'Big G': Scientists Pin Down Elusive Gravitational Constant

gravitational force equation — In equations formulated by Sir Isaac Newton, the force of gravity grows with the mass of two objects and gets weaker the more distant the objects are from each other.

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A fundamental constant that sets the size of the gravitational force between all objects has finally been pinned down using the quirky quantum behavior of tiny atoms.

The new results could help set the official value of the gravitational constant, and may even help scientists find evidence of extra space-time dimensions, said study co-author Guglielmo Tino, an atomic physicist at the University of Florence in Italy. [Twisted Physics: 7 Mind-Blowing Findings]

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According to legend, Sir Isaac Newton first formulated his theory of gravity after watching a falling apple. In Newton's equations, the force of gravity grows with the mass of two objects in question, and the force gets weaker the more distant the objects are from each other. The English polymath knew the objects' masses had to be multiplied by a constant, or "big G," in order to arrive at the gravitational force between those two objects, but he wasn't able to calculate its value. ("Big G" is different from "little g," which is the local gravitational acceleration on Earth.)

Since then, almost every attempt to measure big G has used some variation of Cavendish's method. Many of those experiments got fairly precise values — which didn't agree with one another. That's because it was too difficult to identify all potential sources of error in the complicated systems used, said Holger Müller, an atomic physicist at the University of California, Berkeley, who was not involved in the new study.

To see how the tungsten distorted the gravitational field, they turned to quantum mechanics, the bizarre rules that govern subatomic particles. At small scales, particles such as atoms can also behave like waves — meaning they can take two different paths at the same time. So the team split the paths the rubidium atoms took as they fell, and then used a device called an atomic interferometer to measure how the waveforms of those paths shifted. The shift in the peaks and valleys of the paths when they recombined was a result of the gravitational pull of the tungsten masses.

The findings also could help scientists discover if something more bizarre is at play. Some theories suggest that extra dimensions could warp the gravitational fields in our own four-dimensional world. These distortions would likely be very subtle and would only be noticeable at very small distances. In fact, others have suggested that the different results other labs have gotten were caused by this extradimensional intrusion, Tino said.

Tia is the editor-in-chief (premium) and was formerly managing editor and senior writer for Live Science. Her work has appeared in Scientific American, Wired.com, Science News and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.