Symmetries and wave functions

An important difference between the classical and quantum perspectives is their different criteria of distinguishability. Identical particles are classically distinguishable when separated in phase space. On the other hand, identical particles are always quantum mechanically indistinguishable for the purpose of counting distinct microstates. But these concepts and these distinctions do not tell the whole story of how we count the microstates and determine the multiplicity of a quantized system.

There are actually two different ways of counting the accessible microstates of a quantized system of identical, and so indistinguishable, particles. While these two ways were discovered in the years 1924–1926 independently of Erwin Schrödinger’s (1887–1961) invention of wave mechanics in 1926, their most convincing explanation is in terms of particle wave functions. The following two paragraphs may be helpful to those familiar with the basic features of wave mechanics.

A system of identical particles has, as one might expect, a probability density that is symmetric under particle exchange, that is, the probability density is invariant under the exchange of two identical particles. But here wave mechanics surprises the classical physicist. A system wave function may either keep the same sign or change signs under particle exchange. In particular, a system wave function may be either symmetric or antisymmetric under particle exchange.

Thermodynamics and entropy

The existence of entropy follows inevitably from the first and second laws of thermodynamics. However, our purpose is not to reproduce this deduction, but rather to focus on the concept of entropy, its meaning and its applications. Entropy is a central concept for many reasons, but its chief function in thermodynamics is to quantify the irreversibility of a thermodynamic process. Each term in this phrase deserves elaboration. Here we define thermodynamics and process; in subsequent sections we take up irreversibility. We will also learn how entropy or, more precisely, differences in entropy tell us which processes of an isolated system are possible and which are not.

Thermodynamics is the science of macroscopic objects composed of many parts. The very size and complexity of thermodynamic systems allow us to describe them simply in terms of a mere handful of equilibrium or thermodynamic variables, for instance, pressure, volume, temperature, mass or mole number, internal energy, and, of course, entropy. Some of these variables are related to others via equations of state in ways that differently characterize different kinds of systems, whether gas, liquid, solid, or composed of magnetized parts.

Boltzmann and atoms

The thermodynamic view of a physical system is the “black box” view. We monitor the input and output of a black box and measure its superficial characteristics with the human-sized instruments available to us: pressure gauges, thermometers, and meter sticks. The laws of thermodynamics govern the relations among these measurements. For instance, the zeroth law of thermodynamics requires that two black boxes each in thermal equilibrium with a third are in thermal equilibrium with each other, the first law that the energy of an isolated black box can never change, and the second law that the entropy of an isolated black box can never decrease. According to these laws and these measurements each black box has an entropy function S(E,V, …) whose dependence on a small set of variables encapsulates all that can be known of the black box system.

But we are not satisfied with black boxes – especially when they work well! We want to look inside a black box and see what makes it work. Yet when we first look into the black box of a thermodynamic system we see even more thermodynamic systems. A building, for instance, is a thermodynamic system. But so also is each room in the building, each cabinet in each room, and each drawer in each cabinet. But actual thermodynamic systems cannot be subdivided indefinitely. At some point the concepts and methods of thermodynamics cease to apply. Eventually the subdivisions of a thermodynamic system cease to be smaller thermodynamic systems and instead become groups of atoms and molecules.

One of the most important contributions that the science of astronomy has made to human progress is an understanding of the distance and size of celestial objects. After millennia of using our eyes and about four centuries of using telescopes, we now have a very good idea of where we are in the Universe and how our planet fits in among the other bodies in our Solar System, the Milky Way galaxy, and the Universe. Several of the techniques astronomers use to estimate distance and size are based on angles, and the purpose of this chapter is to make sure you understand the mathematical foundation of these techniques. Specifically, the concepts of parallax and angular size are discussed in the first two sections of this chapter, and the third section describes the angular resolution of astronomical instruments.

Parallax

Parallax is a perspective phenomenon that makes a nearby object appear to shift position with respect to more distant objects when the observation point is changed. This section begins with an explanation of the parallax concept and proportionality relationships and concludes with examples of parallax calculations relevant to astronomy.

Parallax concept

You can easily demonstrate the effect of parallax by holding your index finger upright at arm's length and then observing that finger and the background behind it with your left eye open and your right eye closed.