Debaters like to pretend they know exactly how the world works. So do physicists. I don’t want to take a stance on this issue, but I will say this: the theory of quantum mechanics is so beautiful and surprising that anybody who wants to understand the world should try to understand its basics. No physicist will dispute that this theory is one of the greatest achievements of modern physics, and has revolutionized our understanding of the world. One of its consequences, the fact that our world isn’t deterministic, is already quite surprising, but that is only the very beginning. If you’d keep asking a physicist to explain in everyday terms what it means to live in a quantum world, he might at some point respond with: “That’s a good question, but really that is an issue for philosophers.” Quantum mechanics has another lesson for us, and the world is stranger than anyone could have previously imagined. But apparently even the ones who invented the theory haven’t really been able to put their finger on it. I don’t claim to understand it better than them, but I will try to give you a glimse of the mysterious problem that is behind the weirdness of quantum physics.

In a way, taking my first quantum mechanics course was very different from studying any other part of physics. It started like any unfamiliar theory, with blackboards full of new equations and unfamiliar concepts. But once I started asking questions the answers weren’t as satisfying as I had become used to. Normally I might not understand everything, but I would at least come to understand where the gaps in my knowledge are and slowly become reassured that everything makes sense after all. In quantum mechanics it’s the opposite: at first it appears quite reasonable, but once you start asking further questions the entire thing sort of starts to fall apart, and the more you think about it the further you will fall down the rabbit hole.

I don’t want to give the impression that quantum theory doesn’t work. It works extremely well, explaining everything from the forces acting on the most fundamental particles and the transistors in a computer chip to the chemical reactions in a campfire and the working of a solar panel. And even better, all its predictions that have been tested turned out to be correct.

The problem with quantum mechanics is called the measurement problem. To explain it I first have to talk about superpositions. A well known example is this: suppose somebody would put a cat in a box, with a radiation detector and an unstable particle. During the experiment there is a 1 in 2 probability that the particle decays. If that happens it will trigger the detector, which then opens a valve and releases a poisonous gas, killing the cat. The box is isolated extremely well, so we cannot look inside, hear the cat meow, or see the box shake from its movement during the experiment. We know that when we open the box to look inside after the experiment, there is an equal probability that a happy kitten jumping out or . But the question is this: is the cat dead or alive before the box has been opened? From our everyday experience we would predict that it is alive until the particle decays, and dead afterward. But we would be wrong, because quantum mechanics predicts the cat is in an unfamiliar combination of being dead and alive until we open the box. This state of the cat is called a superposition. Only after we open the box it will instantly change into either of those two states.

You might object that this is just a semantic trick, and that the cat was already dead before the box was opened. But you would be wrong, quantum mechanics tells us the superposition really is a possible state the cat (or an electron, or almost anything else) can be in. The only reason this is hard to accept is that we have never seen a cat in a superposition. In theory we could built a special “superposition-detector-machine”. It will do something similar to us looking in the box and finding a dead or live cat, but instead it will measure that – yes – the cat is in fact in a superposition. For cats such a detector can’t be built in practice, but for electrons (for example) it is, and it is being done routinely in physics labs. This shows there is nothing special about superpositions. In a way they are like a cup of tea, that is not very hot anymore but not yet completely cold. If we had never taken a gulp of lukewarm tea we might not believe that to be possible, but if someone could make it in a lab we would have to agree that, okay, it is possible and there is nothing really special about lukewarm tea.

The strange thing here, from a physicists viewpoint, isn’t that the cat is in a superposition, but that the state of the cat suddenly changes from being in a superposition to being either dead or alive after we perform a measurement by opening the box. Somehow by opening the box we changed it from being in a superposition to being either still alive – with a 50% probability – or dead. By drinking the lukewarm tea, it suddenly becomes burning hot or ice cold, and we cannot predict which it will be before we take a sip. The fact that measurements will often instantly change the state of a system, and will do so in a fundamentally unpredictable way, is what makes quantum mechanics so strange.

In fact, even in quantum mechanics, the state of the system always changes in a completely predictable deterministic way until a measurement is performed. Only the measurements are somehow random and unpredictable. And things get even stranger. Suppose we had a machine that looks at a decaying atom and kills exactly one of two cats in two different boxes. Afterward someone quickly flies one of the boxes to the other side of the galaxy. At that time the cat in that box will be in a superposition. Then the remaining box is opened. When we see a hungry cat jump out, the cat in the other box instantly stops being in a superposition, and becomes dead.

We can conclude that being able to perform a measurement is almost a sort of superpower. We can somehow get random results, while everything else is deterministic. And we can suddenly change the state of a cat on the other side of the galaxy. But what is so special about the experimenter that it gives him this strange superpower? That isn’t really clear from the laws of physics. Two electrons colliding don’t cause a measurement. Can an electron detector perform a measurement, or does it only happen after someone looks at the meter? Does it require being conscious or even having studied physics? None of these questions have a clear answer. Many attempts have been made to reformulate quantum mechanics in a way that doesn’t explicitly mention the experimenter, but to date none of them is really accepted.

Most people using quantum mechanics in their work try not to ponder over such questions. They don’t have to, because quantum mechanical predictions arealwaysaboutspecific measurements. For example, we can predict the likelihood that two colliding atoms will stick together and form a molecule, or that an incoming light particle (photon) is reflected by a glass window. For somebody doing such an experiment there is no ambiguity, he (or she) knows well enough that he is performing the measurement himself. But this means that anybody doing quantum physics gives himself a sort of special status. Heplaces his own mind outside the usual laws of physics, and describes them in an entirely different way. That is a severe break with the tradition in physics, starting with Plato, of putting our personal experience in the second place, and treating it as nothing more than a shadow of the real world out there. Quantum mechanics is a theory of our own experiences, of the cause and effect connecting our consecutive observations. This is the only way in which it currently makes sense. And since all we can ever do is observe the world around us, this can never lead to a contradiction.

It could well be that this strange situation is only temporary. Maybe we just haven’t gasped the full picture. Some people are trying to push the boundary of quantum mechanics, to explain how our subjective measurements can be stripped of their special status. But for a long time idea of an objective truth hasn’t been part of modern physics, and at some point all physics students have to accept that.

Spraakwater article by Josse Muller

ps. There is one common misconception I want to prevent. While performing a measurement on a system does cause an instant change on the other side of the galaxy this can not be used to send information faster than the speed of light. This has to do with the fact that the outcome of a measurement is random. The experimenter cannot choose whether he will find his own cat to be dead or alive, and hence also cannot choose what happens to the other cat.