The butterfly effect is a term used in chaos theory to describe how small changes to a seemingly unrelated thing or condition (also known as an initial condition) can affect large, complex systems. The term comes from the suggestion that the flapping of a butterfly's wings in South America could affect the weather in Texas, meaning that the tiniest influence on one part of a system can have a huge effect on another part. Taken more broadly, the butterfly effect is a way of describing how, unless all factors can be accounted for, large systems like the weather remain impossible to predict with total accuracy because there are too many unknown variables to track.
Origins in Weather Prediction
The concept of the butterfly effect is attributed to Edward Norton Lorenz, a mathematician and meteorologist, who was one of the first proponents of chaos theory. Lorenz was running global climate models on his computer one day and, hoping to save himself some time, ran one model from the middle rather than the beginning. The two weather predictions, one based on the entire process, including initial conditions, and another based on a portion of the data, starting with the process already part way completed, diverged drastically. Lorenz, along with most scientists of his time, had expected the computer models to be identical regardless of where they started. Instead, tiny, unpredictable variations caused the two models to differ.
Intrigued by the results, Lorenz began creating a mathematical explanation that would show the sensitive dependence of large, complex systems like the weather. Sensitive dependence means that the development of the system depends on a wide number of factors. To simplify his findings, Lorenz coined the butterfly explanation that has since become so widely known.
Other Systems
The butterfly effect applies to systems beyond weather; indeed, any complicated system may be vulnerable to seemingly small factors. For example, the travel of asteroids in the solar system can be difficult to predict. This is because the paths of asteroids can be affected by many different gravitational pulls throughout the solar system, including the gravity of the sun, of planets, of moons, and even other asteroids.
In human behavior, it may be possible for small initial changes to render behavior unpredictable. For example, the loved ones of someone who has committed suicide are often left wondering what could have caused the death. They might agonize over the myriad small details they did not see, but which could have predicted the suicide. The butterfly effect might suggest that a huge range of experiences, dispositions, and genetic, physical, and emotional factors were too many to account for in the person's actions.
Counter Theories
Since its development, a number of contradictory theories have been described in opposition to chaos theory and the butterfly effect. These argue that the large systems in question, while being immensely complex, still follow some sort of order, and therefore do not merit the descriptor "chaos." While the number of factors may be large, it is suggested, they are nevertheless quantifiable and finite.
Although the mathematical explanation that Lorenz developed might show the possible effects of a butterfly's wings on weather patterns, there is no evidence that actually proves it. Observation has shown that the effects of a butterfly's wings seems confined to a very small, localized area. Any large-scale effects seem to be dampened by the system at large.
Popular Culture
The concept of small variations producing widespread effects actually predates chaos theory. Writers like Ray Bradbury were particularly interested in the repercussions that might occur if a person traveled back in time and changed one small, insignificant detail. This concept has been the basis of numerous films and stories.