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Sirwin

Problems with the Copenhagen Interpretation

By GrabBag | CrackpotPhysicist | 9 Oct 2020


There's a problem with quantum mechanics. A big one. It's so big that most physicists roll their eyes, wave their hand, and ignore it. And it's such a simple problem even you can understand it.

You probably know what an electron is. At least, what you were taught in school. It's some really small particle with a charge, right? You probably also know what a sound wave is - it's a wave of energy that travels through air and allows us to hear things. A sound wave isn't all that different from a radio wave, which you may also be familiar with. Well, this is all the level of understanding you need to appreciate this conundrum. Buckle down, sit tight, and get ready. This one's a doozey.

You may have heard the term "photon" before, and you were probably told it's some small unit of light. This is true, but you were also told that radio and wifi signals are waves of energy. And you were also told that radio waves and visible light are both electromagnetic waves. So how can something be both a wave and a particle at the same time? Well this, ladies and gentlemen, is the great mystery of quantum physics, and nobody has a definite answer for it.

Let's illustrate it with this analogy. When you speak, your thoughts are being converted into sounds which travel to the eardrum of a listener. Those sounds travel outward from your mouth in all directions, even around and through objects. Speaking a word is not like throwing a baseball, which only goes in one direction and tends to bounce around a few times before stopping. In our experience, it would be very strange indeed if a sound turned into a baseball, or vice versa. But apparently something like this happens at the quantum level, called particle-wave duality, and scientists can't agree on what it means.

It's difficult to perform science at the quantum level. You can't watch quantum phenomena in the same way that you watch a baseball game: you can't be a passive observer. The only way to measure something at this scale is to interact with it and observe the side-effects of the interaction. By making observations, we "collapse" waves into a particles. This is a very strange behavior that doesn't appear anywhere else in nature.

The "Copenhagen interpretation" is probably the most popular way of explaining what the heck is going on with particle-wave duality. This interpretation says things at the quantum scale don't have definite properties. Position, spin, and velocity don't actually have defined values until a measurement is taken. Then, the measurement itself changes the system in a way that grants physical properties. After taking a measurement, the particle may turn back into a wave again, albeit a totally different wave. Quantum physics merely predicts the probability of what those physical properties will be once a measurement is taken.

So, there's a lot of problems with this ideology. Think of billiard balls bouncing off each other. Each ball bounce seems instantaneous, but it's not. It takes about two or three milliseconds for the energy to transfer from one ball to another. This is a continuous process, not an instantaneous one. Mathematicians try to avoid discontinuity because it generally indicates the math is wrong or something important is missing. The problem is: according to the Copenhagen interpretation, switching from wave to particle is not a continuous change. This should raise some red flags.

Here's another thing about billiard balls: their interactions are predictable. Just by knowing the initial conditions of the board and the motion of the cue ball, the outcome is nearly guaranteed. In the Copenhagen interpretation, the very nature of taking a measurement causes a system to instantaneously transform into a specific configuration, chosen at random in a probabilistic way. Albert Einstein famously rejected this idea, saying "God does not play dice!" to which Niels Bohr (one of the original creators of the Copenhagen interpretation) replied, "Einstein, stop telling God what to do." Einstein here is saying hold on, we must be missing something important. And Bohr is saying shut up and accept it.

And finally, the biggest problem with the Copenhagen interpretation: it doesn't define the kind of interaction which causes wave function collapse (a wave turning into a particle). Not just any interaction will do. Waves interact with each other all the time, but they interact as waves, not as particles, like how ripples flow on the surface of water. Experimental evidence has confirmed that even large molecules can interact in this way. So what exactly is different between the interactions scientists have in their experiments and normal everyday interactions? It's not necessarily the electronics in their detectors: some electronics have been designed to prevent waves from collapsing, such as quantum computers. It's really the act of "measuring" or "observing" which causes wave function collapse - and this can't be precisely defined.

There are other interpretations of quantum mechanics. Some interpretations resolve only pieces of the problem, but leave other gaps. Some are downright strange - such as the belief that observation actually creates reality. But then there's one interpretation that, as far as I'm aware, completely resolves all the problems of quantum mechanics without introducing any new ones. And it's arguably the strangest interpretation of all: the many-worlds interpretation. This topic is so amazing that it deserves a full article, so follow me if you'd like to read it!

The thing that amazes me is how physicists really don't care to interpret quantum mechanics. This really goes back to a fear of being wrong. Quantum phenomena really challenge our intuition, and sometimes it's better to have no opinion than a crazy one, especially when your reputation depends on it. But maybe if these topics enter the mainstream, people will be more receptive to them. That's my goal in writing these articles, so thank you for taking the time to read them!

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