In 1977 (still only 17 years after the first laser was built), science fiction brought us a different take on the death ray, crossing it with the sword, the fantasy world’s weapon of choice, in the light saber. These devices wielded by the Jedi in the Star Wars series are supposed to set their bearers apart from the usual, blaster-ﬁring crowd, making them both slightly antiquated and mystical at the same time. But they also bring in a particularly unusual—in fact, some would say ill-thought-out—twist on the physics of light.
There is one prime difference between a beam of light and a piece of metal as the material from which to construct a sword blade: light does not stop of its own accord. For a light saber to come out of its handle for a few feet then suddenly stop, we would expect there to be something there at the end—something from which the light could reflect back. But there isn’t anything. No doubt there are after-the-event justifications of light-saber technology that describe it as some form of plasma beam, limited by a force ﬁeld, but there is little doubt that the initial intent was a laser that just stopped in midair somehow. Everyone who knew anything about physics knew they were impossible. So it was quite a shock when, in 2013, newspaper headlines proclaimed that light sabers had now been made possible.
“Star Wars Light Sabers Finally Invented,” “Scientists Finally Invent Real, Working Lightsabers,” and “MIT, Harvard Scientists Accidentally Create Real-Life Light Saber” were among the dramatic headlines. (I love that use of “finally” as if it is about time that those lazy scientists managed to get around to something so trivial.) This media frenzy was set oﬀ by one of the scientists behind the discovery, Professor Mikhail Lukin of Harvard, who made the, perhaps unfortunate, remark: “It’s not an inapt analogy to compare this to light sabers.” He was, no doubt, encouraged by the Harvard PR department. It all sounded very impressive. And it was an interesting bit of science. But the link to light sabers was tenuous in the extreme.
By using a special, low-temperature material called a Bose-Einstein condensate, the team had been able to produce what they called “light molecules”—pairs of photons of light that were temporarily linked to each other. This is an important break- through because photons generally don’t interact with each other—so any mechanism by which they can be made to work together could be extremely useful when building devices that use light instead of electrons, so-called “photonics,” that could eventually revolutionize computers and other devices based on electronics.
By forming a “Rydberg blockade,” specially excited atoms that prevent photons from progressing, the scientists managed to get the first photon to hold the second photon up brieﬂy, so the two became linked together, pushing and pulling through the medium as they interacted one after the other with excited atoms in their path. It genuinely is interesting stuff. But it has no relevance whatsoever to the practicalities of building a light saber. Unless we have a much more dramatic breakthrough, light sabers will remain stored away in the box labeled “Fantasy.”