This differs from the more accepted version of general relativity, which posits that gravity is constant across the universe. If gravity were a scalar field, then G could potentially have different values across space and time. A scalar field describes a property that can potentially vary at different points in space (an Earthly analogy is a temperature map, where the temperature is not constant, but varies with location). It is even possible that the gravitational constant isn't quite as constant as scientists thought.īack in the 1960s, physicists Robert Dicke - whose team was scooped to the discovery of the cosmic microwave background (CMB) by Arno Penzias and Robert Wilson in 1964) - and Carl Brans developed a so-called scalar-tensor theory of gravity, as a variation of Albert Einstein's general theory of relativity. However, there's also the niggling suspicion that the problem isn't simply experimental, but that there could be some new physics at work. Part of the reason for this is that the gravity of things around the experimental apparatus will interfere with the experiment. Frustratingly, efforts to measure it to greater precision don't agree with one another. For example, the charge of an electron is known to nine decimal places (1.602176634 x 10^–19 coulomb), but G has only been accurately measured to just five decimal points. It is a source of frustration among physicists that "Big G" is not known to as many decimal points as the other fundamental constants. He then used this value for the torque in place of the ' F' in the equation described above, and along with the masses of the weights and their distances, he could rearrange the equation to calculate G. The degree of twisting allowed Cavendish to measure the torque (the rotational force) of the twisting system. When the larger weights were positioned close to the smaller spheres, the gravitational pull of the larger spheres attracted the smaller spheres, causing the fiber to twist. The other dumbbell featured two larger 348-pound (158-kilogram) lead weights that could swivel to either side of the smaller dumbbell. One of the dumbbells had two smaller lead spheres connected by a rod and hanging delicately by a fiber. It involved two dumbbells that could rotate around the same axis. His experiment was referred to as the ' torsion balance technique'. (Image credit: Science & Society Picture Library/Getty Images) The English natural philosopher Henry Cavendish (1731-1810) built a torsion balance to measure the gravitational force between two large masses, so that he could make the first calculation of the mass of the Earth. So Cavendish set about making the measurement, the most precise scientific measurement made up to that point in history. If they knew the size of G, they could calculate the gravitational pull of the mountains and amend their results. In England, the scientist Henry Cavendish (1731–1810), who was interested in calculating the density of the Earth, realized that the surveyor's efforts would be doomed to failure because nearby mountains would subject the surveyors' 'plumb-bob' (a tool that provided a vertical reference line against which the surveyors could make their measurements) to a slight gravitational pull, throwing off their readings. The measurement of G was one of the first high-precision science experiments, and scientists are searching for whether it can vary at different times and locations in space, which could have big implications for cosmology.Īrriving at a value of 6.67408 x10^–11 m^3 kg^–1 s^–2 for the gravitational constant relied on a rather clever eighteenth-century experiment, prompted by surveyor's attempts to map the border between the states of Pennsylvania and Maryland. And knowing the mass of the sun allows us to measure the mass of everything in the Milky Way Galaxy interior to the sun's orbit. Once we know the mass of our planet, then knowing the size and period of Earth's orbit allows us to measure the mass of the sun. For example, once the gravitational constant is known, then coupled with the acceleration due to gravity on Earth, the mass of our planet can be calculated.
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