Physicists have achieved a phenomenon known as subradiance, in which atoms linger in an excited state, in a dense cloud of atoms... Atoms gain energy by absorbing photons (light particles) that cause their electrons to jump from the lowest-energy "ground" state to higher-energy excited states. Once they're in an excited state, atoms spontaneously emit a photon and fall back to the ground state. But this isn't always the case. If many atoms are packed together and separated by a shorter distance than the wavelength of the emitted photon, the light they emit will cancel itself out, and the atoms will...