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The Standard Model Part II: What It Explains

In part I of this series, we toured the "particle zoo" -- the "periodic table" of particle physics. Now, let's ask, what can you actually do with all these strange animals? The Standard Model (SM) was built by patching together several different theories, each of which accounted for some smaller chunk of the overall workings of the world on the small (atomic) scale. These abilities are all combined in the SM.

The part of the model that describes electrical phenomena is called QED-- quantum electrodynamics. It uses the photon and the electrical charges in the table. This theory was the first part of the model to be developed, and has several mind-boggling features. Its calculations are so complex that they can run to several hundred pages. More than a decade of work by Feynman and others was required to keep the theory from giving the answer of infinity for all but the most basic calculations. This is because particles are allowed to interact with themselves, and they can do this an infinite number of times. Since the theory is built on considering every single possible thing that can happen to the particle, if an infinite number of things can happen to it...

This might lead you to suspect that the theory isn't very good. Perhaps the craziest fact of all about QED is that it is one of the the most accurate tools that theoretical physics has ever produced. As with many of the things that Feynman had a hand in, this theory is extremely good at calculating precise answers. It can give the correct answer to certain problems with an accuracy of over 99.999999%!

QCD (quantum chromodynamics), which uses the quarks and gluons, was the next part of the theory to be put in place. This theory is used for calculating many things in nuclear physics. It also leads to some amazing predictions. For example, if you try to pull a group of quarks apart, the strong force holding them together is so strong (Aha!) that instead of being pulled apart, entirely new quarks are formed from nothing which instantly combine into new groups. No one has ever been able to split a proton or neutron and see the individual quarks that make it up!

Neutrinos were added to the model because it appeared that a tiny, tiny amount of matter or energy was disappearing in certain reactions! Matter wasn't really disappearing, it was turning into neutrinos which are almost impossible to see. If mass or energy were actually evaporating in experiments, several centuries of physics and chemistry would have to be rewritten completely!

When you add these basic features and build some more complex ones off of them, you arrive at a fairly comprehensive theory of atomic goings-on. Is the SM complete? The answer is no! There are two major gaps in the model. 

One of these is mass. This is less esoteric; it is quite obvious that things around us have mass. If you push them, they resist you! This is called inertia, and mass tells you how much inertia something has. The standard model predicts a particle that allows for objects to have mass, the "Higgs boson" (also known by the bizarre moniker "God particle"). The problem is that, unlike all of the particles in the table, this one has not yet been seen by anyone in the real world! The primary purpose of the LHC is to allow physicists to find the Higgs. Of course they will be much happier if they do not find it and consequently enjoy years of employment redoing their theories! Very soon they will know.

The other hole in the theory gapes even larger: it does not contain gravity! If the SM were the whole story, we would float into space. (Or, more realistically, the Universe could never form into galaxies and stars and planets at all!) There have been many, many attempts to build gravity into this model. Every single one has failed due to how fundamentally dissimilar the theories of gravity and particle physics are to each other!

Next time: Einstein, gravity, the consequences of Higgs boson/no Higgs boson, string theory and what lies beyond the standard model.

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Tom Hartsfield
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