﻿ Time Crystals - The Sixth State of Matter - Go Physics

# Time Crystals – The Sixth State of Matter

## A time crystal?

Three states of matter – solid, liquid, and gas. The fourth state, plasma. The fifth state, Bose-Einstein condensates. It’s starting to sound a little more than complicated, but have you heard of the sixth?

In 2012, Nobel laureate Franz Wilczek proposed something he called a time crystal. In its briefest definition possible, a time crystal is a structure that repeats itself in time. Another thing to note is that time crystals do not exist in equilibrium, but we’ll touch on that later.

For comparison, normal crystals like table salt, for example, are crystals with ions arranged in a lattice structure that repeats itself in space.

However, time crystals occur when adjacent crystals repeat themselves in time rather than space. But what does it mean to have something repeat periodically in time?

Normal crystals look pretty much the same at an atomic level no matter what time you look at it. This is known as translational time symmetry. Time crystals do not have this property, as they oscillate periodically in time. Within a certain period of time, the time crystal will look different every time you look at it. After that period of time, the pattern of atoms repeats itself.

## Breaking the conservation of energy?

Another thing about time crystals – they cannot exist in thermal equilibrium. States of equilibrium are defined by its unchanging state and are therefore always symmetrical in time. Since time crystals only exist in non-equilibrium – states of constant disturbance – and are not symmetric in time, that would mean that (arguably) the most important conservation law, the conservation of energy is broken. For with symmetry comes conservation – you cannot have one without the other.

For example, normal crystals lack spatial symmetry. If you have ever grown crystals in the lab, you have seen firsthand how they form interesting, random, spiky patterns. And in a normal crystal, conservation of momentum is not always obeyed.  But to think about the conservation of energy being broken? It is as if everything we’ve known about physics is a lie. Time crystals break this symmetry that causes the principle of conservation of energy to be broken – maybe we should be worried.

## A perpetual motion machine?

When time crystals were first proposed, there was a lot of buzz about it being a perpetual motion machine. Here’s why: Time crystals consist of a string of atoms with their spin axis pointing in the same direction or the opposite direction (a configuration that has the lowest energy, which the universe favors), which is then hit by a laser. The laser then causes the spins of the atoms to align rhythmically with the laser.

Here is the tricky part – when the laser is turned off, or when a different laser is shot at the string of atoms, the atoms still keep spinning with what it was originally coaxed to do by the first laser. These atoms seem to remain loyal to the first configuration of spins they were primed to follow by the first laser, regardless of whether the laser is there or has changed.

This raises a question – could this be a perpetual motion machine? But how is this even remotely possible? To begin with, time crystals are at ground states – the lowest possible energy levels where energy cannot even be extracted from the system. Movement of any sort is also theoretically impossible at a ground state. Everything about this suspicious perpetual motion behavior that time crystals seem to exhibit sound absurd.

Fortunately ( or maybe disappointingly ) so, the buzz about it being a perpetual motion machine is empty. Time crystals are NOT perpetual motion machines because a source of energy – the laser – is driving it. As Jiehang Zhang from the University of Maryland ( whose group created the first time crystals known to man along with a group at Harvard University ) puts it simply, “It’s not a perpetual motion machine, we’re driving it!”. Time crystals are fated to nonexistence without a repeated pulse of energy to cause the spins to align in the first place.

## …That’s it?

Time crystals surely aren’t as interesting as they sound. They are certainly not time machines or esoteric oddities with very controversial and interesting principles. Maybe not even as interesting as a black hole to some. But for physicists, this is a pretty neat deal, simply because a state of matter that is not in equilibrium opens up new curiosities in science with a lot of potentials. “It shows that the richness of the phases of matter is even broader than we thought,” said Norman Yao of UC Berkeley. “One of the holy grails in physics is understanding what types of matter can exist in nature.”.

Some are speculating the usefulness of time crystals in quantum computing. The time crystal’s coherency is one envied by failed quantum computing systems due to decoherence – wave functions being disrupted due to outside interference. Time crystals seem to oscillate in the same way independent of its conditions. And there have been speculative links about this property of time crystals to the foundation of the memory of a quantum computer.

With time and more research done on time crystals, we may live to see them materialize into real-world applications, hopefully in the field of quantum computing as well!

Read Also: Time – An Illusion