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Neutrinos: The Breadcrumbs Leading Us to Physics’ Big Answers

If you hold your hand up to the sunlight, billions of neutrinos will pass through it undetectably every second. These subatomic particles rarely interact with other matter because they're electrically neutral and near-massless. But they're the stuff of stars. And supernova. And black holes. Studying them has led scientists to revise the Standard Model of physics and hypothesize about the makeup of the universe.

For a few decades, physicists have hypothesized that neutrinos are the second most common particles in the universe (after photons) because they're a byproduct of common events. During the nuclear fusion that powers stars like our sun, a flavor of neutrino called the electron neutrino is released. They have masses about 0.00000001 times that of electrons. Bigger cosmic engines like supernova and black holes produce other flavors: muon and tau neutrinos. They have masses around two times and four times that of electrons. (And yes, “flavor” is the actual scientific term, because particle physicists are awesome.)

The huge forces that create neutrinos, paired with the particles' super-low masses, shoot neutrinos across space at near the speed of light. And because they don't carry a charge and gravity is a relatively weak force, they can (and do!) pass right through solid planets like nothing's there. Their trajectories are straight lines.

As discussed in the above Fw:Thinking video, by detecting neutrinos and tracing them back to their points of origin, we could learn more than ever before about the nature of cosmic rays, gamma bursts, supernova and other cosmological phenomena. And because neutrinos are so common, their mass – albeit tiny – may explain one of physics' biggest quandaries: dark matter.

Of course, detecting and tracing near-massless particles that rarely interact with anything is the kind of problem that can, to quote researcher Jason Koskinen, “drive experimentalists insane.” For every 100 billion neutrinos or so that pass through Earth, only one is likely to interact with other particles. But physicists have been working on it.

Teams working with detectors (like the IceCube telescope mentioned in the video) painstakingly gather and crunch data, and laboratories across the world have teamed up to prove what we suspect about neutrinos' mass and behavior. Their research won the Nobel Prize and the Breakthrough Prize in Physics in 2015, and led to the realization that humanity's Standard Model of particles and interactions needs to be revised. As they and other teams work, we'll be on the lookout for more information about the big questions that these tiny particles can answer. 

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