The speed of light is the biggest barrier to the dream of intergalactic travel and the chance that we might ever encounter extraterrestrial intelligent beings. Faster than light travel has been the Holy Grail of scientists and science fiction writers and indeed of anyone who dreams of visiting distant stars and galaxies. After the recent unfortunate premature hype over the claims of faster-than-light neutrinos, I thought that we would not hear of such claims for some time. But it appears that scientists at NASA have been working on an idea that would enable faster-than-light travel.
The idea is based on a 1994 paper by Miguel Alcubierre and you can see the abstract of it here. The full paper is available by subscription only but I took a look at it and this article aimed at a general audience (obtained via Machines Like Us) gives a pretty good description of the basic physics and adds more information.
What is interesting is that the proposed idea does not violate any existing laws and lies within the framework of relativity theory and does not require exotic topologies like wormholes, the usual device suggested for getting around the light-speed limit.
What the author points out is that relativity only prohibits an object from travelling locally faster than the speed of light, i.e., it must stay within its local light cone. But by expanding spacetime behind a spaceship and contracting spacetime in front of it, you can propel the spaceship using spacetime itself and thus get motion greater than the speed of light when viewed by an observer who is outside the distorted region of spacetime. The popular idea that most closely resembles this is the ‘warp drive’ of science fiction. (Recall that there are also no limits to the speed with which space itself can expand, which is why distant galaxies are receding from us with speeds greater than that of light, ‘propelled’ by the expansion of space.)
Albucierre does a calculation in which a spaceship leaves Earth and moves towards a star in normal flat spacetime under normal propulsion for a short distance d. It then comes to rest and is then rapidly accelerated by spacetime distortions until it reaches the halfway point of its journey, whereupon the distortions are reversed to slow it down and bring it to rest at a distance d from the star. It then completes the final distance with normal propulsion in flat spacetime again. If the distance d is much less than the total distance to the star D, and the magnitude of the spacetime acceleration/deceleration is a, then the total time for the journal is given by 2√(D/a). By making a suitably large, we can make the time of travel as small as possible. In theory, wee could make a day trip to another galaxy!
Is there a catch? Unfortunately yes. There is always a catch. To get the required spacetime distortions one needs energy densities that are negative. But all the matter-energy forms we are aware of (ordinary matter, anti-matter, dark matter, and dark energy) all produce positive energy densities. Thus it may look hopeless but quantum mechanics comes to the rescue. It turns out that while the vacuum has zero energy density, this is true only for the average energy density and it may be possible to ‘squeeze’ the vacuum to create localized regions of positive and negative energy density such that the average is still zero.
The other major catch is that to produce faster-than-light speeds that can make intergalactic travel feasible requires a large value of a which in turn requires a large negative energy density. What NASA scientists are working on is tweaking Alcubierre’s model by varying the geometry of the spacetime distortions to try and overcome these hurdles and try and produce such motion in the laboratory. If they manage to exceed the speed of light by even a tiny amount, that would be a huge breakthrough in showing the feasibility of the idea. This paper by NASA scientist Harold White describes the attempts to achieve this.
Engineers are ingenious. They tend not to want to waste their time with speculative ideas but once you have shown them that an idea works in practice, however tiny the effect, that provides them with a huge incentive and motivation to begin working on making it workable on a large scale, and once they set their minds to it, they often achieve amazing things.