[ ♪ Intro ]
In the 1990s, engineers poured fifty million
liters of water beneath a mountain in Japan.
In the last 20-some years, that water
has been sitting in a giant tank,
revealing all sorts of things about the subatomic world.
But even after all that time, there is one
thing no one has ever seen happen in the tank,
or anywhere else: a proton decaying.
In other words, no one has ever conclusively
seen a proton turn into other, lighter particles.
That might not seem like much, but it’s
important,
because physicists are pretty sure protons should decay.
So the fact that no one has ever seen it happen
is raising some questions.
Now, when you first start looking at this
research,
it might seem a little weird that scientists think protons decay.
After all, the main theory that explains how
fundamental particles interact,
called the Standard Model, says that they don’t.
Specifically, it says that protons can’t
decay because of what they’re made of.
Protons are made of three smaller building
blocks, called quarks,
two up quarks and one down quark.
These tiny particles can’t exist by themselves,
so it’s not like a proton could just fall to pieces.
Instead, for one to decay, those quarks would
need to turn into something lighter.
The problem is, up quarks are already the
lightest ones out there,
so they can’t get any lighter.
And while down quarks can turn into ups, it
doesn’t happen in protons.
That’s because together, three up quarks
have more energy than two ups and a down quark do.
So for a proton’s down quark to change,
it would need to create that extra energy from nowhere.
Which isn’t a thing.
And that means the proton’s down quark stays
a down, and the particle stays a proton forever.
The Standard Model’s explanation sounds
totally reasonable,
but if you start to look closely at it, it also seems kind of… arbitrary.
See, there are lighter particles than up quarks
that these things could maybe decay into.
They’re all leptons, like electrons and
neutrinos,
which are particles that don’t feel the strong nuclear force that holds atomic nuclei together.
But, according to the Standard Model, quarks
can’t just turn into leptons.
As for why… well, the Model doesn’t really
make it clear.
And that actually opens up the real problem
here.
Because many physicists think the Standard
Model is more complicated than it should be.
Sure, it predicts how the strong, weak, and
electromagnetic forces make particles move and decay,
and it’s one of the best-tested
ideas in the history of science.
But it’s not exactly clean.
Its three forces are transmitted by four types
of particles that come in a dozen total varieties.
The Standard Model also has some weird features,
like separating quarks and leptons even though they have a lot in common.
Plus, it has some gaping holes, like not including
gravity or dark matter.
So many physicists think that some other explanation
has to be out there.
For now, many of them are leaving gravity
out of this debate and are instead searching for GUTs,
or Grand Unified Theories.
These are theories that would combine the
three forces of the Standard Model
into one nice, neat super-force.
The idea behind them is that those three forces
only look so different today
because the modern universe is much colder than the very early universe was.
If temperatures were higher, they would seem
more alike.
And at super high temperatures, they would
all act like the same thing, or, really,
they would be the same thing.
It’s kind of like how researchers in the
eighteen hundreds realized
that electricity and magnetism are just different aspects of electromagnetism.
Or it’s like what happened with electroweak
unification, for you particle physics nerds.
Right now, there are tons of different GUTs,
some more dramatic than others.
But unfortunately, these ideas are hard to
test against each other.
That’s because the Standard Model is so
incredibly well-verified
that all viable GUTs need to be super similar to it in today’s
universe.
These hypotheses can only diverge at much
higher energies,
ones so high that our particle accelerators won’t be accessing them any time soon.
And that’s where protons come in.
Many GUTs predict that the super-force shouldn’t
distinguish between quarks and leptons,
so up quarks in a proton should occasionally
decay into leptons after all.
As a matter of fact, these models suggest
that up-decays would have actually been common
in the super-force’s heyday right after
the Big Bang.
But since things are much colder today, those
decays should happen much less frequently.
The nice thing is, different GUTs disagree
about how long we should have to wait for
this decay to happen.
So watching for that event can help physicists
figure out which ideas could be right.
The simplest GUTs say that these days, your
average proton will decay
after up to a hundred million yottayears or so, a number with 32 zeros in it.
That’s a yotta years, and is a billion trillion
times longer than the age of the universe.
But if you put enough protons in one place,
like, say, by putting 50 million liters of water under a mountain,
you’re more likely
to see a decay much sooner than that.
If a proton in the water decayed, the resulting
leptons would give off light
that would be picked up by detectors around the water.
And we would finally have some answers.
Of course, that hasn’t happened yet.
And to match that result, the project’s
scientists have calculated that proton lifetimes
must average at least a hundred times longer
than the simplest GUTs predicted.
But there are plenty of Grand Unified Theories
still in the running,
so scientists aren’t out of options yet, although the lack of decays is worrying some of them.
For one, we can never use the water tank to
prove that protons can’t decay.
We can just get longer and longer lifetime
estimates.
But also, if protons don’t decay, we’ve
missed something pretty important about the universe.
So maybe we’ll see a proton decay tomorrow
and the Standard Model will finally be overthrown!
Or maybe we’ll never see one, and we’ll
just have to keep working on it.
Thanks for watching this episode of SciShow
Space!
If you liked learning about physics topics
like this,
you can check out our episodes over on the main SciShow channel,
like one about how quarks fixed 
the mess that used to be particle physics.
[ ♪ Outro ]
