Quantum Mechanics and the Mind

In a letter to Schrodinger written in 1950, Einstein stated that that the collapse approach was "refuted most elegantly" (Einstein, pp.39) by Schrodinger's thought experiment. There is no explanation for how quantum states can make contact with anything that obeys classical laws alone and Einstein found the lack of objective reality in quantum mechanics deeply disturbing. He praised Schrodinger for being one of the only physicists "who sees that one cannot get around the assumption of reality, if only one is honest" (Einstein, pp.39) and argued that proponents of the collapse approach "simply do not see what sort of risky game they are playing with reality" by suggesting that it is "something independent of what is experimentally established" (Einstein, pp.39).

Heisenberg and von Neumann both suggested that the collapse occurs when a measurement registers in the mind of a conscious observer (Neumann, 1930) and in 1961, Hungarian-American physicist Eugene Wigner extended Schrodinger's thought experiment to include another human observer, Wigner's friend (Wigner, pp.284-302). If Wigner's friend conducts Schrodinger's experiment whilst Wigner waits outside the laboratory, then the state collapses earlier from the perspective of Wigner's friend than for Wigner, who can consider his friend to be in a superpositional state until he interacts with them. Wigner did not accept this and argued that a self aware consciousness is what causes the collapse. This was considered a possibility because the mind exhibits properties that cannot be explained using classical physical laws, such as subjectivity, and Wigner suggested that scientists search for unusual effects of consciousness acting on matter (Wigner, pp.284-302).

Despite Wigner's suggestion that something must occur at the level of consciousness, there are no collapse dynamics in quantum mechanics and nothing within it suggests that consciousness is special in any way. In 1994, English physicist Roger Penrose suggested that the force of gravity could cause the collapse (Penrose, pp.335-347) and in 1990, Italian physicist Gian Carlo Ghirardi and American physicist Philip Pearle suggested that collapses can occur spontaneously (Ghirardi and Pearle, pp. 35-47). None of these theories have been able to adequately solve the problems that the collapse approach faces and this problem cannot be ignored for much longer, quantum effects are now being demonstrated in larger and larger objects.

In 2009, German physicist Michal Karski and colleagues exhibited quantum effects in a single atom of cesium, allowing "the observation of the quantum-to-classical transition,"(Karski et al., pp.174) and two groups of American physicists headed by John Jost and Keith Schwab have shown quantum effects in simple harmonic oscillators. Jost suggested that "such experiments may lead to the generation of entangled states of larger-scale mechanical oscillators" such as "the vibrations of violin strings" and "the oscillations of quartz crystals used in clocks" (Jost et al., pp.683-685). Schwab argued that "it'd be weird to think of ordinary matter behaving in a quantum way" but agreed that there's no reason it shouldn't. "If single particles are quantum mechanical, then collections of particles should also be quantum mechanical. And if that's not the case-if the quantum mechanical behavior breaks down-that means there's some kind of new physics going on that we don't understand" (Schwab, et al., pp.960-964).

Whilst the measurement problem remains unresolved for the collapse approach, decoherence theory can explain why macroscopic objects appear to exhibit classical behaviour. Decoherence theory was first suggested by German physicist Heinz-Dieter Zeh in 1970 (Zeh, 1970) and this was extended by Polish physicist Wojciech Zurek in 1981 (Zurek, pp.1516-1525). Decoherence theory is the study of how the interference effects of a superposition are suppressed in macroscopic objects. In the case of the double slit experiment, interference effects are suppressed by placing a particle detector at each slit.

When describing the quantum state of a large number of objects, a mathematical device known as a density matrix is used to determine every possibility, some of these possibilities will include the results we expect but some will include the superposition of macroscopic objects. Zeh and Zurek showed that we never observe these possibilities because they decay exponentially, this means that they are not observable long enough for us to notice them. It takes about 10^-27 seconds (less than a millionth of a billionth of a billionth of a second) for the inference effects of macroscopic objects to become unobservable. This process is said to be irreversible because it would be impossible for an observer to reconstruct the superpositional state after it has decayed.

Decoherence shows that the same mechanism that is responsible for the suppression of interference effects in the quantum realm is also responsible for this suppression in macroscopic objects. This occurs when quantum states become entangled with a large number of objects and does not rely on human intervention. Objects on Earth will become entangled with the atmosphere, for example, and Schrodinger's cat would die within 10^-27 seconds of the atom decaying whether it is observed or not. It takes about a year for the inference effects of isolated microscopic objects to disappear and this is why it is much easier to observe them. Decoherence cannot solve the measurement problem for the collapse approach because it does not provide any collapse dynamics, or explain how quantum and classical objects could be composed of two distinct substances.

The measurement problem is partially solved by applying decoherence theory to the Bohm approach because here there is no collapse of the wavefunction. The Bohm approach faces problems of its own however, as it must explain why we only observe one of any number of possible results. We would expect that if the mind of the observer does not collapse into a definite state then it should remain in a superposition and hence observe every possibility. The Bohm approach solves this problem with hidden variables that suppress these other possible brain states, yet these arguments face the same criticisms as collapse dynamics. There is no evidence of these hidden variables, they are not found within quantum theory itself and must be added by hand.

References

Einstein, A., 'Letter to Schrödinger written 22nd December 1950', reprinted in Letters on Wave Mechanics, 1967, Prizbram (ed.), Vision Press, London

Ghirardi G. C. and Pearle, P., 1990, 'Elements of Physical Reality, Nonlocality and Stochasticity in Relativistic Dynamical Reduction Models', Philosophy of Science Association, Vol.2

Jost, J. D., et al., 2009, 'Entangled mechanical oscillators', Nature, Vol.459

Karski, M., et al.,2009, 'Quantum Walk in Position Space with Single Optically Trapped Atoms', Science, Vol.325, pp.174-177

Penrose, 1996, 'Shadows of the Mind', Oxford University Press, Oxford

Schrodinger, E., 1935, 'The Present Situation in Quantum Mechanics', Proceedings of the American Philosophical Society, Trimmer, J.D. (trans.), Vol.124

Schwab, K. C., et al., 2009, 'Nanomechanical measurements of a superconducting qubit', Nature, Vol.459, pp.960-964, quote from 'Mechanics: Nano Meets Quantum', Caltech Press Release, 19 June 2009

von Neumann, J., 1930, 'Fundamentos Matematicos de la Mecanica Cuantica', Institute Jorge Juan, Madrid. See also von Neumann, J., 1996, 'Mathematical Foundations of Quantum Mechanics', Princeton University Press, Princeton

Wigner, E.P., 1962, 'Remarks on the Mind-Body Problem', The Scientist Speculates, Good, I.J. (ed), Heinemann, London

Zeh, H.D., 1970, 'On the interpretation of measurement in quantum theory', Foundations of Physics, Vol.1

Zurek, W., 1981, 'Pointer basis of quantum apparatus: Into what mixture does the wave packet collapse?', Physical Review, Vol.24

Quantum Mechanics and the Mind (1900s)

The collapse approach to quantum mechanics suggests that instantaneous action at a distance should simply be accepted in quantum mechanics as it was with Newton's theory of gravitation. However there are a number of other problems with the collapse approach that need to be addressed, firstly it does not adequately define when the collapse occurs and so cannot be applied to the universe as a whole. Secondly, there are no collapse dynamics within quantum theory itself and so they need to be added by hand and thirdly, it cannot explain how the quantum world can make contact with the classical world at all since both systems obey different physical laws. This is known as the measurement problem and it is analogous to the problem of causal interaction faced by Descartes in 1641.

These problems were highlighted by Schrodinger in 1935, written as a response to the EPR paper which also criticised the collapse approach (Schrodinger, pp.323-38). Schrodinger considered an experiment where a cat is placed in a closed box with a radioactive atom, the atom has a chance of decaying, and if it does it will trigger a device which will kill the cat. The collapse approach suggests that quantum states do not collapse until they are measured and so suggests that the cat is both dead and alive at the same time until the experimenter opens the box, thereby measuring the system. The collapse approach implies that the cat itself cannot count as a measuring device, otherwise it would collapse the state as soon as it became aware of what was happening. Schrodinger argued that this shows the collapse approach cannot adequately describe what happens when macroscopic objects become entangled and Einstein agreed.