Two electrons that leave a metal needle tip almost simultaneously start to repel each other, like two people trying to squeeze through a narrow door at the same time. The closer they are, the more they get in each other's way. For the first time, scientists have fully measured this repulsion in three dimensions, using a needle with an extremely sharp point and ultrashort laser pulses.
They found that if electrons fly close together in the transverse direction, their energies differ noticeably — creating a "gap" of 3.3 electronvolts (a tiny energy, roughly that of a visible-light photon). And if they have nearly the same energy, they scatter more strongly sideways — the divergence angle increases by 34.5%. These results matched calculations that treated electrons as tiny charged spheres.
The experiment showed that with such a filter and additional energy selection, you can make a beam with almost no paired electrons: out of every hundred pulses, only two contained a pair, while retaining 3% of the original beam. This paves the way for sharper images in electron microscopes and for studying quantum entanglement of electrons.
Electrons hate crowding so much that two electrons emitted simultaneously can fly apart at an angle noticeably larger than if they had flown alone.