Thursday, September 26, 2019

Symmetry is the origin of all interactions

In Phil Anderson's review of Lucifer's Legacy: The Meaning of Asymmetry by Frank Close, Anderson makes the following profound and cryptic comment.
In a book focusing, as this does, on symmetry, it seems misleading not to explain the fundamental principle that all interaction follows from symmetry: the gauge principle of London and Weyl, modelled on and foreshadowed by Einstein's derivation of gravity from general relativity (Einstein seems to be at the root of everything). The beautiful idea that every continuous symmetry implies a conservation law, and an accompanying interaction between the conserved charges, determines the structure of all of the interactions of physics. It is not appropriate to try to approach advanced topics such as electroweak unification and supersymmetry without this foundation block.
To see how this plays out in electrodynamics see here.

Tuesday, September 24, 2019

A pioneering condensed matter physicist

In terms of institutional structures, Condensed Matter Physics did not really exist until the 1970s. A landmark being when the Division of Solid State Physics of the American Physical Society changed its name. On the other hand, long before that people were clearly doing CMP! If we think of CMP as a unified approach to studying different states of matter that enterprise began in earnest during the twentieth century.

Kamerlingh Onnes (1853-1924) was a pioneer in low-temperature physics but is best known for the discovery of superconductivity in 1911. In many ways, Onnes embodied the beginning of an integrated and multi-faceted approach to CMP: development of experimental techniques, the interaction of theory and experiment, and addressing fundamental questions.

1. Onnes played the long game, spending years developing and improving experimental methods and techniques, whether glass blowing, sample purification, or building vacuum pumps. He realized that this approach required a large team of technicians, each with particular expertise and that teamwork was important. The motto of Onnes’ laboratory was Door meten tot weten (Through measurement to knowledge). Techniques were a means to a greater end.

2. In Leiden, Onnes sought out theoretical advice from his colleague Johannes van der Waals (1837-1923).  [Almost 10 years ago I gave a talk about van der Waals legacy].

3. Onnes’ experiments were driven by a desire to answer fundamental questions. Questions he helped answer included the following.
Can any gas become liquid?
For gases is there a universal relationship between their density, pressure, and temperature?
How are gas-liquid transitions related to interactions between the constituent molecules in a material? At very low temperatures is the electrical conductivity of a pure metal zero, finite, or infinite?

The first of these questions motivated Onnes to pursue being the first to cool helium gas to low enough temperatures that it would become liquid. At the time all other known gases had been liquified. In 1908 his group observed that helium became liquid at a temperature of 4.2 K. This discovery was of both fundamental importance and great practical significance. Liquid helium became extremely useful in experimental physics and chemistry as a means to cool materials and scientific instruments. Indeed liquid helium enabled the discovery of superconductivity, which resulted from addressing the last question.

The figure shows Onnes (left) in his lab with van der Waals.

The discussion above closely follows Steve Blundell's Superconductivity: A Very Short Introduction.

Friday, September 20, 2019

Common examples of symmetry breaking

In his beautiful book, Lucifer's Legacy: The Meaning of Asymmetry, Frank Close gives several nice examples of symmetry breaking that make the concept more accessible to a popular audience.

One is shown in the video below. Consider a spherical drop of liquid that hits the flat surface of a liquid. Prior to impact, the system has continuous rotational symmetry about an axis normal to the plane of the liquid and through the centre of the drop. However, after impact, a structure emerges which does not have this continuous rotational symmetry, but rather a discrete rotational symmetry.

Another example that Close gives is illustrated below. Which napkin should a diner take? One on their left or right? Before anyone makes a choice there is no chirality in the system. However, if one diner chooses left others will follow, symmetry is broken and a spontaneous order emerges.