The

Casimir effect is a way to produce a

negative curvature of spacetime by creating a negative

energy density, and has been successfully done in

labratory tests. The basic idea is to take two large metal plates and put them facing each other. When you do this the

virtual particles between them will bounce around between the two. The particles that have a wavelength a multiple of the distance between the two plates will get stuck and simply

resonate between the plates. Other virtual particles in between will eventually cancel out since their

wavelengths are not a whole number

multiple, after some time two particles will meet and will cancel out (remember the whole

wave/particle duality thing).

So, after a bit of time what you have is only virtual particles inside the plates whose wavelength is a multiple of the distance between the plates. This means that there will be fewer particles between the plates than outside. This means the energy density in the region between the plates is less than that far away from the plates. Now, since the total energy in the grand scheme of things must be zero, and the energy density inside the plates is less that that far away, the energy density inside the plates must be negative.

So there we basically have the Casimir effect, which proves that virtual particles exist, that there can be negative energy densities, and that theoretically wormholes can be created. Bitchin', eh?