If photons possess more quantum states than expected, it would overhaul our understanding of all particle physics 鈥� and now physicists have identified a way to test this radical idea in an experiment.
Certain elementary particles have a property called spin. For particles of light, or photons, spin can be in one of two potential states, which manifests as the light being polarised in one of two directions. But what if photons had more options 鈥� what if they could be in any of an infinite number of spin states, and thus light could have infinitely many possible polarisations?
That would be the case if a photon were a so-called continuous spin particle (CSP), a massless particle that can carry a force and has infinitely many spin states. Physicists long believed that CSPs shouldn鈥檛 exist: they seemed to be forbidden by the standard model of particle physics, which dictates that force-carrying particles 鈥� such as the Higgs boson, which conveys mass, or the theoretical gravity-carrying graviton 鈥� must have a limited number of spin states.
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However, more than 10 years ago, at the SLAC National Accelerator Laboratory in California and his colleagues were inspired to look more closely at CSPs after they tried to prove that there are no exceptions to this restriction on force-carrying particles鈥� spin states 鈥� but they failed.
鈥淭hat is what completely changed our thinking. I think we kind of realised, 鈥極h, crap! People put these [particles] aside way too quickly,鈥欌�� says Schuster. Now, he and his colleagues have calculated how CSPs could be identified experimentally.
Their work hinges on the massless photon. If it were a CSP, it would have infinitely many spin states, and it would still carry the electromagnetic force. In that case, it would imply that the standard model, which combines all existing particles and their interactions into one internally consistent mathematical theory, is not complete 鈥� that it is merely an approximation of some more complex model of more complicated particles, says Schuster.
He and his team calculated what this would mean for a commonplace physics process: a hydrogen atom emitting a photon as it transitions from a higher to a lower energy state. They found that one specific transition can鈥檛 emit a single standard photon 鈥� but can emit a CSP photon. An experiment that observes that transition could thus prove whether photons are CSPs.
Although their mathematical model for the hydrogen atom was simplified, this result still shows that the mathematics of CSPs have matured enough to make contact with the real world, says Schuster. 鈥淚t鈥檚 exceedingly reasonable to think this is empirically testable, and not in 10, 15 years, but more like now,鈥� he says. He is, however, not involved in any specific experiments yet.
at the University of Paris-Saclay in France says that CSPs are a curiosity in the history of particle physics, and it has remained mathematically challenging to prove whether their existence can be consistent with the standard model. He says the new work is bold and concrete, but still speculative. And no experimental data exists to support it yet.
鈥淢ost physicists, including myself, expect that a fatal inconsistency will eventually be found,鈥� he says. 鈥淭hat said, it鈥檚 important not to conflate expectations with hard facts. A healthy scientific attitude is to remain open-minded 鈥� but not uncritical 鈥� especially in such speculative areas.鈥�
The idea of CSPs is speculative, and many details still must be ironed out. But testing it experimentally could lead to impactful results, says at the University of Tours in France. 鈥淎s [with] other exotic theoretical particles, they could be at the origin of new physics beyond the standard model, and could possibly cure some of the problems faced by the standard model,鈥� he says.
Schuster says this idea motivates his team鈥檚 work. 鈥淭he possibility is kind of radical, and I鈥檓 kind of conservative about it. The responsible thing to do is to really stress-test the theory,鈥� he says. But on the off chance that some future experiment does find evidence a photon is a CSP, not only will the standard model have to be retooled, but 鈥渁 lot of people would eat their hats. I would eat my hat,鈥� he says.
arXiv
