The question that gives the title to this chapter has been with me for a lifetime, from the time I graduated in Milan, Italy, with a thesis on the measurement problem in quantum mechanics. The problem concerns the role of the observer in quantum measurement and therefore, in my view, the precise spot where matter and mind touch each other. It is the at the heart of quantum mechanics. And yet it still does not have a universally accepted solution a hundred years from the inception of the theory.
The difficulty is the fact that the formalism of quantum mechanics includes two parts that seem to be incompatible: one which describes the time evolution of a general quantum system defined by a wave function, the other which describes the measurement process, in which a probability is assigned to each measurement result according to the amplitude of the wave function squared (a prescription called the Born rule). The time evolution applied to the measurement process produces an entangled superposition, typically represented by Schrödinger's famous cat in a box being in a superposition of dead and alive, rather than being either alive or dead.
One of the early proposed solutions to this paradox directly involved the observer's consciousness. When an observer opens Schrödinger's box and looks inside, the superposition of states 'collapses' into a good old either alive or dead cat. In that perspective (called the 'idealist' interpretation), consciousness is an essential ingredient of quantum mechanics. But this solution presents serious paradoxical features and is no longer favored by the majority of physicists.
Modern solutions of the quantum measurement problem, e.g. the so-called decoherence approach, take a different route: they assume that quantum mechanics correctly describes the measurement process and therefore the outcome really is an entangled superposition including a simultaneously dead and alive cat. But the entangled superposition can be shown to be in all respects equivalent to a classical alternative and therefore we can treat the cat as being either alive or dead.
In this perspective, so long as we interpret quantum mechanics as an essentially statistical science, decoherence solves the quantum measurement problem and consciousness does not appear to play any significant role in it.
But quantum mechanics has nothing to say about the individual result of a measurement. It does not tell us whether in any given instance we will find Schrödinger's cat alive or dead. The outcome in a specific instance is intrinsically unpredictable: it is what Wolfgang Pauli called ‘an act of creation’.
This chapter proposes that such acts of creation happen in consciousness. The world unfolds through continuous acts of creation. While the wave function of the universe evolves in a perfectly deterministic way, compatible with all possible experiences, the experienced world is constantly generating new unpredictable events, new ‘acts of creation.’
"Consciousness cannot be accounted for in physical terms.
For consciousness is absolutely fundamental.
It cannot be accounted for in terms of anything else."
Erwin Schrödinger
The quantum measurement problem
The question that gives the title to this chapter (or some variant thereof) has been with me for a lifetime, from the time I graduated in Milan, Italy, with a thesis on the measurement problem in quantum mechanics. Since then the journey of my consciousness unfolding took many forms, most of them not directly related to physics: meditation, community building, psychedelics, translating Eastern wisdom texts, and of course relationships, journeys, the whole phantasmagoria of lived experience.
But from time to time the bug of physics would reawaken and I would fall under the charm the of its detached understanding, looking at the eternal questions from the cool third-person perspective of science.
When that happened, I would inevitably go back to my old quantum measurement problem. For good reasons. Because, if modern science has anything to say about the mind/matter interface, and therefore ultimately about the nature of reality, one of the loci par excellence of that interface is no doubt the quantum measurement process.
As it is well known, this key issue, at the heart of quantum mechanics, is still an open question almost a hundred years from the inception of the theory. We have a wonderful theory, whose predictions, even those that seemed most 'spooky', have been consistently confirmed. But what the theory really means is still in question. Therefore, in order to discuss the possible implications of quantum mechanics for our understanding of consciousness, we need to briefly review the quantum measurement problem.
A concise description of the problem is given in the following paragraphs. The reader desiring a more detailed understanding is referred to the Appendix and to the definition of some key terms given in the Glossary.
The essence of the measurement problem is the difference between the empirically observed outcome of a measurement process and the outcome calculated by applying standard time evolution to the global system involved in the measurement (observed system, measuring apparatus, observer, environment, etc., technically called 'von Neumann chain').
In the first one, the various measurement results appear to be alternative objectively realized facts, each with a definite probability, and the outcome of a further observation on the global system can be calculated by summing the contributions of these independent facts.
In the second one, applying the continuous time development as realized through the Schrödinger equation the outcome of the measurement process appears to be an entangled superposition (see the Glossary) of the various results, and the outcome of a further observation on the global system cannot be simply calculated by summing the contributions of the various results. The calculation involves additional terms, called 'interference terms,' (see the Glossary) connecting the various results and preventing them from being regarded as alternative objective facts realized with certain probabilities given by the Born rule.
The problem then can also be stated as why in the actual outcome of a quantum measurement processes the interference terms do not appear to be there.
The problem is elegantly emphasized by Schrödinger's famous cat in a box (1935). The object system in this case is a radioactive atom, whose general state is a superposition of decayed and not decayed. The von Neumann chain includes a cat in a box with a devilish contraption that links the state of the radioactive atom to the life or death of the cat. By applying standard time evolution to the system, the outcome is an entangled superposition of product states containing the alive cat and the dead cat.
Many solutions have been suggested in the course of the last ninety years but none so far has obtained the unanimous consensus of the physics community. I will describe a few interpretations that seem particularly relevant for our subject.
The Copenhagen interpretation
The Copenhagen interpretation (once also known as 'the orthodox interpretation') of quantum mechanics, solves the riddle by introducing an axiom ad hoc. It assumes that the dynamics of the interaction is such that the entangled superposition which is the outcome of a measurement process spontaneously collapses into the classical alternative of objectively realized results that is observed in practice (collapse of the wave function). This axiomatic step reflects the experimental evidence and offers a convenient frame for practical calculations. But it can hardly be considered a solution of the quantum measurement problem. In the Copenhagen interpretation one assumes a separation between a classical world accessible to us humans and a quantum world accessible only through measurement. As such the Copenhagen interpretation does not consider the measuring apparatus (and the brain of the observer, if we choose to include it in the von Neumann chain) to evolve according to the laws of quantum mechanics. The collapse of the wave function is incompatible with the linearity of the time evolution operator, and as such it amounts to postulating two different time evolution prescriptions, one applying only to quantum measurement and the other to a general quantum system.
The idealist interpretation
Among the early proposed solutions of the quantum measurement problem, the one that stands out for involving consciousness in an essential way is the ‘consciousness causes collapse’ or ‘idealist’ theory. It was hinted at already in von Neumann's Mathematical Foundations and was later developed by London and Bauer in 1939 and by Eugene Wigner in the 1960s. An interesting variation of it is the theory proposed in our times by Henry Stapp. This interpretation accepts the notion of the collapse of the wave function and associates it with a conscious subject experiencing a definite result and thus collapsing all probabilities to zero except for the one corresponding to the actual event.
The idealist interpretation, although not strictly confutable, has some rather weird implications, which probably contributed to its losing favor in the physics community. Consider, e.g., Schrödinger's cat. According to the idealist interpretation the entangled superposition will collapse into the corresponding classical distribution when an observer opens the box and looks inside. But what do we mean by ‘observer’? Should the cat count as an observer? And if so, how far down the chain of life forms does consciousness collapse the wave function? What about an insect? Or a bacterium?
But even if we limit ourselves to human observers, it is easy to imagine paradoxical consequences. E.g., what happens if the observer opens the box without looking inside, takes a Polaroid photo of the content, puts away the photo in a drawer and a month later takes it out and looks at it? For the whole month the cat will have been a superposition of a dead cat in the box and an alive cat going about her business on the roofs.
Also, what about all the cosmological eras before conscious observers appeared in the universe? Was it all an entangled superposition until we collapsed it into an actual universe?
The many-worlds interpretation
A number of recent approaches to the quantum measurement problem give up the notion of collapse of the wave function altogether and assume that quantum measurement is to be entirely described by standard quantum mechanics. The classic example along these lines is the Everett-DeWitt many-worlds interpretation (1957). It takes the entangled superposition that is the outcome of the measurement process at face value, but assumes that each term of the superposition unfolds in a separate universe. In a quantum measurement therefore the world branches off into different worlds, identical except for the different outcomes of the measuring process. The difference involves the whole von Neumann chain, including the observer, who therefore exists in slightly different versions in each universe. Each version has no notion of the existence of other copies of herself or himself and sees the specific outcome of the experiment in her or his world as the only one.
However one might feel about the idea of multiple universes (an idea that is popping up from a completely different context in cosmology) the main difficulty of Everett's theory is probably a technical one, namely how to interpret the Born rule in this context. Why does each result appear with just the correct probability in a sequence of experiments conducted in any one world?
Decoherence
Perhaps the most widely accepted interpretation of quantum mechanics today is the one known as decoherence. That is the interpretation which I have been involved in, in two stages, 40 years apart. The first one was my graduation at the University of Milan in the late 1960s, before the approach was even called decoherence. The second was in the years 2000s, long after I had left the academia.
The core idea of the decoherence interpretation is that in a macroscopic environment the interference terms decay extremely rapidly. The information they contain diffuses through entanglement with the environment and is lost for all practical purposes.
That means the entangled superposition now extends to the entire macroscopic environment (including the macroscopic apparatus), which we are able to observe only to a very limited extent. When the entangled superposition gets projected onto the observations that we are actually able to make, the difference between entangled superposition and classical alternative vanishes. Only a measurement of the quantum state of the entire macroscopic environment would be able to distinguish them.
The decoherence approach can be formulated in a slightly different way, which I have called persistence of information. It is possible to prove that a measurement able to distinguish between the entangled superposition and the classical alternative is necessarily incompatible with all the von Neumann observables. But then such a measurement erases all traces of the previous measurement: it is as if the measurement had not occurred. On the other hand for all the observations that preserve information about the outcome of the measurement the entangled superposition and the classical alternative are equivalent, they give exactly the same results. If we call ‘proper measurement’ a measurement whose outcome is somehow recorded, then we could state that for all proper measurements the entangled superposition and the classical alternative are equivalent, i.e the evolution of the system can be calculated from the classical alternative as well as from the entangled superposition. Persistence of information, the existence of a trace, is the decisive criterion.
In decoherence there is no collapse of the wave function, but it is as if there were. In other words, the collapse of the wave function is an apparent phenomenon. Its appearence is due to:
- the fact that a measurement necessarily involves a macroscopic system (standard decoherence)
- the fact that all proper measurements involve a trace or record (persistence of information)
In all measurements whose outcome we get to know, a human brain is at some point involved. Then the experience gets recorded in the person's brain and the persistence of information criterion is fulfilled. Therefore in all measurements whose outcome we get to know, the outcome can be taken to be a classical distribution. We could extend that argument to perception in general: we perceive the world as classic because all our experiences correspond to trace formation in our brain.
What about ‘improper measurements’, measurements that leave no traces? When no trace is left, the further evolution of the system must be calculated using the full entangled superposition, not the classical distribution. We have a wonderful example of this in the quantum eraser experiments (see Appendix 2).
Acts of creation
Does decoherence solve the quantum measurement problem? It depends on what we require from a solution. If we accept the fact that quantum mechanics is an essentially statistical theory and accept that there is no causal explanation for the single event (no hidden variables!), then yes, decoherence is a complete solution of the quantum measurement problem.
But we may be drawn to investigate further. In decoherence (as in all other non-hidden-variables interpretations) the single event remains unpredictable, indeterminate. You can look at this indeterminacy from two points of view, which are logically equivalent, but have a very different emotional resonance. You can say it is randomness, no meaning attached. Or you can look upon it, as Pauli did, as an ‘act of creation’, or, as Heisenberg said, ‘nature's choice’. All the rest is determined, it is Laplace's clockwork universe ticking away in a perfectly deterministic manner. The quantum state of the universe evolves in a deterministic way (or perhaps does not evolve at all, if we take the Wheeler-DeWitt equation seriously). The only space for creation, for the unpredictable, is the single event that arises at the interface between consciousness and world
We are now ready to discuss the question raised by the title of this paper: has quantum mechanics something to say about consciousness? On one level the answer is no. The observer's consciousness does not play any role in the solution of the measurement problem. From the point of view of persistence of information just the existence of any record or trace will do. And from the point of view of standard decoherence, only the macroscopic nature of the measuring apparatus is required. The solution of the quantum measurement problem does not require the observer's consciousness at all.
But quantum mechanics suggests where we have to look if we want to see consciousness' footprint in matter: it is there where the predictable ends, it is in the act of creation of the single event. We live in an actual world, we experience actual events: out of the statistical picture one definite actuality emerges in any specific instance. As a metaphor, you can think of the algorithm of quantum mechanics (wave function, time evolution, Born rule for calculating probabilities) as the banks of a river, and the indeterminacy of the single event as the water flowing within the banks. The flow is constrained by the banks, but is otherwise unpredictable, ‘nature's choice’. There, I would like to suggest, we have to look if we want to find the expression of consciousness in matter.
The question that naturally emerges at this point is: whose consciousness? There is no 'who' this consciousness belongs to. It is simply consciousness, the consciousness of the universe, the consciousness of Deus sive natura, in Baruch Spinoza's terminology. The consciousness that is involved in the emergence of the actual from the womb of the potential is cosmic consciousness, it belongs to the whole.
Consciousness and life
I would like to propose (this last section is highly speculative) that there is a strict parallelism between consciousness and life and that they are both related to quantum indeterminacy, to the 'acts of creation' that are the unpredictable individual events. When we adopt this perspective the old philosophical conundrum about origins falls away. Where does consciousness arise from unconscious matter? Where does life emerge from the non-living?
There is no 'where'. Consciousness and life are everywhere. We are invited to extend our notion of life and call life all manifestations of freedom. This includes quantum indeterminacy on the microscopic scale as well as quantum indeterminacy manifesting, in ways that we are still far from understanding, in the macroscopic world, in large scale coherent phenomena (some recent brain studies point in this direction).
Therefore, there is no sharp divide between life and non-life. I would like to propose that freedom, life and consciousness are different names for the same thing, and the ‘thing’ is everywhere . It appears as quantum indeterminacy on a microscopic scale, as life on a macroscopic scale, as consciousness on a cosmic scale.
This is a very loose definition of both life and consciousness, probably too loose to be practically useful, e.g. in biology. But it may well be the only philosophically sound definition, because there is no specific attribute ‘life’ that is not there up to a point in the development of an organism and comes in at that point, there is no specific attribute ‘consciousness’ that is not there up to a point in the development of a nervous system and comes in at that point All there is, is degrees of complexity. We recognize life and consciousness when they appear in forms close enough to our degree of complexity. We humans are not special – except for the fantastic complexity of the way in which life and consciousness manifest in us. But everything is special. If there is no specific ‘life’ attribute, there is also no specific ‘non-life’ attribute. We are equally justified saying that life does not exist or saying that everything is alive.
Special box
The Schrödinger quote that opens this chapter captures the essence of what I believe to be a correct rapport between phenomenological experience and philosophical speculation. Phenomenological experience is primary. All the rest is 'about' it. All our notions about matter, including our own body and more particularly our brain, are third person attempts to make sense of our first-person experience of the world.
The fact that such attempts are at all possible is in itself significant (Einstein is quoted to have said that the most incomprehensible fact about the universe is that it is at all comprehensible!). It points to the fact that our first-person experience of the world is structured. The world is a cosmos, not a chaos. Our experience appears to unfold in space and time and to possess that sort of regularity that we call matter. That allows us to construct third person representations of the world and to speculate about the relation between mind and matter. In those third person representations consciousness appears to be located 'in the world', which leads scientific investigation to ask questions like how does consciousness arise from matter (the 'hard problem').
Such questions are not philosophically sound. A better posed question is how does matter arise from consciousness. In Hindu mythology the answer to this question is the god Vishnu dreaming the world.
The notion of this universe, its heavens, hells, and everything within it, as a great dream dreamed by a single being in which all the dream characters are dreaming too, has in India enchanted and shaped the entire civilization. [This picture] is a classic Hindu representation of the ultimate dreamer as Vishnu floating on the cosmic Milky Ocean, couched upon the coils of the abyssal serpent Ananta, the meaning of whose name is 'Unending'. In the foreground stand the five Pandava brothers, heroes of the epic Mahabharata, with Draupadi, their wife... They are those whom the dream is dreaming...
(Joseph Campbell, The Mythic Image, Princeton University Press, Princeton, New Jersey, 1974, p. I.6)
References
Bohr, N. (1928). "The Quantum Postulate and the Recent Development of Atomic Theory". Nature. 121 (3050): 580–590.
Campbell, J. (1974) The Mythic Image, Princeton University Press, Princeton, N.J., I.6.
DeWitt, B.S. (1970). "Quantum mechanics and reality". Physics Today. 23 (9): 30–35.
Everett, H. (1957). "Relative State Formulation of Quantum Mechanics". Reviews of Modern Physics. 29 (3): 454–462.
Jung, C. G. and Pauli, W. Atom and Archetype: The Pauli/Jung Letters, 1932–1958 (2014) ed. Meier, C. A., Zabriskie, B. and Roscoe, D., Princeton University Press, Princeton, NJ.
Lanz L., Prosperi, G.M., and Sabbadini A. (1971), “Time Scales and the Problem of Measurement in Quantum Mechanics”, Il Nuovo Cimento, 2B(2), 184-192.
London, F. and Bauer, E. (1939), "La théorie de l’observation en mécanique quantique", English translation in Quantum Theory and Measurement, edited by Wheeler, J.A. and Zurek W.H. (1983), Princeton University Press, Princeton, NJ, 217–259.
Nadler, S. (2011) A Book Forged in Hell: Spinoza's Scandalous Treatise, Princeton University Press, Princeton, NJ.
Ou, Z.Y., Wang, L.J., Zou, X.Y. and Mandel, L. (1990), Phys. Rev., A 41, 566.
Sabbadini S.A. (2006), "Persistence of Information in the Quantum Measurement Problem", Physics Essays, 2006, 19(1), 135-150.
Schrödinger, Erwin (1935). "Die gegenwärtige Situation in der Quantenmechanik (The present situation in quantum mechanics)". Naturwissenschaften. 23 (48): 807–812.
Stapp, H. (2001). "Quantum Theory and the Role of Mind in Nature". Foundations of Physics. 31 (10): 1465–1499.
Von Neumann, J.(1932), Mathematische Grundlagen der Quantenmechanik (Mathematical Foundations of Quantum Mechanics), Julius Springer, Berlin.
Wigner, E. and Margenau, H. (1967). "Remarks on the Mind Body Question, in Symmetries and Reflections, Scientific Essays". American Journal of Physics. 35 (12): 1169–1170.
Zeh, H.D. (1970), "On the Interpretation of Measurement in Quantum Theory", Foundations of Physics, 1, 69–76.