First result for testing semiclassical gravity effect with a torsion balance

The Schrödinger-Newton equation, a theoretical framework connecting quantum mechanics with classical gravity, predicts that gravity may induce measurable deviations in low-frequency mechanical systems—an intriguing hypothesis at the frontier of fundamental physics. In this study, we developed and operated an advanced optomechanical platform to investigate these effects. The system integrates an optical cavity with finesse over <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mn>3.5</a:mn><a:mo>×</a:mo><a:msup><a:mn>10</a:mn><a:mn>5</a:mn></a:msup></a:math> and a torsion pendulum with an ultralow eigenfrequency of 0.6 mHz, achieving a high mechanical quality factor exceeding <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>5</c:mn><c:mo>×</c:mo><c:msup><c:mn>10</c:mn><c:mn>4</c:mn></c:msup></c:math>. We collected data for three months and reached a sensitivity of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>0.3</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi mathvariant="normal">μ</e:mi><e:mi>rad</e:mi><e:mo>/</e:mo><e:msqrt><e:mi>Hz</e:mi></e:msqrt></e:mrow></e:math> at the Schrödinger-Newton frequency of 2.5 mHz where deviations from the standard quantum mechanics may occur. While no evidence supporting semiclassical gravity was found, we identify key challenges in such tests and propose new experimental approaches to advance this line of inquiry. This work demonstrates the potential of precision optomechanics to probe the interplay between quantum mechanics and gravity.