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-------------------- 2577 - UNIVERSE - What is the universe made of?
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- The Standard Model in physics answers this question: What is everything made of, and how does it hold together? Then you can figure how much it all weighs and even how old it is.
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- The world around us is made of molecules, and molecules are made of atoms. Chemist Dmitri Mendeleev figured that out in the 1860s and organized all atoms, and the elements, into the periodic table. The Periodic Table has 118 different chemical elements.
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- By 1932, scientists knew that all those atoms are made of just three particles, I.e.: neutrons, protons and electrons. The neutrons and protons are bound together tightly into the nucleus. The electrons, thousands of times lighter, whirl around the nucleus at speeds approaching the speed of light.
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- The negatively charged electrons and positively charged protons are bound together by electromagnetism. But the protons are all huddled together in the nucleus and you would think their positive charges should be pushing them powerfully apart. The neutral neutrons can’t help, they have no charge.
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- What binds these protons and neutrons together? Four particles, including the photon, grew to five particles when Anderson measured electrons with positive charge, “positrons” , striking the Earth from outer space. Dirac had predicted these first anti-matter particles long before they were discovered..
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- Five became six when the pion, which Yukawa predicted would hold the nucleus together, was found. Then came the muon, 200 times heavier than the electron, but otherwise a twin particle.
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- By the 1960s there were hundreds of “fundamental” particles. In place of the well-organized periodic table, there were just long lists of baryons (heavy particles like protons and neutrons), mesons (like Yukawa’s pions) and leptons (light particles like the electron, and the elusive neutrinos). Baryons, mesons and leptons were multiplying faster than we could name them.
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- The Standard Model of all the fundamental particles was not an overnight flash of brilliance. No Archimedes leapt out of a bathtub shouting “eureka.” Instead, there was a series of crucial insights by a few key individuals in the mid-1960s that transformed this quagmire into a simple theory, and then 50 years of experimental verification and theoretical elaboration to define the model for all particles.
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- Quarks come in six varieties we call flavors. We have up, down, strange, charm, bottom and top flavors. In 1964, Gell-Mann and Zweig taught us the recipes for these flavors: Mix and match any three quarks to get a “baryon“. Protons are two ups and a down quark bound together; neutrons are two downs and an up.
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- Choose one quark and one antiquark to get a meson. A pion is an up or a down quark bound to an anti-up or an anti-down. All the material of our daily lives is made of just up and down quarks and anti-quarks and electrons. Can’t get much simpler than that!
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- The Standard Model of elementary particles provides an ingredients list for everything around us. Those quarks are tied to one another so tightly that you never find a quark or anti-quark on its own. The theory of what is binding the particles, called gluons , is the quantum chromodynamics theory.
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- The other aspect of the Standard Model is “A Model of Leptons.” That’s the name of the landmark 1967 paper by Steven Weinberg that pulled together quantum mechanics with the vital pieces of knowledge of how particles interact and organized the two into a single theory. It incorporated the familiar electromagnetism, joined 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 for giving mass to fundamental particles.
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- The Standard Model has predicted the results of experiment after experiment, including the discovery of several varieties of quarks and of the W and Z bosons. which are heavy particles that are for weak interactions what the photon is for electromagnetism.
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- The possibility that neutrinos aren’t massless was overlooked in the 1960s, but slipped easily into the Standard Model in the 1990s.
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- Discovering the Higgs boson in 2012, long predicted by the Standard Model, was a thrill but not a surprise. Physicists have made numerous proposals for theories beyond the Standard Model. These bear exciting names like Grand Unified Theories, Super-symmetry, Technicolor, and String Theory.
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- Beyond-the-Standard-Model theories have not yet successfully predicted any new experimental phenomenon or any experimental discrepancy 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.
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- So what happens when we try to weigh almost everything? See Review 2572 to learn the size and age of the Universe to learn the answer.
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- See Review 2574 to get a complete list of reviews about the Universe.
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- January 6, 2020 2577
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--------------------- Tuesday, January 7, 2020 --------------------
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