(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.

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.

This thought experiment uses the fact that radioactive decay is also a quantum phenomenon in which we do not know exactly when an atom will decay but can only assign probabilities that it will do so in a given time interval. So at the beginning, the atom will be undecayed but as time goes on, the probability that it will decay within that time interval increases. While the cat and the rest are in the closed box, since the atom is in a superposition of a decayed and an undecayed state, that must mean that the entire system must also be in a superposition of two states. One is the state of an alive cat and an undecayed atom. The other is a dead cat and a decayed atom. When the box is opened by us the superposition collapses and we see just one of those states.

But we have not really learned anything new. The problem is that the cat, even if found alive, is unable to tell us whether it was in either or both of the two states before we opened the box. So this does not really solve the problem of how we know that there exists a superposition of states before the measurement. I actually find the whole cat paradox distracting and not helpful but I introduced it because everyone expects it to appear whenever we discuss the strangeness of quantum mechanics.

To introduce the possible role of consciousness, I find it clearer to deal with the spin up and spin down particle and a detector. Here too we can discuss the situation in terms of a combined system of the detector and particle. This can be treated as a larger quantum system that consists of a superposition of two states: One state consisting of the detector that shows the result that the particle is spin up combined with a particle that is spin up, the other state consisting of the detector that shows the result that the particle is spin down combined with a particle that is spin down. Now when we make a measurement on this combined detector plus particle system, the wave function of this new enlarged state collapses to one of those two states, which is what we see. But this requires yet another detector/observer to act on the system of the original detector plus particle.

You can see that by introducing a new detector, we are starting a regression, where now the new detector can also be combined with the earlier detector and particle to create a newer, larger, and more complicated superposition of states consisting of two detectors plus the particle. Now we need yet another detector in order to collapse this new enlarged superposition of states. And so on, with the quantum system becoming larger and larger, including more and more detectors, until it finally encompasses the entire universe. At this point the process must stop since there is no room for even a larger detector to serve as the external observer. So the system of the entire universe must be in an uncollapsed superposition of states.

At this point, some physicists say the hell with it, that they have better things to do than worry about something that may not have any consequences in real life. They shrug their shoulders and stop with just the original particle in a superposition of states and a single macroscopic detector and are satisfied that that interaction somehow causes the particle’s wave function to collapse. It does not matter if we don’t know how or why it collapses, it just does, that is the way that the world is, and we should just accept it and move on. The immense success of quantum mechanics despite this big hole in our understanding suggests to them that knowing the answer to this particular question likely does not matter and that we should just use quantum mechanics to calculate observable quantities, something that it has been remarkably successful in doing in the microscopic world. As has sometimes been said, the attitude taken is “Shut up and calculate!”

But others refuse to shut up. They think (and I tend to agree) that this is an interesting question in quantum theory that is worth addressing even if it may not have practical consequences in determining experimental results. But these attempts can lead, if we are not careful, to ideas that can open the door to woo territory. I am not saying that all the people who take these next steps are promoting woo but the fact is that some of these serious suggestions made by credentialed physicists have been exploited by woo practitioners (I am looking at you, Deepak Chopra) to make all manner of outlandish claims, and the introduction of consciousness into the discussion is one of those elements.

Some people suggest that the reason we have the regression that eventually leads to a quantum system consisting of the universe as a whole is because all the detectors we have been talking about so far are inanimate and materialistic objects made up of atoms, and thus are subject to also being treated as quantum systems. They argue that to collapse the original wave function requires at some point the presence of a non-material entity that cannot be considered as a quantum system. They say a conscious observer satisfies this requirement because they claim that consciousness is something immaterial that is not governed by the laws of physics and thus cannot be made part of the ever-expanding detector system. Instead it can act on the system to collapse it. In other words, the presence of a Geiger counter or a camera or a bubble chamber does not collapse the wave function until a conscious observer notes what that detector has recorded. Until that point, the combined system of the detector and particle remains in a superposition of states, uncollapsed, with both options possible. So now we have consciousness as part of the mix and you can see why things can get wild because consciousness itself is a contentious issue quite apart from any role that it might have in quantum mechanics. Once you have introduced a non-material entity that lies outside the laws of physics but can act in the world as an autonomous agent, you are off to the races and pretty much anything goes.

As a hard-core materialist myself, I do not view consciousness as a nonmaterial entity. I view it as an epiphenomenon, a byproduct of the workings of the brain, rather than as some kind of separate entity that can act independently and do things outside of the brain, like collapse the wave function. And since the brain is a material entity, it is subject to the same constraints as any other detector and thus can become part of a superposition of states, like the cat. So, for example, we can treated the particle and the conscious observer as being in a superposition of two states: one consisting of the particle being spin up and the brain registering it as spin up, the other with the particle spin down and the brain registering it as spin down. And so we are back to the regression to the universe issue.

So, in my opinion, introducing consciousness into this discussion does not advance our knowledge and only invites people to include all manner of quasi-supernatural mysticism. But maybe that is what they want. As far as I am concerned, consciousness plays no special role in quantum mechanics.

This ends my planned series of posts triggered by questions about the video of a discussion between physicist Bernard Carr and Robert Lawrence Kuhn about the role of consciousness in measurement. I hope that it answered at least some of people’s questions. This is a topic that has many subtleties and counter-intuitive ideas so I would not be surprised if it led to more questions, though I hope it did not lead to more confusion!

I will be adding more posts to deal with any questions that come up that require extended responses. I already plan to add one on exactly what constitutes ‘objective reality’ and how we can tell if it exists, in response to a comment by bluerizlagirl.