Curved spacetime in a quantum simulator

Newswise — The theory of relativity is good for understanding big things like black holes colliding and creating gravitational waves in space. Quantum theory is good for understanding small things like how electrons behave in an atom. But scientists haven't figured out how to combine these two theories perfectly. They are still searching for a "quantum theory of gravity," which is a big challenge in science that hasn't been solved yet.

The reason it's difficult to solve the problem of combining the theory of relativity and quantum theory is because the math involved is very complicated. Additionally, it's challenging to conduct experiments that can demonstrate both relativity and quantum effects at the same time. To do so, one would need to create situations where the effects of both theories are significant, such as a curved spacetime caused by massive objects and the observation of quantum phenomena like the dual nature of light as both particles and waves.

Scientists at TU Wien in Vienna, Austria, have developed a new approach to tackle the challenge of combining relativity and quantum theory. They use a "quantum simulator" to study the behavior of quantum particles in curved spacetime. Instead of directly studying the actual system of interest, they create a model system that provides insights and understanding through analogy. The researchers have demonstrated the effectiveness of this quantum simulator, and their findings have been published in the scientific journal Proceedings of the National Academy of Sciences of the USA (PNAS). The international collaboration involved physicists from the University of Crete, Nanyang Technological University, and FU Berlin.

Learning from one system about another

The quantum simulator operates on the concept that different physical systems, despite being seemingly unrelated, can follow similar fundamental laws and equations at a deeper level. Even if these systems consist of different particles or exist on different scales, they may share common underlying principles. By studying one system, researchers can gain insights and understanding that can be applied to another system, allowing them to learn about specific phenomena by investigating analogous systems.

Prof. Jörg Schmiedmayer explains that they use ultracold atomic clouds controlled by an atom chip with electromagnetic fields as their quantum system of interest. By precisely adjusting the properties of these atomic clouds, they can establish a correspondence with another quantum system they want to study. By measuring and analyzing the behavior of the atomic cloud model system, they can gain insights and knowledge about the other system. This process is similar to how one can understand the oscillation of a pendulum by observing the oscillation of a mass attached to a metal spring, even though they are different physical systems that can be related to each other.

The gravitational lensing effect

Mohammadamin Tajik, the first author of the paper and a researcher at the Vienna Center for Quantum Science and Technology (VCQ) - TU Wien, explains that they have successfully demonstrated the production of effects that resemble the curvature of spacetime using their approach. In empty space (vacuum), light travels along a "light cone." The speed of light remains constant, and at any given time, it travels the same distance in all directions. However, when light is influenced by the gravitational force of massive objects like the Sun, these light cones become curved. The paths of light are no longer perfectly straight in curved spacetime, resulting in what is known as the "gravitational lens effect."

The researchers have successfully demonstrated the same effect of spacetime curvature or gravitational lensing in atomic clouds using the speed of sound instead of light. In this quantum system, which can be described with quantum field theories, they have discovered a new tool to study the connection between relativity and quantum theory. This provides a unique opportunity to explore the relationship between these two fundamental theories of physics in a novel way.

A model system for quantum gravity

The experiments conducted on atomic clouds have successfully replicated various phenomena observed in relativistic cosmic systems, such as the shape of light cones, lensing effects, and reflections. These findings are not only significant for advancing theoretical research but also have practical applications in areas like solid-state physics and material science. The experiments provide insights into similar structural questions encountered in these fields, allowing researchers to address them through the study of quantum systems. This interdisciplinary approach opens up new possibilities for exploring fundamental physics and discovering new materials.

The researchers aim to further enhance their control over the atomic clouds in order to obtain more comprehensive data. They seek to manipulate the interactions between particles within the system in a precise manner. By doing so, the quantum simulator can replicate highly complex physical scenarios that are beyond the capabilities of even the most powerful supercomputers to calculate. This level of control and simulation opens up new possibilities for investigating intricate quantum phenomena and exploring the behavior of systems that are otherwise challenging to study directly.

Indeed, the quantum simulator serves as a valuable and complementary tool for quantum research. Alongside theoretical calculations, computer simulations, and direct experiments, the study of atomic clouds in the quantum simulator opens up new avenues for exploration. By delving into these systems, the researchers anticipate uncovering novel phenomena that were previously unknown. These phenomena may occur on a cosmic scale, similar to relativistic phenomena, but without the need to focus solely on the behavior of tiny particles. Through this approach, scientists have the opportunity to make groundbreaking discoveries that would have otherwise remained hidden.