Knocking on Heaven's Door - Lisa Randall [44]
The electrons’ quantized levels are essential to understanding the atom. In the early twentieth century, an important clue that the classical rules had to radically change was that classically, electrons circling a nucleus are not stable. They would radiate energy and quickly fall into the center. Not only would this be nothing like an atom, it wouldn’t permit the structure of matter that follows from stable atoms as we know them.
Niels Bohr in 1912 was faced with a challenging choice—abandon classical physics or abandon his belief in observed reality. Bohr wisely chose the former and assumed classical laws don’t apply at the small distances occupied by electrons in an atom. This was one of the key insights that led to the development of quantum physics.
Once Bohr ceded Newton’s laws, at least in this limited regime, he could postulate that electrons occupied fixed energy levels—according to a quantization condition that he proposed involving a quantity called orbital angular momentum. According to Bohr, his quantization rule applied on an atomic scale. The rules were different from those we use at macroscopic scales, such as for the Earth circulating around the Sun.
Technically, quantum mechanics still applies to these larger systems as well. But the effects are far too small to ever measure or notice. When you observe the orbit of the Earth or any macroscopic object for that matter, quantum mechanics can be ignored. The effects average out in all such measurements so that any prediction you make agrees with its classical counterpart. As discussed in the first chapter, for measurements on macroscopic scales, classical predictions generally remain extremely good approximations—so good that you can’t distinguish that quantum mechanics is in fact the deeper underlying structure. Classical predictions are analogous to the words and images on an extremely high-resolution computer screen. Underlying them are the many pixels that are like the quantum mechanical atomic substructure. But the images or words are all we generally need (or want) to see.
Quantum mechanics constitutes a change in paradigm that becomes apparent only at the atomic scale. Despite Bohr’s radical assumption, he didn’t have to abandon what was known before. He didn’t assume classical Newtonian physics was wrong. He simply assumed that classical laws cease to apply for electrons in an atom. Macroscopic matter, which consists of so many atoms that quantum effects can’t be isolated, obeys Newton’s laws, at least at the level at which anyone could measure the success of its predictions. Newton’s laws are not wrong. We don’t abandon them in the regime in which they apply. But at the atomic scale, Newton’s laws had to fail. And they failed in an observable and spectacular fashion that led to the development of the new rules of quantum mechanics.
NUCLEAR PHYSICS
As we continue our journey down in scale into the atomic nucleus itself, we will continue to see the emergence of different descriptions, different basic components, and even different physical laws. But the basic quantum mechanical paradigm will remain intact.
Inside the atom, we’ll now explore inner structure with size of about 10 femtometers, the nuclear size of a hundred thousandth of a nanometer. So far as we have measured to date, electrons are fundamental—that is, there don’t seem to be any smaller components of electrons. The nucleus, on the other hand, is not a fundamental object. It is composed of smaller elements, known as nucleons. Nucleons are either protons or neutrons. Protons have positive electric charge and neutrons are neutral, with neither a positive nor negative charge.
One way to understand the nature of protons and neutrons is to recognize that they are not fundamental either. George Gamow, the great nuclear physicist and science popularizer,