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Massive Neutrino Experiment Undermines Our Sense of Reality

A massive neutrino experiment has produced data which shows that neutrinos must literally be of 2 types that are mutually exclusive at the same time. The result is bedrock quantum mechanics. This is the sort of thing that is typically shown by highly controlled quantum optics experiments, and not with neutrinos that are almost undetectable.

According to quantum theory objects that are minuscule behave quite differently from things on the macroscale. However, those 2-way superposition states are fragile. If a photon, which is simultaneously polarised horizontally and vertically is measured it will “collapse” randomly one way or the other. In quantum theory the polarisation doesn’t exist until it is measured.

It was argued by some physicists that the result of such a measurement is predetermined by some “hidden variable” within the photon.

In 1964 John Bell, a theorist in the UK devised a way to test that suggestion. According to quantum theory, 2 photons in 2-way states can be entangled resulting in a measurement of one determining instantly the polarisation of the other as well as itself, which would occur even when the photons are separated by light years. Bell showed that correlations between the particles are produced by that quantum connection are stronger than hidden variables allow. Hidden variables were disproved in 2015 by physicists in the Netherlands and the US when they performed the best demonstrations yet of those correlations.

Correlations between measurements separated in time, and not in space, are involved in the test with neutrinos. In 1985 theorists Garg and Leggett considered repeated measurements of a single quantum system: a ring of superconductor in which an unquenchable current flows in one direction or the other. The ring acts like a coin, which can be either heads or tails, with the exception that in this case current can also flow in both directions at once.

The current will, according to quantum theory, oscillate between the 2 directions. Therefore a measure will reveal it flowing, say, clockwise, with a probability that depends on the time. It was found by Leggett and Garg that certain correlations among 3 or more measurements would be stronger than is allowed by classical physics – if the current flows in no direction until it is measured.

The Leggett & Garg’s test has been approximated by experiments. The extrastrong correlations in quantum optics were demonstrated by White et al. in 2011, though in an average way and not with a singles photon. Joseph Formaggio et al. have now provided a demonstration using data obtained by the Main Injector Neutrino Oscillation Search (MINOS) experiment at Fermi National Accelerator Lab (Fermilab) which fires neutrinos are close to light speed over 735 km to a 5.4 kiloton detector in the Soudan Mine, Minnesota.

Neutrinos come in 3 flavours that morph into one another. Neutrinos fired from Fermilab start as so-called muon neutrinos and “oscillate” mainly to electron neutrinos in a process resembling the one that was analysed by Leggett and Garg. Individual neutrinos were not measured repeatedly by the MINOS experimenters as a neutrino is destroyed by the detection of it. Each neutrino starts in the same state, with its evolution depending only on the time since it left Fermilab. The measuring of many neutrinos was equivalent to measuring the same one repeatedly.

The neutrinos were also not measured at different distances from Fermilab by the MINOS physicists, so the measurements made by Formaggio et al, could not be compared directly with measurements made by different flight times. Neutrinos oscillate at rates that vary with their energy, with the clock ticking faster for neutrinos that are more energetic. So rather than looking for correlations between neutrinos that were measured at different times, Formaggio et al. looked for correlations in the number of muon neutrinos that arrived at the detector.

The strong correlations that had been predicted by Leggett & Garg were observed, as Formaggio et al. report in Physical Review Letters. According to Formaggio it is a very obvious effect. The data have underscored that the neutrino has no flavour until it is measured.

Sources & Further reading

  1. Cho, Adrian, 2016, Massive neutrino experiment undermines our sense of reality. Science, DOI: 10.1126/science.aag0657


Author: M. H. Monroe
Last Updated 14/07/2016
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