1.1.3. Exotic matter
The vacuum energy density between two parallel plates is relatively "negative" to the vacuum energy density outside the plates. This is the basis for "exotic matter". The negative vacuum energy density in sufficient quantity, when properly engineered by an advanced technology, can curve spacetime into large traversable wormholes that can be used for effective spacetravel that is globally faster than light and backward and forward in time. Trips back in time cannot happen to times before the construction of the wormhole. There is no local faster than light travel in these trips. It is a global effect due to the topology and warping of spacetime by exotic matter. Alcubierre has shown how to use exotic matter to make a Star Trek Warp drive in which the star ship freely floats in zero gravity with no time dilation relative to earth observers. The star ship literally surfs along a gravity shock front. Spacetime contracts ahead of the front and expands behind it. Hawking has conjectured that quantum gravity fluctuations making immense sprays of particle-antiparticle pairs will damp out all of these causality violating time travel effects but no one knows for sure yet.
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