Tuesday, January 06, 2009

Review of Quantum Aspects of Life

Quantum Aspects of Life is a collection of papers edited by Derek Abbott, Paul Davies, and Arun K. Pati which was recently published by the Imperial College Press (this review refers to the paperback edition). The target topic of the book is the role played by quantum mechanics (QM) in living things. Actually, we need to be more specific than that, since it’s accepted that all biology is based on chemistry, and chemistry is inherently based on quantum physics. In the book, the key distinction is described in a couple of different ways: in the foreword, Sir Roger Penrose distinguishes between “strongly” and “weakly” quantum mechanical features utilized by living things; the editors’ preface says the intent is to address the question of “whether quantum mechanics plays a non-trivial role in biology” (p.xiii). In each case the phenomena in question are those which are distinctively quantum mechanical: superposition, entanglement, tunneling, etc.

The volume is very welcome, since the topic seems to demand more focus than it has received. Confirming a significant quantum role could have a huge impact – both on the practical pursuit of biology and the philosophical perspective we take on the nature of life and mind.

There has long been a good circumstantial case to be made that the remarkable nature of non-trivial QM effects may serve to help explain the remarkable capabilities of biological systems. The foreword, preface, and Davies’ opening chapter all invoke Schrödinger’s 1944 book What is Life? as the ur-text exploring this idea. However, experimental confirmation of QM’s role in life didn’t follow, and molecular biology experienced huge growth and success nonetheless. Again repeating themselves a bit, the editors, Penrose and Davies each lament that the science of the intervening decades has been dominated by the “ball-and-stick” model of chemistry. They seem to be implying that scientists (for institutional reasons perhaps) haven’t been bothering to look for quantum effects. Of course, a hurdle for the idea, not as explicitly evident in Schrödinger’s time, is the phenomena of environmental decoherence. An issue running through the book is the challenge posed by the fact that maintaining quantum coherence is very difficult even in carefully controlled experimental settings.

While experimental confirmation of non-trivial QM effects in biology has indeed been elusive, it has not been absent, and a recent result strikes me as not only important, but possibly seminal. I’m referring to the 2007 Engel, et.al. paper in Nature which showed the utilization of quantum coherence in photosynthesis (please see the post “Quantum Biology Goes Mainstream” for links). The timing of this result’s publication was such that it either barely pre-dated or else post-dated the submission of papers to Quantum Aspects of Life. As I read the book, I was often thinking about how this result might change the debate as we move forward: it not only showed the utilization of QM in one of the core processes in biology, it also showed the engineering challenge (and attendant resource demands) which are involved in confirming the presence of such a phenomenon. There are quite a number of papers out there which present theoretical models which postulate QM effects to answer outstanding questions in biology -- here’s a good example of this sort of paper I saw recently – but confirming a theoretical result is another story.

Quantum Aspects of Life begins with a nice foreword by Penrose, whose perspective will be familiar to those who have read his books. He thinks the human mind will demand a quantum-derived explanatory account (which he thinks will be linked to a future theory of quantum gravity). Now, environmental decoherence becomes an increasingly strong obstacle as distances lengthen, so quantum computing in the human brain seems unlikely to be a result of quantum coherence extending across neuronal assemblies. Penrose, and his collaborator Stuart Hameroff (who also contributes a paper to the volume), think, however, that quantum effects may be effected through intra-cellular structures (microtubules) thereby creating an avenue for large-scale computation. Penrose has the following thoughtful observation: “Indeed, there is no question that if the brain does make use of such “strongly” quantum-mechanical phenomena, it must do so through the agency of some very sophisticated organization. (p.viii)” Life and mind might combine small-scale quantum effects with (classically describable) organizational structure.

Chapter one, by Paul Davies, and Chapter three, by Jim Al-Khalili and Johnjoe McFadden both focus on the possible role QM may have played in the origin of life on earth (OOL). Life may have learned to exploit QM phenomena via natural selection after a classically explainable beginning, but it seems plausible that QM may have been key to life from the start. The complexity of the building blocks of life (as we know it) make it statistically very unlikely that the components randomly came together in a primordial soup as was once suspected. Davies explores a model of a quantum replicator as a precursor; both he and Al-Khalili/McFadden discuss the possibility of a quantum-coherent search algorithm which helped lead to an early replicator. These are interesting ideas, although probably a long way from being confirmed or falsified (I have prior posts on work by Davies and McFadden here and here, respectively).

Chapter 2 is from Seth Lloyd, who stays at 40,000 feet by discussing generally how complexity should be expected to arise in a universe which is inherently quantum-mechanical (see my several posts on Lloyd’s “universe as quantum computer” thesis here).

Models of how photosynthesis likely exploits coherence is the subject of Chapter 4 (with the Engel paper validating broadly the idea); the authors of Chapter 5 present models to help quantify the impact of environmental decoherence in biological contexts. This work seems to sharply delimit the potential for coherence; however other authors argue that dynamic systems can foster insulated sub-spaces larger than what would otherwise seem likely.

A quantum role in DNA mutation and replication is a topic which is discussed in several chapters (6,9,10). It seems accepted that tunneling is one avenue to DNA mutation; possibly QM has a more meaningful role to play. The capabilities of artificial quantum systems are explored in chapters 11-14. To the extent QM systems are good at mimicking features of living things, this offers more circumstantial evidence.

One of my favorite parts of the book is the inclusion (as chapters 15 and 16) of the transcripts of two staged debates which took place at conferences: one is on the future of quantum computing (from 2003), and one specifically on the topic of whether life utilizes non-trivial quantum effects (2004). Both debates featured good insights and a good deal of wit. At one point in the latter debate, one of the participants, Howard Wiseman, offered his definition of a non-trivial quantum effect as “something that will make a biologist want to go out and, you know, take a second year quantum mechanics course and learn about Hilbert spaces and operators, so that they understand what’s going on. (p.358)” Again, I thought about how something like the Engel, et.al. result would change the debate if it were held again today. Wiseman and Jens Eisert --another debate participant -- contributed a thoughtful paper (Ch. 17) explaining in a more organized format why they were on the skeptics’ side of the discussion.

Stuart Hameroff gets the last word in the book, offering his positive proposals for how it all might work (Ch.18). He sees in the structure of cells, both cytoskeleton and protoplasm, features which could lead to a larger scale participatory quantum biology. As a layperson, I’m not a good one to offer judgment; my feeling is that Hameroff presents a string of plausible ideas which nonetheless link together to form a very speculative edifice. On the other hand, as far as I know he could be right! Until we have more attention on the topic we will be slow to sort through and find out which quantum biological ideas are fanciful and which are on target.

So, I second the editors of this volume in their hope that its publication will provoke further debate and help motivate experimental research into this fascinating subject.



8 comments:

Anonymous said...

Thanks for your review!
I didn't notice the Engel paper otherwise.

Anonymous said...

...wouldn't have noticed, I mean. Please excuse my English.

Steve said...

You are welcome and thank you for visiting. It is good to try to get some attention to this topic.

Christine Luk said...

One of the authors, Paul Davies is currently the director of a research center at Arizona State "Beyond: Center for Fundamental Concepts in Science" and one ongoing project is the exploration of the foundations and quantum mechanics and quantum biology: http://beyond.asu.edu/research_quantum.php.

Thank you for your review!

Steve said...

Thank you for your comment!
I had seen that he went to ASU. I need to follow up and see what is going on at the center.
- Steve

Jorge said...

Thanks for your review. I encourage you to read The Holographic Universe, by Michael Talbot, ISBN 978-0-06-092258-0, Harper Perennial.

Steve said...

Thank you Jorge.

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