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.

posted over 7 years ago

MikeB‭

11 reputation 1 1 1 0

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.

posted over 7 years ago

LSerni‭

11 reputation 0 1 1 0

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}

posted over 7 years ago

dot_Sp0T‭

26 reputation 46 2 3 0

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.

posted over 7 years ago

M. A. Golding‭

31 reputation 2 3 3 0

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 ""Šrealistic"Š" 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.

posted over 7 years ago

Miguel Bartelsman‭

11 reputation 3 1 1 0

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.

(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.

posted over 7 years ago

Reid Rankin‭

11 reputation 0 1 1 0

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.

posted over 7 years ago

Amadeus‭

31 reputation 1 114 3 0