Einstein’s gravity observed in the quantum world

The press release summarises the study with exquisite precision. However, there are a couple of points that might have benefited from greater clarity, given the importance of both the article and the journal in which it is published. In the press release, when it refers to the pulse being split into two, with one of the parts being pushed upwards by means of a controlled magnetic pulse, and then converted into a state ‘virtually unaffected by the magnetic field’ so that it can fall freely under gravity, what is the residual effect here, expressed as a numerical value or a percentage? Does this affect the purported free fall?

Something similar occurs in the conclusions of the paper (p. 5, 2nd column, at the beginning), where it is stated that ‘we have confirmed the predicted phase with a high level of confidence’; one immediately wonders: how high? I believe this question probably escaped the attention of the article’s referees.

The study is of good quality and the writing is excellent, from the abstract and introduction right through to the conclusions.

The conclusions are supported in the paper by solid data, well-established and corroborated over many years, as is clearly reflected in the publication’s bibliography. The paper fits very well with the existing theoretical evidence. This is a positive point. In any case, so as not to leave anything out, it might be worth recalling the well-known saying that, when an experiment is carried out knowing in advance what the outcome should be, it is easier to adjust the result. Let me be clear: this is a purely general observation; it in no way seeks to call into question the rigour of the experimental design, which I consider, based on the description provided, to be very well controlled.

But it does occur to me to add the following gedanken [thought experiment]. Let us imagine for a moment that, with the same experimental rigour employed here, the result had differed from the theoretical one, after having reviewed the experimental setup time and time again. What would have happened in such a case?

Finally, at first glance, it may be difficult to determine the precise real-world implications of the significant result obtained. Yet, in fact, it does have implications, at least indirectly: it confirms once again, and brilliantly, that the fundamental theories of physics are firmly established. And we can immediately extend this certainty to the vast number of applications these theories have, across a wide range of scales and levels.

The result of this work confirms that we are on solid ground, even when we stand (provided our weight does not exceed certain limits) on the slender bridge that is beginning to connect classical gravity with quantum physics.

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.