In physics, there are two main ways to model the universe. The first is the classic way. Classical models such as Newton’s laws of motion and Einstein’s theory of relativity assume that the properties of an object, such as position and motion, are absolute. There are practical limits to how accurately we can measure the path of an object through space and time, but that’s up to us. Nature knows its movement with infinite precision. Quantum models such as atomic physics assume that objects are controlled by interactions. These interactions are probabilistic and indefinite. Even if we limit an interaction to limited results, we can never detect the movement of an object with infinite precision because nature does not allow it.
These two theoretical worlds, the definite classical and the indefinite quantum, each work very well. The classic for large, massive objects like baseball and planets and the quantum for small, light objects like atoms and molecules. However, both approaches break down when we try to study massive but small things like the inside of black holes or the observable universe in the earliest moments of the Big Bang. Because that has all the properties of the general theory of relativity with all the properties of the quantum theory. This theory is sometimes called quantum gravity, but right now we don’t know it would work.
How different theories are related. Photo credit: B. Jankuloski
This theory is difficult to study because we don’t have experiments to test it directly. However, a new study suggests an experiment that could give us some insight into how quantum gravity works.
The key is to have an object that is quantum in nature, but massive enough that classical gravity has an effect. To do this, the team suggests using a supercooled state of matter known as the Bose-Einstein condensate. This occurs when certain groups of atoms are cooled so much that they are effectively blurred into a single quantum state. If billions of atoms were cooled into a Bose-Einstein condensate, they would form a single quantum object with a mass roughly equivalent to that of a virus. Tiny but massive enough to be able to study the effects of gravity.
The team suggests making such a condensate and then magnetically suspending it so only gravity can interact with it. In their work they show that the shape of the condensate shifts slightly from its “weightless” Gaussian shape when gravity works at the quantum level. If gravity only interacts at the classical level, the condensate remains Gaussian.
This approach could be achieved with our current technology. In contrast to other proposed studies, this experiment would only rely on one fundamental property of quantum systems and not on more complex interactions such as entanglement. If the experiment can be carried out, it could give us a first real look at the fundamental nature of quantum gravity.
Reference: Richard Howl et al. “Non-Gaussian Relationship as Signature of a Quantum Theory of Gravity.” PRX Quantum 2.1 (2021): 010325.
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