# Speed of light and warp drives in general relativity

Velocities can be a tricky thing in general relativity. A cool concept seemly consistent with the Einstein field equations) is an Alcubierre drive, described by the Alcubierre metric.

However, I think I need some clarification of how it is supposed to work. As I understand it, it requires exotic matter to bend space around it to "move faster than light." Exactly what here is referred to be "faster than light?" And how does this work? Are you clumping together a punch of exotic matter at the front/rear and that bends spacetime?

What kind of energy levels are we talking about here? Can we even produce a such power needed to bend spacetime that much today, with current technology? Are the any rest products (exhaust/waste), and how are those handled? In a basic chemical rocket, chemical energy is released (often between kerosene and an oxidizer) and then released as then released as kinetic energy in exhaust molecules in the back of the rocket. The point is, a constant input of energy is needed for a traditional rocket to accelerate. How is this energy provided in a warp drive, and in what form? Does it have to constantly put in more energy to maintain the curvature, or can it just "cruise along" the bubble?

The local speed of light is, as far as I know it, always $c$. So what exactly is that Alcubierre drives implies "FTL travel?" By an observer stationary on Earth? If so, how does that work? If a light beam was sent alongside the warp drive, which would arrive first as measured by observers on the Earth, in the warp drive and at the destination?

• By creating what is basically a "space bubble" Jul 20, 2015 at 23:08
• More on Alcubierre drives. Jul 21, 2015 at 0:01

The Alcubierre drive is like an escalator. Space expands behind the ship, and this is like the moving stairs coming out of the floor behind you, while space contracts in front of the ship, like the moving stairs disappearing into the floor in front of you. This is not, however, a description of the actual Alcubierre process.

We live in de Sitter space, characterized by positive vacuum energy which is powering the expansion of the universe. Negative vacuum energy is characteristic of anti-de Sitter space, which is not in our universe, so far as I know, although Alcubierre argued that the Casimir effect (http://math.ucr.edu/home/baez/physics/Quantum/casimir.html) due to quantum vacuum fluctuations of the electromagnetic field could be a source of negative energy.

As the Casimir effect is a quantum phenomenon, and as general relativity does not incorporate quantum effects, the development of a viable theory of quantum gravity may make the Alcubierre drive more likely.

As the Alcubierre wave is a fluctuation of space time, riders on the wave would not be moving through space and time in their local vicinity. There would be no inertia effects unless the riders moved within the bubble. Because of this, there would be no conflict with general relativity.

Calculations of the amount of negative energy required range from more mass than exists in the entire observable universe (Alcubier's original calculation), down to three solar masses, down to the mass of Jupiter, and down to ~700 Kg. Another possible difficulty is that riders in the bubble may be unable to send signals to the warped space in front of the bubble, perhaps making control of speed and direction impossible. What would be the coordinates through which one would steer in "hyperspace"?

Yet another problem is the interaction of classical phenomena with the warp drive when it is switched on and off, which might be fatal. Hawking radiation might provide extreme thermodynamic conditions (http://journals.aps.org/prd/abstract/10.1103/PhysRevD.79.124017).

The Wikipedia article seems like a good resource, but this may be a more readable introduction: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936.pdf. Also, this paper investigates some ancillary areas of interest using the Alcubierre metric: http://www.earthtech.org/publications/davis_STAIF_conference_2.pdf.

• Thanks. However, I still wonder exactly how the spacetime bending works and what's exactly referred to "FTL" since the speed of light (at least locally) must be equal to c. Jul 22, 2015 at 23:13
• @Madde Anerson: Speed of light inside the bubble would be c, and speed of light outside the bubble would be c. The displacement of the bubble, however, could result in FTL velocity with respect to observers outside the bubble. Here is a link written by a physics prof at U. of Washington (scroll down to the 7th paragraph for an explanation of how the warp works: npl.washington.edu/AV/altvw81.html). It's probably the most accessible description of Alcubierre warp. Jul 23, 2015 at 0:19
• So if we let a light beam travel alongside the bubble, which would arrive first (in different reference frames: according to observers on Earth/in the bubble/at the destination)? Jul 23, 2015 at 12:05
• @Madde Anerson: It would depend on the displacement of the bubble within our universe, and on its velocity measured with respect to observers on Earth. If it moved on a straight path at FTL speed, it would arrive for all 3 observers before the light beam. But a more significant question would be "How to describe the bubble's world line?" I don't think anyone has gone that far with the consequences of the idea yet. NASA is working on the practicality of an Alcubierre drive: gizmag.com/warp-drive-bubble-nasa-interstellar/24392 Jul 23, 2015 at 13:32
• How's that possible? If the world line was above the light cone, the spatial and time coordinates would flip, resulting in imaginary mass. Are you saying that something with real mass would get an imaginary amount of mass in an Alcubierre drive? Jul 25, 2015 at 17:20

Exactly what here is referred to be "faster than light?"

By fast than light it means that if you put a light beam to your left and to your right and you ball take off like racers running towards a take across a finish line that you get to the finish line first.

And how does this work? Are you clumping together a punch of exotic matter at the front/rear and that bends spacetime?

There are different warps drives one of them compresses the space in front of you but presses it out like a pancake so the volume doesn't change, others literally do contract the space in front of you. And yes you do it will exotic matter because spacetime doesn't normally do that on its own.

What kind of energy levels are we talking about here?

Depends how big you want it to be, the issue is the density of energy. Imagine the biggest density you've seen. Now imagine something bigger. Bow imagine something even bigger, an infinite density, now imagine something even bigger still, a negative energy density.

And don't forget that a particular kind of energy density moves according to its own equations so you might have to arrange in advance to get it to show up to the place you want at the time you want.

Can we even produce a such power needed to bend spacetime that much today, with current technology?

If you tried to use, for instance Casmir energy densities then the negative energy densities get larger the closer the plates are together, so maybe you could make a really small one that doesn't go very far since the plates are so close together. I didn't think anyone wanted that so I don't know of anyone trying to build one.

Are the any rest products (exhaust/waste), and how are those handled?

When the metric was written down it was just in the style of "what if there was a spacetime like this what would the metric and hence the stress-energy be like." But the reality is that the stress-energy of known forms of matter have their own evolution equations, dust moves on geodesics, gases follow pressure gradients, charged particles are pushed by electromagnetic fields, etc. No investigation has been made into how to make any of these forms of matter move the right way because they wouldn't work anyway.

If you have some negative energy density you can ask how to make it move. For instance that Casmir energy has to have more positive energy density in the plates than the negative energy density in between the plates so if you want it to move fast you have to make those plates move fast and if you try to accelerate the plates then the Physics is different.

The point is, a constant input of energy is needed for a traditional rocket to accelerate. How is this energy provided in a warp drive, and in what form?

If you have a payload that is not exotic and you are also transporting exotic matter it isn't obvious that you need any fuel since for all we know you are having equal amounts of positive and negative energy arrive and maybe the same for momentum.

The real problems are that space isn't empty and this is going to get in your way. So badly that maybe you just can't go FTL even with exotic matter. And this isn't just space debris, even the quantum vacuum might be a problem. And the original drive couldn't turn and didn't start up or slow down, so even if the original drive was possible if you can't get started that might be a problem.

So how much field to get started or to steer? the original drive had no steering and no start up or slow down. Maybe that can't be done.

Does it have to constantly put in more energy to maintain the curvature, or can it just "cruise along" the bubble?

The part in the middle definitely just cruises, they could be at rest from their own point of view. But the exotic matter has to follow it's own laws which depend on the particular kind of exotic matter.

If a light beam was sent alongside the warp drive, which would arrive first as measured by observers on the Earth, in the warp drive and at the destination?

The warp drive arrives first assuming the are far enough away from each to not affect each other.