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As its name suggests, antimatter is a kind of mirror image of ordinary matter, made up of particles like positrons, with the opposite charge and spin to electrons. (‘Spin’ is a type of angular momentum that all subatomic particles have, spin can have a value of 1/2 or 1.) But most theorists doubt that antimatter also produces antigravity. That’s because the so- called chargeparity-time (CPT) theorem of quantum theory suggests antimatter’s ‘anti-ness’ does not extend to its mass and gravitational effect. That said, it’s always possible there’s a loophole in this theorem: it’s had to be tweaked several times over the decades to explain newly discovered phenomena. Later this year, experimentalists at CERN, home of the Large Hadron Collider, plan to look for signs of strange behaviour when particles of antimatter are released in a vacuum. If the particles rise, antigravity may be the explanation.