John Oliver: So, roughly speaking, what are the chances that the world is going to be destroyed? One-in-a-million? One-in-a-billion?
Walter Wagner: Well, the best we can say right now is a one-in-two chance.
Walter: Yeah, 50-50… It’s a chance, it’s a 50-50 chance.
John: You come back to this 50-50 thing, what is it Walter?
Walter: Well, if you have something that can happen and something that won’t necessarily happen, it’s going to either happen or it’s gonna not happen. And, so, it’s kind of… best guess at this point.
John: I’m… not sure that’s how probability works, Walter. –The Daily Show
At its simplest, most fundamental level, it’s hard to imagine anything simpler than the particle physicist’s favorite toy, the collider. Just take two things moving very quickly in opposite directions, and smash them together!
Okay, okay, two particles. Take two particles and smash them together. Why would you want to do that?
Because the more energy you give these guys, the more energy is available to create new matter, perhaps even to create some rare, never-before-seen particles of ultra-high, unstable masses!
The key, of course, is twofold: getting to those incredibly high energies and also being able to detect the stuff that comes out! Now, the big collider that everyone knows about is, of course, the Large Hadron Collider, or LHC for short.
At 26 kilometers in circumference, it’s certainly one of the largest particle accelerators in the world. But, I’m sure you’re curious, how does it work?
Well, any type of accelerator has three things to worry about:
- You need to accelerate this particle, and that’s going to take a force. For all the accelerators we use, that force comes from electric fields acting on the particle’s charge. The more massive the particle, the bigger the electric field you need to accelerate it.
- You need to make sure you can steer these particles, and that takes magnetic fields. If you want your particles to go faster and you need to bend them into a circle, you’re going to need ever stronger magnetic fields. And…
- You need to overcome the limits of synchrotron radiation. (This one’s tricky; more on it later.)
So, you ask, how do we do all this? Let’s head on into the tunnel and have a look!
This is the huge, curved tunnel that houses the LHC. (The second most powerful collider on Earth, Fermilab, has one that looks just like this, but it’s only about 7 km in circumference.)
Once you’ve got your big circle, you need to accelerate your particles up to speed and keep them moving in the circular path you need! How do we do that?
Giant electromagnets! Yes, seriously. An electromagnet is special, because you can vary the amount of magnetism it generates simply by varying the amount of current in the electromagnet. This means, as you get that particle moving faster and faster, you can ramp that magnet up to keep it moving in the same circle! And, as long as you’ve got an electric field going in there, too, you can get that particle going as fast as you want!
That is, of course, limited by the strength of your magnet and the radius of your collider. With superconducting electromagnets, it’s going to be tough to beat what they’re doing at the LHC. Even at that, what they’re doing at the LHC barely beats what they’ve been doing at Fermilab for nearly 20 years!
So, you say, maybe I’ll just build a bigger collider!
Why not build one around the circumference of a whole planet, in fact?! Well, that’s where prohibitive cost comes in. Can we find a clever way to probe the nature of matter without having to spend hundreds of billions of dollars to do it?
Those of you who’ve been following particle physics for some time, you may remember that the LHC hasn’t always been the rage at CERN. There was a large collider in the same tunnel there before.
The Large Electron Positron (LEP) collider! However, instead of using protons, LEP used electrons and anti-electrons (positrons). At first thought, you might think this is a huge advantage, and in a lot of ways, it is.
Unlike protons, where you’ve got three quarks and a myriad of gluons splitting up the energy, the electron (and the positron) is just a single point, containing all of the energy and momentum. Also, although they have the same charge, the electron is only 1/1836th as massive as the proton, meaning that if you apply the same force to an electron and a proton, the electron will accelerate 1836 times as much!
How can you go wrong, you ask, and why aren’t all colliders using electrons and positrons?
It wasn’t just LEP, many of the older colliders did. The Stanford Linear Accelerator Center (SLAC), above, was one, and the proposed International Linear Collider (ILC), below, would be another.
But they’re straight lines! Sure, you don’t need the fancy electromagnets, but you only get one chance to accelerate them! Why would I spend all my money trying to build a super-long, super powerful beam, wasting it all on these accelerating cavities:
Why not just put these in a giant circular path? Well, that’s where synchrotron radiation will get you. You see, when you put a charged particle in a magnetic field, the field doesn’t just steer it. It also causes it to emit radiation. For a proton, it isn’t a big deal; the fact that the proton is so massive means the radiation emitted is negligible. But for an electron…
Well, an electron simply can’t handle a strong magnetic field; it will radiate its energy away and will simply refuse to go faster unless you turn the field off. Which is why we need to put it in a straight line.
And for some reason, those are the only options people talk about. Either go from the Large Hadron Collider up to a Very Large Hadron Collider for protons (which people have been talking about since at least 1997, when I first started out in physics), or go back to electrons and build a huge Linear Collider.
There is, of course, another option. What if we could find a particle that had the best of both worlds? What if there were a single particle (not a composite one, like the proton) that carried all the mass, energy, and momentum of a system, would respond to electric and magnetic fields, but wouldn’t give off synchrotron radiation like an electron. Ideally, this would be a particle that was the same (magnitude) charge as either an electron or a proton, but with a mass in between the two; much heavier than an electron but still lighter than the proton.
That’s what a muon is! Why not go and build a giant muon collider?!
At 207 times the mass of the electron, synchrotron radiation is no longer an issue. But there is an issue: the muon is unstable!
They’re easy enough to make, but with a mean lifetime of only 2.2 microseconds, what chance will we have to accelerate them and collide them before they decay away?
It’s actually really simple. Make them moving relativistically, or close to the speed of light! Because of time dilation, as long as you get them moving fast enough, they’ll live long enough to do whatever you want them to.
Now, if only we had a place to make them relativistically and then put them, immediately, into some sort of accelerator. If only…
Why hello there, Fermilab. With your high-luminosity main injector, looking like the perfect candidate to create relativistic muons, and your soon-to-be-decommissioned main ring (a.k.a. Tevatron) just waiting to be repurposed for such an exciting project.
There’s already a team trying to figure out how to make this happen, and I admit, there are many technical challenges to overcome. But the future of colliders isn’t in building them ever bigger and more expensive, it’s in building them smarter. As long as you can get an interesting luminosity — or a high enough collision rate (which, with the main injector now in place, shouldn’t be a problem) — this could very well be the future of particle physics!