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).
Please click here to download the paper.
NOTE: If you can't read
ps-files, click instead to download
a PDF version of the paper. This can be viewed and printed
on any PC or Macintosh
using Acrobat Reader, downloadable for free from
Adobe.
Click
if you are interested
in other research of mine.
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 (~0.001-0.1 seconds), both for regular
neuron firing and for kink-like 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 top-level multi-neuron 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.
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.
Here's an article about my paper that appeared in the the February 4, 2000 issue
of Science:
Return to my home page