If a neutrino was a superhero, its special power would be passing straight through solid
objects.
An ability I'm sure many of us wouldn't mind having when we're locked outside our house.
But how come neutrinos can walk through walls and we can't?
Get ready for fun with forces, today on Even Bananas!
One of my favorite particle physics facts is this: at the atomic level you've never
actually touched anything in your life.
I know what you're thinking, but it's true.
When we touch something, a couple of things are actually happening.
First, since the electrons in our atoms feel the electromagnetic force, they repel one
another.
Second, electrons actually aren't little spheres moving around the nucleus like a miniature
solar system. Instead, the electrons are everywhere, forming an electron cloud.
Sometimes, atoms get close together and share electron clouds, forming molecules.
But if you keep smushing atoms closer together, you run into the Pauli Exclusion Principle,
which basically means electrons with the same exact set of properties can't occupy the same
space.
So you get these electron clouds pushing against one another and that is how we get our impression
of solidity, the feeling of touch.
We're made of electrons, and unfortunately, that means we can't stick our head through
a wall.
Now, let's turn to our super-powered neutrino.
Neutrinos are neutral; they don't feel the electromagnetic force or the strong force
which applies to quarks inside the nucleus.
They only feel the weak force and gravity.
As a neutrino travels along, it also doesn't care about the Pauli Exclusion Principle for
the electron cloud because it already has different quantum properties.
So the neutrino basically only has to worry about the atomic nuclei and we know there
are big gaps between them.
This is the origin of their superpower that lets them pass more easily through things.
Let me try to give you a sense of scale. Pretend you're a neutrino jogging through a forest
of atomic nuclei.
To be at a scale, if a nucleus was the size of a redwood tree, a neutrino would be about
as big as a virus.
Now, you're trying to avoid hitting any trees as you run.
Turns out, you could do this blindfolded.
That's because at this scale, the trees would be about 31 kilometers apart. The obstacles
are few and far between.
It's really more like a desert than a dense forest.
This is why neutrinos pass through matter without issue.
It's not that they won't ever interact, but it's really unlikely for that to happen and
this is one reason researchers building neutrino detectors often fill them with heavier elements.
By picking materials with bigger nuclei, they increase the chances of a neutrino getting
close enough to it to interact.
When neutrino does interact it, does so by exchanging the particles that carry the weak
force: the w and z bosons.
Most of the time, the neutrino interacts with a proton or a neutron as a whole. But sometimes
if a neutrino has enough energy, it can interact with the quarks inside a proton and neutron.
Although that does have the unfortunate side effect of blowing the particle apart.
One interesting thing to note: we've been talking about neutrinos more generally, but
there's an extra special feature for electron neutrinos.
As you know, matter contains electrons rather than their heavier cousins, the muon and the
tau, which rapidly decay.
The electron neutrino has a special extra possibility to exchange a w boson with the
electrons in matter.
This extra exchange possibility causes what you can think of as "drag" on the electron
neutrino and it's more pronounced when you send neutrinos through a big mass of matter.
For example: the earth.
This "matter effect" is actually useful for studying one of the biggest questions about
neutrinos: their masses.
We know that there are three kinds of neutrinos, but we don't know which one is heaviest and
which is lightest, and that's pretty bizarre; we have a good sense of the masses of basically
all the other particles.
Studying the matter effect on electron neutrinos, combined with a lot of maths, can help researchers
figure out this basic property.
Maths! That's our superpower.
Move over MCU, for more tales from our neutrino extended universe series, check out the full
even bananas playlist below in the description.
And don't forget to like, share, and subscribe.
Fun fact: in preparing for this video, we calculated the size of neutrinos and nuclei
at a bunch of different scales.
For example, if a nucleus was the size of the earth a neutrino would be about the size
of a parking space.
Or, if a nucleus was the size of the sun, a neutrino would be about the size of a comet.
Trying to wrap your head around the scale of the different things in the universe is
really mind-blowing.