In this Universe there seems to be a conspiracy between the quantum and the general relativistic regimes preventing one from curving space-time and zipping around Faster-than-Light. Some 15 years ago Miguel Alcubierre caught the attention of the physics and SF communities by proposing a warp-metric of space-time that allowed one to move around at speeds above lightspeed. Very quickly several arguments against this prospect were advanced, and almost as quickly several work-arounds past the difficulties were counter-proposed.
However, as the authors discuss, a somewhat more serious difficulty arises because of the horizons that the warp-metric creates. These horizons act just like black-hole Event Horizons and so they produce Hawking radiation. Hawking radiation is normally quite benign as most horizons are pretty large – for example, a collapsed star’s horizon is typically 10 km across. But in a warp-metric there are two horizons – forward and aft, contracting and expanding – and to produce them very thin shells of “negative energy” are needed. And because they’re so very thin (~10-35 metres) the Hawking temperature is very, very high – i.e. a large fraction of the Planck Temperature (1032 K.) Ouch!
All is not lost. If the space-warping horizons could be made thicker, then their temperature will be much, much lower. A metre wide and they’re cooler than the CMB. But how to do so??? Originally the warp-bubble walls were close together to create a negative energy density via the Casimir Effect. If we don’t create negative energy that way, then how?
On the bright side, the “Toaster effect” doesn’t occur when going slower than light. So “field drives” based on warping space-time are still OK, if you can make the negative energy to create a warp and you stay under ‘c’. So a “Star Trek” style impulse engine is still a possibility.