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The Standard Model of particle physics, that old workhorse of a theory, has dodged another bullet. The model lays out the properties of all known elementary particles and describes three of the four fundamental forces that govern nature (gravity is left out—finding a home for it is one of the most pressing problems in physics). But it also raises some questions—for instance, why should protons and neutrons, which make up atomic nuclei, be so heavy when their constituent parts, quarks and gluons, are so light? (In fact, the Standard Model holds that gluons are massless.)
So a team of European researchers undertook the gargantuan task of calculating the mass of so-called hadrons, such as protons and neutrons, from the bottom up, using the basic assumptions of quantum chromodynamics, the theory of how gluons bind quarks together via the strong nuclear force. (Though quarks and gluons don't have much mass on their own, they do pack energy; using Einstein's famous equation, E = mc2, that energy can be converted to mass.)
This is no easy task, as even empty space in quantum theory is permeated by fields that fluctuate over time—so-called virtual particles pop into existence before disappearing just as quickly. Those virtual particles can wreak havoc on small-scale computer models, throwing off any mass or energy estimate unless properly accounted for, something computer power has not been able to do until recently.
Physicist Stephan Dürr of the John von Neumann Institute for Computing in Jülich, Germany, and his collaborators broke down space and time into a four-dimensional lattice, then extrapolated what the subatomic world would look like, virtual particles and all, as the spacing of the lattice shrank to zero—as it approached a continuous spacetime fabric like that of the real world. The masses Dürr and his co-authors found through their supercomputer number-crunching aligned remarkably closely with experimental observation, providing yet another indication that the Standard Model is on the right track, if somewhat incomplete. Their results appear today in Science.
CREDIT: Image courtesy of Forschungszentrum Jülich/Seitenplan, with material from NASA, ESA and AURA/Caltech
