by Jim Baggott
As space missions go, MAQRO is ‘medium-sized’. It was first proposed to the European Space Agency (ESA) in 2010, and this proposal was substantially updated and resubmitted in 2015.26 In September 2016 it was resubmitted in response to the ESA’s call for ‘New Science Ideas’, and was selected for further detailed investigation by the ESA’s Concurrent Design Facility during 2018.*
If the mission gets the go-ahead, it will build on all the knowledge from the recent highly successful LISA Pathfinder, launched in December 2015, which was designed to test technology capable of detecting gravitational waves in space.†
The cost of the LISA Pathfinder mission was estimated to be €400 million back in 2011.* Given that, if it is commissioned, the MAQRO mission is unlikely to launch for another ten years or more, its budget requirements will inevitably be substantially larger than this. As I’ve said before, the metaphysical preconceptions that give rise to realist convictions among quantum physicists tend to come with a hefty price-tag.
For sure, the MAQRO mission will do more than test the representation of quantum reality in macroscopic superpositions, but the price to be paid refers to more than just a few philosophical conundrums (and the occasional headache).
* Which is why both Rovelli and Smolin—who have long collaborated on loop quantum gravity—have in their own individual ways sought to come to terms with the interpretation of quantum mechanics.
* For more details and to follow the project’s progress, see http://maqro-mission.org/.
† LISA stands for Laser Interferometer Space Antenna.
* https://spacenews.com/lisa-pathfinder-proceed-despite-100-cost-growth/
9
Quantum Mechanics is Incomplete
Because We Need to Include My Mind (or Should That be Your Mind?)
Von Neumann’s Ego, Wigner’s Friend, the Participatory Universe, and the Quantum Ghost in the Machine
Of course, the problem of the collapse of the wavefunction didn’t originate in 1932 with the publication of von Neumann’s Mathematical Foundations of Quantum Mechanics. But it’s fair to say that his approach to quantum measurement really dragged the problem into the open, from where it has proceeded to torture the intellects of quantum physicists and philosophers for the past 90 years or so.
As we’ve seen, the anti-realists dismiss the collapse as a non-problem, no more difficult to understand than the abrupt change in our knowledge when we gain some new information. A few theorists more inclined to realist preconceptions have sought to identify physical mechanisms that act on physically real wavefunctions, designed in attempts to explain how and becomes or.
But what did von Neumann himself think was going on?
In his classic text, von Neumann clearly distinguished between two fundamentally different types of quantum process. The first, which he referred to as process 1, is the discontinuous, irreversible transformation of a pure quantum state into a mixture, involving the ‘projection’ of some initial wavefunction into one of a set of possible measurement outcomes, with an accompanying increase in entropy. We now call this the collapse of the wavefunction, although von Neumann himself didn’t use this terminology.* Process 2 is the continuous, deterministic, and completely reversible evolution of a wavefunction, governed by the Schrödinger equation according to Axiom #5 (see Appendix). These two processes are distinct: process 1 cannot happen in process 2, and vice versa.
He then looked at quantum measurement from the perspective of three fundamental components, which he labelled I, II, and III:
I is the quantum system under investigation;
II is the physical measurement; and
III is the ‘observer’.
He proceeded to demonstrate that if a quantum system I is present in a superposition of the measurement outcomes (for example, particle A is in a superposition of ↑ and ↓ states), then this will evolve smoothly and continuously according to process 2. On encountering the measuring device, the wavefunction becomes entangled, but von Neumann saw no reason to suppose that quantum mechanics ceases to apply at this classical scale. The entangled wavefunction must then continue to evolve smoothly according to the Schrödinger equation. Process 2 still applies. If we further entangle the device with a gauge, we know well by now that this gives rise to another superposition consisting of components A↑ and A↓.
Von Neumann could find no reason, based purely on the mathematics, to suppose that process 1 would have any role to play in the composite system I plus II. Process 2 applies equally to classical measuring devices and gauges as it does to quantum systems.
Schrödinger wouldn’t publish the paper containing the reference to his famous cat for another three years, and von Neumann was already aware of the implications for an infinite regress. But his resolution of the problem was quite straightforward. If the quantum mechanics described by process 2 applies equally well to classical measuring devices, then there is again no good reason to suppose that it ceases to apply when considering the function of human sense organs, their connections to the brain, and the brain itself. Suppose that the laboratory has an overhead light which illuminates the screen of the gauge and some of the reflected light is gathered and focused at the observer’s retinas. This triggers electrical signals in the observer’s optic nerves, which travel to the visual cortex located at the back of the observer’s brain.
We can choose simply to expand the definition of the ‘quantum system’ in I to include the particle A, the classical measuring device, the gauge, and the reflected light. Component II—the ‘physical measurement’—then includes the observer’s sensory apparatus and brain. The result is yet another superposition:
This implies that the observer enters into a superposition of ‘brain states’, and .
Von Neumann wrote:1
Now quantum mechanics describes the events which occur in the observed portions of the world, so long as they do not interact with the observing portion, with the aid of process 2…, but as soon as such an interaction occurs, i.e. a measurement, it requires the application of process 1.
So what, then, did he have in mind with regard to the ‘observing portion’ of the world? Based on conversations he had had with his Hungarian compatriot Leo Szilard, von Neumann suggested that component III consists of the observer’s ‘abstract ego’. In other words, process 1—the collapse of the wavefunction—only occurs when the measurement outcome is registered in the observer’s conscious mind.
The logic is pretty unassailable. No observer has ever reported experiencing a superposition of brain states (or, at least, anyone declaring that they have directly experienced such a superposition wouldn’t be taken very seriously). Components I and II are entirely ‘mechanical’ in nature—they involve physics and biochemistry. We’re left to conclude that because III is not mechanical, then this must be the place where the continuous evolution of the wavefunction—process 2—breaks down, to be replaced by process 1.
This conclusion is nevertheless quite extraordinary considering von Neumann’s mission in the Mathematical Foundations, which was to provide a much more secure mathematical basis for quantum mechanics using Hilbert’s axiomatic approach. As I’ve already mentioned, these axioms (especially Axiom #1) served to entrench the prevailing Copenhagen interpretation directly in the formalism itself. Although Bohr was more ambiguous about what this actually meant, Heisenberg’s perspective was firmly anti-realist. Yet in his theory of quantum measurement, von Neumann went substantially beyond the Copenhagen interpretation, ignoring Bohr’s insistence on an arbitrary boundary between the quantum and classical worlds.
I would argue that the introduction of a role for consciousness in the measurement process represents an addition to the conventional quantum formalism, seemingly at odds with the ‘nothing to see here’ axiom. I guess von Neumann would have responded that Axiom #1 refers only to the mathematical structure, and the proposed addition of component III is decidedly non-mathematical. He wrote that: ‘III remains outside of the calcula
tion’.2
As it stands, von Neumann’s theory can still be interpreted in two fundamentally different ways. An anti-realist would agree with the description of components I and II and interpret III not as a physical collapse, but as the registering of the measurement outcome and the updating of the observer’s state of knowledge about it. This most definitely involves the observer’s conscious mind, but only in a passive sense, and takes us back to relational quantum mechanics, or information-theoretic interpretations, or QBism (take your pick).
But it seems that von Neumann held a different view. Component III is intended as the place where process 1 occurs, considered as a real physical collapse. His long conversations with Szilard concerned the latter’s work on entropy reduction in thermodynamic systems through interference by intelligent beings, a variation on Maxwell’s Demon.3 The philosopher Max Jammer notes that this kind of paper ‘marked the beginning of certain thought-provoking speculations about the effect of a physical intervention of mind on matter’.4
A real physical collapse implies a real wavefunction, and therefore a much more active role for the observer’s conscious mind. It is for this reason that I include consciousness-causes-collapse theories in my collection of realist interpretations of quantum mechanics. That von Neumann wasn’t specific on how he thought the wavefunction itself should be interpreted just adds to the confusion (but is pretty much par for the course in this business).
But now we need to ask ourselves: Just who is the observer? Let’s return to the scenario we’ve considered a few times already, in which Alice makes a measurement in the laboratory but Bob is delayed in the corridor. I’m going to make one small adjustment. I’m going to replace Bob with renowned theorist Eugene Wigner. Alice and Wigner are close friends.5
We recall that Alice performs a measurement on a quantum system consisting of an ensemble of A particles prepared in a superposition of ↑ and ↓ states. Instead of a gauge the measuring device is now connected to a simple light switch. If the device records an ↑ result, the switch is not thrown and the light doesn’t flash (). If the device records a ↓ result, the switch is thrown and the light flashes (). She runs the experiment once, and observes the light flash.
Wigner is still in the corridor. As far as he is concerned, the total wavefunction that Alice just experimented on has the form of another superposition involving the measurement outcomes A↑ and A↓, the two possible states of the light, and , and Alice’s possible brain states, and :
Wigner now enters the laboratory. The following conversation ensues.
‘Did you see the light flash?’ asks Wigner.
‘Yes,’ replies Alice.
As far as Wigner is concerned, the measurement outcome has just registered in his conscious mind and the wavefunction collapses into the state described by A↓.
But, after some reflection, he decides to probe his friend a little further.
‘What did you feel about the flash before I asked you?’
Understandably, Alice is starting to get a little irritated. ‘I told you already, I did see a flash,’ she replies, testily.
Not wishing to put any further strain on his relationship with Alice, he decides to accept what she’s telling him. He concludes that the wavefunction must have already collapsed into the state A↓ before he entered the laboratory and asked the question, and the above superposition that he took to be the correct description is, in fact, wrong. This superposition ‘appears absurd because it implies that my friend was in a state of suspended animation before [she] answered my question’.6 He wrote:
It follows that the being with a consciousness must have a different role in quantum mechanics than the inanimate measuring device…. It is not necessary to see a contradiction here from the point of view of orthodox quantum mechanics, and there is none if we believe that the alternative is meaningless, whether my friend’s consciousness contains either the impression of having seen a flash or of not having seen a flash. However, to deny the existence of the consciousness of a friend to this extent is surely an unnatural attitude, approaching solipsism, and few people, in their hearts, will go along with it.
This is the paradox of Wigner’s friend. To resolve it we must presume that the irreversible collapse of the wavefunction is triggered by the first conscious mind it encounters.
There’s more. Nowhere in the physical world is it possible physically to act on an object without some kind of reaction. This is Newton’s third law of motion. Should consciousness be any different? Although small, the action of a conscious mind in collapsing the wavefunction produces an immediate reaction—knowledge of the state of a system is irreversibly (and indelibly) generated in the mind of the observer. This reaction may lead to other physical effects, such as writing the result in a laboratory notebook, or the publication of a research paper, or the winning of a Nobel Prize. In this hypothesis, the influence of matter over mind is balanced by an influence of mind over matter.
If we introduce a role for consciousness in our representation of quantum mechanics, then we must acknowledge the truth of one of Wheeler’s favourite phrases. He argued, paraphrasing Bohr, that ‘No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon.’7 Rather than interpret this registration process simply as an irreversible change in our knowledge of the system, Wheeler explored a more realistic interpretation in which the process is actually an irreversible act of creation. ‘We are inescapably involved in bringing about that which appears to be happening.’8
Wheeler took some pains to separate the notion of a ‘quantum phenomenon’ from consciousness, arguing that it is the physical, irreversible act of amplification that brings the phenomenon about. But his phrase includes the word ‘observed’, and if we accept a role for conscious observation in quantum physics, then we arrive at more or less the same conclusions. If consciousness is required to collapse the wavefunction and ‘make it real’, then arguably the quantum state that it represents does not exist until it becomes part of the observer’s conscious experience. It’s a relatively small step from this to the rejection of Proposition #1. Nothing exists unless and until it is consciously experienced.
And, indeed, in 1977 Wheeler himself elaborated what was to become known as the ‘participatory anthropic principle’:9
Nothing speaks more strongly for this thesis than…. the anthropic principle of [Brandon] Carter and [Robert] Dicke and…. the indispensable place of the participating observer—as evidenced in quantum mechanics—in defining any useful concept of reality. No way is evident to bring these considerations together into a larger unity except through the thesis of ‘genesis through observership’.
Here anthropic means ‘pertaining to mankind or humans’. Although Wheeler would subsequently declare that the ‘eye’ of the observer ‘could as well be a piece of mica’,10 it’s virtually impossible to read this 1977 essay without concluding that this is about ‘us’, participants of a universe that we create by observing it. In their comprehensive review of anthropic reasoning, called The Anthropic Cosmological Principle, John Barrow and Frank Tipler accepted Wheeler’s sentiments to be consistent with a version of what they call the strong anthropic principle: ‘Observers are necessary to bring the universe into being.’11
Okay. So on this particular visit to the shores of Metaphysical Reality, we’ve settled ourselves comfortably in a deckchair on the beach, with the Sun shining, shades on, sipping a margarita. We’re here to stay a while. Whilst the logic is clear, what we’re trying to do here is resolve two very deeply rooted philosophical conundrums—the collapse of the wavefunction and the nature of consciousness—simply by bringing them together. I have to say that this has never struck me as a particularly productive way to go.
By introducing consciousness into the mix, we invite an awful lot of further difficult questions. What is consciousness and how does it work? What does it mean when we say that consciousness is something other than ‘mechanistic’ and what evidence do we have f
or this? How is the collapse of the wavefunction supposed to be triggered by consciousness? Is the collapse of the wavefunction actually responsible for consciousness? Is the mind a quantum computer?
These questions are no doubt thought-provoking, but don’t expect to find too many ready answers. The study of consciousness is the only discipline I’ve come across that is structured principally in terms of its problems. We have the ‘hard problem’ of consciousness, the ‘mind–body’ problem, the problem of ‘other minds’, and many more. These problems have sponsored much philosophical reflection and many words but—at present—there appears to be no consensus on the solutions.
We find ourselves in quite a curious situation. Consciousness is very personal. You know what it feels like to have conscious experiences of the external world and you have what I might call an inner mental life. You have thoughts, and you think about these thoughts. You know what your own consciousness is or at least what it feels like. So what’s the problem?
To answer this question it’s helpful to trace the physical processes involved in the conscious perception of a red rose. Now, roses are red because their petals contain a subtle mixture of chemicals called anthocyanins, their redness enhanced if grown in soil of modest acidity. Anthocyanins in the rose petals interact with sunlight, absorbing certain wavelengths and reflecting predominantly red light, electromagnetic radiation with wavelengths between about 620 and 750 billionths of a metre, sitting at the long-wavelength end of the visible spectrum, sandwiched between invisible infrared and orange. Of course, light consists of photons but, no matter how hard we look, we will not find an inherent property of ‘redness’ in photons with this range of wavelengths. Aside from differences in wavelength (and hence energy, according to the Planck–Einstein relation), there is nothing in the physical properties of photons to distinguish red from green or any other colour.