Friday, June 20, 2014

What is Gravity?

Not in the sense of describing how gravity works, but measuring the strength of gravity. Apparently, even after new measurements, the value is still somewhat vague. From Ars Technica:
The gravitational attraction being studied here is that between a cloud of cold rubidium atoms and a 500 kg tungsten weight. The tungsten was arranged in a cylinder that surrounded the device that contained the rubidium atoms. It could be shifted up to pull the atoms back against the downward force of the Earth's gravity or shifted down to accelerate the atoms further. 
Using light, a single cloud of atoms was split into two populations and propelled upward through a vacuum chamber—the authors refer to "using the ‘moving-molasses’ technique" to shift the atoms. The two populations move to different heights and then return to the bottom of the chamber. There, due to the quantum nature of the atoms, they create an interference pattern. If anything alters the trajectory of the atoms, such as the gravitational attraction of the large chunk of tungsten used in these experiments, it will show up as changes in the interference pattern. 
... In the end, the experimental uncertainty is 150 parts-per-million, a value the authors think they can reduce considerably in future work. The value of G ends up being 6.67191 x 10-11 cubic meters per kilogram per second-squared. 
That's in keeping with fewer than half of the previous measurements, most of which were made using torsion bar devices. Of course, those measurements don't all agree with each other

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