John Archibald Wheeler (1911-2008) occupies a pivotal place in the history of quantum foundations because he pushed the Copenhagen intuition to its most radical and unsettling conclusions. His later work represents the beginning of the paradigm shift I am describing in this book. Earlier in his career he was the Ph.D. advisor and mentor of two other key figures in the history of quantum mechanics: Richard Feynman and Hugh Everett.
In the late 1940s Feynman came up with a fourth version of quantum theory, called “sum over histories”. To calculate an electron's fate, Feynman adds up all its possible histories, cancels everything out, and whatever is left represents what will actually happen, expressed as a pattern of probabilities. This is a computational reformulation which has no impact on the measurement problem. Feynman’s formulation makes the ontology even more mysterious: what does it mean for a particle to ‘take all possible paths’? The sum over paths gives a probability amplitude, but to connect it to definite outcomes, we still need a Born rule interpretation and a collapse or selection mechanism. The Born rule, named after physicist Max Born, states that if a quantum system is described by a wave function, the probability of finding the system in a particular measurable state is proportional to the square of the magnitude of the probability amplitude for that state. In the context of Feynman's formulation, the process calculates a probability amplitude for a specific history or final state. The Born rule is then the essential step that converts this calculated amplitude into a real-world probability that the outcome will be observed in a measurement. Feynman's interpretation retains unitarity (the mathematical equivalent of a complete, uncollapsed wave function), and doesn’t explain how or why a single outcome emerges in a measurement, so the collapse problem remains.
In 1951 David Bohm's book Quantum Theory argued “electrons are not things” and the following year Bohm appeared to do what von Neumann had claimed was impossible: he created a model that allowed electrons to be ordinary objects without contradicting quantum theory. Bohm's theory is very strange. It involves something called “pilot waves” – a new sort of physical entity, with its own fundamental field and a new law of motion. Quantum entities “ride” on the pilot wave, which is aware of everything else going on in the universe, including “measurements”, and communicates this to the electron. Now the electron can be a normal electron, and the unordinariness is in the wave. Had the world's greatest mathematician made a mistake?
In 1957 Everett invented a radically new interpretation: Many Worlds. Since measurement devices are no different to anything else in the world, measurement interactions cannot be special. Bohr had to assign a special status to measuring devices, conferring on them a classical-style existence that is not possessed by the entities they are measuring. Von Neumann didn't consider measuring devices special, describing them in terms of possibility waves just like any other sort of matter. But in order to justify this, von Neumann had to make the act of measurement metaphysically special – he had to remove it from the rest of reality. MWI gets rid of the act of measurement altogether, by positing that where von Neumann thinks there is an act of measurement or observation, reality splits into multiple diverging timelines.
By the mid 1960s there were already at least five metaphysical theories competing to be the one true interpretation of quantum theory. The first was the original form of the Copenhagen Interpretation. This was Bohr's view, and maybe still is the default position for physicists if you ask them what quantum mechanics tells us about reality. Electrons have no definite attributes when unmeasured. This position flatly contradicts the Newtonian conception of a material reality made of microscopic entities that are themselves ordinary objects with ordinary properties, but fails to replace it with anything but a mystery. This might work if all you're interested in is the practical application of quantum theory, but it raises major philosophical questions that it makes no attempt to answer. Reality is “out there” – it is local – but at the smallest level it is fuzzily undefinable. But what does “smallest level” mean? This interpretation requires a completely arbitrary “Heisenberg Cut” – a fundamental division between two radically different sorts of reality, without any explanation as to why two different sorts of reality exist, or why the line between them should be in any particular place. The big problem with Bohr's view is that he treats measuring devices differently from everything else in reality: everything is treated as a probability wave except for the measuring device. But why should measuring devices be granted immunity to the quantum laws that apply to everything else? If your answer is that the measuring device should be treated as a quantum system that is measured by another measuring device then you have an infinite regress – what is the real measuring device? This is called “von Neumann's paradox of infinite regress”, because von Neumann was obliged to break the infinite regress by postulating the collapse of the wave function.
The second was Heisenberg's view, which had already started drifting towards a more observer-centric stance. Quantum theory, according to the CI, represents the world in two different ways: the observer's experience is expressed in the classical language of actualities, while the unmeasured quantum realm is represented as a superposition of possibilities. Heisenberg suggested we take these representations literally as a model of the way things really are. Thus, according to Heisenberg, the unmeasured world actually is what quantum theory represents it to be: a superposition of mere possibilities. Heisenberg called them potentia: unrealised tendencies for action, awaiting the magic moment of measurement that will grant one of these tendencies a more concrete style of being that we humans experience as actuality. It is worth noting that while this view resembles that of von Neumann in important respects, Heisenberg thought of “measurement” as something more epistemological than ontological – measurement as a change in knowledge rather than in reality.
Von Neumann's theory was being championed by Wigner, who in 1961 came up with another thought experiment, which is now known as “Wigner's Friend”. Wigner asked: what happens when the observer himself is observed? Imagine an observer (Wigner's "friend") inside a sealed laboratory, performing a standard quantum measurement, such as observing whether an atom is in a superposition of decayed and not-decayed states. According to the standard CI, before observation the atom exists in a superposition of both states. The friend opens the detector, makes an observation, and records the outcome. From the friend's perspective, the superposition collapses: the atom is definitely decayed or not decayed. However, from Wigner's external perspective, the friend himself, along with the atom, the detector, and the friend's brain, can be described by a global wave function that remains in superposition. Until Wigner opens the laboratory and asks the friend for the result, quantum theory (in its unitary evolution form) insists that the entire lab-friend system exists in a combined superposed state. So when does the collapse actually occur? Is it when the friend sees the outcome? Or is it only when Wigner gains knowledge of what his friend has seen? Wigner affirmed von Neumann's view that consciousness plays a special, essential role in collapsing the wave function. When the friend becomes aware of the measurement outcome, reality snaps into place: collapse occurs not by interaction with a macroscopic device, but by the intervention of a conscious observer. He wrote: “It is not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to consciousness.” Physics, on this view, is not a detached description of a mind-independent world, but is inherently observer-relative. Wigner’s conclusion was bold, but it carried a heavy price: it seemed to imply a hierarchy of observers that led straight into the jaws of solipsism. If the "Friend" is in a superposition until Wigner looks at him, is Wigner himself in a superposition until his wife looks at him? This "infinite regress" suggests that nothing is ever truly real until the "Ultimate Observer" at the end of the chain takes a peek. In the late 1970s, Wigner began to retreat from his 1961 position. His change of heart wasn't sparked by a sudden love for materialism, but by a deeper realisation about the nature of physical laws. He began to fear that if consciousness were the sole cause of collapse, then the laws of physics would be "nonlinear" in a way that would make objective science impossible
In Bohm’s theory, the quantum potential is nonlocal, meaning the behaviour of one particle can depend instantaneously on the configuration of the entire system. Quantum attributes reside in “the entire experimental arrangement”... which has to end up meaning the whole of reality, since the whole universe could be implicated in a single measurement. Entanglement plays a central role: the pilot wave evolves in configuration space, so subsystems are generally non-locally correlated. And completing this picture was MWI, which denied collapse and thereby attempted to make the whole concept of “measurement” redundant.
In 1964 John Bell set out to answer the question raised by Bohm's theory: had von Neumann made a mistake? He had not. Von Neumann's proof involved the caveat that no theory involving ordinary objects combining in reasonable ways is consistent with quantum theory. There is nothing ordinary about pilot waves, The wave is nonlocal: its evolution depends on the entire configuration of the system, so it encodes global information instantaneously, which involves faster-than-light transmission of information, which is prohibited by special relativity.
In his attempt to understand what had gone wrong, Bell made the most important advance since 1925. He came up with a theorem (not a mere theory, but a logical proof that will never be overturned) that demonstrates that any model of reality (not just quantum mechanical, but any model at all) must be non-local. Bell proved that Bohm's superluminal connections are unavoidable. If you believe objective reality is non-local (as Kant did) then this is no problem at all, because the superluminal connections can exist at the noumenal level – they can be part of non-spatio-temporal “reality as it is in itself” rather than part of reality as it appears to us. In contrast, anybody who believes in a local objective reality and accepts special relativity has got some major rethinking to do. After Bell, any theory of reality has to either be explicitly non-local, or somehow make the local/non-local distinction irrelevant. This was already known in 1966, but is worth mentioning that the 2022 Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for their works on “quantum nonlocality”.
Bell's theorem has implications for the options listed in the previous section. MWI avoids Bell’s conclusion by denying that measurements have single outcomes – a move that sidesteps the theorem rather than resolving the underlying conceptual tension. Bohr rejected the idea of an underlying reality, but he still treated measurement outcomes as local events, which is incompatible with Bell’s result. It cannot work without breaking Einstein's speed limit, so (assuming Einstein was right), Bell’s result renders Bohr’s local interpretation untenable. Heisenberg's view survived, provided the noumenal “potentia” are considered to be connected superluminally rather than locally. Bohm's view remained alive for the same reason. A recent Nature paper (2025) challenges key assumptions of Bohmian mechanics, though debate is ongoing 1 Bell’s theorem marks the point at which the old metaphysical scaffolding collapses. After Bell, the question is no longer whether reality is no-local, but what non-locality means.
1Sharoglazova, V., Puplauskis, M., Mattschas, C. et al. Energy–speed relationship of quantum particles challenges Bohmian mechanics. Nature 643, 67–72 (2025). https://doi.org/10.1038/s41586-025-09099-4.