When you think of a particle accelerator,  you probably picture something like this:   A machine the size of a city with thousands of  giant magnets and massive underground tunnels.

Because for decades, physicists have been  obsessed with building bigger and more   powerful accelerators to unlock deeper  and deeper secrets about the universe. But recently, at least two teams of  scientists have gone the opposite   direction: They've built particle  accelerators the size of a penny.

And if that seems useless, well, so far it is. But eventually, mini accelerators just  might revolutionize how we treat cancer,   manufacture electronics, and even explore  the fundamental nature of reality. [♪ INTRO] Particle accelerators aren’t just  for physicists smashing protons   together and creating exotic forms of matter. At their core, they’re just machines  that get particles moving fast.

That’s it. And there are all sorts of ways that’s useful. If you’re a doctor, you can annihilate tumor  cells by blasting them with electron beams.

If you work in a semiconductor factory,   you can fine-tune the properties of silicon  by bombarding it with charged particles. We even use accelerators to  produce high-energy X-rays for   scanning cargo coming in and out of the country. Depending on what they’re meant for, accelerators  can take all kinds of shapes and sizes.

But at your most basic level, all you really  need are some particles coming from somewhere,   a tube to shoot them through, and some  kind of mechanism for accelerating them. Scientists built the first-ever particle  accelerators back in the late 1800s, hoping to   explore the basic principles of electricity  and figure out what matter was made of. One of these was a little table-top  machine called a cathode ray tube.

This was a glass tube that had all the air  sucked out and two metal pieces on either end. One of them, called a cathode,   was negatively charged, while the anode  on the other end was positively charged. That created an electric field between  the two ends, and if it was strong enough,   it would peel electrons off the cathode and pull  them through the tube, smack into the anode.

Some physicists used these devices  to explore electricity and matter. But others put the technology right to work. They used it to shoot electrons at the backs  of old-school TV screens and computer monitors.

These were coated with a material  like phosphorus that would glow   when the electrons hit them, creating an image. But this was just the beginning. In the 20th century, particle accelerators  started getting bigger and more powerful.

By the late 1920s and 30s, newer  accelerators used a special type   of circuit called a Cockroft-Walton generator. This machine was the size of a  large room, and it worked like   a kind of electrical ladder that gradually  raised electrons to higher energy levels. It did this by sending an alternating current,   or a current that keeps reversing  direction, through the circuit.

But the way the circuit was designed,  electricity could only flow one way. So when the current flip-flopped,  the electrons couldn’t travel back. They kept working their way up the circuit,   picking up energy until they flew  off the top in a high-energy beam.

In 1932, one of these accelerators  succeeded in smashing protons into   a lithium ion, breaking it into two helium nuclei. It was the first time anyone ever  changed one element into another. And this breakthrough, called transmutation,   paved the way for the nuclear age  that would begin a decade later.

But physicists weren’t about to stop there. They weren’t just thinking about  making TV screens or nuclear power. They dreamed of uncovering what the universe  was made of on the most fundamental level.

But they were stuck. No matter how strong your microscope is,   it physically can’t make out details smaller than  the wavelength of light you’re observing with. That means anything smaller  than an atom is just invisible.

But scientists knew that if they could  smash atoms open and trace the fallout,   they could indirectly figure  out what it was made of. To do that, they needed to get particles  moving at some seriously high speeds. And this is when accelerators started getting big.

In the early 1930s, one scientist at  UC Berkeley came up with a new kind of   accelerator that he called  a “proton merry-go-round.” The first one he made was  only about the size of a CD. It worked by whirling protons around in a  circle, using powerful magnets to steer them. Each time they came around, they got  a little kick and picked up speed,   until finally the accelerator  flung them off like a slingshot.

But the problem was, particles get harder to   steer as they move closer and  closer to the speed of light. That’s caused by relativistic  effects, but essentially,   it’s just like how turning a car at highway  speeds is harder than turning at slow speeds. So, to pull this off in an accelerator, you  either need more force to make the beam turn,   or a bigger circle so the  turn is less sharp … or both.

As physicists broke through to higher  and higher energies, they began building   bigger magnets and bigger accelerator  rings just to keep the beam on track. And their work paid off! They were able to slam protons into a target  so hard that their nuclei smashed open.

Meanwhile, other kinds of  accelerators were getting bigger too. Some researchers started stacking  accelerators together in long   straight lines to create what’s  called a linear accelerator. In these machines, particles would get a boost  of energy at different points along the way.

And the limit was really just how  many tubes you stacked together. In 1962, the U. S. government began building  the Stanford Linear Accelerator in California.

It stretched over 3 kilometers,  and it got electrons whizzing up   to just 2 centimeters per second  slower than the speed of light. And accelerators were about to get even bigger. In the mid-twentieth century, governments  around the world were pouring money into   huge research projects with lofty goals — things  like the Manhattan Project and the Moon Landing.

It was the era of Big Science. Giant teams of scientists were coming together  to make impossible-seeming things happen. And it was happening in the world  of particle accelerators too.

In the 1960s, the U. S. established a national laboratory  in Illinois called Fermilab. It eventually became home to a circular  accelerator called the Tevatron that was   over 6 kilometers long and boosted protons  to over 99.9999% the speed of light.

Then in 2008, the Large Hadron Collider in  Switzerland overtook the Tevatron as the   world's most powerful accelerator,  with a loop 27 kilometers long. These accelerators did some incredible work. They recreated energies that last  existed at the origin of the universe.

And they detected rare particles,   like the Higgs boson, that only exist  under the most extreme conditions. Over the years, these discoveries helped us  build a picture of what underpins the universe,   and left physicists dreaming about what they could  discover with bigger and bigger accelerators. But meanwhile, in the medical field,  researchers were pivoting the other way.

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The sale ends August 15th, so don’t wait! Time, and magnets, are running out! Scientists wanted to find a way to use beams  of electrons to destroy cancerous tumors.

And there, the challenge wasn’t  reaching record-breaking energies. It was producing a controlled beam … with  a machine that could fit inside a hospital. So, over the next few decades, as  physics accelerators were growing   to the size of cities, medical  ones gradually got smaller.

One newer innovation in accelerator  technology helped with that. It turned out that the same pulses of  microwave radiation that had produced   radar signals in World War II could also be used  to create electric fields inside an accelerator. That’s because any kind of light wave  is an oscillating electric field.

That’s just what it means to be light. And you can use this field  to push electrons around,   just like in a cathode ray  tube or any other accelerator. But the benefit is, there’s a simple  way to amp up that electric field: If you aim a beam of microwaves  into a little chamber, you can   trap those waves between the walls  in what’s called a standing wave.

And then, if you keep sending  in pulses of microwaves,   that standing wave will get bigger and bigger. It’s kind of like if you have  a swing going back and forth,   and then you keep pushing it  so it goes higher and higher. This setup lets you squeeze a bunch  of energy into that one standing wave.

So you can create a powerful electric field in a  much smaller space than previous accelerators did. Machines like the Tevatron and  the LHC were already using this   technology — that’s what let them  reach absolutely colossal energies. But the designers of medical accelerators began  adopting it too, just on a much smaller scale.

And over time, that scale continued shrinking. Physicists began setting up electric  fields with higher-frequency waves,   which were physically smaller, so  accelerators could be smaller too. Meanwhile, a bunch of different tweaks  to parts like vacuum pumps, magnets,   and energy sources also helped  make accelerators more compact.

Over 70 years, they went from the size  of entire rooms to the size of closets. But some researchers were still aiming smaller. They wanted to build accelerators the  size of microchips, so they could apply   radiation precisely where it needs to  go and avoid damaging healthy cells.

And in 2023, a team of researchers based  mostly in Germany made some exciting progress. They borrowed some technology  that’s used to build microchips   and used it to build an adorable little  accelerator that would fit on a penny. Clearly there’s not much of a  runway for boosting particles here,   so these scientists had to get really creative.

For their accelerator tube, they built  a tiny channel out of silicon pillars. The whole thing was only half a millimeter long. So we’re talking tiny here.

Then they shot laser pulses at the pillars. That created an electric field that  whisked electrons down the tube. And in that short distance, they  were able to boost their electrons   by 12,000 electronvolts over just  that half-millimeter-long channel.

That’s about as powerful as one  of those old-school TV monitors,   but nowhere near the millions of electron  volts needed for clinical purposes. Then, soon after that, a separate research  group based at Stanford published an article   about a similar device that managed to reach  double the energy of that first accelerator. And what’s exciting about that  is that the two tiny accelerators   could work together as a mini linear accelerator.

The first accelerator can get things going  and then pass the electrons off to the second   accelerator, kind of like an egg-and-spoon relay  race, except with electrons instead of eggs. And this setup is extremely efficient. Even though the Large Hadron Collider is 54  million times the size of this accelerator,   the little guy is around 100 times more efficient.

The challenge now is just scaling up the power and  intensity without scaling up the size too much. Eventually, accelerators like these  could find their way into hospitals. These days, high-energy electrons can’t be used in  radiation treatment because they’d burn the skin.

But in tiny tubes, accelerators  like this could go under the skin   to target tumors without harming the patient. That could be huge for cancer treatment. But these miniature accelerators  are about more than just that.

More broadly, they’re a new  way of accelerating particles. If physicists scale up the basic technology,   maybe we could make massive accelerators  even more powerful than the LHC. For now, these mini-accelerators don’t really do   anything… other than take a bunch  of electrons on a very short ride.

But honestly, the first-ever accelerators  didn’t do much more than that either. And yet, without them, we’d never have developed   radiation treatment or built  giant machines like the LHC. So sometimes, we need an  invention that does nothing.

Because it can lead to inventions we  couldn’t have imagined without it. [♪ OUTRO]