The researchers have been inspired by one of the most beautiful experiments in the history of mankind. British scientist Henry Cavendish “probably spoke fewer words throughout his life than any man who has lived for eighty years, including the Trappist monks,” his contemporary Lord Brougham described with guile. Cavendish, born in 1731 and deceased in 1810, was effectively introverted and lonely. He was “the richest of all the wise and the wisest of all the rich”, in the words of the French astronomer Jean-Baptiste Biot. But, silently and locked in his mansion, he discovered hydrogen and the composition of water. And, in 1798, he conceived one of the most audacious experiments in the history of mankind. Today, a team of Chinese scientists has climbed on their shoulders to redefine, with unprecedented precision, one of the most important constants to describe our universe, together with the speed of light.
Cavendish was almost 70 years old and had set out to find out the density of planet Earth. For this he needed the universal gravitation constant (G) postulated by Isaac Newton a century earlier. The old man, always quiet, built a kind of scale in the basement of his house in South London: two small spheres, fixed to the ends of a horizontal rod suspended from the ceiling by a thin fiber. When approaching two lead spheres larger, about 160 kilograms each, the force of attraction that the other two balls suffered made the rod rotate, and all this in a perceptible way thanks to a set of mirrors, lights and telescopes installed by Cavendish. In his book Mathematical Principles of Natural Philosophy, published in 1686, Newton had formulated that the gravitational interaction between two bodies could be expressed as a force directly proportional to the product of the masses of those bodies and inversely proportional to the square of the distance that the To stop. Using this formula and the observations in his basement, the shy Cavendish came to the conclusion that the average density of the Earth was 5.48 times greater than that of water. And it did not fail very much: today it is calculated that the correct figure is 5.51.
A team led by Luo Jun, of the University of Science and Technology of Huazhong (China), has refined Extremely Cavendish experiment, with steel balls and vacuum chambers, and has reached two similar measurements with two independent devices : 6,674184 × 10-11 and 6,674484 × 10-11 cubic meters match kilogram per second squared. It is “record accuracy,” according to physicist Stephan Schlamminger of the National Institute of Standards and Technology. The new measures are published today in the prestigious journal Nature.The search for the greatest possible accuracy is not a whim. Geophysicists use the constant G to study the structure and composition of the Earth. And it is also essential in fields such as particle physics and cosmology, the part of astronomy that studies the origin and future of the universe.
“The true value of G is still unknown,” admits, however, Professor Luo. The difficulty of measuring the constant is devilish. The gravitational force exerted by the Sun is so great that it prevents the planet Earth from fleeing through space. However, in a laboratory, the gravitational force between two objects of one kilogram separated by one meter is equal to the weight of a handful of bacteria. It is an “extremely weak” force, in Luo’s words.The Information Committee for Science and Technology (CODATA), based in Paris, is the international reference body for this constant. In 2014, its experts adopted 14 G values determined in the last four decades in different laboratories around the world. “The relative difference between the highest and the lowest value of G is close to 0.055%.” This situation does not allow us to obtain a G value with high precision, laments Luo.Despite the accuracy of their results, Chinese scientists have obtained two different data with two slightly different and independent devices. The team does not know how to explain this discrepancy. “There is something that we still do not know and we need more research,” says Luo. Or, maybe, we need another Henry Cavendish.
“It is a scandal that the unit of mass is still a physical object,” William Daniel Phillips, Nobel Prize in Physics, lamented a month ago at an international conference on atomic physics held in Barcelona. He was referring to the kilogram, whose reference prototype is a cylinder of platinum-iridium -custodied in a basement in Paris- that defines the unity of mass since the 19th century in the so-called international system.
Already in 1899, the German physicist Max Planck suggested ending this arbitrariness and proposed creating a system of units based on the constants of nature, alien to human constructs. “He proposed to use the speed of light, Planck’s constant and Newton’s universal gravitation constant,” says Chinese physicist Jun Luo. “However, this system of units is not completely competitive against the current international system, due to the poor accuracy of the gravitation constant,” laments the researcher at the Huazhong University of Science and Technology.