Scattering

Scattering

Scattering has a very general meaning. Any type of collision can be called a scattering. Another example of scattering is the reflection of light on a mirror, in which case the light (more accurately, the photons) is scattered. In the most general sense, any change of state can be considered as a scattering.

Scatterings can be treated using either classical mechanics or quantum mechanics, depending on the size of the scattering objects. In scattering, the momentum of the system is always conserved. This can be derived easily using Newton's third law in classical mechanics. In the microscopic world where we must use quantum mechanics, there are subtle issues because of the uncertainty principle. The momentum of the system long before and long after the scattering is always equal. "Long" means that the period of time is large compared with the scattering time.

The energy of the system is not always conserved. If the total energy of the system is conserved, the scattering is called an elastic scattering. If total energy is not conserved, it is called an inelastic scattering. Depending on the actual scattering process, the total energy may increase or decrease. An explosion, for example, is a process where the total energy is conserved. If energy decreases in a collision, it is lost to the environment. For instance, when cars collide some of the energy is lost to deforming the body of the car. Likewise, when energy increases in a collision, it comes from the environment. In the case of explosions, it comes from the stored chemical energy of the explosive material.

An important quantity in scattering is the scattering cross section. It characterizes how easily scattering can occur. The larger the cross section is, the stronger scattering we have. The scattering cross section is generally very difficult to calculate because of the complicated interactions during the scattering. In classical mechanics, depending on the number of objects involved in the scattering (few-body system or many-body system), we can use different approximations. If we have two objects, for instance, we can use classical mechanics to calculate the cross section. For many objects, we can use numerical methods. In the case of quantum mechanics, things become much more complicated because of the interaction of quantum waves. One famous approximation tool is Fermi's golden rule.

The molecules in a gas or liquid are constantly colliding, or scattering, with one another. Their scattering is dependent, however, on many variables, including temperature, pressure, and volume. Some scattering parameters, such as cross section, are directly relates to the thermodynamic properties of the substance. For instance, molecules with a larger cross section should reach thermal equilibrium faster than molecules with a small cross section at the same temperature, pressure and volume. In high energy physics, or particle physics, scientists accelerate particles and collide them at very high speeds. By calculating the cross section of the scatterings and comparing them to the values measured from experiment, scientists can discover many details about the particles they are studying. Scattering is a basic tool in high energy physics. Almost all subatomic particles have been discovered using scattering.