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Photo, caption follows:

Hurricane Isabel churns through the Caribbean on September 14, 2003. A hurricane is a classic example of self-organization: air and water molecules form into a whirling maelstrom spontaneously, without any one molecule being in charge. A hurricane is also a classic example of emergence: the behavior of the storm as a whole is nowhere to be found in an air or a water molecule by itself.
Credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC


Understanding Emergence
Emergence, loosely speaking, is what happens when the whole adds up to much, much more than the sum of its parts.

A classic example is water. A water molecule in isolation is just an oxygen atom with Mickey Mouse ears: two little hydrogen atoms attached at an angle. But collect a few zillion of the things, under the right conditions of temperature and pressure, and you’ve suddenly got a substance that sloshes, sparkles, gurgles and swirls—not to mention having the ability to freeze, thaw, evaporate, boil and support life. Water in its bulk form exhibits behaviors that are nowhere to be found in a molecule by itself.

Another classic example is superconductivity. Take a metal such as lead or tin and cool it below a certain critical temperature: many of the individual electrons in that metal will suddenly start to march in step, so to speak—a collective motion that allows them to flow as if the metal offered no electrical resistance whatsoever. Again, electrons in the aggregate exhibit behaviors that are nowhere to be found in one electron alone.

Similar examples of emergence abound in science. As the Nobel laureate physicist Philip Anderson put it in a 1972 article in Science, "more is different."

This is especially true when it comes to the inner workings of solids, liquids and other kinds of condensed matter. (Rutgers University physicist Piers Coleman maintains an online textbook on this subject, along with articles and presentations aimed at a broader audience.) Indeed, NSF-funded scientists are currently studying a remarkable variety of emergent phenomena in condensed matter, including exotic, heavy-Fermion superconductivity, quantum phase transitions at ultra-low temperatures, Bose-Einstein condensation—and not least, high-temperature superconductivity, which still resists scientists’ full understanding nearly two decades after its celebrated discovery in 1987.

Another rich source of emergent phenomena is self-organization: a general name for what happens when the components of a system create complex structures on their own, spontaneously, without anyone or anything being in charge. (Think of water vapor crystallizing into a snowflake.) Scientists have studied self-organization in many contexts, from the dynamics of earthquakes to the flocking of birds. But the process seems to be particularly important in biology, where—to take just one example—structural molecules inside the cell will spontaneously assemble themselves into cell membranes, ribosomes and other components. Indeed, a host of NSF-funded researchers are now trying to adapt this kind of molecular self-assembly for practical use in nanotechnology.

Finally, yet another rich source of emergent phenomena is network theory. This is a relatively new field that looks at issues ranging from the vulnerability of the Internet to the maze of chemical interactions inside the cell—anything having to do with networks of connections in general. Among the leaders in network theory are NSF-funded researchers such as Albert-Lazlo Barabasi at the University of Notre Dame, and Jean Carlson at the University of California, Santa Barbara.

Creating Molecules and Materials by Design [Next]