On Tuesday and Wednesday, the first stable proton beams were injected into the Large Hadron Collider (LHC). Here’s a nifty videomajigger that explains the basic functions of the various devices which make up the LHC:

A few ‘quick’ explanations from someone who should totally not be giving them (welcome to the internets):

The “hadron” in Large Hadron Collider refers to a type of subatomic particle which is made up of “smaller” particles called quarks. These quarks are bound together by something called the strong force, and are pretty damned tough to pry apart (that’s why we have to collide ’em). The hadrons used in the LHC are mostly single protons, which, if you remember your high school chemistry, are hydrogen ions.

The video mentions that the goal of the LHC is to recreate the conditions present very shortly after the Big Bang. Why do this? The idea goes like this. An infinitesimal time after the Big Bang, the universe (as it was) was so hot and so dense that everything was a big homogeneous mess. If you were able to pick out any two particles and look at them, they would be exactly the same. Within a very small time period, however, the universe expanded and cooled. This cooling “created” from that one homogeneous mess four fundamental forces which we recognize today: gravity, the strong force, the weak force, and electromagnetism.

Bluntly put, all of these fundamental forces were once the same thing. The goal, then, is to figure out how they were unified and what exact mechanism led to their separation in the first place. This mechanism is referred to as “spontaneous symmetry breaking”. The classic way to explain symmetry breaking is with the following example. Say you have a ball sitting precisely at the top of a hill jutting up from the ground. Any very small movement of the ball will cause it to plunge down one of the sides of the hill. Before this movement, the ball had no preferred side. It was indifferent towards falling in any one direction. Thus ‘direction’ was a symmetry of the system. Once the ball falls, however, it prefers the direction it happens to be falling in. In that case, symmetry is broken under the action of falling. It now prefers a direction. Similarly, at some time very shortly after the Big Bang, the four fundamental forces disambiguated and became their own unique selves. They chose a “direction”, and have had to stick to that direction ever since.

The current model of particle physics is called the Standard Model, and it describes how three of the fundamental forces–the weak force, the strong force, and the electromagnetic force–are mediated via fundamental particles. It’s actually better than that, because we’ve been able to unify two of these forces–electromagnetism and the weak force. This unified theory is called the electroweak theory.

One prediction of the Standard Model is a particle called the Higgs boson. This is a pesky little part of the standard equations (developed by a guy named Higgs, obviously) which we’ve not yet been able to experimentally verify. To understand why physicists think such a particle exists, you have to know a little bit about the Standard Model. This model is what’s called a quantum field theory. Quantum field theories postulate that the universe is composed of a sea of ‘virtual particles’ (quick aside, there is some evidence for these virtual particles). The actual particles that we observe–electrons and protons and so on–are simply eddies or bubbles in that sea. (Pick your analogy, because none of them are exactly apropos.) This means that given any quantum field, there should be an associated particle.

But why should the Higgs boson exist? Well, the Standard Model predicts something that is.. uh, very wrong. It predicts that most things in the universe do not have mass. Yeah, kind of a big deal. Well, a guy named Peter Higgs thought, “Hey, you know these quantum field theories have been really successful. I bet you anything that mass results from another one of these fields.” Direct quote, no lie. (Ok, lie.) So Higgs developed the framework for this, and a guy named Steven Weinberg integrated it into the model. Lo and behold, when you plug everything in things suddenly get mass again. And it doesn’t eff up any of the other parts of the model. Bonus. Well, if there’s a field, there’s an associated particle. Hence there should be a Higgs particle.

Well what about gravity? The Standard Model describes three of the four forces as exchanges of quantum particles. The electromagnetic force results from the exchange of photons. The strong force results from the exchange of particles called gluons. The weak force results from the exchange of two particles called the W and Z bosons. We know how these particles work (more or less), and we’ve verified their existence experimentally. Presumably there exists a particle which mediates the gravitational force, too (which is tentatively called the graviton). Unfortunately, when we try to write down a theory for such a particle, it doesn’t pan out. It turns out that it works when you’re talking about everyday distances, but not for quantum-sized distances. Which is a problem, because all of the other forces work perfectly fine at any distance.

All that flapdoodle you hear about string theory is exactly the attempt to solve the problem of quantum gravity. The LHC comes into this because there’s a non-zero chance that it will verify one or two of the predictions of string theory (if string theory is in fact true). There are other non-trivial things physicists will be looking for as well. Symmetry Magazine is a good place to start if you want to learn a whole lot more.


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