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.