摘要：We are finally testing the ideas that quantum collapse in the brain gives rise to consciousness and that consciousness creates the reality we see from the quantum world.

If physics explains all the phenomena in the universe, and if consciousness is part of the universe, then is seems that physics can explain consciousness.

Of course, this assumes that consciousness isn’t separate from the material reality that physics explains – which runs counter to René Descartes’s dualist view of mind and matter. Some have no problem with that. They include Daniel Dennett at Tufts University in Massachusetts and Michael Graziano at Princeton University, who argue that our intuitive sense that consciousness needs an explanation that goes beyond objective descriptions of the physical world is misplaced. Consciousness is a mirage produced by sophisticated neural mechanisms in the brain, they contend, so we need no new physics to explain it. Rather, we need a better understanding of how the brain creates models: of the world, of a self in the world and of a self subjectively experiencing the world.

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Other non-dualists don’t outright deny that consciousness may have unusual properties that need explaining. If they are correct, then quantum mechanics may offer an explanation.

Quantum systems can exist in a superposition of all possible states simultaneously, and classical reality emerges when this superposition collapses into a single state. One idea is that this happens when the mass of a quantum system crosses a threshold. According to theoretical physicist Roger Penrose at the University of Oxford and anaesthesiologist Stuart Hameroff at the University of Arizona, consciousness emerges as the result of such collapses occurring in the brain. In their model, called orchestrated objective reduction (Orch OR), microscopic structural elements within neurons, called microtubules, enter into states of quantum superposition. These span networks of neurons and when the mass of the microtubules in a superposition exceeds some threshold, it collapses, producing conscious moments.

There are many dots to be connected for this idea to be taken seriously. One is to show that microtubules can attain states of quantum superposition in the relatively balmy temperatures of the human brain. According to Hameroff, preliminary and unpublished work by Jack Tuszynski at the University of Alberta, Canada, and Gregory Scholes at Princeton University has achieved that. “They have found evidence for quantum states in microtubules persisting for up to 5 nanoseconds,” he says.

The next step is to expose microtubules to anaesthetics, which selectively disrupt consciousness while leaving non-conscious brain functions intact. “The prediction is that anaesthetics would dampen microtubule quantum states proportional to known anaesthetic potencies in putting humans and animals to sleep,” says Hameroff. This provides a way to test Orch OR, he says, and Bruce MacIver at Stanford University in California is looking into conducting such an experiment.

But what if consciousness is separate from material stuff and so outside the purview of physics? If so, Orch OR probably has it backwards, says Johannes Kleiner at the Munich Centre for Mathematical Philosophy in Germany.

In a yet to be published paper, he and Kobi Kremnitzer at the University of Oxford show mathematically that if non-material consciousness were to affect material reality, in our physical reality, superpositions would appear to spontaneously collapse. “So the arrow is from consciousness to collapse,” says Kleiner. “In Orch OR, it’s from collapse to consciousness.”

Does consciousness make reality?

To answer this question, we must first consider how the reality of everyday experience emerges from the more esoteric quantum reality.

A quantum system exists in a superposition of all possible states, which collapses into a single state (classical reality) only when someone or something observes or measures it – at least, according to standard quantum theory. However, this theory fails to define exactly what constitutes a measurement or an observer, and how exactly that collapse happens. Since the1950s, many theorists have tried to solve this “measurement problem” by getting rid of the need for collapse-causing measurements and observers. The Many Worlds interpretation of quantum mechanics, for example, says that there is no collapse and that each classical state in the superposition is physically realised in a different world. So-called collapse theories, meanwhile, propose that superpositions collapse randomly into one of the many possible classical states once the mass of the matter in the quantum system crosses some threshold, and so resists superposition. There is no empirical evidence to prove or disprove these ideas.

Another possibility is that consciousness causes collapse. The Nobel prizewinning theorist Eugene Wigner speculated just this in the 1960s. He eventually disavowed his own meanderings, however, and the idea went out of favour, partly because there was no way to formulate such a theory with mathematical precision. Recently, however, David Chalmers at New York University and Kelvin McQueen at Chapman University in California came up with an admittedly speculative but mathematically grounded account of how consciousness can cause collapse.

Quantum collapse

They start with integrated information theory (IIT), taking it as an exemplar of a mathematical account of consciousness. IIT says that any system that integrates information is conscious. Chalmers and McQueen consider IIT as applied to quantum systems, reasoning that any such system that integrates information can enter a superposition of conscious states. They then posit that conscious states are resistant to superposition, in much the same way that, in other models, mass that crosses some threshold resists superposition and causes collapse. So if a quantum system enters a superposition of states in which at least one of the states is conscious (according to IIT), then that consciousness will collapse the system.

In this way of thinking, “consciousness creates classical reality”, says McQueen. “But it doesn’t create quantum reality. It’s converting quantum reality into classical reality.”

Previous attempts to solve the measurement problem by appealing to consciousness have run into a major problem: if classical reality requires the presence of conscious humans, how did the universe evolve classically to the point where human consciousness appeared? The new idea avoids this because IIT doesn’t limit consciousness to biological beings. The universe could have begun as a quantum system and continued evolving quantum mechanically until matter first became able to integrate information. This consciousness then started to collapse quantum reality, creating the sort of classical reality we experience today.

Is the universe conscious?

For some, the question of whether the universe is conscious makes little sense. “It presupposes a view of consciousness as some special something which is detached from psychology,” says Daniel Dennett at Tufts University in Massachusetts. “Is the universe embarrassed? Is the universe happy? If it can’t be any of those things, then the claim that it’s conscious is, I think, vacuous.”

Others don’t dismiss the question outright, however. In particular, the integrated information theory (IIT) of consciousness raises the spectre that any physical system can be conscious. A metric called phi measures how much integrated information a system possesses, and any with a phi that is even a smidgen more than zero is conscious. That would include, for example, a thermostat, which might simply be conscious of being on or off. Hard-line proponents of IIT are led towards panpsychism, the idea that everything in the universe is conscious – including, possibly, the universe itself.

In February, mathematician Johannes Kleiner at Ludwig Maximilian University of Munich, Germany, and Sean Tull at Cambridge Quantum Computing Limited, UK, came a step closer to a formal analysis of the idea. They published what they call the mathematical structure of IIT. They identified the mathematical space that describes the states of physical systems on the one hand, and the mathematical space that describes experiences that physical systems may be having on the other. They then developed an algorithm to map the elements of one space to the other. If IIT says a physical system is conscious because its phi is greater than zero, then this new maths lets you map its physical state to an experience.

So, in this way of thinking, is the universe conscious? “The interesting bit is that you can ask that question of the theory,” says Kleiner. “You can plug in the state of the universe, if you happen to be able to describe it, and then the mathematical mapping would give you an answer.” Unfortunately, he adds, right now such calculations are possible for only the trivially simplest of systems.

Philosopher Kelvin McQueen at Chapman University, California, is more circumspect about IIT and its implications for panpsychism. Panpsychism as an idea originated because some philosophers went searching for the intrinsic nature of the material world and came up empty, he says. Take an elementary particle’s inertial mass, which is defined as its disposition to resist acceleration given an applied force. “But what is mass in and of itself that gives rise to that disposition? What’s its intrinsic nature?” he says. “The panpsychists like to say, ‘well, the only thing that’s intrinsic that we know of is consciousness, so mass is ultimately really a kind of consciousness.’ They imbue consciousness in elementary particles.” However, according to IIT, an individual elementary particle isn’t a system that is integrating information. So IIT isn’t synonymous with panpsychism. “It doesn’t say everything is conscious,” says McQueen. And the universe could be one of those things that isn’t, even though it has conscious systems within it.