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The gravitational attraction between any two objects, including Earth and Newton's proverbial apple, is proportional to their masses (and to the inverse-square of their distance) by a proportionality factor known as G. But measurements of this fundamental constant have given incompatible results.
Physicists have used the quantum nature of matter to obtain a highly precise value for the universal gravitational constant, the 'big ' that appears in Isaac Newton's law of how gravity pulls together everything, from planets to apples. Although the technique still needs refinements, physicists believe that in the future it will beat the precision of conventional methods — and hopefully solve apparent discrepancies between measurements that have long puzzled physicists.
Nature's Noah Baker asked Guglielmo Tino why the gravitational constant is so hard to measure.
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In a study described today in , researchers measured the minuscule gravitational tug between rubidium atoms and a 516-kilogram array of tungsten cylinders. The uncertainty in the latest measurement is 150 parts per million, or 0.015% — only slightly larger than that of the conventional method of determining , which is to quantify the mutual pull of two macroscopic masses.
The measurement is “a marvellous experimental achievement and an important contribution to the knowledge of ”, says Holger Müller, a physicist at the University of California, Berkeley, who was not involved in the study.
The technique, which exploits the ability of matter particles such as atoms to behave as waves, could provide fresh insight on a problem that has frustrated physicists for years. The conventional method measures the torque caused by gravitational attraction on the weights attached to a rotating balance, an experiment that was first conducted by English scientist Henry Cavendish in 1798. But despite the increasing precision of some 300 modern-day experiments using Cavendish's set-up, different labs have found slightly different values for , and in recent years the discrepancy has widened rather than narrowed (see 'G-whizzes disagree over gravity').
What is the Universal gravitation constant?
Universal gravitational constant is the universal constant relating force to mass and distance in Newton's law of gravitation.