This post builds on two ideas we explored in previous posts: Quantum Suicide and Personal Identity. You should definitely read those posts first if you’d like the full picture.

Hugh Everett was an American physicist known for his radical interpretation of Quantum Mechanics called the Many-Worlds interpretation. Everett challenged the validity of leading interpretations of Quantum Mechanics at the time, first and foremost the Copenhagen Interpretation. Under the Copenhagen Interpretation, the true behavior of a quantum system can be inferred by an observer independently of its own state. Everett’s big idea was that observers are part of the quantum system. Both are entangled such that one cannot be defined without the other. The false notion that observations are independent of the observer leads to one of the thorniest issues in Quantum Mechanics, a conundrum known as the measurement problem.

The Many-Worlds interpretation, published in 1957, holds that the probabilistic equations used to predict quantum phenomena continue to hold after an observation is made — it is just that every time a measurement is made, all the possible outcomes actually occur in different branches of reality, creating a multitude of parallel worlds, or a “multiverse.”

The notion of the world continually splitting at every quantum decision point across a multiverse of parallel universes has significant implications for personal identity. This is because individuals are deemed to be in continual superposition with themselves because they exist in many parallel universes simultaneously. In his dissertation, Everett summarizes the implication of this contention as follows:

“The price, however, is the abandonment of the concept of the uniqueness of the observer, with its somewhat disconcerting philosophical implications.”

Everett proposed a thought experiment to better illustrate the state of the observer:

“As an analogy one can imagine an intelligent amoeba with a good memory. As time progresses the amoeba is constantly splitting, each time the resulting amoebas having the same memories as the parent. Our amoeba hence does not have a life line, but a life tree. The question of identity or non-identity of two amoebas at a later time must be rephrased. At any time we can consider two of them, and they will have common memories up to a point (common parent) after which they will diverge according to their separate lives after this point. It becomes simply a matter of terminology as to whether they should be thought of as the same amoeba or not, or whether the phrase “the amoeba” should be reserved for the whole ensemble.”

Everett’s experiment is astonishingly similar to another thought experiment titled People Who Divide Like an Amoeba by british philosopher Derek Parfit, which we have discussed in a previous post. Everett then suggests a closer analogy:

“If we were to take one of these intelligent amoebas, erase his past memories, and render him unconscious while he underwent fission, placing the two resulting amoebas in separate tanks, and repeating this process for all succeeding generations, so that none of the amoebas would be aware of their splitting. After awhile we would have a large number of individuals, sharing some memories with one another, differing in others, each of which is completely unaware of his “other selves” and under the impression that he is a unique individual. It would be difficult indeed to convince such an amoeba of the true situation short of confronting him with his ‘other selves’.”

Everett claims that the same is true for our own existential state under the Many-Worlds approach to quantum mechanics. Thus, he contends:

“The same is true [if] one accepts the hypothesis of the universal wave function. Each time an individual splits he is unaware of it, and any single individual is at all times unaware of his “other selves” with which he has no interaction from the time of splitting.”

The term Wave Function refers to the equation used by quantum theory to predict the probability of possible outcomes in a quantum system. Everett suggested the term Universal Wave Function to describe the equation governing the totality of existence.

The Many Minds interpretations examines the consequences of the Everett Many-Worlds interpretation from the perspective of the mind. Rather than many worlds branching at each quantum decision point, it is the observer’s mind that is branching. It was first introduced in 1970 by H. Dieter Zeh as a variant of the Everett interpretation and explicitly termed “Many Worlds” by American philosophers David Albert and Barry Loewer in 1988. It was re-formulated again by the British Philosopher Michael Lockwood (Many-Minds Interpretations of Quantum Mechanics, 1996).

According to Lockwood, the significance of the Many-Minds interpretation is that it is the first scientific theory to recognize the necessity of subjective experiences. This is because Many-Minds leads directly to the following scenario:

“[a view] of the world as, in some sense, a sum of perspectives…the inevitable selectivity involved in a point of view is automatically accommodated, via the idea that consciousness is tied to one amongst a potential infinity of what, in the context of quantum mechanics, are known as representations.”

Here is Lockwood’s account of the Many-Minds interpretation:

“A many minds theory, like a many worlds theory, supposes that, associated with a sentient being at any given time, there is a multiplicity of distinct conscious points of view. But a many minds theory holds that it is these conscious points of view or ‘minds,’ rather than ‘worlds,’ that are to be conceived as literally dividing or differentiating over time.”

When an observer interacts with a quantum system, they become a part of a larger quantum system. Each possible outcome predicted by the equations of quantum mechanics has a corresponding mental state, or mind, in the brain of the observer. Ultimately, only one mind is experienced. The minds corresponding to outcomes which did not occur simply become inaccessible. This leads to a scenario under which every conscious being is endowed with a large number of minds, corresponding to all possible outcomes. As an observer interacts with a quantum system, the probabilities of realizing specific outcomes directly correlates to the number of minds they have.

In the end, Lockwood concludes that it is senseless to ask which, of the many possible minds, are “yours” following a specific quantum interaction. His conclusion is similar to the one arrived at by Parfit, as we saw in the post about Personal Identity. Parfit was not concerned with Quantum Mechanics, but rather with a series of thought experiments under which a person’s mind is duplicated or split either by surgery or some other means. These experiments demonstrate that, following such a split, questions such as “which person will I become?” are senseless. His conclusion is that we should discard the notion of personal identity. There is no determinate answer to these questions, even if we know everything there is to know. In a way, I will become none of them and, in a way, I will become all of them.

I find it fascinating that two thinkers from two unrelated disciplines arrive at such similar conclusions. It is even more fascinating that both lines of thought suggest that it is the way we speak about something as seemingly trivial as our own identity which ought to change.