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Conservation Laws, Symmetry and Particle Physics. Modified from Dr. Allen I. Mincer’s webcast from NYU Jan ‘05. A bit of motivation. SJS science teacher Harry Portwood encourages his students to ask “How do you know that?”
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Conservation Laws, Symmetry and Particle Physics Modified from Dr. Allen I. Mincer’s webcast from NYUJan ‘05
A bit of motivation SJS science teacher Harry Portwood encourages his students to ask “How do you know that?” He hopes that they will get in the habit of asking that question in all matters of science.
Game Plan • Our task must be defined • We must discover the rules of this game we play: Symmetries and conservation laws • We need some practice before we play the game: Measurement of particle interactions • Let’s play! Experiments to do
Our task:Explain the structure and properties of the sub-atomic world. Unfortunately, we don’t ever get to see any of these things! This is starting to sound like its going to be hard…
However, nature plays by some very consistent rules If we discover the rules, our task simplifies (?) to the reconstruction of what has happened based upon what we observed. Example: coin exchanges
A quantity that does not change is sometimes called an ‘invariant’ (fancy word alert) Transactions that have an invariant in the face of a change of time, place, order, etc, have the property known as symmetry.
Richard Feynman quotes Prof. Hermann Weyl: “a thing is symmetrical if one can subject it to a certain operation, and it appears exactly the same after the operation.”
Different types of transactions have different invariants? Does that mean there are different types of symmetries? Hmm…. If we observe an invariance, can we deduce a specific symmetry? But what does any of this have to do with Physics? It’s time for that story…
Have you ever heard of one of the most important, yet mostly unknown, female mathematicians of the 20th century?
Emmy Noether (1882-1935): • Educated as a language teacher, but she preferred mathematics • Granted permission in 1907 to study mathematics under Hilbert, Klein, Minkowski. • Became a lecturer in mathematics in Vienna, 1913 • Granted faculty status at Gottingen in 1919
Noether’s Theorem (1915): For every continuous symmetry in nature, there is a corresponding conservation law. Every conservation law has a corresponding symmetry.
Conservation Laws! At last, some physics … • We can predict the final value from the initial value without knowledge of “transaction” details • Doing many experiments and seeing what is conserved gives information about the “transactions”even if details are not known
Come to think of it, we also know some symmetries: Snowflakes are symmetric under 60 degree rotations, but this is a discrete symmetry, rather than a continuous symmetry.
Einstein included some of Noether’s work with invariants in his 1916 General Relativity Paper Hey! That was my idea! Now, Albert!
Noether’s Theorem, derived from Classical Mechanics, emerged intact from the ‘Quantum Mechanical Revolution’ It’s the one thing I’m certain of! Now, Werner!
So when we observe symmetries in nature, Noether tells us to look for a conservation law – a big payoff: At last we found the rules of the game… and by applying conservation laws, we can reduce the number of possible interpretations of our experiments!
Or given a conservation law, we can use symmetry principles to predict the unobservable! Pi=Pf There must be some unseen collision products!
Example: linear momentum and total mechanical energy • KE = ½ mv2 for each object in system • PE depends on position of each object • p = mv for each object • KE and p may be summed over the entire system
If our system is symmetric with respect to time, mechanical energy will be conserved If our system is symmetric with respect to position, momentum is conserved. If our system is symmetric with respect to rotation, angular momentum is conserved.
There is also symmetry of reflections – ‘parity’ The Marx brothers do an early experiment with parity.
Should an object and its reflection follow the same physical laws? Until you realize that your right hand is your mirror image’s left hand!
What is so special about Right Handedness? A particle’s ‘spin’ directioncan be definedin a right-handed sense.
Experiments have shown that ‘handedness’ is not always symmetric The primary sense of the beta rays here is left-handed; its mirror image is right-handed. This form of radioactive does not conserve parity.
And this particular asymmetry led to an understanding of a key reason why we can exist! Our universe is a ‘weak left-hander,’ resulting (thankfully) in a preference for matter over anti-matter!
Conservation laws assure us that the interaction still must play by the established rules. Conservation Laws are obeyed!
Some Familiar Conserved Quantities • Energy = mc2 + kinetic energy • Momentum = m0v/ Ö(1 - v2/c2) • Angular momentum • Electric charge
Some not-so Familiar Quantities • Baryon number (number of quarks minus number of anti-quarks) • Lepton number (number of e- mu- tau- and neutrinos minus anti-particles)
Another type of scattering measurement If we shoot a sufficient number of particles at a target, we can determine its size (area) by counting the number of hits and misses.
The Rutherford Experiment From: The Discovery of Subatomic Particles by Steven Weinberg
Gold foil Radium Zinc sulfide screens Lead collimator The Rutherford Experiment After The Discovery of Subatomic Particles
The Rutherford Experiment “… the chance of an alpha particle being scattered backwards was very small. … It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Sir Ernest Rutherford, quoted in The Discovery of Subatomic Particles
Enough theory … Play time!
Sources • Dr. Allen Mincer, NYU Physics Dept. • Harry Portwood, St. John’s School • Symmetry and the Beautiful Universe, Leon Lederman • http://www.emmynoether.org • http://www.eftaylor.com • http://xroads.virginia.edu/~MA01/Cober/marx/mirrormovie.html • The Discovery of Subatomic Particles, Steven Weinberg • Six Not so Easy Pieces, Richard Feynman
The players: Our friends, the particles • Atom = nucleus + electrons • Nucleus = protons + neutrons • Neutrons, protons and hosts of other particles now known to be made of quarks! • Leptons • And of course, anti-matter (but we’ll save that for another day).
The interactions (forces) • Gravity (How small can we make Newton’s apple?) • Electromagnetic force (like charges repel, etc) • Strong nuclear force (keeps nuclear protons from repelling each other) • Weak nuclear force (radioactive decay) • ??? Higgs Boson ???
