10.3 Quantum Physics Interpretations

Life was simple when we just believed in classical physics. Classical physics did have its own problems around the question of free will if determinism was true. But quantum physics threw even bigger questions about reality itself.

To remind you of what we learned in the chapter about quantum physics, unlike classical physics which allowed us to measure the state of things and predict accurately(in theory) the position and momentum of objects in time, quantum physics defined the state of particles in terms of wave functions and equations. The wave functions which describe the state of the particles evolve with time. But they don’t tell us the exact state. They only tell us the probability of what we will find when we observe the state of the particle. Two big questions arise from this:

If the state of the wave function “collapses” only when we observe, what is its state before we observe it? What is “real” before we make observations? This is called the Reality Problem.

What exactly happens when we make observations that make the wave function collapse? This is called the Measurement Problem.

Firstly, what does “observation” or “measurement” mean here? If I had a measuring device that measures the position of a particle but I look at the measurement on the device only the next day, which of these two activities count as observation? Is it when the device made the measurement or is it when I looked at the device reading. Most scientists stick to the former definition. Some philosophers argue that the “observation” only happens when a conscious observer reads the measurement because no one knows the state of the device unless a conscious observer is able to look at it.

When we say that quantum theories only give us the probabilities of finding something somewhere, the first conclusion that we reach is that we just don’t know the correct equations. There is something missing in our equation that makes it probabilistic rather than being exact. Maybe if we find what is missing, we will be able to find an equation that makes accurate predictions. Even Einstein was a staunch supporter of this idea. He rejected the idea that nature is fundamentally probabilistic with the popular quote - “God does not play dice with the universe.” Considering all the things that happen around us, if a God does exist, I wouldn’t put it past him to play dice with the universe. I will also add a disclaimer that the statement made by Einstein was metaphorical and not literal.

Anyway Bell’s theorem proved mathematically that a local hidden variable theory cannot be true. I stressed the word local because it is an important caveat in the Bell’s theorem. It adds a condition that nothing breaks the principle of locality, which means that nothing can travel faster than the speed of light. If this is the case, then the hidden variable theory cannot be true. So far we haven’t found anything that breaks the speed of light and hence we are inclined against believing that the hidden variable theory is true.

If the hidden variable theory is not true, then what are our other options? The two widely accepted ones are the Copenhagen Interpretation of Quantum mechanics and the Many-worlds interpretation.

The Copenhagen Interpretation states that there is no point in assigning definite physical properties to a particle before it is observed. It is just in a quantum superimposition, which is a mathematical state. We cannot say how or what it is, in reality. It is only when we observe that we can assign meaningful properties. Until we observe it, we can’t explain what state it was in.

This explanation(though it doesn’t really explain and just gives up) wasn’t satisfactory to many physicists who wanted a sense of understanding of reality. The other interpretation that is The Many-Worlds Interpretation gave them that. According to this interpretation, at every measurement, the universe splits into multiple universes and depending on the universe, the observed measurements differ. How is probability explained here? Why does one outcome have a higher probability over the other? It is explained by a concept called quantum amplitude which I won’t try to explain as I am pretty sure that I will get it wrong. It is a fun theory though. The multiple universe theory is a staple in many science fiction and even in superhero stories these days.

With all these interpretations, one can see the impact of Quantum Physics on the field of Philosophy. If the Copenhagen interpretation were to be believed, then what exactly is reality? How can we believe in Physicalism if even Physics doesn’t say anything about the nature of reality? If the many-worlds theory is to be believed, it gives rise to a lot of moral questions - I may be a good person in this branch. Does that mean there are other branches where I am an evil person who uses loudspeakers in public transport? But physicalism does survive in the many-worlds theory though it leads to a lot of moral questions.