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The whole point of physics has always been to understand what the universe is made of and how the stuff in it interacts. You know, like how Isaac Newton wanted to figure out why an apple always falls straight toward the ground.

And we've come a really long way over the years. These days, after about a century of incredible research in fundamental physics, we have a pretty good idea of the building blocks that make up everything — and the rules that describe how they interact. In fact, our fundamental picture of the universe seems so nearly complete that it's led some people to suggest that we're arriving at some version of “the end of physics.” And for sure, physics is at a turning point, but before researchers pack it up and head home, it's worth understanding what the so-called “end of physics” is really all about.

As far as anyone can tell, every single thing in our entire world is made up of a small handful of elementary particles, like electrons and quarks. And they obey very strict rules when they interact with each other. Starting with those basic particles and the rules they follow, you can build up to all sorts of things—like the physics of baseball, the chemistry of pie-baking, or the biology of cell division.

Of course, it doesn't make much sense to explain how something like the brain works using elementary particles like quarks. It usually makes more sense to describe reality with bigger things, like molecules or cells. But the point is, no matter what unit makes the most sense to use, that unit still obeys the same basic principles.

Like, you're not going to need a brand-new elementary particle to make sense of some everyday thing, like how a bird flies. Most of the basic rules that describe the stuff in our everyday world are part of a framework called the Standard Model. This framework is essentially a set of math and physics principles that describe the fundamental structure of the world as we know it.

It includes three of the world's four fundamental forces and the couple dozen particles related to them. Those three forces are the electromagnetic force, the strong force, and the weak force, and they fit neatly into the Standard Model. But there is one more fundamental force: gravity.

And it's a little bit of an oddball because it's not neatly described by the physics of elementary particles, like the other forces are. So it doesn't fit into the Standard Model. And that's part of the reason there's no theory of everything that neatly ties up all the forces and particles in the universe.

But, even without a theory of everything, the Standard Model and general relativity do a solid job of describing almost everything in our world. Which is a pretty tall order, all things considered. These theories are the culmination of a century's worth of research into fundamental physics.

And it can be tempting to see fundamental physics as a puzzle with almost all the pieces in place. Which kind of sounds like the end of the road for physicists. But before anyone calls it a day or, like, converts to a biologist, there are a few things to consider.

First of all, there were plenty of times throughout history when scientists thought physics was basically complete… and each time, they were extremely wrong. For instance, in the 1920s, even after we discovered the mysteries of quantum mechanics and relativity, the physicist. Max Born still had the nerve to say that “physics, as we know it, will be over in six months.” Spolier!

It wasn't. Then, there was that time at the end of the 19th century when the physicist Albert Michelson said, “It seems probable that most of the grand underlying principles have been firmly established.” This guy was ready to call it quits before we even knew about quantum stuff and relativity! Physicists at the time had been so used to the laws of motion discovered by Isaac Newton that they expected them to work forever, for everything.

The thing was, by the time he'd said that, Michelson himself had already conducted an experiment that would go on to prove him —and the whole Newtonian worldview—wrong. The Michelson-Morley experiment, as it's called, provided strong evidence that there isn't any universal, absolute reference frame that everything can be measured relative to, as Newton's way of thinking suggested. That experiment was crucial in paving the way for relativity.

And the move from an absolute to a relativistic way of thinking about motion totally upended what we'd thought for centuries about how the universe works. As a result, Einstein had to come in and invent a whole new way to describe space and time. So, back in Michelson's day, the “grand underlying principles” of physics still had a long way to go.

Basically, every time someone like Born or Michelson thought they had all the answers, they realized that their framework was just a really good approximation of reality. Once you got out to certain extremes, that approximation started to break down. In other words, their frameworks did a good job of describing the world under certain conditions, but they weren't perfect explanations.

And the same thing is likely true of the Standard Model and general relativity. They seem fundamental because they describe the universe really well, but in extreme environments like black holes or the Big Bang, those frameworks still seem to break down. And there are still some problems with the laws we call fundamental.

I mean, just the fact that gravity doesn't mirror the other three fundamental forces suggests that some piece of the story is still missing. And there are other imperfections that constantly remind us that our fundamental theories are just a really good approximations of reality. Meaning it's almost certainly not the end of the road.

Instead, we're in a kind of weird, unprecedented era where physicists know our theories aren't complete, but they also have very little evidence for anything beyond it. That makes it much harder to make progress now than it was a hundred years ago. Because the thing is, Michelson and Morley were able to disprove the fundamental laws of their time using a lab experiment that can now be done in a college classroom.

But the times have changed. People have plucked all the low-hanging fruit. If you want to make discoveries about fundamental physics these days, you need really big experiments.

In 2012, the Large Hadron Collider at CERN in Switzerland discovered the last particle missing from the Standard Model, the Higgs boson. That was a gargantuan international collaboration involving billions of dollars, thousands of scientists and engineers, and an army of support staff. You're simply not going to find an undiscovered particle without that kind of tech, because you need something that can create conditions way more extreme than you get in everyday life.

And as much as theoretical physicists hoped that the Large Hadron Collider would find evidence of physics beyond the Standard Model, it simply hasn't. But the fact that it's difficult doesn't mean that there is nothing left to find. And the good news is, modern physicists understand this better than people like Michelson did back in the day.

So they're not claiming that the job is done. They know that there are still “grand underlying principles” that haven't been discovered, and tons of ways that our current theories are incomplete. Just like what happened with relativity back in the day, the solution to the problem will likely be a whole new theory that only looks like the Standard Model or general relativity under the right conditions.

Fundamental physics has reached a very high plateau with our current theories. But without enough experimental evidence to guide it higher, it's also kinda stuck in a rut. The good news is, there are still lots of ways theoretical physicists are pushing forward.

For one, finding ways to unify the fundamental laws of physics is still a huge area of research. In some cases, physicists know where the fundamental laws break down—like in black holes. There, both relativistic effects and quantum mechanical effects are relevant, but they don't agree on what should happen.

General relativity says black holes should evaporate into nothing, but quantum mechanics says that's not possible. So we simply don't know what's right. But scientists have begun figuring out ways to research those kinds of environments.

Like, to get around the fact that they can't exactly study things like black holes in labs, they sometimes use what are called analogous physical systems. So rather than study, say, a black hole directly, physicists study a system that has similar properties. For instance, one of the most important properties of black holes is that they don't let light escape.

So physicists found a way to make a similar system in a lab —except instead of trapping light, their system traps sound. This is called an acoustic black hole. And these things actually reproduce properties we'd expect to see in real black holes.

Scientists are hoping that they can help figure out the real fate of black holes, since relativity and quantum mechanics disagree. Another team of researchers found a way to put ultra-cold helium atoms in a state where they behave like Higgs boson particles, and they were able to use that to study properties of the Higgs even before they had discovered the Higgs boson itself. In general, you can use the concept of analogous systems to invent all sorts of unusual environments that'll make particles behave in ways they normally wouldn't, and that's been one way for physicists to push the limits of fundamental physics.

But these days, it's not the only way to study the fundamentals of reality. As computers get more and more powerful, simulations of physical systems have become much more common in fundamental physics research. And simulations have been a total game-changer, because in the past, just knowing the basic rules that govern a system wasn't enough to tell how the system would behave in practice.

For instance, it would take way too much number-crunching for a human to figure out how the laws of physics would play out over billions of years of galactic evolution. But with simulations, computers do the work of figuring out how the laws of physics play out under certain conditions —in a fraction of the time. Based on the results, scientists make predictions about how those systems behave in the real world.

And that can get us a long way! Like, predicting the chaotic movement of weather systems would be close to impossible without advanced computer simulations, no matter how well you understand how particles work. And simulations are currently the only way to test theories about things like the evolution of the early universe.

So even without new particles or forces, fundamental physics is still pushing forward. Clearly, theoretical physics isn't done. But it has changed, for sure.

The next plateau in our search for the theory of everything isn't going to be reached by a lone maverick working alone in a lab. It's going to take contributions from thousands of researchers across the globe, doing everything from writing equations to examining astronomical data to programming computers. The plateau we've reached with our latest theories is exciting, but in some ways the things a theory can't explain are more exciting than the things that it can.

Thanks for watching this episode of SciShow! If you're interested in learning more about fundamental physics, you can check out our four-part series that covers each one of the fundamental forces. It begins with the strong force, and you can get started with that video right after this. [♪ OUTRO].