Most physical objects in everyday life maintain
the same identity whether we’re interacting with them, or not. Like, a baseball is a baseball whether you're
holding it in your hand or it's flying through the air. Even electrons are electrons whether they're
part of an atom or flying freely through interstellar space; they have the same mass, same charge:
they are electrons. But neutrinos, those weird, super light, super
fast, electrically neutral, hard-to-interact-with particles - they are identity-agnostic. Neutrinos have different identities depending
on whether they’re interacting with other particles, or traveling freely, and on top
of that their identities can change over time! Here’s what I mean: when they interact with
other particles, like when they're produced in fusion in the sun or in radioactive decay,
there are three different kinds of neutrinos, characterized by the particles involved in
their creation or annihilation. And when they're traveling through space,
there are three different kinds of neutrinos, characterized by their masses. But these two sets of identities don't match
up in a one-to-one correspondence; instead, each of the "interaction" identities is actually
a mix of the three "traveling" identities. And this weirdness allows neutrinos to change
their identities. That's because the traveling identities have
different masses, so they travel differently from each other – technically what happens
is they each pick up a complex-numbered phase depending on their mass and how far they’ve
traveled, but I’ll just show that using arrows that rotate at different speeds, which
is essentially the same thing. Anyway, because the arrows rotate at different
speeds, over time a combination that initially looked like an electron-interacting neutrino
might become the muon-interacting combination. And then, if you wait longer, the combination
will look like an electron-interacting neutrino again, then muon-interacting, and so on, back
and forth – all happening as the neutrino flies super fast through space. It’s kind of like if I took my violin and
played an A , and somehow by the time the sound waves reached your ear, the relative
strengths of the frequencies had shifted so you heard the sound as an E, or a D, or an
A, depending on how far away you were. In fact, the oscillation of neutrinos back
and forth between different identities was discovered in part because we didn’t see
as many neutrinos coming from the sun as our understanding of fusion suggested. It turned out that about 2/3 of the electron-interacting
neutrinos had turned into muon and tau-interacting neutrinos en route to the earth, in a very
real, very long-range example of quantum superposition! Oh, and one other little technicality: even
though the three “interaction”-neutrinos are named after the specific electron-family
particles that are involved in their creation and annihilation, they can still interact
with (that’s physics-speak for “bounce off of”) other members of the electron family
as well as quarks . Ok, I’d like to thank the Heising Simons
foundation for their support of this video and of neutrino research! They put me in touch with some of the neutrino
researchers they help fund, who were awesome to talk to and learn from. And Heising Simons also funds a variety of
research in other fields, like exoplanets, microscale gravity, climate change, and so
on. To learn more about Heising Simons, check
out heisingsimons.org.
