Leonard Erickson wrote:
 > Slight problem. the ship's kinetic energy depends on which frame of
 > reference you use... It's energy with respect to the star and the
 > various other bodies in a system will be different for each.

Yes. Is it numerically significant?

In special relativity, the relationship:

(kinetic energy)^2 = (momentum x c)^2 + (rest mass x c^2)^2

is true in all frames of reference.

[Where the ship's kinetic energy is (gamma-1) x m x c^2, and gamma is
the Lorentz factor for the velocity.

Similarly, the momentum is (gamma) x m x v].

At worst, you will get variances comparable to the Shapiro effect (delay
in radar travel time between planets caused by solar gravity) of a few
percent between observers.

I am arguing for a worst case velocity of ~0.26c, which makes the
variation between observers even smaller.

The effective "preferred" frame of reference would be the biggest star
in a planetary system given that's where most of the local distortion of
space-time is going to come from.

(General relativity is the best fit to the situation of high-speed
interplanetary travel, but the math required is way beyond my minimal
skills).

 > Reaction drives don't have this problem, because the increase in KE
 > in any frame is matched by an equal and opposite change in KE on the
 > part of the exhaust.

It looks like you are confusing momentum with kinetic energy here.

I readily concede that momentum will not be conserved unless we expand
the system. For example: if the spacecraft harvests momentum somehow
from all the other bodies in-system, momentum is then conserved.

Energy is a scalar quantity, so directions don't matter.

In the absence of external influences (a closed system), the total
amount of energy is conserved at all times: kinetic energy of moving
objects [rocket + exhaust] plus potential energy of the reaction mass is
a constant.

The energy-momentum relationship given above applies.

 > Reactionless drives don't have that factor. so they will *always*
 > violate conservation of energy, conservation of momentum, and even
 > conservation of angular momentum in at least one frame of reference.

Correction:
You will violate at least one of these quantities in at least one frame
of reference.
Momentum is the hardest one not to break; energy is a lot easier.

But if someone in Andromeda eventually sees an apparent causality or
conservation violation, does it matter for the folks in the spaceship
and the near vicinity that do not?

Rob O'Connor