elastic vs inelastic scattering

When two billiard balls collide and simply bounce apart, unchanged, that is an elastic collision — the same balls leave, just headed in new directions. But imagine the balls were lumps of clay that smash, stick, shatter, or spawn new pieces: the things that come out are different from the things that went in. That is an inelastic collision. Particle physicists draw exactly this distinction, and it turns out to carry deep information about what particles are made of.

In elastic scattering the same particles emerge as entered; only their directions and momenta change, and the total kinetic energy is preserved. An electron glancing off a proton and leaving the proton intact is elastic. In inelastic scattering, by contrast, some of the collision energy is converted into making new particles or exciting the target into a different state — the outgoing set is genuinely different. Because energy can turn into mass (E equals m c squared in action), a hard enough collision can create heavier or more numerous particles than it started with. The harder you smash, the more inelastic the outcome tends to be.

This distinction is not mere bookkeeping; it is how physics probes the inside of matter. Gentle, elastic collisions map the overall shape and size of a target, like feeling its outline. Violent, inelastic collisions break in and reveal the constituents — they are how we discovered that the proton is not a solid speck but a swarm of quarks and gluons. The deepest version, deep inelastic scattering, was the experiment that revealed quarks for the first time.