God and Dice and Quantum Physics

Wednesday May 4 2016
by Maddy

Albert Einstein. Everyone has heard of him. From a young age he is presented to children as the epitome of genius. His image is often what comes to mind when one is told to picture a scientist. Old, grey haired, moustachioed, a little bit crazy. A simple equation that is only useful to a handful of people known and repeated by children. E=mc2. It rolls off the tongue. One doesn’t need to know the meaning of it just that it had a huge impact on physics at the time of its conception.

The deeper and deeper you get into physics the more and more stories you hear about this marvelous human. It forms a beautiful picture of intelligence, wit, humility and admiration of the physical world. If you want to know what Albert Einstein contributed to science you can get that information almost anywhere. Try trusty old Wikipedia. But to really get an idea of how amazing he was you need to be finding anecdotes and letters written between him and other great minds of the time. It’s particularly hard to find true anecdotes about Einstein. His name is synonymous with ”genius” and many stories have popped up over the years about Einstein’s cheeky antics and witty comebacks. Most of these are just reworded jokes, all of which are dubious as to their veracity.

There is one quote from Einstein that often pops into my head when thinking about quantum physics. Though it has been translated from German it is a genuine thought from the man himself:

I, at any rate, am convinced that He does not throw dice.”

This is often shortened to “God does not play dice”.

What does this quote mean?

More often than not to a non-physicist it sounds like a beautiful metaphor about fate. Like God does not leave anything to chance or Everything happens for a reason. But to be very frank this is wrong. Sorry. Einstein was a physicist after all.

The first misinterpretation is that Einstein believed in a God. In a way he did. It is widely interpreted that Einstein was what we might call a Pantheist. Pantheists believe in the divinity of nature, not in a personal God. It is a belief that goes well with mathematics and science and discovery. But I’m not here to discuss religious beliefs of historical figures. If you want to get an idea of Einstein’s beliefs check out this letter Einstein wrote to Erik Gutkind after reading his book, Choose Life: The Biblical Call to Revolt. I’m here to talk about quantum physics.

You see, when Quantum theory was first revealed, Einstein had a problem with it. He had a problem with the indeterminacy: the idea that before we measure something, it's everywhere and doing everything it possibly can. Not because we don't know, but because we can't know.

It’s probably worth going into the physics a little bit here so bear with me. For example, let's take an electron, the negatively charged sub-atomic particle that “flies” around the positively charged nucleus of an atom.


Fig. 1 Really crude Rutherford model of an atom. Protons are positively charged sub-atomic particles. Neutrons are neutral, they have no charge. Electrons are negatively charged. Protons and neutrons live in the centre, the nucleus, while electrons are much lighter and faster and fly around the outside.

Now imagine we want to know something particular about it. Like, what's the spin? An electron will act as if it is spinning (spinning doesn’t really doesn’t really make sense for an elementary particle but it still has this property). The ‘spinning’ can be either clockwise or counter-clockwise which we label up and down.


This is where the word Quantum comes in, when we measure the spin of an electron it can only ever be these two values and nothing in between we say spin is quantised. As a classical analogy, that’s like saying your car can only go 10km/hr or 20km/hr and nothing in between. Nonsense!

But I glossed over a little point here, it’s not that it can only BE up or down it’s that we can only MEASURE up or down. The electron is often in a superposition of states until we measure it. This is an idea you may have heard of in terms of the thought experiment (or in German, Einstein’s native tongue a Gedanken) Schroedinger’s Cat. “The cat is both alive and dead until the box is opened” is what people often take away from it. While this is not the correct interpretation of the problem it does help us to grasp the concept of superposition a little more easily. The particle is in this weird superposition of up and down until we measure it and it picks one.

My car analogy breaks down here because it’s not that the car can only go one of these two speeds it’s that whenever we check the speedometer it ‘jumps’ to either 10km/hr or 20km/hr.

Weird, I know.

Now imagine I prepare an electron in a specific way that I can replicate over and over again. I do this and measure the spin with my SPIN-DETECTORTM. Let’s say I get spin up. Now I have a cup of tea and a biscuit and after by break I set up the experiment again in EXACTLY the same way with a new electron. It’s important that it is exact, that’s what makes it an exciting result. I don’t mean like an experimental ‘I did it the same’ but a theoretical ‘exactly the same preparation’. Now I measure the spin of this electron. It is spin down.

What? ‘That doesn’t make sense’, I say. I repeat the experiment again and again. Each time preparing it in exactly the same way. And each time if find I cannot predict whether I will read spin up or spin down. That is the indeterminacy of nature. When SPIN-DETECTORTM looks at the electron God rolls his dice to decide between up and down and the little blue LED screen blinks with his answer.

If this doesn't make you go “Wowzers, my brain is imploding!” then consider this classical analogy: I set up a cannon to shoot a cannonball in the air and paint a big old ‘x’ where it lands. After the obligatory cup of tea I set up this experiment again. Same cannon, same cannonball, same direction, same wind. Everything exactly the same. I shoot the cannonball and it lands precisely on the ‘x’ I drew before. No matter how many times I repeat this it will always land on that ‘x’. Its future is determined by the laws of classical physics laid down for us by Isaac Newton in the 17th century.

When you get down to atomic scales there is a fundamental limit on what you can predict. It’s not that we don’t have good enough equipment it is just not possible. This is what Einstein had a problem with. It just didn’t sit right with him. He, Boris Podolsky and Nathan Rosen came up with the EPR paradox. Another Gendanken that was thought at the time (1935) to disprove this “quantum indeterminacy” stuff. He believed in the “Hidden variables” interpretation.  That there was some other factor that we didn’t know that could fill in all that gaps a number maybe that would allow us to predict everything exactly.

That is the meaning behind the quote and this is the reason it is so intriguing to consider: Einstein was not right about this one.

In the 60s a physicist named John Stewart Bell mathematically proved that no theory of hidden variables could ever reproduce all of the predictions of quantum mechanics. Amazing! It took thirty years but we now know that this is the way nature works. It is indeterminate. Unpredictable.

In no way does this mean Einstein wasn’t an extraordinary scientist and a great thinker. The moral to take is definitely not “Einstein wasn’t so great after all” but to accept quantum physics as strange. So strange that on a philosophical level it is hard to accept as truth.

This weird unpredictable trait of nature is known as the Copenhagen interpretation. If you'd like the read more about it and other theories check out the Wikipedia article on Interpretations of quantum mechanics.▼

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