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, but it works. The problem is it is so deeply alien to our current way of thinking it seems inconceivable. It 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)

Bayesian Probability

The science of probabilities goes back centuries, using basic methods of counting how often something specific happens. You do not have to flip a coin very many times to see that each side comes up half the time, so the probability is one half. This is called frequentist probability. Bayesian probability, based on Bayes’ theorem, is a well-established technique that is more sophisticated. Putting it very simply, this is based on relevant data of events in the past, and an analysis of how this data affects the probabilities of possible events in the future. If you think the person pressing you for a coin toss on some issue looks dodgy, you are likely doing some Bayesisan probability assessment on whether his coin is fair.

Naturally, 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 are no facts of the matter about the world unobserved. In other words, the facts that are not known, meaning 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. 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. Indeed, the assumption is made there is no physical reality actually there. In other words, QBism holds that quantum theory should not be thought to offer a description or representation of physical reality itself. (This kind of view is called pragmatist, and it appears in various forms in interpretations of quantum theory as described at the Stanford Encyclopedia of Philosophy.)

On this view, a quantum state just represents probabilities that a specific agent will experience a specific event. Thus the quantum theory is considered to be concerned solely with the interactions of an agent with the world. This view is adopted in order to get away from the issues of objectivity which have haunted quantum theory from the beginning. The whole issue with the collapse dynamics is that it seems to have no physical reality. (Decoherence does solve one aspect of this issue, but not the big one, as described in Measurement Problem.) The whole problem in the measurement problem is it seems the things experienced cannot happen in physical reality, so this view holds there is no real physical reality. This does not go down well in physics.

For concepts like this to challenge the current paradigm, an explanation would be required. If the world is to be understood as personal in this extraordinary way, how can that be explained in the view of what we know the real world to be like? The idea of world superposition resolves exactly this issue. The world encountered as physical reality is a many-worlds reality, the effect of world superposition, as described in The Indeterminate World. 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. The key point is that this does not invalidate the traditional view of the objective physical world. Both are true. The world described by QBism is an emergent ‘second-logical-type’ phenomenon, as described in Logical Types. Critics say that such a radical viewpoint cannot be adopted without an ontology for the theory, meaning an explanation of how and why this works with regard to the real physical world. In other words, objectivity must be addressed. World superposition is the ontological explanation of QBism.

The QBism World

Given that the QBism world is the correct description of the world one encounters, fantastic implications follow automatically. Firstly this means that the world is bizarrely undefined, until it is observed. The world at large is indeterminate, meaning it exists 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.

The really strange implication is that this makes a difference to everyday life. In this kind of reality, confirmation bias makes it more and more likely that one will experience the events in the world one expects, as described in Interactive Destiny.

The Born Rule

In a fascinating development, Fuchs has discovered that the Born rule can be rewritten almost entirely in terms of this Bayesian probability. 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 Bayesian 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 many-worlds reality of the inside view, rather than in the objective physical world of the outside view.

This result strongly suggests that QBism is correct, in that probabilities of what happens in the physical reality of each individual are indeed defined solely by the observations made by the individual. This is of tremendous importance because it tells us the change apparently induced in a physical system under observation is nothing of the sort. It means this can only be a phenomenon operating purely in the subjective frame of reference, the inside view of the many-worlds reality. As stated by mathematician Euan Squires, in a quantum experiment with two outcomes:

The complete description of the “physics” in orthodox quantum theory … contains both terms, i.e. both “results”. The unique result of which I am aware does not exist in physics – but only in consciousness. The Born rule does not have anything to say about physics – it says something about consciousness. (1996, 3)

As Everett shows, there is only the appearance of this change of the physical world. In other words, this happens only in experience, on the inside view. As he demonstrates, this is a central aspect of any successful formulation of quantum mechanics. This is all described in detail in Many Worlds. If the Born rule turns out to be purely an expression of Bayesian probability, this would seem to show that the observation dynamics operates solely on the inside view, in experience.

The Born rule has never been derived from any principles of physics. It 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 The Indeterminate World,  this is not exactly correct. The information that defines the determinacy of the personal world is the record of observations, which is not just imaginary. This defines the class of worlds in which this individual exists. Fuchs holds that “Quantum mechanics is a law of thought” (Gefter, 2015) in his interview with Quanta Magazine. However, it is here proposed that it is more correctly considered as a law of observations, as shown in the Many Worlds page.