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

[vicky_molokh]

Originally Posted by Ulzgoroth

One of the basic points of pseudovelocity is that you don't need to break momentum conservation, because pseudovelocity moves you without actually involving momentum.

EDIT: Which actually gives me the thought that 'inertialess' technology along the lines of the Lensman books might give you results usefully similar to pseudovelocity without actually using reactionless drives.

On the contrary, it seems to break conservation of momentums because it 'just moves'. Like this:

Originally Posted by lwcamp

Angular momentum is also a vector (classically, anyway - relativity adds some additional components that are hard to visualize, but I'll just stick with the classical 3-vector version). Angular momentum is defined as the magnitude of the momentum times the perpendicular distance from the observer to the object (in a direction mutually perpendicular to the momentum vector and the direction from the observer to the object). If something isn't moving, it has no angular momentum (assuming no internal motions - a rotating object has angular momentum but not everything with angular momentum needs to be rotating). If something is moving but is headed straight toward you or straight away from you, it also has no angular momentum. The further its distance of closest approach becomes (assuming a straight line trajectory) and the faster it is moving and the more massive it is, the greater its angular momentum. So, consider an observer moving toward the spacecraft, and then have the spacecraft warp away in a direction perpendicular to the direction to the observer. From the observer's point of view the spacecraft starts off with non-zero momentum (it is moving in his reference frame) but zero angular momentum (it is going straight toward him). After the warp we know that it must be going the same speed and direction from conservation of momentum, but now it is not headed straight toward the observer. Consequently it has non-zero angular momentum. Since you can always find a frame of reference that does this, free-form warping violates the conservation of angular momentum and is thus disallowed in general relativity.

How do we get around this? I can think of three ways. First, the spacecraft can dump all of its energy when it turns on its warp drive. Without energy it has no mass and thus neither momentum or angular momentum. It leaves the energy behind in some form - matter or radiation (if it is in the form of radiation, you get the release of 20,000 megatons of energy per ton of spacecraft mass). Then, when it gets to its destination it absorbs as much energy as needed to precipitate the spacecraft out of warp (that is, a 100 ton spacecraft would need to have its warp bubble gobble up 100 tons of mass at its destination in order to release the spacecraft). The spacecraft will end up with the velocity of the mass the warp field gobbled up. Until the spacecraft precipitates out of warp it cannot exchange energy with the rest of the universe (doing so would add or subtract mass to the warp bubble, which would then violate angular momentum conservation). This probably means the spacecraft is flying blind and will have a difficult time finding the mass it needs to exit warp.

Second, you can have a prepared pathway of highly curved spacetime, and all warp travel is along this path. Since the spacetime geometry of the pathway is highly curved, we are outside of the Newtonian limit and angular momentum (and energy and so on) is not localized along the path. This lets you go from one point on the path to another, but you can't warp to places off the path.

Third is that the warp drive emits some sort of radiation which never decays away to the weak field limit. This last possibility is not very good for adventure fiction since strong field gravitational radiation smashing through the universe out to arbitrary distances is rather hard on the setting.

And now I wonder just how bad the second solution is. As in, can the drive itself be the creator of such highly-curved spacetime?