A space-time crystal or four-dimensional crystal is a theoretical structure periodic in time and space. It extends the idea of a crystal to four dimensions. The idea was proposed by Frank Wilczek in 2012. His speculation was that a construct would have a group of particles that move and periodically return to their original state, perhaps moving in a circle, and form a time crystal. In order for this perpetual motion to work, the system must not radiate its rotational energy. This type of motion is distinct from that of persistent currents in a superconductor, wherein the rotating Cooper pairs are not time crystals because their wave functions are homogenous, meaning time translational symmetry isn't broken. Symmetry would be spontaneously broken in Wilczek's ring system if its ground state still involves continuous movement. Tongcang Li and others proposed a system with beryllium ions circulating in a magnetic ion trap at about 10−9 K. Wilczek also suggested that a computing device could be possible with different rotational states representing information, and maybe different kinds of ions. Since this construct is in the lowest energy state it could in principle survive the heat death of the universe and continue forever. In May 2013 researchers announced they will attempt to build a component of a space time crystal, by making a rotating ring of calcium ions. Their location will be confined by electric field, and rotation in a ground state will be forced by a magnetic field. Unwanted disturbances will be minimized by reducing the temperature to 1 μK by way of laser cooling. The ion trap will be 100 μm wide. Possible rotation of the ion ring will be demonstrated by using a laser to electronically excite one of the trapped ions. Patrick Bruno has criticized this concept, arguing that Wilczek's rotating state is not the ground state of the system. He derives the supposed true, non-rotating ground state. In August 2013 Bruno presented arguments that indicated rotating ground-state systems are impossible.