notes

Forces and Particles

Electric and magnetic forces were added to Newton's picture in the 19th century, most famously by James Clerk Maxwell (shown here).  Later, the last two nuclear forces, the "strong" and "weak" nuclear forces came along to explain very small phenomena.  The whole modern picture of forces consists of Einstein's general relativity, which dominates explanations on the largest scales, and the quantum theory plus special relativity, combined into the standard model of particle physics, whiich is applied to the smallest scales.   It all fits on a T shirt.  But gravity is not integrated properly yet.

James Clerk Maxwell Birthday - Namesake of Mauna Kea Telescope

One way to organize the chain is to ask which of the basic forces dominates at each scale.  Then the various explanations have to be fitted together, and this is sometimes difficult.  The standard model does not include gravitation.  The transition from the quantum world to the classical (Newtonian) world is notoriously problematic.

Much of what we know about the nucleus and the fundamental particles comes from scattering experiments.  The grandaddy of all scattering experiments is the Rutherford Scattering Experiment.  Studies of cosmic rays were also important, before particle accelerators took over.

Today, giant accelerators like the Large Hadron Collider (LHC) at CERN create powerful beams of particles which are used to probe down to scales much smaller than protons.  Huge detectors like the ATLAS detector at the LHC are required to measure all the particles that emerge from collisions. Here is a good diagram of the detector.  You can watch the data arrive live here.  The Standard Model of Particle Physics describes the dynamics of the various types of elementary particles which inlcude leptons (like the electron) and hadrons (like the proton and neutron, which are composed of quarks).  Hadrons are further classified into mesons and baryons:  2 quarks for a meson, and 3 for a baryon.

List of mesons, List of baryons. Most of these particles are unstable and decay into simpler stable particles. All particles made of quarks are collectively called hadrons.

Recently, an experiment involving neutrinos made the news because it appeared that the neutrinos had been caught travelling faster than light.  This result would be earth shattering, if it is true. You can read some nice news stories about it here, and here, and many other places.  You can have a go at reading the original scientific paper on the experiment here.  Needless to say, people are quite skeptical about the correctness of this experiment and it will certainly be closely tested before being accepted.


Tablet notes for lecture 5 here.  Lecture 7, here.



Sep 25, 2011 1:37 PM

The modern "great chain of being" and emergence

The modern great chain of being links scales from the size of the universe to the size of the most findamental particles we know.  Explanation in terms of smaller simpler parts is the standard reductionistic method of science.  Going backward, up the chain, involves emergence.  "Weak emergence" is the claim that everything is reducible in principle, even non-obvious properties of complex systems --- but that such reduction is merely impractical.  "Strong emergence" is the claim that some properties are irreducible --- that sometimes the whole is different than the parts, even in principle.  This idea is much less accepted scientifically.

How was this chain assembled?  The idea of atoms --- as ultimate unbreakable particles --- goes back to the ancient greeks, like Democritus.  The roman poet Lucretius gives an account of the universe in terms of differently shaped atoms.  But atoms actually came along much later via Chemistry.  They were not really incorporated into Physics until the quantum revolution in the 1920s.

The real Physics synthesis behind the chain begins properly with Isaac Newton.  He formulated the first "universal" laws and emphasized the idea of forces.  His laws of mechanics and gravitation stood until Einstein modified them in the early 20th century.  Most famously, Newton showed that his law of gravity resulted in orbits that are conic sections.  Conic sections are a staple of ancient greek mathematics, once again showing the "unreasonable effectiveness of mathematics".



Class notes from the tablet are posted here. And here.

Sep 19, 2011 10:07 AM

Lectures 1+2: Introduction

In the first meeting, we will discuss the general idea of the course and cover some housekeeping items.

The first part of the course will discuss how Physics (and many other sciences) have constructed a modern sythesis somewhat analogous to the medieval idea of the the Great Chain of Being.  This organization spans the widest possible range of scales, something like the old movie called Powers of Ten. [See also here, and pop cultural versions here, and here].



It is interesting to note that this movie, which was released in 1966, covers a range of scales much smaller than we think are relevant today.  At the outer limit, the size of the observable universe, is now thought to be  many hundreds of times larger than in the movie, at least.  How much larger is a complicated question, since the universe has expanded considerably since the light was emitted by the farthest objects we can see with telescopes.  And even then, there are probably even larger parts of the universe over the horizon (whose light has not had time to reach us since the Big Bang).  On the other end of the scale, the movie stops at the size of the quarks inside of a proton --- whose existence was still tentative in 1966.  Today, quarks and the consitution of protons is well established and the size limit extends much much smaller than quarks.  String theory, a much debated tentative theory of the ultimate structure of matter, posits objects on the Planck scale, which is 19 powers of 10 smaller than reached in the movie. A string is much much smaller compared to a proton than a proton is to a person. The movie would have to be more than twice as long to reach this whole range, even doing every decade in 10 seconds.  Long stretches of the movie would be very boring (or even more boring than they already are).


Class notes from the tablet are posted here.


Sep 11, 2011 10:45 AM