In 1994, a physicist called Miguel Alcubierre proposed a “warp drive” solution resolving the prohibition in Einstein’s special relativity on traveling faster than light. The problem turns out to be that anybody in the path of the incoming space ship gets fried.
That’s the conclusion of a group of University of Sydney physics students who have re-examined the maths of the Alcubierre drive, and found that when it decelerated below faster-than-light speeds, it would emit gamma rays sufficient to be seriously unhealthy to anybody waiting in the arrivals lounge.
(OK: I realize that I am straying here. Nobody’s solved Alcubierre’s equations in a way that actually permits the construction of any kind of warp drive. But bear this in mind: both satellite communications and lasers have one foot in Special Relativity, which puts a gap of half a century between equation and machine. Alcubierre’s equations are less than 20 years old. Anyhow, you’re just as interested in this as I am.)
For those unfamiliar with the Alcubierre Drive, there’s a Wikipedia, or my compressed explanation: mathematically, it’s possible to create a bubble of space, with spacetime compressed in front of the bubble and expanded behind it; that bubble will do the traveling on behalf of the spacecraft it encloses, as it were. Inside the bubble, no faster-than-light prohibition is broken. But in the – for want of a better word – “unbubbled” universe, the Alcubierre region can follow a path between two points that allow it to apparently travel between them faster than light.
However, according to Brendan McMonigal, Grant Lewis and Philip O’Byrne of the University of Sydney, in this paper, Alcubierre overlooked what happens to ordinary matter that the warp-drive spaceship would encounter on its travels.
Hitch-hiking matter becomes planet-killer
The problem is that the Alcubierre Drive spaceship is going to encounter matter during its trip: space is only nearly empty, not completely empty. Matter traveling towards the ship, the paper says, will become “time locked” with the ship. When the ship decelerates, these hitch-hikers are released from the bubble emitting huge amounts of energy as gamma rays and high-energy particles.
“The amount of energy released infront of the ship is unbounded, as we can increase the energy of the released radiation and particles simply by travelling across a larger distance,” McMonigal explained.
“Even across trivially small distances the energy is enormous. We would need to do more detailed calculations to work out the particular shape of the beam, but it would definitely zap the relatives, and could quite possibly fry their planet.”
This probably makes it a good thing we can’t build an Alcubierre drive yet, even though, as McMonigal explained to The Register, the distortion of spacetime is not so exotic as it sounds:
“Einstein's General Relativity tells us that gravity is the result of warped spacetime,” he told El Reg. “This means that simply by being in the gravitational field of the Earth as we are now, we are experiencing warped spacetime (the specific nature of the warp for the Earth has been measured to match Einstein's predictions to high accuracy).
“What the warp drive equations tell us is what distribution of "stuff" we would need (in this case a particular negative energy density distribution) to create the spacetime deformation which would result in a ship travelling to a distant location in a short amount of time.
“In fact, the question of how we would generate this distribution is the main barrier to this technology.”
In other words, it’s not that the “bubble” can’t ever be created, just that we don’t quite know how to do it yet (remember what I said about lasers?).
And how large does the space-time bubble have to be? Only a bit larger than the ship it encloses: “The deformation is only in a small sphere around the ship itself,” McMongal told The Register.
“The size of this sphere is not defined by the theory, we only require that it is large enough to completely encompass the ship, but clearly a smaller sphere would be better (so that less spacetime is affected during the journey).”
Doesn’t this break Relativity’s prohibition on faster-than-light travel? Keep in mind that relativity comes in two parts – and it’s Special Relativity that prohibits faster-than-light travel.
“It is incorrect to look at such a scenario using Special Relativity, what we must use instead is Einstein's General Relativity,” McMonigal explained.
“This theory tells us that we can bend and warp spacetime, such that we can reach a distant location quickly by contracting the spacetime between us and the destination (and expanding the spacetime behind the ship).
“It is actually a highly desirable aspect of the Alcubierre warp drive that there is no time dilation [Reg: that is, people inside the ship don’t experience time dilation], as this means that you can travel to a distant location and back again without your friends and family dying of old age in the meantime.”
If all of this sounds as if it belongs in the “why bother?” world of science, here’s McMonigal’s final comment to The Register:
“Einstein's theory of relativity will be 100 years old very soon, and while some of the best minds on the planet have looked at it, its equations are complex and still throw up surprises.
“The warp drive itself is an example of this, with it not being discovered until the 1990s. The importance of relativity continues to grow - every GPS enabled mobile phone contains a relativity corrector to ensure it takes into account the time difference between the ground and satellite - and we should continue to explore what the equations can tell us.” ®