

Let's face it: quantum mechanics is hard. The idea that we can't simultaneously know a particle's position and velocity is preposterous. The particle exists, so it must be somewhere, doing something. Unfortunately, even that simple assumption prevents us from describing the results of wellknown experiments. One problem is that the very act of observing an electron, literally bouncing a photon or other particle off of it and then recording the photon's arrival in a sensor, changes the electron's position and velocity in unpredictable ways. The upshot is that the more certain you are of an electron's position, the less certain you are of its velocity, and viceversa. Brian Cox and Jeff Forshaw, both professors at the University of Manchester and popular presenters, are clearly used to explaining quantum mechanics to their students and the public. In The Quantum Universe, they describe quantum objects as waves, which they notate using a series of clocks. Each clock's size represents the probability that the particle will be found at a specific location, while the clock's hour hand represents the particle's state. You add up the values of all of the clocks, essentially an infinite series with progressively smaller (yet stubbornly nonzero) values, to describe the particle. I don't mind admitting that I'll have to revisit this introductory section and subsequent chapters using clocks to more fully understand what the authors are getting at. Fine by me. The book operates on two levels — one where you can accept the implications of the authors' explanations without truly understanding their methodology, but also another where you are invited to have a go at the real stuff. I intend to do so, mainly because I feel that, given a bit more time, I could grasp Cox and Forshaw's explanation. Even when a reader focuses only on the implications of quantum mechanics, The Quantum Universe is a very interesting book. The authors begin with the Double Slit Experiment, which is standard fare for such works, before moving on to more challenging implications. For example, they argue that quantum theory allows a particle to move from one position to any other position in the universe instantly. Every clock in the infinite series of clocks has a nonzero probability assigned to its position, so it's possible, though extremely unlikely, that a particle on Earth could appear in Alpha Centauri an instant from now. It seems outlandish, but the theories that explain observed phenomena demand that result be true. Through a fortunate coincidence, material in some of the chapters in The Quantum Universe correspond to information presented in lectures from Cosmology: The History and Nature of Our Universe, a DVD lecture course from The Great Courses I'm working through. For example, Cox and Forshaw have a chapter named "Empty Space Isn't Empty", which corresponds to theories about dark matter and dark energy that, in part, explain why light from distant sources is bent through gravitational lensing while passing through the vacuum of space. The Quantum Universe is a very good book, one that I plan to revisit several times. I highly recommend it to anyone who wants to take on quantum mechanics without a strong background in math or physics, or to anyone who studied those subjects in college and wants to take the next step into the quantum world. 
