Theory · Advanced · 2:06

Warp drive

Carrying a ship faster than light by moving space itself: what it would take, and the view from inside

Theory, not observation. Nothing here has ever been seen. Each idea comes from real physics and says how it could be made, and what it would look like. Light is bent as the equations say it would be; the things themselves are drawn.

Below are the 9 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the What if tab

1. Moving space, not the ship

Nothing can outrun light through space, but space itself can stretch. In 1994 Alcubierre showed that a bubble of flat space could be carried faster than light by squeezing space ahead of it and stretching it behind. Here it is 200 m across.

Relativity's speed limit is for things moving through space. It says nothing against space itself moving: distant galaxies recede faster than light because the space between them keeps growing. Alcubierre's idea is to make a small patch of that happen on purpose, and take a ship along in it.

The bubble is a region of flat space with a wall round it, in which space is squeezed in front and stretched behind. The ship sits at rest in the middle, like a surfer carried by a wave. Its crew would be in free fall in flat space: no push, no time dilation, clocks running as they do outside.

It is a solution of Einstein's equations, not a machine. The chart in the corner is the solution's own: how fast space is stretched (above the line) or squeezed (below it) along the line of travel. The picture is what an onlooker behind the bubble would see as it races off at twice the speed of light: a disc of warped sky ahead, shrinking as it goes.

2. What holds the wall up

To hold the wall, matter with negative energy is needed all round the bubble, most at its sides. In 1997 Pfenning and Ford found that a bubble to carry a ship across the Milky Way in ten years needs about −10⁶⁴ kg, far more than the observable universe holds.

Alcubierre's formula for the energy has a minus sign everywhere: the same kind of exotic matter that holds a wormhole open. It is zero along the line of travel and greatest in the wall round the ship's middle, the violet ring. The ring is drawn.

In 1997 Michael Pfenning and Larry Ford applied the limits quantum theory puts on negative energy to the bubble. They found its wall could be only a few hundred Planck lengths thick, and the energy for a bubble big enough to carry a ship far about −10⁶⁴ kg: some hundred billion times the mass of the observable universe, about 10⁵³ kg.

Later designs cut the bill. Van Den Broeck's of 1999 needs under three Suns' worth for a bubble carrying something as small as an atom. In 2021 Bobrick and Martire showed that bubbles slower than light can be built from ordinary matter, and Lentz proposed a faster one that needs only positive energy, which others dispute. None is cheap, and none is agreed.

3. How one would be built

On paper a warp drive is built in three steps: make a great deal of negative energy, shape it into a thin wall all round the ship, and switch it on ahead of the ship. No one knows how to do any of them. The ring here is drawn.

1. Make negative energy. The only kind we can make is tiny: the Casimir effect gives about −4 × 10⁻⁴ joules per cubic metre between plates a micrometre apart, and squeezed light makes brief pulses. Limits found by Ford and Roman allow only so much for so long. The bubble needs about −10⁶⁴ kilograms.

2. Shape it into a wall. Alcubierre's formula puts the matter in a shell round the ship: none along the line of travel, most at the sides, with a smooth wall. Pfenning and Ford found the wall would have to be only a few hundred Planck lengths thick, about 10⁻³³ metres, thinner than any matter we know can be made.

3. Switch it on ahead of the ship. The ship cannot send a signal to the wall in front of it, so it cannot start the bubble from inside. Krasnikov suggested in 1995 that a slower ship could prepare the route as it goes, leaving a tube of altered space behind it, so that later trips along it would be fast.

None of this exists as a machine. Each step is what the mathematics asks for, not a technology. The ring of negative energy in the picture and the route ahead are drawn.

4. Inside the bubble

Inside, you would feel nothing. The ship is at rest in flat space: no push, no time dilation, clocks running as everywhere else. What changes is the sky. Turn the drive up and the stars ahead crowd together, brighten and turn blue.

At the centre the space is flat and at rest. Nothing pushes you as the bubble speeds up, since you are not accelerating; the bubble's wall does the moving. Your watch keeps the time everyone's does.

Only the light coming in shows what is happening. It has to cross the wall, where space is flowing, and comes out shifted: light from ahead is raised in frequency by 1 plus the speed, light from behind lowered by 1 minus the speed. Every ray here is followed through the warp geometry.

The tints are a simple stand-in for shifted colours, and the brightness follows the Doppler factor. The ship itself, at the centre, is not drawn.

5. What the ship sees

At twice the speed of light, the sky within 60° of straight behind cannot reach the ship at all: its light never catches up. Ahead, starlight arrives 3 times higher in frequency: bluer and brighter.

Clark, Hiscock and Larson followed light through the bubble in 1999, and Thomas Müller and colleagues in Stuttgart later made films of the view. Beyond the speed of light, stars in the forward half look closer to the direction of travel than they really are, and grow brighter and bluer towards it.

Behind the ship the Doppler factor, 1 minus the speed, falls to zero at the speed of light, and beyond it a cone opens round straight behind from which no light arrives at all. Its half-angle is the arc whose cosine is 1 over the speed: 60° at twice light's speed, more as the ship goes faster.

The cone is a horizon, like the Mach cone behind a supersonic jet: what is inside it is outrun by its own light. Looking straight back, the crew would see only dark, with a few dim red stars towards its edge.

6. What an onlooker sees

From outside, a warp bubble is all but invisible: no glow, no wall to see. From behind it shows as a disc of warped sky ahead that shrinks as the bubble races away; from the side you would notice nothing until it had gone past.

Light from stars far ahead crosses the bubble's wall before it reaches you, and the wall turns it by tens of degrees and shifts its colour. What you see is a disc of sky that is really from a wide stretch of the heavens, packed into a small circle round the direction of travel: the farther the bubble, the smaller the disc. The camera here is at rest in space, behind the bubble.

The bubble moves faster than any light could show it: its image would reach you only after it had gone past. Anyone in its path would be passed before they saw it coming, as a sonic boom follows a supersonic plane.

The ship in the middle is not drawn, and the wall's own glow, if it had one, is not either: only what the equations do to starlight is shown.

7. No steering, no stopping

A ship cannot send a signal to the front of its bubble, so it cannot switch the bubble on, steer it or stop it. Ahead lies a cone, 60° round the line of travel, that no signal from the ship can enter.

In 1995 Sergei Krasnikov pointed out that nothing the ship sends can reach the front of the bubble faster than light, so the crew can neither create the wall nor steer it nor stop. Everything about the wall would have to be arranged ahead of the ship, before it came.

Clark, Hiscock and Larson found the two cones drawn here: ahead, one into which the ship can send no signal, and behind, the one from which no light can reach it. Each has a half-angle whose cosine is 1 over the speed.

So a ship in a warp bubble could not stop at its destination unless machinery already there did it. Krasnikov's own answer was a tube of warped space laid along the route beforehand. These are ideas, not designs.

8. Two more problems

If the bubble is switched on from flat space, calculations say quantum effects pile up on its front wall without limit and destroy it. And a way of going faster than light can also be a way of going back in time.

Finazzi, Liberati and Barceló showed in 2009 that a faster-than-light bubble made from flat space gets a horizon at its front wall, and that the energy of quantum fields there grows exponentially in time. Their result treats gravity classically and matter quantum-mechanically, so a full theory of quantum gravity could differ.

Allen Everett showed in 1996 that a simple change to the model lets a ship travel a loop that brings it home before it left. Like the wormhole, a faster-than-light bubble is a time machine as well as a drive.

Stephen Hawking's chronology protection conjecture says nature always stops such loops from forming. Whether it holds for warp bubbles is not known. The pulsing glow drawn on the front wall stands for the energy piling up; it is drawn.

9. Has anyone built one?

No. A NASA team led by Harold White looked for a tiny warp field with a laser interferometer in 2011 and 2012 and found no convincing sign. A warp drive stays in the equations, waiting for the negative energy it would need.

At NASA's Johnson Space Center, White and Richard Juday built an interferometer to look for a warp distortion from a tiny ring of ceramic capacitors. The results were inconclusive, and a 2014 analysis argued that the distortions such a set-up could make are far too small for it to see.

General relativity allows the shape of the bubble; what is missing is the matter. No known material has negative energy in the amounts needed, and quantum theory allows only tiny amounts, like the Casimir effect's, for short times.

The idea still does useful work. It shows what relativity permits and forbids, and it has pushed physicists to ask which of the rules about energy nature really keeps. Bubbles slower than light from ordinary matter are studied now for what they say about gravity, not as engines.

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