- 4091 - UNIVERSE - What is the universe made of?
---------------- 4091 - UNIVERSE - What is the universe made of?
- 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, 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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!
-
- 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.
-
- 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.
-
- The possibility that neutrinos aren’t
massless was overlooked in the 1960s, but slipped easily into the Standard
Model in the 1990s.
-
- 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.
-
- 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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July 14, 2023 UNIVERSE - What
is the universe made of?
2577 4091
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Friday, July 14, 2023
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