Electrons in Concert: A Simple Probe for Collective Motion in Ultracold Plasmas
(PhysOrg.com) — Collective, or coordinated behavior is routine in liquids, where waves can occur as atoms act together. In a milliliter (mL) of liquid water, 1022 molecules bob around, colliding. When a breeze passes by, waves can form across the surface. These waves are not present in the same volume of air, where only 1019 gas molecules randomly move about.
Do such waves occur in plasmas, the most prevalent state of matter in the universe? Like gases, they are made of particles bumping around in a shapeless glob. However, plasma densities can range from 1026 atom/mL all the way down to much less than 1 atom/mL. Wave-like features that occur even at such miniscule densities are one key feature of plasmas.
Unlike in liquids, these “waves” happen in plasmas because the particles are charged, thus exerting strong forces on each other, even at large distances. But not all seas of charged particles are plasmas. What makes a plasma a plasma is the organized behavior of the charged particles.