[Spaceships] Your preferences regarding plausible/playable Reactionless Drives

[vicky_molokh]

Originally Posted by Anthony

Black holes can also have electromagnetic charge and thus engage in electromagnetic interactions, which we know are moderated by photons.

*Starts wondering whether the entire Lovecraftian Horror genre was vaguely inspired by the slow discovery of QM*

[teviet]

Originally Posted by sir_pudding

This a problem with any model of gravity that requires particles. I think it's becoming increasingly clear that "gravitons" probably don't exist.

"Gravitons", like "photons", are just a way to describe the quantization of energy and angular momentum in a field. We don't yet have a full theory of quantum gravity, but such a theory most likely will include gravitons. From "semiclassical" models of gravitational waves on a nearly-flat nearly-static background, it's predicted that gravitons are "spin-2 tensor bosons"; that is, they carry twice as much angular momentum as a photon.

Regarding the tractor/pressor beam, this will violate local energy/momentum conservation unless the beam itself has significant rest mass in the frame of whatever you're pushing against. (That is, it's more like the "beam" of a building than the "beam" of a laser.)

Otherwise what you have is a gravitational rocket, which has the same performance characteristics as a photon or a neutrino drive, except that the exhaust stream consists of gravitons. For a given wattage, a graviton beam is even less detectable than a neutrino beam, so a graviton rocket is actually an ideal "reactionless" thruster, if you want something whose exhaust passes intangibly through the interior of your ship, planet, or star.

TeV

[teviet]

Originally Posted by vicky_molokh

Gravitons? Who knows. Supposedly time slows to a halt in a singularity, yet singularities emit gravitons, which somehow manage to 'get somewhere' in time. So they seem to be able to jump over the fences that slow down or stop even the mighty photon.

This is actually one of the standard misconceptions of black hole physics. The static field of a star or planet or black hole does not propagate from the object, and does not involve the emission of gravitons. Or, looking at it another way, it involves the transmission of "virtual gravitons", which are not limited to the speed of light (and carry no information). Really, "virtual particles" are just a mathematical bookkeeping device.

In the same way, the force between two static electric charges can be described as an exchange of "virtual" photons, but no actual photons are emitted.

TeV

[Anthony]

Originally Posted by teviet

Regarding the tractor/pressor beam, this will violate local energy/momentum conservation unless the beam itself has significant rest mass in the frame of whatever you're pushing against.

Pressor beams don't require rest mass, they just require the same amount of energy as creating photon pressure between two mirrors (which is a special case of the photon rocket, and can theoretically be vastly more efficient than a conventional photon rocket). Tractor beams are messier, because what they really require is an energy debt of the same magnitude.

[Nymdok]

I havent run a sci-fi game, but if I did, Id use Paul March/Woodward/Mach's reactionless thrusters.

Heres an Interview
http://nextbigfuture.com/2009/09/mac...aul-march.html

The Second PArt of the interview

http://nextbigfuture.com/2009/09/mac...t-part-ii.html

with comments from Paul March in the comments section.

Also a lengthy slide show presentation with lots of pictures :)

http://www.cphonx.net/weffect/STAIF-...20Appendix.ppt

Im not nearly well enough researched to speak on it fully which is why I leave the links.

I attended a 'brown bag talk' over lunch on it he had one day (Paul March works in my building and is a nice guy!) and was pretty impressed with the idea.

To use this in a game, the only leap you need to make is that SOMEONE gets a functional version fo this working and that you can extrapolate it to the nth degree of speed.

Nymdok

ETA: When I saw the devices that March was building and then I saw the primitive arc-reactor in the Iron Man movie, I freaked!

[teviet]

Originally Posted by Anthony

Pressor beams don't require rest mass, they just require the same amount of energy as creating photon pressure between two mirrors (which is a special case of the photon rocket, and can theoretically be vastly more efficient than a conventional photon rocket). Tractor beams are messier, because what they really require is an energy debt of the same magnitude.

Well, technically an EM standing wave or laser cavity does have rest mass (standing wave energy/c^2) and a rest frame (the frame in which the wave is stationary). So what you've really got is a maximally stiff medium, exerting a a pressure of 2*(energy density)*c on either end.

So to get some thrust T, you need to build up a stored energy of 0.5*T*D where D is the distance to the "anchor". And of course there's the startup time of 2*D/c while the beam is "reaching out" to the anchor and reflecting back, during which time you don't get the full reaction force, but you do still get the photon rocket thrust.

So suppose you've got a 1MW repulsor drive some distance from the Earth, say one lightsecond (about as far as the Moon). You turn it on. For the first two seconds, you just get the 0.003N photon drive thrust. Then the return wave reaches you. Thrust builds up steadily: after a minute it's at 0.2N, after an hour it's at 12N, after a day it's at 288N (and the total rest mass of the beam is about a microgram). This can increase indefinitely, except that more and more of the energy is going into the ship's kinetic energy and not into increasing the stored energy of the beam, eventually asymptoting at a velocity of sqrt(2*P/M) where P=1MW is the power and M is the mass of the ship.

For those of you that care, the motion of the ship is a hyperbolic function:

x(t) = sqrt[ D^2 + 2*(P/M)*t^2 ]

The tractor beam effect is harder to model, but will probably scale similarly.

TeV

[cmdicely]

Originally Posted by vicky_molokh

*Starts wondering whether the entire Lovecraftian Horror genre was vaguely inspired by the slow discovery of QM*

That is well-known as an influence on Lovecraft's work, and explicitly referenced in some of them.

[sir_pudding]

Originally Posted by cmdicely

That is well-known as an influence on Lovecraft's work, and explicitly referenced in some of them.

His problems with relativity are known from his letters, what evidence do you have that he had any understanding of quantum physics?

[Anthony]

Originally Posted by sir_pudding

His problems with relativity are known from his letters, what evidence do you have that he had any understanding of quantum physics?

Who says you have to understand QM to find it disturbing?

[teviet]

Originally Posted by Anthony

Who says you have to understand QM to find it disturbing?

"Anyone who claims to understand quantum mechanics has not studied it deeply enough"

--Richard Feynman (?)