[Spaceships] Your preferences regarding plausible/playable Reactionless Drives

[vicky_molokh]

Greetings, all!

Reactionless Drives are obviously considered TL^. However, either to satisfy physics buffs, defend against munchkins, and just to avoid inconsistencies and unwanted uses/consequences, GMs often prefer to carefully select the way reactionless drives work.

AtomicRocket/ProjectRho mentions the problem with 'realistic' reactionless drives being de-facto fancy Photon Drives. The alternative is to use some sort of special frame of reference. Question #1 for the experts and Space GMs: what are the consequences of doing it with a universal frame of reference, butchering relativity?

The other option is to tie the reference frame to the mass within a certain volume. With a small enough volume, this gives us weaponized Tractor Beams/Spheres, which may or may not be what you want. However, I'm looking at a different idea:

The sphere of volume is increased, until its total mass, excluding the spaceship, is so large that as a result of the 'Reactionless' Drive's work, the mass gains an acceleration relative to the spaceship of less than one Planck length per Planck time. In other technobabble, the universe allows a 'reactionless' drive due to 'rounding errors'. This probably butchers Quantum Mechanics, but might have fewer consequences for the game world, since the differences might be written off as humans misunderstanding QM.

Finally, there was some talk of a project named for the ST drive, i.e. Warp Drive. How hard is to playabilitate/plausibilitate it to acceptable levels?

Awaiting comments and a discussion.
Thanks in advance

[Fred Brackin]

Originally Posted by vicky_molokh

Question #1 for the experts and Space GMs: what are the consequences of doing it with a universal frame of reference, butchering relativity?

I can only say that I have never seen any consequences to the game from either butchering or complying with relativity.

[Pomphis]

Originally Posted by vicky_molokh

Question #1 for the experts and Space GMs: what are the consequences of doing it with a universal frame of reference, butchering relativity?

Depends. Do you mean an entirely newtonian universe ? No time dilation at all ?

[malloyd]

Originally Posted by Fred Brackin

I can only say that I have never seen any consequences to the game from either butchering or complying with relativity.

Neither have I. The negative consequences of reactionless drives are the missile with ridiculous kinetic energy (which happens with any fast spacecraft drive that cares about physics at all), and the cosmic power plant (from their violation of conservation of energy, which always happens in the Newtonian case and happens when they are better than photon drives in the relativistic one).

Ditching relativity, at least if you do it with a preferred frame, isn't likely to have any effect on the game. OK, technically magnetism goes away, light vanishes as electromagnetism becomes impossible, nuclear reactions stop working and the sun goes out for lack of an energy source, and atoms fall apart when the binding energy disappears, but you can simply declare Maxwell's equations and mass to energy equivalence fundamental laws rather than logical consequences of relativity.

A quantum mechanical handwave is really no better. It solves neither of those actual problems and causes just as much damage to the rest of physics - modern physics is pretty highly interconnected. And if you are concerned about your players exploiting the loopholes (though you probably shouldn't be, if they aren't willing to ignore them by GM fiat they clearly don't want to play in a campaign with good space drives), it is likely to have more easily abused ones, since it isn't likely to require difficult to engineer tremendous speeds, distances or energies to produce them.

[lwcamp]

Originally Posted by vicky_molokh

AtomicRocket/ProjectRho mentions the problem with 'realistic' reactionless drives being de-facto fancy Photon Drives. The alternative is to use some sort of special frame of reference. Question #1 for the experts and Space GMs: what are the consequences of doing it with a universal frame of reference, butchering relativity?

A universal frame of reference makes the spacecraft something like an airplane or submarine operating it its respective medium. When the drive is turned "on" with respect to the frame of reference, it will experience drag that accelerates it to the velocity of the reference frame. The drive could presumably also apply thrust (otherwise it wouldn't be much of a drive), which will give a force that you can approximate as the lower of a static force (the force the "propellers" produce when the craft is at rest with respect to the reference frame) and the power divided by the craft's velocity with respect to the reference frame. Note that since the force decreases with increasing velocity, and since drag tends to increase with increasing velocity, you will get a maximum velocity your craft can travel at.

In addition to drag and thrust, you might also be able to get lift - that is, a force perpendicular to the direction of travel relative to the special frame of reference. This would allow your craft to make banking turns.

In some sense, this type of drive might not be reactionless at all - it may simply be interacting with some sort of atmosphere in hyperspace, or cosmic ether, or something, and like a boat or jet plane it sends jets of the fluid shooting away using its "propellers" and uses those jets as the reaction mass it is pushing off against.

Originally Posted by vicky_molokh

Finally, there was some talk of a project named for the ST drive, i.e. Warp Drive. How hard is to playabilitate/plausibilitate it to acceptable levels?

There has been work on a particular kind of distortion of space-time called the warp drive by the guy who discovered it (Alcubierre). So far, all worked out warp drive examples I have seen have had zero mass. This is probably a necessity, because in general relativity if any disturbance in space time can be localized, then energy, momentum, and angular momentum are conserved (note, I had previously thought that angular momentum might not be conserved. I have since revised that opinion). Any movement of a mass without a reaction will always result in non-conservation of angular momentum (just as any change in velocity of a mass will always result in non-conservation of linear momentum). Because of the conservation of energy, if you put anything into a warp bubble, it will acquire the energy and hence mass of the thing inside of it, so either you have a warp bubble that can't warp (because it has mass) or you somehow take energy out of it until it has zero mass. If it has zero mass, then the thing that is warping cannot come out into the rest of the universe until it can acquire energy from outside equal to its mass (with enough factors of c to satisfy the units you are working in). A warp drive that must be kept at zero mass also can't see or communicate.

Also note that if you consider the magnitudes of the energies involved (rather than the net energy), the original Alcubierre design had you working with many galaxies worth of energy (both positive and negative). work by others (such as Van der Broeck) have gotten the energy magnitudes down to just several times that of a star (but you have to be inside a pocket universe connected to the normal universe by a wormhole with a proton-sized diameter). If you have civilizations that can throw around energies of that magnitude, the scale of effects will probably be well beyond the merely human and matter will be as fragile as cobwebs and tissue paper are to our steel and bullets and engines.

Now if you don't care about real physics, you might choose to abandon the conservation of angular momentum. You can keep the conservation of energy and linear momentum, to prevent your reactionless drive craft from accelerating to ludicrous speeds and turning planets into rapidly expanding clouds of plasma and vapor. By choosing these sets of conservation laws, it allows you to move your craft without acquiring velocity. If you turn off the drive, you will have the velocity you originally started with assuming that nothing exerted any forces on you while the drive was on (if forces were exerted on you, you would be accelerated by them as normal, as would the thing exerting the forces on you in the usual non-reactionless way). A craft with a reactionless drive can still use outside forces to alter its velocity - for example, it might hover over a planet. Gravity would keep accelerating it down, but it could keep moving away from the planet to counter its velocity so that it doesn't crash. Note that since we are still conserving energy, this would require power from the craft's power plant equal to the gain in kinetic energy (neglecting inefficiencies which would appear as heat). Because it would need to be subject to an external force to gain energy, and because it would require the craft's on board power source, this makes it more difficult to create planet destroying weapons of mass destruction from any equivalent of a space-yatch or tugboat. It also makes spacecraft less maneuverable near strong sources of gravity since they need more power to warp outwards.

Luke

[Pomphis]

Just one addition: note that ships with such pseudo-velocity drives will still need some "real velocity" drives if you want to be able to get off without a spectacular explosion on another planet or docked to a space station or ship. After all, leaving the effect of your ship´s drive means that you regain the velocity you had when the drive was switched on. Which is very unlikely to be close enough to the one of the planet/station/ship you are just embarking.

[teviet]

Alcubierre warp drives explicitly require contramatter (negative mass) of some form. You can also use them as time machines. So I'm not sure if that's what you want.

Originally Posted by vicky_molokh

... one Planck length per Planck time...

In other words, the speed of light? :)

Basically you have two possibilities: either energy/momentum conservation is truly being violated, or there's some mystery field or intangible medium that absorbs the difference. Neither of these is a fundamental impossibility. The first obviously rules out general relativity, so you are basically left with a contrivance: "The Universe has a preferred frame, and certain interactions depend on this frame, but the rest of the laws of physics have been arranged in such a way as to conceal its existence". Still, it's not impossible.

Also, if you can create momentum, you can create energy for free. That much is a mathematical certainty, or else modern-day generators wouldn't function as they do.

An "intangible medium" does less damage to the laws of physics, and you can tune the "intangibility" so that the medium is dragged with local matter. But if it comprises any substantial fraction of the mass of a galaxy (say the cold dark matter component), we would notice it. And cold dark matter is already so tenuous that you either have to reach out to huge distances to grab enough of it to impart significant momentum, or you need to throw the smaller amount near you at relativistic speeds. Which basically gives the same operating characteristics as a photon/neutrino drive.

I didn't read the whole atomic rocket site; what problem, if any, do they have with non-reactionless "reactionless" drives? My preferred handwavium is a mechanism that converts baryons into leptons, specifically neutrinos, and preferentially emitted in one direction with the recoil passing to the mechanism. Technically this is three handwaves, but it gives you a "reactionless" total conversion drive (i.e. no noticeable exhaust) that is inefficient as a power plant.

TeV

[Sunrunners_Fire]

Or you can go, "These are the observed traits of the drive. As you are interested in the hows and whys of the drive, insert-player-name-here, please feel free to come up with a satisfactory explanation that satisfies your requirements so I can look it over." And then hand them a game mechanic description of what the drive does.

Let them figure out the details. If they can't come up with something that satisfies them, well, then it is doubtful that you could either.

[Diomedes]

The other not-quite reactionless drive I can think of is the repulsor/tractor drive: use a tractor beam to pull yourself to your destination planet (or push of from your planet of origin). Using an entire planet for reaction mass means it won't move very much, but you don't have to carry your propellant with you.

[Crakkerjakk]

Originally Posted by Sunrunners_Fire

Or you can go, "These are the observed traits of the drive. As you are interested in the hows and whys of the drive, insert-player-name-here, please feel free to come up with a satisfactory explanation that satisfies your requirements so I can look it over." And then hand them a game mechanic description of what the drive does.

Let them figure out the details. If they can't come up with something that satisfies them, well, then it is doubtful that you could either.

This is my preferred method. If me waving my hands wildly isn't enough to satisfy their SoD, then they get to make up their own explanation.