[Spaceships] Your preferences regarding plausible/playable Reactionless Drives

[lexington]

Here's an idea that just occurred to me. In quantum mechanics you can apparently have a charged vacuum. How much technobabble/handwavium would be involved in having an engine that is repelled by that charge?

[Anthony]

Originally Posted by teviet

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.

Any drive that lets you usefully travel around the solar system will let you make a pretty big boom. If it can reach earth escape velocity, that's 15x object mass in TNT; if it can reach solar escape that's 200x object mass in TNT.

[lwcamp]

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?

There already is one, but not they way you seem to think.

Originally Posted by ehrbar

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.

The first part of the first paragraph is absolutely correct. The stress energy tensor is one physical "thing," just like an objects position is one physical "thing" that can be described by three numbers.

All the conservation laws are summed up in the statement that the four-dimensional divergence of the stress energy tensor is zero. What this means is that the rate of change of the energy in a volume is equal to the amount of momentum flowing through the surface of the volume, and the rate of change of momentum in a volume is equal to the net force on the surface of the volume. When combined with the symmetry of the stress energy tensor, this also gives you that the rate of change of angular momentum inside the volume is equal to the net torque on the surface. And there you have it - a single unified law of conservation of mass, energy, and momentum summed up by the single equation grad * T = 0.

Luke

[teviet]

Originally Posted by 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.

<SNIP>

Okay, before Luke beats me to it, I'll add that a pseudovelocity drive still violates energy/momentum conservation. Why? Well, consider a closed surface around the spaceship. It contains the spaceship's rest energy. Now the spaceship moves outside: the surface now contains no energy. But because the velocity of the spaceship at any moment is zero, the energy flux across the surface at any moment is also zero. Hence: no local energy conservation.

So probably we do need something like an Alcubierre negative-energy field to nullify a ship's mass first. Before I'd said that Alcubierre drives could violate causality, that's without quantum gravity! According to some studies, the Alcubierre warp bubble may quantum-mechanically unstable (it could be disrupted by amplified vacuum fluctuations in the same manner as a wormhole-based timewarp). So, (hands start waving) maybe only particular configurations can be made stable, and (hands waving faster) these might correspond to discrete pseudovelocities that are less than c.

TeV

[lwcamp]

Originally Posted by teviet

Okay, before Luke beats me to it, I'll add that a pseudovelocity drive still violates energy/momentum conservation. Why? Well, consider a closed surface around the spaceship. It contains the spaceship's rest energy. Now the spaceship moves outside: the surface now contains no energy. But because the velocity of the spaceship at any moment is zero, the energy flux across the surface at any moment is also zero. Hence: no local energy conservation.

You are quite correct with this assertion. It is interesting to note, though, than in general relativity the conservation laws based on the stress energy tensor are only true under certain specific circumstances. Namely, the surface across which you are considering your flow of energy, momentum, and angular momentum must be what they call "asymptotically flat," which is basically a fancy way of saying that gravity must be approximately Newtonian. This gives you an out when considering the conservation of energy and momentum - when the warp drive is crossing the surface gravity is very far from Newtonian - the curvature of space-time due to the warp is extreme and under these circumstances, you do not expect momentum or energy to be locally conserved.

So what do you do? You can consider a surface large enough that the warp drive doesn't move through it. So long as you consider surfaces this big (and assuming that you are smart enough to avoid crossing pulsars or black holes or cosmic strings or other gravitational disturbances when choosing your boundary) then energy and momentum within this surface must be conserved.

Unfortunately, unless it has zero mass, angular momentum cannot be conserved. For those who do not already know (and I'm guessing teviet already does know), the magnitude of the angular momentum can be found my multiplying the linear velocity by the distance of closest approach if the object takes a straight line path (this follows from the fundamental definition of the angular momentum as the cross product of the distance vector with the momentum vector). Note that while every rotating object has angular momentum, objects can have angular momentum without rotating.

So consider if you are drifting straight toward a spacecraft equipped with a warp drive at rest. At this point in time the warp drive is turned off. From your point of view, the spacecraft is drifting straight toward you. Since it is moving directly toward you, it has no angular momentum from your point of view.

Now suppose that the spacecraft turns on its warp drive and moves away perpendicular to the line between you and its original position, and then turns its warp drive off. In order for angular momentum to be conserved, it must have zero angular momentum when it finishes. That means it must either still be moving straight toward you, or it must have zero velocity, or it must have zero mass. But it can't have zero velocity with respect to every coordinate system, and it can't be moving straight toward the origin of every coordinate system. Thus, if the mass is non-zero, angular momentum will be non-conserved for all but a small set of observers. Note that you could have surrounded the spacecraft by a closed surface defining a volume which the spacecraft never leaves during its warp, so in this case general relativistic conservation of angular momentum should hold. This indicates that under general relativity, warp drives with non-zero mass cannot actually warp.

Originally Posted by teviet

So probably we do need something like an Alcubierre negative-energy field to nullify a ship's mass first.

That is certainly a valid seeming solution as far as we currently know. Although there still seem to be problems if you warp faster than the speed of light.

Also note that if the warp drive has a net mass/energy of zero, and the stuff being warped around has a non-zero mass, then in order to satisfy the conservation of energy when you get where you want to go you must absorb as much mass/energy from the environment as is held inside the warp field in order to stop warping!

Luke

[sir_pudding]

Originally Posted by lwcamp

That is certainly a valid seeming solution as far as we currently know. Although there still seem to be problems if you warp faster than the speed of light.

There isn't any reason why it should have to be able to attain FTL is there?

Also note that if the warp drive has a net mass/energy of zero, and the stuff being warped around has a non-zero mass, then in order to satisfy the conservation of energy when you get where you want to go you must absorb as much mass/energy from the environment as is held inside the warp field in order to stop warping!

Can you do that by crashing into stuff? Interacting with the interstellar medium?

[lwcamp]

Originally Posted by sir_pudding

There isn't any reason why it should have to be able to attain FTL is there?

Not that is currently known - sub-luminal warping of zero mass warp regions doesn't have any absolute deal-breakers that I am aware of.

Originally Posted by sir_pudding

Can you do that by crashing into stuff? Interacting with the interstellar medium?

Yup. Pretty much.

Also note that once you crash into something, even a single atom of the ISM, or absorb even a single photon (to sense your environment, perhaps), they your warp drive will have mass and you can't warp any more. Maybe you could radiate away the excess energy or something, as long as it is small enough, and then you could warp again.

Luke

[teviet]

Originally Posted by sir_pudding

There isn't any reason why it should have to be able to attain FTL is there?

No, but nor is there any reason why you can't. The translational pseudovelocity of the warp drive is determined by the configuration of positive and negative energy density along its surface, and does not seem to care whether the resulting velocity is more or less than c. But if you set it up so that it's greater than c, then a pair of such devices could route a message backwards in time. So you need some extra handwaving to place limits on its speed.

Originally Posted by sir_pudding

Can you do that by crashing into stuff? Interacting with the interstellar medium?

This could be a problem, but need not be. To establish the negative-energy field you just need to create equal amounts of positive and negative energy, and dump the positive energy. So assume you can disperse the positive energy as something harmless, like gravitational waves why not.

Then when you stop, you just release the negative-energy contramatter or whatever and let it disperse. Bob Forward assumed that "negmatter" or contramatter was dangerous stuff to leave lying around, as it would nullify any ordinary matter that it touched, but that's actually not likely: more likely it would be weakly interacting, and you'd need some exotic equipment to even notice it passing by. (Well, but who really knows?)

This means after starting and stopping your drive, you'll end up with two expanding clouds of positive and negative energy, each equal in magnitude to the rest energy of your ship. That's fine, in fact it's necessary; the offset between the centres of these clouds will be just right to avoid the angular momentum problem that Luke pointed out.

You'll note that all of these mechanisms involve creating and manipulating energies at least as large as the rest energy of your ship, so this really high-power stuff. But you can arrange your technobabble so that the final output of these vastly energetic reactions is something innocuous, like neutrinos or gravitational waves.

TeV

[Anthony]

Originally Posted by teviet

This could be a problem, but need not be. To establish the negative-energy field you just need to create equal amounts of positive and negative energy, and dump the positive energy. So assume you can disperse the positive energy as something harmless, like gravitational waves why not.

Or just a chunk of matter. You can handle all of your conservation issues (even stuff like baryon number) if you just leave behind a cloud of matter with the same chemical composition as the ship (you need enough energy to cover the change in form, but that's not all that much). Thus, the field you're using to negate your mass is the exact negative of your ship (reversed baryon number, lepton number, etc). To exit your pseudovelocity field, you just need to contact baryonic matter which you can convert into the form of your ship. This has some implications for use as weapons, but they're relatively modest compared to a lot of other weapons options. Of course, this seems more like teleportation than like flying around, and in fact you can't stop in space, and we're still probably going to be violating the Shannon-Hartley theorem since we're transmitting information without transmitting much energy, but we're getting closer.

[jason taylor]

What I'd like to know, is whether reactionless drives really get enough plot mileage to justify the bending of normal physics from the authorial stand point. How much do they do that another kind of reaction drive does not? If the idea is to economize space or fuel or reduce heat then ano ther kind of reaction drive can be arranged that does the same thing. FTL is an absolute necessity(space opera never seems to work without FTL, so we just have to live with it). Grav control is very much of a convenience(there are only a few times we really want to think about the yucky consequences of being without gravity for weeks at a time, and anyway it's an extra headache for the props department in a stage rather then literary forum). But why reactionless drive?