New time crystal built using sound waves breaks Newton’s third law

Physicists at New York University in the US have built a new kind of time crystal using sound waves, where particles appear to break Newton’s famous third law of motion. Particles in this time crystal move in an irregular, nonreciprocal manner, opening new possibilities for technology and industry. 

Newton’s third law of motion, where every action has an equal and opposite reaction, is the basis for many applications in the world. From rocket engines to bouncing a ball, a fish swimming, to children jumping on the trampoline, everything can be explained using this law.

In technical terms, the law also states that forces always occur in pairs. When forces are unbalanced, it causes a change in the object’s state of motion. But if an object does not follow Newton’s third law, then it can open up newer frontiers in science and technology. 

What is a time crystal? 

Time crystals are an unusual form of matter composed of particles that oscillate, much like a pendulum. After years of prediction that time crystals could exist in the real world too, scientists made the first-ever time crystal nearly a decade ago and have found several types of time crystals each with unique properties that could be used for different applications. 

“Time crystals are fascinating not only because of the possibilities, but also because they seem so exotic and complicated,” said David Grier, a professor of physics at the NYU’s Center for Soft Matter Research. 

Under Grier’s leadership, researchers Mia Morrell and Leela Elliot made a new version of a time crystal where tiny crystals made from styrofoam beads floated on a cushion of sound and can be displaced using sound waves. In this setup, sound serves as an acoustic levitator, keeping the beads suspended in mid-air. 

How does the crystal work? 

“Sound waves exert forces on particles — just like waves on the surface of a pond can exert forces on a floating leaf,” explained Morrell in a press release. “We can levitate objects against gravity by immersing them in a sound field called a standing wave.”

The levitating beads interact by scattering sound waves between themselves. Larger beads scatter more sound that the smaller ones and therefore have a greater effect on smaller beads than vice versa. This creates an imbalance in how the beads can influence each other. 

“Think of two ferries of different sizes approaching a dock,” added Morrell. “Each one makes water waves that pushes the other one around — but to different degrees, depending on their size.” Since these are carried out by sound waves, they are not limited by Newton’s Third Law. 

While previous experiments with time crystals have been done, this one is also different because it operates in a device about one foot tall and is visible to the naked eye. “Our system is remarkable because it’s incredibly simple,” said Grier in the press release. 

The study and device are likely to help researchers understand biological systems, such as circadian rhythms, and nonreciprocal processes, such as how the body breaks down food. 

The research findings were published in the journal Physical Review Letters