Historical research guide · 0781 · 2012
Alcubierre warp drive: On the matter of matter
Brendan McMonigal · Geraint F. Lewis · Philip O’Byrne
In one minute
An Alcubierre warp bubble carries a ship by deforming spacetime around it, so the ship never moves through local space and its crew feels no acceleration at all. Brendan McMonigal, Geraint Lewis and Philip O’Byrne, at the University of Sydney, asked the question the design literature had largely skipped: what happens to the ordinary matter the bubble meets on the way — the dust, the gas, the starlight. They integrated the free-fall paths of light and of massive particles across the full range of starting speeds, through bubbles travelling both below and above the speed of light, and through bubbles that speed up and slow down mid-flight. The picture that comes out is specific and useful. Particles the bubble catches from the front pile up against an inner horizon, gain enormous energy and become ’time locked’, ageing almost not at all while inside. Particles that pass through come out with the energy they went in with, displaced a little. And when the bubble drops back below light speed, everything it has collected is released forward at once.
Connection
Chapter 4 treats metric engineering as engineering, and this is the paper that first mapped, in detail, how a warp bubble interacts with the ordinary matter it flies through — which makes shielding and beam aiming design parameters rather than afterthoughts. Every result in it is a geodesic integration in a written-down metric, so anyone can repeat it.
From the source
The Alcubierre spacetime is asymptotically flat except at the walls of a small spherical bubble around the ship: spacetime is deformed so that the chosen path becomes a timelike free-fall geodesic, the travellers therefore experience no inertial effects at all, and their proper time equals the coordinate time of observers at the origin and destination however large the global velocity.Section II, line element Equation 1 and the shape function Equation 2
Horizons form only above light speed. Solving the null condition gives two positions, one on the side the bubble is moving towards and one behind it, at a separation from the ship set by the reciprocal of the bubble velocity; when the global velocity is subluminal these positions lie outside the bubble radius and the horizons do not exist.Section III, Equation 14; Figures 1 and 2 for subluminal velocity 0.5 and superluminal velocity 4
Particles that pass clean through the bubble are blueshifted at the ship by a factor of one minus the product of the ship velocity and the particle velocity, but leave the far side with their original velocity and energy, displaced in space by an amount that grows with the time spent inside and with the ship velocity.Section III A, null and massive particles; Section IV, Conclusions
Particles moving in the same direction as a superluminal bubble are captured instead: they asymptote towards the front horizon, obtain extremely high blueshifts, and become ’time locked’ for the duration of their stay, experiencing very little proper time between entering and eventually leaving the bubble.Abstract; Section III A 2; Section IV, Conclusions
Changing the bubble’s speed shifts the particles it is carrying. Under acceleration a particle inside gains or loses velocity magnitude according to whether it is moving towards the front or the rear of the bubble, and deceleration reverses the effect, with the size of the boost scaling with the size of the acceleration — while Eulerian matter, stationary in the rest frame of the origin and destination, is left unaffected by bubble accelerations and decelerations.Abstract; Sections III B and III C, one-way and round trips; Section IV, Conclusions
The engineering consequence the authors state plainly: the space behind a superluminally travelling bubble is left almost entirely devoid of forward-travelling particles and holds only a sparse population of greatly reduced energy, while the space in front of a bubble decelerating from superluminal to subluminal velocity receives a concentrated beam of extremely high energy particles — so a crewed Alcubierre ship requires shielding, and where the released beam is pointed at arrival is a design parameter to be set.Section IV, Conclusions, final two paragraphs
The way in
https://doi.org/10.1103/PhysRevD.85.064024How to cite it
Brendan McMonigal, Geraint F. Lewis, Philip O’Byrne (2012) Alcubierre warp drive: On the matter of matter doi:10.1103/PhysRevD.85.064024