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.