Thursday March 27, 2008 5:30 am Lethbridge, Alberta

Chapter 7 The Subatomic World

Having skim-read this chapter, my initial impression is that this chapter contains the most detail and will require the most time to understand.

• "The majority of physicists could not care less about the issues surrounding the interpretation of quantum mechanics, and rightly so. They are too busy using the theory to understand the structure and properties of the subatomic world." [p. 156]

At the moment I see three distinct topics: quantum mechanics structure of the atom structure of the nucleus Quantum mechanics refers to the mathematics (particularly Schrodinger's wavefunction) used to describe and predict various physical phenomena (particularly matter and energy). The idea of quanta refers to the critical idea that energy exists in discrete levels rather than as a continuous range. Let me try to summarize the book so far. Chap. 1 - describes the two-slit experiment and suggests that light has properties of both a wave and a particle. Chap. 2 - Max Planck (German) suggests that energy given off by a black-body as it is heated can only occur in discrete steps. Albert Einstein (Swiss) shows that light must be considered as both a wave and a particle. Niels Bohr (Dane) creates a model of the atom involving orbiting electrons about a nucleus. Louis de Broglie (French) suggests that all matter should be considered as being both a wave and a particle. Chap. 3 - Schrodinger (Austrian) develops the equation of a wavefunction for matter waves. This involves the idea of probability. Werner Heisenberg (German) derives the uncertainty principle. Chap. 4 - describes some weird consequences of quantum mechanics Chap. 5 - discusses the problem of what is really going on Chap. 6 - discusses the problem of interpretation Chap. 7 - returns to the issue of describing the structure of the atom and nucleus.

This chapter summarizes what we now know about the structure of the atom and the nucleus.

• 1895 - Wilhelm Rontgen (German) discovers x-rays.
• 1896 - Henri Becquerel (French) discovers radioactivity
• 1897 - J. J. Thomson (English) discovers the electron
• 1911 - Ernest Rutherford (New Zealander) suggests atomic model of a nucleus with orbiting electrons
• 1912 - Niels Bohr (Danish) elaborates on atomic model

"We must regard every electron as spread throughout the volume of its atom. ... The different regions within the atom where each electron prefers to spend its time depend on the shape of its wavefunction." [p. 162]

Here is a superb illustration of the electron clouds for hydrogen, silicon, iron and silver [p. 163]

Now the story focuses on the structure of the nucleus.

• 1932 - James Chadwick (English) discovers the neutron.
• 1932 - Heisenberg proposes that the nucleus is made up of protons and neutrons.
• 1935 - Hideki Yukawa (Japanese) suggests mechanism for nuclear "glue" to hold protons and neutrons together.

"... the uncertainty principle is more generally a statement about our inability to assign precise values to two complementary quantities simultaneously, such as a particle's position and its momentum. ... There are other pairs of quantities that fall into this category, such as a particle's energy and the precise duration that it has that energy. ... In the quantum realm, the time scale of events is very short indeed. ... This short time scale allows particles such as protons and neutrons to utilize the uncertainty principle in a neat yet crucial way. They can borrow energy from literally nowhere for a very short duration, provided the energy gets paid back before the uncertainty principle is violated. ... Einstein's equation tells us that mass and energy are interchangeable, so the borrowed energy can be used to creat a particle of a certain mass. Yukawa proposed the creation of such a particle, now called a pion, inside nuclei. This particle, he suggested, was responsible for the attractive force holding protons and neutrons." [p. 170]

"Virtual particles [ i.e. one's that only exist for very short periods of time] that can be created out of pure energy are known as bosons. They are also referred to as force-carrying particles, in the sense that when they are exchanged between two other particles the process leads to a force between them. Bosons obey different quantum rules to proper matter particles, known collectivley as fermions, like electrons, protons and neutrons, which make up atoms and hence all the matter we see around us." [p. 171]

• by the second half of the 20th century a large number of sub-atomic particles had been discovered.
• this led to the idea that these particles may themselves be composed of something smaller.
• Murray Gell-Mann and George Zweig proposed the idea of more elementary particles called quarks.

This has been a lot of fun. The fine detail may still be missing, but the overall sense of the picture is fairly clear.

Tags: physics, science, quantum