Ours is a quantum world, but the discovery of that world took many decades of experimental and theoretical work. Starting in 1900, a radical new theory was developed that explained the chemistry of the elements, the periodic table, the sizes of atoms, why we are the size that we are, and various phenomena that had defied explanation using the conventional classical view of the world that existed until that time (including, for instance, Newton's laws of motion that describe the falling apple and the orbits of the planets).

The new conceptual ideas are referred to broadly as quantum theory, and the mathematical approaches that were developed to describe and integrate these ideas into a generally applicable method of calculation are known as quantum mechanics. Collectively this body of work has been called “the most successful set of ideas ever devised by human beings”1 and “the most powerful physical theory that has ever been devised.”²

Until 1925, quantum theory was an assemblage of ad hoc postulates, assumptions, and quasi-classical constructs that managed to explain experimental findings. But in the next several years a firm foundation for the overall effort was laid in place when three young scientists developed separate mathematical constructs that accurately described the one-electron hydrogen atom.

In the fall of 1927, some twenty-four of the top scientists from around the world, “the greatest gathering of physicists ever,”³ met for nearly a week in Brussels for the fifth conference sponsored by the Belgian industrialist Ernest Solvay, this one devoted exclusively to examining these exciting, new, and continuing developments in quantum mechanics. This group and five other guests are shown in Figure 1.1. Seventeen of this group were by then or would later be recipients of the Nobel Prize in physics or chemistry. (Note: the Nobel Prize is awarded only to scientists who are alive at the time that the award is to be given. And the honor is usually bestowed many years after the work that merits it has been done. So many a worthy scientist has died before he might have received the prize.)

By the time of this Solvay conference, the physics community had divided into two camps with dramatically opposite views on the interpretation and implications of the theory: one camp was led by Albert Einstein (at the center of the first row in the figure) and the other was led by Niels Bohr (at the far right in the second row). The opposing views ran so deeply as to dispute the meaning of reality and physics itself. These views had recently been defined, but this was the first time that all of the major players on both sides would be assembled to present and discuss them. It was to be a clash of titans.

In Part One, I explain the lead-up to the meeting and how quantum theory and quantum mechanics were developed. I describe experiments, ideas, and the people involved. In Part Two, I describe the meeting and the controversy over the new theory's arrival, explore its mind-boggling implications, and tell of much later definitive experiments that would judge the debate.

Note that in providing the historical narrative for these Parts One and Two I draw heavily on the excellent book Quantum—Einstein, Bohr, and the Great Debate about the Nature of Reality, by Manjit Kumar.⁴ And, at appropriate points throughout these same parts and also in Part Three, I indicate the awards of Nobel Prizes in Physics and recite the rationale that the Nobel Foundation gave for each award. My source for all of these citations is a section titled “Nobel Prize Winners,” in Physics: Decade by Decade, by Alfred B. Bortz, a book of the Twentieth Century Science series.⁵

SCIENTIFIC NOTATION AND SCIENTIFIC SHORTHAND

Throughout this book, I avoid all but the simplest of mathematics. Instead, I use a “scientific shorthand” to describe a few simple physical relationships. And, because you will encounter here and there some very large and some very small numbers, I simplify by using a “scientific notation.” A couple of examples of both the shorthand and the notation are provided and explained in the indented paragraphs that follow. I suggest that you take a couple of minutes now to look at these examples so that you will be familiar with both conveniences and can easily proceed in those instances where you may need them.

For example, I will refer to the speed of light as c, which we know from Einstein's formula E = Mc² for the energy equivalent of a mass of material. (This formula, or equation, is just a scientific shorthand.) And c is just a shorthand for the number and the units describing the speed of light. c = 299,793,000 meters per second, where 299,793,000 is the number and meters per second contains the units (meters and seconds) and is written in shorthand as m/s. (Note that a meter, the standard international [SI] unit of length, is just three inches longer than a yard. You may be familiar with the terms “yardstick” or “meter stick” to measure length.) The superscripted 2 after the c simply means that c is multiplied by itself, c × c.

To concisely and sometimes approximately express large numbers such as c, we'll be using a scientific notation in which we: (a) round off to the first few digits expressed in decimal form (2.998, for our example), and then (b) multiply by the number ten raised to a superscript (“power”) given by the total number of digits that would follow the decimal point. (This is the same as the number 10 multiplied by itself that many times.) So, in this notation, c = 2.998 × 10⁸ m/s. The number of digits shown may depend on the precision to which a quantity is known or the degree of precision needed in its use. Rounding off further, we would have the easy-to-remember c = 3 × 10⁸ m/s. (Here 10⁸ represents ten multiplied by itself eight times.)

We will also encounter some very small numbers. For example, the mass of an electron is 0.00000000000000000000000000000091083 kilograms, where one kilogram is about 2.2 pounds. We write this as 9.1083 × 10^–31 kg, where the minus sign in 10^–31 indicates that 9.1083 is divided by 10³¹, that is, divided by ten thirty-one times.