Everettian quantum theory, also called the many-worlds interpretation (MWI) or relative-state interpretation, is a framework for quantum mechanics proposed by Hugh Everett III in 1957. It rejects the idea of wavefunction collapse and instead asserts that the universal wave function evolves strictly unitarily according to the Schrödinger equation, applying equally to all systems — including observers and measurement devices.
Universality – Quantum mechanics applies to the entire universe as
a closed system, with no privileged “external” observe.
Strict Unitarity – No collapse postulate; the state vector evolves deterministically via the Schrödinger equation.
Multiplicity – Every possible outcome of a quantum measurement is realized in a separate, dynamically independent branch of reality.
Branching and Decoherence
When a measurement occurs, the global state becomes entangled between the system, apparatus, and observer:
|s1⟩⊗|φ0⟩→c1|s1⟩⊗|φ1⟩+c2|s2⟩⊗|φ2⟩
Here, |φi⟩ are pointer states of the apparatus, each correlated with a distinct outcome.
"The reasons why this is so are worth understanding, because they go much deeper than the usual objections, and they’re grounded definitively in a result known as Haag’s theorem, first proved by Rudolf Haag in 1955, a connection that the usual criticisms of Everettian quantum theory never mention.
In the Everettian interpretation, the quantum wavefunction never collapses but rather branches endlessly into parallel histories. Marletto makes her case in favor of this approach on the strength of one claimed virtue, universality. Quantum theory, she opines,
“doesn’t come with a label saying here I cannot be applied.” In other words, if you take the formalism at face value and apply it to everything, starting with the subatomic realm and on up through observers like us to the universe as a whole, and you reject the artificial addition of collapse postulates or hidden variables of any kind that end or supplement the continuous dynamics, then you arrive at the many-worlds version of quantum mechanics as the pure essence of the theory.
Marletto herself is aware of the extravagance and recoils from it. When pressed by Kuhn on the issue of an endlessly dividing reality, she demurs, insisting that this “sci-fi” version of MWI is a misunderstanding and that no universes are multiplied, no particles are created, and no conservation laws are violated. Rather, “it’s simply that the state changes” through the entanglement of systems.
This response is fair game since it is the going position among sophisticated Everettians who, following David Wallace, treat worlds as emergent patterns within the one evolving quantum state, real in
the way tigers and hurricanes are real emergent patterns in the world. But the analogy conceals the difficulty rather than resolving it. Tigers and hurricanes are patterns within an already actual world, whereas branches are supposed to be worlds. An emergent pattern inherits its actuality from the arena in which it emerges, and in the Everettian case the actuality of the branch is the very thing in question. If what MWI refers to as “branching” is really nothing more than an entanglement structure developing within a single quantum state, then the question of how a branch is actually a world constituting a concrete reality with you in it has not been answered; it has been dropped from the picture entirely.
You can’t have it both ways. Either the branches are, in fact, substantial parallel realities and the ontological inflation is real, or they’re merely bookkeeping structures in a mathematical object and you’re keeping the label “many worlds” while jettisoning the worlds.
In fact, if you are a theist, it is natural to read it in terms of divine omniscience, that is, as exhaustive conceptual knowledge of every history that could be experienced, of which one history, ours, is actual. The mathematical elegance and parsimony the Everettians so desire are preserved in this reading, but the quantum multiverse is not.
What needs attention here is what these phenomena actually show. Superpositions yield interference effects that no single component could produce by itself, which means the phenomenon cannot be dismissed as a mere indicator of epistemic ignorance. But it is also the case that in the experimental manipulation of superposed states we stand outside the superposition observing it, so in these cases, at least, its components are demonstrably real without being co-actual parallel realities in which different versions of ourselves observe different things. Nothing in the physics compels their promotion to concrete worlds elsewhere, either.
Examined through a quantum lens, physical objects cast modal shadows. The quantum wavefunction maps this possibility structure, decoherence traces its path into the single stable history of our experience, and the choice we are presented with is not whether the structure is real, but where it resides. The Everettian finds a place for it by actualizing all of it at a heavy price that we are about to evaluate; the theist houses it in the mind of God, and pays nothing but recognition of the One “in whom we live and move and have our being” (Acts 17:28).
Let’s start with the branching process itself. Consistent with the mathematics of waves, the quantum state can be decomposed into a sum of component waves in infinitely many different ways, each way defining a set of “worlds” that is wholly dependent on the choice of its components. For example, decompose the wavefunction one way and Schrödinger’s cat bifurcates into a living-cat branch and a dead-cat branch; but decompose it differently in an equally legitimate mathematical way, and the branches are exotic superpositions that have no classical description at all. Mathematics is no guide here.
The situation regarding quantum probabilities is even worse. To illustrate this, suppose that a quantum event has two outcomes with
probabilities of one-third and two-thirds.
The currently favored patch for this problem is due to David Deutsch and David Wallace. What they do is reinterpret the quantum probabilities as betting odds that a rational agent should adopt with respect to his branching future. This is a clever proposal, but there are compelling reasons to think it ultimately fails. As David Baker has argued, there is a vicious circularity in the repair. The branches must exist before the bets are placed, but these branches are created by decoherence, and the decoherence required has already helped itself to the probabilities the decision-theoretic analysis was supposed to supply.
Finally, let’s not forget the matter of persons. Marletto’s suggestion that MWI treats observers “democratically as qubits” sounds enticingly egalitarian until one asks “What happens to me?” at a branching event. Aside from our earlier point that in the experimental manipulation of superposed states we stand outside the superposition observing it, so its components are not co-actual parallel realities in which different versions of ourselves observe different things, the fact remains that the first-person perspective each of us occupies is an unassailable datum before we do any physics at all. It is not the sort of thing that can be partitioned. Furthermore, if the branching structure contains continuers of you making incompatible decisions, so that choices that you make are unmade by you elsewhere, then moral responsibility, which requires you (and not quantum dynamics) to be the actualizing source of the choice that is made, is left without a basis.
When we are dealing with finitely many degrees of freedom in ordinary quantum mechanics, the classic Stone-von Neumann theorem guarantees that, under standard regularity assumptions, the theory has essentially one Hilbert space representation in which it functions. Different formulations amount to nothing more than different notations that represent the same physics. When we move to quantum field theory (QFT), however, fields have infinitely many degrees of freedom — roughly, one for each point of space — and the guarantee of a unique representation collapses. There are infinitely many mathematically legitimate representations and they are inequivalent in the sense that the vacuum state from which the representation is built is not even a state in any other representation.
The fact remains, however, that this success is purchased by quietly setting aside premises that cannot hold exactly in the interacting
relativistic theory, and even after seventy years of effort, no nontrivial interacting QFT describing our four-dimensional world has ever been rigorously constructed. In fact, for the simplest candidate, approached through its standard lattice formulation, the mathematicians Aizenman and Duminil-Copin showed in 2021 that the exact interacting theory does not exist at all: take the continuum limit and the interaction completely disappears.
Now, consider the many-worlds picture: the Everettian asks us to be face-value realists about the “universal wavefunction,” the quantum state of life, the universe, and everything. In this picture, the state of everything must be a state of quantum fields. But Haag’s theorem tells us that no unique, dynamics-inde. At bottom, nature behaves in ways we can count on, but it does so for no discernible physical reason.
This is no mere gap awaiting some clever equation to close it. If we follow the two explanatory demands — the first being the existence of experiencers, and the second being the existence of a ground for their experiences — to their logical and ontological conclusion, declining, with the tradition of sufficient reason, to treat this structure as a brute fact, we arrive at the inversion of materialism. The possibility structures of this world reside in a Mind, and the actual world of our experience is that Mind’s communication of one unified history to the finite minds who share it. pendent arena exists for there to be such a state."
Universality – Quantum mechanics applies to the entire universe as
Strict Unitarity – No collapse postulate; the state vector evolves deterministically via the Schrödinger equation.
Multiplicity – Every possible outcome of a quantum measurement is realized in a separate, dynamically independent branch of reality.
Branching and Decoherence
When a measurement occurs, the global state becomes entangled between the system, apparatus, and observer:
|s1⟩⊗|φ0⟩→c1|s1⟩⊗|φ1⟩+c2|s2⟩⊗|φ2⟩
Here, |φi⟩ are pointer states of the apparatus, each correlated with a distinct outcome.
In the Everettian interpretation, the quantum wavefunction never collapses but rather branches endlessly into parallel histories. Marletto makes her case in favor of this approach on the strength of one claimed virtue, universality. Quantum theory, she opines,
Observers get treated “democratically, all in the same way as qubits.” It is, she states, “the interpretation with the least amount of problems.” This praise is fainter than it sounds, rather like commending a ship as the one with the fewest catastrophic leaks.
The mathematics of quantum theory does not wear its ontology on its sleeve. The claim, based on its mathematics, that every branch of the wavefunction is a real material universe containing a real material copy of you simply does not follow. It is an extravagant metaphysical gloss on the formalism that postulates an innumerable proliferation of unobservable worlds to avoid the conclusion that measurement outcomes are in any way special.
Marletto herself is aware of the extravagance and recoils from it. When pressed by Kuhn on the issue of an endlessly dividing reality, she demurs, insisting that this “sci-fi” version of MWI is a misunderstanding and that no universes are multiplied, no particles are created, and no conservation laws are violated. Rather, “it’s simply that the state changes” through the entanglement of systems.
This response is fair game since it is the going position among sophisticated Everettians who, following David Wallace, treat worlds as emergent patterns within the one evolving quantum state, real in
You can’t have it both ways. Either the branches are, in fact, substantial parallel realities and the ontological inflation is real, or they’re merely bookkeeping structures in a mathematical object and you’re keeping the label “many worlds” while jettisoning the worlds.
In fact, if you are a theist, it is natural to read it in terms of divine omniscience, that is, as exhaustive conceptual knowledge of every history that could be experienced, of which one history, ours, is actual. The mathematical elegance and parsimony the Everettians so desire are preserved in this reading, but the quantum multiverse is not.
What needs attention here is what these phenomena actually show. Superpositions yield interference effects that no single component could produce by itself, which means the phenomenon cannot be dismissed as a mere indicator of epistemic ignorance. But it is also the case that in the experimental manipulation of superposed states we stand outside the superposition observing it, so in these cases, at least, its components are demonstrably real without being co-actual parallel realities in which different versions of ourselves observe different things. Nothing in the physics compels their promotion to concrete worlds elsewhere, either.
Examined through a quantum lens, physical objects cast modal shadows. The quantum wavefunction maps this possibility structure, decoherence traces its path into the single stable history of our experience, and the choice we are presented with is not whether the structure is real, but where it resides. The Everettian finds a place for it by actualizing all of it at a heavy price that we are about to evaluate; the theist houses it in the mind of God, and pays nothing but recognition of the One “in whom we live and move and have our being” (Acts 17:28).
Let’s start with the branching process itself. Consistent with the mathematics of waves, the quantum state can be decomposed into a sum of component waves in infinitely many different ways, each way defining a set of “worlds” that is wholly dependent on the choice of its components. For example, decompose the wavefunction one way and Schrödinger’s cat bifurcates into a living-cat branch and a dead-cat branch; but decompose it differently in an equally legitimate mathematical way, and the branches are exotic superpositions that have no classical description at all. Mathematics is no guide here.
The situation regarding quantum probabilities is even worse. To illustrate this, suppose that a quantum event has two outcomes with
Q: If MWI is correct, then both outcomes occur, so what sense can be made of the assertion that their probabilities are unequal?
Q: In fact, if everything that can happen quantum-mechanically does happen, in what sense is anything improbable at all?
The currently favored patch for this problem is due to David Deutsch and David Wallace. What they do is reinterpret the quantum probabilities as betting odds that a rational agent should adopt with respect to his branching future. This is a clever proposal, but there are compelling reasons to think it ultimately fails. As David Baker has argued, there is a vicious circularity in the repair. The branches must exist before the bets are placed, but these branches are created by decoherence, and the decoherence required has already helped itself to the probabilities the decision-theoretic analysis was supposed to supply.
Finally, let’s not forget the matter of persons. Marletto’s suggestion that MWI treats observers “democratically as qubits” sounds enticingly egalitarian until one asks “What happens to me?” at a branching event. Aside from our earlier point that in the experimental manipulation of superposed states we stand outside the superposition observing it, so its components are not co-actual parallel realities in which different versions of ourselves observe different things, the fact remains that the first-person perspective each of us occupies is an unassailable datum before we do any physics at all. It is not the sort of thing that can be partitioned. Furthermore, if the branching structure contains continuers of you making incompatible decisions, so that choices that you make are unmade by you elsewhere, then moral responsibility, which requires you (and not quantum dynamics) to be the actualizing source of the choice that is made, is left without a basis.
When we are dealing with finitely many degrees of freedom in ordinary quantum mechanics, the classic Stone-von Neumann theorem guarantees that, under standard regularity assumptions, the theory has essentially one Hilbert space representation in which it functions. Different formulations amount to nothing more than different notations that represent the same physics. When we move to quantum field theory (QFT), however, fields have infinitely many degrees of freedom — roughly, one for each point of space — and the guarantee of a unique representation collapses. There are infinitely many mathematically legitimate representations and they are inequivalent in the sense that the vacuum state from which the representation is built is not even a state in any other representation.
The fact remains, however, that this success is purchased by quietly setting aside premises that cannot hold exactly in the interacting
Now, consider the many-worlds picture: the Everettian asks us to be face-value realists about the “universal wavefunction,” the quantum state of life, the universe, and everything. In this picture, the state of everything must be a state of quantum fields. But Haag’s theorem tells us that no unique, dynamics-inde. At bottom, nature behaves in ways we can count on, but it does so for no discernible physical reason.
This is no mere gap awaiting some clever equation to close it. If we follow the two explanatory demands — the first being the existence of experiencers, and the second being the existence of a ground for their experiences — to their logical and ontological conclusion, declining, with the tradition of sufficient reason, to treat this structure as a brute fact, we arrive at the inversion of materialism. The possibility structures of this world reside in a Mind, and the actual world of our experience is that Mind’s communication of one unified history to the finite minds who share it. pendent arena exists for there to be such a state."
MindMatters
