Relativity reveals the fundamental nature of the world, the four-dimensional space-time universe. This, however, is only part of the picture. Taking quantum theory at face value, it means we live in the domain of all possible worlds. This is the meaning of Everett’s famous many-worlds theory. So these are the essential, imagination-wrenching adjustments to our worldview. It means not only is the universe incomprehensibly vast, but also every possible version of the world exists within it. The latter may sound too weird to be believable, but this is increasingly understood to be the only tenable interpretation of quantum theory, as described in The Case for Parallel Universes in Scientific American.
The next point, which is no less tricky to grasp, is that when it comes to a quantum universe, which is what we live in, all these worlds are here and now. They are not somewhere else, off in the distance. They are parallel realities, slightly different versions of the whole world, that exist exactly where we are. In this sense it is just like the parallel worlds in The Long Earth science-fiction series by Terry Pratchett and Stephen Baxter. The final point is the strangest of all. As described by Max Tegmark in Parallel Universes (pdf), one actually exists in a very large number of these worlds; this is an inevitable consequence of the new physics.
In the domain of all possible worlds, there are a great number in which there is an identical copy of you. And in the quantum universe, they are all here and now, like the worlds in The Long Earth. All standard physics so far. There is a potential implication of all this which has not been aired before. As the individual you know yourself to be, you exist, simultaneously, in all these worlds. And this in turn means that your physical reality is the superimposed sum of all these words, a world superposition. And this kind of world operates precisely as described in QBism. It is indeterminate except where you have observed it. In the more technical pages here the logic of this is described in some detail, starting in The Indeterminate World. Here we are concerned with what this actually means.
Suppose there is a butterfly perched behind me sunning himself; I cannot see the position of his wings. For every possible position he may have adopted, there is a version of the world in which I exist. The net result in my physical reality is the sum of all these versions, as illustrated by the last image in the picture below. In my physical reality, the position of his wings is indeterminate.
This is the physical reality of the world of the individual. Everything I observe is determinate, but everything unobserved is a superposition of all possible variations of how it could be. This is what quantum theory is telling us. Here, however, we have a simple rationale for this weird concept, and a physical basis for the bizarre definition of the world in QBism.
This of course sounds all wrong because it seems so obvious the world we are actually living in must be an ordinary world, all there, real and solid. And it is. But the physical reality you encounter, the world superposition, is the simultaneous existence of a great number of these worlds; and in this domain only what you have observed is determinate. The things you have not observed are different in these different worlds, therefore, like the wings of the butterfly, these aspects of the world are all the different possibilities superposed; and the net result is indeterminate. In other words, exactly as described in QBism, other than that which is directly observed, everything is defined solely in terms of probabilities. In all likelihood the butterfly has his wings wide open for display, so this is the most likely observation for me if I turn and look. But before I do, the real physical situation is not just unknown, it is indeterminate. This is the QBism world, the result of world superposition.
The key point is that this resolves the great paradoxes of quantum theory, as the authors of QBism show. What is more, nothing else does according to a hundred years of research. In other words, taking physical reality to be a world superposition resolves the fearsome paradoxes that have defeated comprehension of the quantum theory all this time. It is certainly very weird but it is enormously significant. As stated by von Baeyer, quoted on the Home page, the general consensus is that the deep things quantum theory is so urgently trying to tell us about our world are irrelevant to everyday life, and too weird to matter. As shown here such perceptions are utterly false. The things the science is trying to tell us are certainly very weird because they are exactly the opposite of what seems so obviously to be true. But they could not be more relevant. As described in the following pages, one is deeply connected with the personal world; and one is constantly interacting with what is likely to be experienced happening in this world. All this is simply taking quantum theory at face value, as described in detail in the second category of pages here. In this section of the site the implications are described in detail.