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Physicists have been trying to measure the <\/span>fundamental gravitation<\/span>al constant<\/span> for well over two centuries. The current <\/span>accepted value of big G<\/em><\/em><\/a>, as it\u2019s known, is 6.67430 \u00d7 10<\/span>-11<\/span> cubic meters per kilogram per square second. It also has an uncertainty of \u00b10.00015 \u00d7 10-11<\/sup><\/span> <\/span>m<\/span>3<\/sup><\/span>\/(kg s<\/span>2<\/sup><\/span>). As far as constants of the universe<\/a> go, that\u2019s very uncertain.<\/span><\/p>\n <\/p>\n Schlamminger<\/a> is a physicist at the U.S. National Institute of Standards and Technology.<\/p>\n Stephan Schlamminger recently completed a 10-year effort<\/a> at the U.S. National Institute of Standards and Technology to replicate an earlier measurement of big G<\/em><\/em> from the International Bureau of Weights and Measures<\/a>, or BIPM (located near Paris) that\u2019s notably higher than most measurements. He spoke with IEEE Spectrum<\/em><\/em> about why it took so long to get a number\u20146.67387 x 10-11<\/sup> m3<\/sup><\/span>\/(kg s2<\/sup>)\u2014and why it\u2019s notably lower than the BIPM result, to the tune of 0.0235 percent<\/a>.<\/p>\n Why is it so difficult to measure big <\/strong>G<\/em><\/strong><\/em>?<\/strong><\/p>\n Stephan Schlamminger: <\/strong>Gravity is very weak. When you were a kid, you probably played with fridge magnets, and it was a force you could feel. But if you have two coffee cups, you can try all you want\u2014you can\u2019t feel the force between them. It is there, but it\u2019s so, so weak.<\/p>\n How did you attempt to measure big <\/strong>G<\/em><\/strong><\/em>?<\/strong><\/p>\n Schlamminger: <\/strong>We used what\u2019s called a torsion balance. The key idea in the torsion balance is that it decouples vertical gravity that you have from Earth from horizontal gravity, and that makes it sensitive to masses that are around the torsion balance but not<\/em><\/em> the Earth below.<\/p>\n Ours had a fourfold geometry. It has a very thin torsion strip, then four cylinders in a \u201cplus sign\u201d arrangement. All of this is inside a vacuum. Outside, we have four larger cylinders that gravitationally attract the four smaller masses to them. If I move the outer masses just a tiny little bit, the plus sign will rotate, and we measure that angle that it moves. That angle is proportional to the gravitational torque.<\/p>\n Why try to replicate the BIPM value?<\/strong><\/p>\n Schlamminger: <\/strong>We could move the field forward. The measurements have been plagued with inconsistencies, so by redoing an experiment, we hoped to shed light on the inconsistencies.<\/p>\n We did not find a smoking gun, so there\u2019s no single reason why it\u2019s different\u2014our value versus their value. It\u2019s still a big question mark.<\/p>\n What was it like spending 10 years on this?<\/strong><\/p>\n Schlamminger: <\/strong>It\u2019s a bit like herding cats. I\u2019ve measured other fundamental constants, like Planck\u2019s constant<\/a>, and for most experiments, they have some sort of self-calibration built in. But with the gravitational constant, you have to keep track of every single mass that moves\u2014where they are, how big they are, and weigh them.<\/p>\n How does your result compare to the rest?<\/strong><\/p>\n Schlamminger: <\/strong>Our result is a little bit below the standard accepted literature value. I was disappointed because it doesn\u2019t agree with the BIPM value, nor with the literature value. If there\u2019s something wrong with the BIPM experiment, then the literature value\u2014which includes that result\u2014probably ought to come down a bit. But that is not for me to say. I think somebody else, independent, should figure out what the new mean value ought to be.<\/p>\nStephan Schlamminger<\/h3>\n
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NIST used a torsion balance with a fourfold geometry. This animation shows an exaggerated version of how the outer green masses gravitationally attract the inner blue masses.<\/small>S. Kelley\/NIST<\/small><\/p>\n