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# 14: Relativistic Collisions

• 14.0: Prelude to Relativistic Collisions
• 14.1: Photons
Before we dive into the examples, there’s one particle that requires special attention: the photon, or quantum of light - from that other early-20th-century theory known as quantum mechanics. Photons travel (by definition) at the speed of light, and need therefore be massless. They do carry energy though, which is related to their frequency.
• 14.2: Totally Inelastic Collision
In a totally inelastic collision, particles stick together. A possible example is the absorption of a photon by a massive particle, resulting in an increase in its mass, as well as possibly a change in its momentum.
• 14.3: Radioactive Decay and the Center-of-Momentum Frame
Radioactive decay is the process by which unstable particles with high mass; fall apart into more stable particles with lower mass. Although the process itself is quantum mechanical in nature, the dynamics of radioactive decay are described by special relativity and are essentially identical to those of an inelastic collision in reverse.
• 14.4: Totally Elastic Collision - Compton Scattering
As a final example of a collision in special relativity, we consider the totally elastic case: a collision in which the total momentum, total kinetic energy, and the mass of all particles are conserved.
• 14.E: Relativistic Collisions (Exercises)

Thumbnail: Black hole.