The big physics story over the weekend was the re-start of the Large Hadron Collider at CERN, the world’s largest and highest-energy particle accelerator. It was initially started in 2008, but some key circuits failed shortly after it was switched on. A relatively quick patch job allowed it to operate at half its designed energy for a few years, long enough to discover the Higgs boson and secure a Nobel Prize for two of the half-dozen theorists with a claim to have invented it, then it shut down for two years of more comprehensive repairs. It’s back now, and better than ever, hopefully able to begin colliding protons at its original specs within the next several months.
But you might be asking “Why is this a big deal, anyway?” Well, it’s a big deal because our very best theory of fundamental physics is wrong, and everybody knows it. We just don’t know how it’s wrong.
The theory in question is called the “Standard Model” because physicists are bad at names, and it’s wrong not in the boring sense of being incorrect– the subtitle of Robert Oerter’s excellent book rightly calls it “The Unsung Triumph of Modern Physics,” and it ranks as one of the greatest intellectual achievements in human history–but “wrong” in the sense of being incomplete. As Oerter’s title puts it, it’s a theory of almost everything, brilliantly successful at describing the things that it includes, but unable to explain a number of features of the universe in which we live.
The Standard Model consists of a collection of material particles– six types of quarks (up, down, strange, charm, top, and bottom), the electron, muon, and tau particles, and electron, muon, and tau neutrinos (these last six are collectively called “leptons”)– and a collection of force-carrying particles associated with the fundamental interactions of physics– the photon conveys electromagnetism, the W and Z bosons carry the weak nuclear force, and eight types of “gluons” that carry the strong nuclear force. The Higgs boson completes the package, the last piece of the theoretical edifice tying the weak and electromagnetic forces together.
The Standard Model, however, is unable to explain some of the properties of these particles– why neutrinos have mass, for example. It also offers no explanation for dark matter (which we know is real because it’s like a card game), or even the fact that everything we see in the universe is made of matter, and not antimatter. In the very simplest theories of physics, the Big Bang should’ve produced matter and antimatter in equal amounts, which would then annihilate each other leaving nothing but energy. We understand the basic mechanism by which the Big Bang might’ve produced a slight excess of matter that survived to become, well, everything we see, but the properties of the Standard Model particles don’t lead to a big enough excess.
Those are pretty big gaps, and trying to fill them is a big part of the activity of modern theoretical physics. There are at least as many ways of extending the Standard Model as there are theoretical physicists trying to extend the Standard Model, and essentially all of them predict the existence of new types of particles. Looking for evidence of these new particles is a big part of the activity of modern experimental physics, and the Large Hadron Collider is the most spectacular part of that effort.
It is, however, only one piece of that effort, and other pieces are interesting in their own right. The overall search for exotic physics can be broken down into three broad classes of experiments: colliders, observatories, and precision measurements.
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