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How could a Faster than light drive be limited to only travelling between different solar systems, and not within a solar system? I don't really want to allow travel between planets in the same solar system using FTL or allow things like Kinetic Weapons to be equipped with FTL to destroy a planet by slamming into it.

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    $\begingroup$ What would keep people from jumping to a different system and right back (to a different point in the original system)? $\endgroup$ – Aganju Jun 3 '17 at 16:44
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    $\begingroup$ Politics, caution as a result of failures in history; The Treaty of Far Reach 28 all are agreed that all FTL traffic into known systems must only use FTL in permitted FTL lanes. Essentially traffic control, in spaaace. Treaties are a very workable solution that are ignored too often, and ignoring them can actually be a plot point. $\endgroup$ – dcy665 Jun 3 '17 at 23:14
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    $\begingroup$ It really depends on the nature of your FTL drive. For example, mine obeys conservation of energy: jump too far "downhill", and the excess energy gets dumped into your ship as heat, with catastrophic consequences. For safety reasons, you want to jump to reasonably flat parts of spacetime. $\endgroup$ – Mark Jun 4 '17 at 0:54
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    $\begingroup$ @cde imagine trying to explain air traffic control and battlefield real time communications together to someone from 400AD. That is the minimum delta in our comprehension of dealing with FTL. So don't do that. Explain the parts that are interesting to the plot. The technical details will kill the pacing of the story, I don't care how accurate the science is. The readers will be shed like rain from a windshield. An FTL windshield. $\endgroup$ – dcy665 Jun 5 '17 at 6:49
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    $\begingroup$ Read some Neal Asher, specifically the Cormac/Polity novels. Basically, more gravity makes FTL for ships harder. You can decide how much harder you need to suit your purposes. Whatever answer you pick, it's basically going to be "something because gravity something handwave". $\endgroup$ – Grimm The Opiner Jun 5 '17 at 9:36

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A guy named Arioch has a webcomic called Outsider (highly recommended, BTW) with a simple and elegant solution to this problem. Ships in that universe use a drive that jumps them through hyperspace, but the drive only provides the initial impulse to break the ship out of the plane of normal space; the hyperspace trajectory afterwards is purely ballistic.

By ballistic, I mean affected by gravity. Gravity affects objects in higher dimensions as well, and to return to normal space the ship must be pulled back down by the gravity of a large body like a star. If it is not, the ship will be stuck in hyperspace, unable to get rid of its higher-dimensional inertia.

ballistic hyperspace trajectory

(Arioch has posted an in-depth discussion of this system here, from which I have shamelessly stolen the image above.)

As a corollary, ships cannot jump from deep in a gravity well, as they would be pulled immediately back down into real space; they cannot jump too deep into a system either, because they will experience sufficient force to snap them back into normal space from the star.

Jump ranges are naturally limited by the precision of the drive's initial impulse and the precision with which the desired ballistic hyperspace trajectory can be calculated. Since there are only so many stars within range of any given starting point, this means that ships cannot jump past the front lines of a battle, and there is a meaningful concept of distance between two stars, measured not just in travel time, but in uncertainty about the state of the territory you will be passing through.

An interesting side-effect of this mechanism is that despite the existence of galaxy-wide FLT travel there remains no way to get to arbitrary points in interstellar space. The vast, vast majority of space is empty and far away from an anchoring gravity well, and thus inaccessible. Almost anything could be out there...

This mechanism fits in well with the IRL hypothesis is that gravity penetrates into higher dimensions, simultaneously explaining observed the stronger-than-predicted observed gravitational effects that are typically attributed to "dark matter" as gravity leakage into our dimensional plane and gravity's relative weakness as leakage out.

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    $\begingroup$ Wouldn't this method make it trivial to aim & drop kinetic weapons (mentioned by the OP) into a system's star? The whole system is based on homing in on stars after all :/ $\endgroup$ – dot_Sp0T Jun 4 '17 at 15:25
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    $\begingroup$ @dot_Sp0T The idea is that objects in hyperspace will be "captured" into normal space and lose their hyperspace velocity when they pass close enough to a gravity well. Thus, a kinetic weapon would come back into normal space with precisely the real-space velocity it had at the start. Given the high, fractional-c starting realspace velocity a hyperspace-based kinetic weapon would need to be effective as such, the delta-v requirements for course corrections on final approach would be astronomical (no pun intended). $\endgroup$ – Reid Rankin Jun 4 '17 at 15:58
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    $\begingroup$ For space (sic) constraints I won't go into why this creates a huge suspension of disbelief right from the very first mention. It is the old problem: that approach tries to come over as very scientific, real-world based; but at the same time it seems so obviously to be wrong (not impossible, just wrong - invalid internal logic) that it just puts me off as a reader. Like reading those very old classics (somewhere between J.Verne and P.K.Dick) which were scientific in their day but all those explanations are just wasted today. Good if the reader is fond of something like that. Not me. ;) $\endgroup$ – AnoE Jun 4 '17 at 22:12
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    $\begingroup$ @Neil yet to get to the other side of the Earth, a suborbital flight is by far the fastest way to get there, going by land/sea (or even a standard passenger jet) is far slower. Remember that the parabolic path is done at FTL speeds, and that it is not a spatial path. $\endgroup$ – Baldrickk Jun 5 '17 at 9:26
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    $\begingroup$ So replace "speed" with "alternate travel method" the point still is that the diagram provided is conceptial, it doesn't actually show a longer route to reach the destination, more that you are travelling outside of normal spacetime. $\endgroup$ – Baldrickk Jun 5 '17 at 9:40
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It is common to require that FTL only operate at some distance from large mass. So, the ship must withdraw (say) beyond the orbit of Neptune before engaging. Likewise, ships will arrive at this distance.

In some of my other posts I also mention the idea of needing to match velocity with the special preferred reference frame. This would require months of travel and conventional acceleration prior to jumping.

Another idea, sometimes used by the same authors, is to have special jump points associated with the stars. You can only jump between the paired “Crazy Eddie” points, not arbitrarily.

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    $\begingroup$ said Crazy Eddie points could be the mouths of a wormhole. Super-expensive to build but the usage fee makes them worth building anyways if the usage fee is set so high it's only worth paying if you're jumping between stars. $\endgroup$ – John Dvorak Jun 3 '17 at 13:48
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    $\begingroup$ This could be combined with dot_Spot's answer, by making it so that the drive has an unfortunate tendency to put you in large gravitational masses unless you steer clear of them! $\endgroup$ – Cort Ammon Jun 4 '17 at 0:09
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    $\begingroup$ See also: Mass Relays, as used in Mass Effect game franchise. masseffect.wikia.com/wiki/Mass_Relay $\endgroup$ – NVZ Jun 4 '17 at 3:08
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    $\begingroup$ An interesting side-effect of how things worked in Mote in God's Eye is that there existed systems where the shortest way to travel between two planets was to travel in realspace to a nearby Alderson Point/Crazy Eddie Point, jump to another system, travel in realspace to another Crazy Eddie Point that leads to a different part of the original system, and jump back. As long as the total realspace distance was less than the direct route, they'd save fuel and/or travel time. $\endgroup$ – Ray Jun 4 '17 at 21:22
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    $\begingroup$ @NVZ Not quite so sure on the ME comparison. FTL was possible through Eezo within the Gravity Well of a star. The Mass Relays are simply a massive scale Eezo drive capable of reaching much higher speeds than in system FTL. $\endgroup$ – SGR Jun 5 '17 at 9:50
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Make the aiming difficult

If the lowest precision of coordinates you can aim at still has an error margin greater than the typical size of a solar system your method would not be usable for travelling distances shorter than said margin.

Example: Aiming at the sun would result in arriving anywhere between the Sun and the Kuiper Belt (some ~50 AU error-margin).


On the chances of hitting anything: Using the rules defined above we can calculate the likeliness of hitting something as follows:

  1. Define an error margin; we'll go with 50 AU
  2. Calculate the volume of a sphere with radius r = error-margin:
    523'598.775598 AU^3
    or 1'752'971'514'717'000'000'000'000'000'000 km^3
    or 1.752971514717e+30 km^3

  3. Calculate the volume of all things inside this sphere; we assume the sphere to be centered on the Sun and, for simplicity's sake, sum up the volumes of the stellar object on this list:
    1'416'691'835'910'000'000 km^3
    or 1.416691835910e+18 km^3

  4. Divide the volume of all objects by the volume of the sphere:
    8.0816592e-13
    or 0.00000000000080816592
    or 8 in 10'000'000'000'000
    or 1 in 1'250'000'000'000

So in contrast, if everyone of the ~8 billion people on earth would do 1'000 of these jumps, then ~6 people would hit something.

I'm using the google calculator for all calculations

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    $\begingroup$ This gives you the chance of ending up inside the sun or a planet though (if FTL works out-of-dimension) or slamming against them (if it works in-dimension). $\endgroup$ – Siguza Jun 3 '17 at 14:05
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    $\begingroup$ @Siguza yes, unless you plan for the error and aim somewhere safe.... that's the whole point of it! $\endgroup$ – dot_Sp0T Jun 3 '17 at 14:07
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    $\begingroup$ @dot_Sp0T you could then end up some 100AU from the solar system's centre. W/o ftl it will take months/years to get to the inner planets from that. $\endgroup$ – theonlygusti Jun 3 '17 at 14:13
  • $\begingroup$ @theonlygusti that's the gist, yeah. I mean eventually it depends on the error-margin the author is comfortable with, but yeah. so Travel time between two Earth and Kepler II might amount to a year. Sounds fine to me for somewhat-hard-scifi $\endgroup$ – dot_Sp0T Jun 3 '17 at 14:25
  • $\begingroup$ @dot_Sp0T Okay, I read your answer as "make it unreliable" rather than "make it unsafe". However this does only partially solve the problem of slamming something into the planet. If you miss, you just try again. And you should still be able to aim in a way that leaves it possible to end up inside the planet but not inside the sun. Also, I could imagine you might even wanna try and aim for the sun... I'm not sure what is physically possible here, but... antimatter? Something that changes the balance that makes the planet inhabitable? $\endgroup$ – Siguza Jun 3 '17 at 14:37
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First off, there is no realistic way to limit an FTL drive because there is no such thing as a realistic FTL drive in the first place. That is to say, any drive that allows you to physically move faster than light is not only impossible but nonsensical. This is due to a few reasons such as time dilation (time would stop for you), space contraction (you'd go through all space at once), and energy expenditure (there's not enough energy in the universe to get to light speed, let alone surprass it).

What you could do is arrive at a destination faster than light would, by taking a shortcut. This is the principle behind many of the more " realistic " interstellar drives. That shortcut may be another dimension (hyperdrives), bending space to make distances shorter (warpdrives) or opening a hole in space to your destination (jumpdrives). All of these have the added bonus that they do not turn your spacecraft into a relativistic apocalypse bullet.

Now onto methods to limit these drives to "safe zones":

  • Limit through accuracy - let's say that your drive is extremely inaccurate, that is to say, that if you aim at a star, you may end up in said star or you may end up in the fringes of that star system. Given how vast space is, it's fair to say that the chance of accurately delivering a missile to its target is less than slim. The main disadvantage of said drive is its inherent danger, so to do regular trips you'll have to aim far from your actual target lest you want to end up inside it.

  • Limit through resources - make the drive need a special resource to work, some kind of exotic energy only found far from stars. This resource would be needed not only to start the jump but to end it, so people can only jump from the edge of a system to the edge of a system. This can be expanded if you are using warpdrives or hyperdrives, so that you can only travel through "paths" of this energy. This would allow you to fine-tune how you want your spaceships to function and where you want them to go. While I say resource, this can also be the lack of such, like gravity.

  • Limit through infrastructure - make your drives function with gates or portals. You can set it up to work such that you can only jump from, to or both. The main disadvantage of this method is colonization and exploration. Since you'd need a gate to whatever destination you want to go to, you'd need to physically move such gates to every new system you discover. Alternatively, make it so an ancient civilization did all that hard work already and you just discovered the gates (like Mass Effect).

  • Limit through countermeasures - set up "jump inhibitors" near important locations. These inhibitors would limit jump/warp activity in an area near them, like the other methods, they could prevent jumps from, to or both. A network of these inhibitors in a star system would prevent rogue jumps to undesired locations and, depending on the size of the devices, you may implement them in spacecraft to prevent enemies from escaping or from surprising your troops.

  • Bonus - combine these ideas to make things more interesting. Like low accuracy + inhibitors for extra safety or resources + infrastructure for less handwaving.

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    $\begingroup$ Fear of surprise attacks? maybe they are big, expensive and dangerous, so you only set up one per system, so the most central location would be near the star. Maybe the power requirements are so big that you need to be really close to the star to get enough sunlight for those solar panels. $\endgroup$ – Miguel Bartelsman Jun 3 '17 at 10:42
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    $\begingroup$ Good to see someone point out the barriers special relativity puts up in attempting to cross the lightspeed barrier. Conceptually to achieve FTL some way would need to be found to go around c. Your answer mentions some of the common ploys used in science-fiction. $\endgroup$ – a4android Jun 3 '17 at 13:18
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    $\begingroup$ Karl Schroder's Permanence has an FTL drive that requires ships to get very close to sufficiently massive stars before going superluminal or breaking out of it. Of course, you can only travel from one star to another. Any interplanetary FTL travel was impossible. $\endgroup$ – a4android Jun 3 '17 at 13:22
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    $\begingroup$ I like the idea of an exotic energy that only exists far from gravity wells. The inhibitor idea is cool too. If you throw in a fictional gravity field generator (like most sci-fi), and say the exotic energy is repelled or consumed by gravity fields, then you just created an inhibitor for every ship with artificial gravity, which would help prevent collisions at FTL speeds. Of course, the downside is that you wouldn't be able to use your artificial gravity while travelling FTL, so maybe collisions would still be possible (but unlikely, because space is HUGE). $\endgroup$ – Kyle A Jun 3 '17 at 13:32
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    $\begingroup$ This assumes that relativity is reality rather than a model to explain reality. The fact that we haven't found sufficient reason to doubt it doesn't guarantee we won't. A hundred years ago, someone could have said the same thing about the Newtonian model. $\endgroup$ – WGroleau Jun 3 '17 at 22:55
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The most used options in literature to spice up space travel are:

FTL drive requires local space to be "flat".

This can be played in several different ways. The most direct way (e.g. in the Honor Harrington novels by David Weber) is that there is an absolute limit of spacetime distortion, the hyper limit or Warshawski limit, above which the hyperspace transition simply cannot work. This can have several sub-possibilities such as far-out gas giants or other space phenomena throwing a gravitational shadow of their own.

A clever twist of the same concept is at the bottom of Ian M. Banks' The Algebraist.

On a grander scale, and without gravitation being explicitly added to the mix (actually, the ending of A Fire upon the Deep shows that the effect is not gravitational), the Zones of Thought of Vernor Vinge's mark areas of the galaxy where hyperlight is - or is not - possible, whether a star is present or not.

A more complex setup has the spatial "bending" as a less-than-insurmountable obstacle to FTL due to several reasons, so that more advanced ships or more powerful "compensators" can jump nearer to a star (or a planet). This happens with Raymond Weil's Slaver Wars (where different model of engines allow getting deeper and deeper in a planet's gravity well). Even so, the inner solar system can be off limits to everyone.

And of course, you cannot reintegrate inside a solid object as the gravitational potential is too steep.

A twist on this latter concept is that you cannot reintegrate anywhere unless the mass density is very very low, because all pre-existing matter (say gas molecules) will mix with your matter at such a negligible distance that it will cause a nuclear fusion event.

So you can reintegrate outside Pluto and risk no more than a slight increase in your chances of getting bone cancer. Reintegrate inside a comet trail or inside a cloud of stellar dust, and you'll be literally boiled alive by the waste heat.

FTL is only possible between massive objects such as stars

This is, I believe, Niven and Pournelle's Alderson Drive limitation. Gravity eddies caused by stellar objects give rise to Alderson Points and "tramlines", and you don't get them except near stars (or possibly gas giants).

FTL requires equipment at both ends

This is the more "realistic" explanation - FTL violates the currently known laws of physics, and no known natural phenomenon is FTL. It stands to reason that if FTL is possible at all, it requires transition to a normally detached dimension/brane/space/whathaveyou, and real-space equipment to perform de-transition; be it a F'Sherl-Ganni's wormgate, a Fourth Empire's hyper receiver, or higher-dimensional wormholes. It makes sense to only install these terminals near habitable (actually, inhabited) stars.

Space flatness influences hyperspace transition

In this scenario, any object immersed in a gravitational field will experience two different "tugs", a larger attraction in the part nearer to the field source, a weaker attraction on the opposite side. This difference is greater the larger the object and the steeper the gravitational gradient.

If this difference translates e.g. in different hyperspatial acceleration given a fixed hyperspatial translative force, any spaceship will be subject to a force that will try to rip it at each Jump. The nearer the Sun, the larger the ship, and the faster the hyperspeed, the greater the force. In the end, the maximum ripping force that can be tolerated by a human being will limit the distance from the Sun and the hyperspeed that can be attained. If the maximum hyperspeed, at the distance of Pluto, is very little above light speed, then FTL travel inside a solar system is possible and yet it makes no economic sense.

Sidestep the problem entirely - just make transluminal kinetic kill weapons impossible

You can use the "Vegas rule" -- whatever happens in hyperspace, stays in hyperspace. As soon as your kinetic missile becomes transluminal, it stops interacting with the subluminal Universe, and is no longer a weapon. When it reappears, its superluminal speed does not transition with it (not like the hyperplanetary projectiles of Doc Smith's Space Patrol), and the missile reacquires the same speed it had before going transluminal.

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  • $\begingroup$ Good answer! As an Honorverse aficionado, I want to point out that the Warshawski limit (colloquially, the hyper limit) is not the same thing as the alpha wall. The alpha wall refers to the distance between the lowest (alpha) band of hyperspace and normal space. See this answer for more details. $\endgroup$ – Reid Rankin Jun 3 '17 at 20:47
  • $\begingroup$ I could repeat what Dan Alderson told me about his tramlines; it's too long for a Comment but I'm not sure it's worth an Answer. $\endgroup$ – Anton Sherwood Jun 6 '17 at 7:55
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    $\begingroup$ This only solves the problem if your enemy is stupid. Take a month or two ramping your kinetic projectile up to a significant fraction of the speed of light in some arbitrary, well-defended place, and only then jump it through hyperspace to its target where it will arrive going absurdly fast because it was already going that fast before it jumped. $\endgroup$ – Perkins Aug 26 '17 at 23:29
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Say that the run-up acceleration to FTL has to be about 10% of light speed, that FTL depends upon a discovery involving a violation of symmetry in the Lorentz contraction (a consequence of relativity), but 10% of C is the minimum sufficiency to exploit this and make the FTL mechanism work.

That is 67 million MPH, and it takes about the entire width of the solar system to get that fast and the only known way is too slingshot around the Sun several times, taking a month. And once a ship is at FTL, it is not in normal space and passes through normal matter like neutrinos: But like them can be influenced by gravity, meaning the only way they can hit the brakes and drop out of FTL is by using the gravity of another star; they must navigate through the star; which will pop them back into normal space at their original speed, a few million miles from the center of the star. (Ours has a radius of about 440,000 miles; but pick your stars carefully, or make sure you have good shielding if you will still be inside it; or since it is just sufficient gravity that is required, pass through non-center).

In response to comment about stopping by gravity assist: If FTL is possible, getting into or out of it is likely a violation of the physics we know, and exploiting fictional physics of the future. I'll accept achieving relativistic speed may not be possible with gravity assist, but certainly achieving the maximum possible gravity assist is a start before we kick in the nuclear bomb pulse propulsion.

Of course my 10% of C is just made up too; the author can make it 2% of C. As far as getting out by flying through a star, I'd say that is not a form of gravity assist. I will instead say the FTL drive is exploiting a form of space-time resonance, the type (in sound) that can break glass; and it takes the gravity of at least a red dwarf star to disrupt that resonance; because that gravity bends space-time, remember? So the resonance breaks due to that much (or more) space-time distortion. Disrupting the resonance is what drops us out of FTL; not the slowing down due to the star.

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    $\begingroup$ While the idea of phasing through matter is really cool, I don't think it's possible to decelerate from relativistic speeds using gravitational assists. A perfect gravity assist can only give you twice the speed of the orbital body, even if you managed to pull off a perfect assist through every single planet in the solar system it would barely suffice, not to mention that to pull that off planets would need to be aligned. $\endgroup$ – Miguel Bartelsman Jun 3 '17 at 11:01
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    $\begingroup$ @MiguelBartelsman Check edit. $\endgroup$ – Amadeus-Reinstate-Monica Jun 3 '17 at 13:44
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    $\begingroup$ @MiguelBartelsman right, and slingshots have been discussed in more depth here on WB if you can find it. You won’t slingshot around the sun several times — for that to work the first time you are already in a galactic orbit! In orbit around the sun (bound to it) you cannot use it as a slingshot. $\endgroup$ – JDługosz Jun 3 '17 at 19:14
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    $\begingroup$ @JDługosz you're correct about slingshots, but if you're trying to achieve a really high velocity it still makes sense to enter an elliptical orbit around the Sun and make repeated burns at perihelion in order to maximise use of the Oberth effect. $\endgroup$ – Nathaniel Jun 4 '17 at 1:25
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    $\begingroup$ @JDługosz fair point, yes, the Sun's escape velocity is way less than 10% of the speed of light, so there's no way my suggestion would help reach such a speed. $\endgroup$ – Nathaniel Jun 8 '17 at 0:40
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Establish FTL highways that need to be constructed (I don't mean physical roads or rails but some distortion in time and space or something - sci fi!!). While in theory they could go through a star system, they didn't for the reasons you mentioned. This is somewhat similar to real highways which also don't go through cities or only at a limited speed. Well, that's how they do it in my country at least. Radiation alone should be a problem for the inhabitants of the solar system.

The advantage of this method would be that there still could be such paths to the planets, but at a different speed. You can make your spaceships pretty slow unless they are on one of those roads and avoid the problems that come with a FTL engine.

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    $\begingroup$ This is all well and good until they plan to build an intergalactic super-highway right through your planet. Just don't blame me, the plans have been on display for a month. $\endgroup$ – gmiley Jun 7 '17 at 12:31
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    $\begingroup$ @gmiley Don't worry, I already started the paperwork to block the highway. Now I just need to hitchhike to the department of space vehicles to push it through. $\endgroup$ – Loduwijk Jun 9 '17 at 20:23
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Do you want them to be able to jump to ANY star or just some specific stars?

In the latter case, you have Mass Effect's mass relay.

The basic rule is: You can only "jump" from relay to relay.

Hence, you can't jump to a star system that doesn't have a relay built yet. This makes FTL not viable for "exploring" but once some colonies have been established in a system, they could start building their relay to be connected to the "relay network", so the FTL trading and massive inmigration may start.

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  • $\begingroup$ That's not how Mass Effect FTL works. They still have ship-based FTL, which they use to get between nearby systems, but the operative word is "nearby". The relays allow for essentially instantaneous jumps between them regardless of distance. While the games have been inconsistent in how fast non-relay FTL, the principle is still there. That's why the Reaper invasion was delayed by 6 months when Shepard destroyed the Alpha Relay; the Reapers had to travel to another system to use another relay to launch the attack, and that travel took time. $\endgroup$ – Keith Morrison Feb 26 '18 at 16:23
  • $\begingroup$ @KeithMorrison I am aware that ships could travel to nearby system, though I thought they did that through SSTL (Slightly Slower Than Light) :P. What you say is interesting, though. If you provide any sources backing that and explicitely categorizing that travel as FTL (the scope of this question), I will make sure to ammend my answer and credit you :) $\endgroup$ – xDaizu Feb 26 '18 at 16:29
  • $\begingroup$ "In space, low-mass fields allow FTL travel and inexpensive surface-to-orbit transit" - (Codex entry on Mass Effect Fields); "...allowing instantaneous transit between locations separated by years or even centuries of travel using conventional FTL drives" - (Codex entry on Mass Relays); "Faster-than-light drives use element zero cores to reduce the mass of the ship, allowing higher rates of acceleration" - (Codex entry on FTL Drive). Also, there's a Codex entry called "FTL Drive". $\endgroup$ – Keith Morrison Feb 26 '18 at 20:49
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Since everyone else is saying gravity, I will suggest ELECTRICITY. Make your FTL drive dependent on leveraging a sun's magnetic field lines into a particle accelerator. Similar to all the gravity-well suggestions except the ionized solar wind is the polarizing force that aims the ship away from a star during acceleration, and towards the other star during deceleration.

It only works on the star, not a planet, because the star is the dominant magnetic field. At FTL speeds you can't stop halfway to visit a planet, it would be like a ball stopping in mid-air.

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    $\begingroup$ wrote "It only works on the star, not a planet, because the star is the dominant magnetic field." Except when you are close enough to the planets. When you are close enough to them, there comes a point where their magnetic field is stronger for you than the sun's. This is the same for gravity too though, so my comment also applies to the other answers you mention. If magnetism and gravity were to suddenly not work this way, we would all start falling immediately off Earth and toward the sun; ow. ;) $\endgroup$ – Loduwijk Jun 9 '17 at 20:29
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Some interpretations of Einstein's field equations allow for FTL by forming a "bubble" of curved space around a ship (check out the Alcubierre drive on Wikipedia). Forming such a bubble would require some exotic matter, which may not play nicely with particles of the solar wind, random bits of debris, or anything else you might encounter inside of the boundary shock of a solar system. Additionally, it isn't well understood what would happen to any matter that crosses the boundary of the bubble, as it would experience some awesome tidal forces, so you could reasonably assume whatever happens would be bad for all concerned.

If your FTL technology is based on folding space or warp bubbles, these considerations could provide a very practical limitation on when and where you could safely use the tech. FTL is pretty exotic stuff, so it isn't at all unreasonable to think the limitation would be one of safety, either for the occupants of the ship, or the destination.

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  • $\begingroup$ Anything even approaching lightspeed would have a huge issue with running into random interstellar particles, so that's an issue. But this answer was more along the lines of what I was thinking: if you're accelerating a bubble of space, it's not entirely implausible to suggest that this interstellar detritus runs into your warp bubble, and is similarly accelerated. So you arrive with a spray of extremely high-velocity particles: hopefully nothing important is directly in front of you. $\endgroup$ – canhascodez Jun 5 '17 at 23:36
  • $\begingroup$ @sethrin Not necessarily - if space outside the bubble is curved around it, particles could pass "around" the bubble without being affected. Maybe? $\endgroup$ – Samthere Jun 6 '17 at 11:06
  • $\begingroup$ @Samthere you could posit that, but it would rather defeat the purpose of this exercise. We're not trying too hard to have real physics here, since the topic is FTL. As it happens, others have suggested that the Alcubierre drive does indeed have this defect. $\endgroup$ – canhascodez Jun 6 '17 at 14:34
  • $\begingroup$ @sethrin So to be clear for the context of the question, is the suggestion that people jump to stars knowing they can "discharge" these particles into the star? I was reading it in reverse; you'd want to jump somewhere away from established areas where you can do damage, which might be near stars. $\endgroup$ – Samthere Jun 6 '17 at 14:43
  • $\begingroup$ @Samthere you would want to be very far away from everything. For a spacecraft with a 10m^2 cross section and a 1 ly journey, you're going to hit somewhere between 15.84 micrograms and 158.4 kilograms of interstellar hydrogen. If we assume that this is directly converted to mass energy (may or may not be true, but this is sci-fi), you're probably going to outshine the sun briefly. If you hit anything else, well...it's not going to be a directed beam and it's not going to be pretty. Having the spray hit any matter is just going to spread the problem around. Feel like sterilizing a solar system? $\endgroup$ – canhascodez Jun 6 '17 at 23:33
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Perhaps your faster than light drive can go "through" objects or pass "through" their spatial coordinates without interacting with them. Thus you can't harm a planet or a star by hitting them. Instead you pass harmlessly through them like neutrinos. Thus people might say that an ship is "neutrinoized" when using FTL drive, and some of your characters might think that the ship and themselves are turned into neutrinos when the drive is on until someone explains what actually happens.

Possibly the FTL drive is so fast and the mechanisms for turning it on and off are so slow that the shortest possible distance you can travel using it is one trillion miles (1,000,000,000,000 miles). The distance between one star and its closest neighbor star is usually about 5 light years or about thirty trillion (30,000,000,000,000) miles.

So they aim their starship at the destination star and then turn on the FTL drive and emerge from the drive within a trillion miles of the star. But if their slower than light (STL) drive is fast enough to travel up to a trillion miles in a short enough time for story purposes then a ship on slower than light drive could destroy a planet by ramming it.

So if the ship emerges from the drive too far away from the destination, they turn on the FTL drive again for the shortest possible time and travel about a trillion miles. This may take them to the other side of the destination sun. If they are still too far away from the destination planet, they travel back approximately a trillion miles. If they are still too far away from the destination planet, they try again.

And sooner or later the variation in the distance traveled in the shortest possible FTL trip will enable the to comes out of FTL drive close enough to reach the destination in a short enough time using the slower than light drive.

Or maybe the shortest FTL trip is always exactly 1,000,000,000,000 miles. Thus after they make the long interstellar trip they see how far they are from the destination planet. They calculate the surface of a sphere exactly 1,000,000,000,000 miles above low orbit around the planet. And they calculate the surface of a sphere exactly 1,000,000,000,000 miles from their present position. And maybe there will be positions where those two spheres intersect. If so they will make a FTL trip to one of those positions and then another one to low orbit around the destination planet and use their slower than light drive to match speeds and then land.

Or maybe they will have to make one or more intermediate FTL trips 1,000,000,000,000 miles long zig zaging around to reach a position exactly 1,000,000,000,000 miles from low orbit around the destination planet.

If you keep adjusting the parameters of the FTL drive and the STL drive you can make a trip across the galaxy take much less time than an interplanetary trip.

But I kind of doubt you can make the STL drive slow enough that it will be impractical to devastate a planet by ramming it under the STL drive. The energy needed to match speed and direction of a planet orbiting around a different star with a different velocity than the star and planet of origin will make ramming with a starship on STL drive very dangerous.

Perhaps the starship will be legally required to travel by FTL drive to a spot in empty interstellar space and match its intrinsic velocity to the velocity of the destination star and maybe even the orbital velocity of the destination planet before resuming the FTL trip. Thus any ship that stops its FTL drive in a solar system and has an unmatched velocity will be assumed to be hostile and the planetary defenses will be activated.

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It requires a whole planet to launch the FTL drive, which makes the planet difficult to be used for anything else. To use the spaceship, first you go to that planet using conventional ways. There are armies and technicians from each fraction on that planet to make sure it won't be used as weapons. And there are not enough planets for each fraction to build their own.

It's like why you don't go somewhere 1km away by air.

Alternatively, the same device can be used to slow down an FTL object or change its path near the planet it is based on.

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A solution from Larry Niven's Known Space universe is to have some form of life that lives in hyperspace and hangs out in gravity wells. Any ship that enters hyperspace too near a sun stands a good chance of getting eaten.

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    $\begingroup$ Originally it was the singularity associated with the gravity wells of stars, but then it got silly. Hyperspace monsters, indeed! Sometimes second thoughts aren't wiser. This shouldn't detract from your answer, pleurocoelus. I'm just grumbling about Larry Niven. $\endgroup$ – a4android Jun 3 '17 at 13:08
  • $\begingroup$ Then there's his idea to make the Galactic Core Explosion a lie and part of a conspiracy theory... larryniven.net/stories/downinflames.shtml Fortunately, he never wrote it. $\endgroup$ – pleurocoelus Jun 5 '17 at 22:35
  • $\begingroup$ Fortunately!? I think Down in Flames would have been a fitting conclusion to Known Space. It wasn't entirely his idea. This was devised in a discussion with Norman Spinrad about how Known Space (KS) was accumulating more and more contradictions. Thus reducing and restricting possible stories that could be set in KS. Instead he wrote Ringworld and only made it worse. $\endgroup$ – a4android Jun 6 '17 at 1:46
  • $\begingroup$ @a4android To each his own, but I interpreted it as taking a huge dump over his previous work and thumbing his nose at the fans. Kind of a "ha ha you believed it" experience. All fictional universes will be full of contradictions so they don't bother me so much. Up to a point, at least. I much prefer his explanation that the Known Space stories are histories told by different people who may not know all the facts. It is an interesting idea, though. $\endgroup$ – pleurocoelus Jun 6 '17 at 12:37
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Not sure I fully understood all the other answers, so forgive me if I'm repeating someone.

I suggest the achievable speed and/or the achievable acceleration is inversely proportional to the magnitude of the net gravitational attraction on the drive itself. Set the multiplier such that the point at which a conventional drive is faster is close enough for the conventional drive to be practical.

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Hyperspace.

Your FTL requires travel through a different dimensional space that does not interact with normal space. Once you drop out of it, no momentum is maintained, making using FTL un-weaponizable.

Said hyperspace is also not a 1:1 mapping of normal space, and the minimal travel distance in hyperspace amounts to 35~70 or more AU. Meaning the smallest jump clears you from the Sun to Pluto or farther, making intra-system jumps unlikely.

Due to how freaking huge space is, dropping out in the middle of an object is already near zero in probability, but you could also have dropping out of hyperspace clear any matter in its way, ala the Stargate woosh.

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Use an Alderson Drive

It opens existing wormholes at the ends of the Alderson Tramline (in the Pournelle and Niven Universe of Mote in God's Eye) and other books by different authors using the same concept. These wormholes only form away from the star, and between two nearby stars. Not all stars form tramlines, and some wormholes - notably the one in the book, end up being within the atmosphere of a Red Giant star. Travel between wormholes within a system is via standard reaction drives, so no FTL within a system, and once at a wormhole, it only links to the destination star, so you need to travel to the next wormhole in the destination star system if that is not your final system. Hence, travel takes months, but travel between star systems is instantaneous.

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    $\begingroup$ Welcome to Worldbuilding! You should start with the tour. You should read the old answers before posting — at least be mindful of who mentioned what already if you add to it. $\endgroup$ – JDługosz Jun 5 '17 at 7:38
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I wrote a short story a while ago where the FTL was based around space folds, which could only be done at Lagrange points. Essentially, the gravity-based drive could only cope with so many calculations, so you needed to be moving very slowly and in a point where the gravity from the two closest gravity sources would cancel each other out.

It didn't allow for any speed in or out, as low speed was a prerequisite for the gravity drill to work. That takes care of your hypervelocity thing. This admittedly allowed for FTL from planet to planet, but you could handwave that hyperspace tunnels are only stable outside of systems, necessitating inter-system use - perhaps it's only safe to do from fringe to fringe of each system due to shallower gravity wells?

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Imagine that there exists a background magnetic field across the galaxy. A result of traveling through a very large volume of space in a very short time via FTL drive; (whether warp drive or jump drive) Is an enormous electromagnetic potential is developed across the ships hull. If arriving to close to a star the ejected hydrogen nuclei or solar wind creates a conductive path for electrical discharge. And suddenly you have performed electrical discharge machining to drill a huge hole in the hull of the ship because you arrived too close to a star before bleeding off the electromagnetic potential acquired during travel. This idea also gives you some story bylines for finding shipwrecks just inside a solar system.

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Energy

No one mentioned it but in RL you needs a infinite amount of energy to achieve something with mass to reach LV that's main reason why FTL in RL is not possible.

In fiction FTL can be possible but the amount of energy generated can be engouh to increase the mass of the ship (relative to the "still" sol) to a point anyone in earth frame rest can think of your ship as a massive star.

This can pertubate the planets, asteroid's belts, Oort cloud coments and the sol orbit.

These pertubations cannot be used per si as a weapon because those pertubations can needs decades to cause nasty effects like meteor storms, etc. But can be a issue enough to not jump from or to a habited star system. Also at this tech level vaporise a possible earth colliding comet can be trivial.

Also jumping too near of a star can be dangerous for the ship because exiting from FTL to a system with every object (star, comet, planets) being hurled at your exit positions can creat a hazard also note it can pertubate those orbits enough to make impossible to accurate calculate a planet position before FTL jump.

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  • $\begingroup$ It does not require an infinite amount of energy, it requires an amount of energy equivalent to converting the object's entire mass into kinetic energy. Which means what it takes to get going really fast is an external power source that can burn up more than your ship's mass in fuel. A simple solar sail could easily get you going a substantial fraction of C by the time you left the solar system if you sized it appropriately. $\endgroup$ – Perkins Aug 26 '17 at 23:34
  • $\begingroup$ @Perkins I'm not sure how to follow it. If you convert all the mass in energy ther's not more mass, only energy and energy can travel at c sure but you cannot re convert the energy in mass (you can hand wave that of course). Yeah a solar sail and another methods can (in theory) get you in a fraction of c if... you got a immense mass/energy ratio and enough time (decades, centuries?) $\endgroup$ – jean Aug 28 '17 at 10:43
  • $\begingroup$ As you get closer to the speed of light, accelerating takes more energy until, at the speed of light, going any faster would require infinite energy. Getting up to that speed merely requires an enormous amount of energy. Since you can't get more efficient than total conversion, and since the energy to reach the speed of light is equivalent to conversion of the craft's entire mass, you need an external power source to attain such speeds. But, if you have such a power source, the only limit on your velocity is the fact that we know of no particles faster than light to do the pushing with. $\endgroup$ – Perkins Aug 28 '17 at 21:36
  • $\begingroup$ @Perkins Just a bit correction. You do need Infinite Energy to reach speed of light to anything with rest mass > 0. To go FTL you need more (more than infinite) that's the catch. Also for the point of depart frame the ship mass ill increase with speed. At realtivistic speeds anyone in the original planet ill observe the ship as a massive object. $\endgroup$ – jean Aug 29 '17 at 10:35
  • $\begingroup$ It's worse than that. Since you must have an external power source, and since that power source's transmitted energy cannot travel faster than light, even having more than an infinite amount of power at your disposal wouldn't actually let you exceed the speed of light. Doing so would require a particle that travels faster than light to use as a transfer medium, doing something horrible to spacetime, or otherwise finding a gap in the rules that we haven't stumbled upon yet. It's rather like trying to build a super-sonic aircraft that's powered and propelled by sound waves... $\endgroup$ – Perkins Aug 30 '17 at 17:14
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I've seen two systems that came at this problem from different angles.

The first is (I believe used in David Weber's Honorverse?) that gravity wells from large objects (like stars and planets) throw off the calculations required to make sure you come out at the proper place at the other end. In addition, stars have a lot of matter orbiting about them. If you miscalculate, its unlikely you could end up either inside the star, or some other bit of matter (planet, asteroid, etc), but it is possible, and essentially instant death. The better your computers and operators, the closer in you can safely get try to get, but diminishing returns on that effectively force everyone to warp to and from the outskirts of the star system, not directly inside it.

The other one I've seen, featured in K.D. Wentworth and Eric Flint's Jao Empire series, is that the stars themselves are required to travel FTL. Each star essentially acts as a jumpgate, and you have to take (heavily-shielded) ships inside its photosphere in order to activate the FTL drives. That one's so distinctive, I'm not too sure how much you can borrow of it though. I suppose if you tweaked it a bit (eg: make it about the star's magnetic field or something, which would require approaching from a magnetic pole)

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Gravity warps hyperspace, decreasing the distance between points in hyperspace, the more gravity on a region the further you have to travel in hyperspace.

Making travel in system via hyperspace as show if not slower than real drives.

I think i read about drives that work this way.

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Gravity

You can say, that, if you're to close to a planet, your ship is under the influence of the gravity of said planet and accelerating to light speed would throw the ship off the course to an degree, that the ship may even crash into an other planet, unable to slow down fast enough.

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  • $\begingroup$ That would mean courses needed to be plotted above or below the ecliptic. It also doesn't solve the issue of just aiming something at a planet. $\endgroup$ – dot_Sp0T Jun 3 '17 at 10:42
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  • For ships: Too many things which fly around in the solar system (Kuiper belt).

  • Why would an kinetic anit-planet weapon in a solar system profit from FTL? The planet is not going to jump away, and splitting a kinetic weapon into parts doesnt help the defender. That means that the only relevant value is the input of energy, which probably would be the limiting factor. Assuming that these are strategic weapons (on the uselessness of Planet destroyers: watch "The Lexx"), quickly hitting makes no sense.

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Hmm. How about this system:

FTL are done through creating directed space distortions. You create it, it explodes, creating a cylindrical space distortion, which carries a gravitation wave. This wave creates reality-free bubbles on a front of a wave because of cavitation, and within those bubbles ships travel. But problems is:

1) There's minimal size for usable distortions, if too small - it will not produce reality-cavitation bubbles, and minimal power level sends you at least lightyear away.

2) It eats a lot of energy. Ships can't do it on their own, requiring some sort of gate. Maybe, even planet-based.

3) Accuracy - bubbles are unpredictable, ship computer just hops into one it can detect, and in the end ship pops out within wide area at the end, which massively limits precision.

4) They are basically harmless on the recieving end, due to gravity wave dissipation within the same wide area.

So, for these reasons, it's hard to directly weaponise from distance, and no slugfest. Too random, too costly. But slamming a planet through is possible, if you drag your distortion generator to your target. Or ypur planet to your target.

It also makes such gates dangerous to place close to your own planets - if it's captured by enemy, they will basicaly get a cannon right under your nose. If they are planet based, they shoul be guarded like nuclear storages.

Problem is, there's no explainable reason to stop cruise missles, but allow spaceships. But cruise missles at least can be detected and counter-measured, just as ships.

Don't forget that FTL is not real, so apply your artistic license, but support imaginary physics with creative side-uses. Whatever principle you decide to use, it will be more believable if unusual technology applied not only to FTL travel, but also to other areas of life. This part makes them more believable than any amounts of physical terms - if believability is your target.

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In a story I am trying to write ,gravity from stellar objects in normal space causes the dimensions of hyper space to constrict exponentially as you approach the area in hyper space that corresponds to the area in normal space where large gravity wells exist.

So just popping out of hyper space and entering orbit around the Earth would be like trying to park a semi truck in a parking garage made for small economy cars.

To look at it another way, as normal space expands into hyper space and the universe expands ,gravity from objects in normal space causes hyper space to be nearly non existent in the areas close to large objects like stars or black holes .

I hope this gives you some ideas.

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Perhaps telemitry computers do not function at light speed. You may be looking at the nearest star, but it's data that is light years away at best... orbit inside the star's system is not up to date and while larger bodies can be accounted for, you cannot account for the like of asteroids, commets, OTHER SPACE TRAFFIC which is data which is at best years out of sync when you make your jump calaculation. Best to make the jump to the edge of the gravety well and use sublight to manage the interior.

As for FTL in systems, it would be like taking a 747 from Baltimore to Washington D.C. Doable, yes... fuel-economical and profitable? Nope. To say nothing of the other craft in system which are zipping around and might get in the way.

It could even allow you to jump from inside the system to a nearby system, as the stuff in the way is easier to calculate a viable window. But inbound traffic must stop at the boarder's edge. This could be stopped by indicators which are set up to prevent the catastrophy of an offensive attack or a busted nav system.

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Cheap and effective FAD systems (FTL Area Denial)

Any target worth protecting has one, and keeps it turned on unless they have a good reason to do otherwise. Anything using standard FTL drives entering the exclusion zone gets dragged back to sublight speeds in a decidedly violent manner.

Note that this won't 100% prevent using FTL in-system, but you'd have to get the people in your flightpath to let you through, which they would likely be disinclined to do.

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I don't think you can, but not because of science but because of logic.

Let A and B be planets in the same solar system, and C be the nearest neighboring star in another solar system. If I can use FTL travel to travel from A -> C, I can use FTL to go from C -> B. You could set it up such that an FTL trip A->C->B takes longer than a non-FTL trip A->B, but then you're constraining FTL in your world to be pretty slow.

One way you could address this would be for the original departure location to be temporarily "tainted" somehow such that FTL drives don't work in the vicinity for some period of time, but that would probably have a lot of side effects that you'd need to account for (e.g. if I can't travel to B for 5 years after an FTL trip from A->C because my departure affected the region, then no one can travel to either A or B via FTL for that period either).

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This is my first attempt at an answer, so I apologize in advance if I missed a guideline. Everyone so far has had some fantastic insight! I'll contribute what I know (which isn't much) and see where it goes.

  • FTL travel is still a disputed area of science. Shortly after Einstein's Theory of Relativity, the early scientific consensus was that light speed as a "barrier." Anything capable of traveling faster than light is already doing so. And everything slower than light is (theoretically) doomed to stay there. But this rule only applies to matter.

  • I understand the popular opinion that the energy requirement to travel FTL is really high, but it IS within our reach. About 10 years ago, some guy actually got a wind-powered vehicle to travel FASTER than the speed the wind was blowing. And he did it on land! (It would've been easier with a properly sized hydrofoil on a sail boat.) Read the story if you wish.

  • If we ever make contact with intelligent alien life, there's no guarantee that they'll be based on Carbon, or share our environment. For all we know, they could be electrical, living on a planet with no atmosphere at all, and be incapable of leaving the planet. Kind of like a plasma ball. So let our trading partners supply the energy. And whatever we're using as a delivery vehicle should be a drone! It eliminates the need for life support, and drones don't care if they travel through time!

Regardless of what you decide, let me know when it's done! I'll buy the first copy.

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  • $\begingroup$ It is some potentially useful insight, but I don't see how exactly it answers the question. $\endgroup$ – Mołot Feb 26 '18 at 8:40
  • $\begingroup$ I thought you might say that. I didn't want to say too much before because it's not my story to write. The idea I was going for, in order to prevent weaponization and other hazards, is that in order to achieve FTL, it needs to depend on the trading partner. For whatever reason you choose, we can only make half the jump. And here's "why":Ever shine a flashlight into dark woods? No matter how good your flashlight is, it can only illuminate so far. But if your partner is on the other side of that darkness, with a flashlight of his own, you might be able to illuminate the entire path. $\endgroup$ – Thomas Rigger Feb 26 '18 at 9:32
  • $\begingroup$ If both parties create half the jump, attacking becomes impossible. And the flashlight explanation is compatible with everything on this page: gravity, energy requirements, navigation (especially! because it's really hard to calculate distance over infinity), etc... $\endgroup$ – Thomas Rigger Feb 26 '18 at 9:36
  • $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$ – rek Feb 26 '18 at 10:05
  • $\begingroup$ @rek Thanks for the feedback. I should've reviewed the question after reading all the responses. This topic has generated a shocking amount of material, and it's ALL intelligent! Obviously, I got distracted by something. Re-reading the question will help me prevent this. $\endgroup$ – Thomas Rigger Feb 27 '18 at 3:15

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