Even when you close your eyes at night, 100 billion neutrinos produced in the sun will pass through them – traveling close to the speed of light, but never hitting anything. Neutrinos are extremely elusive and only weakly interact with matter around them: nature’s true ghosts. Until very recently, these tiny particles were believed to be massless. In the late 1990s, researchers demonstrated that neutrinos constantly change between three different types (flavors or species), which affects how they interact with matter. This is something they can only do if they have mass – a discovery that was granted the Nobel…

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