Shadows of the Mind: A Search for the Missing Science of Consciousness
is a 1994 book by mathematical physicist
Roger Penrose that serves as
a followup to his 1989 book The Emperor's New Mind: Concerning
Computers, Minds and The Laws of Physics.
Penrose hypothesizes that:
Human consciousness is non-algorithmic, and thus is not capable of
being modelled by a conventional
Turing machine type of digital
Quantum mechanics plays an essential role in the understanding of
human consciousness; specifically, he believes that microtubules
within neurons support quantum superpositions.
The objective collapse of the quantum wavefunction of the microtubules
is critical for consciousness.
The collapse in question is physical behaviour that is non-algorithmic
and transcends the limits of computability.
The human mind has abilities that no
Turing machine could possess
because of this mechanism of non-computable physics.
1.1 Mathematical thought
1.2 Objective reduction
1.3 Orchestrated objective reduction
2.1 Gödelian argument and nature of human thought
3 See also
4 Notes and references
Orch-OR § The Penrose–Lucas argument
In 1931, the mathematician and logician
Kurt Gödel proved his
incompleteness theorems, showing that any effectively generated theory
capable of expressing elementary arithmetic cannot be both consistent
and complete. Further to that, for any consistent formal theory that
proves certain basic arithmetic truths, there is an arithmetical
statement that is true, but not provable in the theory. The essence of
Penrose's argument is that while a formal proof system cannot, because
of the theorem, prove its own incompleteness, Gödel-type results are
provable by human mathematicians. He takes this disparity to mean that
human mathematicians are not describable as formal proof systems and
are not running an algorithm, so that the computational theory of mind
is false, and computational approaches to artificial general
intelligence are unfounded. (The argument was first given by Penrose
The Emperor's New Mind (1989) and is developed further in Shadows
of The Mind. An earlier version of the argument was given by J. R.
Lucas in 1959. For this reason, the argument is sometimes called
the Penrose-Lucas argument).
Main article: Penrose interpretation
Penrose's theory of Objective Reduction predicts the relationship
between quantum mechanics and general relativity. Penrose proposes
that a quantum state remains in superposition until the difference in
space-time curvature reaches a significant level. This idea is
inspired by quantum gravity, because it uses both the physical
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. It is an alternative to the Copenhagen interpretation, which posits
that superposition fails under observation, and the many-worlds
hypothesis, which states that each alternative outcome of a
superposition becomes real in a separate world.
Penrose's idea is a type of objective collapse theory. In these
theories the wavefunction is a physical wave, which undergoes wave
function collapse as a physical process, with observers playing no
special role. Penrose theorises that the wave function cannot be
sustained in superposition beyond a certain energy difference between
the quantum states. He gives an approximate value for this difference:
Planck mass worth of matter, which he calls the "'one-graviton'
level". He then hypothesizes that this energy difference causes the
wave function to collapse to a single state, with a probability based
on its amplitude in the original wave function, a procedure taken from
standard quantum mechanics.
Orchestrated objective reduction
Main article: Orch-OR
When he wrote his first consciousness book,
The Emperor's New Mind in
1989, Penrose lacked a detailed proposal for how such quantum
processes could be implemented in the brain. Subsequently, Hameroff
The Emperor's New Mind and suggested to Penrose that certain
structures within brain cells (microtubules) were suitable candidate
sites for quantum processing and ultimately for consciousness.
Orch-OR theory arose from the co-operation of these two scientists
and was developed in Penrose's second consciousness book Shadows of
the Mind (1994).
Hameroff's contribution to the theory derived from studying brain
cells (neurons). His interest centred on the cytoskeleton, which
provides an internal supportive structure for neurons, and
particularly on the microtubules, which are the important component
of the cytoskeleton. As neuroscience has progressed, the role of the
cytoskeleton and microtubules has assumed greater importance. In
addition to providing a supportive structure for the cell, the known
functions of the microtubules include transport of molecules,
including neurotransmitter molecules bound for the synapses, and
control of the cell's movement, growth and shape.
This section needs expansion. You can help by adding to it. (October
Gödelian argument and nature of human thought
Penrose's views on the human thought process are not widely accepted
in scientific circles (Drew McDermott, David Chalmers and
others). According to Marvin Minsky, because people can construe false
ideas to be factual, the process of thinking is not limited to formal
logic. Further, AI programs can also conclude that false statements
are true, so error is not unique to humans. Another dissenter, Charles
Seife, has said: "Penrose, the Oxford mathematician famous for his
work on tiling the plane with various shapes, is one of a handful of
scientists who believe that the ephemeral nature of consciousness
suggests a quantum process."
In May 1995,
Solomon Feferman attacked
Penrose's approach on multiple grounds, including the mathematical
validity of his Gödelian argument and theoretical background. In
1996, Penrose offered a consolidated reply to many of the criticisms
John Searle criticises Penrose's appeal to Gödel as resting on the
fallacy that all computational algorithms must be capable of
mathematical description. As a counter-example, Searle cites the
assignment of license plate numbers (LPN) to specific vehicle
identification numbers (VIN), to register a vehicle. According to
Searle, no mathematical function can be used to connect a known VIN
with its LPN, but the process of assignment is quite simple—namely,
"first come, first served"—and can be performed entirely by a
Stuart Hameroff have constructed the
Orch-OR theory in
which human consciousness is the result of quantum gravity effects in
microtubules. However, in 2000,
Max Tegmark calculated in an article
he published in Physical Review E that the time scale of neuron
firing and excitations in microtubules is slower than the decoherence
time by a factor of at least 1010. Tegmark's article has been widely
cited by critics of the Penrose-Hameroff hypothesis. The reception of
the article is summed up by this statement in his support: "Physicists
outside the fray, such as IBM's John Smolin, say the calculations
confirm what they had suspected all along. 'We're not working with a
brain that's near absolute zero. It's reasonably unlikely that the
brain evolved quantum behavior', he says." In other words, there
is a missing link between physics and neuroscience, and to date,
it is too premature to claim that the
Orch-OR hypothesis is right.
In response to Tegmark's claims, Hagan, Tuszynski and Hameroff
claimed that Tegmark did not address the
Orch-OR model, but instead a
model of his own construction. This involved superpositions of quanta
separated by 24 nm rather than the much smaller separations
stipulated for Orch-OR. As a result, Hameroff's group claimed a
decoherence time seven orders of magnitude greater than Tegmark's,
although still far below 25 ms. Hameroff's group also suggested
that the Debye layer of counterions could screen thermal fluctuations,
and that the surrounding actin gel might enhance the ordering of
water, further screening noise. They also suggested that incoherent
metabolic energy could further order water, and finally that the
configuration of the microtubule lattice might be suitable for quantum
error correction, a means of resisting quantum decoherence.
In 2007, Gregory S. Engel[who?] claimed that all arguments concerning
the brain being "too warm and wet" have been dispelled, as multiple
"warm and wet" quantum processes have been discovered.
The Emperor's New Mind
Computational theory of mind
Alan Turing, creator of the Turing test
Notes and references
^ Minds, Machines and Gödel
^ a b Penrose, Roger (1999) ,
The Emperor's New Mind (New
Preface (1999) ed.), Oxford, England: Oxford University Press,
pp. 475–481, ISBN 0-19-286198-0
^ 'Folger, Tim. "If an Electron Can Be in 2 Places at Once, Why Can't
You?" Discover. Vol. 25 No. 6 (June 2005). pp33-35.
^ Hameroff, S.R. & Watt, R.C. (1982). "Information processing in
microtubules" (PDF). Journal of Theoretical Biology. 98 (4):
549–561. doi:10.1016/0022-5193(82)90137-0. PMID 6185798.
^ a b c Hameroff, S.R. (1987). Ultimate Computing. Elsevier.
ISBN 0-444-70283-0. Archived from the original on 6 April
^ Penrose, Roger (1989). Shadows of the Mind: A Search for the Missing
Science of Consciousness. Oxford University Press. p. 457.
^ Penrose is Wrong Drew McDermott, PSYCHE, 2(17), October 1995
^ Minds, Machines, And Mathematics – A Review of Shadows of the Mind
Roger Penrose David J. Chalmers, PSYCHE 2(9) June 1995
^ Penrose's Gödelian argument (PDF) Feferman, PSYCHE 2(7) May 1995
^ Beyond the Doubting of a Shadow – A Reply to Commentaries on
Shadows of the Mind
Shadows of the Mind Roger Penrose, PSYCHE, 2(23), January 1996
^ Searle, John R. The Mystery of Consciousness. 1997.
ISBN 0-940322-06-4. pp 85–86.
^ Tegmark, M. (2000). "Importance of quantum decoherence in brain
processes". Phys. Rev. E. 61 (4): 4194–4206.
arXiv:quant-ph/9907009 . Bibcode:2000PhRvE..61.4194T.
^ Tetlow, Philip (2007). The Web's Awake: An Introduction to the Field
of Web Science and the Concept of Web Life. Hoboken, NJ: John Wiley
& Sons. p. 166. ISBN 978-0-470-13794-9.
^ Maurits van den Noort; Sabina Lim; Peggy Bosch (2016-10-28).
"Towards a theory of everything: The observer's unconscious brain".
Nature. 538: 36–37. Bibcode:2016Natur.538...36D.
doi:10.1038/538036a. Retrieved 2016-05-11.
^ Hagan, S., Hameroff, S., and Tuszyński, J. (2002). "Quantum
Computation in Brain Microtubules? Decoherence and Biological
Feasibility". Physical Review E. 65 (6): 061901.
arXiv:quant-ph/0005025 . Bibcode:2002PhRvE..65f1901H.
doi:10.1103/PhysRevE.65.061901. Retrieved 16 March 2014. CS1
maint: Multiple names: authors list (link)
^ Hameroff, S. (2006). "Consciousness, Neurobiology and Quantum
Mechanics". In Tuszynski, Jack. The Emerging Physics of Consciousness.
Springer. pp. 193–253.
^ Engel, Gregory S.; Calhoun, Tessa R.; Read, Elizabeth L.; Ahn,
Tae-Kyu; Mančal, Tomáš; Cheng, Yuan-Chung; Blankenship, Robert E.;
Fleming, Graham R. (12 April 2007). "Evidence for wavelike energy
transfer through quantum coherence in photosynthetic systems". Nature.
446 (7137): 782–786. Bibcode:2007Natur.446..782E.
doi:10.1038/nature05678. PMID 17429397. Retrieved 7 December
^ Panitchayangkoon, Gitt; Dugan Hayes; Kelly A. Fransted; Justin R.
Caram; Elad Harel; Jianzhong Wen; Robert E. Blankenship; Gregory S.
Engel (6 July 2010). "Long-lived quantum coherence in photosynthetic
complexes at physiological temperature". Proceedings of the National
Academy of Sciences. 107 (28): 12766–12770. arXiv:1001.5108 .
PMC 2919932 . PMID 20615985.
This article includes text originally by Philip Dorrell[permanent dead
link] which is licensed under the GFDL
Works by Roger Penrose
The Emperor's New Mind (1989)
Shadows of the Mind
Shadows of the Mind (1994)
The Road to Reality
The Road to Reality (2004)
Cycles of Time
Cycles of Time (2010)
Fashion, Faith, and Fantasy in the New Physics of the Universe
Fashion, Faith, and Fantasy in the New Physics of the Universe (2016)
The Nature of Space and Time
The Nature of Space and Time (with Stephen Hawking) (1996)
The Large, the Small and the Human Mind (with Abner Shimony, Nancy
Cartwright and Stephen Hawking) (1997)
White Mars or, The Mind Set Free (with Brian W. Aldiss) (1999)
Techniques of Differential Topology in Relativity (1972)
Spinors and Space-Time: Volume 1, Two-Spinor Calculus and Relativistic
Fields (with Wolfgang Rindler) (1987)
Spinors and Space-Time: Volume 2, Spinor and Twistor Methods in
Space-Time Geometry (with Wolfga