(See Part 1, Part 2, and Part 3. Also I am going to suspend the limit of three comments per post for this series of posts because it is a topic that benefits from back and forth discussions.)

Even if we decide to treat the microscopic and macroscopic worlds as separate and governed by different laws, they is one place where the two world collide that we cannot ignore. Recall that we said that in the quantum world, many results do not come into existence until they are measured. Any contact at all of a quantum superposition of states with a macroscopic object, however small, can cause the collapse of the wave function. But in order for it to be useful to us, we need to know what the final result was, and that means we need a measurement involving a measuring device whose results we can see, such as a detector like a fluorescent screen, photometer, bubble chamber, geiger counter, and so on. So when we measure (say) the spin or location of an electron, we unavoidably have an interaction of an object that belongs to the classical world (the detector) with an object that belongs to the quantum world and this leads to what is called the measurement problem.

To understand the measurement problem, recall that we start with a quantum system that is prepared so that a particle (say an electron or photon) is created such that we cannot predict which state (spin up or spin down) it will be found in upon measurement. We describe the wave function of this particle as being in a superposition of two states, one spin up and one spin down. (Such a superposition of states is said to be coherent.) This superposition will continue to exist as long as the particle does not interact with anything that can be considered macroscopic, however small. When it does, the wave function is said to abruptly shift from being in a superposition of the two states to just one of the states. (This process is referred to as decoherence.) We can’t predict with certainty which state it will collapse into but if we know the initial wave function (say because it is a solution of the Schrodinger equation that we are able to obtain), we can predict the probability of collapsing into each one.
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(See Part 1 and Part 2. Also I am going to suspend the limit of three comments per post for this series of posts because it is a topic that benefits from back and forth discussions.)

Einstein was a firm believer in what we call objective reality, the idea that objects have properties that exist independently of, and prior to, any observer measuring them. As fellow physicist Pascual Jordan recalled, “We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it.” In this case Einstein, who is so often associated with turning our views of space and time upside down, was firmly on the side of the ordinary person in the street in believing in objective reality. He felt that the nature of objective reality required the particle to be spin up or spin down even before any measurement on it and so a complete theory should give solutions that contain that information. The fact that quantum mechanics stopped short of doing so meant, he felt, not that it was wrong but that it must be incomplete, the stepping stone to a more comprehensive and better theory that encompasses it.

But after more than a century, no such theory has emerged and many (probably the overwhelming majority) of physicists have come to accept that the lack of more information than is provided by quantum mechanics is not a failure of the theory but is because there is no more information to be had. In short, there is no objective reality, at least in the quantum world. The theory is indeed telling us everything that we can know and so is complete.
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(See Part 1 here. Also I am going to suspend the limit of three comments per post for this series of posts because it is a topic that benefits from back and forth discussions.)

It looks like I may have not been sufficiently precise in my first post, leading to some confusion, so I am going to take a slight detour from my series of posts on this topic to address an issue that came up in the comments about the nature of the probability and statistics that is used in quantum theory and how it differs from what we use in everyday life, in particular, the nature of the uncertainty in predicting outcomes. (As always with quantum mechanics, since the phenomena involved are invisible to our senses and often counter-intuitive, we have to use analogies and metaphors to try and bring out the ideas, with the caveat that those never exactly represent the reality.)

Let’s start with classical statistics that we use in everyday life in a situation where the results of a measurement are binary. Suppose that we want to know what percentage of a population has heights less that five feet. If we measure the height of a single person, that will be either more or less than five feet. It will not give us a probability. How do we find that? We take a random sample of people and measure their heights. From those results, we can calculate the fraction of people less than five feet by dividing the number in that category by the size of the sample. That fraction also now represents the probability that if we pick any future person at random, that person will be shorter than five feet. When we pick a random person, we do not know which category they will fall into but we do know that it will be either one or the other. What we also believe is, that in the classical world, each person’s height was fixed before we measured it. We just did not know it beforehand.
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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.
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I like this discussion because it does not try to hide the fact that the interplay of the observer and the wave function in quantum mechanics is a fundamental unresolved question in physics.

Are observers central to physics, or are they more accurately framed as bystanders to and byproducts of phenomena that exist independently of consciousness? In this interview from the long-running series Closer to Truth, Bernard Carr, an emeritus professor of mathematics and astronomy at Queen Mary University of London, traverses the double-slit experiment, the fine-tuning argument and more to explore what significance, if any, first-person observation holds in the realm of fundamental physics. In his conversation with the US presenter Robert Lawrence Kuhn, he doesn’t adopt a personal stance. Instead, he considers these persistent questions through a contemporary frame, assessing how discussions around them have evolved and where they stand among physicists today.