“Our common sense evolved in a highly unusual, obscure part of the universe, Earth; it is not surprising that our common sense fails to grasp the true universe. The problem lies not in relativity but assuming that our common sense represents reality.” -Michio Kaku

*Parallel worlds*, by theoretical physicist Dr. Michio Kaku, explores the intensely science fiction sounding, yet mathematically viable concept that we are living in not the *only*, but merely one of *countless* other universes.

Many quantum theories rely on multiple iterations of this concept as part of a potential explanation, including inflation theory, superstring theory, and M-theory. One interpretation even attempts to use quantum decoherence to explain particle physics’ baffling wave-function collapse by applying a mind boggling fractal multiverse model dubbed the many-worlds interpretation.

Having read this book a decade ago and struggling with some of the concepts at the time, I wanted to see if I would absorb anything new from it today. This time I paid closer attention to Dr. Kaku’s detailed explanations concerning quantum physics, as I had taken a more general approach to the book the first time around.

Quantum physics, simply defined, is the physics of matter and energy at its most base level. Three of the four fundamental forces, electromagnetism, the strong nuclear force, and the weak nuclear force, are all governed by quantum physics. Gravity, the first fundamental force, appears to be the only force governed by classical physics. However, it is not quite that simple. Dr. Kaku addresses the difficulty in understanding quantum phenomena in this book, one reason being that we have no applicative frame of reference for its properties and behavior. Not to mention, on the quantum level, much is simply *unknowable*, as true randomness and uncertainty is the standard.

While physicists are still searching for a unified theory to pull the four forces together under the same laws, quantum physics may very well *be* the correct answer to unification, and merely *appears* to operate under a different set of laws on the macroscopic level, causing the perception of a division. Nonetheless, despite its multiple perplexing properties, much *is* known about quantum physics. Understanding the basics of it is important to understanding how a multiverse is mathematically feasible.

I have to commend Dr. Kaku, as both a physicist and as an educator. These are some of the most difficult to understand concepts that we’ve ever explored as humans, however, he finds a way to effectively explain, to entertain, and to enthrall readers, without it ever feeling as though he is talking down to, nor pushing a personal agenda on them.

So,* are* there parallel worlds? It’s no surprise that Dr. Kaku does not have a definitive answer as to whether any of these alternative models of the cosmos actually represent our reality. Many quantum concepts are far out of reach of the capabilities of human math and sciences, and therefore remain entirely untestable. However, if you give the book a chance, you may realize new horizons of wonder, as you ponder the exciting possibilities and curiosities about this place in which we all reside.

I highly recommend this to readers with any interest in physics or applied mathematics.