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Seeking New Laws - the Character of Physical Law

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Seeking New Laws - the Character of Physical Law Week 7 - The Character of Physical Law Seeking New Laws - The Character of Physical Law - Guess Feynman’s Method Measure Calculate Week 7 John Anderson The Academy of Lifelong Learning 1 Finding a New Law o “First we guess it. Then we compute the consequences of the guess to see what would be implied if this law that we guessed is right. Then we compare the result of the computation to nature, with experiment or experience, compare it directly with Bad Example: The Muon g-2 observation to see if it works. experiment at Fermilab. If it disagrees with experiment Video: Don Lincoln, FNL, 7 mins. it is wrong.” https://www.youtube.com/watch?v=UckuqHDB08I Page 156 2 1 Week 7 - The Character of Physical Law Observations, rules, ideas o If we put all these principles together, we discover that there are too many. They are inconsistent with each other. It seems that if we take quantum mechanics, plus relativity, plus the proposition that everything has to be local, plus a number of tacit assumptions, we get inconsistency, because we get infinity for various things when we calculate them, and if we get infinity how can we ever say that Tacit assumption this agrees with nature? example: Probabilities sum to 100% Page 155 3 Observations, etc. o “There is always the possibility of proving any definite theory wrong; but notice that we can never prove it right.” – p. 157 o “I am sure that history does not repeat Creativity: build on and itself in physics. The reason is this. Any beyond past successes. schemes – such as ‘think of symmetry laws’, or ‘put the information in mathematical form’ or ‘guess equations’ – are known to everybody now, and they are all tried all the time.” – p. 153 4 2 Week 7 - The Character of Physical Law Beauty in Laws of Physics o “You can recognize truth by its beauty and simplicity.” p. 171 o “I think it is because nature has a simplicity and therefore a Should the math itself be great beauty.” p. 173 beautiful? 5 Paul Adrien Maurice Dirac o Born in Bristol, Gloucestershire o Introvert. French at home, problems o Shared 1933 Nobel Prize o Father encouraged math education o Insisted on ‘beauty’ in his equations o Lucasian Prof. Cambridge ‘32-68 o Dirac equation predicts spin and anti- particles. 1902 - 1984 o Invented notation still used in physics. Modern particle physics begins with Dirac. 6 3 Week 7 - The Character of Physical Law Special Relativity - Results 1 For a stationary particle this 2 2 2 4 c p m c 2 is just the best know equation H 2 in history: E mc •Dirac adds Relativity to Quantum Mechanics. •Coefficients are really matrices not numbers. •Two positive energy solutions Spin! •Correct magnetic moment Wow! •What is negative energy solution Antimatter! 7 Shelter Island - 1947 Note: Feynman is holding Lamb’s forth. Experimental Measurements Lead to Breakthrough Standing: Willis Lamb, K. Darrow, Victor Weisskopf, George Uhlenbeck, Robert Marshak, Julian Schwinger, David Bohm. Seated: J. Robert Oppenheimer, Abraham Pais, Richard Feynman, Herman Feshbach 8 4 Week 7 - The Character of Physical Law Standard Model – Took Time Crown Jewel of 20th • Three generations of Century Physics quarks and leptons. • Quarks – 1968 • Higgs – 2012, • Tau – 1975 • Top quark – 1995 • W, Z boson – 1983 Compare to Feynman’s Chart 9 The Standard Model - Pithy "Matter particles (quarks and leptons) interact with one another by various forces, each of which is mediated by the exchange of force particles (photons, gravitons(?), gluons, W's, and Z's) in accordance with various conservation laws." - Sheldon Glashow Nobel Prize 1979 Note: Need to add the Higgs Field/Particle to this description. 10 5 Week 7 - The Character of Physical Law The Particle Zoo Post War Detectors and Technology Cyclotron Betatron Bevatron National Laboratories We discover a lot of E.O Lawrence Bevatron at Lawrence particles. Nobel Prize - 1939 Berkeley Lab 1954 11 Symmetry and the Particle Zoo gree n red SU(3) Lie Group - QCD • 1st direct use of Lie Groups • 8 force quanta - gluons How to bring order out of chaos? Quarks. Pattern recognition. Predict missing particle. 12 6 Week 7 - The Character of Physical Law Extend to Nuclear Forces Can you derive Equations from a Principle of Local Invariance? Yang and Mills – Yang knew Pauli’s work on QFT – trouble makers! impressed that gauge invariance determined all E&M interactions. A Symmetry: Strong force same: nn, np, pp. Apply E&M gauge invariance to isospin? •More complex: Field itself must have “isospin” property to keep identity of a proton or neutron the same everywhere. Millennium Prize – “Yang–Mills Existence and Mass Gap. Prove that for any compact simple gauge group G, a non-trivial quantum Yang–Mills theory exists on four-space and has a mass gap Δ > 0.” Worth $1 million. 13 7.
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