Friday, July 11, 2014

Science Friday: And Inside the Neutron Is...

Second to last post on George Gamow's classic, Thirty Years That Shook Physics.

The previous posts were:

1a. Planck's Quantum
1b. Jumping Photons (Einstein and the Photoelectric Effect)
1c. The Compton Effect (Proof of Energy Packets)

2a. Thomson and Rutherford's Atoms
2b. Bohr's Contributions (How electrons fill the atom)

3a. Pauli Exclusion Principle (no two electrons at any one energy state)
3b. The Pauli Neutrino

4a. De Broglie's Wavy Particles
4b. Schrödinger's Wave Equation

5. Heisenberg's Uncertainty Principle
6. Dirac's Anti-Particles

Chapters 7 and 8 of Gamow's book are fairly short. We have more or less reached the end of the thirty years that shook physics from 1900 to the early 1930s. I've pretty well decided which book to follow Gamow with, and I'll announce it next week.

With chapters 7 and 8, we are beginning to see the first hints of what will become the standard model of physics, with its panoply of particles. Little did we know when we learned about the proton, electron, and neutron, that we were just scratching the surface of what lies below.

[As an aside, my Dad remembered talking to army buddies in Europe about whether they would ever split the atom. Little did he know that Hiroshima was less than a year away!

Actually, it had already been done. Gamow mentions more than once the telegram Niels Bohr received at a 1939 conference at George Washington University relaying that two professors at Berlin University had split a uranium atom using a slow neutron bombardment, liberating vast amounts of energy.]

1. You might remember that Wolfang Pauli had suggested the existence of a very small, chargeless particle that accounted for some of the energy missing when "beta decay" takes place. Beta decay is basically when an unstable, radioactive nucleus emit an electron. In the mid-20s, they could account for where some of the energy went from the decay but not all of it.

Pauli suggested there might be a very, very small particle that carried off the missing bit of energy. In his conversations and discussions, he was calling it a "neutron." But in 1932, when James Chadwick actually discovered a neutral particle in the nucleus of almost all atoms, Chadwick called that particle a "neutron." And Chadwick called it a neutron in print.

So it was the Italian, Enrico Fermi, who called Pauli's particle a "neutrino" (a "little neutron" in Italian). Its existence wasn't finally confirmed until 1955. But long before all the details were experimentally demonstrated, Fermi was accurately predicting how such decays would play out.

It's not exactly accurate to say that a neutron has a proton and an electron inside it, but it works for a blog. In a certain state, a neutron will decay into a proton, while releasing an electron and a neutrino. Next to photons, neutrinos are the most populous particles in the universe.

This decay relates to one of what are currently considered the four basic forces of nature--the so called weak nuclear force (the others are gravity, electromagnetism, and the strong nuclear force). The details weren't worked out when Gamow wrote this book. A big piece of the puzzle fell into place in 1968, the year he died.

2. Chapter 8 is very short and is about the suggestion of Hideki Yukawa in 1935 that there was another yet undiscovered particle that created the "strong force" that holds protons together in the nucleus without them flying apart (because they have the same charge). This strong force that sticks them together only operates at a very, very short distance (10-13cm).

They knew about what mass the "meson" (started out as "mesotron," the idea being a middle sized particle, in between the mass of an electron and a proton) should be. In 1937, such a particle was discovered among cosmic radiation from the sun. But it turned out to be a little too light. Finally, in 1947, balloons captured an earlier version of this meson in the upper atmosphere amid cosmic radiation, and it had the right mass.

Thus two more particles in the growing pantheon: the pi meson (pion, the heavier) and the mu meson (muon, the lighter).

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