Entanglement; Amir D. Aczel; 2002; Raincoast Books; 252 pages; Note, index

​This book is about quantum mechanics and entanglement.  You will know more about QM after reading it, but trust me, you will not understand it any better.  It is too weird and there are no analogies in our macroscopic world that apply… not even causality.  The book is a good history of the development of this crazy part of physics, from Newton and Young through Heisenberg, Bohr, Einstein and Bell.

QM seems to be saying that  two particles that are separated by great distance can, some how, instantly influence another. This seems to violate relativity (which it kind of does). Einstein rejected the QM model because it was proposing "spooky action at a distance". For most of the 20th century, no one knew who was right: Bohr, the champion of QM, or Einstein, the master of relativity. Einstein  is famous for saying "god does not play dice with the universe".  He was wrong and Bohr was right.

The book spends quite some time on the infamous two slit experiment.
Richard Feynman had this to say about the two slit experiment:

"We choose to examine a phenomenon  (the two slit experiment) that is impossible, absolutely impossible, to explain in any classical way. And which has in it the heart of quantum mechanics. In reality, it contains the only mystery."

The ideas in this book are far too deep to explore in a short piece like this. So rather than trying to write notes about the contents, I am going to describe the famous two slit experiment. 

Newton showed that light was a wave using a prism. Waves display interference. Click the link to see water wave two-slit interference. Young's two slit experiment shined  monochromatic light (light of one colour) through two vertical slits set a certain distance apart. When the light passes through the two slits and appears on a screen, it shows patterns of light and dark, aka "interference". Liight was a wave. Then Einstein came along and explained the photoelectric effect in which light is clearly acting like a particle (called a photon). Light is both a wave and a particle, and neither, at the same time! This is known as "wave-particle duality".

Flash forward a few years. The experiment is refined. With modern tools, they can see where and when a single photon hits the detectors. Laser light is used to create the interference pattern. Billions of photons interfering with each other  to create an interference pattern on the detector.

So far, so good. Nothing odd is happening here.

If we turn the brightness down, or in other words, fire fewer and fewer photons of light, we would expect to see interference pattern disappear because there will not be enough photons to interact with each other to create interference.  In the extreme case, experimenters can fire one photon at a time, separated by as much time as desired.  In this scenario, it is obvious that the photon must pass through one of the slits and, since it is alone, no interference can appear. But… it does! As each photon is fired, slowly over time,  a diffraction pattern emerges! This makes no sense. The only way to explain this is to say that the photon goes through both slits at once and interferes with itself!  That is three exclamation points in one paragraph. 

It gets stranger. If the experimenter makes any attempt to figure out which slits the photons go through, the pattern disappears. The reason for this is partly explained by  what the word "look" means in this context. To "look" at a photon or any subatomic particle, we must  bounce something off it. Doing this at the atomic level is like firing a bullet at a BB.  If you hit the BB, you know where it was, but you now have no idea in which direction the BB is now travelling. To know more precisely where a particle is, you must hit it harder, and that act upsets the QM applecart. This is just a restatement of the Heisenberg Uncertainty Principle. QM effects are reliable enough to make your smart phone work, but only if you do not look too closely at the mechanisms. If you look, QM hides.

No one has been able to  explain this in terms people can understand, and no one has been able to find an exception to the rules.  

The remainder of the book discusses the weird world of entanglement., focusing on Bell's Theorem which ultimately  defeated Einstein's world view.

Dumb asses light Deepak Chopra like to read into QM some magical effect that binds us all together. I am tempted to call the "the Force". That is rubbish. It should be noted that entanglement does seem to work at infinite speed, but it cannot be used to communicate instantly over vast distances. Einstein is still right about that. 

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