That's water vapor produced by condensation from the plane's hot exhaust. Until the water particles evaporate, you can follow the streak to track where the plane is flying.
A similar visual method allows scientists to track the motion of electrically charged particles inside a container of liquid hydrogen, or bubble chamber.
Charged particles inside such a container leave tiny bubbles behind them as they zoom through the liquefied hydrogen. Once the atomic particles get moving, researchers can snap photographs to document the particles' paths.
Donald Glaser (1926 - ), a University of California-Berkeley professor, won the 1960 Nobel Prize in Physics for creating the first bubble chamber. Today's scientists, however, often rely on larger and more advanced containers for their atomic experiments. Wire chambers and spark chambers have largely taken the place of bubble chambers.