Historical research guide · 0355 · 2021
Introducing Physical Warp Drives
Alexey Bobrick · Gianni Martire
In one minute
Alexey Bobrick and Gianni Martire, at the Advanced Propulsion Laboratory of Applied Physics in New York, take the warp drive out of the exotic-solutions box and give it a general definition: a shell of material, ordinary or exotic, wrapped around a flat passenger region and moving inertially through flat space. Every warp metric in the literature, Alcubierre’s 1994 solution included, sits inside that definition. Two results follow. Subluminal warp drives can be built from purely positive energy — they give the first explicit spherically symmetric example, whose signature effect is that time inside the shell runs slower than outside. And because a warp drive is a shell like any other object, it cannot accelerate itself: it needs propulsion, and Alcubierre’s time-varying velocity quietly broke energy conservation. They also cut the negative energy an Alcubierre drive demands by about two orders of magnitude, by flattening the bubble and choosing a better shape function. Warp geometry, stated as ordinary general relativity.
Connection
Chapter 4 treats the metric as something you build rather than something you accept, and this is the paper that turns the warp bubble into an engineering object: a mass shell, described by known physics, with a price tag you can compute. It also names the bill honestly — the shell still needs a power source and a way to push, which is where chapters 6 and 13 pick the thread up.
From the source
A warp drive spacetime is defined generally as an asymptotically flat vacuum background enclosing a compact curved shell, which in turn encloses a flat extended passenger region; every existing warp drive metric, the Alcubierre solution included, falls inside this single definition.Section 2.1, definition of a general warp drive spacetime; Figure 1
Stationary warp drive spacetimes split into exactly four classes, according to whether the remote comoving observer moves subluminally or superluminally, and whether the internal observer has the same character as that comoving observer (mild) or a different one (extreme).Section 2.1, Classes I to IV
The first manifestly positive-energy warp drive spacetime is constructed: a spherically symmetric subluminal shell. For such a shell built of non-exotic matter the only achievable modification of the interior is that time inside passes more slowly than for a remote comoving observer — an Earth-mass shell of 10 metre radius slows the rate of time by about four parts in ten thousand.Section 3.1, Equations 5 to 9
Any warp drive is a shell of regular or exotic material moving inertially, so it has no way of changing its own velocity: every warp drive requires propulsion, and the time-variable velocity assumed in the original Alcubierre construction does not satisfy the continuity equations.Sections 5.1 and 5.2, what is a warp drive and constructing warp drives
Flattening the Alcubierre bubble along its direction of motion by a factor reduces its total energy by the same factor, and a variational choice of shape function — the slower-decreasing profile that minimises the energy integral — cuts the requirement by a further factor of about three.Section 4.2 and Section 5.3; derivation in Appendix A.3
Extreme flattening yields superluminal solutions that satisfy the semiclassical quantum inequalities without extreme energy densities; the authors note these solutions still violate the averaged null energy condition, so they may be probing the limits of the inequalities themselves — a question they hand to analogue-gravity experiments and numerical relativity.Section 4.2, third implication of Equation 12; Section 5.4
The way in
https://arxiv.org/abs/2102.06824How to cite it
Alexey Bobrick, Gianni Martire (2021) Introducing Physical Warp Drives doi:10.1088/1361-6382/abdf6e