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I have this idea for a science fiction story (It actually came to me in a dream.). In the distant future, the inhabitants of the Milky Way have formed a galaxy-spanning civilization similar to the Republic in Star Wars. However, instead of using FTL starships to travel between planets in different star systems, they move the planets. Each member world has been outfitted with a powerful jump drive allowing it to travel tens of thousands of light-years in the blink of an eye. It might work by temporarily enlarging microscopic primordial wormholes (Einstein-Rosen bridges) and "warping" through them, but that's just one option.

A planet will use its jump drive to teleport from its current star system to another one, where it will assume an orbit in its habitable zone. A given planet will undergo about 1 to 4 jumps per day, with each jump being planned weeks or months in advance. When two planets are in the same star system, people use sub-light spaceships to travel between them. After a ship makes the trip from one planet to another, it can land or it can enter orbit and "ride out" the next jump. Since there may be a dozen or so member worlds in a given star system at a given time, a single ship can reach any other member world within only a few jumps (Think Six Degrees of Kevin Bacon.).

My question is, how can I justify this system? What principles, based in real physics or sci-fi logic, can I use to explain why using a starship for FTL travel is impossible/more difficult/less efficient but using something as massive as a planet to do it is possible/less difficult/more efficient?

Keep in mind that FTL communication exists in this setting, and may use a technology similar to that of the planetary jump drives or something completely different.

Edit: I might call my story "The Planetary Exchange" as a parallel to the Stock Exchange. And the jumps would be coordinated via a central computer network; it wouldn't be a chaotic free-for-all.

Another Edit: Oh, and during a jump any moon(s) in orbit around a planet will also be taken along for the ride, not just spaceships.

Update: Thank you everyone for all of your excellent answers! I have chosen BBeast's answer as the most helpful. My favorite is their "economy of scale" theory, though I also really like the heat sink explanation. But feel free to continue speculating. This is all really interesting! :)

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Jul 9 at 14:45
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    $\begingroup$ I feel like adding a planet to any solar system would be quite disruptive to everything around it—but one option is that each planet is roughly the same size, and the jump switches places (like musical chairs) $\endgroup$ – Mirror318 Jul 9 at 21:03
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    $\begingroup$ @MarkMoralesII well that could be the villain's plan all along... $\endgroup$ – Mirror318 Jul 9 at 22:39
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    $\begingroup$ "1-4 jumps per day, planned weeks or month in advance" ? Did you mean years? $\endgroup$ – Aganju Jul 10 at 1:51
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    $\begingroup$ @Aganju Or maybe you were asking if I meant 1 to 4 jumps per year rather than per day. I meant per day, despite how crazy that may sound. $\endgroup$ – Mark Morales II Jul 10 at 3:58

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There are two broad reasons why the jump drives are only used on planets. The first reason is that some property which planets have but spaceships lack is required for the jump drive to work successfully. Two such properties come to mind: gravity and thermal mass.

The second reason is that it is possible to move smaller objects, but due to some scaling property of the jump drive technology it is not practical to move small things. This works best if we assume the jump drive to require a large amount of external infrastructure to operate.

In all scenarios, we require that the energy to jump scales slowly or not at all with the volume of space or amount of matter that is jumping. If this is not the case, then moving planets would be virtually impossible and the technology would instead focus on making spaceships suitable for the jump drives.

For this, we shall assert that the majority of the energy used in the jump drive is to initiate the connection between two distant points in space. The energy to envelope a larger region of space or transport additional mass does not grow too quickly, up to at least planetary scales.

You can use choose one or a combination of all of the below factors for your jump drives, adjusting their significance as you see fit.

Jump Drives Require Gravity Wells

Planets, by definition, have substantial mass and produce substantial gravity. Spaceships, on the other hand, are rarely large enough to have any noticeable gravity (except with very sensitive instruments); any exceptions to this would be plot fodder for your story.

By general relativity, gravity bends spacetime, with regions of high mass (such as around a planet) having high curvature. A wormhole requires bending space to the extreme. So let us postulate that the jump drive generates some sort of wormhole.

This jump drive requires existing curvature in spacetime (that is, an existing gravity well) to generate or contain a wormhole. Of course, small objects still have some gravity, so we have to apply some form of scaling. Either the amount of energy required to perform a jump goes up prohibitively quickly if there is too little gravity, or the distance of the jump is smaller with less gravity.

This scenario, where the jump drive converts an existing gravity well into a wormhole, has an interesting consequence. Compact objects such as neutron stars and black holes have extreme gravitational curvatures while often being only a few kilometres in diameter. This means that, if you could equip a neutron star or black hole with a jump drive, you could use it as a sort of super jump drive. I would posit that this would give the jump drive a ludicrous range, suitable for intergalactic travel. It may also be able to create a very large wormhole which could cover an entire solar system.

If a spaceship wants to jump independently of a planet, it would need to somehow generate a strong gravitational field. It might be able to do this with a miniature black hole, although the black hole can't be too small otherwise Hawking radiation would lead to its untimely evaporation. Creating and containing a black hole is a non-trivial exercise, though, so even if it is possible it won't be common.

Jump Drives Require Heat Sinks

Thermodynamics is a harsh mistress. As a consequence of the second law of thermodynamics, every process which does useful work also produces some amount of waste heat. The second law also dictates that you can't turn the waste heat into a more useful form (unless you produce even more waste heat to compensate). By the first law of thermodynamics, it is impossible to make that heat simply disappear. As such, you have to put that heat somewhere.

In space, the typical thing to do is radiate away that heat, using radiators and black-body radiation. However, there is a limit to how fast you can radiate your heat before your radiators have to be so hot that they'll melt.

The solution to temporarily having more heat than you can get rid of is to have a heat sink. This is something which can absorb a large amount of thermal energy while only rising a little bit in temperature. A good heat sink has a lot of mass.

The jump drives require an enormous amount of energy to be expended in a relatively short amount of time. This produces a stupendous amount of waste heat. So much waste heat, in fact, that any object as small as a spaceship would instantly vaporise.

Planets are far more massive than any practical spaceship, thus have a much larger heat sink than any spaceship could have. If you need something approaching a planetary mass of matter for a heat sink for your jump drives, then only planets can have jump drives.

Such a jump drive would have most of its infrastructure on the planet (although it can have power beamed to it from a local Dyson swarm), distributed across the entire planet as many nodes (the more the better). Each node of the jump drive would have 'roots' made of some material with superlative thermal conductivity stretching deep into the planet's crust and/or oceans. When the jump drive is operating, each node sends the heat it produces through its roots into the planet, increasing the planet's temperature by a tiny amount. Each node would also have a vast array of radiators ('leaves', perhaps) which it uses to get rid of that heat in between jumps.

This jump drive, which requires massive heat sinks, precludes their use with non-terrestrial objects (such as gas giants, stars or black holes) unless you build a megastructure with the mass of a terrestrial planet surrounding that object. This would not be practical for regular transport and trade, making it only useful if you need to move the gas giant/star/black hole somewhere else for whatever reason.

If someone wanted to use this jump drive with a spaceship, they would have to do one of two things. Either they would have to find a sufficiently massive heat sink (an asteroid, perhaps, or maybe some unobtainium bricks), or invent a vastly more efficient jump drive to reduce the amount of waste heat.

A consequence of this jump drive is the accumulation of heat if you jump too rapidly. While multiple jumps in rapid succession may be possible (recharging of capacitors/batteries permitting), that won't allow enough time to radiate away all the heat produced in the previous jump. To avoid jump drive induced global warming, you would probably need to have an average frequency of jumps much less than once per day (although this can involve multiple jumps in a couple of days followed by a few weeks or months of no jumping, although I would imagine general transit would prefer to space out the jumps).

Economy of Scale

The prior reasons are physical constraints which might be used to make jump drives impossible for small objects. However, we can instead invoke the economy of scale which can make it impractical to routinely transport small objects while still leaving open the possibility.

Let us suppose that the member solar systems each have Dyson swarms or some similar technology which harnesses an appreciable fraction of their star's power, and that this is the largest source of power in the solar system. This is expected for a Type III civilisation.

This jump drive requires an enormous amount of energy to function. So much energy that it requires most of the power of the Dyson swarm. As such, a given solar system can only operate a limited number of jump drives in a given period of time. This makes jumps per day a limited resource which must be managed carefully.

We require the scaling of energy with the size of the jump to be very small compared to the baseline energy needed to initiate the jump.

Because the number of jumps is limited, but the amount of stuff you can take per jump is not limited (or at least has a much looser limit), we want each jump to take as much stuff as possible. A planet probably has the most amount of civilisation-useful stuff in one place, so the optimal solution is to move the whole planet and everything on and around it.

Note that if this economy of scale is the only constraint, then equipping spaceships with jump drives is still possible. If the jump drive instead operates purely on external hardware (with a solar system spanning wormhole projector, perhaps), then any objects can be transported without having to equip them with a jump drive.

However, using a star system's power to operate a jump drive for a mere spaceship (or fleet of spaceships) would be the equivalent of shutting down a nation's airports, seaports and major roads for several hours. If a character or entity is able to get authorisation for such a move, then that character or entity (or their task) must be of galactic importance.

With this jump drive, if someone were to invent a compact and portable power source which rivalled that of a star and equipped a spaceship with that power source, then that spaceship could jump at will. But such a power source would be a game-changing technology.

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    $\begingroup$ The potential to scale up the jump mechanism to harness a black hole has real season finale potential. Second season guaranteed! $\endgroup$ – Willk Jul 8 at 14:50
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    $\begingroup$ Two early-ish examples of "require gravity wells" are Cities in Flight by Blish (the spindizzy drives mentioned in comments to OP), and Macroscope by Piers Anthony. $\endgroup$ – Ross Presser Jul 8 at 15:10
  • $\begingroup$ Spindizzies initially worked on the scale of a city like Pittsburgh or NYC; in a later book they started moving an entire planet. (The larger the spindizzy object, the faster FTL it could go. Spindizzies were not a jump drive.) $\endgroup$ – Ross Presser Jul 8 at 15:12
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    $\begingroup$ Of course, if it's a technical reason, i would assume that they jump otherwise uninteresting mule planets, not the nice habitable ones. all of the jumping around has to play havoc on vegetation, weather, and biorhythms. $\endgroup$ – ths Jul 8 at 15:40
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    $\begingroup$ I would add that a possible mechanism is that the drive is capable of increasing an existing gravity well, but not creating one from scratch. So the bigger the well they start from, the less energy it is required to reach wormhole levels. Biggest thing we can easily move is the whole planet :) $\endgroup$ – bracco23 Jul 9 at 9:05
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Jump drives are just plain huge

The reason they move entire planets around instead of smaller ships is because, by the time you've put all necessary equipment for a functioning jump drive into your ship, the ship's size is edging into planetary scales. ("That's no moon... it's a space station!") Putting the jump drive onto a planet is easier and cheaper than building a ship that large.

There may still be some moon-sized starships out there, but they would be built for very specific purposes and rare enough that the average person might question whether they actually exist or if something that outlandish is "obviously" an urban legend.

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    $\begingroup$ Adding unto this, maybe the difference in cost of moving a lot of mass versus just the jump drive itself is negligible, making it more sensible to just bring along everyone, so you have everything ready in case you need it. $\endgroup$ – William Mariager Jul 8 at 20:05
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Hyperspace is dangerous and/or power threshold

Danger Will Robinson! There's gravel, small boulders, charged particles and pirate ships (maybe even Space Hulks!) in hyperspace. A planet with fully certified Van Allen belts and a decent atmosphere will have no trouble with the natural hazards and has the firepower to stomp aggressors flat. (Ships riding out the trip in low orbit must ensure that their orbital path puts them "behind" the planet during the jump window in order to be protected.) A hyperspace-capable ship, however, has no chance of surviving even the brief (a few minutes?) transit and can be pulled off course and captured by pirate tractor beams.

Alternatively or additionally, the threshold energy to "jump" and maintain a body in hyperspace is huge, but once the threshold is reached the size of the "jump bubble" can easily accommodate a planet and everything else to a considerable orbital distance. So, if the power plant requirements are such that the minimum ship size would be hundreds of kilometres across, why not take the entire planet?

One point re transfers between planets - the planets cannot be too close together or there will be destructive tidal effects. Let's say a safety distance of at least 20 light seconds between each planet at all times, assuming that jumps are always accurate to within a light second. In order for a planet jumping four times per day to have any transfers there will need to be very fast sublight ships - assuming that the ships start in low orbit they still need a constant 6G acceleration for five and a half hours to get from one planet to the other. (What happens if a drive fails and a ship cannot make it before the planet jumps?) Required technology for the parameters to be met is extremely efficient and or reactionless sublight drives with massive delta v and artificial gravity control to protect the passengers. Or reduce the frequency of the jumps to 1/day maximum.

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    $\begingroup$ I'll note that if hyperspace's hazard is only radiation (such as charged particles), then any amount of radiation which can't be stopped by some decent radiation shielding and artificial magnetic fields would be a danger to a planet too, especially after many exposures. The other hazards (like space debris and pirates) are well blocked by a planet, though. The danger of hyperspace can be adjusted to allow spaceships a chance (albeit not good odds) of surviving the trip if it suits the story. And if pirates can survive hyperspace, then natural hazards can't be that bad. $\endgroup$ – BBeast Jul 8 at 11:37
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    $\begingroup$ Planets could jump on a schedule. If you miss the jump you just wait a couple days. In fact we don't wait for our ships to get back before jumping, we just pick them up next time. 1-4 jumps per day planned months in advance? Sounds a lot like a train schedule! $\endgroup$ – user253751 Jul 8 at 14:24
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    $\begingroup$ In addition, any communications / GPS/ other satellites in orbit would likley need to be deorbited and relaunched every jump $\endgroup$ – ContextSwitch Jul 8 at 17:05
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    $\begingroup$ Re sublight ships being too slow: This is a recurring problem in most of the space opera genre. Planetary systems are much bigger than most writers seem to think. You should realistically budget a minimum of several hours to move between planets (and preferably quite a bit longer), or make your ships FTL-but-not-fast-enough (e.g. if they go at 150% of light speed, they're not making it to a neighboring system in a reasonable amount of time). $\endgroup$ – Kevin Jul 8 at 17:34
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    $\begingroup$ I would guess in a situation like this, assuming there aren't Star Trek style inertial dampeners, there would be fixed space stations in each system that don't jump. You want a flight from Planet A to Planet B, what you actually do is take a flight from Planet A to Station Z. Planet A jumps out of the system, Planet B jumps in at roughly the same place in orbit around the star, and you take a flight from Station Z to Planet B. If you want Planet C instead which is scheduled on the opposite side of the star, you have to enter the system a day early. $\endgroup$ – CPomerantz Jul 8 at 18:43
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Accuracy is related to mass

The accuracy of the jump drive is inversely proportional to the mass. There's a limit to how accurately we can control them; and for smaller objects, this translates into a dangerously large error margin on where the object ends up. A ship could be jumped, but the risk of ending up inside a star or planet core, or exiting the jump with completely the wrong velocity vector, is too large. Larger objects can be jumped more accurately, as the increased mass means that the error becomes small enough that the risk is low.

That's also why they jump 4 times a day; whilst it's accurate enough to jump planets without risk of collision, it's not accurate enough to put them into good orbits (and anyway, jumping planets into other systems really messes up orbits – check out the three body problem). So they arrive in systems moving on random stupid vectors – not safe orbits, but on collision courses with each other or the star. But they're far enough apart that they're safe for a day or two. Hence they're jumped 4 times a day (well before there'd be any risk) to keep a good safety margin.

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  • $\begingroup$ Your comment about the arrival vector is very neat... and it adds an interesting twist. If your arrival vector is uncontrolled then you are safe location but random "orbit", which is fine for a few days... but if the planet isn't jumped again to another "goldilocks zone" around a different star (or possibly the same star) then bad things will happen to the planet over time. Basically, once a planet starts jumping.. it must KEEP jumping until it gets lucky and arrives somewhere in a safe orbit. $\endgroup$ – Roger Lucas Jul 10 at 9:24
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Violent necessity

Given the absurd amount of energy available to a type 3 civilization, I find the efficiency arguments presented here very hard to swallow. With galactic energy levels, it would be feasible to just build a planet-sized spacecraft, complete with artificial oceans and biosphere if necessary, and jump that instead. Furthermore, moving an entire planet thousands of light years sounds very risky. Even if the odds of failure are one in a trillion, are you really prepared to risk your entire planet on those odds, with its billions of lives and tens of thousands of years of cultural history? Even if you would, would the general population? At first, I thought there was no way to justify this risk. But then I realized: all you need to do is make the alternative even riskier.

Your republic is beset by a rival type 3 civilization, perhaps from a neighboring galaxy, that for whatever reason would be much happier if the republican planets were replaced with their equivalent mass in rubble and plasma. Perhaps you're an affront to their gods, or perhaps they just want the raw materials locked beneath your crust. Their attacks are too powerful to block (offense tends to scale up faster than defense) and too fast to dodge (depending on the details of your FTL communication), so your only option is to flee--preferably before the enemy armada even arrives.

So, it's unsafe for any planet to stay in the same system for long. Space is big, and the Andromedan armada can't be in every system at once, so as long as its member worlds keep continuously on the move, the republic can continue to grow and prosper in relative safety. They'll need to be careful however: dozens of worlds in the same system will present a very juicy target.

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  • $\begingroup$ A Type III civilisation in its totality might be able to wield the power of a galaxy, but I would posit that even they would not be able to deliver all that energy to a single point. Even if they somehow had a way to channel all that energy to a single jump drive, it sounds like every member planet wants their own jump drive, which means each jump drive can take at most the power of one star. When ripping holes in space time, this isn't a lot of energy. As for risk, that can be made as low as necessary for the story. But +1 for war as an explanation. $\endgroup$ – BBeast Jul 9 at 0:19
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    $\begingroup$ @Ángel The question is explicit that planets are used instead of spacecraft. This would make no sense if ambiguity was introduced by allowing the categories of "planet" and "spacecraft" to overlap. $\endgroup$ – ApproachingDarknessFish Jul 9 at 1:05
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    $\begingroup$ @Ángel In a (rather contrived) sense, you could consider these planets as 'natural' spaceships, but only to the extent that a rocket motor strapped to an asteroid turns that asteroid into a spaceship. But, of course, why would you ever build a planet-sized spaceship when you can just use a pre-existing planet? $\endgroup$ – BBeast Jul 9 at 4:53
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    $\begingroup$ @BBeast More efficient use of space/matter, perhaps. A planet is an awful lot of solid rock, while a ship of the same size could have multiple decks. $\endgroup$ – nick012000 Jul 9 at 16:53
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    $\begingroup$ @nick012000 This is true (it's the idea behind a Dyson swarm, after all); if you disassemble a planet you can make a lot of stuff from the remains, and a Type III civilisation has the power to do that. But if you're fleeing a powerful enemy then you'd generally want to take your homeworld without blowing it up and rebuilding it first. $\endgroup$ – BBeast Jul 10 at 12:14
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Ships are not permitted to have jump drives, due perhaps to:

  • Jump drives are an artefact of an older civilisation who for an unknown reason built them on planets. No-one is allowed to attempt to pull one out from the planet.
  • Jump drives are hideously expensive and/or difficult to construct. The materials and work involved is horrendous, and only a planet-sized economy can afford one.
  • Jump drives are dangerous, and only world governments are permitted to build and use them. Their production is strictly controlled. (works for nuclear weapons!)
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FTL does not exist

Even a Type III civilization isn't enough to break through 300,000 km/s barrier because it is physically impossible - and as such there is no 'faster than light' movement for anyone or anything. Wormholes are the only way of bridging huge distances.

'FTL communications' are really just ansibles (two releays that are quantum-entangled so that any state change in one is mirrored by the other in real-time, no matter the distance) or microscopic wormhole generators that don't transport mass but electromagnetic radiation (can be much smaller than mass transporters due to E=mc^2).

Why not outfit ships with jump drives?

Because (choose one or more options)

  • they are huge and clunky and thus make sublight navigation a problem. After all, the mass of the jump generator has to be sublight-accelerated, too, kind of like the rocket equation. Meaning: Only a couple percent of the ship's mass is NOT jump-generator mass - and sublight engines would have to maneuver all of that. Not economically feasible.

  • jump generator efficiency is inversely proportional to mass transported. Meaning: if you want to jump 1kg mass, you'd need a jump generator of 100t. If you want to jump 1000kg mass (1t), you'd need a jump generator of 1,000t. If you want to jump 1000t mass (medium-size space ship), you'd need a 10,000t jump generator. If you want to jump 5.97x10^24kg (mass of earth), you'd need a 5.97x10^13kg jump generator. In comparison: the world's heaviest building as of 2018, the bucharest palace of parliament, weighs in at about 4.1x10^9kg. place 10,000 of those buildings next to each other, and you'd have a world-class jump generator

  • jump drives work by making use of a gravity well that has to be at least as deep as the one generated by a planet of 80% earth mass, but produce toxic waste at the leaving point. Meaning: if you are on a ship, you'd only be able to jump if you were within LEO at most, better on the planetary surface. However, you'd be leaving so much toxic waste (burst of high-energy radiation) behind that even jumping from LEO would mean severe environmental damage to the planet you are jumping from. You'd only be able to jump from uninhabited planets.

  • jump drive transport delimiters aren't accurate at ship size. The jump drive transports everything within a certain gravity well, and ship gravity wells are too small to register properly. (as a result of this, you can easily transport entire planets and their LEO environment, maybe even out to half the distance to the moon). Meaning: If you're lucky, the entire ship gets transported. If you're unlucky, only the core around the jump drive gets transported. Or the jump drive accidentally latches on to the next bigger gravity well and you end up transporting not only your ship but also near-by asteroids or it tries to transport the next best planet (and fails catastrophically).

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    $\begingroup$ That FTL communications concept is a very bad misunderstanding of the entire concept of quantum entanglement. For starters, any measurement of an entangled particle breaks the entanglement, so you could at best use a single pair of particles to send one bit (except per Bell, you can't even do that). In order to have continuous communications, you would need to generate a steady stream of pairs, sending half of on a millennia-long sublight journey to destination before they could be used. $\endgroup$ – Paul Sinclair Jul 8 at 17:43
  • $\begingroup$ @PaulSinclair Indeed, my FTL communications concept would not be based on entanglement. It would use something else, like the aforementioned microscopic primordial wormholes or some kind of subspace/hyperspace radio. $\endgroup$ – Mark Morales II Jul 8 at 22:12
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You don't need a physical explanation, rather a psychological one. People are generally bad at planning what they need. Taking a planet along ensure you have all you might need at reach. A starship might not fit all.

I am a cyclotourist and in my free time I enjoy doing multi days trip. This has teached me to travel light: in 2 panniers I can take all the needed for a 2-3 weeks trip.

Two years ago I was in Japan, climbing on the top of a mountain to visit a temple-city, where visitors are allowed to spend the night in the temple premises and practice meditation. As said, I was there with my bike and two panniers. Next to me there were 3 women, all in their late 20es, each puffing and panting to move 2 extra large suitcases where they supposedly had packed all they "might" need for just one overnight stay.

Some years ago I was to a 2 days off site with several colleagues: I had all the needed for 2 days in 1 sport bag. Some colleagues had 2 stuffed trolleys with them.

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  • $\begingroup$ This would be good, if the planet stayed where it was for a few years, or even days at a time. The set up as described doesn't allow for this as a plan, because if you want to achieve anything other than a new night sky from your position shift, you have to pack and be on an interplanetary flight within your six hour window. $\endgroup$ – Jontia Jul 8 at 15:00
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Planets are self-sustaining. Ships are not.

Keeping a ship running is expensive. You need CO2 scrubbers, water purifiers, regular resupplies of food and removal of waste, and a massive number of employees. Don't forget the massive cost of making one either. But none of that applies to planets. As long as you jump to the appropriate "Goldilocks Zone" for each star you visit, food and oxygen take care of themselves. Plus they're already staffed with all the employees you could ever need (and they keep making more). Combine it with KerrAvon2055's suggestion that it takes just as much energy to transport a ship as it does a planet and you'd be a fool not to take the whole thing.

This gives you a good origin story too: Scientists on Earth discovered how to create wormholes long before we started colonizing other planets. In fact, it was only because of warping technology that we were able to do so. Our ships didn't have the energy to power the wormhole-maker so we just took the whole planet!

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  • $\begingroup$ It makes sense? For Ming the Merciless, maybe, but imagine the voting campaigns needed to get a majority of people to agree on any series of destinations. $\endgroup$ – NomadMaker Jul 10 at 11:05
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The diameter of a wormhole is proportional to the mass that passes through it. For planet-sized masses, spacetime up to a few times lunar distance is smooth enough to make the stresses of passage through it negligible. But trying to push an asteroid through one would result in significant disruption of its surface. Ships on scales we are used to would be pulled apart into separate atoms by the stresses of the passage.

Elementary particles are subject to enormous stresses, but they have no physical size, and there is nothing to pull apart. Thus FTL communications is possible by sending streams of particles. But to send anything larger, you have to be near planet-sized before it results in anything other than total destruction.

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Life support

It proves to be very, very, very hard to outfit starships with long-lasting life-support systems. Particularly if you send them to a system that does not have planets so they have to last particularly long.

If gravity proves necessary for life in the long run, and no artificial gravity is possible, the planet provides it for free.

Transportation costs

The big reason to go is to mine all the easily available asteroids. Carrying all that stuff back through space is expensive, and it turns out the drives have economies of scale -- moving one planet, once, is cheaper than moving little batches of stuff.

Furthermore, with the planet right there, you have everything you could have. You don't have to curse yourself for forgetting the one vital thing.

Communication

FTL communication is one thing, but being the man on the spot gives you more knowledge than anything else.

Personnel

Recruiting staff is hard when they get torn from their families for long periods. Everyone on the planet is an easier sell given they are all together.

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Jump drives are hideously complex to maintain

Because the infrastructure to build and maintain one is huge. The systems to control one require 1000s of people to run. Each world's jump drive company employs a significant proportion of the population.

Sure, you could build on in a ship, but you'd be building something the size of a death star to house the systems and maintenance staff. Why bother, when you can start with a nice planet instead?

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Surface tension

In order to jump, you need to make a field around the object. It might be some dimensional distortion field, or radiation shielding against hyperspace, or other protective shield. The thing is, this field wants to disperse. To keep it together, you need to generate some surface tension that holds it together. "Focusing" it on a bigger volume is easier, but for smaller ones you need to compress it very hard and that takes high amounts of energy and specialised equipment. The planet's own gravitation field can also help in this, and thus needed to be supplied artifically.

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Piers Anthony used a similar approach in his Sci-Fi novel Macroscope. The idea being that it wasn't using a "jump drive" per se but rather that the planet was collapsed into a singularity only to pop out again on the other end. If some manner can be found to control this effect then you have a controllable jump drive which completely explains why worlds must be used but ships cannot.

Also with a technology such as this what is to prevent the civilization from developing Dyson Sphere's and jumping the entire solar system at once?

Sounds like a good book and I look forward to reading it when you are done.

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  • $\begingroup$ Hmmm, jumping Dyson Spheres might work, perhaps for a story set even further in the future when they can enlarge wormholes to that scale. And I'm not sure if or when I'll actually write a book; right now I'm just exploring the concept. But thank you. :) $\endgroup$ – Mark Morales II Jul 9 at 23:24
  • $\begingroup$ Maybe you could jump a star system but if all other travel is sublight it would take forever to go visiting, because you could not jump in very close without causing havok with planetary orbits. $\endgroup$ – Zan Lynx Jul 10 at 23:11
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Air traffic control for this stuff is a logistical nightmare. This is going to quickly become impossible without a planet-busting disaster scenario at least every few years of subjective time. And the lesser catastrophes will be quite apocalyptic as well. Get the orbit wrong (where in the hell does the velocity come from to match the new stars ecliptic plane and orbits v?) and your people are freezing to death while the planet is ejected out into that star's Oort cloud. Atmospheres shredded away, water boiling off.

While moving planets will still be an option in your universe, this won't be for road trips... the technology will be reserved for terraforming and similar large-scale engineering projects where they attempt to minimize the number of trips any large astronomical body ever makes. Trips will be planned out decades in advance, and once the planet reaches its final destination there it will sit forever excepting some emergency that prompts hasty action.

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  • $\begingroup$ +1 for recognizing the traffic control problems of small ships. But I disagree that planetary movement is necessarily rare. $\endgroup$ – SRM Jul 9 at 5:55
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Microbiology is crueler than we thought, and it makes all biospheres lethally incompatible with all others.

Interplanetary tourism is generally lethal. Communication from orbit is pretty safe, and telepresence is easy, but actually breathing the air of other worlds is suicide.

Food production for natives of one planet can only happen on that planet. Reproduction in general can only happen on that planet.

So, if you have to travel, you have to take the whole thing.

c.f. War of the Worlds, and a more recent book, but, spoiler.

Now let's talk about traffic control: wow, risky! And the gravity waves?

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Ships are tiny compared to a planet, and will have a negligible effect on the existing orbits of whatever system it enter or leaves.

A planet, on the other hand, could disrupt the existing orbits greatly, and not necessarily in predictable ways. If this technology is used, it will likely only be used for moving between systems where such disruptions are deemed inconsequetional: ones with no planets, or at least no inhabited planets.

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  • $\begingroup$ While this is not an answer it is, I think, a valid objection, maybe the death knell for the idea. The orbital mechanics of star systems are more subtle than meets the eye. $\endgroup$ – Peter - Reinstate Monica Jul 9 at 4:30
  • $\begingroup$ I posted a question on Astronomy SE asking about the effects on the star systems. $\endgroup$ – Peter - Reinstate Monica Jul 9 at 5:13
  • $\begingroup$ @Peter Hmmm, maybe Dan W is right and the planets are really just falling/hurtling through space in between jumps, as scary as that sounds. Like a galactic game of hot potato. $\endgroup$ – Mark Morales II Jul 10 at 0:00
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The planets don't have the jump drive, it's just a hole in space.

Your civilisation has figured out how to make artificial wormholes by ripping holes in space. Tearing space open requires a colossal amount of energy just to pierce the cosmic event horizon, but after that your problem is stopping the tear continuing indefinitely rather than making it big enough to transport things.

So, planet size holes around the universe become the thing. These are used in two manners.

  1. Temporary construction/terraforming methods. You use your hole in space to grab interesting looking rocks and stick them into orbits much more convenient to your day to day business to get on with terraforming them (adding a new planet to the Exchange) or stripping them for raw materials, like the unobtainum needed to create the holes!

  2. Permanent structures, that sit in place on a handy orbital plane (for some reason not moving with the same orbital velocity) and they become part of extraordinarily complex multi-system orbital paths for the planets in your civilization. Moving 4 times a day might be excessive for this set up, but you could just hop between systems on a regular basis as part of your planetary "year". Always circling back to the same place, but with different neighbours as other routes have shorter years.

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Gravity Equivalent of Thunderclap

When a bolt of lightning strikes in a storm, it creates a vacuum. When that vacuum collapses, there’s a massive boom of thunder that is quite disturbing to hear up close. It is a rolling wave of sonic energy.

When an jump drive activates, it leaves behind a true vacuum: a volume completely devoid of all particles that otherwise never occurs in nature. Space itself bends to fill the vacuum. This is somewhat destructive to matter nearby because it turns out to be the single most powerful pull possible for energy input.

Simply put: no planet will allow any jump ships anywhere near its orbit. Any ship that arrives is immediately destroyed so that it cannot jump away again and damage the planet.

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    $\begingroup$ This establishes planetary space as a no warp zone, although it doesn't make clear why it is more efficient to warp the whole planet rather than simply move your ships far from planets before they can warp. It also sounds like the bigger the thing warped, the bigger the thunderclap, so warping planets might destroy the entire solar system unless you can posit that the effect decays rapidly with distance (although a force which decays sufficiently rapidly probably doesn't reach very far anyway, rendering the thunderclap not as dangerous). $\endgroup$ – BBeast Jul 9 at 6:55
  • $\begingroup$ Note: If we are in a false vacuum, creating a true vacuum is expected to lead to the complete destruction of the universe. Yes, really, all of it. en.wikipedia.org/wiki/False_vacuum#Existential_threat $\endgroup$ – user253751 Jul 10 at 10:43
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    $\begingroup$ @user253751 'False vacuum' does not refer to the soup of real and virtual particles which tends to occupy space where there is nominally a vacuum, but rather about the ground state of the quantum fields which define matter and forces in our universe. So the "true vacuum" SRM talks of has nothing to do with the quantum field theory true vacuum. $\endgroup$ – BBeast Jul 10 at 12:19
  • $\begingroup$ Although if it were me, I would explain the "thunderclap" by saying it produces a bucketload of gravitational waves, which is has real-life precedent (especially if the jump drive functions by warping space somehow). Then you don't have to contrive some quantum field theory technobabble. $\endgroup$ – BBeast Jul 10 at 12:23
  • $\begingroup$ @BBeast I was aiming at something like the gravity waves but leaving the idea more open-ended for whatever effect such a vacuum might have, which depends somewhat on how the author makes the jump drive work. $\endgroup$ – SRM Jul 10 at 14:11

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