On 12/29/2016 12:31 PM, Vibrator ! wrote:
So, there's an intriguing thought to end on - if an EM-driven
spacecraft subsequently decelerates again by simply performing a 180°
rotation and continuing to apply constant thrust, all of the
'anomolous' momentum and energy is neatly returned to source.
Well, no, actually, it wouldn't be.
You've neglected angular momentum, which isn't so easily patched up as that.
In an inertial frame in which the craft is initially at rest, it's all
good. But if we assume that we're viewing it from a frame in which the
craft was originally travelling on a line passing through the origin,
/and/ we assume that its initial acceleration took place /perpendicular/
to that line, /and/ we assume (just to keep it simple) that it
accelerated very hard for a very short time /just as it passed through
the origin/, then, though its linear momentum changed, the initial
acceleration didn't affect its angular momentum.
However, the final acceleration, which takes place after it has
travelled a significant distance from the origin, will not be parallel
to its radius vector, and hence will change its angular momentum but a
significant amount.
Consequently, angular momentum won't be conserved in this scenario.
This is, BTW, one of the issues with teleportation as it commonly
appears in sci-fi. You can patch the linear momentum pretty easily but
unless you want to throw CoAM overboard you've got a problem.
As I said to start with, none of this "proves" the EM drive can't work.
However, it makes the /likelihood/ that it's anything more than bad
measurements seem very small, as CoAM, CoM, and CoE have been verified
many times over, in the exact realm the EM drive operates in. It's
reflecting reasonably garden-variety EM radiation in a cavity, which is
well within the region where classical physics is most solid. It's not
like the thing has a black hole on board or something.
