*Since the dawn of quantum mechanics, physicists and philosophers have argued that solving the measurement problem requires an appeal to the minds of conscious observers. This is still the case today, argues Shan Gao.*

Quantum mechanics is a very successful physical theory due to its accurate empirical predictions. But at its core remains a key conundrum: the problem of measurement. There seems to be a conflict between what the Schrödinger equation tells us, i.e. that a system described by the wave function can be in a state of superposition, instantiating seemingly conflicting properties, and what experience tells us, i.e. that when we take a measurement of such a system, we get a definite result, not a superposition. To use Schrödinger’s thought experiment, quantum mechanics would seem to imply that a cat can be both alive and dead at the same time, but we only experience cats that are dead or alive.

There are a number of proposed solutions to the measurement problem, including what has come to be known as hidden variable theory, which suggests that the wave function is not a complete description of a system and eliminates superposition states, and the many-worlds interpretation , which suggests that overlaps are expressed across different universes, each with a definite outcome. But since the dawn of quantum mechanics, physicists and philosophers have argued that the human mind is needed to solve the measurement problem. To understand why this is still the case, we need to take a closer look at the measurement problem and the proposed solutions.

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**Formalization of the measurement problem**

In 1995, Tim Maudlin provided a precise formulation of the measurement problem. According to this formulation, the measurement problem arises from the incompatibility of the following three statements:

(C1) The wave function of a physical system is a complete description of the system;

(C2) the wave function always evolves according to a linear dynamical equation, such as the Schrödinger equation;

(C3) a measurement produces a single defined result.

There are three main approaches to solving the measurement problem thus formulated. The first approach is to negate the statement (C1) and add certain hidden variables and corresponding dynamics to explain the definite results of the measurement. A well-known example is the pilot wave theory of de Broglie and Bohm, or Bohmian mechanics. The second approach is to deny statement (C2) and revise the linear and deterministic Schrödinger equation by adding some nonlinear and stochastic evolution terms, which describe the dynamic collapse of the wave function, to explain the definite results of the measurement . Such theories are called collapse theories. The third approach is to deny the statement (C3) and assume the existence of many equally real worlds to accommodate all possible measurement outcomes. In this way, it can also explain the definite results of measurement in every world, including our world. This approach is called Everett theory or the many-worlds interpretation of quantum mechanics.

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When you observe that the hand of a measuring device points to a definite position after a measurement, are you really sure that the needle actually points to a definite position?

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**Where does the mind enter quantum mechanics? **

So far we haven’t talked about the observers and their minds. And it seems that quantum mechanics has nothing to do with consciousness. However, there are two well-known and intriguing conjectures that claim that quantum mechanics and consciousness have intimate connections. These two conjectures are both related to collapse theories. The former suggests that consciousness causes the wave function to collapse. This conjecture has a long history and can be traced back to von Neumann (1932), London and Bauer (1939), Wigner (1961) and Stapp (1993). Recently Chalmers and McQueen (2022) give a more rigorous and complete formulation of the conjecture based on the Integrated Information Theory (IIT) of consciousness. The other argues that the collapse of the wave function creates consciousness. This conjecture is proposed by Penrose and Hameroff (1996, 2014) and has been called the orchestrated objective reduction model. Admittedly, many researchers in this field think these two conjectures are too radical to be true, although experiments have not yet given us a definitive answer either way.

So, does the mind really matter in quantum mechanics? My answer is yes, at least for now. Let me ask you a simple question. When you observe that the hand of a measuring device points to a definite position after a measurement, are you really sure that the needle actually points to a definite position? Unfortunately, I must say that physicists still do not know whether the indicator in this case points to a definite location. In fact, what we know for sure, from experience, is only that we as observers get a definite recording after a measurement by having a definite mental state of that recording. But we don’t know for sure whether a measuring device really achieves a definite result after a measurement. For example, if mental state is randomly determined from a branch resulting from post-measurement superposition as in single mind theory (Albert and Loewer, 1988), then the pointer of a measuring device does not indicate a definite location after a measurement, but an observer will get a definite record after an observation of the pointer’s position.

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So for now, we have to ASSUME some form of psychophysical connection in quantum theories. Without a hypothesis about the connection between mind and quantum mechanics, we also cannot test the predictions of a quantum theory, since measurements are made by an observer in the final analysis, and a measurement ends only when the result is consciously perceived by an observer. This motivates a more fundamental formulation of the measurement problem for observers (Gao, 2019), which asserts the incompatibility of the following three assumptions:

(A1) An observer’s mental state is determined by his wave function;

(A2) the wave function always evolves according to a linear dynamical equation, such as the Schrödinger equation;

(A3) a measurement by an observer produces a single mental state with a definite record.

This formulation of the measurement problem highlights the important role of the psychophysical connection in causing the problem. With this new formulation, we can look at the solutions of the measurement problem from a new angle. In particular, the three main quantum realist theories, namely Bohm’s mechanics, Everett’s theory and collapse theories, correspond to three different forms of psychophysical connection. In fact, there are only three types of physical states that can determine the mental state of an observer, which are (1) the wave function in collapse theories, (2) some branch of the wave function in Everett theory, and (3) other hidden variables such as particle configuration in Bohmian mechanics.

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Despite the great advances in neuroscience and quantum technologies, quantum mechanics and consciousness remain two mysteries of our times

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As an optimist, I believe that eventually we will know which form of psychophysical connection and which quantum theory is true. This is one of the fundamental reasons why I qualify the claim that mind is important in quantum mechanics *currently*. Another, deeper reason is that the form of psychophysical connection assumed by a quantum theory must be consistent with our current scientific and philosophical understandings of the conscious mind. In other words, an analysis of observers’ minds can help determine which quantum theory is false.

Take for example Bohmian mechanics. In Bohmian mechanics the physical state, which determines the result of a measuring device and the mental state of an observer, is not the wave function but the configuration of the Bohmian particles. These particles have only positions and velocities in three-dimensional space, have no mass or charge, and also have no interactions with each other (when their wave function is not an entangled state). An interesting question then arises: Can these Bohmian particles compose brains that create conscious minds? Or, to put it differently, do observers have conscious minds in Bohmian mechanics?

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It is a fundamental postulate in neuroscience and philosophy of mind that the parts of a system must be strongly connected to each other in order for it to generate the conscious mind. A typical example is the integrated information theory of consciousness, which is one of the leading theories of consciousness. According to this theory, consciousness requires a grouping of elements within a system that have physical cause-and-effect power over one another, and the level of consciousness of a system is described by the system’s integrated information, which can be represented by a precise mathematical quantity Φ. A system whose elements have strong connections will have Φ high, while a system whose elements have weak connections will have Φ low. Our brain has very high Φ, and is therefore highly conscious. Conversely, if the components of a system have no connections, the system will have zero Φ, meaning it is not conscious at all.

Now let’s move on to the key question: what if our brains were composed only of Bohm particles? Could this Bohmian brain generate the conscious mind? We need a common assumption in neuroscience and philosophy of mind to answer this question. It is that our conscious mind is generated by the activities of some almost classical systems such as neurons in our brain without involving quantum entanglement. In this case, the effective wave function of these quasi-classical systems is a product state, or in other words, each system has its own effective wave function or wave packet, and the motion of its Bohm particles is guided only by its wave packet.

Hence, we can argue that such a Bohmian brain cannot generate the conscious mind. For example, according to the Integrated Information Theory of consciousness, a Bohmian brain will have zero Φ and thus have no conscious experiences, since there are no connections between the Bohmian particles of non-entangled quasi-classical systems in the brain, and the whole system does not integrate the information.

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Without a hypothesis about the connection between mind and quantum mechanics, we can’t even test the predictions of a quantum theory

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The Bohmian mechanics example above demonstrates that the mind does indeed matter in quantum mechanics for now. According to our current understandings of the conscious mind, our bains cannot be composed of Bohmian particles alone since they cannot create the conscious minds we have, and therefore some versions of Bohmian mechanics (whose ontology consists only of particles) are probably false.

Despite the great advances in neuroscience and quantum technologies, quantum mechanics and consciousness remain two mysteries of our times. In my opinion, a careful and thorough examination of the possible connections between them is not only necessary but also urgent to unravel these two mysteries. I really hope this article inspires more people to join the quest for the ultimate reality of the universe.