The first thing to understand about Quantum Mechanics is that it was invented backward.
Normally a physicist will devise a theory of what is going on, and then work to test that theory, and see if it fits. If it doesn't, they try to come up with a better theory. The goal is to refine the theory to such precision that it can be expressed mathematically, in clear equations that successfully predicts what happens in experiments.
With Quantum Mechanics, however, the equations came first. No theory existed to suggest them. No theory has yet been proposed that makes sense of them.
A physicist known as Niels Bohr stitched together parts of two different equations to try to make sense of some strange results he'd gotten from one specific experiment.
The result was a set of equations that have literally never failed to predict the exact outcome of any experiment in which they have been tested.
Now, let's just put that into context a little. It's a bit like losing the combination to your bicycle's padlock, and then coming up with a new combination made up of half the number for your bank card, and half the combination for your luggage, and then finding that this new code not only opened your bicycle lock, but opens every combination lock of any kind that could ever exist, even in theory.
Niels Bohr had the magic code, but no idea of why it worked. Just the code, just mathematics. And from that day till this, physicists have struggled, with varying degrees of success, to work out why it is that these equations are so extremely powerful. What do they actually describe?
This point matters, especially if you want to understanding the unique position Quantum Mechanics occupies — that even now, well over half a century after the devising of these equations, there is no conclusive theory of what is really going on in reality to make these equations work.
Something clearly is. You don't just stitch together numbers like magic incantations and produce results like that. Whatever the Quantum equations are describing has to be very fundamental.
And this is because there are some extremely strange results to account for. Deep down at the level of atoms and electrons, things don't behave at all like they should.
There have, of course, been several attempts to interpret the results. Some are highly abstract. One famously involves multiple universes. The most widely accepted one, however, is what's called the “Copenhagen Interpretation”. It was named after the city where Niels Bohr and his colleague Werner Heisenberg put it together. It was their attempt to make sense of the experiments, of what the equations meant. And the Copenhagen Interpretation is useful because it really zeroes in on the strangeness revealed by the experiments.
It's also important to look at it here, because we need to see clearly how this whole thing is understood, so we can go deeper, and look at what's happening from a fresh angle, and for the first time see a new way that sense can be made of all this.
At its heart, the Copenhagen Interpretation isn't really that hard to understand, so much as it is strange. It's far more of a strange thing, than a complex thing. And of course something to always, always remember is that reality doesn't care about how strange we think it is. It does not tip-toe around our expectations of it.
Rejecting or sidelining ideas just because they are intellectually wrenching, or confound our assumptions, is, almost every time it occurs, a step backward, and almost always taken in fear. So let's put our science hats on, and take a look at what the Copenhagen Interpretation has to say for itself.
Inside atoms, electrons spin around and around. They spin around the core of the atom, which is called the nucleus (which is, for some reason, is Latin for ‘nut’). Now, a classical understanding of this looks something a little like a small solar system, with the electrons spinning around like planets around the sun, and that's not a bad place to start.
It's not that hard to understand, or to picture, that electrons and nuclei are doing the same, or a very similar kind of thing, to what planets and suns do. Just spinning around a central point.
But that's not actually what's happening. And to get some sense of what is happening, we're going to look at a simple, experimental issue.
Electrons are very, very small, even by the standards of subatomic particles. This makes them, for want of a better way of putting it, quite delicate.
Now, say the lights go out in your house, and you need to find your way through it in the dark to the fusebox, you might light up a candle, or pick up a torch. What that candle or torch is doing is (obviously enough) emitting light. The light bounces off your surroundings, comes back to your eyes, and you can see where things are.
If you shine a light onto an electron, the particles of light (photons) will smack into the electron, bounce off, and you can (basically) see what the electron's up to. But the electron is so small that the energy of the photon melds with the electron, and energises it.
And because it now energised, it cannot exist in the state in which it was, when you were looking for it. And so it vanishes from that place.
So it's a little bit of a problem. You can't discover the properties of an electron without changing those properties.
Still, okay, just a technical annoyance. Something to annoy experimenters. No major problem here, nothing big, nothing too strange.
Now, of course, you can guesstimate where that electron would be without shining a light on it. You can expand that guess and refine it, and chart the probability of where the electron might be, and how it might be moving. You'll never fix its position and speed entirely, but you can get an idea. You can narrow the field of possibility, and you can chart that field on a graph.
You can say — “Ok, well, there's about a 40% chance that it's spinning in this position, and a 30% chance it's spinning at this speed. And it's less likely to be over here, and more likely to be over there.” And you can draw the probability of it like a curve — the high point of the curve is the place where it's most likely to be, the low point of the curve is the place where it's less likely to be.
Simple enough. Just dealing with curves and experiments and such. No big strangeness with any of this, not really.
Of course, once you actually get your sleeves rolled up and “go in there” to find what this electron is actually doing, you energise it, and it's not doing that anymore.
But's it's ok, you've found out what it was doing. So you go back to the graph. You don't need to have that curved line charting where the probabilities lie. You know exactly where it was, so there's no need for them now.
So what you can do is get an eraser, rub out that curve on the graph, and just mark specifically what the electron was doing. Your graph looks a lot neater now. It's just a point, and you can mark it very accurately.
The actual electron of course, is now gone. More energy has been added to it by the photon, so it's not doing what it was doing.
This make sense?
We're just talking about a graph, remember. You can just draw on a graph a curve that represents where the electron probably is, and then once you know exactly, you rub out the line, and put in a little dot. Except to get the information to put that dot in the right place, you have to disrupt the electron.
Nothing too complex, nothing too taxing, nothing too strange.
And if all it were was just a graph, it would just be one of those things, were it not for the fact that…