Decades ago, this table of fundamental particles was incomplete. Several of the quarks and leptons had not yet been discovered. But physicists looked at the patterns in the table and used them to go searching for the missing particles. For example, many years ago scientists knew that there had to be a sixth quark because there was an empty spot in the table. Even though it had never been found, people were so confident it existed that it was included matter-of-factly in many textbooks along with its predicted mass. After twenty years, the top quark was finally found (sort of—its mass was much
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