shock wave

A shock wave is a propagating abrupt change in a fluid's pressure (and temperature and density). They can be caused by an object moving faster than the fluid's speed of sound (or a fluid moving past something faster than its speed of sound), and can also be caused by an explosion. Shock waves travel at a faster rate than normal waves. Waves analogous to shock waves can occur in other situations, such as the breaking waves on the ocean, an analogous gravity-wave phenomena. Shock waves create substantially more drag than "normal" waves and raise the pressure and temperature, with kinetic energy transformed into heat energy (shock heating). A sharp object moving supersonically through a fluid can create an attached shock wave (attached to the object) which is oblique, but propagates along straight lines. A blunt object causes a detached shock wave or bow shock ahead of the object. An explosion can produce a non-oblique shock wave, at right angles from the direction of motion. When the fluid is a conductor or is detonating, additional factors determine the character of a shock wave.

In astrophysics, objects passing through material faster than the material's sound speed, as well as explosive and/or magnetic phenomena pushing fluid past its sound speed can cause shock waves. Shock occurs within supernova explosions, and also results from the material driven outward into surrounding interstellar medium (ISM) (e.g., supernova remnant). Jets cause such shock as well. Planets cause shock passing through the interplanetary medium, an example of bow shock. Much of the shock studied within astrophysics is gas/plasma moving against other gas/plasma.

Identifying shock waves from an astronomical distance is a challenge. One method used when observing gas clouds is to look for molecules likely formed by the heat of the shock (i.e., a shock tracer). This can be a sign of star formation. EMR appearing to be thermal radiation associated with a high temperature can be observable evidence of such heating, and thus of a shock wave. Physical characteristics of Earth- or meteorite-material that indicates a past episode of sudden heating suggest some past event that included a shock wave. Shock involving relativistic speed also produces identifiable synchrotron radiation.

Within a plasma where magnetism is a factor (described by magnetohydrodynamics), a classification of shock waves is a C-type shock (continuous shock or slow shock) versus a J-type shock (discontinuous shock or fast shock). C-type shock waves have been modeled and the determined characteristics have been used to explain various jet, ISM, and molecular cloud phenomena. The term collisionless shock refers to a kind of shock within a collisionless plasma, in which individual particle interactions ("collisions") are insignificant compared to the dynamics of the physical fields.