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The absolutely amazing theory of almost everything

Abstract particle physics illustration

How does our world function at the subatomic level?

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

More than a quarter of the Nobel Prizes in physics in the last century are direct input to or direct results of the standard model. Still, the name suggests that if you can afford a few extra dollars a month, you should buy the upgrade. As a theoretical physicist, I would prefer The Absolutely Amazing Theory of Almost Everything. That's what the standard model really is.

Many remember the excitement among scientists and the media over the discovery of the Higgs boson in 2012. But the very troubled event did not come out of the blue - it ended a five-decade undefeated series for Standard Model. Every basic force except gravity is included in it. Any attempt to overthrow it to demonstrate in the laboratory that it needs to be significantly reworked - and there have been many of these over the last 50 years - has failed.

In short, the Standard Model answers this question: What is everything made of and how is it related?

The smallest building blocks

You know, of course, that the world around us is made of molecules, and molecules are made of atoms. Chemist Dmitry Mendeleev figured out in the 1860s how to organize all the atoms - that is, the elements - into the periodic table that you probably studied in middle school. But there are 118 different chemical elements. There are antimony, arsenic, aluminum, selenium… and 114 more.

The periodic system

However, these elements can be further degraded. Credit: Rubén Vera Koster

Physicists like things simple. We want to boil things down to their essence, a few basic building blocks. Over a hundred chemical elements are not simple. The ancients believed that everything is made of only five elements - earth, water, fire, air and ether. Five is much simpler than 118. That is also wrong.

In 1932, scientists knew that all these atoms are made up of only three particles - neutrons, protons and electrons. The neutrons and protons are bound tightly together into the nucleus. The electrons, which are thousands of times lighter, swirl around the nucleus at speeds approaching that of light. Physicists Planck, Bohr, Schroedinger, Heisenberg, and friends had invented a new science - quantum mechanics - to explain this motion.

It would have been a satisfying place to stop. Only three particles. Three is even simpler than five. But how to keep together? The negatively charged electrons and positively charged protons are bound together by electromagnetism. But the protons are all gathered in the nucleus, and their positive charges should push them sharply apart. The neutral neutrons can not help.

What binds these protons and neutrons together? "Divine intervention" a man on a street corner in Toronto told me; he had a booklet I could read all about it. But this scenario seemed like a lot of trouble even for a divine being - to keep track of each and every one of the universe's 108 ° protons and neutrons and bend them to its will.

Expansion of the zoo of particles

Meanwhile, nature cruelly refused to keep its zoo of particles at only three. Really four, because we have to count the photon, the light particle, as Einstein described. Four grew to five as Anderson measured positively charged electrons - positrons - hitting Earth from outer space. At least Dirac had predicted these first antibody particles. Five turned into six when the peony that Yukawa predicted would hold the core together was found.

Then came the muon - 200 times heavier than the electron, but otherwise a twin. "Who ordered it?" II Rabi joked. That sums it up. Number seven. Not only not simple, superfluous.

In the 1960s, there were hundreds of "fundamental" particles. Instead of the well-organized periodic table, there were just long lists of baryons (heavy particles like protons and neutrons), mesons (like Yukawa's pioneers) and leptons (light particles like the electron and the elusive neutrinos) - without organization and no guiding principles.

The standard model came into this fracture. It was not a glimmer of brilliance overnight. No Archimedes jumped out of a bathtub and shouted "eureka." Instead, there were a number of crucial insights from a few key figures in the mid-1960s that transformed this swamp 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 an ingredient list for everything around us. Credit: Fermi National Accelerator Laboratory

Quarks. They come in six varieties, we call flavors. Like ice cream, except not so tasty. Instead of vanilla, chocolate and so on, we have up, down, strange, charm, bottom and top. In 1964, Gell-Mann and Zweig taught us the recipes: Mix and match all three quarks to get a baryon. Protons are two ups and a down quark bound together; neutrons are two downs and one up. Choose a quark and an antique quark to get a meson. A peony is an up- or a down-quark bound to an anti-up or an anti-down. All the material in our daily lives is made of only up and down quarks and antiquarks and electrons.

Simple. Well, simple-ish, because keeping those quarks tied is a feat. They are tied together so tightly that you will never ever find a quark or anti-quark alone. The theory of the bond and the particles called gluons (the bell) that are responsible is called quantum chromodynamics. It is an important part of the standard model, but mathematically difficult, and even poses an unsolved problem with basic mathematics. We physicists do our best to reckon with it, but we are still learning how.

The second aspect of the standard model is "A model of leptons." It is the name of the landmark paper from 1967 by Steven Weinberg that brought together quantum mechanics with the vital pieces of knowledge about how particles interact and organized the two into a single theory. It incorporated the well-known electromagnetism, combining it with what physicists called "the weak force" that causes certain radioactive decays, and explaining that they were different aspects of the same force. It incorporated the Higgs mechanism to provide mass to fundamental particles.

Since then, the standard model has predicted the results of experiment after experiment, including the discovery of several variants of quarks and of the W and Z bosons - heavy particles that are for weak interactions, what the photon is for electromagnetism. The possibility that neutrinos are not massless was overlooked in the 1960s, but slipped easily into the standard model of the 1990s, a few decades too late for the party.

CERN Particle Accelerator SM Higgs Boson Decay

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

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 decisive victory for the Standard Model over the dark forces that particle physicists have repeatedly warned of towering up on the horizon. Concerned that the standard model did not adequately embody their expectations of simplicity, worried about its mathematical self-consistency, or looking forward to the possible necessity of bringing gravity into the fold, physicists have put forward several proposals for theories beyond the standard. Model. These bear intriguing names such as Grand Unified Theories, Supersymmetry, Technicolor and String Theory.

Unfortunately, at least for their followers, beyond-standard model theories have not yet successfully predicted any new experimental phenomenon or any experimental inconsistency with the standard model.

After five decades, far from requiring an upgrade, the standard model is worthy of celebration as the absolutely amazing theory of almost everything.

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

This article was first published in The Conversation.The conversation

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