Overview
"The Quantum Universe" is an approachable book that attempts to explain the mathematical ideas underpinning modern quantum theory. In this regard, it is quite different than most other books of its kind. Take Brian Greene's brilliant "The Fabric of the Cosmos," for instance: whereas Greene attempts to provide intuitive descriptions of quantum phenomena, Cox and Forshaw attempt to provide intuition for the mathematics of quantum theory. In other words, whereas most pop modern physics books strive to explain how the quantum world behaves, "The Quantum Universe" is more concerned with explaining the mathematical formalisms we use to describe quantum behavior.
Complex ideas are explained by analogy
If this sounds a bit intimidating, don't worry: there's very little math to be had. As in every good science book for the masses, the authors explain by way of analogy. You're invited to think of "quantum interference" as water waves canceling each other out; of "propagating probability waves" as a set of clocks moving through space; and of logic gates in semiconducters as simple hydraulic valves. For the most part, the analogies are enlightening, and they make a lot of intuitive sense.
That said, this book IS a lot more technical than most of its compatriots. If you've never picked up (and read through!) a popular presentation of modern physics, this probably isn't the place to start. The quantum world is a strange place, and you'll want to make sure you've had some exposure before you let Cox and Forshaw guide you through the particulars. On the other hand, if you've ever felt like every pop physics book tends to rehash the same analogies - and you're ready to probe a bit deeper - "The Quantum Universe" is a breath of fresh air.
Short summary of content
About half of the book is spent explaining electron propagation. The authors do a great job of explaining deeply mathematical ideas without invoking any complex equations. Schrodinger’s wave equation is describe in terms of clocks: "numbers in the complex plane" are replaced by the "clocks," complex addition amounts to "combining clocks," and so. We're told to imagine a propagating probability wave (read: traveling electron) as the aggregate sum of a bunch of winding, traveling clocks, and to find the probability a certain electron will show up at a certain place, you learn to add up all of its clocks and take the height of the resulting hour hand. It all sounds pretty confusing and nonsensical, summarized in a paragraph like this, but the authors really do use this analogy to great effect.
The rest of the book uses this simplified model of electron propagation to explain some pretty deep concepts. The highlight of the book comes when the authors effectively derive the Heisenberg uncertainty principle using nothing but said "clocks" and a clever bit of reasoning. The authors go on to explain why electrons only swirl about atomic nuclei in quantized (read: "discrete") energy levels; they introduce you to the Pauli exclusion principle, and use it to explain why we don't simply fall through the floor; and they attempt to convey the utility of quantum theory by explaining how it led to the development of semiconductors.
Conclusion
The authors probe ambitious depths, and for the most part, they succeed. Unfortunately, they sometimes over-reach. Certain topics, it seems to me, simply cannot be penetrated without fully delving into some serious mathematics. Clocks can only do so much, and as a result, you often have no choice but simply take the authors’ word as they ask you to make one unmotivated leap of faith after another.
Despite this occasional wand-waving, "The Quantum Universe" is a great book. Most books like it focus on describing the quantum weirdness we observe, they as such, they tend to leave you a bit bewildered. Here, Cox and Forshaw focus on explaining how it we're able to model this weirdness, and as a result, you'll leave feeling like it at least makes sense that someone out there really understands this stuff. "The Quantum Universe" appeals to your intuition without dumbing down its subject matter, and if you're willing to delve a bit deeper than usual - and you're still able to surface at the end - you'll come out with a profoundly deeper understanding of the weird, weird ways in which our world works.
Quotes:
"Quantum theory is perhaps the prime example of the infinitely esoteric becoming the profoundly useful" (2)
"The job of quantum theory should be to predict directly observable things... it should not be expected to provide some kind of satisfying mental picture for the internal workings of the atom, because this is not necessary and it may not even be possible." (13)
"We have learned that our perception that objects move smoothly from point to point is, form the perspective of quantum theory, and illusion. It is closer to the truth to suppose that particles move from A to B via all possible paths." (90)
"...point-like particles are really of no size and to as 'What happens if I split an electron in half?' makes no sense at all - there is not meaning to the idea of 'half an electron'." (116)
Appendix: what is a "propagating probability wave"?
To begin with, let's briefly talk about "wave-particle duality." The idea here is that electrons sometimes behave like particles, and other times behave like waves - in other words, electrons are both particles and waves. Huh?
The idea that electrons are particles is fairly intuitive. It’s pretty easy to imagine an electron point orbiting an atomic nucleus. In fact, it turns out that electrons look like particles whenever they're observed: that is, we never detect electron waves. We only detect electron points.
On the other hand, electrons sometimes behave like waves, in that they 'interfere' with one another. If you shoot a bunch of electrons at two tiny side-by-side holes in front of an electron-sensing plate, the electrons hit the plate in a 'striped' pattern. It's as if the electrons were waves - some entering through the right hole, and others through the left - where at some places, the waves cancel out, and at others, they combine. The end result of this ' wave interference' is the observed striped pattern.
But it gets even weirder that this: apparently even a single electron interferes with ITSELF. Let’s say you shoot an electron through the two holes. Once it hits the plate on the other side, you shoot another. You repeat this for a while. At the end of this one-electron-at-a-time experiment, you'll see the same stripped pattern as before. Again, here electrons were all fired in series, and yet, at the end we observe a pattern that indicates interference.
To explain this so-called double-slit experiment, we assume that a single electron is actually many places - at the same time. Essentially, we assume that an electron is a PROBABILITY wave. An electron travels as a sort of "wave of possible electron locations." The wave is "highest" at locations the electron is most likely to be found. And as soon as the wave is observed, or interacts with something, the electron simply manifests itself at a SINGLE location, where the location was picked, at random, from the probability distribution.
This basically means that it makes no sense to ask "what path did the electron take to arrive at the location of measurement"? In a sense, it took EVERY path touched by its probability wave, and in doing so, it "interfered with itself. "
Weird? Totally. True? Almost certainly. Interesting? That’s for you to judge, but cool shit, as far as I’m concerned.