Phrase of the Week: Fermi Acceleration

February 8, 2013

Fermi acceleration, so named because it was first theorized by Italian-American physicist Enrico Fermi in 1949, is one process by which tiny particles in the vast reaches of space can be accelerated to amazing speeds.

The process described by Fermi involves a charged particle like a proton or an electron bouncing off a magnetic field, often one associated with an interstellar cloud of dust and gas. This accelerates a particle only a tiny amount; but in the late 1970s, theorists showed that the shock waves thrown out by supernova explosions can accelerate particles much more efficiently and to much higher energies.

Charged particles ahead of the shock front can pass through it and get bounced about – “scattered” is the physics term – by magnetic fields that travel with the shock front. Often they careen back through the shock into magnetic fields on the other side, which ping-pong them across the shock again.

The particles gain energy every time they cross the shock front. By chance, a small number of particles cross the shock front many, many times, gaining energy until they’re zooming back and forth at close to the speed of light. Some of these escape the shock front altogether, to go zipping through the cosmos. This is thought to be a source of cosmic rays.

“An analogy that is sometimes used is of two tennis players, one rushing towards the net and the other receding more slowly,” said Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC and Stanford – and one of the theorists who originally proposed this second, more efficient type of acceleration with collaborator Jeremiah Ostriker, now of Columbia University.

“They volley the tennis ball back and forth with it getting faster and faster until the approaching player hits it out," he added. "I like this analogy because I played tennis regularly with Jerry when I was thinking about this!”