Last week, I gained a phenomenally geeky claim to fame: I once made a musical instrument out of a piece of apparatus that has just been used to shed light on a mystery that has hung over physics for the past 25 years. Allow me to explain, as I will at every dinner party I attend for the next eight years.
Let’s start with the mystery. We’ve known for just over a century about strange materials called ‘superconductors’—materials that, below a certain temperature, allow an electric current to flow inside them indefinitely without losing any energy. As well as being a fascinating scientific curiosity, superconductivity also has a wealth of practical applications: perfectly efficient power lines, hugely powerful electromagnets for particle accelerators or hospital scanners, or even levitating trains (see the video at the foot of this article for a demonstration).
Superconductivity was first observed in mercury in 1911. Mercury superconducts at −269°C, which is just four degrees above absolute zero. This is a bit cold to be of widespread practical use, and so scientists scrambled to discover new superconductors that work at higher temperatures. In the decades that followed, we discovered that most of the elements in the periodic table superconduct, and some alloys do too.
Then, in the 1950s, Bardeen–Cooper–Schrieffer (or BCS) theory explained the strange behaviour of electrons in these materials that allowed them to become perfect electrical conductors. BCS theory was a coup of theoretical physics: it explained all superconductors observed so far, perfectly. However, its success was bittersweet for technologists hoping to make use of superconductors, because it decreed that none could work above around −240°C.
Then, in 1987, a totally unexpected revolution took place: a new family of superconductors based on copper and oxygen smashed all records, taking the highest known superconducting temperatures up to a balmy −150°C. BCS theory, which worked so well for simple, elemental superconductors, couldn’t explain the behaviour of these new materials.
A paper published last week brings us a step closer to understanding how these new superconductors work. Physicists, including my old officemate Paul, used some of the biggest magnetic fields on Earth to try to work out what’s happening inside these materials. In fact, the pulsed magnets used burn through enough energy to melt a tonne of steel every second.
The best way to record data about what your sample is doing during this massive burst of magnetism is to use a sensor, separate from the magnet, comprising a tiny coil of wire placed around the sample. As the enormous magnetic field briefly suffuses the sample, its electrical properties change; and because the sample is enveloped by the coil, the coil’s properties change slightly too. By looking for these subtle changes in the coil’s electrical properties, we can deduce what the electrons in the sample are up to.
It turns out that you can turn this scientific instrument into a musical one: a theremin. You’ll know the theremin as the fretless analogue-electronic instrument responsible for the wailing, other-worldly title music of every 1960s sci-fi B-movie ever made. You play it by moving your hand near an aerial; subtle changes in the electrical properties of the aerial as you move your hand alter the pitch of the note the theremin produces.
Hopefully you’ve just come to the same realisation we did one rainy afternoon during my PhD: the mechanism that underlies this advanced physics apparatus for measuring materials in massive magnets is almost identical to a theremin.
So we rushed upstairs to the lab, grabbed a few bits of electronics and a ghetto blaster with an audio jack, and got soldering. Within an hour, we were basically Portishead.
Well, nearly. Our music could certainly be described as experimental, if not rock. The problem with our high-tech knock-off was that the coils were optimised for measuring magnetic materials with exquisite precision, rather than measuring the proximity of the fumbling hands of novice thereminists. This "musical" instrument covered three octaves in less than a centimetre of hand movement. It could probably have been played by a talented gerbil, but we were neither rodents nor, it seemed, sufficiently talented.
If you’re dying to hear it, don’t worry: we made an MP3. A 17-minute MP3 (see below). Some of the better bits sound like a Clanger trapped in a crisp packet.
Several years later, this same apparatus has allowed the behaviour of the electrons in these materials to be nailed down. We now know that the electrons arrange themselves in waves throughout the material, and we think that these electrical ripples are somehow crucial for high-temperature superconductivity. So, while we haven’t superseded BCS theory yet, we do know that it’s worth looking for other materials containing these so-called "charge-density waves" to see if we can smash the −150°C barrier, and maybe even find superconductors that work at room temperature.
So that’s how I made music with the apparatus used to solve one of the most famous and longest-standing mysteries in physics. I’m off downstairs now to play a next-generation DNA sequencing machine like a bongo in case it turns out to be instrumental in curing cancer.