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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 ].