[Spaceships] Reactionless Drives, Pseudovelocity, and NOT breaking a setting (much)

[lwcamp]

Originally Posted by vicky_molokh

But the other idea I had is some sort of 'bleed over' effect, such that when one pushes away from a gravitational well, some of the work the drive performs behaves as if the drive is 'pushing' against the planet, thus spending energy on reducing/compensating real downward velocity. Sort of like the Spooky Traction At A Distance Drive. Of course, this still leaves the ability to theoretically get real velocity by matching with a big celestial body on a fast orbit elsewhere, but that seems highly impractical. Also, maybe some sort of bleed-over that makes real velocity gradually match that of a close big body while the drive is active (again, not a panacea). Are these two bleed-overs useful / subtly harmful / what else can be said about these two ideas?

Warp out to go find a binary pulsar (not hard, they broadcast their location far and wide). Match speed with one of the neutron stars. Now warp back to your target and slam into it at relativistic speeds.

Originally Posted by vicky_molokh

Also, what are the consequences of the ability to break conservation of angular momentum with TL^ drives?

I haven't been sufficiently clever to figure out game-breaking applications. Energy is conserved, linear momentum is conserved, so you don't get cheap and easy r-bombs. It makes station-keeping a bit easier.

Luke

[vicky_molokh]

Originally Posted by lwcamp

Warp out to go find a binary pulsar (not hard, they broadcast their location far and wide). Match speed with one of the neutron stars. Now warp back to your target and slam into it at relativistic speeds.

Well yeah, that works if FTL is both available and doesn't have some sort of velocity-matching requirement. So I guess that's safe enough for my single-solar-system-setting, but something to worry about in Space Operas.

Originally Posted by lwcamp

I haven't been sufficiently clever to figure out game-breaking applications. Energy is conserved, linear momentum is conserved, so you don't get cheap and easy r-bombs. It makes station-keeping a bit easier.

Luke

Hmm. So, 'conservation of angular momentum holds under standard conditions, but can be bent with intricate devices' looks reasonably safe enough for gaming, yes? Or are there some other physical laws that produce drastic cascade effects from the mere possibility of bending this law?

[lwcamp]

Originally Posted by lwcamp

Suppose you can pseudospeed yourself to 0.01 c. Now go park yourself over a planet. Say, just above Earth's exosphere. Make sure you're not in orbit - stay stationary and hover over the planet with pseudospeed. Earth's gravity is pulling you down. It is exerting a force on you, and since by definition your change in momentum is force integrated over time, you are gaining momentum toward the center of the Earth. No problem, though - you can pseudospeed upward to counter your downward real velocity. Every second, you gain another 9.8 m/s of downward velocity. After about 85 hours of this, you have picked up nearly 0.01 c of real velocity. Beyond this point, your pseudovelocity drive is going to have trouble keeping up, so maneuver away from Earth. Of course, you conveniently positioned yourself so that the real velocity you built up helps you get to your real destination - say, it points towards the eeevil base of eeevilness on Callisto. So you zip over to Callisto with real and pseudo velocity, and then plow into your target at an actual speed of 0.01 c. Kaboomie!

Just remembered - power is force times speed. So while you could pull the hovering trick at low altitude for a while, eventually the power requirement would exceed your reactor's output. Then you need to move further out in order to decrease the acceleration and reduce the power requirement, which slows down your accumulation of velocity.

Luke

[malloyd]

Originally Posted by Ulzgoroth

That wouldn't help, since you can just as easily (if a bit less efficiently) toggle the drive off to accumulate real speed and then back on to recover altitude.

Depends on how you violate physics when you toggle the drive - if you keep your momentum or energy through a toggle, you of course are moving toward the center of the planet infinitely fast when you reach zero mass, which exceeds your pseudovelocity speed limit by a lot.

I...do not think this is true. Can you show it?

The simplest way is to imagine you have a generator, attach a long rod perpendicular to the shaft and glue the reactionless engine to the tip. Power it with the output of the generator. If there is some speed at which the tip of the rod moves at which the output of the generator exceeds the power consumption of the drive, you have net energy creation. If there isn't you destroy energy. Since the rod can be arbitrarily short, a top speed limit won't help. If you add in relativity, the breakeven power requirement for the reactionless thruster turns out to be the same as for a photon drive. If you insist on pseudovelocity, the drive can't move the rod at all, which is functionally identical to the inertialess condition, and essentially useless as a drive, since you can never move again once you encounter a free atom.

Physics is highly interconnected, there really is no way to make miracle SF propulsion systems - whether they're reactionless thrust, FTL, or just superefficient drives - useful only for propulsion. There are always severe side effects, and if you are going to have to ignore them eventually it might as well be from the start rather than trying to come up with some sort of justification that pushes them somewhere you can't see them but somebody else can find them and surprise you with them.

But that's a pretty specific sub-case of energy from nothing.

So what kind of energy are you imagining which is not convertible into kinetic energy?

[Ulzgoroth]

Originally Posted by malloyd

Depends on how you violate physics when you toggle the drive - if you keep your momentum or energy through a toggle, you of course are moving toward the center of the planet infinitely fast when you reach zero mass, which exceeds your pseudovelocity speed limit by a lot.

I don't know why you've decided that pseudovelocity drives mean you have zero mass.

Originally Posted by malloyd

The simplest way is to imagine you have a generator, attach a long rod perpendicular to the shaft and glue the reactionless engine to the tip. Power it with the output of the generator. If there is some speed at which the tip of the rod moves at which the output of the generator exceeds the power consumption of the drive, you have net energy creation. If there isn't you destroy energy. Since the rod can be arbitrarily short, a top speed limit won't help. If you add in relativity, the breakeven power requirement for the reactionless thruster turns out to be the same as for a photon drive. If you insist on pseudovelocity, the drive can't move the rod at all, which is functionally identical to the inertialess condition, and essentially useless as a drive, since you can never move again once you encounter a free atom.

So, no, the rod can't be arbitrarily short because generators and reactionless drives are not usually arbitrarily small (but I don't see how a speed limit would help anyway). And the 'how can a pseudovelocity ship move in a universe where vacuum isn't absolute' is not a hard problem.

If all you were going for is that an otherwise physics-compliant reactionless drive that can impart more kinetic energy on itself than it consumes energy input allows a perpetual motion generator, I'd agree. But 'reactionless motion' includes a variety of things that won't play nice with your generator at all.

Originally Posted by malloyd

Physics is highly interconnected, there really is no way to make miracle SF propulsion systems - whether they're reactionless thrust, FTL, or just superefficient drives - useful only for propulsion. There are always severe side effects, and if you are going to have to ignore them eventually it might as well be from the start rather than trying to come up with some sort of justification that pushes them somewhere you can't see them but somebody else can find them and surprise you with them.

You claimed a specific consequence.

Originally Posted by malloyd

So what kind of energy are you imagining which is not convertible into kinetic energy?

Taking out the rest of the italicized condition really changes the statement.

Even supposing that your reactionless drive is used as a high-power perpetual-motion generator, if your only way of using it to produce a relativistic projectile is using that power to run a rocket, it's not really more of a problem than a conversion power plant of the same output would be.

[malloyd]

Originally Posted by Ulzgoroth

I don't know why you've decided that pseudovelocity drives mean you have zero mass.

It's the only way of avoiding violating conservation - you change velocity without changing energy or momentum, you must have zero mass. It's not necessary if you don't care about the infinite energy issue.

So, no, the rod can't be arbitrarily short because generators and reactionless drives are not usually arbitrarily small (but I don't see how a speed limit would help anyway).

If it is sufficiently low and the minimum size of the drive is sufficiently small, then it prevents you from *creating* energy with this scheme. You can still destroy it, but that's much less a problem.

Even supposing that your reactionless drive is used as a high-power perpetual-motion generator, if your only way of using it to produce a relativistic projectile is using that power to run a rocket, it's not really more of a problem than a conversion power plant of the same output would be.

Well slightly, because energy from nothing is theoretically worse than energy from mass conversion, but as a practical issue, yeah not really. The problem with kinetic kill projectiles is an unavoidable consequence of any space drive that is any good. If you are travelling between planets is periods substantially shorter than a year, your body has more energy relative to those planets than a small nuclear weapon. *Any* good SF drive system is a weapon of mass destruction. Which is sort of my point, if you want to ignore that, just do it. Trying to alter the way the drive works so it isn't true without breaking the rest of physics in exploitable ways is doomed to fail.

[Ulzgoroth]

Originally Posted by malloyd

It's the only way of avoiding violating conservation - you change velocity without changing energy or momentum, you must have zero mass. It's not necessary if you don't care about the infinite energy issue.

Or, as I thought was pretty obvious considering the title of the thread we're in, you have a thing called pseudovelocity which acts like real velocity in some ways (such that you can use it to go places) but not in others.

Originally Posted by malloyd

If it is sufficiently low and the minimum size of the drive is sufficiently small, then it prevents you from *creating* energy with this scheme. You can still destroy it, but that's much less a problem.

Presumably you mean 'sufficiently large', since minimum size being small is a lack of constraint?

I'm not seeing why a low input RPM to your generator would preclude energy creation.

Originally Posted by malloyd

Well slightly, because energy from nothing is theoretically worse than energy from mass conversion, but as a practical issue, yeah not really. The problem with kinetic kill projectiles is an unavoidable consequence of any space drive that is any good. If you are travelling between planets is periods substantially shorter than a year, your body has more energy relative to those planets than a small nuclear weapon. *Any* good SF drive system is a weapon of mass destruction.

Any space drive that allows you to run up kinetic energy at an uncomfortably high rate, certainly. Even if, like the perpetual-motion-spinner-plus-photon-drive-array system it is probably useless for anything other than kinetic-kill applications.

But your conversion from reactionless travel to powerful free-energy source is very incomplete. A stutterwarp or pseudovelocity-bubble doesn't work with your generator. You can't even drive an RKV with a basic reactionless drive that has a sub-relativistic etheric speed limit...because the generator won't generate when it's moving at the drive's speed limit already.

Originally Posted by malloyd

Which is sort of my point, if you want to ignore that, just do it. Trying to alter the way the drive works so it isn't true without breaking the rest of physics in exploitable ways is doomed to fail.

I'm aware that's your point. I'm not seeing a compelling case for it.

[malloyd]

Originally Posted by Ulzgoroth

But your conversion from reactionless travel to powerful free-energy source is very incomplete. A stutterwarp or pseudovelocity-bubble doesn't work with your generator. You can't even drive an RKV with a basic reactionless drive that has a sub-relativistic etheric speed limit...because the generator won't generate when it's moving at the drive's speed limit already.

We must not be picturing the same thing. In the limit you can do exactly the same trick as with the potential energy teleport scheme upthread, use the drive to accelerate, turn it off and pull out the energy.

On the more general issue, I'm uncertain where we are looking at this differently. Are you imagining a preferred frame? A pseudovelocity (or for that matter teleport) scheme can look less abusable that way - the sub-relativisitic etheric speed limit part certainly suggests that - having the ether as a preferred frame. Though it doesn't make the potential energy issue go away.

[Ulzgoroth]

Originally Posted by malloyd

We must not be picturing the same thing. In the limit you can do exactly the same trick as with the potential energy teleport scheme upthread, use the drive to accelerate, turn it off and pull out the energy.

If you can climb out of a gravity well for less than the potential energy gain, you can probably leverage that into free energy. The generator isn't really portable (because gravity wells are heavy), so you can't use that power to drive an independent RKV. Whether you can use the perpetually-falling ship as an RKV depends on the drive principles.

Originally Posted by malloyd

On the more general issue, I'm uncertain where we are looking at this differently. Are you imagining a preferred frame? A pseudovelocity (or for that matter teleport) scheme can look less abusable that way - the sub-relativisitic etheric speed limit part certainly suggests that - having the ether as a preferred frame. Though it doesn't make the potential energy issue go away.

Any speed limit (which you brought up first) on a drive implies some kind of at least locally preferred frame, surely.

I wouldn't be too surprised if it's impossible to 'safe' a reactionless drive that doesn't imply a preferred frame at some point in its operation.

(And of course you can likely 'unsafe' many drives by configuring the preferred frames sufficiently wrong. A galactic-center relative Immovable Rod, and all that.)

[malloyd]

Originally Posted by Ulzgoroth

If you can climb out of a gravity well for less than the potential energy gain, you can probably leverage that into free energy. The generator isn't really portable (because gravity wells are heavy), so you can't use that power to drive an independent RKV. Whether you can use the perpetually-falling ship as an RKV depends on the drive principles.

This might be a good example of the sort risks you run into with these things. If you assume you are "safe" from an abuse because planetary gravity wells are not portable, you may end up in trouble when a clever player attempts to build an equivalent potential well generator with a pair of (much more portable) charged spheres.

I wouldn't be too surprised if it's impossible to 'safe' a reactionless drive that doesn't imply a preferred frame at some point in its operation.

Probably. Actually I suspect a preferred frame may be necessary for a reactionless drive to make sense at all - the relationship between your momentum and kinetic energy will not transform the same way between different frames as the energy consumed by your drive - so if it's "safe" in one frame, it isn't in others. Of course that's a contradiction - it's the same system - so relativity is going to have to give somewhere. With a reaction drive, the relative momentum and kinetic energy of both your ship and your exhaust change, and not in the same way (their relative velocities are necessarily different relative to any other frames you are considering), allowing for some cancelling out.