The lure of G

It was back in 1982, during his PhD, when Clive Speake first understood the true nature of gravity. “It was amazing,” he recalls fondly. “I remember going out the night I had done the experiment for the first time, and I just looked up at the Moon. It had given me a completely different feel for what the Moon is, you know? Why it’s there.”

Such epiphanies are not uncommon among those who perform measurements of the gravitational constant. Gravity is the most famous of nature’s forces, because it is the one that is most obviously present in our lives. Ever since Isaac Newton was inspired by a falling apple, everyone has known that it is gravity behind the inevitable phrase “what goes up, must come down”. Yet despite our familiarity with gravity, few of us have experienced the force other than when it is directed towards the ground beneath our feet.

To see why, you need only look at the numbers. Gravity is the weakest force – 10^36 times weaker than electromagnetism, the force that governs most other everyday phenomena. The only reason we can feel gravity on Earth is because it scales with mass: our planet’s mass of five zetta-tonnes (5 × 10^21 tonnes) is enough to bring gravity into the realm of normal human perception. But the force still exists between all other objects, and if you do ever witness it with Earth out of the equation – for example, in the faint shift of two suspended metal weights – it might at first seem like magic. “It’s a liberating experience,” says Speake, who is now based at the University of Birmingham in the UK.

Liberating – and infuriating. The gravitational constant – “big G”, as it is commonly known – is what characterizes the strength of gravity according to Newton’s law, and it is fiendishly difficult to measure. Experiments struggle to deliver uncertainties much smaller than one part in ten thousand – compare that, for instance, with the proton–electron mass ratio, which is known to four parts in ten billion.

Low precision alone is enough to keep a metrologist up all night. But in recent years, a much more serious problem has arisen: measurements of big G are in wild disagreement with one another. Since the turn of this century, values recorded by some of the best labs in the world have been spread apart by more than 10 times their estimated uncertainties. Something is amiss – yet no-one is quite sure what. “You go over it, and over it, and over it,” says Speake. “And there comes a time when you say, I just can’t think of anything we’ve done wrong.” […]

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