# In a new study, quantum simulator sheds light on curved spacetime and quantum gravity

Quantum mechanics and the theory of relativity are pillars of modern physics. Quantum mechanics explains all the phenomena and behavior of particles at the smallest of scales, whereas relativity is on the other end of the spectrum and explains cosmic structures at the largest of scales.

However, combining the two theories for a quantum theory of gravity has eluded physicists for decades.

Now, a group of scientists from TU Wien in Vienna have used a quantum simulator to study the effects of curved spacetime. The simulator allows scientists to create a model system resembling quantum particles in curved spacetime, combining aspects of both theories.

## How a model system can help to understand quantum particles in spacetime

For the study, the team examined the speed of sound in ultracold atomic clouds controlled by electromagnetic fields.

But, you might be wondering how studying the speed of sound helps us understand quantum particles in spacetime. The answer lies in the fact that many physical systems are similar. This is because all systems obey the same laws and equations of science.

In a vacuum, light travels at a constant speed in a path called a *light cone*. However, when light passes near heavy objects such as planets and stars, its path, or the light cone, gets bent. The distorted light causes background objects to enlarge and appear as a ring around the object being observed, this is known as *gravitational lensing*.

Therefore, the researchers adjusted the properties of the atomic clouds so that their properties were similar to quantum particles.

By observing the speed of sound in the atomic clouds, analogous to the speed of light in spacetime, the team could observe phenomena such as bent light cones, lensing effects, and reflections in the quantum simulator.

## What does this mean for the quantum theory of gravity?

The findings from studying the quantum simulator offer a fresh perspective on the quantum theory of gravity and shed light on how quantum particles behave in curved spacetime. It gives us the ability to explore the interplay between relativity and quantum mechanics in a more accessible and controlled setting.

While the quantum simulator does not directly simulate the full complexity of quantum gravity, it serves as a valuable tool for studying specific aspects and gaining a deeper understanding of the fundamental principles involved.

This opens up new avenues for physicists to explore quantum field theories in spacetime and takes us closer to understanding the fundamental forces at play in the universe.

The study was published in the journal *Proceedings of the National Academy of Sciences*.

**Study abstract:**

We investigate signal propagation in a quantum field simulator of the Klein–Gordon model realized by two strongly coupled parallel one-dimensional quasi-condensates. By measuring local phononic fields after a quench, we observe the propagation of correlations along sharp light-cone fronts. If the local atomic density is inhomogeneous, these propagation fronts are curved. For sharp edges, the propagation fronts are reflected at the system’s boundaries. By extracting the space-dependent variation of the front velocity from the data, we find agreement with theoretical predictions based on curved geodesics of an inhomogeneous metric. This work extends the range of quantum simulations of nonequilibrium field dynamics in general space-time metrics.