "throw" another object into hyperspace without entering hyperspace itself. There is no method known to the major combatants to project an external jump field that does not include the generator itself. You can use another ship to tow the object into the correct vector, and perhaps use some kind of attachable "jump pack" to perform the jump, but whatever generator that creates the jump field is going into hyperspace along with the object.

Wouldn't a viable defensive tactic to deter invasion be to place mines or debris around a jump point? 

Space is big, and debris is small. The unpredictability inherent in a jump means that even the optimal jump "point" is really a zone almost 1 AU across. That's a lot of space to fill with debris, and there's no way to make the debris stay there; the gravity of the system primary will make it either fall in toward the star or orbit out of the zone.

A possible exploit would be trying to hit enemy planets with guided FTL missiles, in order to cause the previously-mentioned "high-energy explosion". That being said, this depends on three factors-- how high-energy is the high-energy explosion, how accurate your hyperspace drives are, and how cheap the drive is.

Such a collision delivers normal kinetic energy, determined by the hyperspace momentum of the transiting object plus the difference in velocity between the two objects. Since a typical pre-jump velocity is 3,000 km/s, that kinetic energy is usually enough to vaporize both objects if they are of a similar size. If the object in realspace is very large, however, like a planet, this damage is not likely to be significant, especially since the transiting object will usually impact somewhere deep inside the obstructing planet. Also, because it's not possible to accurately predict exactly where the transiting object will re-enter realspace, it's very difficult to hit a planet-sized target.

• 

Synopsis

• Wormholes connect across space and time, not just space.
  
  
• Nevertheless, you can't use wormholes to build a time machine.
  
  
• A consequence of this is that interstellar wormhole networks form branching, tree-like structures with no closed loops.
  
  
• You can try to steal nodes (worlds) from a neighboring branch by forming a closed loop. The weakest link will break, and if that link is in the other branch you will have stolen the nodes left stranded by the break. This is called a causality attack.

---

When you project a wormhole from a metropole world to a colony, you almost always exploit relativistic time dilation to reduce the perceived time to reach the colony from the metropole. For example, if the colony—metropole distance were 100 light years and the wormhole was projected at 99.9999% the speed of light, then it would take 100.0001 years for the wormhole mouth to reach its destination in the reference frame of the metropole and the colony. But due to relativistic time dilation, in the reference frame of the projected wormhole mouth it only takes 0.1414 years. Since you can look through the wormhole, you will see that 0.1414 years after launch, it arrives at its destination. At that point, you can go through and get to your new colony – you only need to wait about a month and a half to feel alien soil under your feet, rather than a century. In the metropole's reference frame, going through the wormhole takes you 100 light years away, and 99.8596 years into the future (going the other way, from the colony to the metropole, takes you 100 light years away and 99.8596 years into the past).

One might think that these time warps would let you engage in all kinds of time travel. It is easy to see that the metropole—colony situation described here doesn't allow these kinds of shenanigans. For practical purposes, you only have a time machine when you can go back to the place you left at a time before you left. And you can't do that here. Go from Colony to Metropole and you go back in time 99.8596 years. Go back to Colony through the wormhole, and you go forward in time the same amount, plus any time you spent on Metropole, so you get back after you left. If you go back through flat space-time, it will always take at least 100 years since you can't go faster than the speed of light so you also get back after you left. No paradoxes for you!

However, it is easy to imagine situations where a wormhole, or a configuration of wormholes, does make a time machine. Imagine that there are two colonies, Colony A and Colony B, each 100 light years away from Metropole, and 100 light years away from each other. The wormholes to both colonies go 99.8596 years into the future when traveling from Metropole to either colony. Now Colony A sends a wormhole to Colony B. The Colony A wormhole also goes 99.8564 years into the future when going from Colony A to Colony B. This means if a traveler at Colony B went through the Colony A wormhole he would go back in time 99.8564 years. Then going from Colony A to Metropole he would go back in time another 99.8564 years. Then he could go from Metropole to Colony B and go forward in time 99.8564 years. The net result is that he ended up back where he started nearly a century before he left.

It seems that nature really doesn't like time machines. Here's why. Think about what happens when the Colony A – Colony B wormhole has gone just far enough that a light signal going through the wormholes can get back to where it left just as it is leaving. Now, since the propagating signal and the newly transmitted signal are both leaving at the same time, you have double the intensity. So this doubled intensity signal goes around and meets itself again, quadrupling its intensity. And so on. At this point, just as the configuration is on the verge of becoming a time machine, it becomes a perfect resonator for light signals, which then build up to arbitrarily high intensities until something breaks and you don't have an incipient time machine any more.

Now clever people will try to come up with ways around this — like putting a lead shield in the way of the signal's path. It turns out these tricks don't work. When you pull quantum mechanics into the picture, what get amplified are virtual fluctuations in the electromagnetic field and those can go around and anything it is possible to go around and through anything it is possible to go through. And it's not just light. All other particles behave the same way, so even if you somehow got the wormhole past the point where light would destroy it, it would be ruined by all kinds of other quantum fluctuations. You can't beat nature. And nature doesn't like time machines.

The consequence of this is that if you have closed loops in your wormhole network, it is really hard to keep time machines from forming. There are tricks you can play on a planet, but all interstellar wormhole networks form tree-like branching patterns without closed loops for just this reason.

But you can exploit this no-time-machine property. It is called a causality attack. If you are on one branch of a wormhole network and want to expand a bit but are blocked by a neighboring branch, you beef up all your wormholes and then send a wormhole to the neighboring branch. Something will break, but if it is a wormhole connection in the neighboring branch that is the weakest link, the network will break there. Now you have just stolen all the nodes (worlds) in the network that had been cut off by the break, and you can get to them using the wormhole you just sent.

In the Human expansion into the Milky Way, these sorts of attacks were common between the Americans and Chinese, Americans and Europeans, Europeans and Indians, and Indians and Chinese. This jockeying for territory was considered just the way things were done, and generally accepted back on Earth. The American president might engage in trade negotiations with the Chinese premiere the day after a causality attack stole a dozen American worlds and all their colonists for the Chinese, with little more than lodging and official complaint. The colonists, on the other hand, usually get pretty pissed off about such things.

• 

A description about how two wormhole networks would fight in my Vergeworlds setting

Synopsis

• It is difficult for an attacker from one wormhole network to capture worlds from another network in a timely manner.
  By destroying the wormhole to a conquered world, the defender can greatly slow down the attacker.
  
  
• The best way for an attacker to attack across a broken wormhole is to exploit any back doors that also link back to the same world.
  
  
• The Squirm used the wormhole communication system of the Mants and Gummis to get around broken wormhole links.
  
  
• Humans don't use wormholes for communication in the same way, so Squirm have been known to clandestinely abduct Humans, infect them with a mind-control parasite, implant a miniature wormhole in them, and set them loose to return home before beginning their invasion, in order to prevent a broken wormhole from stopping their advance.

We have already seen that when a metropole projects out wormholes to colonies, the connection from the metropole to the colony takes you considerably forward in time as well as through space. For example, if the metropole and colony are 100 light years apart, going from the metropole to the colony will take you nearly a century into the future.

Now suppose the Squirm capture the colony. The metropole will want to immediately break the wormhole to the colony. They can always project another wormhole if they want to counter-attack, which will connect across the same time-lag … from the point of view of the colony and the metropole, only as much time will have elapsed as it takes for the wormhole mouth to travel there (0.1414 years, in the above example). On the other hand, if the Squirm want to advance onto the metropole, they will need to project their own wormhole. But they're already nearly 100 years ahead in time compared to the metropole – projecting their own wormhole back the other way would result in connecting to a time coordinate nearly another 100 years ahead of the colony's time. This gives the metropole two centuries to prepare for the invasion. Two centuries of industrial output and military buildup, to fight off an invasion force which the Squirm only have a month and a half to prepare for. Therefore, the major objective of the Squirm will be to capture the wormhole before it can be destroyed, and clamp it open. Otherwise, their invasion of the metropole will almost always fail.

In the wars of the Squirm with the Zox and the Gummis, the Squirm were able to exploit a back door. Both the Zox Hierate and the Gummis used Antecessor technology, including ultra-miniaturized wormholes. These tiny wormholes were used for wormhole phones. A phone connected to a base station via a wormhole, allowing nearly instantaneous communication between the phone and the station. The station can then route your call to any other wormhole phone connected to the same station, or to other stations using wormholes between stations. The station ensures time-balancing via time dilation of either end (using miniature cyclotron-like devices based on affectors) or, in extremis, lengthening the wormhole throat. This avoids causality-related collapse in a wormhole-rich environment.

Taking a wormhole phone through another wormhole automatically avoids issues with causality and allows you to communicate instantly across interstellar distances, since the phone wormhole picks up the exact same time lag as the transit wormhole as it goes through. This made them popular with travelers. When a Squirm captured a phone a traveler was using, however, this gave them a wormhole link back to the phone's station which they could enlarge and send an invasion through.

Humans don't use wormhole phones. They use microwave transmissions and fiber-optic guided lasers. At first, this stymied the Squirm in their war with the Humans of the Indian bough. They hit upon an insidious solution, though. They developed a mind-control parasite that infected Humans and Pannovas. Before beginning an invasion, they would covertly locate a Human (Pannovas were very rare in the Indian bough) from a different world, abduct him or her, infect him or her with the parasite, and implant a miniaturized wormhole. The parasite instilled an overwhelming desire to return home. Only when the target arrived home would the Squirm invasion begin, providing them with a concealed route to different Human worlds.

In the Verge, all polities forbid taking wormhole phones through wormholes. Human technology is used exclusively for inter-world communication (usually routed through the main transport wormhole). This is one of the few Human technologies the Zox will use. Even the Gummis, with their lack of most authoritarian political organizations, form public health and safety committees that enforce this ban – usually with the enthusiastic support of the public since Gummis remember all too well the devastation of the Squirm. In the Verge Republic, Transit Law and FERA are responsible for detecting, tracking down, and neutralizing parasite-controlled travelers. Transit Law is also tasked with identifying and neutralizing covert Squirm operations before they can capture and infect citizens. Local law enforcement and health authorities also act to prevent and intercept infected people in their jurisdiction before they can become a threat.

It is worth noting that if two distinct networks connect to each other with more than one wormhole, it will form a closed loop. This initiates a causality attack, and the weakest link within that loop will break. This may well result in some of the worlds changing which network they belong to. The Gummis used this tactic extensively in their war with the Squirm. This allowed them to break pieces off the Squirm network and defeat them piecemeal, isolated from assistance of the concentrated force of the Squirm armed forces.

• 

• 

(An in-universe explanation as to why the Empire, et. al., prefer to use their special – i.e., yes, per here, space-magic enhanced – wormhole technology.)

A common question among newcomers to the field of spacetime engineering, especially as it applies to wormholes, is the reasoning behind our use of dynamic wormholes (i.e., those that are created, used, and collapsed in the course of a single gating) rather than static wormholes, permanently inflated to allow passage and held open by exotic mass-energy “frames”. This seems, to these questioners, more elegant: being less wasteful in terms of energy (although the cost of maintaining the unstable exotic mass-energy frames should not be undercounted; the analogous Andracanth ram is not designed for continuous operation), and requiring nothing on the part of transiting vessels.

Sadly, this is prevented by the interaction of static wormholes with relativistics. Transporting a wormhole end incurs the time dilation of relativistic flight, such that one can travel through the wormhole to the destination system, in the reference frame of the transported end, years or decades before the wormhole end is delivered in the reference frame of the sender; sometimes, indeed, while the linelayer would still be visible leaving the origin system! This means, in effect, that outgoing travelers through the wormhole are stepping years or decades into the future, while returning travelers are likewise passing into the past, vis-à-vis flat space-time.

While this has interesting astrophysical and galactopolitical consequences (amply dealt with elsewhere), it alone does not cause issues from the point of view of infrastructure; since a return through flat space-time must require (per the Luminal Limit) more time than the wormhole’s time differential, the block universe is preserved.

However, it is easily demonstrable that the only topology which guarantees this is a pure directed acyclic structure, or tree, in which only one path is available to outgoing and returning traffic.

This is undesirable from an infrastructure point of view, since it greatly limits the capacity of the network given the bottleneck links near its core; forces all otherwise cross-link traffic, even between nearby systems, through a single distant core node (likely to be, as a strategic aside, near to if not within the builders’ most important star systems); and causes both of these issues to expand geometrically with scale.

More importantly, while there are a few primarily theoretical exceptions, almost any alternative structure containing cross-links (and therefore cyclic structures) enables certain routes to function as closed timelike curves (i.e., a time machine), allowing particles, even virtual vacuum fluctuations, to return to their origin point at or before the time of their entry into the route. Such a path doubles the intensity of transiting particles with each retraversal (which all occur effectively instantaneously), thus creating arbitrarily high peak intensities, in turn resulting in the catastrophic resonance collapse of at least one of the wormholes along the critical path. Quite apart from the loss of route, the energies involved in this collapse along with those likely to be liberated from damaged stargate systems are such as to pose a significant hazard to the systems containing the mouths of the collapsing wormhole.

(This is also, as we will see later, perhaps the most important reason for the Imperial Timebase system being intertwined with stargate control systems at a very low level, and for the various sequencing and safety protocols encoded therein. While the wormholes used for gating are ephemeral, it would be possible – without coordination – for a simultaneous set of openings to form such a closed causal loop, which would then undergo such catastrophic collapse.

Bear in mind that, while we are able to lock the emergence of dynamic wormholes onto the empire time reference frame, the natural phenomenon of drift (q.v.) along t axis guarantees nonidentity, and as such this does not immunize loops of such wormholes from the catastrophic resonance collapse phenomenon.)

Since the point of collapse is controllable to a limited extent by the “strength” of the links along the CTC route, this effect is also weaponizable by hostile powers with wormhole capability (a causality attack, recognized by the Ley Accords as one prohibited form of causal weapon).

For these reasons, Imogen Andracanth’s team considered the static wormhole to not be viable as a large-scale interstellar transport technology.

 

– The Stargate Plexus: A Journeysoph’s Guide

• 

• 

(ed note: The novel uses the Foldspace system of FTL. The human empire had been expanding peaceably for a couple of centuries, encountering no alien species. Then the star Antares went supernova, which altered the topology of the foldspace lines. That's when the xenophobic Ryall aliens made their appearance. Up until now there were no foldspace lines linking the two networks. The supernova made a few, and the war was on.

The humans figured that the Ryall empire was larger than theirs, since in battles the Ryall always seemed to show up with more starships. As it turns out, that wasn't quite the case...

A human boarding party under command of Philip Walkirk captures a Ryall ship. A corporal mentions that one of the Ryall was acting funny.)

      “I found him amidships in one of the equipment rooms. He had this big bar he’d ripped out of some machinery and was using it to beat holy hell out of some access panel. Looked to me like he wanted to get through it and into the machinery beyond.
     “What did you say just now, Corporal?” he asked.
     “I said this damned crazy centaur attacked me, sir...”
     “No, about his trying to smash a machine. What machine?”
     “‘Fraid I don’t recognize this alien machinery too good, sir.”
     “Take me to it.”
     Sayers led the way, followed by Philip Walkirk and Sergeant Barthol. They moved through gloomy corridors until they reached a small compartment almost at the very center of the spherical ship.
     “Yonder machine over there, sir!” Sayer said, playing the beam from his hand lamp over a dented access panel.
     Philip gazed at the panel, blinked, and then emitted a low whistle.
     “This thing important, sir?” Barthol asked.
     “You might say that,” Philip replied. “What Corporal Sayers refers to as ‘yonder machine’ is their astrogation computer. The fact that he was trying to beat it to death may mean that their normal destruct mechanism failed to operate properly.”
     “That good, sir?”
     Philip Walkirk’s sudden laughter startled the two noncoms. “That box, Sergeant, may well contain information vital to the conduct of the war.”
     “What information, sir?”
     “If we’ve been very, very lucky, we may just dredge up a foldspace topology chart for the whole damned Ryall hegemony!”

---

     “Greetings, Captain Drake, Captain Dreyer, officers of Discovery and Terra, and colleagues,” Alvarez began in a loud, strong voice.  “As you know, ten days ago, Miss Lindquist and I went down to Corlis.  Our task there was to see if we could extract the astrogation data we captured with the Ryall ore carrier.  I am pleased to announce that we were successful!”
     Alvarez manipulated the screen control and caused the holoscreen to light up.  Near the bottom of the screen were two star symbols.  The larger was labeled ‘ANTARES NEBULA’; the smaller, ‘EULYSTA/Corlis.’  Between the two symbols was the dotted line marking an active foldline link.
     “Here you have the path by which we entered this system.  The link between Antares and Eulysta is quite recent.  It was originally formed when Antares exploded.  Because the Eulysta-Antares foldpoint leads directly into the heart of the nebula, the Ryall apparently consider it impassable.”
     Alvarez touched the control and a third star suddenly materialized in the depths of the screen.  “This is Carratyl.  It is the next system in from Eulysta.”  Alvarez paused and looked up from his notes.  “I hope everyone realizes that these names are translations of Ryall originals.  The original of ‘Carratyl’ sounds like someone clearing his throat.
     “It was Carratyl toward which the Ryall ore carrier was fleeing when we caught up with it.  Unlike Eulysta, which is virtually uninhabited, Carratyl is a bona fide system of the Ryall Hegemony.  It possesses a single inhabited world, Kalatin, which has a population of approximately one billion.  Kalatin is an agricultural world.  The pilot’s ephemeris from which we obtained this data indicates that there is a small naval base on the largest of its three moons.
     “Which brings us to the next system of interest,” Alvarez said as he caused a fourth star symbol to appear on the screen.  “I won’t trouble you with the Ryall name, for we humans have known this star since ancient times.  I give you Spica!”
     There was a sudden silence throughout the wardroom, followed by a low muttering from the astronomers present.  Drake gazed with amazement at the flock of symbols that had suddenly appeared on the screen.  From somewhere nearby, a voice muttered, “My God!  Six ... seven ... eight foldpoints!”
     Up on the podium, it was obvious that Boris Alvarez was enjoying their reaction to his sudden revelation.  He grinned as he said,  “As most of you have already noticed, Spica possesses a total of eight foldpoints.  This makes it the hub star of the largest foldspace cluster ever discovered.  More importantly, however, Spica is the premier system of the Ryall Hegemony, as you will shortly see.”
     Alvarez manipulated his control and the other stars of Ryall space began appearing.  Even before the full diagram was completed, the pattern was clear to those who knew how to read a foldspace topology chart.  Drake scanned the diagram in growing disbelief.  Short strings and branches of foldlines tied the individual star systems of the Ryall Hegemony to one another.  Here three stars were strung together; there two others branched away from a third.  In another case, four stars were connected in a rare closed ring pattern.  As he searched the diagram, however, Drake could find no telltale line connecting any two of the small groupings save through the central hub system of Spica.  Alvarez confirmed his growing suspicions a few seconds later.
       “There are twenty-two separate systems in the Ryall Hegemony.  Every single one of them belongs to the Spica Foldspace Cluster!”

---

     After long minutes spent staring at the blue-white star, Drake cleared the screen and brought up another view.  This time the screen was filled with more than a hundred stars.  Having discovered that he could not sleep after a long, eventful day that had included the fleet’s departure from Corlis, Drake had put his insomnia to good use.  He plotted the positions of the twenty-two stars that Boris Alvarez had identified as being part of the Ryall Hegemony, and then color-coded them a bright crimson for easy identification.
     After studying the shape of Ryall space for a moment, he had called up Discovery’s astrogation database and input the same data for the nearly eighty inhabited stars that comprised human space.  He had colored the human stars green, and then merged them with the Ryall star map.  Finally, he had reduced the scale of the display to the point where he could take in the two realms at a single glance.
     For five hundred years, the human race had expanded out along the foldlines, eventually occupying an ellipsoid-shaped region of space some 500 light-years long by 200 light-years in diameter.  The Ryall had been expanding as well, occupying their own region of the galaxy.  Ryall space was approximately one-third the volume of human space, and nearly a perfect sphere.  The two realms shared a common boundary, and actually interpenetrated in the region around Antares.  Displayed as they were on the same screen, it was obvious why humanity was slowly losing its war with the centaurs.
     The long history of warfare on Earth had taught generations of generals the value of seeking favorable terrain on which to fight a battle.  From Waterloo to Little Round Top to the Battle of Prudhoe Bay, the victors owed their success more to the lay of the land than to their military superiority over the vanquished.  What the star map did for Drake was prove that the same could well be true of humanity’s war with the Ryall.
     Ryall space, localized as it was within the Spica Foldspace Cluster, was blessed with short, internal lines of communication and a high degree of inter-connectivity.  No system in the hegemony lay more than six foldspace transitions from any other system.  Human space, on the other hand, was strung out along the axis of the galactic spiral arm.  The distance between the two farthest human systems was fifteen transitions.
     The tactical and strategic value to the Ryall of their foldspace cluster was substantial.  If attacked, they could spread the alarm and rush reinforcements anywhere in their realm much more quickly than could Homo sapiens in similar circumstances.  Once mustered, their forces could shift rapidly from trouble spot to trouble spot, allowing each ship to do the work of two or more human craft.

---

     “At least we have Professor Alvarez’s data.”
     “I only wish it weren’t quite so distressing. Drake.”  It had not taken long for Gower to recognize the problems associated with fighting an enemy whose home territory was contained entirely in close-coupled foldspace cluster.  “My battle staff estimates the force multiplier effect of their internal lines of communication to be at least two, and possibly as high as three.”
     “Yes, sir.  That was my conclusion as well.”

---

     “Yes, sir,” Belton replied.  The admiral got to his feet and walked over to a bookcase that covered one entire wall of the first coordinator’s office.  He manipulated a control and several things began to happen simultaneously.  The window behind the coordinator’s desk turned opaque while a section of the bookcase swung forward to reveal a wall-mounted holoscreen.  The screen came alight to reveal a foldspace map of the Ryall Hegemony.  The map had been color coded to show the various interconnecting paths between Ryall stars.
     “The data you gentlemen provided has been a godsend,” the admiral began, gesturing toward the foldspace topology chart.  “In the nearly two weeks since we received this new information, our analysts have been working round the clock to incorporate it into our strategic and tactical doctrines.  In order to accomplish this, we have been reanalyzing practically every engagement we have ever fought with the Ryall.  In so doing, we have understood things that have puzzled us for the better part of a century.  In short, gentlemen, we have been learning the advantages that the Spica Foldspace Cluster confers on the Ryall.
     “The most important advantage our enemies derive from the cluster comes from its unusually high connectivity quotient. This close coupling of the Ryall stars manifests itself in a number of ways, most of them bad from our point of view.  As has already been noted by Captain Drake and others, the Spica Cluster allows the Ryall to utilize their forces much more effectively than can we.  In other words, they are able to do the same job with far fewer ships.”
     “Do you have any quantitative figures on that?” Gower asked.
     Belton nodded.  “We think the factor is approximately two point seven.  For the non-military men among us, that means that 100 Ryall starships can do the work of 270 human ships.”
     A low whistle emanated from somewhere on Drake’s left.  He was not sure, but he thought it came from Stan Barrett.
     Belton continued.  “Nor is force multiplication the only manifestation of a high connectivity quotient.  For with their short travel times, the Ryall have no need to defend in depth.  They can concentrate their forces in those systems where they dispute with us.  Should we open up a new front anywhere else, it is a relatively easy matter for them to rush forces to the new battle zone.
     “Lastly, of course, there is the advantage that their tightly bound foldspace cluster confers on their industrial capabilities.  The short travel distances and times, plus the numerous opportunities for transshipment, allow their planetary economies to be integrated with one another whereas our own worlds’ economies are only loosely bound together.  With their low transportation costs, Ryall worlds can afford to specialize.  We see this in the Ryall system of Carratyl, whose primary activity is the production of agricultural products for the rest of the hegemony.  Presumably, there are Ryall worlds that specialize in the production of starships, and still others who are heavy or light industry specialists.”
     “So far, Admiral,” Drake said, “you haven’t said anything we didn’t already know.”
     “Quite true, Captain.  I have been discussing the strategic consequences of the fact that the Ryall Hegemony occupies the Spica Foldspace Cluster.  These are obvious to anyone who cares to think about them.  Now, let us turn to the tactical advantages, which aren’t so easily determined.”
     Belton picked up a screen control and punched a number into its keypad.  The Spica foldspace topology chart disappeared, to be replaced by one showing the relationship of the Hellsgate, Aezer, and Hermes systems, including all the foldpoints of each.  Belton continued:
     “Let us consider our battle plan for breaking the Ryall blockade of Aezer.  A Grand Fleet battle group will launch a diversionary attack against the Aezer-Hermes foldpoint in the hope that the Ryall commander will choose to strip his Aezer-Hellsgate defenses to provide reinforcements.  Some forty hours later, a mixed force of Grand Fleet, Sandarian, and Altan starships will launch an all out assault against the weakened Aezer-Hellsgate foldpoint.  Once Aezer-Hellsgate is open, our ships will race to blockade the foldpoint leading back to Ryall space in order to cut off the flow of Ryall reinforcements, and to attack the Aezer-Hermes defenses from behind.”
     Belton turned to face Gower and Drake.  “It’s a good plan, gentlemen.  It has the elegance of simplicity and just the right touch of genius.  Unfortunately, it has one minor defect.  It won’t work!”
     There was a long pause in which Drake looked at Gower, and then both men locked eyes with their respective diplomatic representatives.  Finally, Gower cleared his throat and said, “I fail to see a flaw in our thinking, Admiral Belton.”
     “The flaw,” Belton replied, “is in the assumption that the Ryall will denude the Aezer-Hellsgate defenses in response to a threat against Aezer-Hermes.  That seemed a logical assumption two weeks ago when you first presented your plan to us.  However, now that we understand the hegemony’s topology, we no longer believe the Ryall will choose to reinforce from within the Aezer system.
     “Rather, we believe the Ryall commander will call for reinforcements from the home worlds, which means that our force attacking from Hellsgate will be thrown against full strength foldpoint defenses.”
     “The Ryall won’t have time to get ships from the heart of Ryall space,” Admiral Gower replied.
     “I wish that were true,” Belton responded.  “However, we have simulated it a hundred times using a hundred different scenarios.  Like us, the Ryall use communications relays between their front lines and their home worlds.  It will take them less than eight hours to get word of the initial attack back into the heart of Ryall space.  Even if we launched simultaneous assaults against both foldpoints and were able to punch through without unacceptable losses, by the time we reach the third Aezer foldpoint, we’ll find it boiling with reinforcements.”
     The silence was even longer this time.  Coordinator Gellard was the first to speak.  When he did so, there was great sadness in his voice.  “I’m sorry, gentlemen, but under the circumstances, we will have to withdraw our support from the plan to attack Aezer.”

---

     “Gentlemen, three days ago you presented the results of a Grand Fleet analysis concerning the Altan-Sandarian plan to drive the Ryall from the Aezer star system.  At that time, you pointed out that the plan’s basic assumption – that the Ryall would strip the Aezer-Hellsgate foldpoint defenses to reinforce Aezer-Hermes – was incorrect.  By utilizing the astrogation data we provided, you proved that the fast communications and travel times within the hegemony made it likely that the centaurs would reinforce directly from their home stars.  Since such reinforcement makes a diversionary attack worse than useless, you recommended that the attack not be carried out as planned.
     “Now, the obvious solution is to such a predicament would be a strategy of simultaneous large scale attacks against both foldpoints.  Unfortunately, your analysis proved once again that such a tactic has little chance of working.  The problem is that we would not be able to seize the system quickly enough to prevent the Ryall reinforcements from entering it.  In such a situation, superior Ryall mobility would likely allow them to overpower any of our ships that survived the initial assault.
     “Finally, you recommended that we accept the fact that our situation is hopeless and abandon our homes while there is still time to do so.”
     “Everyone in this room is well aware of recent events, Captain Drake.  What is your point?”
     “I am merely reminding you, Coordinator, that as things stand, no course of action appears likely to break the Ryall stranglehold on the Aezer system.  I propose that we accept this unpleasant fact and look elsewhere for the solution to our dilemma.  When solving a problem, gentlemen, it is always useful to step back a bit and look to fundamentals.
     “Several weeks ago, Coordinator, I sat in Grand Fleet Headquarters and listened to your chief strategist attempt to explain away the obvious fact that humanity has been losing ground to the Ryall for much of the past century.  What I heard was that the Ryall have more ships and a larger resource base than we do.  Yet, the Ryall data we captured shows this not to be true.  The Ryall fleet is not larger than our fleet.   Their ships are no better equipped.  Indeed, the Ryall Hegemony is substantially smaller than human space, and Ryall warriors are neither smarter, more tenacious, nor braver than human warriors.”
     Drake paused in his recitation and looked at his audience.  “So why, gentlemen, are we still losing this war?”
     “They have Spica,” Coordinator Blenham said.
     “Correct!  At the risk of disagreeing with the poet, the fault lies not in ourselves, but in our stars.  The Ryall are the beneficiaries of a simple accident of nature.  They inhabit the Spica Foldspace Cluster.”
     Drake turned abruptly and activated the control that brought the holoscreen to life.  In the screen’s pseudo-depth was a diagram very like the one Drake had developed that first night he had learned the Ryall secret.  Scintillating in the blackness of space lay all the stars of human space and the Ryall Hegemony.  Each star was color coded, and had a tiny line connecting it to its neighbors.
     “Here we see the problem displayed in a form which is relatively easy to understand.  Where we humans are spread along the spiral arm in a collection of stars only barely related to one another, the Ryall inhabit a compact ball of stars, each tied closely to Spica.  As Admiral Belton explained in our last meeting, the advantages of this arrangement include substantially faster communications between systems, a more efficient utilization of natural resources, and a degree of industrial integration which our own worlds can only dream about.  While none of these factors is decisive in and of itself; taken in toto, they give the Ryall an advantage that we find nearly impossible to overcome.”
     Drake stabbed out with a finger and pointed to the star that was at the center of the ball of red threads that permeated the Ryall portion of the screen.  “If we are ever to win this war, we will have to counter the advantages which the Ryall derive from their foldspace cluster.”
     “And how do you propose to do that, Captain?” Blenham asked.
     Drake grinned.  “Quite easily, Coordinator.  All we need do is capture and hold Spica!”

---

     There was a sudden silence from each of the listeners.  Even Bethany was surprised to the point of speechlessness.  Finally, Admiral Belton came alive.  He looked from the screen to Drake and back again.
     “Captain, I hope you don’t take offense at my next remark. Whether you do or not, however, I must ask it.  Are you drunk, or just plain crazy?”
     “Neither, Admiral.  It can be done.  I know because I’ve spent the past two days proving that it can be done!”
     “If you can’t dislodge the Ryall from Aezer, how the hell do you propose securing the central star of their whole damned hegemony?  For Christ’s sake, how do you propose getting to Spica in the first place?”
     “By using the back door, Admiral.  Specifically, the transition sequence will be Antares, Eulysta, Carratyl, Spica.”
     “They’ll slaughter you before you get halfway there.”
     “No they won’t.  Remember, the entire Ryall fleet is centered in the Aezer, Constantine, and Klamath systems.  Eulysta, being one of their interior stars, is virtually uninhabited.  Even if they are rebuilding the Corlis complex, there will not be more than half-a-dozen commercial starships in the system.  As for Carratyl, it is a backwater agricultural system with a single naval base and no foldpoint defenses at all.  If we can only get to the Carratyl-Spica foldpoint before the alarm is spread throughout the hegemony, we’ll be able to pour an overwhelming force of ships into Spica before they’ll be able to react.”
     “Let’s say we succeed in capturing Spica,” Belton said.  “What’s to stop the whole damned Ryall navy from pouncing on us immediately thereafter?”
     “Nothing, Admiral.  In fact, you can expect them to do just that.  Remember our own problems in attempting to break the Ryall blockade of Aezer.  By capturing Spica, we turn the existing tactical equation on its ear.  This time it will be the human forces that possess interior lines of communication, superior coordination, and mobility.  For once, it will be the Ryall who will have to fight blind.  They will be forced to feed their fleets through the various foldpoints piecemeal, and we will destroy them the same way.”
     “How long do you think we can hold eight separate foldpoints against determined attack?” Ryerson asked.
     “As long as necessary, sir.  Our assault force will hold only long enough for us to bring up orbital fortresses.  We will get those by stripping some of the foldpoint defenses here in human space.  Once the fortresses are in place, we will be able to hold Spica as well as we hold our own systems.”
     “Hold it for how long?”
     “Until they either learn civility or run out of ships.”
     “That could take the better part of a thousand years!”
     “I don’t think so, sir.  You see, the centaurs’ great advantage is also their Achilles’ heel.  The hegemony depends on fast, inexpensive star travel.  Their industrial base is highly integrated, with each world specializing in what it does best. The moment we succeed in blocking the hub system of their foldspace cluster, their industrial machine begins to fall apart.  If we maintain our choke hold long enough, the hegemony will suffer a catastrophic economic collapse.  Once that happens, their ability to wage war will be gone.  We can then capture their home systems one at a time.  We’ll force them back to their home worlds until they learn to accept our right to exist.”

The Solar Government was to expend several trillion Labor Credits before it discovered that...

• (a) the discontinuity window could not reliably be produced on or near a planetary mass
• (b) only 139 people out of 19 billion could produce the effect
• (c) they were all women
• (d) they were all powerfully telesthetic (i.e., clairvoyant), and mildly telekinetic
• (e) a window could only be created between two positions in space that the Telesthetic was "comfortable" in and felt she "knew"
• (f) a Gnostech (computer with artificial intelligence) initiated by the using Telesthetic was required
• (g) bionic/electronic techniques could be used to amplify and refine the effect, but no pure-machine system could create it
• (h) the range of the effect was theoretically unlimited but its accuracy was subject to degradation with the square of the distance.

Psionic linking techniques and the Telesthetics founding of the Telesthetic Guild was the response. It is probably the heavy use of empathetic bridging in these techniques that explains the remarkable fact that no member of the Guild, even while on opposing combat teams, has ever deliberately caused another member's death.) This solidarity of Telesthetics was almost totally responsible for the virtually bloodless conduct of the Intra-Specific Wars of Autonomy in the 25th Century.

In a sense the Outleap itself was responsible for the Wars of Autonomy: it dispersed and enlarged the human community into a multi-system race which was heavily dependent upon one socioeconomic factor, one resource that could not be synthesized by technology — the Telesthetics. The number of Telesthetics available to a given system was almost purely a function of how much population was contained within or controlled by that system.

The freedom from birth-controls in the colonized systems did have the desired effects of providing the population basis for "home-grown" Telesthetic crews to operate the Star Gates and the increasing number of Teleship.

It also, however, had several counter-productive side effects: (a) The vastly increased and dispersed human population became ungovernable by the institutions of the Solar Hegemony, (b) the "frontier" societies tended to produce divergent eco-political systems that either wanted independence, or worse, attempted to impose their provincial "solutions" on the rest of humanity.

All these factors conspired to produce a number of essentially pointless wars.

• 

Fuller still looked puzzled. "See here; with this new space strain drive, why do we have to have the molecular drive at all?"

"To move around near a heavy mass — in the presence of a strong gravitational field," Arcot said. "A gravitational field tends to warp space in such a way that the velocity of light is lower in its presence. Our drive tries to warp or strain space in the opposite manner. The two would simply cancel each other out and we'd waste a lot of power going nowhere. As a matter of fact, the gravitational field of the sun is so intense that we'll have to go out beyond the orbit of Pluto before we can use the space strain drive effectively."

“What in blazes has that to do with your failure to obey orders?” he demanded, with explosive vehemence. “That ship must have used an interstellar space-warp drive to appear out of nowhere in the middle of the Asteroid Belt. And you deliberately let it slip away from you!”

Langford shut his eyes before replying. He saw again the myriad stars of space, the dull red disk of Mars and the far-off gleam of the great outer planets. He saw the luminous hull of the alien ship looming up out of the void. An instant before, the viewpane had been filled with a sprinkling of very distant stars with a faint nebulosity behind them. The ship had appeared with the suddenness of an image forming on a screen, out of the dark matrix of empty space.

Langford leaned forward, a desperate urgency in his stare. “Mere alienage doesn’t justify the crime of murder , sir!” he said. “Attacking an alien race without weighing the outcome would have been an act of criminal folly, charged with great danger to ourselves.”

(ed note: this is the earliest reference I've mananged to find of the term "space warp drive")

• 

"Ladies, gentlemen! We are ready for our first Jump. Most of you, I suppose, know, at least theoretically, what a Jump is. Many of you, however—more than half, in point of fact—have never experienced one. It is to those last I would like to speak in particular.

"The Jump is exactly what the name implies. In the fabric of space-time itself, it is impossible to travel faster than the speed of light. That is a natural law, first discovered by one of the ancients, the traditional Einstein, perhaps, except that so many things are credited to him. Even at the speed of light, of course, it would take years, in resting time, to reach the stars.

"Therefore one leaves the space-time fabric to enter the little-known realm of hyperspace, where time and distance have no meaning. It is like traveling across a narrow isthmus to pass from one ocean to another, rather than remaining at sea and circling a continent to accomplish the same distance.

"Great amounts of energy are required, of course, to enter this 'space within space' as some call it, and a great deal of ingenious calculation must be made to insure re-entry into ordinary space time at the proper point. The result of the expenditure of this energy and intelligence is that immense distances can be traversed in zero time. It is only the Jump which makes interstellar travel possible.

"The Jump we are about to make will take place in about ten minutes. You will be warned. There is never more than some momentary minor discomfort; therefore, I hope all of you will remain calm. Thank you." The ship lights went out altogether, and there were only the stars left.

It seemed a long while before a crisp announcement filled the air momentarily: "The Jump will take place in exactly one minute." And then the same voice counted the seconds backwards: "Fifty...forty...thirty...twenty... ten...five...three...two...one..."

It was as though there had been a momentary discontinuity in existence, a bump which joggled only the deep inside of a man's bones.

In that immeasurable fraction of a second, one hundred light-years had passed, and the ship, which had been on the outskirts of the solar system, was now in the depths of interstellar space.

Someone near Biron said shakily, "Look at the stars!"

In a moment the whisper had taken life through the large room and hissed itself across the tables: "The stars! See!"

In that same immeasurable fraction of a second the star view had changed radically. The center of the great Galaxy, which stretched thirty thousand light-years from tip to tip, was closer now, and the stars had thickened in number. They spread across the black velvet vacuum in a fine powder, back-dropping the occasional brightness of the nearby stars.

• 

The warp theory of my esteemed colleagues (and I am sure they will correct me if I am wrong) is based on the principle that two separate units of anything cannot exist in the same place at the same time; nor can they coexist without each having an effect upon the other. When the units are energy fields, the effect is supposed to be spectacular. (The effect is spectacular —I will admit that. As my esteemed colleagues have already so admirably demonstrated, the effect is certainly spectacular... though I somewhat doubt that this was the specific effect they had hoped for.)

Theoretically —at least, as their theory says —when two continuous fields are overlapped, it will cause a wrinkle in the fabric of existence. Unfortunately, the continuous energy field is only a myth —a mathematical construction. It is a physical impossibility and cannot exist without collapsing in upon itself.

Of course, there are still some members of this learned academy who insist on remaining doggedly skeptical of this fact of life. It is almost pitiful to watch them continue these attempts to generate an energy field that is both continuous and stable. So far, the only thing that they have succeeded in doing is to convert several million dollars' worth of equipment, buildings, and surrounding property into so much slag. (Oh, and incidentally, in doing so, they have also proven me correct.)

DR. J. JOSEPH RUSSELL, PH.D., M.A.. etc., comments to the Board of Inquiry into the Denver disaster

---

Insufferable old windbag!

ANONYMOUS "ESTEEMED COLLEAGUE"

---

Dammit! It's like trying to stack soap bubbles!

DR. ARTHUR DWYER PACKARD, remark overheard by lab. assistant and quoted by Duiy Hirshberg in "Packard —Behind the Myth"

---

In light of events, it would be criminal to let them continue.

DR. J. JOSEPH RUSSELL, comment to newsmen after appearing before the Board of Inquiry

---

Actually, they were on the wrong track to begin with. The problem was not to create a continuous and stable energy field at all —but only to overload a section of space. Once they began thinking of it in those terms, the. solution was obvious —and even practical, considering the then existing technology.

The answer lay in the use of a series of interlocking noncontinuous fields. The noncontinuous field gives the illusion of continuity, but like a strobe light, the field is actually a very rapid series of ons and offs. Several noncontinuous fields working in phase can create a stable continuous field. Each of the separate noncontinuous energy fields fills in the gaps of the others.

Three noncontinuous fields can dovetail their functions to make one continuous one, and two continuous energy fields can be overlapped to generate the much sought after warp.

When six field generators are working in phase and all on the same section of space, a great pressure quickly builds up. Something has got to give. Usually space does.

HOWARD LEDERER, Encyclopedia of 1,000 Great Inventions

---

Dammit! Why didn't I think of that?!!

Remark attributed to DR.ARTHUR DWYER PACKARD

---

Because, I did.

Remark attributed to DR. J. JOSEPH RUSSELL

---

The warp has no relation at all to normal space. It is a bubble, or miniature universe. Within it a ship still obeys all the known laws of physics, but it is totally separated from the outer universe.

The bubble, or warp, is made up of great energies locked together in a titanic embrace. The potential power inherent in that embrace is far greater than the sum of the component energy fields —not just because the bubble is a stable construct, but because it is a dimple in space itself. The very structure of existence is pressing against it, trying to restore itself to a condition of minimum distortion. With such an infinite store of unexpressed force to draw upon, the potential power of the system is almost unlimited. (In practice the limit is the size of the ship's generators.)

If a secondary set of fields is superimposed across this point of pressured space —that is, the warp —it acts to liberate some of this great power and simultaneously provides a focus for it. As every second sees the warp restored to stability, the bubble cannot collapse; but this continued release of energy must be somehow sublimated —and it is; the effect is the introduction of a vector quantity into the system.

Because the shape of the secondary fields can be controlled, they can be used to produce a controllable velocity in any direction. The warp can be made to move at velocities many times the speed of light.

The Einsteiniun time-distortion is neatly sidestepped, as the ship is not really traveling faster than light—only the warp is. The ship just happens to be inside it. It is the warp that moves, the ship moves within the warp and is carried along by it. Consequently, a starship has two velocities, one is the realized faster-than-light velocity; the other is the inherent normal space velocity....

... For maneuvering within a planetary system, inherent velocity is an important resource; but unless it is compensated for, it can cause havoc to a ship in warp....

JARLES "FREE FALL" FERRIS, Revised Handbook of Space Travel