At CERN's antimatter factory, antiprotons are born with the energy of a bullet from a rifle. You can't just put them in a box: they instantly annihilate upon touching ordinary matter. So physicists devised a way to catch them in electromagnetic traps — invisible cages made of fields.

Previously, antiproton beams were slowed by passing them through a thin metal foil. Particles lost energy, but many were lost. The GBAR experiment took a different route: the beam is decelerated by an electric field, like an elevator that gently lowers a passenger instead of a sudden crash to the ground. After such deceleration, the antiprotons enter a trap where they are further cooled by a cloud of cold electrons: these carry away excess energy, just as cool water cools a hot kettle.

Why so many antiprotons? They are used to assemble antihydrogen atoms — the mirror twin of ordinary hydrogen. Then physicists want to drop these atoms and see whether they fall downward like everything else, or perhaps levitate. The answer will tell us whether antimatter truly obeys the same gravity as matter.

Trapped antiprotons can not only be stored but also accelerated back to the desired energy, forming a dense bunch. It's like compressing a spring and releasing it at the right moment. This produces an antiproton beam for creating antihydrogen, the building blocks for the experiment on antimatter falling.