Ultracold atoms could test relativity in the quantum realm

Tiny Ferris wheels made from light, extremely cold particles could allow researchers to test one aspect Albert Einstein's theory of relativity on an unprecedentedly small scale.

The theories of special and general relativity, which Einstein formulated in the early 1900s, changed our understanding of time by revealing that moving clocks could run slower than those that stood still. If you move fast enough or accelerate enough, the time you measure will stretch out; the same thing can happen if you find yourself walking in circles. These phenomena were observed for relatively large objects, but Vasilis Lembessis at King Saud University in Saudi Arabia and his colleagues have now developed a way to test them, and on a very small scale.

To study the rotation and timing of the smallest objects we can control—atoms and molecules—they turned to ultra-cold kingdomjust a few millionths of a degree higher lowest possible temperature. Here, quantum properties, as well as the movement of atoms and molecules, can be manipulated extremely precisely using laser beams and electromagnetic fields. In fact, in 2007, Lembessis and several other colleagues developed a method for tuning laser beams to hold atoms in the shape of a cylinder and rotate inside it. They called it an “optical Ferris wheel,” and Lembessis says his team's new calculations show it can be used to observe relativistic time dilation, as measured by ultracold particles.

Their calculations suggest that nitrogen molecules may be a good candidate for testing rotational time dilation in the quantum world. Considering the movement of electrons inside them as ticking internal clockThe researchers were able to detect a shift in tick frequency of just one part in 10 quadrillion.

At the same time, Lembessis says that experiments with optical Ferris wheels have so far been relatively rare. Because of this, the new proposal opens the door to testing the theory of relativity in unknown conditions, where new or unexpected effects may arise. For example, the quantum nature of ultracold particles could challenge the “clock hypothesis,” which determines how much speeding up a clock changes its clock rate.

“It is important to test and confirm our understanding of physical phenomena in nature. It is when we get a surprise, something unexpected, that we need to revise our understanding and gain a deeper understanding of the Universe. This work offers an alternative way to test relativistic systems with some clear advantages over mechanical setups,” says Patrick Oberg at Heriot-Watt University in the UK.

For example, although relativistic effects such as time dilation typically require very fast movement, using an optical Ferris wheel would make them accessible without the need for impractical speeds, he says. Aidan Arnold at the University of Strathclyde in the UK. “Given the incredible accuracy of atomic clocks… the change in time 'felt' by the atoms of the Ferris wheel should be noticeable. Moreover, since the accelerated atoms do not fly very far, there will be enough time to measure this change,” he says.

Changing the focus of the laser beams could also control the size of the Ferris wheel that would hold the particles, thereby testing the time dilation effect for different rotations, Lembessis says. But there will also be technical challenges, such as ensuring that the atoms or molecules don't heat up and become uncontrollable as they spin.