- Crystals are orderly states of matter in which the arrangements of atoms take on repeating patterns. In the language of physics, they are said to have “spontaneously broken spatial symmetry.”
- Time crystals, a newer concept, are states of matter whose patterns repeat at set intervals of time rather than space. They are systems in which time symmetry is spontaneously broken.
- The notion of time crystals was first proposed in 2012, and in 2017 scientists discovered the first new materials that fully fit this category. These and others that followed offer promise for the creation of clocks more accurate than ever before.
...I must clarify what, exactly, a crystal is. The most fruitful answer for scientific purposes brings in two profound concepts: symmetry and spontaneous symmetry breaking.
In common usage, “symmetry” very broadly indicates balance, harmony or even justice. In physics and mathematics, the meaning is more precise. We say that an object is symmetric or has symmetry if there are transformations that could change it but do not.
...We say a law has symmetry if we can change the context in which the law is applied without changing the law itself. ...
...Time translation symmetry expresses a similar idea but for time instead of space. It says the same laws we operate under now also apply for observers in the past or in the future. In other words, the laws we discover at any time apply at every time.
...Without space and time translation symmetry, experiments carried out in different places and at different times would not be reproducible.
...Whereas ordinary crystals are orderly arrangements of objects in space, spacetime crystals are orderly arrangements of events in spacetime....We are looking, then, for systems whose overall state repeats itself at regular intervals.
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