What are time crystals? And why are they so weird?
Physicists in Finland are the latest scientists to create “time crystals,” a newly discovered phase of matter that exists only at tiny atomic scales and extremely low temperatures but also seems to challenge a fundamental law of nature: the prohibition against perpetual motion.
The effect is only seen under quantum mechanical conditions (which is how atoms and their particles interact) and any attempt to extract work from such a system will destroy it. But the research reveals more of the counterintuitive nature of the quantum realm — the very smallest scale of the universe that ultimately influences everything else.
Time crystals have no practical use, and they don’t look anything like natural crystals. In fact, they don’t look like much at all. Instead, the name “time crystal” — one any marketing executive would be proud of — describes their regular changes in quantum states over a period of time, rather than their regular shapes in physical space, like ice, quartz or diamond.
Some scientists suggest time crystals might one day make memory for quantum computers. But the more immediate goal of such work is to learn more about quantum mechanics, said physicist Samuli Autti, a lecturer and research fellow at Lancaster University in the United Kingdom.
And just as the modern world relies on quantum mechanical effects inside transistors, there’s a possibility that these new quantum artifacts could one day prove useful.
“Maybe time crystals will eventually power some quantum features in your smartphone,” Autti said.
Autti is the lead author of a study published in Nature Communications last month that described the creation of two individual time crystals inside a sample of helium and their magnetic interactions as they changed shape.
He and his colleagues at the Low Temperature Laboratory of Helsinki’s Aalto University started with helium gas inside a glass tube, and then cooled it with lasers and other laboratory equipment to just one-ten-thousandth of a degree above absolute zero (around minus 459.67 degrees Fahrenheit).
The researchers then used a scientific equivalent of “looking sideways” at their helium sample with radio waves, so as not to disturb its fragile quantum states, and observed some of the helium nuclei oscillating between two low-energy levels — indicating they’d formed a “crystal” in time.
At such extremely low temperatures matter doesn’t have enough energy to behave normally, so it’s dominated by quantum mechanical effects. For example, helium — a liquid at below minus 452.2 Fahrenheit — has no viscosity or “thickness” in this state, so it flows upward out of containers as what’s called a “superfluid.”
The study of time crystals is part of research into quantum physics, which can quickly become perplexing. At the quantum level, a particle can be in more than one place at once, or it might form a “qubit” — the quantum analog of a single bit of digital information, but which can be two different values at the same time. Quantum particles can also entangle and teleport. Physicists are still figuring it all out.
Time crystals are among the many strange features of quantum physics. In normal crystals like ice, quartz or diamond, atoms are aligned in a particular physical position — a tiny effect that leads to their distinctive regular shapes at larger scales.
But the particles in a time crystal exist in one of two different low-energy states depending on just when you look at them — that is, their position in time. That results in a regular oscillation that continues forever, a true type of perpetual motion.
However, such perpetual motion only truly exists forever in ideal time crystals that haven’t been fixed into one state or the other, and since the time crystals in the Aalto University experiments were not ideal, they lasted only a few minutes before they “melted” and started behaving normally, Autti said.
The same limitation means there’s no way to exploit the perpetual motion: A time crystal would just stop — “melt” — if an attempt were made to extract physical work from it, he said.
Time crystals were first proposed in 2012 by the American theoretical physicist Frank Wilczek, who was awarded the Nobel Prize in physics in 2004 for his work on the subatomic “strong” force that holds quarks inside the protons and neutrons of atomic nuclei — one of the fundamental forces of the universe. They were first detected in 2016 in experiments with ions of the rare-earth metal ytterbium at the University of Maryland.
Time crystals have only been made a handful of times since then, as just creating them is extremely difficult. But the Aalto University experiments hint at a way for making them more easily, and for longer. This was also the first time that two time crystals have been used to form any kind of system.
Physicist Achilleas Lazarides, a lecturer at Loughborough University in the U.K., did theoretical research on time crystals that helped in the creation of a working quantum simulation of them in a specialized quantum computer operated by the tech giant Google.
Lazarides, who wasn’t involved in the latest study, explained that the perpetual motion in time crystals takes place at the margins of the laws of thermodynamics, which were developed in the 19th century from earlier ideas about the conservation of energy.
It’s usually stated that the total working energy of a system can only decrease, which means perpetual motion is impossible — something borne out over centuries of experiments.
But the quantum changes in the low-energy states of the nuclei in time crystals neither create nor use energy, so the total energy of such a system never increases — a special case that’s allowed under the laws of thermodynamics, he said.
Lazarides acknowledged that the current experiments with time crystals are far from any practical applications, whatever they might be, but the chance to learn more about quantum mechanics is invaluable.
Time crystals are “something that doesn’t actually exist in nature,” he said. “As far as we know, we created this phase of matter. Whether something will come out of that, it’s difficult to say.”