杏吧原创

Update: On 2 June 2011, Barry Wood鈥檚 committee for the fundamental physical constants. The value for 鈥渂ig G鈥� was reduced by 66 parts per million, and the uncertainty on it increased from 100 parts per million to 120 parts per million.

Original article from 23 April 2011 issue:

We鈥檝e been measuring gravity for 200 years, but we鈥檙e still not sure how strong it is. Meet the metrologists striving to find out

HAROLD PARKS鈥橲 belongings were already leaving for France when he realised gravity had given him the slip. 鈥淭he movers were in my apartment taking my stuff away,鈥� he says. He was in his lab at the research institute in Boulder, Colorado, making the final checks on an experiment that had taken up the past two years of his life 鈥� to precisely measure the strength of gravity. 鈥淭he signal shouldn鈥檛 have changed,鈥� he recalls. 鈥淏ut it did.鈥�

That was 10 years ago. Having relocated, for a while Parks was tempted to give up on gravity. But the force exerts a mysterious pull on those who measure it. After a sojourn at the high temple of metrology, the (BIPM) in Paris, France, Parks was back in Boulder, rebuilding and improving his old experiment.

By the time he and his supervisor, Jim Faller, published their measurement of gravity鈥檚 strength in Physical Review Letters (), it was September 2010. The intervening years of scrupulous checking and rechecking had given them reasonable confidence in their result. 鈥淚t was beautiful,鈥� says Parks. Beautiful, but for one thing: it didn鈥檛 agree with anyone else鈥檚.

Sitting in his office 2000 kilometres away in Ottawa, Canada, Barry Wood has the task of mulling such problems over. A metrologist at Canada鈥檚 , he chairs the international that decides the values for the fundamental physical constants. These are numbers such as the Planck constant 鈥� which determines the size of a quantum of energy 鈥� or the charge of an electron. Or indeed Newton鈥檚 constant of gravitation, the object of Parks and Faller鈥檚 attentions.

Every four years, Wood鈥檚 committee revises these numbers, and the results of the latest overhaul are due any day. Normally it is a routine celebration of the onward march of precision metrology, as the constants are redefined with accuracies that edge ever closer to 1 part in a billion.

All except gravity, that is, which has always been a bit of a party pooper. More than 200 years of measurements of Newton鈥檚 constant 鈥� 鈥渂ig G鈥� to its friends 鈥� have delivered a number accurate to a measly 1 part in 10,000. The latest round of experiments, Parks and Faller鈥檚 included, raise the dismal prospect that this could get even worse. Wood鈥檚 jury is still out, but depending on what they agree this month it could become official: gravity is going down.

Gravity鈥檚 ways are as mysterious as its effects are ubiquitous. Ever since Isaac Newton grappled with an apple over 300 years ago to produce the first quantitative description of the force, we have been puzzling over what it means. While Newton had no answer, he at least came up with a definition 鈥� a handy formula that allows us to work out its effects. According to his inverse square law, gravity makes any two objects attract one another with a force proportional to their masses divided by the square of the distance separating them. Riding at the front of that equation, telling us how big that force should be, is big G.

Pending a shiny new quantum depiction of gravity, no theory tells us where this number comes from, or what its value should be. 鈥淏ig G is just there,鈥� says of the University of Birmingham, UK, who has spent 30 years investigating the ups and downs of gravity. It is there in Newton鈥檚 inverse square law and in Einstein鈥檚 general theory of relativity, our most accurate description of gravity to date.

The only way to reveal gravity鈥檚 true worth is to measure it in highly controlled experiments 鈥� which is easier said than done. Earth鈥檚 hulking mass sucks everything towards it, masking a fundamental truth: gravity is by far the weakest of nature鈥檚 four fundamental forces. And that means it is by far the trickiest to measure.

Since Henry Cavendish to measure it a little more than two centuries ago, progress has been painfully slow (see 鈥淏ig G, little g鈥�). 鈥淚t is not a case of someone pops into a lab, stays a week and comes out with a number,鈥� says Wood. 鈥淢ost of the experiments take a decade.鈥� Few people have the time, resources or nerve to stick at it that long.

It doesn鈥檛 help that there is no obvious pay-off to pinning down big G. A precise number would help us predict the motions of planets and stars more accurately, but no applications depend on it in the way that, say, GPS depends on knowing how long a second lasts to 1 part in a trillion.

Even so, our apparent impotence in the face of gravity raises hackles. 鈥淚 have been heckled at meetings by people who said it was a scandal big G was so imprecise,鈥� says Speake.

The latest round of experiments designed to remedy gravity鈥檚 shortcomings was inspired by the last notable episode in big G鈥檚 history. In 1995, a team from , the PTB in Braunschweig, published a new value using a new version of the apparatus that Cavendish had used 200 years earlier. 鈥淭hey had the equipment, they had good people, they had the time and the money,鈥� says Terry Quinn, who at the time was head of the BIPM. 鈥淓veryone assumed this was going to be the 别虫辫别谤颈尘别苍迟.鈥�

In the event, it was a disaster. The PTB team delivered a value that was 6 parts in 1000 higher than the then-accepted value. In metrological terms, that鈥檚 off the scale. No one could believe that gravity had suddenly become that much stronger, or that all the experiments before had been so consistently wrong. There had to be a mistake.

鈥淓veryone thought, 鈥榳e must be able to do better than that鈥�,鈥� says Quinn. So teams from all over the world set out to find their own number for gravity, which Wood鈥檚 committee would then combine to give the official value 鈥� as it has done since 1969. In the meantime, the committee opted to keep big G unchanged. To reflect the large spread of measurements, however, they increased the uncertainty by a factor of 10.

Cruel tricks

By the time the fundamental constants were up for review again in 2002, a consensus had been reached that excluded the PTB measurement. The value for big G was increased a little and the official error was set back to where it had been before 1998.

鈥淭he fundamental constants are accurate to one part in a billion, all except gravity鈥�

Meanwhile, Parks was beavering away in Boulder. His and Faller鈥檚 experiment was a variant of an apparatus that had been used to try to pin down big G before. It consisted of two free-hanging pendulum bobs surrounded by four massive stacks of tungsten. Moving the tungsten masses inwards (see diagram) draws the bobs closer together by an amount 1000 times smaller than the diameter of a human hair. Still, the shift is large enough to be picked up by a laser interferometer.

Not that it is easy to be sure the movements are down to gravity alone. 鈥淚t鈥檚 about thinking of all the things the world can do to you to muck up your experiment,鈥� says Parks. The pair set up the pendulums in a vacuum to avoid the effects of temperature changes and air resistance slowing the pendulums鈥� movements. They also floated the tungsten stacks on a thin layer of air to stop them vibrating unexpectedly. Even so, tiptoeing anywhere near the experiment was a no-no: the additional mass of a person would weigh down one side of the floor and nudge the apparatus ever so slightly.

鈥淕ravity is so tricky to measure that even tiptoeing anywhere near the experiment is a no-no鈥�

The problems didn鈥檛 stop at the doors of the lab. Next to, and towering over, the basement where the experiment was situated was a high-rise block. As the sun crept across the sky during the day, it warmed first one side of the tower and then the other, causing it to expand unevenly. The effect was to imperceptibly tilt the tower and everything attached to it, including Parks鈥檚 lab, first one way and then the other.

Even that cruel trick was nothing compared to what was unmasked the day the fire alarm sounded. 鈥淭here had been regular spikes in data taken during the day,鈥� says Parks. 鈥淭hey just went quiet.鈥� It turned out that a surge in current each time the elevator moved in the tower caused a slight change in the magnetisation of the pendulum bobs, moving them ever so slightly and skewing the results.

After years of noticing such effects and removing or compensating for them, Parks could be forgiven for a moment of self-doubt when he had what he thought was his final number: some 0.03 per cent below the official value for big G, and three standard deviations away from the previously accepted mean.

鈥淧hysicists like to get the right answer,鈥� says Faller. Parks鈥檚 response was to sit on his work while he ran through every other source of error he could think of. 鈥淚 assumed I鈥檇 come up with something where I鈥檇 say, 鈥極h that鈥檚 stupid. Why didn鈥檛 you see that?鈥�,鈥� he says. But nothing came.

By the time he had regained his mojo it was clear he wasn鈥檛 the only one getting anomalous results. In June 2009, a group at the in Wuhan, China, measured big G by timing the swing of a pendulum suspended in a vacuum as large masses rotated around it. That, too, represented the culmination of more than a decade鈥檚 work 鈥� and came in lower than the accepted value ().

Back in Ottawa, Wood鈥檚 committee is now chewing over the figures, wondering how strong gravity should be for the next four years. With such a slippery customer, however, there is no clear right answer. Now that two of the latest results disagree with the previous best judgement, the likelihood is that the new value will, like that in 1998, be less accurate than the one before 鈥� and possibly even a bit lower.

Lifelong obsession

Is gravity actually changing? Probably not. Exotic theories of gravity do exist which postulate that its strength should vary from day to day, place to place or season to season (New 杏吧原创, 18 April 2009, p 28). But the state of our experimental art means we are not yet able to see the tiny shifts predicted, even if they do exist. 鈥淎t the moment big G just looks like a boring constant in Newton鈥檚 law,鈥� says Speake. Quinn goes further. 鈥淲e just don鈥檛 discuss these things,鈥� he says.

Which leads to the obvious question: why bother to accurately measure big G if it鈥檚 so tricky and no one鈥檚 expecting unusual gravitational effects anyway? One answer is the hoary chestnut, 鈥渂ecause it is there鈥�. 鈥淧art of the fun of it is doing something really hard you鈥檙e not sure you can do,鈥� says Parks.

鈥淚t鈥檚 like weeding the garden,鈥� says Speake. 鈥淎s soon as you start paying attention to weeds, they start bothering you.鈥� He and Quinn are getting ready to publish their latest determination of big G 鈥� which builds on work started in 2001 and is the product of years of Quinn鈥檚 retirement and Speake鈥檚 Christmas and summer holidays.

鈥淚f you look at people from Cavendish onwards who鈥檝e done this, we鈥檙e all a bit crazy,鈥� says Quinn. But there is more to it than physicists with their heads in the clouds, he says. Aside from the measurements leading to advances in other areas of precision metrology, it is important to know what the gravitational constant is, says Quinn 鈥� not today, perhaps, but certainly in the future. A possible quantum theory of gravity could one day predict precisely what big G鈥檚 value should be. 鈥淭hen we鈥檒l need a robust experiment to test whether it鈥檚 right or not,鈥� says Quinn. 鈥淭hat鈥檚 ultimately the point.鈥�

That is a good few years away yet. But judging by history, we will probably need that long to get to grips with gravity. Parks remembers attending a conference in London in 1998 to mark the bicentenary of Cavendish鈥檚 original experiment, at a time when he was just starting out on his own measurements. 鈥淪omeone looked around and said, 鈥楴ice to see all these young physicists working on gravity again鈥�,鈥� he recalls. 鈥淎nd Terry Quinn said, 鈥榊es 鈥� but they鈥檒l all be old physicists before they鈥檙e done.'鈥�