How We Came to Know the Cosmos: Light & Matter

Discover How We Came to Know the Cosmos

Chapter 29. The Mind and Quantum Mechanics

29.1 The many minds interpretation

Any material theory of the mind will have to explain why people have subjective experiences (discussed in Chapter 28). This might be a problem that we’ll never be able to solve. However, there might already be a theory that explains the subjective nature of consciousness. This is Hugh Everett’s many worlds interpretation of quantum mechanics[1,2] (discussed in Chapter 20).

The many worlds interpretation of quantum mechanics suggests that everything evolves in accordance with Schrödinger’s wave function. This means that when someone measures the spin of an electron, for example, then there is no collapse of the wave function as it is measured as being either up or down. Instead, both possibilities happen and the observer observes both, although they are not aware of it. They are said to ‘branch’ into two.

The many minds interpretation of quantum mechanics looks at the consequences of the many worlds interpretation from the perspective of the mind. This is because the many worlds interpretation challenges classical notions of identity. Everett first considered this in an early draft of his 1957 proposal using the analogy of an intelligent amoeba that is constantly splitting, so it has a life tree rather than a life line.[2]

A similar situation sometimes arises in science fiction when two copies of an individual are created. It could be argued that they both have an equal right to existence, and all of their former privileges and belongings, yet they may go on to lead very different lives, and become very different people.

There appear to be three options for what could happen to us when this sort of branching takes place:

  • We become one of the potential observers after branching.
  • We become all of the potential observers after branching.
  • We become none of the potential observers after branching.

29.1.1 Single minds and ‘mindless hulks’

The American philosophers David Albert and Barry Loewer coined the term ‘many minds’ in 1988.[3] Albert and Loewer relied on a dualistic theory of the mind (discussed in Chapter 26) and first suggested that we must have one mind before and after branching.

The problem with this is that the other observers will be ‘mindless hulks’, people who act in every other way like a person but are not ‘ensouled’, and there would be no way to know the difference between the two.

In response to this problem, Albert and Loewer suggested that every brain must be associated with an infinity of minds so that they can be distributed after a split in accordance with the Born rule.

29.1.2 A sum of perspectives

In Mind, Brain & the Quantum: The Compound “I”, first published in 1989, Michael Lockwood suggested that the many minds interpretation does not have to rely on a dualistic theory of the mind.

Lockwood claimed that the many minds interpretation is the first scientific theory that implies subjective experiences are necessary and that the “irreducibly perspectival character to what is revealed in consciousness” arises “quite naturally, and independently” within it.

This is because the many minds interpretation leads directly to,

[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.[4]

Lockwood referred to the superpositional brain as the ‘Mind’ and to our subjective consciousness as ‘maximal experiences’. He described the sum of maximal experiences that correspond to one Mind as a ‘biography’.

Branching occurs relative to a biography when there is a branching of maximal experiences. This happens when we become aware of the macroscopic effects of a quantum interaction. Lockwood stated that it’s already accepted that we can have mutually incompatible experiences that are wholly our own, they simply occur at different times.

Lockwood showed that biographies can be represented by a cylinder - with time as the vertical dimension and biographies as the horizontal dimension. The distribution of maximal experiences at any particular time is seen by looking at a cross-sectional area of the cylinder. This will be made up of many different colours, each representing different experiences. Before the measurement, the surface could be green, for example, but after the ‘split’, it will divide into yellow and blue regions.

A cylinder that is green at the bottom and then spits into yellow and blue. The height of the cylinder represents time and the colours represent biographies.

Figure 29.1
Image credit

After measuring a quantum system, the maximal experiences of the Mind branch into two. Here, maximal experiences are represented in different colours.

In contrast to Albert and Loewer, who claimed that you will only become one of the observers present after a quantum interaction, Lockwood appears to suggest that you become them all. In fact, Lockwood claimed that it does not make sense to ask who you will become.

In 1986, Derek Parfit considered thought experiments involving the mind being duplicated or modified until it’s no longer logical to ask when ‘you’ cease to exist.[5] Parfit concluded that before branching it's meaningless to ask which of a multiple set of people you will become. This is an empty question because there is no determinate answer even if we know everything there is to know. In a way, you will be none of them, and, in a way, you will be them all.

Parfit suggested we replace the notion of identity with that of personal survival, here we care about what happens to our future-selves but recognise that they’re not us.

It seems that there’s no room for subjective probability if we accept Lockwood’s interpretation, but this may not be the case. Lev Vaidman offered an analogy: if a person is given a sleeping pill before they interact with a quantum system and are then placed in one of two rooms, corresponding to the outcome, it’s meaningful to ask the person on awaking what the probability of them being in either room is.[6]

Lockwood’s many minds interpretation suggests that there’s an importance attached to the probability, or ‘weight’ of a world. Lockwood claimed that saying that a world has twice the weight as another is,

parallel to saying, for example, that this pain lasted twice as long as the last one...[or that] this pain is, superpositionally speaking, twice as extensive as that.[7]

Lockwood went on to state that,

[It’s] rational for an instantaneous mind to care equally about all of its successors...this is analogous to preferring the longer to the shorter of two alternative pleasures, when they are of equal intensity.[8]

Vaidman claimed that someone should not agree to play games like quantum Russian roulette because the version of them that survives will be in a world with a low weight.[9] Quantum Russian roulette is similar to Russian roulette, except that the gun is triggered by a quantum event. This means that the players will branch every time they pull the trigger so that they both live and die.

The concept of quantum suicide extends the idea of quantum Russian roulette to differentiate between the collapse approach, the Bohm interpretation, and different versions of the many worlds interpretation. The idea behind quantum suicide is that if you are somehow all of the observers after branching, then you would never observe a world where you instantaneously die and so, from your perspective, every time you pull the trigger you must live.

This idea was first proposed by the roboticist and futurist Hans Moravec[10] and the philosopher Bruno Marchal[11] in 1988. Max Tegmark extended the idea in 1998.[12] Moravec considered a situation where scientists are unable to turn on a particle accelerator; every time they try something stops them. He suggested that this is the only thing we could possibly observe if the effect of the particle accelerator was to instantly destroy the Earth.

One advantage of Lockwood’s many minds interpretation is that it can give a definite solution to the preferred basis problem, whereas decoherence is only approximate. Lockwood’s theory resolves the preferred basis problem by stating that the conscious mind is only capable of comprehending definite states. He claimed that the reason why consciousness selects this particular basis is no more of a mystery than why we are unaware of other parts of our mind (discussed in Chapter 28).

This basis is subjective, however, and so it is still relevant to ask why our minds behave this way. Lockwood considered whether creatures could have evolved to observe macroscopic objects in superpositional states, but it is debatable as to whether this is physically possible since it’s difficult to understand how they could interact.

The anthropic principle

The anthropic principle was devised to explain why the universe appears to be perfectly designed for our existence, in a way that seems highly improbable.

The strong anthropic principle

The strong anthropic principle was first suggested by the Australian physicist Brandon Carter in 1974. This implies that life is somehow needed for the universe to exist.[13] The British writer Douglas Adams described the main problem with this in 1998,

Imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’ This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it’s still frantically hanging on to the notion that everything’s going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise.[14]

The weak anthropic principle

The weak anthropic principle was first suggested by the American physicist Robert Dicke in 1961. This states that this is a selection bias, as we couldn’t observe any world that is incapable of containing life.[15]

Proponents of the weak anthropic principle argue that the universe would not be hospitable to life if any of the universal constants were different. These include: the ratio between the strength of the force of gravity (discussed in Book I) and the nuclear forces (discussed in Chapter 23), the proportionality constant between energy and mass, and the ratio between the rate at which dark energy is accelerating the expansion of spacetime and the rate at which the universe would otherwise collapse due to gravity[16] (all discussed in Book I).

The evolution of life was also reliant upon quantum fluctuations that occurred during the inflationary epoch of the early universe[17] (discussed in Chapter 22). If there were no fluctuations, then everything would be equally dispersed. If the fluctuations in symmetry were too vast, however, then everything would be distributed too closely together, creating another universe full of black holes.

A final important factor for any universe that contains life is the number of dimensions that we experience (discussed in Chapter 25). If we lived in a world with only two spatial dimensions, then the tubes that carry our food, oxygen, and blood, would cut us in half. If we were to experience four spatial dimensions, however, then the force of gravity would be proportional to the cube of an object’s mass, rather than the square. This would mean stars would be heavier, and so have shorter lifetimes, and life would not have time to evolve.

The weak anthropic principle states that the universe must be the way it is because we wouldn’t be able to observe it if it were any other way. Yet this still doesn’t explain why it happened to be suitable for life.

The many worlds interpretation provides a natural extension to the weak anthropic principle by suggesting that there are an infinite amount of worlds and so, however improbable our existence is, we know that life must exist somewhere. An unusual event occurred because it had a non-zero probability and a vast number of trials were run.

29.1.3 An infinity of minds

The British philosophers Simon Saunders and David Wallace claimed that Lockwood was wrong to say that there’s no answer to who we will become upon branching.[18,19] Instead, they suggest that we’ll become one particular mind after we branch in agreement with Albert and Loewer.

Saunders and Wallace agreed that this means we must have an infinite amount of minds inside of us at all times, but unlike Albert and Loewer, they accepted a material theory of the mind. Wallace described how “when I say ‘who will I become’” before initiating a quantum experiment with two possible results, “that statement should actually be ascribed to two versions of me”.[19] We don’t notice the fact that we have numerous minds because, from a subjective point of view, they will be indistinguishable, all seeing the same things and thinking the same thoughts at the same time.

This approach to consciousness was first suggested by the American philosopher David Lewis in 1976,[20] and so Saunders and Wallace advocated a Lewisian rather than Parfitian notion of personal identity.

Wallace did not agree with Lockwood’s interpretation of 'weight' and argued that there’s no observable difference between worlds with a low weight or a high weight, and so pain will not be more intense in a world with a higher weight.

It would also be impossible to try to maximise the weight of the world we’re in because branching occurs all the time and we have no way to keep track. Wallace described how it would lead to a person being,

faced with the impossible task of calculating how much branching will occur across the entire lifetime of the Universe (contingent on [their] choice of action) in order to weigh up the value, now, to [them] of carrying out a certain act.[21]

Wallace also showed that we don’t need Lockwood’s many minds interpretation to account for the preferred basis problem. Wallace suggested that it doesn’t matter that the basis of decoherence is approximate if you accept a functional definition of the mind.

Wallace claimed that all macroscopic objects should be understood “in terms of certain structures and patterns which emerge from quantum theory”.[22] Wallace argued that we already use the concept of patterns to provide functional definitions and gave the example of our definition of a tiger. We can define the word ‘tiger’ from many perspectives, and choose the one that is easier or most useful for us.

We could, for example, try to learn about the behaviour of tigers by studying them at an atomic level, but this would be overly complicated. We could study them at a cellular level, but again this would provide us with too much irrelevant information. We learn most about the behaviour of tigers by studying them in terms of single, individual tigers, patterns that arise from a background of energy and matter. Wallace claimed that we define a tiger as “any pattern which behaves as a tiger”.[22]

In the case of Schrödinger’s cat, the superpositional state of the inside of the box can be described as containing patterns of both a dead and alive cat as well as “all possible macroscopic objects made out of the cat’s subatomic constituents”.[22] Cats are described as patterns, and so cannot be in superpositions themselves. Cats can only be duplicated. This level of description is not usually needed, however, because decoherence will quickly remove all observable effects of the superposition.

Wallace agreed with the American philosopher Daniel Dennett, who claimed that a pattern is real if it’s useful for us to refer to it when explaining theories.[23] Patterns are subjective, just as worlds are, and so it’s possible that different types of minds perceive patterns, and hence define macroscopic objects, in different ways.

A photograph of the sculpture ‘Quantum Cloud’ by Antony Gormley, showing what looks like a person made from pieces of metal

Figure 29.2
Image credit

Quantum Cloud by Antony Gormley. We are patterns that arise from a background of energy and matter.

29.2 References

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