Ellis, George -Nature Article
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NATURE|Vol 435|9 June 2005 ESSAY 743 Physics, complexity and causality Although the laws of physics explain much of the world around us, we still do not have a realistic description of causality in truly complex hierarchical structures. George F. R. Ellis The atomic theory of matter and the periodic table of elements allow us to understand the physical nature of material objects, including living beings. Quantum theory illuminates the physical basis of the periodic table and the nature of chemical bonding. Molecular biology shows how complex molecules underlie the development and functioning of living organisms. And neuro- physics reveals the functioning of the brain. In the hierarchy of complexity, each level links to the one above: chemistry links to biochemistry, to cell biology, physiology, psy- chology, to sociology, economics, and politics. Particle physics is the foundational subject underlying — and so in some sense explain- ing — all the others. In a reduc- tionist world view, physics is all there is. The cartesian picture of man as a machine seems to be vindicated. But this view omits important aspects of the world that physics has yet to come to terms with. Our environment is dom- inated by objects that embody the out- comes of intentional design (buildings, books, computers, teaspoons). Today’s physics has nothing to say about the intentionality that has resulted in the existence of such objects, even though this intentionality is clearly causally effective. A simple statement of fact: there is no physics theory that explains the nature of, or even the existence of, football matches, teapots, or jumbo-jet aircraft. The human mind is physically based, but there is no hope whatever of predicting the behaviour it controls from the underlying physical laws. Even if we had a satisfactory funda- mental physics ‘theory of everything’, this situation would remain unchanged: physics would still fail to explain the out- comes of human purpose, and so would provide an incomplete description of the real world around us. Can we nevertheless claim that the underlying physics uniquely causally determines what happens, even if we can- not predict the outcome? To examine whether we can, contemplate what is required for this claim to be true within its proper cosmic context. The implication is that the particles existing when the cos- mic background radiation was decou- pling from matter, in the early Universe, were placed precisely so as to make it inevitable that 14 billion years later, human beings would exist, Charles Townes would conceive of the laser, and Edward Witten would develop string the- ory. Is it plausible that quantum fluctua- tions in the inflationary era in the very early Universe — the source of the pertur- bations at the time of decoupling — implied the future inevitability of the Mona Lisa and Einstein’s theory of relativ- ity? Those fluctuations are supposed to have been random, which by definition means without purpose or meaning. However, such meaning did indeed come into being. Ever higher levels of interaction and causality arose as com- plexity spontaneously increased in the expanding Universe, allowing life to emerge. Darwinian processes of selection guided the physical development of living systems, including the human brain. It is possible that what actually hap- pened was the contextual emergence of complexity: the existence of human beings and their creations was not uniquely implied by the initial data in the early Universe; rather the underlying physics together with that initial data created a context that made the existence of human beings possible. Conditions at the time of the decoupling of matter and radiation 14 billion years ago were such as to lead to the eventual development of minds that are autonomously effective. Such minds are able to create higher-level order, such as the Hubble Space Telescope and Kurt Gödel’s incompleteness theorem, that embodies a purpose and meaning not in existence before. With this view, the higher levels in the hierarchy of complexity have autonomous causal powers that are functionally inde- pendent of lower-level processes. Top- down causation takes place as well as bottom-up action, with higher-level con- texts determining the outcome of lower- level functioning, and even modifying the nature of lower-level constituents. Stored information plays a key role, resulting in non-linear dynamics that are non-local in space and time. Brain functioning is causally affected by abstractions, such as the value of money, the rules of chess and the theory of the laser. These abstractions are realized as brain states in individuals, but are not equivalent to them — James Clerk Maxwell’s theory of electro- magnetism is not the same as any individual’s brain state. Although such concepts are causally effec- tive, they are not themselves physical variables. Consequently physics per se cannot causally determine the outcome of human crea- tivity; rather it creates the ‘possibility space’ to allow human intelligence to function autonomously. This situation is not dependent specifi- cally on human intentionality. Physics by itself cannot explain any behaviour that is adaptive and depends on context, for example, beaver dam-building and the dances of bees. It is plausible that these also emerged at late times in the expand- ing Universe as higher-level autonomous behaviours, made possible but not causally determined by the underlying physics and chemistry of matter. If this is the case, the challenge to physics is to develop a realistic descrip- tion of causality in truly complex hierarchical structures, where top-down causation and memory effects allow autonomous higher levels of order to emerge with genuine causal powers. So far, attempts to relate physics to com- plexity — such as the reaction–diffusion equation, chaos theory, the renormaliza- tion group, complexity theory — take us only a small step on this road. ■ George F. R. Ellis is at the Mathematics Department, University of Cape Town, Rondebosch, Cape Town 7701, South Africa. FURTHER READING Ellis, G. F. R. Phys. Today (in the press). Bishop, R. C. Phil. Sci. (in the press). Intelligent design: no physics theory is able to explain a teapot. THE MUSEUM OF EAST ASIAN ART/HERITAGE IMAGES Nature Publishing Group ©2005
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Transcript of Ellis, George -Nature Article
NATURE|Vol 435|9 June 2005 ESSAY
743
Physics, complexity and causalityAlthough the laws of physics explain much of the world around us, we still do not have a realisticdescription of causality in truly complex hierarchical structures.
George F. R. Ellis
The atomic theory of matter and the periodic table of elements allow us tounderstand the physical nature of materialobjects, including living beings. Quantumtheory illuminates the physical basis of the periodic table and the nature of chemical bonding. Molecular biologyshows how complex molecules underliethe development and functioningof living organisms. And neuro-physics reveals the functioning ofthe brain.
In the hierarchy of complexity,each level links to the one above:chemistry links to biochemistry,to cell biology, physiology, psy-chology, to sociology, economics,and politics. Particle physics is thefoundational subject underlying— and so in some sense explain-ing — all the others. In a reduc-tionist world view, physics is allthere is. The cartesian picture ofman as a machine seems to bevindicated.
But this view omits important aspectsof the world that physics has yet to cometo terms with. Our environment is dom-inated by objects that embody the out-comes of intentional design (buildings,books, computers, teaspoons). Today’sphysics has nothing to say about theintentionality that has resulted in theexistence of such objects, even thoughthis intentionality is clearly causallyeffective.
A simple statement of fact: there is nophysics theory that explains the nature of,or even the existence of, football matches,teapots, or jumbo-jet aircraft. The humanmind is physically based, but there is nohope whatever of predicting the behaviourit controls from the underlying physicallaws. Even if we had a satisfactory funda-mental physics ‘theory of everything’, this situation would remain unchanged:physics would still fail to explain the out-comes of human purpose, and so wouldprovide an incomplete description of thereal world around us.
Can we nevertheless claim that theunderlying physics uniquely causallydetermines what happens, even if we can-not predict the outcome? To examinewhether we can, contemplate what isrequired for this claim to be true within itsproper cosmic context. The implication is
that the particles existing when the cos-mic background radiation was decou-pling from matter, in the early Universe,were placed precisely so as to make itinevitable that 14 billion years later,human beings would exist, CharlesTownes would conceive of the laser, andEdward Witten would develop string the-ory. Is it plausible that quantum fluctua-tions in the inflationary era in the very
early Universe — the source of the pertur-bations at the time of decoupling —implied the future inevitability of theMona Lisa and Einstein’s theory of relativ-ity? Those fluctuations are supposed tohave been random, which by definitionmeans without purpose or meaning.
However, such meaning did indeedcome into being. Ever higher levels ofinteraction and causality arose as com-plexity spontaneously increased in theexpanding Universe, allowing life toemerge. Darwinian processes of selectionguided the physical development of livingsystems, including the human brain.
It is possible that what actually hap-pened was the contextual emergence ofcomplexity: the existence of human beingsand their creations was not uniquelyimplied by the initial data in the early Universe; rather the underlying physicstogether with that initial data created acontext that made the existence of humanbeings possible. Conditions at the time ofthe decoupling of matter and radiation 14 billion years ago were such as to lead to the eventual development of minds thatare autonomously effective. Such mindsare able to create higher-level order, suchas the Hubble Space Telescope and KurtGödel’s incompleteness theorem, thatembodies a purpose and meaning not inexistence before.
With this view, the higher levels in thehierarchy of complexity have autonomouscausal powers that are functionally inde-pendent of lower-level processes. Top-down causation takes place as well asbottom-up action, with higher-level con-texts determining the outcome of lower-level functioning, and even modifying thenature of lower-level constituents.
Stored information plays a key role,resulting in non-linear dynamicsthat are non-local in space andtime. Brain functioning is causallyaffected by abstractions, such asthe value of money, the rules ofchess and the theory of the laser.These abstractions are realized asbrain states in individuals, but arenot equivalent to them — JamesClerk Maxwell’s theory of electro-magnetism is not the same as anyindividual’s brain state. Althoughsuch concepts are causally effec-tive, they are not themselves physical variables. Consequentlyphysics per se cannot causally
determine the outcome of human crea-tivity; rather it creates the ‘possibility space’ to allow human intelligence to functionautonomously.
This situation is not dependent specifi-cally on human intentionality. Physics byitself cannot explain any behaviour that isadaptive and depends on context, forexample, beaver dam-building and thedances of bees. It is plausible that thesealso emerged at late times in the expand-ing Universe as higher-level autonomousbehaviours, made possible but not causallydetermined by the underlying physics andchemistry of matter.
If this is the case, the challenge tophysics is to develop a realistic descrip-tion of causality in truly complex hierarchical structures, where top-downcausation and memory effects allowautonomous higher levels of order toemerge with genuine causal powers. Sofar, attempts to relate physics to com-plexity — such as the reaction–diffusionequation, chaos theory, the renormaliza-tion group, complexity theory — take usonly a small step on this road. ■
George F. R. Ellis is at the MathematicsDepartment, University of Cape Town,Rondebosch, Cape Town 7701, South Africa.
FURTHER READINGEllis, G. F. R. Phys. Today (in the press).Bishop, R. C. Phil. Sci. (in the press).
Intelligent design: no physics theory is able to explain a teapot.
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