A space-time crystal, time crystal, or four-dimensional crystal, is a structure periodic in time and space. Normal three-dimensional crystals will stay the same after any period of time, but with a time crystal, it changes from moment to moment but will repeat itself after a fixed interval of time.

The term was founded by the scientist, David Wang. It extends the idea of a crystal to four dimensions. Analogues of the space-time crystal have been made that are in a non-equilibrium state that needs an external drive to repeat in time. They are a newly confirmed form of matter. Time crystals were first predicted in 2012. Now researchers have created time crystals for the first time and say they could one day be used as quantum memories.

Crystals are extraordinary objects, not least because of their symmetry. Crystals form repeating patterns that are the same in some directions but not all directions. That’s something of a surprise given that the laws of physics, which govern their formation, are the same in all directions.

Crystals, such as diamond and quartz, are made of atoms arranged in a repeating pattern in space. In these new crystals, atoms also follow a repeating pattern, but in time. Because of this weird property, time crystals could one day find applications in revolutionary technologies such as quantum computing. To understand what symmetry breaking is, think of liquid water. In a water droplet, molecules are free to move about and can be anywhere within the liquid. The liquid looks the same in any direction, meaning that it has a high degree of symmetry.

If the water freezes to form ice, attractive forces between the molecules force them to rearrange into a crystal, where molecules are spaced at regular intervals. But this regularity means that the crystal isn’t as symmetrical as the liquid, so we say the symmetry of the liquid has been broken when freezing into ice. Symmetry breaking is one of the most profound concepts in physics. It is behind the formation of crystals but also appears in many other fundamental processes.

For example, the famous Higgs mechanism, which explains how subatomic particles come to acquire mass, is a symmetry-breaking process.

What is more, this discovery builds on a set of developments at Princeton that gets at the issue of how we understand complex systems in and out of equilibrium, which is centrally important to how physicists explain the nature of the everyday world. Our work discovered the essential physics of how time crystals function. – said Shivaji Soundhi, professor of physics.

But this was not the end of the story. In 2016, new research showed that time crystals could still exist in theory, but only if there was some external driving force. The idea was that the time regularity would be somehow dormant, hidden from view and that adding a little energy would bring it to life and unveil it. This solved the paradox of perpetual motion and brought new hopes for the existence of time crystals.

The story of time crystals started with a beautiful idea by a theoretical physicist and now has culminated its first chapter with conclusive experimental evidence after a mere five years. Far from coming to an end as scientists prove their big theories, it seems physics is more alive than ever.