The Objections of an Unashamed Reductionist
Hawking (Stephen)
Source: Penrose - The Large, the Small and the Human Mind, Chapter 7
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  1. To start with, I should say I’m an unashamed reductionist. I believe that the laws of biology can be reduced to those of chemistry. We have already seen this happening with the discovery of the structure of DNA. And I further believe that the laws of chemistry can be reduced to those of physics. I think most chemists would agree with that.
  2. Roger Penrose and I worked together on the large-scale structure of space and time, including singularities and black holes. We pretty much agree on the classical theory of General Relativity but disagreements began to emerge when we got on to quantum gravity. We now have very different approaches to the world, physical and mental. Basically, he’s a Platonist believing that there’s a unique world of ideas that describes a unique physical reality. I, on the other hand, am a positivist who believes that physical theories are just mathematical models we construct, and that it is meaningless to ask if they correspond to reality, just whether they predict observations.
  3. This difference in approach has led Roger to make three claims in Chapters 1-3 that I strongly disagree with.
    1. The first is that quantum gravity causes what he calls OR, objective reduction of the wavefunction.
    2. The second is that this process has an important role in the operation of the brain through its effect on coherent flows through microtubules.
    3. And the third is that something like OR is needed to explain self-awareness because of the Gödel Theorem.
  4. To start with quantum gravity, which is what I know best. His objective reduction of the wavefunction is a form of decoherence. This decoherence can come about through interactions with the environment or through fluctuations in the topology of spacetime. But Roger seems to want neither of these mechanisms. Instead he claims that it occurs because of the slight warping of space-time produced by the mass of a small object. But, according to accepted ideas, that warping will not prevent a Hamiltonian evolution1 with no decoherence or objective reduction. It may be that accepted ideas are wrong but Roger has not put forward a detailed theory that would enable us to calculate when objective reduction would occur.
  5. Roger’s motivation in putting forward objective reduction seems to have been to rescue Schrodinger’s poor cat from its half-alive, half-dead state. Certainly, in these animal liberation days, no one would dare suggest such a procedure, even as a thought experiment. However, Roger made a point of claiming that objective reduction was so weak an effect that it could not be experimentally distinguished from decoherence caused by interaction with the environment. If that is the case, then environmental decoherence can explain Schrodinger’s cat. There’s no need to invoke quantum gravity. Unless objective reduction is a strong enough effect to be measured experimentally, it can’t do what Roger wants it to do.
  6. Roger’s second claim was that objective reduction had a significant influence on the brain, maybe through its effect on coherent flows through microtubules. I’m not an expert on the operation of the brain, but it seems very unlikely, even if I believed in objective reduction, which I don't. I cannot think that the brain contains systems that are sufficiently isolated that objective reduction could be distinguished from environmental decoherence. If they were that well isolated they wouldn’t interact rapidly enough to take part in mental processes.
  7. Roger’s third claim is that objective reduction is somehow necessary because Gödel’s Theorem implies that a conscious mind is not computable. In other words, Roger believes that consciousness is something special to living beings and that it couldn’t be simulated on a computer. He didn't make it clear how objective reduction could account for consciousness. Rather, his argument seemed to be that consciousness is a mystery and quantum gravity is another mystery so they must be related.
  8. Personally, I get uneasy when people, especially theoretical physicists, talk about consciousness. Consciousness is not a quality that one can measure from the outside. If a little green man were to appear on our door step tomorrow, we do not have a way of telling if he was conscious and self-aware or was just a robot. I prefer to talk about intelligence2 which is a quality that can be measured from the outside. I see no reason why intelligence3 should not be simulated on a computer. We certainly can't simulate human intelligence4 at the moment, as Roger showed with his chess problem. But Roger also admitted that there was no dividing line between human intelligence5 and animal intelligence6. So it will be sufficient to consider the intelligence7 of an earthworm. I don't think there’s any doubt that one can simulate an earthworm's brain on a computer. The Gödel argument is irrelevant because earthworms don’t worry about Π1-sentences.
  9. The evolution8 from earthworm brains to human brains presumably took place by Darwinian natural selection. The quality selected for was the ability to escape enemies and to reproduce, not the ability to do mathematics. So again the Gödel Theorem is not relevant. It is just that the intelligence9 needed for survival can also be used to construct mathematical proofs. But it is a very hit and miss business. We certainly don’t have a knowably sound procedure.
  10. I have told you why I disagree with Roger’s three claims that there is objective reduction of the wavefunction, that this plays a role in the operation of the brain and that it is necessary to explain consciousness. I had better let Roger reply.


See "Penrose (Roger) - The Large, the Small and the Human Mind: Responses to Shimony, Cartwright & Hawking" for a reply.

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