The name QBism is a contraction of Quantum Bayesianism. This stands for applying Bayesian probability, a technical method of calculating probabilities in everyday situations, to quantum mechanics. The authors demonstrate that the probabilities of events in the world, even the outcomes of subatomic experiments, are defined only by what the individual has directly experienced and observed in the world up until now. Yes, it seems to make no sense whatsoever.

Firstly this means that the world is bizarrely undefined, until it is observed. The world at large is ‘indeterminate’, meaning it exist in a quantum superposition of possible states. This is a big deal. In this kind of world, the physical reality ‘out there’ beyond one’s immediate perception is not all solid and real as we automatically think it is. It is not all there, waiting to be seen. Whether or not something ‘out there’, unseen, elsewhere in the world, is one way or the other is not a certainty. Rather than being actually one way or the other, the real world is probabilistic, in physical reality. So whether or not the news is accurate on a specific point, whether or not your computer has been hacked or your apartment bugged, are not certainties one way or the other. Everything not directly observed and confirmed is probabilistic.

Secondly, this means that the world is bizarrely personal. As the authors state:

What is real for an agent rests entirely on what that agent experiences, and different agents have different experiences. (2013, 3)

Clearly it is a major and fundamental change to our current worldview.

Bayesian Probability

The science of probabilities goes back centuries, using basic methods of counting how often something specific happens. You don’t have to flip a coin very many times to see that each side comes up half the time, so the probability is one half. Bayesian probability, based on Bayes’ theorem, is a well-established technique that is more sophisticated. Putting it very simply, this is based on the existing data of all the things known to have happened in the past, which might be relevant, and an analysis of how this relates to possible events and situations in the future.

Nonetheless, of course, in our ordinary worldview, Bayesian probability is still just an assessment of probability, the best guess. In most cases, there are attributes of a situation that are not known; so however sophisticated the method, there is no way to predict what is likely to happen with certainty because not all the facts are known. However, QBism takes the view that there is no fact of the matter about such facts. In other words, the facts that are not known, meaing attributes of a situation that are unobserved, are indeterminate – not just unknown but ‘physically uncertain’. As in the Copenhagen interpretation, there is no determinate objective reality ‘out there’, waiting to be observed. Only what is experienced and observed is determinate.

As shown by Christopher Fuchs, the lead author of recent papers, and his colleagues, now supported by David Mermin, this approach works. It resolves all the dreadful problems that have plagued quantum theory since the beginning. If we assume that physical reality is determinate solely where observed, the physics makes perfect sense. However, this is of course a huge leap away from the current worldview, so it seems incredibly weird.

It gets weirder. It means the world itself is personal, defined solely by what has been observed by the individual. As the authors state, this carries the bizarre implication that different people actually live in different versions of the world:

This means that reality differs from one agent to another. (2013, 3)

Epistemology Ontology

QBism is an epistemology, meaning it addresses only what we know of reality, not what is actually there in physical reality. Critics say that such a radical viewpoint cannot be adopted without an ontology for the theory, meaning an explanation of how and why the real physical world should have such a bizarre origin, and be personal in this extraordinary way. The idea of Universe Superposition presented here resolves exactly this issue. In a nutshell, taking quantum theory at face value, as proposed by Hugh Everett in the many worlds theory, the universe defines all possible physical worlds. The physical reality of the conscious individual is the superimposed sum of a vast number of such worlds; and the net result is a physical world determinate only where directly experienced and observed, exactly as defined in QBism. So the world of QBism has a simple and explicit ontology.

Given that QBism is correct, fantastic implications follow automatically. Firstly, as stated by Fuchs et al, the record of observations defines physical reality. In this case, the only definition of probabilities of events in the world is Bayesian, determined by past experiences. The crucial implication for everyday life is that in this kind of reality, confirmation bias effectively gives rise to an alteration of what is likely to happen in the world, as described in Interactive Destiny, meaning that which version of events is likely to be encountered is altered by what one expects.

In a fascinating development, Fuchs has discovered that the Born rule can be rewritten almost entirely in terms of this Bayesian probability. This result strongly suggests that QBism is correct, and that probabilities of what happens in the physical reality of each individual are defined solely by observations made by the individual. The determinacy of reality is defined solely by the record of observations – as explained in The Perceiving Subject, this is the same thing as the known world of the individual.

The Born rule is the law in quantum mechanics that gives the probability of a specific outcome to an observation of a quantum system. Fuchs has demonstrated that the rules of everyday probability, based solely on the observations made, apply directly to the fundamental components of physical reality. So it would seem we have evidence that this dynamics is in fact a phenomenon operating in the effective physical reality of the inside view, rather than in the physical world of the outside view. The Born rule, addressing collapse, has never been derived from any principles of physics. It is considered random. The Born rule is simply a formula that fits all the observations we make. Fuchs is showing this is a Bayesian rule. This remarkable result is described in a very accessible manner by Hans von Baeyer in his article Quantum Weirdness? It’s All in Your Mind (2013). As von Baeyer states in the overview, QBism sweeps away the bizarre paradoxes, but the cost is that “… quantum information exists only in your imagination”. However, as explained in Universe Superposition, this is not the case at all. The ‘quantum information’ is the determinacy of the world, and this is a property of the set of worlds in which this individual exists. Fuchs holds a similar line saying “Quantum mechanics is a law of thought” (Amanda Gefter, 2015) in his interview with Quanta Magazine. However, it is here considered to be a law of observations, as shown in the Many Worlds page.

Perhaps the greatest obstacle in the way of QBism being adopted is the apparent craziness of observations being so significant. How can observations, just looking at something, possibly influence objective physical reality? The answer, of course, is that they do not; but it is exactly as if they do. Exactly why and how this happens is explained in Universe Superposition. In a nutshell, the personal reality, the inside view, is a superposition of worlds. And the experience of this world is a sequence of such moments, the 4D matter& energy hologram movie of life. It is not the physical world, the definition of one frame of the movie, that is altered. What is affected is which path the movie is taking, going forward through the infinite possibilities of the quantum universe. This is what is defined by Bayesian probability; and this is what is altered as observations are made. Technical details are given in the Measurement Problem.