Warped Passages - Lisa Randall [58]
Kandinsky’s reaction was a bit extreme. Radical as the fundamentals of quantum mechanics were, it’s easy to overreach when applying them in nonscientific contexts. I find the most bothersome example to be the frequently abused uncertainty principle, which is often misappropriated to speciously justify inaccuracy. We will see in this chapter that the uncertainty principle is, in fact, a very precise statement about measurable quantities. Nonetheless, it is a statement with surprising implications.
We’ll now introduce quantum mechanics and the underlying principles that make it so different from older, classical physics that came before. The strange and new concepts we’ll encounter include quantization, the wavefunction, wave-particle duality, and the uncertainty principle. This chapter outlines these key ideas and gives a flavor of the history of how it was all worked out.
Shock and Awe
The particle physicist Sidney Coleman has said that if thousands of philosophers spent thousands of years searching for the strangest possible thing, they would never find anything as weird as quantum mechanics. Quantum mechanics is difficult to understand because its consequences are so counterintuitive and surprising. Its fundamental principles run counter to the premises underlying all previously known physics—and counter to our own experiences.
One reason that quantum mechanics seems so bizarre is that we are not physiologically equipped to perceive the quantum nature of matter and light. Quantum effects generally become significant at distances of about an angstrom, the size of an atom. Without special instruments, we can see only sizes that are much larger. Even the pixels of a high-resolution television or computer monitor are generally too small for us to see.
Furthermore, we see only huge aggregates of atoms, so many that classical physics overwhelms quantum effects. We generally also perceive only many quanta of light. Although a photoreceptor in an eye is sufficiently sensitive to perceive the smallest possible units of light—individual quanta—an eye typically processes so many quanta that any would-be quantum effects are overwhelmed by more readily apparent classical behavior.
If quantum mechanics is difficult to explain, there is a very good reason. Quantum mechanics is sufficiently far-reaching to incorporate classical predictions, but not the other way round. Under many circumstances—for example, when large objects are involved—quantum mechanical predictions agree with those from classical Newtonian mechanics. But there is no range of size for which classical mechanics will generate quantum predictions. So when we try to understand quantum mechanics using familiar classical terminology and concepts, we are bound to run into trouble. Trying to use classical notions to describe quantum effects is something like trying to translate French into a restricted English vocabulary of only a hundred words. You would frequently encounter concepts or words that could be interpreted only vaguely, or which would be impossible to express at all with such a limited English vocabulary.
The Danish physicist Niels Bohr, one of the pioneers of quantum mechanics, was aware of the inadequacy of human language for describing the inner workings of the atom. Reflecting on the subject, he related how his models “had come to him intuitively…as pictures.”* As the physicist Werner Heisenberg explained, “We simply have to remember that our usual language does not work any more, that we are in the realm of physics where our words don’t mean much.”*
I will therefore not attempt to describe quantum phenomena with classical models. Instead, I will describe the key fundamental assumptions and phenomena that made quantum mechanics so different from the classical theories that came before. We’ll reflect individually on several of the key observations and insights that contributed to quantum mechanics and its development. Although this discussion follows a roughly historical outline, my real purpose is