Former US Defense Secretary Donald Rumsfeld famously distinguished between "known knowns", "known unknowns" and "unknown unknowns". It's a distinction that should ring clear bells with Harvard physicist Howard Georgi, because he's choosing to make the same distinction between different types of matter. Most of us are familiar with the "known knowns" of matter - ordinary stuff, such as tables, chairs, quarks and electrons. Many physicists spend their days hunting for "known unknowns", like the Higgs boson and the particles that make up dark matter. But Georgi has gone a step further. He is dabbling in the world of the "unknown unknowns" by proposing the existence of an entirely new type of matter unlike anything we have encountered before. He calls it the unparticle.
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