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30 October 2014

The vanishing particle physicist and the puzzle he left behind

Ettore Majorana was an Italian physicist, the best of his generation, who disappeared in mysterious circumstances in 1938.

By Michael Brooks

You probably haven’t heard of Majorana fermions. You probably haven’t heard of fermions, for that matter – nor of Ettore Majorana. Prepare yourself: it’s a story of mystery and frustration.

Let’s deal with the man first. Majorana was an Italian physicist, the best of his generation, who disappeared in mysterious circumstances in 1938. He emptied his bank account, bought a ticket for a boat ride from Palermo to Naples and left a note for his colleagues apologising for the inconvenience he was about to cause. He may have committed suicide but his body was never found. There are rumours of subsequent sightings in Buenos Aires. Some think that he abandoned physics and disappeared to live out his days in a secluded monastery.

Majorana’s specialism was quantum theory and it is tempting to think that he engineered his disappearance to reflect the subject’s strange ambiguities. A quantum particle such as an electron can be in two places at once, or simultaneously moving in two different directions; Majorana seemed to want his friends to wonder whether he could, too.

And so to his fermions. Just before his vanishing trick, Majorana predicted the existence of a particle with highly unusual properties. Although we have never seen one directly, there is every reason to think that it does exist and scientists on the 77-year-long hunt are tantalisingly close to pinning it down. This month, a group of Princeton University researchers added to the accumulating pile of evidence that the Majorana fermion is a real and fundamental building block of nature.

A fermion is one of the two types of particle that make up all matter. You have probably heard of the other one, the boson, because of all the Higgs fuss. Fermions are just as interesting. The building blocks of the atom – protons, neutrons and electrons – are all fermions. Majorana fermions have an added twist. Unlike protons, neutrons and electrons, they are their own antiparticle.

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That is very odd. Matter and antimatter annihilate each other when they come into contact. Bang an electron and its antiparticle, the positron, together and they will disappear in a flash of energy. So how can a Majorana fermion be its own antiparticle?

To find out, we have to get one we can observe, hence the hunt. And in a peculiar case of life (or death) imitating art (or science), Majorana fermions are offering glimpses of their properties in materials crammed with ghostly particles that don’t actually exist.

When a solid conducts electricity, it does so because it contains negatively charged electrons that can move through the material. As they leave their usual position, the electrons leave behind a network of “holes”. Viewed from one perspective, these behave like real particles that have a charge opposite to the electron. The details get complicated but, in some materials, this can create a situation in which the moving charges and holes amount to something that behaves exactly like a Majorana fermion. That’s what the Princeton researchers have seen.

It has been very difficult to get this far. The material containing the Majorana fermion was just one atom wide and three atoms thick and had to be cooled to -272°C, one degree above the coldest temperature allowed by the laws of physics. The fermion was so small that it required a scanning-tunnelling microscope two storeys high to see it.

That said, although the observed particle is a good facsimile, it is frustratingly still more like a Majorana fermion’s shadow puppet than the real thing. Majorana’s legacy still eludes us. That note was not apologetic enough.