In 2009, the British physicist Stephen Hawking performed a variation of this experiment, holding a party for time travellers, but sending out the invitations after the event had taken place, so that only visitors from the future could possibly attend. But nobody came. Dr Hawking said this constituted “experimental evidence that time travel is not possible”.
Normal crystals have an atomic structure that repeats in space - just like the carbon lattice of a diamond. But, just like a ruby or a diamond, they’re motionless because they’re in equilibrium in their ground state. But time crystals have a structure that repeats in time, not just in space. And it keep oscillating in its ground state. Imagine it like jelly - when you tap it, it repeatedly jiggles. The same thing happens in time crystals, but the big difference here is that the motion occurs without any energy. A time crystal is like constantly oscillating jelly in its natural, ground state, and that’s what makes it a whole new form of matter - non-equilibrium matter. It’s incapable of sitting still.
via http://www.sciencealert.com/scientists-have-just-announced-a-brand-new-form-of-matter-time-crystals