Gold, dense, precious and enduring, is the material of choice to see if quantum gravity is real, a pursuit that may finally reconcile relativity to quantum mechanics in hopes of creating a unified theory that has eluded giants like Einstein and Bohr for well over 100 years.
Physicist Markus Aspelmeyer vividly remembers the day, nearly a decade ago, that a visitor to his lab declared the gravitational pull of his office chair too weak to measure. Measurable or not, this force certainly ought to exist. Ever since the work of Isaac Newton in 1687, physicists have understood gravity to be universal: every object exerts a gravitational force proportional to its mass on everything around it. The visitor’s comment was intended to bring an increasingly fanciful conversation back down to Earth, but Aspelmeyer, a professor at the University of Vienna, took it as a challenge. “My resolution was ‘Okay, I am going to not only measure the gravitational field of this chair, but we are going to go small, small, small!’” he recalls.
The research effort born on that day has now produced its first result: a measurement of the gravitational force between two tiny gold spheres, each about the size of a sesame seed and weighing as much as four grains of rice—the smallest masses whose gravity has been measured to date. The results, published in Nature today, bring physicists one step closer to the distant goal of reconciling gravity with quantum mechanics, the theory underlying all of nongravitational physics.
Gravity can be understood as originating from a warping of spacetime, which is shown in this artist’s impression. Credit: Arkitek Scientific
Why gravity's so hard to measure ...
It is hard to fathom just how extraordinarily weak gravity is for such small masses. The gravitational pull of one sphere (the “source mass”) on the other (the “test mass”) a few millimeters away is more than 10 million times smaller than the force of a falling snowflake. The central challenge facing Aspelmeyer’s team was to design a detector exquisitely sensitive to this gravitational force yet totally insensitive to much larger background forces pushing and pulling on the test mass from all sides.
The end goal ...
The observation of gravitationally induced entanglement would be groundbreaking. But a conclusive demonstration that gravity is quantum mechanical would require proving that the two particles interacted only through gravity. Aspelmeyer’s efforts to isolate gravitational forces between progressively smaller masses are a critical step toward such a definitive test. “Since quantum is going from small to big, there’s a chance for gravity and quantum to meet somewhere in the middle,” says Sougato Bose, a theoretical physicist at University College London, who co-wrote the other proposal with nine collaborators.
No doubt, Einstein and Bohr would be all in on this endeavor without question.
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