Max Tegmark's quantum mechanics library: brain
I've added a rejoinder to a temperature critique below.
This paper was covered in
Science,
Cern Courier +
critique,
Physics Web and
Dagens Nyheter (Swedish).
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The importance of quantum decoherence in brain processes
Author:
Max Tegmark
Abstract:
Based on a calculation of neural decoherence rates, we
argue that that the degrees of freedom of the human brain
that relate to cognitive processes should be thought of as a
classical rather than quantum system, ie, that there is
nothing fundamentally wrong with the current classical
approach to neural network simulations. We find that the
decoherence timescales ~10^{13}10^{20} seconds are
typically much shorter than the relevant dynamical
timescales (~0.0010.1 seconds), both for regular
neuron firing and for kinklike polarization excitations
in microtubules. This conclusion disagrees
with suggestions by Penrose and others that the brain acts
as a quantum computer. Our result is also difficult to
reconcile with the Stapp model where thought processes
correspond to toplevel multineuron quantum events.
To place this problem in context, we also discuss
a decomposition of the global density matrix into three subsystems
as in the figure on the right, where the three interaction
Hamiltonians cause qualitatively different effects.
Reference info:
quantph/9907009, Phys. Rev. E, 61, 41944206
Related papers:
In this paper,
Hagan, Hameroff and Tuszynski argued that my calculation must be flawed because
the decoherence timescales that I derived decrease as you lower the temperature of the brain, whereas you might intuitively expect the opposite.
The point they overlooked is that as soon as you drop the absolute temperature by about 10%, below 0 Celsius, your brain freezes and the decoherence time grows dramatically.
The slight decrease in decoherence time for tiny temperature reductions simply reflects the fact that the scattering cross section grows as you lower the temperature,
just as slow neutrons have larger cross section than fast ones in a nuclear reactor.
This site also contains the latest versions of some closely related papers
of mine:

Tegmark 1993 describes how decoherence looks
like, feels like and smells like wavefunction collapse, thereby eliminating
one of the main motivations for the Copenhagen interpretation.

Tegmark 1998 describes the
socalled manyworlds interpretation of quantum mechanics.

Tegmark 1997 proposes a theory with an even larger
ensemble, making even the many worlds of quantum mechanics seem plain and
close to home in comparison!

Tegmark & Yeh 1994 and Tegmark
& Shapiro 1994 are marginally related, giving explicit examples
of the effects interaction with the environment.

Tegmark 1996 is also marginally related, discussing
how decoherence can be measured in practice.

Tegmark & Wheeler 2001 places this work in a broader context and is much more readable if you're not a physicist.
Popular articles:
Here's an article about my paper that appeared in the the February 4, 2000 issue
of Science:
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This page was last modified October 16, 2005.
max@physics.upenn.edu