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What goes up, must… be quantum? Experiment to test gravity’s quantum chops

A new experiment will test the notion that gravity, one of the fundamental forces in the physical world, relies upon quantum physics to work.

If so, it would further indicate the centrality of quantum mechanics in the universe and begin to explain the previously unknown underpinnings of a natural phenomenon so ubiquitous that most people take its existence for granted.

Yale’s David Moore, an associate professor of physics in the Faculty of Arts and Sciences, is part of an international research team that will conduct the experiment, called “Macroscopic Superpositions Towards Witnessing the Quantum Nature of Gravity,” or MAST-QG. The five-year project, funded by the Gordon and Betty Moore Foundation and the Alfred P. Sloan Foundation, aims to lay the foundations for the experiment.

MAST-QG will attempt to link two foundational descriptions of the universe. General relativity, which is Einstein’s theory that gravity is created by objects with mass that curve or warp the space around them, is famously incompatible with quantum mechanics, which explore the strange behavior of atoms and particles.

Measuring the feeble gravitational interactions between quantum mechanical particles has previously been impossible due to their tiny masses.

Moore, a member of Yale’s Wright Lab, will work with researchers from University College London, the University of Warwick in England, Northwestern University, and the University of Groningen in the Netherlands, to develop the experiment. The lead investigator is Gavin Morley from the University of Warwick.

We caught up with Moore recently to discuss the experiment and how it is likely to work.

Is it surprising to you that we don’t already know whether gravity is a quantum phenomenon?

David Moore: Gravity is this force that seems like the most apparent force in our everyday lives, but it’s a complete mystery. It’s embarrassing to say, really. Despite the fact we’ve been studying it for hundreds of years, we don’t understand gravity at a microscopic level in any detail at all.

We have this beautiful theory about how gravity works in astrophysical distances, thanks to Einstein’s theory of relativity. But we also know it doesn’t work when we apply that same theory to quantum mechanics and the particles that make up the universe. Why? For many of us in physics, that’s just an exciting question to try to answer.

Do we know how quantum mechanics influence other fundamental forces, such as electromagnetism and the “strong” nuclear force?

Moore: Yes, we’ve learned a huge amount about all the other fundamental forces — all but gravity, the one we’ve known about the longest. It’s the one force that doesn’t fit into our quantum picture of the world.

What makes gravity different in this regard?

Moore: Gravity is incredibly weak compared to all the other fundamental forces. For example, the attraction force between the electron and proton in the hydrogen atom, due to their electric charge, is nearly 40 orders of magnitude — 10 thousand trillion trillion trillion times — stronger than gravity. The other three fundamental forces that govern the universe — electromagnetism and the strong and weak nuclear forces — are all much closer in their intrinsic strength. The weakness of gravity is itself a major puzzle, but also makes experiments extremely challenging.

The only reason we know anything about gravity is because we have an entire planet pulling gravity down on us. The atoms in our shoes alone are enough to hold us up against the entire planet. It’s pretty impressive.

But we’ve never been able to put big objects, those objects we can see in the visible world, into a quantum state and witness the behavior of gravity at that tiny scale. So that’s precisely the idea of our experiment. We’re going to put some of the biggest objects ever into a quantum state and attempt to see their gravity.

What are those objects?

Moore: They’re called microdiamonds. There’s a very special, atom-sized impurity in diamonds called a “nitrogen vacancy center,” in which one of the diamond atoms is missing and a nitrogen atom is next to this vacancy. This strange type of impurity acts as nearly a perfect quantum system, which we can embed into a diamond crystal that is one-fiftieth the width of a hair.

We want to take the microdiamonds and trap them in a laser that will levitate them in the center of a vacuum chamber. We have technology that can “talk” to the quantum part of the diamond and use it as a handle to manipulate the mass of the entire diamond to see the effects of gravity.

What is Yale’s role in the project?

Moore: We have traps for these microparticles here at Yale, at Wright Lab. We typically use them for glass, not diamonds. Our lab has done the world’s most sensitive search for dark matter particles that could have a tiny electric charge — a millionth of an electron’s charge. We can control the exact number of electrons and protons in a sphere held inside the trap. That’s exactly the kind of stuff we want to do with this gravity experiment.

What might prevent you from getting a quantum measurement of gravity?

Moore: We have to eliminate all interactions other than gravity. Gravity is so weak, if you have even one extra electron on this diamond, in addition to the gravitational force, you’ll never have any chance of seeing gravitational entanglement. We’re working hard not just on making these tiny quantum systems, but also keeping out all other forces.

Is there an aspect of this research that drew you in on a personal level?

Moore: This is really just about trying to understand how the world works. It’s like asking an artist why they do art. A lot of us are just very interested in knowing the fundamental building blocks of the universe. How do they work together? It’s a sufficiently important enough question that if we can learn anything in my lifetime, it would be exciting for a lot of us.




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