The subatomic world is full of all sorts of unfamiliar particles, with names like quarks
The subatomic world is full of all sorts of unfamiliar particles, with names like quarks

(which is not the sound an Australian duck makes) to bosons (which isn't a clown particle).
(which is not the sound an Australian duck makes) to bosons (which isn't a clown particle).

Each have their respective personalities and behaviors. However, one of the particles is
Each have their respective personalities and behaviors. However, one of the particles is

particularly curious. This class of particles is called the neutrinos.
particularly curious. This class of particles is called the neutrinos.

Neutrinos are the ghosts of the subatomic world. They interact less than any of the
Neutrinos are the ghosts of the subatomic world. They interact less than any of the

other known particles. Even though we are constantly bombarded by them...in fact
other known particles. Even though we are constantly bombarded by them...in fact

something like 650 trillion of them are hitting you every second, it took thirty years to
something like 650 trillion of them are hitting you every second, it took thirty years to

prove that they exist at all. So let me tell you their story.
prove that they exist at all. So let me tell you their story.

To start with, we need to go back to 1930, when they were first proposed. It turned out that
To start with, we need to go back to 1930, when they were first proposed. It turned out that

there was a maddening little problem with a kind of radioactive decay called beta decay.
there was a maddening little problem with a kind of radioactive decay called beta decay.

Beta decay is when an atomic nucleus emits an electron and the nucleus changes from one
Beta decay is when an atomic nucleus emits an electron and the nucleus changes from one

element to another. This is kind of like the alchemist's dream of changing lead into gold,
element to another. This is kind of like the alchemist's dream of changing lead into gold,

although the actual elements were different. At the time, the emitted electrons were called
although the actual elements were different. At the time, the emitted electrons were called

beta particles, which is where the decay got its name.
beta particles, which is where the decay got its name.

The problem was that when the energy of the original nucleus was compared to the combined
The problem was that when the energy of the original nucleus was compared to the combined

energy of the daughter nucleus and electron, the books didn't balance. After the decay,
energy of the daughter nucleus and electron, the books didn't balance. After the decay,

energy was missing.
energy was missing.

Since one of the most dearly-held ideas of physics is that energy can't be created or
Since one of the most dearly-held ideas of physics is that energy can't be created or

destroyed, the observation of the loss of energy in beta decay sent the research world
destroyed, the observation of the loss of energy in beta decay sent the research world

into a tizzy. Lots of ideas were considered, including the possibility that perhaps in
into a tizzy. Lots of ideas were considered, including the possibility that perhaps in

the quantum world that energy could disappear.
the quantum world that energy could disappear.

In 1930, physicist Wolfgang Pauli came up with the idea that the reason the energy was
In 1930, physicist Wolfgang Pauli came up with the idea that the reason the energy was

missing was that because there was an unobserved particle emitted in beta decay. If that was
missing was that because there was an unobserved particle emitted in beta decay. If that was

true, the unobserved particle could carry away the energy and the mystery was explained.
true, the unobserved particle could carry away the energy and the mystery was explained.

These particles have come to be called neutrinos.
These particles have come to be called neutrinos.

There are four known forces; two familiar ones called gravity and electromagnetism,
There are four known forces; two familiar ones called gravity and electromagnetism,

and two ones that aren't familiar called the strong and weak nuclear forces.
and two ones that aren't familiar called the strong and weak nuclear forces.

Neutrinos do not interact with the strong and electromagnetic forces. We know that because
Neutrinos do not interact with the strong and electromagnetic forces. We know that because

if they did, they would be very easy to see. Neutrinos do experience the weak nuclear force
if they did, they would be very easy to see. Neutrinos do experience the weak nuclear force

and gravity. Of the two, the weak force is the stronger of the two by far. For all practical
and gravity. Of the two, the weak force is the stronger of the two by far. For all practical

purposes, neutrinos interact only via the weak nuclear force.
purposes, neutrinos interact only via the weak nuclear force.

And the weak force is aptly named. To give a sense of how weak it is, let me give you
And the weak force is aptly named. To give a sense of how weak it is, let me give you

an example. Neutrinos originate from nuclear reactions and the biggest nuclear reactor
an example. Neutrinos originate from nuclear reactions and the biggest nuclear reactor

around is the Sun. The Sun emits so many neutrinos that even though it is about 93 million miles
around is the Sun. The Sun emits so many neutrinos that even though it is about 93 million miles

away, something like 650 trillion neutrinos from the Sun hit you every second.
away, something like 650 trillion neutrinos from the Sun hit you every second.

So suppose you wanted to shield yourself from that steady barrage of neutrinos. How would
So suppose you wanted to shield yourself from that steady barrage of neutrinos. How would

you do that? Well, if you took a huge block of solid lead that extended to the nearest
you do that? Well, if you took a huge block of solid lead that extended to the nearest

star...and I'm not talking about the Sun, I'm talking about Alpha Centauri, which is
star...and I'm not talking about the Sun, I'm talking about Alpha Centauri, which is

about 5 light years away, you'd be able to block something like half the neutrinos from
about 5 light years away, you'd be able to block something like half the neutrinos from

the Sun.
the Sun.

If five light years of solid lead is such a poor shield, then there is no way that you
If five light years of solid lead is such a poor shield, then there is no way that you

can stop them...even if you used the entire Earth as your protection. Most neutrinos blow
can stop them...even if you used the entire Earth as your protection. Most neutrinos blow

right through the planet like it's not even there. Neutrinos >>REALLY<< don't interact
right through the planet like it's not even there. Neutrinos >>REALLY<< don't interact

very much.
very much.

So how did we detect them? Since neutrinos are emitted from nuclear interactions, one
So how did we detect them? Since neutrinos are emitted from nuclear interactions, one

early idea was to put a neutrino detector near a nuclear detonation. However, that idea
early idea was to put a neutrino detector near a nuclear detonation. However, that idea

was quickly nixed, as being unnecessarily dramatic. So no bomb.
was quickly nixed, as being unnecessarily dramatic. So no bomb.

Instead, in 1955, physicists put a neutrino detector near a nuclear power plant and basically
Instead, in 1955, physicists put a neutrino detector near a nuclear power plant and basically

turned the plant on and off. On, and the detector saw more neutrinos. Off, and it saw less.
turned the plant on and off. On, and the detector saw more neutrinos. Off, and it saw less.

On and off, on and off. That's essentially all it took.
On and off, on and off. That's essentially all it took.

In the last half century and more, neutrinos have been studied in great detail. There is
In the last half century and more, neutrinos have been studied in great detail. There is

no longer any question of their existence. We have observed them from many nuclear reactors
no longer any question of their existence. We have observed them from many nuclear reactors

and can even generate beams of them and shoot them hundreds of miles through the Earth to
and can even generate beams of them and shoot them hundreds of miles through the Earth to

distant detectors. We have seen neutrinos that were created in distant supernovae. We
distant detectors. We have seen neutrinos that were created in distant supernovae. We

have even observed neutrinos emitted from the radioactive material inside the Earth.
have even observed neutrinos emitted from the radioactive material inside the Earth.

Neutrinos are now familiar denizens of the particle world.
Neutrinos are now familiar denizens of the particle world.

Now this doesn't mean that just because we have studied them in great detail, doesn't
Now this doesn't mean that just because we have studied them in great detail, doesn't

mean that neutrinos have told us all of their secrets. Neutrinos are unique in the subatomic
mean that neutrinos have told us all of their secrets. Neutrinos are unique in the subatomic

realm in that they can actually change their identity. In another video, I'll tell you
realm in that they can actually change their identity. In another video, I'll tell you

about this totally-crazy behavior.
about this totally-crazy behavior.