Gravity is, at bottom, a mystery. Yes, we can talk about the curvature of space-time and perhaps make analogies with stretched rubber sheets. But we don’t know why mass causes spacetime to curve.
To put it another way, in our theory of gravity, matter is the stage and space-time is the cast and crew of the stage. But the behavior of matter is described by quantum mechanics, which takes space and time as given. For quantum mechanics, space and time are the stage in which matter presents the best show in history. How do we get these two theories to make a single move?
Maybe that’s why the two theories just don’t get along: No show can have two main actors, right? We can finally find out thanks to a new experimental device that can make it possible for both gravity and quantum mechanics to play leading roles.
Uniting gravity and quantum mechanics
Physicists think that it should be possible to unite gravity and quantum mechanics – a common theory from which flows the sweet sap of quantum mechanics and the slightly mind-blowing nectar of general relativity. The problem is that theories are based on data, and we have very little data on how quantum gravity modifies quantum mechanics (or vice versa).
Observations that quantum mechanics and gravity are simultaneously important are difficult because of scale. When the forces that govern quantum mechanics are important, gravity is incredibly weak. At the other end of the scale, gravity dominates at very long distances, while quantum effects disappear entirely at these distances. It may be that the single event horizon of a black hole provides a natural experiment in which you can observe quantum mechanics working alongside gravity. Fortunately, there are no nearby black holes that allow us to make detailed observations.
The alternative is to create our own hardware where we can directly observe the quantum behavior modified by gravity. In addition to technical difficulties, this also has a drawback: the gravitational effects will be Newtonian in nature and will likely be subject to the vagaries of Earth’s mass distribution. It will take some first-class models to show that any observed deviation from the predictions is due to new physics. However, researchers are building experiments that can at least provide data.
Shh, swinging pendulums
How do we obtain this data? Essentially, we need a massive, massive object that also has observable effects in a quantum system. This is exactly what researchers are on track to create. It is a pendulum, but not just any pendulum.
An ordinary pendulum, a mass on a string, will stop quickly if it does not receive an additional input of energy. Even in a vacuum, the pendulum chord will bend and flex, dissipating its momentum. Similarly, the connections between the rope and the dough, and the rope and the supporting frame, will also bend and flex. These imperfections not only absorb energy from the swinging motion, but also return it, usually at the wrong time, by changing the swinging motion of the pendulum. The result is that the pendulum movement is not perfectly predictable. These imperfections may be incredibly small, but they are enough to remove the influence of quantum mechanics.
To make an almost perfect pendulum, the researchers soldered a small silicon disk (3mm in diameter) to a thin fiberglass, which was then welded to a large glass block. The fiber stretching process and welds were the critical steps. By paying attention to how the fiber is drawn, the researchers were able to create a fiber 5 cm long and 1 µm in diameter that was highly uniform. Welds (as opposed to glue or fastener) provide the type of rigid fastener that doesn’t flex too much. To give you an idea of how good this pendulum is, once it got moving, it was still moving 15 hours later without any additional power. If it had been glued and attached, that time would only be two to three hours.
Fire the photon cannon
The pendulum itself is not a quantum object. But it is operating at the interface where gravity and quantum mechanics can play together. The natural movement of the pendulum is given by the laws of gravity (Newtonian gravity, in this case). But the silicon disk is an excellent mirror: it can be pushed by light.
The basic idea is that as the mirror sways, it will also collide with and reflect the photons. That reflection kicks the mirror. If the mirror moves towards the incoming light, the kick will slow it down; If the mirror moves away from the incoming light, the kick will speed it up. This is only observable for very clean mechanical oscillators, like the one that researchers have built.
The quantum mechanics of these interactions is well understood and the measurements are very accurate. The oscillator properties are given by gravity, so any deviation may be attributable to deviations from Newtonian gravity. However, as I said earlier, this is going to need a good model of the quantum and gravitational aspects of the entire system.
In the coming years, the researchers will build the optical configuration to allow them to drive and measure the pendulum with high-precision lasers. After that, the real fun will begin, and we will be able to see that the ascending photon meets the descending graviton.
Physical Review Letters, 2020, DOI: 10.1103 / PhysRevLett.124.221102 (About DOIs)
