Quantum for dummies – Part 2 – Interpretation

Back to Introduction

Back to Part 1 – The Basics

The nature of reality

In this week’s installment, I want to discuss what those two key attributes (wave-particle duality and the uncertainty principle) mean in the real world. There’s some really cool and exotic mathematics to describe the quantum world, but the physical interpretation is open to conjecture.

What it really means is that in the quantum world, there are no solid particles flying around other particles, as pictured in text books on classical physics. Rather, the positions of particles are ‘smeared’ over a piece of space in a kind of cloud, jiggling about madly. Picture standing in the middle of a kids’ jumping castle with a bunch of over-excited 5 year olds bouncing around. Or maybe cooking popcorn is a better image.

Anyway, it means that it is not possible to describe that state of a quantum particle with 100% certainty. We can talk only of probabilities. So each particle has a probability curve which tells us the likelihood of, say, being at a particular point in space. If you’re like me, you get bored with probability very quickly, so let’s just agree that quantum particles can be described by probability curves – also known as their Wave Function (hey, nice jargon!).

Fortunately, this world of uncertainty only applies at quantum scales. When we clump many particles together, the probabilities are aggregated, which is quite handy. At that point, we can treat these aggregated particles as an object in the macroscopic world, which brings us back into our comfort zone. This aggregate behaviour is difficult to grasp, but is better described by an everyday analogy. Take a kettle boiling on the stove. At any point in time, if we examine the water molecules we’ll find that they are zipping around frantically in random directions and speeds, bumping into each other and the wall of the kettle. Trying to describe the state of the water by measuring the features of each molecule would be a nightmare. On the other hand, we can describe the aggregate state of the molecules by the position of the kettle and the temperature of the water. Cool? Well, it will eventually.

In summary, the behaviour of quantum particles is certainly not something we see in the macroscopic world, so it’s very different from our day to day experience. But make no mistake, it has been verified experimentally over and over again, and has withstood scrutiny for 80 or 90 years now.

What was that thing about a cat?

OK, now that we’ve covered the basics and the reality of the quantum world, let’s have some fun and discuss some of the really weird stuff. What I mean by ‘weird’ is that quantum effects can be challenging to the imagination – our intuition about how things work doesn’t always give us the right answer. As a result, many of the implications of quantum effects have resulted in ongoing philosophical debates, and unfortunately, opened the door to pseudo-scientists to mis-use the science to con people. Yes, I’m looking at you, Deepak Chopra, you serial quantum-abuser.

So let’s discuss Schroedinger’s cat. Erwin Schroedinger was one of the giants of the physics world and contributed to the foundations of quantum mechanics. In fact, the Wave Function I mentioned earlier is known as Schroedinger’s Wave Equation, and is fundamental to the mathematical edifice of quantum mechanics. But enough gushing and adulation. He proposed a ‘thought experiment’ to explain quantum uncertainty by using macroscopic objects – a cat, to be specific. It goes like this.

We have a sealed box in which there is a cat. It shares the box with a container of poison gas, and an atom of a radioactive element, which is capable of piercing the container of poison gas. (I will spare you the details of radioactive decay – suffice to say that it is a quantum process, and therefore we can’t predict when, or even if, radioactivity will pierce the gas container). Looking at the box, there are two possibilities – the gas has escaped and the cat is dead, or, the gas is still contained, and the cat is, well, confused. Confused but alive.

In quantum-speak, the cat is both dead and alive, until we open the box to examine it. Put mathematically, the wave function of each possibility still exists – they are ‘superposed’ or added together.

Only when we open the box, is the wave function said to ‘collapse’, resulting in certainty instead of a range of probabilities.

Cruelty to animals aside, this thought experiment has been mis-used by many to justify various existential questions about the nature of reality – for example, does something exist if no-one is there to perceive it (i.e. collapse it’s wave function). In reality, it was proposed as a simple discussion tool using a scale more familiar to us. Nonetheless, Schroedinger’s Cat is now arguably more famous than Felix and Sylvester combined.

So how many realities are there?

The notions implied in the previous section on Schroedinger’s Cat have led to a number of theorists, most famously Hugh Everett in 1957, proposing that the two possibilities facing the confused feline are actually alternate realities. That is, both realities exist, but when we open the box, we only experience one of the realities, while somewhere in another reality, an alternate ‘us’ experiences the other reality.

Put in quantum terms, Everett proposes that the Wave Function is an objectively real thing, not just a statistical tool. And instead of the Wave Function collapsing as I said above, we are simply selecting one reality from the many on offer. This implies that all possible histories and futures are objectively real.

This is a bit mind-bending, and although supported by the mathematics, is still very much a theoretical construct and not supported by mainstream physics. There is certainly no evidence to support it, and it’s hard to imagine what such evidence would look like. Still, this interpretation has been a goldmine for sci-fi writers.

If you want to pursue this a bit further try this primer here, or better, Brian Greene’s ‘The Hidden Reality’, which I have reviewed here.

Next week we’ll have  short intro to the real world applications made possible by quantum physics, including the phenomenon of  ‘quantum locking’ which you just have to see to believe.

On to Part 3 – Applications