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THE next time you get caught in a downpour, don’t think about how wet you are getting – but how you are getting wet. Rain is, after all, just molecules composed of hydrogen and oxygen atoms, and there is nothing wet about hydrogen or oxygen on their own. There isn’t even anything wet about a single water molecule. Put lots of them together in the right conditions, however, and you will get wet.
The wetness of water is an example of an “emergent” property: a phenomenon that can’t be explained by the fundamental properties of something’s constituent parts, but rather manifests only when those parts are extremely numerous. Emergent phenomena are ubiquitous in nature and a proper grasp of how they come about could hold the key to solving some of our biggest mysteries.
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“There is a sense in which nothing in science makes sense without emergence,” says Erik Hoel, a neuroscientist and author based in Cape Cod, Massachusetts.
In physics, for example, some materials exhibit superconductivity, where large numbers of electrons can move without resistance, and yet it isn’t always clear why. Neuroscientists, meanwhile, find that consciousness seems to emerge from some collective behaviour of neurons. In both cases, simply understanding the basic constituents of the system doesn’t explain the phenomenon in question, never mind allow you to recreate it from scratch.
“Ultimately, we want to explain under which circumstances we will see novel properties,” says Larissa Albantakis, a computational neuroscientist at the University of Wisconsin-Madison.
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But the study of emergence is, by turns, promising and maddeningly difficult. The standard “reductionist” approach to scientific investigation breaks large-scale, or macroscopic, systems down into their constituent microscopic parts to deduce the laws that govern those parts. It doesn’t work when we are faced with emergent systems, which may explain why we haven’t yet cracked room-temperature superconductivity or consciousness.
“We have to get a better understanding of how the interactions between more microscopic parts of a system connect with the macroscopic behaviour of the same system,” says Albantakis. The problem is that “while the word ’emergence’ gets thrown around a lot, there is not enough rigorous, quantitative work on the subject”, she says.
How does emergence work?
Jessica Flack at the Santa Fe Institute in New Mexico is among those aiming to change that. For starters, she wants to better characterise the different degrees of emergence, so we have a better chance of coming up with an explanation of how it works. Some systems, for example, are strongly “screened off” – meaning that changes at the micro-scale make very little difference to the emergent behaviour. “One task for science is to start quantifying this variation in screening off,” says Flack.
This way, a form of reductionism would work on emergent systems. It would be possible to do experiments that knock out certain variables, meaning researchers could characterise macroscopic behaviour in a way that allowed them to work backwards to microscopic characteristics.
Flack says it is similar to inferring the statistical properties of gas molecules by examining how pressure, temperature and volume affect each other. We might not be able to identify a single or precise cause, but we can identify a family of microscopic configurations or patterns of interactions that underlie the emergent behaviour.
Hoel has a different approach, known as “causal emergence”, which seeks to identify the scale at which a system’s variables have the strongest causal effects on each other and hence on the emergent property. A model of the system at this scale will be more informative about important features of the system, he says.
The goal, of course, is to crack mysteries like superconductivity and consciousness. And Hoel reckons there is no reason to think there is anything special about those systems. “I don’t think emergence is some rare or magical quality, but almost stupidly common,” he says.
This story is part of a special package in which we explain 13 of the most mind-bending concepts in science. See the other entries below
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