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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Modified from Dr. Allen I. Mincer’s webcast from NYUJan ‘05
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.
Unfortunately, we don’t ever get to see any of these things! This is starting to sound like its going to be hard…
If we discover the rules, our task simplifies (?) to the reconstruction of what has happened based upon what we observed.
Example: coin exchanges
Transactions that have an invariant in the face of a change of time, place, order, etc,
have the property known as symmetry.
“a thing is symmetrical if one can subject it to a certain operation, and it appears exactly the same after the operation.”
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…
For every continuous symmetry in nature, there is a corresponding conservation law.
Every conservation law has a corresponding symmetry.
Snowflakes are symmetric under 60 degree rotations, but this is a discrete symmetry, rather than a continuous symmetry.
Hey! That was my idea!
It’s the one thing I’m certain of!
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!
There must be some unseen collision products!
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.
The Marx brothers do an early experiment with parity.
Until you realize that your right hand is your mirror image’s left hand!
A particle’s ‘spin’ directioncan be definedin a right-handed sense.
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.
Our universe is a ‘weak left-hander,’ resulting (thankfully) in a preference for matter over anti-matter!
Conservation Laws are obeyed!
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.
From: The Discovery of Subatomic Particles by Steven Weinberg
“… 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