[extropy-chat] bidirectional thrust

[Hal Finney]

Russell Wallace writes:
> ... frame dragging? So if a spaceship is trying to accelerate, the
> presence of massive objects nearby will increase the spaceship's
> effective inertia (unless they're moving/accelerating in the direction
> the ship is trying to go)?

Yes, this happens, but only to an extremely small extent unless you are
dealing with relativistic masses, i.e. situations where you are getting
into black hole territory.  Frame dragging with respect to the earth
or the sun will be undetectable except with very sensitive experiments,
or over very long periods of time.

Frame dragging can be considered the gravitational equivalent of
magnetism.  In electrodynamics, we have electric fields and forces,
which are pretty simple.  Force is proportional to charge and inversely
proportional to distance squared.  But then when charges are in motion,
new forces are created, magnetic forces.  These are vector fields rather
than scalar fields and the effects are more complex.

The same thing happens in relativity.  With slow motions (relative to the
speed of light) you get simple, Newtonian forces.  With fast motions,
and correspondingly strong fields, you get new effects including frame
dragging and other kinds of forces. 

Trame dragging doesn't work as an explanation of inertia.  The forces
are too weak in normal situations to have a significant effect.

Hal