Testing whether gravity obeys the laws of classical physics or quantum mechanics is no easy matter. Discovering mineral deposits from space and etching atom-sized lithographs aren’t exactly simple either. But all three tasks may soon belong to a quantum phenomenon called the Bose-Einstein Condensate (BEC).
Perhaps most dramatic is the downsizing BECs enable of a role physicists had mused might require an absurdly huge and expensive particle accelerator the size of the Milky Way galaxy. Yet, recent research proposals suggest that a tiny, super-cooled cloud of atoms could provide a more practical tabletop experiment that probes the mystery of whether gravity is classical or quantum in nature.
The proposed experiment would harness a Bose-Einstein condensate—billions of atoms cooled to near absolute zero so that they behave like a single large atom from a quantum mechanics standpoint—within a millimeter-sized spherical trap and see how it reacts to its own gravitational pull. If gravity is classical in nature, the chilled cloud of atoms can only follow a classical-like probability distribution under the influence of its own gravitational pull. But if gravity is quantum in nature, the cloud of atoms could change from its initial classical-like probability distribution to a quantum-like probability distribution.
“Using tabletop experiments seems like the only foreseeable possibility to test quantum gravity in the lab,” says Richard Howl, a physicist and research fellow at the University of Hong Kong.
Since Nobel-winning physicists first created Bose-Einstein condensates (BECs) back in 1995, they have become important parts of both lab experiments and commercial applications. BECs are already deployed in gravimeters that can detect small changes in Earth’s gravity for the purpose of oil exploration and other geophysical surveys. Researchers have been investigating their usefulness in optical computing. They can form the basis for atom-lasers and atom-scale lithography—with obvious possible applications in nanotech and nano-sized electronics. And they have helped measure the Newtonian gravitational constant and tested the equivalence principle of general relativity for physics experiments, Howl says.
Such knowledge helped inspire Howl and an international team of colleagues to propose how BECs could help measure gravity’s possible quantum nature in a paper published on 17 February 2021 in the journal PRX Quantum. Howl originally conducted the research while at the University of Nottingham in the UK.
While much more feasible than building a galaxy-sized successor to the Large Hadron Collider in Switzerland, a proposed tabletop experiment would come with its own complications. The biggest challenge would likely involve trying to measure the relatively weak gravitational interactions within the BEC while minimizing the effect of residual electromagnetic interactions between atoms, says Simon Haine, a physicist and research fellow at Australian National University in Canberra. He developed his own tabletop experiment proposal that was first uploaded to the preprint server arXiv back in 2018.
In his paper that was also more recently published on 15 March 2021 in the New Journal of Physics, Haine calculated that the electromagnetic interactions are 1015 times larger than the gravitational interactions in his proposed version of the experiment. This can differ somewhat based on various experimental configurations, but it generally means researchers will need to improve the technological state-of-the-art for controlling and minimizing such electromagnetic interactions.
“Recently there has been some progress into controlling these interactions with lasers rather than magnetic fields,” Haine explains. “It’s possible that this could overcome some of these hurdles, but this research is still in its infancy.”
Another challenge is that such an experiment would need to be repeated many times—Haine estimated 105 repetitions for his experimental proposal—to collect enough statistical evidence to resolve the tiny gravitational effects. And researchers must create an incredibly stable environment to minimize other disruptions. “A BEC experiment is very complicated and is susceptible to things like tiny temperature drifts in the building, fluctuating water pressure in the pipes used to cool the magnetic coils, and other mundane things,” Haine says.
Despite some slight differences in approach, the papers from both Haine and Howl’s group of colleagues share a similar vision for how to enable a tabletop experiment. This could help resolve conflicting theories about whether gravity is classical or quantum by providing the first solid experimental evidence one way or the other, along with inspiring new research directions for quantum gravity and quantum theory. Howl and several of his colleagues have become part of an international consortium called the Quantum Information Structure of Spacetime (QISS) that is drawing inspiration in part from such tabletop experiment proposals.
There are also possible technological spinoffs such as improving commercial gravimeters used in detecting the gravitational signature of minerals, according to both Haine and Howl. The latter also noted that the experimental setup could aid additional research focused on looking for gravitational waves.
The proposals for tabletop testing of quantum gravity may not become technically feasible for another decade or two. And even if they would be more practical than building a gargantuan particle accelerator in this case, Howl believes that particle accelerators are still worth building for many other types of fundamental research involving Higgs physics, supersymmetry and dark matter.
Still, research organizations may start shifting some resources toward tabletop experiments that are comparatively far cheaper than building a successor to the $5-billion Large Hadron Collider in Geneva, Switzerland.
“When it comes to funding,” Howl says, “I suspect that funding bodies will be looking closely at moving some resources to tabletop experiments as they are currently a fraction of the cost of large particle accelerators.”
Editor’s Note: This story was updated to clarify that the 1015 comparison for electromagnetic vs. gravitational interactions and the estimated requirement of 105 experimental repetitions refer specifically to the Haine proposal. The name of the journal that Howl and colleagues published in has also been corrected.
Jeremy Hsu has been working as a science and technology journalist in New York City since 2008. He has written on subjects as diverse as supercomputing and wearable electronics for IEEE Spectrum. When he’s not trying to wrap his head around the latest quantum computing news for Spectrum, he also contributes to a variety of publications such as Scientific American, Discover, Popular Science, and others. He is a graduate of New York University’s Science, Health & Environmental Reporting Program.