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The emergence of emergenceUsing chemical intuition often allows one to predict what might transpire on throwing a batch of chemicals into a beaker, but sometimes the unexpected can occur. Bruce c. Gibb discusses how you define an ‘emergent phenomenon’, recognizing that it’s not a simple exercise and can actually be different for each of us.

One of the boons of writing essays for Nature Chemistry is the comments you receive from people who have read an article and have some feedback to provide or question to pose (yes, gentle reader, you are not alone!). More often than not these catalyse further thought on the topic in question or its re-evaluation. And occasionally, these thoughts morph themselves into a new essay. Case in point: what you are now reading.

Among the privileges of an academic job is the opportunity to visit other institutions. There is nothing like it for garnering new ideas and ensuring that your research endeavours are in their proper context. Anyway, a short while back I had the opportunity to visit the chemistry department at Northwestern University and was chatting with some graduate students and post-docs at a function when the topic of emergent phenomena came up. Ah, light dinner conversation! The topic had arisen because I had briefly mentioned emergent phenomena while discussing complex systems in an essay or two some time back1,2. The question was straightforward enough: “How do you define an emergent phenomenon?” The answer was not, and probably never will be, straightforward. It has a distinct philosophical hue and many shades of uncertainty, qualities that are antithetical to the rigorous brain of a chemist tuned to balancing chemical reactions, counting electrons, writing equations and generally quantifying phenomena.

Let me start with a step back. A complex system — quite distinct from one that is merely complicated — is a system with an array of intertwined parts that as a whole is out of equilibrium from its surroundings and exhibits one or more properties (known as ‘emergent phenomena’) not obvious from the properties of the individual components3,4. Three examples on very different scales are illustrative. Hurricanes and other tropical cyclones are a product of gravity, the spin of the Earth, and the properties of gases and bulk water. They are born, they live and they die. Here

death is a good thing, for the thought of a terrestrial equivalent of Jupiter’s red spot moving randomly between continents ad infinitum is not a pleasant one. Tropical cyclones are compartmentalized with a well-defined boundary, at least when viewed with visible or infrared light from a considerable height. They turn enthalpy into entropy (hurricane Katrina pummelled the Gulf Coast of the United States with 54 trillion watts of power dissipated over a 5-hour period — that’s around 1018 joules!). Let’s consider two emergent phenomena of tropical cyclones: are the beautiful spiral bands of clouds or the hideous destruction wrought by the high winds apparent when considering the latent heat of water, the Coriolis effect and fluid dynamics? Perhaps. If you have no concept of latent heat, the Coriolis effect and fluid dynamics, are they still apparent? Probably not. Moving down in scale, consider also yourself. You are born, you live, and then — sad truth of sad truths — you die. While you are on the planet you are compartmentalized, and do your utmost to make sure it remains like that… trypanophobia (fear of injections) is perfectly normal. You also turn enthalpy (food and oxygen) into entropy (I don’t

need to say it do I?) to maintain your non-equilibrium state. And of course, you display many emergent phenomena: physical growth, thoughts, desires, friendships, singing in the bathtub, the list goes on and on. Are these phenomena apparent considering your composition (65% oxygen, 18% carbon, 10% hydrogen, 3% nitrogen and so on)? Moving further down to the chemical scale of things we have, to take just one simple example, a combination of iodine and hydrogen peroxide under acidic conditions. Many of you probably know what happens with this simple mixture, but if you don’t, go and make yourself a cup of tea and think what could happen…

Got the tea? Good! This mixture is of course the Bray–Liebhafsky reaction, the first homogeneous isothermal chemical oscillator discovered. In a closed system, this non-equilibrium chemical process is created, goes through a period of existence, and then ceases to exist5. And while it does exist, it exhibits temporal oscillations in oxygen and iodine concentration; as the oxygen level increases so the concentration of iodine decreases, then as the oxygen levels begin to drop the iodine concentration increases, a process that continually repeats itself. Yet, and it is worth stating this again, it has the audacity of being initially composed of ‘only’ iodine and hydrogen peroxide! From a simple beginning, the reagents generate (at least): iodide, hypoiodite, iodite, iodate and periodate (and their corresponding conjugate acids) linked through a multitude of intertwined steps that eventually lead to the decomposition of hydrogen peroxide. Was the oscillation phenomenon apparent? Or was it emergent? If we go back to the middle of the twentieth century then the idea of a chemical oscillator was heresy. It was a violation of the second law of thermodynamics. If there is a swing from the initial state to one state of affairs this must be downhill in terms of overall free energy. To swing back to another state, and then back again, must therefore be a violation of the second law. Based on

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Homo sapiens

Typical orangutan

Einstein and his ilk

God-like figure

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The chances that a phenomenon is described as emergent versus the percentage intelligence of the person or creature perceiving the phenomenon.

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4 nature chemistry | VOL 3 | JANUARY 2011 | www.nature.com/naturechemistry

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this type of argument the whole idea of chemical oscillators was simply nonsense. Indeed, the detractors of this idea spent considerable time and intellectual capital debunking the work of Boris Pavlovich Belousov, the inorganic chemist who had the good fortune of initiating modern nonlinear chemical dynamics. And yet they had missed one tiny little point. All oscillating chemical systems, from the most famous Belousov–Zhabotinsky (BZ) reaction, through the Bray–Liebhafsky reaction, and on to the systematically designed oscillators of Epstein, Kustin and De Kepper6,7, are not at chemical equilibrium. While oscillating, they never pass through their respective equilibrium points and so are governed by the laws of non-equilibrium dynamics8. Only in a closed system, where there is no input of fresh reagents and no outflow of products, will a chemical oscillator ultimately reach equilibrium. The reaction is ‘born’, it exists, and then it ‘dies’. But while it exists, it oscillates. And then, much to the disappointment of the chemistry class witnessing the demonstration, the oscillations between red and blue in the BZ reaction slowly fade away as the system reaches equilibrium.

So, back to the question: “How do you define an emergent phenomenon?” To answer, one needs to appreciate that there are both temporal and personal components to what one describes as emergent. Although it was way back in 1828 that Fechner first described an electrochemical cell that produced an oscillating current, in 1951 when Belousov first tried to publish his work detailing what would ultimately become the BZ reaction, he was met with such resistance that he ultimately gave up trying to publish the paper in peer-reviewed journals. As Sagúes and Epstein (among others) have noted9, it finally appeared in the unrefereed abstracts of a conference on radiation biology. Buried, but not dead; the original manuscript had been in circulation among colleagues, and the curiosity of Anatol Zhabotinsky and a slowly expanding group of scientists identified new variations and details about chemical oscillators. By the time Ilya Prigogine received his Nobel Prize in Chemistry in 1977 for his work

on systems far from equilibrium, chemical oscillators were, if not de rigueur, certainly well established. So over time this chemical phenomenon has gone from being heresy, to emergent, to something we have a reasonable (although far from complete) grasp on. Today, would it be reasonable to describe the oscillation of a BZ reaction as emergent? That depends on whom you ask. If you ask a hard-core chemical oscillator designer the answer would probably be no. If you ask one of the students who witnessed the BZ reaction in the aforementioned class demonstration, the likelihood of a yes response would be much higher. And if you ask someone in between, me for example, I would probably say “No”. Or maybe “Yes”. I’m not sure.

And so to a graph… because most chemists enjoy them now and again. You know the one, it’s what you probably scanned before even getting into the text. It’s a graph of the chances that a phenomenon is described as emergent versus the percentage intelligence of the person or creature perceiving the phenomenon. Now you have to forgive the liberties I’ve taken in presenting these metrics (and the linear relationship between them). For instance, what is intelligence? Another difficult question, but I think we can all agree that it has something to do with the ability to assess a situation — whether it is one we find ourselves in or an abstract set of circumstances — and act in such a manner as to improve upon or solve the state of affairs. Intelligence comes in many guises, of course; a chemist can make an intelligent decision to isolate a compound, a soccer player can make an intelligent pass to a team-mate, or a politician can communicate effectively with their electorate to promote re-election. However you qualify intelligence, I’ve quantified it on the y axis from, for example, an inanimate brick (0% intelligence) to some sort of

supreme being or deity (100% intelligence). The former is completely incapable of even perceiving a phenomenon; the latter is all-knowing and all-seeing. He or She is not surprised by anything because He or She designed and constructed everything. If there is such a being (and I put my money down against Pascal’s Wager), then he or she has probably got the Universe more or less down pat. Sounds pretty boring to me.

Somewhere in between these two extremes are Homo sapiens. Many phenomena are apparent, others are emergent; and the cleverer we are, the more experienced we are, and the wiser we are, the less the chance a previously unobserved phenomenon would be described as emergent. Experience and wisdom have temporal components of course, so the longer we stay far from equilibrium (alive), and the longer a society holds itself together, the more we can slide the x axis to the right. Could we ever move the axis far enough so that nothing is emergent? I have my doubts, because as the old saying goes, if the human brain was so simple that we could understand it, we would be too stupid to understand it. But that doesn’t detract from the scientific challenge. And to respond to the young chemists who posed the question central to this essay: move that x axis as much as you can! Embrace the unknown and never be afraid to say, “I don’t know!” For, as Einstein once famously said, “If we knew what we were doing, it wouldn’t be called research.” ❐

Bruce C. Gibb is in the Department of Chemistry at the University of New Orleans, Louisiana 70148, USA. e-mail: bgibb@uno.edu

References1. Gibb, B. C. Nature Chem. 1, 17–18 (2009).2. Gibb, B. C. Nature Chem. 1, 252–253 (2009).3. Balazs, A. C. & Epstein, I. R. Science 325, 1632–1634 (2009).4. Whitesides, G. M. & Ismagilov, R. F. Science 284, 89–92 (1999).5. Sharma, K. R. & Noyes, R. M. J. Am. Chem. Soc. 97, 202–204 (1975).6. De Kepper, P., Epstein, I. R. & Kustin, K. J. Am. Chem. Soc.

103, 2133–2134 (1981).7. De Kepper, P., Epstein, I. R. & Kustin, K. J. Am. Chem. Soc.

103, 6121–6127 (1981).8. de Groot, S. R. & Mazur, P. Nonequilibrium Thermodynamics

(Dover, 1984).9. Sagúes, F. & Epstein, I. R. Dalton Trans. 7, 1201–1217 (2003).

One needs to appreciate that there are both temporal and personal components to what one describes as emergent.

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