Our description of a physical system is always relative to a reference system. For example, there is a bouncing ball on the railway station platform. We can observe it from the reference system of the platform itself or from the reference system of a passing train.

The generalized covariant principle is a basic principle in physics. It says that the laws of physics describing the motion of the ball do not depend on the reference system of the observer. Whether the observer is in a moving train or on a platform, the same laws of physics are used to describe the ball on the platform, that is, the laws of physics have nothing to do with the selection of the reference system.

Since Galileo, this principle has been the key to describing motion. At the same time, it is also the core of Einstein’s theory of relativity. The information it contains is that the laws of physics observed from different reference systems are symmetrical.

On the other hand, reference systems are also physical systems, and they all need to follow the laws of quantum mechanics in the end. Therefore, some physicists wonder: can we formulate laws of physics from the perspective of an observer ” attached” to quantum particles, thus introducing a quantum reference system?

Časlav Brukner, professor of physics at the university of vienna, and his research team are interested in this issue. In a recent paper published in Nature Communications, they proved that any quantum system can be regarded as a quantum reference system.

▲ Quantum features, such as quantum superposition, are defined only relative to the observer. When we look at the train from the perspective of the observer standing on the platform, the train looks like a quantum superposition of different positions. In the figure, the superposition state is represented by multiple fuzzy phantoms.

Taking the train and platform as examples, they are both physical systems that follow the laws of quantum mechanics and can be in superposition quantum states at different positions at the same time. When the observer on the train sees that the platforms are in superposition at different positions at the same time, the observer on the platform sees that the train is also in superposition. Therefore, whether an object like a ball exhibits quantum or classical properties depends on the reference system of the observer. The covariant principle can be extended to such quantum reference systems. This means that the form of the laws of physics can be independent of the choice of the quantum reference system.

However, the observer sitting on the train saw that the observer on the platform and the ball were in a quantum superposition state.

The first author of the paper, Flaminia Giacomini, said: ” Our research results show that the symmetry of the world must be expanded at a more basic level.” The interaction between quantum mechanics and gravity is still an area to be explored. Scientists believe that the classical concept of reference system will not be enough in this field, and the reference system must be fundamentally quantum.

This new insight is expected to play a role in this field.