kw: book reviews, nonfiction, physics, popular treatments
Physics is the intersection of mathematics with observations of nature. So a book that promised an entirely non-mathematical presentation of the deepest puzzles of physics was impossible for me to pass by. In Seven Brief Lessons on Physics, Carlo Rovelli aims not so much for non-physicists to understand the great theories of physics, but for them to become intrigued by them.
Optimistically enough, he begins with "The Most Beautiful of Theories", discussing Albert Einstein's two related theories of relativity, the Special Theory, which treats of the effects of relative motion on time and space, and the General Theory, which unifies space with gravity. He discusses the problems left unsolved by Newton's mechanics, and at least helps us get a glimpse of the way that these two theories resolve them, at least in part.
Many people think that Einstein's Nobel Prize was for one of this theories of relativity, but it was instead for his work on the Photoelectric Effect, with which he demonstrated that light is quantized, or made up of particles. Newton had thought this might be so, calling the particles "corpuscles", but had no way at the time to prove it one way or another. Albert Einstein did so, and then worked on quantum theory for many years. Today, many, at least many of those with some scientific training, are more or less comfortable with light's having both a wave nature and a particle nature. Not only that, but elementary particles such as protons are found to also have a wave nature, though it takes subtle apparatus to winkle out the evidence for it.
Eventually, Einstein was dissatisfied with quantum mechanics, not least because his theory of general relativity and the developing theory of quanta were in fundamental conflict. General relativity requires that space and time be continuous. All aspects of quantum theory require them to be "chunked". Is this just another duality we simply have to accept, like the particle-wave duality of light and even matter? Dr. Rovelli is clear: At the moment we don't know, and nobody is sure how to resolve the dilemma. I like that about him. He doesn't sweep the problems under the rug. They are just there, waiting for someone to hit upon the right approach to straighten them out.
Rather than discuss each of the following chapters, I think it best to leave folks with the following picture of the way light behaves as it enters our eyes and is perceived. Once light is on its way to us, either directly from a source such as the sun or an artificial lamp, or indirectly after bouncing off something, whether it travels as a wave or as a stream of particles is not important. But as it reaches the cornea of the eye, and before that the very thin film of tears on the cornea, it behaves as a wave and is refracted. There is no equation in quantum mechanics which can adequately describe refraction. This shows us that quantum theory is still not complete. During the tenth of a nanosecond that the light is traveling through the eyeball, it is refracted several times, as it passes from one thing to the next: the tear film, the cornea, and aqueous humor in the front of the eye, the crystalline lens behind the iris, the vitreous humor that fills the rest of the eye, and a very thin film of liquid between that and the retina. At the retina, all of a sudden, the light behaves like a stream of particles. The "color" of light depends on the kinetic energy of those particles, the photons, the quanta of light. The cone cells in our retina come in three varieties (for most of us). The cones that respond only to a range of higher energy photons stimulate the color "blue", those that respond best to lower-energy photons stimulate "red", and those with a medium energy preference stimulate the color "green". Thus the particular mix of variously-energetic photons in the beam of light striking a particular patch of cone cells stimulates a color response, which may differ quite a lot from the response of the next patch over, depending on the energy mix of photons that reach that spot.
An interesting side note is that the solution to a quantum mechanical event requires an "observer", and in a simple way, we humans are typically considered the observers. But if phenomena such as diffraction occur when none of us is watching, as we think is true, then the "observer" is actually the whole of the universe, which responds at some level (usually a very, very, very low level) to every quantum event. So we aren't really the "observers" of quantum theory, but those who have figured out that whatever happens in the universe seems to matter to all the universe. At that point physics begins to border on metaphysics. By definition, science gets left behind if we go further.
The other matters covered in the book, cosmology and the shape of space, the resolution of the "particle zoo" that first emerged from our early cyclotrons and synchrotrons, what black holes might really represent, and where we fit into all of this, are each treated succinctly. Dr. Rovelli revels in the beauties of natural science as studied by theorists. His little book is a "good college try" at helping some of the rest of us respond to that beauty.