(See Part 1, Part 2, Part 3, Part 4, Part 5, and Part 6. 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.)

In order to fully appreciate the role of Heisenberg’s uncertainty principle on the question of objective reality and measurement, a highly truncated history of quantum mechanics might help.

The theory traces its beginnings to 1900 when Max Planck decided to assume that the material that made up the walls of the cavity inside a body that was at a uniform temperature (called a blackbody) could be treated as oscillators that could only absorb and radiate energy in discrete amounts (‘quanta’) and not continuously as had been previously assumed. The size of these quanta depended upon the frequency of oscillation as well as a new constant he introduced that has come to be known as Planck’s constant h. The value of this constant was very small, which is why it had long seemed that the energy could be absorbed and radiated in any amount. This was a purely ad hoc move on his part that had no theoretical justification whatsoever except that it gave the correct result for the radiation spectrum of the energy emitted by the blackbody. Planck himself viewed it as a purely mathematical trick that had no basis in reality but was just a placeholder until a real theory came along. But as time went on and the idea of quanta caught on, he began to think that it could represent something real.

In 1905 Einstein proposed that light energy also came in quanta and this was used to explain the photoelectric effect, which was what he was awarded the Nobel prize for in 1921. Then Niels Bohr in 1913 used the idea of quantization to come up with a model of simple atoms that explained some of their radiation spectra. Both of their works used Planck’s constant.

Erwin Schrodinger’s eponymous equation was proposed by him in 1926 and set in motion the field of quantum mechanics because it laid the foundations of a real theory that enabled one to systematically set about making calculations of observables. Almost simultaneously, Werner Heisenberg came up with alternative formulation based on matrices. (Later on P. A. M. Dirac showed that the two formulations were equivalent.) But Schrodinger’s theory was in the form of a differential equation that enabled one to calculate the wave function of a particle that was moving under the influence of a potential. Differential equations and wave behavior were both very familiar to physicists and thus Schrodinger’s approach was more easily accessible and used more widely.
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(See Part 1, Part 2, Part 3, Part 4, and Part 5. 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.)

As promised, here is a follow-up post to discuss how we know whether an ‘objective reality’ exists in the quantum world or not. It took me longer to write than I anticipated because the issues are subtle and I had to be careful in how I try to explain them. It is also a little long.

To refresh some ideas, ‘objective reality’ means that a measured quantity exists before we measure it. i.e., the measurement merely tells us what already existed. By contrast, the standard interpretation of quantum theory says that for certain properties of a particle, the measured value only comes into existence upon measurement and does not exist before. Hence the quantum world does not demonstrate objective reality. The problem is that since we seem to need the measurement in order to know what the value of the quantity is, it looks like we cannot say whether it existed before the measurement or not.

So how can we know something without in fact measuring it? Einstein suggested that if we can predict the outcome of a measurement with 100% accuracy, then that property has an objective reality, in that it exists before the measurement. i.e., it is as good as having been measured even though it has not been directly measured.

Let us now look at the scenario described by bluerizlagirl in a comment to Part 4 in this series.

How is this different from taking a red card and a black card from a deck; having someone select one at random, climb in a spaceship and travel halfway across the universe; and as soon as I look at my card, say it happens to be red, I know at once that their card is black? They have always been opposites from the outset, so as soon as you know the state of either one, then you automatically know the state of the other one, by the property of oppositeness.

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(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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