If you want a working lightsaber, try one made of glueballs -particles consisting entirely of force. There's just one problem. Although theorists are adamant glueballs must exist, experimentalists are equally adamant we're unlikely ever to prove it. "You can't do an experiment and know they exist," says Frank Close of the University of Oxford. Glueballs are bundles of gluons, the particles that transmit the strong nuclear force between quarks, sticking them together into things like protons and neutrons within the atomic nucleus. Gluons have an odd quirk: they themselves carry strong-force charge, meaning they can also stick to each other. Simulations of a world chock-full with gluons suggest that an energy of around 1500 megaelectronvolts (MeV), or about one-and-a-half times the energy contained in a proton, should be enough to stick a load of them together into a glueball. In 1995, Close and fellow theorist Claude Amsler of the University of Zurich in Switzerland showed that two particle "resonances" with energies of 1370 and 1500 MeV, which had just been discovered at CERN, might fit that bill. They have since been joined by a third candidate at 1710 MeV.
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