Physicist Richard P. Feynman once said that if he could boil down the most important principles of physics to a single sentence, it would be, “All things are made of atoms.” (He actually goes on to expand on this, meaning that he actually boiled physics down to a compound sentence. For our immediate purposes, this first bit is enough.) The structure of atoms determines fundamental properties of matter in our universe, such as how atoms interact with each other in chemical combinations. The study of physics at the scale of an atom is called atomic theory, or atomic physics. Though this is several scales above the scale that string theory operates on, understanding the smaller structure of matter requires some level of understanding of the atomic-level structure.
Ancient Greeks considered the question of whether you could divide an object forever. Some — such as the fifth century B.C. philosopher Democritus — believed that you would eventually reach a smallest chunk of matter that couldn’t be divided any more, and they called these smallest chunks atoms.
Aristotle’s view that matter was composed of five basic elements was adopted by most philosophers of the time and remained the dominant way of thinking for many years, well into the time that “natural philosophy” began its transition into “science.” After all, no scientists or philosophers had ever seen a smallest chunk of matter, so there really wasn’t any reason to suppose they existed.
This began to change in 1738 when Swiss mathematician David Bernoulli explained how pressurized gas behaved by assuming that gas was made up of tiny particles. The heat of a gas was related to the speed of the particles. (This built on the work of Robert Boyle, nearly a century earlier.)
In 1808, British chemist John Dalton tried to explain the behavior of elements — substances that can’t be chemically broken down into simpler substances — by assuming that they were made up of atoms.
According to Dalton, each atom of an element was identical to other atoms of the same element, and they combined together in specific ways to form the more complex substances we see in our universe.
Over the next century, evidence for the atomic theory mounted (see the sidebar “Einstein’s contribution to atomic theory”). The complex structures formed by different atoms were called molecules, though the exact mechanism for how atoms formed molecules was still unclear.
Einstein’s contribution to atomic theory
As if he weren’t credited with enough, Albert Einstein is also frequently cited as the person who provided some of the last definitive support for the atomic theory of matter in two of his 1905 papers.
One of the papers was his PhD thesis, in which he calculated the approximate mass of an atom and the size of sugar molecules. This work earned him his doctorate from the University of Zurich.
The other paper involved analyzing random motion in smoke and liquids. This type of motion is called Brownian motion and had puzzled physicists for some time. Einstein pictured the motion as the result of atoms of smoke or liquid being jostled around by atoms of the surrounding gas or liquid, which explained the phenomenon perfectly. His predictions were supported by experimental findings.
It took more than 150 years from the time of Bernoulli for physicists to fully adopt the atomic model. Then, as you find out in the next section, after it was finally adopted, it was found to be incomplete! The complications arising in the study of string theory may well prove to take just as long, and perhaps ultimately be just as incomplete. But that doesn’t mean they’re necessarily “wrong,” any more than atomic theory is “wrong.”
