[extropy-chat] bidirectional thrust

[scerir]

[Mike]
... you can't talk about 'conservation of momentum' 
with a field thruster like this if it is thrusting 
against the entire universes inertial frame, 
because you have no external point of reference 
to base such a judgement on.

[Hal] 
Physics is local. That's the lesson from general relativity.  
[...] Now, if you wanted to claim that this device 
is reaching out to the distant stars, physically grabbing 
onto them and pulling them backwards as it goes forwards, 
at least that would not violate conservation of momentum.  
But it does violate the principle of locality of physics ...


Good points.

There have been (at least) two views on the origin of
inertia (instantaneous opposition to acceleration, or to
rotation, of a "material" object). Inertia has been assumed 
to be an essential property of "matter" or, according 
to Mach, a property that originates "externally", as 
a linkage among all the "matter" (_and fields?_) of 
the universe.

According to Mach a single object, in an empty universe,
should be devoid of inertia. And inertia is, according to
Mach, a sort of asymptotic function of the surrounding matter,
or of the gradually surrounding matter. According to Mach
(if I remember well) an empty universe cannot exist, and
a single object in an empty universe cannot accellerate, 
and cannot rotate. (According to GR empty and rotating 
universes do exist, they make sense. Taub, Goedel, that
sort of universes.)

According to Mach whether one regards acceleration (or
rotation) as a motion of a specific object, or as a
"counter" motion of the surrounding matter, is _arbitrary_.
(As Mike was, perhaps, saying).

The problem is, however, that inertia (as a reaction force
to acceleration, or rotation) occurs at the same moment
that acceleration, or rotation, is applied to the
specific object. Thus, Mach principle should imply
a sort of instantaneous back-reaction propagating field,
from the distant stars to the specific object, involving 
superluminal velocities. (As Hal was, perhaps, meaning).
Something like the advanced vs. retarded Dirac-Feynman-Wheeler 
radiation fields. Or something like the advanced vs. retarded 
actions between two photons, entangled in a singlet, 
according to Klyshko, or Shumacher, or Bennett, sometimes 
known as "ghost" effects.

But it is late now. And I've lost what I was going to
say (if anything). And I realize that the above is
already known by anyone! 
:-)

Ahhh, I was going to talk about the "Sakharov drive".
Next time.