  The Alcubierre warp drive is an exotic solution in general relativity. It
allows for superluminal travel at the cost of enormous amounts of matter with
negative mass density. For this reason, the Alcubierre warp drive has been
widely considered unphysical. In this study, we develop a model of a general
warp drive spacetime in classical relativity that encloses all existing warp
drive definitions and allows for new metrics without the most serious issues
present in the Alcubierre solution. We present the first general model for
subluminal positive-energy, spherically symmetric warp drives; construct
superluminal warp-drive solutions which satisfy quantum inequalities; provide
optimizations for the Alcubierre metric that decrease the negative energy
requirements by two orders of magnitude; and introduce a warp drive spacetime
in which space capacity and the rate of time can be chosen in a controlled
manner. Conceptually, we demonstrate that any warp drive, including the
Alcubierre drive, is a shell of regular or exotic material moving inertially
with a certain velocity. Therefore, any warp drive requires propulsion. We show
that a class of subluminal, spherically symmetric warp drive spacetimes, at
least in principle, can be constructed based on the physical principles known
to humanity today.
