Basic Concepts

Abstract

Technically speaking, physics deals only with one-body and many-body problems (because the two-body problem reduces to the one-body case, and the three-body problem does not, and is already insolvable). Still, what an average physicist thinks of as “many” in this context is probably something of the order of 10¹⁹-10²³the number of particles in a cubic centimeter of a gas or a solid, respectively. When you have this many particles on your hands, you need a many-body theory. At these densities, the particles will spend enough time at several de Broglie wavelengths from each other, and therefore we need a quantum many-body theory. (A good thing too: What we really should not mess with is the classical chaos!)

Article Note

When asked to calculate the stability of a dinner table with four legs, a theorist rather quickly produces the results for tables with one leg and with an infinite number of legs. He spends the rest of his life in futile attempts to solve the problem for a table with an arbitrary number of legs. A popular wisdom. From the book “Physicists keep joking.”