What seems like bending the laws of physics is actually just using them to our advantage.

Gravity is the force with which we’re most familiar. It’s what’s keeping you on the planet, even as you’re reading this. Despite gravity feeling deeply intertwined with our daily lives, scientists are still unable to quantize it—or test general relativity (our best theory of gravity) and quantum physics in a single experiment.

This is, without a doubt, the biggest open problem in physics at present. But one experiment could change that—and unlock a new theory of physics with mind-blowing applications.

This is the BVM experiment. My colleague Chiara Marletto (the M in the experiment’s title) and I—and, independently, Sougato Bose (the “B”)—proposed an idea for testing the quantum nature of gravity even with objects far smaller than the Earth; ones that are roughly the size of a biological cell, or perhaps a bit smaller.

We suggest placing two masses in a superposition—or the quantum property of simultaneously being in many states—of two locations each. If gravity is quantum, the two masses would then become entangled, meaning their positions would be intertwined. We would then see four distinct ripples in the gravitational field, two for each mass. If instead gravity were classical, there would be no entanglement, and thus only one ripple for each mass.

Think of it in terms of dropping beach balls into a swimming pool. Here, our superposed mass would be the ball and the gravitational field would be the pool’s surface. Drop the beach ball and you would see waves in both the deep and shallow ends of the pool—if gravity truly is quantum, that is. With two quantum balls there would be four ripples, leading to them being entangled.

Imagine the BMV experiment is performed and entanglement confirmed—suggesting that gravity is quantum. What practical applications could this result have? Well, for starters, we could build quantum computers using gravity. Given that black holes pack more information than any other known object in the universe, perhaps we could one day use them as the ultimate quantum supercomputers. However, there is another possibility that sounds even more science-fictional.

Gravity is the weakest of our four fundamental forces—the other three being electromagnetism and the strong and weak nuclear forces. Gravity is unusual among forces, not only because of its weakness, but also because it’s always attractive between all objects. In other words it acts as a universal glue. That’s exactly what keeps us stuck to Earth, keeps Earth in orbit around the sun, and keeps the sun in its own orbit around the center of our Galaxy, and so on. But gravity’s quantum nature could actually be used to make gravity repulsive. This is my latest work with Marletto and another colleague, Pablo Saldanha, in which we designed an antigravity machine.

The machine works similarly to the BMV experiment. Imagine that one of the two gravitating masses (the “source”)—or a beach ball, in terms of our layman’s experiment—is in a superposition of states, while the other (the “probe”) is localized in one place. In the part of the superposition where the source is closer to the probe, the gravitational attraction is stronger. Meanwhile, in the other part where the source is farther away from the probe, the gravitational attraction is weaker. In both branches, the force of gravity is still attractive, so how do we make this into repulsion?

For starters, every quantum experiment has three parts:

1. Prepare a superposition.
2. Let it evolve in time.
3. Finally, measure in another superposition.

It’s this last part that gives us the repulsion, but it only does so for one of the outcomes of the final measurement. So what matters is the post-selection; we have anti-gravity only if the right outcome is observed. On average, if both outcomes are included, gravity is always attractive, just as it is in the classical world. So, we need to discard one of the outcomes of the final measurement, which is what gives us repulsion.

While some might argue that anything can happen if we post-select—or observe the most favorable outcome—that isn’t necessarily the case. Indeed, discarding “bad” outcomes (such as those where the particles attract) leads us to observe what we want (in this case, repulsion). However, classically, this isn’t possible, no matter how much we post-select. If our experiment is confirmed, it would therefore show that gravity can act from two different points on the source at the same time. In other words, only if gravity is quantum could we have an antigravity machine.

How realistic are the BMV and antigravity experiments to perform? Pretty difficult. Luckily, a number of world-leading quantum groups are racing to implement these experiments, and I’m optimistic that we will have conclusive results in the early 2030s. Even more excitingly, I am collaborating with Marletto and my Italian colleagues Marco Genovese, Fabrizio Piacentini, and Ettore Bernardi, who are wizards in the lab, to try to get there first. We are on the cusp of solving one of the biggest mysteries of physics, and, as a bonus, might be able to develop technology that even renowned science-fiction writer Arthur C. Clarke couldn’t have imagined.