Though physicists have competing ideas of what quantum gravity could be like, they have yet to definitively determine whether the gravity that we experience is quantum at all. A new proposal lays out a way to dispute or affirm this by observing whether a quantum object’s state is affected when its gravity is measured.
Physicists have repeatedly shown that tiny objects are subject to quantum effects, but for large objects whose behaviour is highly affected by gravity – with black holes being the most extreme example – the same task has been extremely difficult.
at University College London has been chipping away at this problem one quantum phenomenon at the time. In 2017, he and his team proposed an experiment to test gravity’s ability to create quantum entanglement. Now, they are taking on the issue of measurement. The states of quantum objects change when you measure their quantum properties. So, could the same thing happen when you measure their gravity?
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Something classical can be measured as precisely as the scientist would like without changing the outcome of that measurement, says Bose. “It’s similar to how cheering on your favourite football team on TV does not change anything.”
In his team’s new proposal, a macroscopic object like a crystal would play the role of the football team, while “cheering” for that object would be measuring its gravitational field. But there is a twist: at the beginning of the experiment, the object would be put through a procedure that left it in a state of quantum superposition.
Specifically, it could move towards the detector at the end of the experimental apparatus by taking one of two paths. But in this special quantum state, it would be impossible to tell which path it actually followed – until it reached the detector. By measuring the crystal’s quantum properties, like the spin of some of its atoms, the detector would interact with it in a way that changed its quantum state. The resulting measurement would then reveal which path it took. In other words, the measurement both collapses the superposition of potential paths and tells the observer which one it collapsed into.
But in some runs of this experiment, the researchers would also add a detector for the crystal’s gravitational field. Before the crystal reached the quantum detector, the extra detector would map the strength of the gravitational force that the crystal would exert on some other object.
If adding this extra detector – cheering on the football team – changes what the final detector reads out, then gravity isn’t impervious to measurement and therefore isn’t classical. If the final detector shows the same readings regardless of whether the crystal’s gravity was measured, then gravity isn’t quantum. Bose says that this makes the proposed experiment a yes-or-no test for gravity’s quantumness.
“Quantum gravity is a high-stakes question,” says at the Lawrence Berkeley National Laboratory in California. This idea adds one more option to a growing body of experimental and theoretical proposals for answering it, he says. Though many physicists agree that a quantum theory of gravity is essential for understanding our world, an experiment like this could eliminate some remaining proposals in which gravity isn’t quantum, says Carney.
Team member , also at University College London, says there are numerous practical challenges associated with building experiments like the one in the new proposal. For instance, the crystal and the detectors would have to be extremely well-isolated from all environmental disturbances. Yet making the proposal a reality is still feasible with near-term technologies – and several projects that could get there are already under way worldwide, he says.
“This kind of experiment is a grand goal of the community at the moment,” says Bose.
Journal reference:
Physical Review Letters,
