George Levy wrote:
Negative matter/energy however are different. If negative matter/energy
could exist they would give space a negative curvature.
The issue of negative curvature is somewhat separate from negative mass,
though--if the density of matter/energy in our universe was below the
critical density Omega, the universe would have negative curvature, no need
for negative mass (see http://tinyurl.com/9ox67 ). It might be true that
adding a certain density of negative mass/energy would have the same effect
on spacetime curvature as subtracting the same amount from the density of
positive mass/energy though, I'm not sure.
Negative matter/energy may be identical to dark energy.
I think dark energy has negative pressure (tension, basically), but not
negative energy--see section 6 of the article at http://tinyurl.com/8kepb ,
the one titled "Negative Pressure". But the "Casimir effect" that pulls two
parallel plates together (see http://tinyurl.com/anoky ) might qualify as
negative energy--at least, the energy density between the plates is lower
than the energy of the ground state of the quantum vacuum, but whether this
would actually have the same effect as "negative energy" in GR is probably
something physicists can't be sure of without a theory of quantum gravity.
Here's an article on negative mass/energy, and its relevance to keeping
wormholes open in GR: