The Absolutely Incredible Theory of Almost Everything

How does our world work at the subatomic level?

The standard model. What a boring name for the most accurate scientific theory known to human beings.

More than a quarter of the Nobel Prizes in Physics of the last century are direct inputs or direct results of the Standard Model. Still, its name suggests that if you can afford a few extra bucks a month, you should buy the upgrade. As a theoretical physicist, I would prefer The Absolutely Incredible Theory of almost anything. That’s what the standard model really is.

Many remember the enthusiasm of scientists and the media over the 2012 discovery of the Higgs boson. But this high-profile event didn’t come out of nowhere — it capped a five-decade unbeaten streak for the standard model. All fundamental forces except gravity are included. All attempts to overthrow it to demonstrate in the laboratory that it needs substantial reworking – and there have been many over the past 50 years – have failed.

In short, the standard model answers this question: what is it all made of and how does it fit together?

The smallest building blocks

You know, of course, that the world around us is made of molecules, and molecules are made of atoms. Chemist Dmitri Mendeleev discovered in the 1860s how to arrange all of the atoms – aka the elements – in the periodic table that you probably studied in college. But there are 118 different chemical elements. There’s antimony, arsenic, aluminum, selenium… and 114 others.

Periodic table

But these elements can be broken down further. Credit: Ruben Vera Koster

Physicists like simple things. We want to boil things down to their essence, a few basics. More than a hundred chemical elements is not simple. The ancients believed that everything was made up of only five elements: earth, water, fire, air and ether. Five is much simpler than 118. It’s also wrong.

In 1932, scientists knew that all these atoms were made up of only three particles: neutrons, protons and electrons. Neutrons and protons are tightly bound in the nucleus. Electrons, thousands of times lighter, whirl around the nucleus at speeds close to the speed of light. The physicists Planck, Bohr, Schroedinger, Heisenberg and their friends had invented a new science – quantum mechanics – to explain this movement.

It would have been a satisfying place to stop. Just three particles. Three is even easier than five. But how to hold together? Negatively charged electrons and positively charged protons are bound by electromagnetism. But the protons are all packed into the nucleus, and their positive charges should powerfully push them away. Neutrons can’t help.

What binds these protons and these neutrons? “Divine intervention” a man told me on a Toronto street corner; he had a pamphlet, I could read all about it. But this scenario seemed to pose a lot of problems, even for a divine being – to keep an eye on every one of the 108° protons and neutrons in the universe and bend them to his will.

Expand the Particle Zoo

Meanwhile, nature has cruelly refused to limit its particle zoo to just three. Really four, because you would have to count the photon, the particle of light described by Einstein. Four changed to five when Anderson measured electrons with a positive charge – positrons – hitting Earth from space. At least Dirac had predicted these first antimatter particles. Five became six when the pawn, which Yukawa believed would hold the core together, was found.

Then came the muon – 200 times heavier than the electron, but otherwise a twin. “Who ordered this? II Rabi joked. That sums it all up. Number seven. Not only not simple, redundant.

In the 1960s, there were hundreds of “fundamental” particles. Instead of the neatly organized periodic table, there were only long lists of baryons (heavy particles like protons and neutrons), mesons (like Yukawa pions) and leptons (light particles like the electron and the elusive neutrinos) – with no organization and no guiding principles.

Into this breach has slipped the standard model. It wasn’t an overnight flash of brilliance. No Archimedes jumped out of a bathtub shouting “eureka”. Instead, there was a series of crucial ideas from a few key people in the mid-1960s that turned this quagmire into a simple theory, and then five decades of experimental verification and theoretical elaboration.

Standard model of elementary particles

The standard model of elementary particles provides a list of ingredients for everything around us. Credit: National Fermi Accelerator Laboratory

Quarks. They come in six varieties which we call flavors. Like ice cream, except it’s not as tasty. Instead of vanilla, chocolate and so on, we have high, low, weird, charm, low and high. In 1964, Gell-Mann and Zweig taught us the recipes: mix and match three quarks to get a baryon. Protons are two ups and one down quark bound together; neutrons are two lows and one high. Choose a quark and an antiquark to get a meson. A pion is an up or down quark bound to an anti-up or an anti-down. All the material of our daily life is composed only of quarks, anti-quarks and electrons.

Simple. Well, it’s simple, because keeping those quarks bound is a feat. They are so closely related to each other that you never find a single quark or antiquark. The theory of this binding, and the particles called gluons (laughs) that are responsible for it, is called quantum chromodynamics. It is an essential piece of the standard model, but mathematically difficult, even posing an unsolved problem of basic mathematics. We physicists do our best to calculate with it, but we are still learning how.

The other aspect of the standard model is “a lepton model”. It’s the name of Steven Weinberg’s landmark 1967 paper that brought together quantum mechanics with the essential knowledge of how particles interact and organized the two into a single theory. He incorporated the familiar electromagnetism, associated it with what physicists called “the weak force” that causes certain radioactive decays, and explained that they were different aspects of the same force. It incorporated the Higgs mechanism to give mass to fundamental particles.

Since then, the Standard Model has predicted the results of experiment after experiment, including the discovery of several varieties of quarks and W and Z bosons, heavy particles that are to weak interactions what the photon is to electromagnetism. The possibility that neutrinos are not massless was overlooked in the 1960s, but slipped easily into the Standard Model in the 1990s, a few decades later.

CERN particle accelerator Higgs boson decay SM

3D view of an event recorded at the CERN particle accelerator showing the expected characteristics of the decay of the SM Higgs boson into a pair of photons (dashed yellow lines and green towers). Credit: McCauley, Thomas; Taylor, Lucas; for the CMS CERN collaboration

Discovering the Higgs boson in 2012, long predicted by the Standard Model and long sought after, was a thrill but not a surprise. It was yet another crucial victory for the Standard Model over the dark forces that particle physicists have repeatedly warned were looming on the horizon. Concerned that the Standard Model did not adequately embody their expectations of simplicity, worried about its mathematical consistency, or anticipating the possible need to incorporate the force of gravity, physicists made numerous proposals for theories beyond of the norm. Model. These go by exciting names like Grand Unified Theories, Supersymmetry, Technicolor, and String Theory.

Unfortunately, at least for their proponents, theories beyond the Standard Model have not yet successfully predicted a new experimental phenomenon or an experimental divergence from the Standard Model.

After five decades, far from needing an upgrade, the Standard Model deserves to be celebrated as an absolutely incredible theory of almost everything.

Written by Glenn Starkman, Distinguished University Professor of Physics, Case Western Reserve University.

This article first appeared in The Conversation.The conversation

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