A group of fundamental particles, long theorised but thought to be physically impossible, might exist after all. Known as paraparticles, they could one day have exotic applications if we ever manage to detect them.
Paraparticles aren’t a new idea, but physicists have previously dismissed them as having no relevance to physical reality. The concept has its origin in a division between the known fundamental particles that are always classified as belonging to one of two groups, either a fermion or a boson.
The difference comes down to what happens when two particles in the same group swap places – exchanging bosons, such as photons, doesn’t alter their quantum wavefunction, which is a complete mathematical description of their properties. But swapping two fermions, for example electrons, turns their wavefunction negative.
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This seemingly simple difference has profound physical consequences. It means that an infinite number of bosons are allowed to occupy the same space – the principle by which lasers work, where many photons can pile up and create extremely high energy densities. By contrast, fermions must always remain separated in space, which is what keeps neutron stars stable.
In the 1950s, British physicist Herbert Green put forward a more complicated model of particle swapping, called parastatistics, where the quantum wavefunction can be altered in ways other than turning negative. Green showed that this implied many new classes of particles could exist, which he called parabosons and parafermions. These would allow only a certain number of particles to exist in the same state, rather than just one or infinitely many. But physicists that examined the idea more closely found that these paraparticles wouldn’t function in detectably different ways from fermions or bosons, rendering the theory seemingly irrelevant.
Now, and at Rice University in Texas have found that paraparticles could actually be physically detectable. “Our paper proves, for the first time, that there is actually something beyond fermions and bosons,” says Wang.
The duo first came up with a new mathematical description of paraparticles, which included stricter rules than previous definitions, such as making sure that information couldn’t travel faster than light. They then showed that there were specific quantum systems where these paraparticles should emerge as vibrations and be physically detectable.
“It’s a very daring thing, because everybody will take the folklore that parastatistics are not physical and they will work with that and bypass the topic,” says at the University of Leeds, UK. “But these guys went back to it and they proved they are possible to exist, which is fantastic.”
Not everyone is convinced, though. “It’s an interesting observation, but it’s not clear how easy this will be to realise in nature,” says at the University of Oxford. Observing these particles would require advances in our ability to control quantum states that are likely many years away, he says. “It’s a proof of principle, but the principle doesn’t cover anything, at least at the moment, close to reality.”
One limitation of the new work is that the pair have so far only shown that paraparticles can exist in one or two dimensions, though there is nothing ruling out their existence in three. Another wrinkle is that the paraparticles Wang and Hazzard propose are technically quasiparticles which, as the name suggests, aren’t fundamental particles like electrons or photons, but are instead collective vibrations acting as if they were a particle. Wang and Hazzard also speculate that paraparticles could exist as fundamental particles because the maths doesn’t rule it out, but they don’t have any concrete evidence to show where or how they might show up.
It isn’t clear whether these paraparticles, if they are ever observed, could have some practical use, but another kind of quasiparticle that has been observed in two dimensions, called the anyon, might be useful for quantum computing systems, because of its unique property that it has a memory of its swapping behaviour. “We don’t have an obvious application [for paraparticles] at the moment, it’s just something exciting and exotic, but that has proven in the past to be a good enough reason to invest research in it,” says Pachos.
Nature
