John Smith I wrote:
For example, a "gravity engine" could be possible if we only knew of a
way to "shield" things from gravity with little cost in energy.

Don't forget zero point energy. http://www.calphysics.org/zpe.html.

More than half a century ago, I remember the conservation
of energy principle being modified to: "Energy cannot be
created or destroyed *by ordinary chemical means*" - right
after WWII. It took some time for the meaning of e = mc^2
to soak in. :-)
--
73, Cecil http://www.w5dxp.com