Reacción a "Einstein’s gravity observed in the quantum world"

The principle of equivalence, as formulated by Einstein, states that the only reference frames that are truly free of forces are those in free fall. This runs counter to our intuition that, if we are at rest on the Earth’s surface, no force is acting upon us. In reality, gravity is pulling us downwards, which is counterbalanced by an opposing reaction force from the ground. That effect is cancelled out when we let ourselves fall. Therefore, the true privileged reference frames, in which the principle of relativity holds strictly (that is, the laws of physics take the same form), are those in free fall. By contrast, those at rest on Earth are accelerated systems and are therefore not equivalent. If we throw a basketball, the free-fall reference frame will measure a straight-line trajectory, as befits a trajectory at constant velocity—that is, one free of forces—whilst an accelerated reference frame will measure the curved trajectory with which we are familiar. These differences can be calculated using coordinate transformations within the framework of the theory of relativity.

The experiment published today in Science Advances is the quantum equivalent of this idea, as it measures the differences between a reference frame in free fall and one at rest on the Earth’s surface, but carried out using physical systems governed by quantum physics—in this case, rubidium atoms cooled to ultra-low temperatures. What is significant, then, is the effect on the quantum wave function. To measure the differences between an atomic wave function in free fall and one at rest, the experiment uses an interferometer: a device that guides wave functions along different paths depending on their quantum state. Gravity in quantum systems has been observed many times in various experiments over recent decades, and on some occasions even using similar atomic interferometers. That is not what is new, however; rather, in this case, it is specifically the effect on the wave function — in other words, the phase difference — that is measured, rather than, for example, the value of the acceleration due to gravity, as in previous experiments. To achieve this, the interferometer must be modified so that, in one of its arms, gravity is counteracted by magnetic fields – the equivalent of anchoring an object to the Earth’s surface – whilst in the other arm it acts freely. The authors refer to this ingenious device as the Galileo Quantum Interferometer (QGI). The results appear to agree well with relativistic predictions based on the principle of equivalence. It is a beautiful and highly sophisticated experiment, which provides an elegant theoretical complement to the already known results on gravity in quantum systems.