My link to a video of a discussion between physicist Bernard Carr and Robert Lawrence Kuhn generated a request for me to to clarify what was being said about the possible role of consciousness in quantum measurements. With me, you have to be careful about what you wish for because as so often happens, my attempts to explain difficult physics concepts leads to multi-part posts because of all the subtleties involved. I hope that readers will think and discuss each part and clarify it in their minds before moving on to the next section.

Since this is a tricky topic, before I give my views, let me state my background in this area so that you can judge for yourselves whether to give any credibility to my opinions. I have worked all my professional life in the area of quantum physics, and thought and read about the measurement problem a lot and have even taught about it as part of quantum mechanics courses. But I have not published any papers in this particular area of quantum mechanics. I also apologize in advance for some oversimplifications that I will make in order to make the subject more intelligible to people without a background in quantum mechanics or even physics. I will also, where appropriate, include the technical terms for various processes. It is not important that you know this jargon. I only include it so that people who read other articles that use those terms will have a better idea of what is being talked about.

To start off, let us postulate that the universe can be split up into two categories: a microscopic world (roughly of the order of atoms and smaller) and a macroscopic one of large objects. How we describe how things behave in the two worlds is quite different. The behavior of entities in the microscopic world is governed by the laws of quantum mechanics. That world is not accessible to our senses but has to be probed with sophisticated instruments and so our knowledge about it is necessarily indirect. The behavior of the macroscopic world is governed by classical physics (such as Newtonian and Einsteinian dynamics) and much of it is accessible via our senses.

The macroscopic (or classical) world can be roughly defined as such that at any given time, entities are presumed to have specific properties such as an exact location is space and an exact velocity, even if we do not actually know them. The classical laws are such that those two things, combined with knowledge of the forces acting on the entities, completely determine their subsequent behavior. As a result, they behave deterministically and in principle if not in practice, we can predict with 100% accuracy where they will be at a later time. This is called classical determinism. So if I flip a coin, the outcome of whether it will lands heads or tails was 100% determined by the type of coin, how it was tossed, and the environment. Since we do not have perfect knowledge of those conditions, we cannot predict the outcome, even though it was completely fixed by the toss.

The microscopic (or quantum) world is very different. The equivalent of a coin toss is the creation an elementary particle that could have two different properties. We usually discuss it in terms of objects (say electrons and photons) that can rotate in two different ways about an internal axis (like the Earth spinning): clockwise (we can call it ‘spin up’) or counterclockwise (we can call it ‘spin down’). (To be precise, the objects are not thought to be actually rotating. They have a property that has the dimensions of rotational motion so ‘spin’ is an appropriate metaphor to describe it.) When we measure the spin (the equivalent of looking at the flipped coin) we always find it to be either spin up or spin down, just like the coin is always found to be either heads or tails.

So far, so good. We are on firm ground, I hope

The catch is that the popularly accepted interpretation of the laws of quantum mechanics (known as the Copenhagen model) that is used to describe the microscopic world say that prior to the measurement of its spin, the entity was in both states and that it was the act of measurement that forced it into one or the other state. We cannot predict the state into which it will be forced but it is not completely arbitrary. The outcomes are determined statistically, depending on the experimental set up, with fixed probabilities of being found spin up or spin down. Thus the best we can hope are probabilities, not certainties. This is called quantum determinism, to distinguish with classical determinism.

To expand on this difficult idea, the laws of quantum physics say that entities (such as the electron or the photon) do not have a specific location and velocity either. The most information that we can get about them is what is called the wave function which only gives us the probabilities that the entity will be is a given spin state (or location or velocity) if we decide to measure any of those things. If we repeat the identical experiment (the equivalent of repeating the coin toss), sometimes we will find one spin state (or location or velocity depending on what we choose to measure) and sometimes another. But again, it is not completely random. If we know the wave function, then we can predict the probability that we will find each state. The difference with the classical coin case is that in that case, each individual result (heads or tails) existed before we looked at it but in the quantum case the result does not come into existence until we make the measurement. If we applied that same idea to the classical world, it would be like saying that the coin flip outcome is both heads and tails until the moment when we look at it when, by some mysterious mechanism, the coin abruptly goes into the heads or tails state, which is what we always find.

In short, in the classical case, we lack knowledge of something that already exists but in the quantum case, the result itself does not exist until we measure it.

This is what boggles people’s minds. And it was this particular feature of quantum mechanics that Einstein found unsatisfactory, though he acknowledged that quantum mechanics was the best theory that we had so far. Even during his lifetime, the successes of quantum mechanics in explaining so much had convinced him and pretty much everyone that it was the best way we had to describe the microscopic world and the evidence in support of quantum mechanics has only increased since then.

Next: Attempts to bridge the gap between the microscopic and macroscopic descriptions of the world.