[Spaceships] Your preferences regarding plausible/playable Reactionless Drives

[teviet]

Okay, so let's take another look at Molokh's "quantum roundoff" drive. Why not? Lots of people interested in quantum gravity have imagined it as some sort of Planck-scale gridwork (pre-geometry) or cellular automata.

Normally position can be measured to infinite precision, but now we have a handwavy device that introduces roundoff bias in position every "clock cycle" of the Universe. Maximum roundoff error is +/- half a Planck length per Planck time, for a maximum speed of 0.5c. Perfect: we don't have to worry about time travel.

For consistency with relativity, it would be a pseudovelocity: that is, the device would retain its proper rest frame, and the extra translational motion would be relative to that frame. Since the device isn't actually accelerating, it could jump to 0.5c instantly.

Power requirement: well, who knows? If we assign an energy cost based on fundamental physics, the only values we have to work with are the rest energy of the spaceship (too big!) or something like one Planck mass per Planck volume of the field (even worse!). Better to say that the fundamental cost of powering the drive is zero (you're tweaking the laws of physics, not adding energy to anything), but the operating equipment has some arbitrary overhead that can improve with better technology. Or you can use some other mechanism (computation time?) to limit changes in pseudovelocity.

How about speeds other than 0.5c? Well, that's based on going from infinite precision to minimum precision. Maybe you can adjust the number of bits of roundoff error, allowing speeds of c/4, c/8, c/16, etc. Whether these are any "cheaper" is up to you. Simplest is to make cost proportional to pseudovelocity (which comes in discrete increments, above). Or you could try something fancy like make cost inversely proportional to the number of bits of precision that you keep. Then you end up with something like:

cost = constant * log(0.5) / log(v/c)

This gives an expensive startup followed by a steady increase to the maximum pseudovelocity.

Are we having fun yet?

TeV

[Anthony]

Originally Posted by teviet

Power requirement: well, who knows?

You need enough to account for changes in gravitational potential energy.

[ehrbar]

We used to have separate laws of conservation for mass, energy, linear momentum, and angular momentum. We now have a unified law of conservation of mass-energy. Is there any grounds to suspect it could be modified into a single law of conservation of mass-energy-momentum?

Maybe. As Robert L. Forward pointed out in Indistinguishable From Magic, General Relativity holds that mass, energy, linear momentum, and angular momentum compose a single "thing" for purposes of causing the gravitational field—the stress-energy tensor. If they all contribute to a gravitational field, he suggests that maybe they're all fundamentally the same thing, and maybe they can be inter-converted.

Which, of course, gets you your reactionless drive — mass/energy/angular momentum converted directly into linear momentum.

[lexington]

Originally Posted by ehrbar

We used to have separate laws of conservation for mass, energy, linear momentum, and angular momentum. We now have a unified law of conservation of mass-energy. Is there any grounds to suspect it could be modified into a single law of conservation of mass-energy-momentum?

Maybe. As Robert L. Forward pointed out in Indistinguishable From Magic, General Relativity holds that mass, energy, linear momentum, and angular momentum compose a single "thing" for purposes of causing the gravitational field—the stress-energy tensor. If they all contribute to a gravitational field, he suggests that maybe they're all fundamentally the same thing, and maybe they can be inter-converted.

Which, of course, gets you your reactionless drive — mass/energy/angular momentum converted directly into linear momentum.

Mass/energy to linear momentum is just a photon drive or in GURPS a total conversion drive.
Angular momentum to linear momentum is the Dean Drive, which violates Newton's Third Law.

[Anthony]

Originally Posted by ehrbar

Which, of course, gets you your reactionless drive — mass/energy/angular momentum converted directly into linear momentum.

Mass and energy can be converted into linear momentum; the conversion factor is c.

[teviet]

Originally Posted by ehrbar

As Robert L. Forward pointed out in Indistinguishable From Magic, General Relativity holds that mass, energy, linear momentum, and angular momentum compose a single "thing" for purposes of causing the gravitational field—the stress-energy tensor. If they all contribute to a gravitational field, he suggests that maybe they're all fundamentally the same thing, and maybe they can be inter-converted.

That's not really what the science suggests. I was a bit surprised that Bob goofed up like this in that book; he's usually a lot more careful with his facts. True, conservation of a unified stress-energy tensor is enforced by field equation of general relativity, but this doesn't mean that the different parts are interconvertable; it means that each component is individually conserved. Mass and energy are "interconvertable" because they fall within the same component of the stress-energy tensor.

Bob made a big point of "converting" mass into linear momentum with the proportionality constant c. But that's not really how it works. You conserve mass/energy and momentum by throwing some of your mass/energy away at the speed of light and absorbing the recoil. You get the same conversion factor, but you're not really destroying energy or creating momentum. The only "conversion" is between mass and energy.

There is no analogous mechanism for angular<->linear momentum. Again, mass<->energy conversion has to be involved somewhere, because those are the only things that can interchange (because they're really the same thing).

TeV

[Fred Brackin]

Originally Posted by lexington

Mass/energy to linear momentum is just a photon drive or in GURPS a total conversion drive.

Note that a total conversion drive without obnoxious exhaust comes quite close to being as good as a reactionless drive for most rpg purposes i.e. the PCs whoosh off towards Planet X without complicated concerns about fuel or delta-v.

[malloyd]

Originally Posted by Fred Brackin

Note that a total conversion drive without obnoxious exhaust comes quite close to being as good as a reactionless drive for most rpg purposes i.e. the PCs whoosh off towards Planet X without complicated concerns about fuel or delta-v.

Which is sort of its point. It works just like a reactionless drive until you try to get up to speeds that are a large fraction of the speed of light, then you run out of fuel first. Since you can't get up to those speeds inside a solar system unless you've tweaked the thrusters so you can somehow manage hundreds of gravities, the only stuff this is likely to prevent are the worst of the planetbuster applications, which you didn't want in your game anyway, right?

[teviet]

Originally Posted by malloyd

Which is sort of its point. It works just like a reactionless drive until you try to get up to speeds that are a large fraction of the speed of light, then you run out of fuel first. Since you can't get up to those speeds inside a solar system unless you've tweaked the thrusters so you can somehow manage hundreds of gravities, the only stuff this is likely to prevent are the worst of the planetbuster applications, which you didn't want in your game anyway, right?

An efficient photon or neutrino drive will end up converting about half your fuel's rest energy into payload kinetic energy (for large fuel-to-payload mass ratios). So you can still make a pretty big boom.

TeV

[Fred Brackin]

Originally Posted by malloyd

Since you can't get up to those speeds inside a solar system unless you've tweaked the thrusters so you can somehow manage hundreds of gravities, the only stuff this is likely to prevent are the worst of the planetbuster applications, which you didn't want in your game anyway, right?

No, I am blithely unconcerned about planet-destroying nihilists.

I do have some concerns about an overwhelming effect of kinetic energy weapons on space combat but very few space drives avoid those.

There's also the question of ships actually maneuvering against one another as opposed to whooshing past each other at insane velocities.

It is these concerns and not theoretical planetbusters that cause my preference for hyperdynamic and other "inertialess" drives.