At the center of a distant galaxy, where a supermassive black hole weighing a hundred million suns spins, space is threaded with magnetic fields. They coil into tight vortices, like a drain in a sink, but instead of water, there's plasma whipped up to near-light speeds. It's here, at the shock waves, that particles are born with energies no earthly accelerator can match.
The mechanism responsible is called first-order Fermi acceleration. A particle, like a ball bouncing between two approaching rackets, ricochets off moving magnetic irregularities and gains energy. But there are brakes: synchrotron radiation (when a particle spirals around a magnetic field line and loses energy as light) and inverse Compton scattering (collisions with photons that sap speed).
Electrons are less fortunate: they're lighter and lose energy faster, so their ceiling is about 120 gigaelectronvolts. These numbers are for a typical black hole of a hundred million solar masses. That's how nature builds its own accelerator, and we're just beginning to read its blueprints.
An energy of several hundred petaelectronvolts is comparable to the kinetic energy of a flying mosquito, but packed into a single subatomic particle, a proton.
In Carl Sagan's novel Contact, the alien signal comes from the vicinity of a supermassive black hole — perhaps that's where natural accelerators hide.