One of the toughest questions ever asked came up in 1887, when Swedish King Oscar II for his 60th birthday offered a prize for a mathematical competition that could best answer the problem of how stable the solar system is. The contest was won by Henri Poincaré, the French scientist and mathematician, who rattled the foundations of stability by concluding that the orbits of the planets around the sun were stable in a sense, but that there were different kinds of stability and, of all things, stability itself was variable.
By most accounts, stability is generally assumed to be a good thing, and instability is suspicious, like left-handedness. Instability is commonly understood to be an imperfection, as for example, when it describes economic or political circumstances, or a nuisance, as when a tire or disk drive wears out from misalignment. Human behavior and cognition can be traced to the need to comprehend complex problems of dynamic perception, including disequilibrium and abrupt change.
In physics, instabilities are increasingly understood to be a fundamental part of the order of things, whether they are troublesome or not. They have names. They provide explanations for observable phenomena. In the history of science, solid objects in motion were relatively easy to understand, but scientists ran into new dimensions of complications when liquids came into play, which makes sense because liquids are not as stable, and they have more boundaries especially when mixed together.
The sum of our instabilities in a sense can be seen throughout nature — in the ocean, clouds, in our familiar landscapes and in textures of landscapes on other planets, in the origins and structures of the galaxies. They are emblematic of nature’s capacity to mix and change. An instability in our climate pattern is contributing to global warming.
A classic instability and one of the first that was formally identified can be observed when oil is poured into a cylinder, covered with a partition of some kind, and then water is poured on top of that. When the division between the two substances is removed, the observer sees the boundary between the two different densities become unstable, enabling an assortment of strands and blobs of oil to infiltrate upward into the water.
The oil and water trick is a classic kind of instability, known as a Rayleigh-Taylor instability, after John William Strutt, the third baron of Rayleigh for his findings that were later elaborated by G.I. Taylor. Lord Rayleigh won the Nobel Prize in Physics in 1904 for the discovery of the element argon. In its commendation, the Nobel Committee praised Rayleigh’s “capacity for understanding everything just a little more deeply than anyone else.” Among many scientific contributions, G.I Taylor was a military expert in the characteristics of blast waves and fluid dynamics. Although he did not win a Nobel Prize, he left his mark in many other ways. He was a member of the British delegation to the Manhattan Project at Los Alamos.
There are surely as many kinds of instabilities as there are interfaces between dynamic forces and objects that are provisionally balanced. A partial list would also include the Kelvin-Helmholtz instability that predicts the transition between the billows produced by winds blowing over water and the turbulence of a more energetic phase that causes waves to form. Even the familiar mushroom cloud commonly associated with a nuclear detonation is a variation on the Rayleigh-Taylor instability. The image is shaped when hot, thin gasses that are produced by a large explosion pour upward into a cooler, denser surrounding atmosphere. In recent weeks, news has arrived that certain kinds of gamma ray explosions, among the most powerful in the universe, may be triggered by instability in the interior of neutron stars that set up extraordinarily powerful magnetic fields.
At a laser conference in Santa Fe in April, Dr. Ray E. Kidder, formerly a chief scientist at the National Ignition Facility at Lawrence Livermore National Laboratory in California, in a video message, blamed instabilities for the agonizing failures of that project’s 40-year effort to achieve fusion energy — in theory, a virtually unlimited supply of atomic energy derived from fusing atoms together, the way the sun creates its energy.
“The enemies of symmetry are fluid instabilities, and I will never forgive Rayleigh and Taylor for theirs,” he said in the transcript of the message. “As we know, and I regret, the NIF at Livermore has not achieved ignition. I believe this is due to insufficient implosion symmetry caused by one or more of those instability rascals.”
He assured the audience that those problems were being “skillfully observed and attacked,” and then he also paid tribute to the “worldwide or even greater significance of malicious instability,” noting very recent developments suggesting that the origin of the universe might be traced to instability, sometimes referred to as a quantum fluctuation, in the structure of space itself.
Not only does instability play a central role in the universe, one might say, but we may owe our existence to one.