(See Part 1, Part 2, Part 3 and Part 4. 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 is time to finally get to the issue that triggered this series of posts and that is why some people think that consciousness plays a role in quantum mechanics.

When Einstein asked his friend Pascual Jordan whether he really believed that the moon exists only when he looked at it, he was undoubtedly being facetious. It is like asking if, when we enter a completely dark room and turn on the light and see all that is there, whether the furniture was not there before but only appeared because we observed it. It is not necessary that I must observe it, just that someone has observed it. In the case of macroscopic objects like the moon and room furnishings, the state had been observed before and thus it is no longer in a superposition of states. Thus the world of macroscopic objects is classical. The issue only arises when we talk about something that has not been observed before, such as the spin of a particle that has been created in a superposition of two states.

The big unanswered question is: What is it about a macroscopic object (the detector) that triggers the collapse of the wave function from a superposition of states to a single observable state? We have talked glibly about this interaction of the state with detectors somehow being the cause but we can also ask what makes something a detector. The detector could be something like a camera or a geiger counter or anything that macroscopically registers the state that the particle is found in so that we can know it. But if we believe that everything in the world is ultimately governed by the laws of quantum laws, which most physicists do, then the detector should also in principle be governed by quantum laws even if it is technically impossible to carry out the calculations.

This brings us unavoidably to the famous (or infamous) Schrodinger’s cat.

In Schrödinger's original formulation, a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal radiation monitor such as a Geiger counter detects radioactivity (a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. If no decaying atom triggers the monitor, the cat remains alive. Mathematically, the wave function that describes the contents of the box is a combination, or quantum superposition, of these two possibilities. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality resolves into one possibility or the other.

This cat is perhaps the most famous cat in history, heard of even by people who have no idea who Schrodinger is or what the cat is supposed to have done. I have never quite understood the fascination with this story. It was created by Schrodinger because he intensely disliked the idea of a states being in a superposition and he felt that by making the state macroscopic, the absurdity of the idea of a cat being in both dead and alive states would be manifest and people would reject it. But his cat is not an argument for or against superposition and is thus an irrelevancy.
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(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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