For two atomic nuclei to fuse, they must overcome their fierce electric repulsion — like trying to bring two angry bulls together. Normally, only monstrous energy does the trick, as in stellar cores, where Hans Bethe explained fusion back in the 1930s. But recently, scientists found a way to build a metallic 'pen.' They soaked thin palladium and titanium foil with deuterium, then fired slow deuterium nuclei at it. In the tight weave of metal atoms, free electrons act like a tranquilizer: they dampen the charge, weakening the repulsion. And so the bulls, with no room to back off, collide and fuse into helium nuclei.
But the real surprise came next. When the energy of the bombarding nuclei was lowered to tiny values, the laws of physics demanded the reaction vanish. Instead, the fusion yield stopped dropping and froze at a constant level, as if the bulls in the pen found each other regardless of their swagger. This plateau shatters established ideas and gives physicists a working platform to study 'material' catalysis of nuclear reactions. It might just pave the way to terrestrial fusion energy.
The fusion probability in the foil turned out to be a quintillion (10¹⁸) times higher than for the same nuclei in a vacuum. That's roughly like a single grain of sand suddenly becoming the size of Mount Everest.