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We’re sorry if it makes you feel a bit squeamish, but trillions of ghostly particles from beyond Earth zip through your body every second. Known as neutrinos, they’re not harmful, because they almost never interact with other matter. In fact, they can pass through an entire planet or even a star without so much as a wiggle.

That’s good for planets and stars, but not so good for astronomers. They want to study neutrinos because they’re produced in stars, exploding stars, and other powerful objects and events. So they can reveal details about what’s going on inside those objects. If neutrinos aren’t affected by other matter, though, it’s impossible to catch them. And if you can’t catch them, you can’t really study them.

Every once in a while, though, a neutrino does cause a reaction from another bit of matter — a faint flash of light.

And 25 years ago today, scientists announced something new revealed by the flashes: neutrinos have mass. Before that discovery, it was thought that neutrinos had no mass at all.

It was known that neutrinos come in three varieties, called flavors. Using a detector buried far beneath a Japanese mountain, the scientists found that some neutrinos can change flavor. The only way that could happen is if neutrinos have mass.

The mass is tiny. Still, there are so many neutrinos that it adds up — a bit of substance from some ghostly particles.

Script by Damond Benningfield

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