Supercomputers in the USA, Europe, and Japan have proved indispensable for research into the effects of quantum mechanics at hugely different length scales. One investigation, just published in the journal Science, confirms the theory of the strong interaction in particle physics by exactly calculating the mass difference between the proton and the neutron. The other two investigations employed high-performance computing to look at the properties of superconductors, and to simulate from first-principles the dynamics of very large systems, potentially of millions of atoms.
Protons and neutrons are the main components of atomic nuclei and have almost but not quite the same mass: the neutron is about 0.14 per cent more massive than the proton. It is not entirely stable but can decay into a proton and a pion. The mass difference is key to the stability of atoms and thus the structure of matter as we know it, but it is paradoxical. Because the proton is electrically charged it should be the heavier particle, deriving a small extra contribution to its mass from the electromagnetic field.
Now, some 80 years after the discovery of the neutron, a team of physicists from France, Germany, and Hungary, headed by Zoltán Fodor, a researcher from Wuppertal, has finally calculated the tiny neutron-proton mass difference from first principles, using one of the most powerful computers in the world, Juqueen at the Forschungszentrum Jülich in Germany to perform the lattice quantum-chromodynamics and quantum-electrodynamics calculations.
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