Through out the development, and indeed the popularisation of physics apparent paradoxes have had a prominent place. These paradoxes are usually the product of applying false logic to physics and/or a misunderstanding of what the mathematical theories are really telling us about the Universe. Prof. Jim Al-Khalili in his latest book, Paradox: The Nine Greatest Enigmas in Physics, takes us on a mind-bending tour of some of the perplexing puzzles that have shaped our understanding of the natural world.

This book is about my own personal favourite puzzles and conundrums in science, all of which have famously been referred to as paradoxes, but which turn out not to be paradoxes at all when considered carefully and viewed from the right angle.

Jim Al-Khalili

 Photo by Andy Miah Some of the paradoxes covered are well known. For example, Schrödinger’s famous cat, Olbers’ paradox and the time-travelers favorite, the grandfather paradox.

In resolving our paradoxes we will have to travel to the furthest reaches of the Universe and explore the very essence of space and time. Hold on tight.

Jim Al-Khalili

In my opinion Al-Khalili presents a rather readable and light-hearted review of some rather interesting issues in physics. It was an enjoyable read overall.

Outline of the book

Chapter 1, The Game Show Paradox, discusses the now infamous Monty Hall problem. Without giving the game away too much, one has to think about the role of prior knowledge in probability theory. This is more a problem in basic probability theory than physics, but it illustrates the need for careful thinking.

Achilles and the Tortoise is the title of chapter 2. Here the topic is Zeno’s paradoxes-“motion is an illusion”. Clearly, motion is not an illusion, so what is going wrong? What false logic and/or mathematics is being applied here?

Chapter 3 deals with Olber’s paradox: why is the night sky dark? The resolution takes us to the forefront of cosmology. How does this support the Big Bang?

Maxwell’s demon is the title of chapter 4. Maxwell imagined a hypothetical experiment describing how to violate the Second Law. In his thought experiment a container is divided into two parts by an insulating wall, with a door that can be opened and closed by some entity known as “Maxwell’s demon“. The demon is able to view the molecules and opens the door only for “hot molecules”. Thus, one side of the container heats up and the other cools, causing a decrease in entropy in violation of the Second Law. How is this resolved?

The idea of length contraction and simultaneity in special relativity are the subject of chapter 5. The Pole and the Barn Paradox, which is always used in university classes is discussed and resolved.

Chapter 6, The Twin Paradox, deals with the very famous paradox of relativistic twins. How can they age differently? What is time dilation and is it real?

Time travel and the paradoxes it creates have fascinated people for a long time. Do we know if time travel is allowed or not anyway? Chapter 7, The Grandfather Paradox, discusses the issues here and suggests some resolutions to this. What happens if you went back in time and killed your grandfather? Maybe we live in a multiverse and time travel means we enter a parallel world? Maybe the laws of physics just don’t allow time travel? Non-one really know and this is an unresolved paradox.

In chapter 6, The Paradox of Laplace’s Demon, the idea of determinism is discussed along with the technical issue of chaos. Determinism says that if we know all the positions and momenta of all the particles in the Universe at a given time, then we can use classical mechanics to predict exactly the state of the Universe at some later time. Any being that could do this would loose freewill as everything is totally predetermined, this is the demon. This is not compatible with quantum mechanics (as we know it), but even more basic than this classical systems do exhibit chaos- “sensitivity to initial conditions”. Can we have determinism, but not predictability?

The Paradox of Schrödinger’s Cat is the subject of chapter 9. According to quantum mechanics, is the cat when we are not actually looking at him alive and dead at the same time? What is the role of the observer? How come, if everything is made of quantum particles do macroscopic objects behave classically? Not everything here is that well understood and quantum decoherence, that is many quantum objects becoming a classical object, is an ongoing area of research, both mathematically and experimentally.

Are we alone in the Universe? As the Universe is large and old there must be other forms of life out there? Some of these must be intelligent, capable of space flight and given enough time colonise the Galaxy? In chapter 10, Fermi’s Paradox Al-Khalili asks these questions. Fermi highlighted the contradiction between high estimates of the probability of the existence of an extraterrestrial civilization and the lack of evidence that they exist.

The book finishes off in chapter 11 entitled Remaining Questions. Al-Khalili lists some of the open problems in science that today are at the forefront of human knowledge. One should take away from this chapter the sense that despite what we do know about the Universe, questions remain.

UK publication

Hardcover: 256 pages
Publisher: Bantam Press (12 April 2012)
Language: English
ISBN-10: 0593069293
ISBN-13: 978-0593069295

 US publication Paperback: 256 pages Publisher: Broadway; Reprint edition (October 23, 2012) Language: English ISBN-10: 0307986799 ISBN-13: 978-0307986795