Our basic understanding of core-collapse supernovae hasn't changed in decades. Large stars burn through all the fuel at their cores and start creating heavier elements in reactions that consume energy. The lack of energy from these reactions allows gravity to pull the interior of the star in on itself, collapsing it into a neutron star or black hole. The energy released by this process then blows the remainder of the star apart.

And, generally, that's right. But there's an entire busload of devils in the details. The statistics of supernovae that we've observed indicate that the model may be seriously incomplete. And on the theoretical side, there are still plenty of uncertainties, including over some of the basics, such as whether all core collapses actually result in a supernova.

A paper being released by Physical Review D provides what might be a potential explanation for the discrepancy: flavor-changing neutrinos. Neutrinos play a key role in our current models of supernovae, and right now, those models don't take into account one of neutrinos' most striking features: their ability to change identity.

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Almost a hundred years ago, physicists theorized out what free fall should do to a quantum wave. If the solution they came up with is wrong, quantum mechanics and Einstein's theory of gravity flatly contradict each other. But testing it has been impossible because nobody has managed to build an interferometer that could perform the necessary measurement.

Now, a team led by Ron Folman, a physicist at Ben-Gurion University of the Negev, with collaborators in Germany, the UK, and the US, including Nobel laureate Roger Penrose, has done it. They built a new interferometer that gives a single atom two possible paths at once: one that involves a free fall, and another where it is held perfectly still. Both paths end at the same place at the same moment, allowing the team to measure what the fall does to a wave-like property of the atom.

Long time coming

Ever since Galileo, physicists have known how to describe a falling object—where it is, how fast it goes, or how quickly it accelerates. Quantum mechanics, though, insists that every object is also a wave. “Every particle, doesn't matter if it's a car or a spaceship or an atom, is a wave,” Folman says. “Everything that is a wave, like sea waves or sound waves, goes up and down. And if you're up or down, this is measured by something called a phase. A phase just tells you if you are at the top of the wave or at the bottom.”

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Physicists with the Beijing Spectrometer III (BES III) experiment have uncovered convincing new evidence of the existence of so-called glueballs, an elusive composite particle made entirely of gluons predicted by quantum theory. The results appeared in a preprint posted to arXiv last month and were also presented last week at the International Conference on High Energy Physics (ICHEP).

All the stuff we see around us is made up of quarks held together by gluons (carriers of the nuclear strong force) to form protons and neutrons, which comprise the core of every single atom. The Higgs boson, discovered in 2012 after decades of searching, was widely touted as the final missing piece of the Standard Model of Particle Physics. But there are still plenty of unanswered questions, including whether or not glueballs really exist. They should, if the Standard Model is correct; they're a direct prediction of quantum chromodynamics, i.e., the theory of the strong nuclear force. There should even be several kinds of glueballs.

As Matthew Francis wrote for Ars in 2015:

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A photon is a single particle of light, and under normal circumstances, it can't be divided. But a photon is also not a particle, in the sense that it does not have a specific location. Instead, it is an extended object.

So if a photon is only partway through the process of reflecting from a perfect mirror and you yank the mirror away, what happens? The answer, from a trio of Norwegian physicists, turns out (arxiv.org link) to be more complex than I expected.

A photon divided?

Let’s first talk briefly about dividing and combining photons. If this were a common experience in our lives, then shining a single color of light through a piece of glass or reflecting it from a surface might cause photons to divide or combine. This would lead to an amazing array of colors: Our universe would be the most fantastic and legal LSD trip you could imagine. But this doesn't generally happen, hence LSD.

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