On Thu, Apr 20, 2017 at 01:38:03PM -0400, Richard Aiken wrote:
> NASA has refined the mathematics since the initial efforts. They've
> got it down to sixteen hundred pounds. Since the shape of the rings
> got a lot bigger in the same refinements, I rather think that the
> density isn't very high.

The required density is not just "very high", but utterly ridiculous.
Gas giant cores are vacuum compared with the minimum densities needed.
So are white dwarf stars.  Even neutron stars don't come close, and
they're the densest things that we can find in the universe.

Moving into the realms of theoretical objects, we would need to go to
black holes to find comparable masses per unit volume.  They get less
dense with size, so we would need to look at the very smallest ones,
which as far as we can tell don't actually exist and would evaporate
instantly if they did.  Then we need to create matter with more
negative energy than the positive energy released in hundreds of Tsar
Bomba detonations, and somehow pack it into a size as much smaller
than an atom, as an atom is smaller than our solar system.

This is not surprising.  Any such warp field needs enough density of
matter to deform spacetime at least as much as a black hole does.
Worse, it requires it to be deformed in very specific ways, which
require even more.  The only way that could be possible while not
requiring more than stellar magnitudes of negative energy is if it
occupies an incredibly tiny volume.

We have trouble keeping solid matter compressed to even twice its
ordinary density, which is a factor of about 1 000 000 000 000 000 000
000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 less than
required for any FTL Alcubierre-like field.  Even without the
"negative energy" problem, I doubt that this type of warp drive would
ever exist.  If we ever achieve FTL, it will be by some other means.

- Tim