Kepler-11

anticharm:

I have a problem. I hardly reread what I type, and tend to forget what has been discussed.  So forgive me if I repeat myself, if I repeat myself, if I repeat myself. I typically just tell people that I like emphasizing different principles or factoids when I do this. I also like tying everything together, and in physics, everything does tie together for the most part. I’m telling you guys this because I’ll be turning my attention to describing the fundamental particles and forces. Some of what I’ll be talking about, I’ve already mentioned (in detail or in passing) other things we’ve went over in detail, and I probably will go over them again, just from another angle. Anyway, this isn’t about my sporadic lecture methods, this is the beginning of the fundamental particles.

I hate to pull a Sheldon and Penny scene, but we must ask ourselves, what is physics? I’m kidding, but we really must start on a warm spring day in ancient Greece. Well, I imagine it as such anyway. The notion that the world is an arrangement of fundamental particles has a extended history. Around 400 B.C.E., the Greek philosophers Democritus and Leucippus proposed that mater is made of indivisible particles that the called atoms, a word derived from a- (not) and tomos (cut or divided). Not much else was produced in terms of scientific discovery and theory about this idea until the early 1800s when John Dalton discovered that many chemical phenomena could be described if atoms of each element are the basic, indivisible building blocks of matter.

Near the end of the 1800s it became obvious that atoms are not indivisible. The idea of an internal structure of the atom was being thrown around, and J. J. Thomson’s discovery of the electron in 1897 showed that atoms could be divided into charged particles. The hydrogen nucleus was identified as a proton, and in 1911 the sizes of nuclei were measured by Ernest Rutherford’s experiments. Quantum mechanics, including the Schrödinger equation (which describes how the quantum state of a physical system changes with time), flourished over the next 15 years. Scientists were on their way to understanding the principles that underline atomic structure.

The discovery of the neutron was a vital breakthrough. In 1930 Walther Bothe and Herbert Becker observed that when beryllium, boron, or lithium was blasted by αparticles from radioactive polonium, the material produced a radiation that had much greater penetrating power than the original α particles. Experiments by James Chadwick in 1932 revealed that the produced particles were electrically neutral, with mass roughly equal to that of the proton. Chadwick called these particles neutrons.

It was challenging to identify neutrons directly because they have no charge. Because of this, they yield little ionization when they pass through matter and they are not deflected by electric or magnetic fields. Neutrons usually interact only with nuclei; they can slow during scattering, and they can penetrate the nucleus. Slow neutrons can be detected by means of a nuclear reaction in which a neutron is absorbed and an α particle is produced. The expelled α particle is easy to identify because it is charged. Later experiments showed that neutrons, like protons and electrons, are spin-½ particles.

The neutron cleared up a mystery about the arrangement of the nucleus. Before 1930 the mass of a nucleus was believed to be due only to protons, but no one knew why the charge-to-mass ratio was not identical for all nuclides. It became obvious that all nuclides (except hydrogen-1) comprise both protons and neutrons. In fact, the proton, the neutron, and the electron are the building blocks of atoms. You may think that would be the end of the adventure. Sorry, this is barely the beginning. These are not the only particles, and they can do more than build atoms.

Sources:

1. Martin, B.R. and G. Shaw. “Particle Physics.” Wiley Publishing, 2008.
2. Bauer, Wolfgang & Gary D. Westfall. “University Physics with Modern Physics.” McGraw-Hill, 2011.
3. Basu, Dipak. “Dictionary of Material Science and High Energy Physics.” CRC Press LLC, 2001.

Image(s) credit:

1. “Four Elements” McGraw-Hill. “The four basic elements of ancient Greek philosophy, each shown with its regular polyhedron, as assigned by Plato.”
2. “Scale Comparison” McGraw-Hill.