What is Big G? The Mystery of the Gravitational Constant
- 11 hours ago
- 3 min read
Big G (the universal gravitational constant) is one of the most widely known values in physics, and one of the first you learn at school. It determines the strength of attraction between two masses and applies anywhere in the entire universe. Scientists have been trying to measure the exact value of Big G for over 225 years (since Isaac Newton first suggested the concept of gravity) but still its value remains one of the least well known out of nature's fundamental forces. In April, scientists at NIST (National Institute of Standards and Technology) in the US published a paper outlining the results of their experiment trying to better determine this constant. The study was lead by physicist Stephan Schlamminger who has been working on this problem for the past 10 years, and although experiments have become more sensitive and sophisticated over this time, how to find the exact value of this constant to the same accuracy levels as the other fundamental constants still remains a mystery - either because of experimental error or, more intriguingly, a fundamental lack of our understanding of gravity.
Before we get into the nitty gritty of the study lets quickly recap what Big G actually is. Firstly, it is not to be confused with little g. Little g is the value of acceleration due to gravity. On Earth that is 9.81m/s^2, but it is different on all planetary bodies depending on their mass, which controls their gravitational strength. For example, the value of little g on the Moon is 1.62m?s^2 because it is much smaller than the Earth. On the other hand, Big G is a universal constant, meaning it's value is the same everywhere in the universe (as far as we know). It can be used in an equation (F= Gm1m2/r^2) to measure the gravitational attraction between tow objects such as between the Sun and the Earth or the Earth and the Moon. This is a very important equation in the Earth Sciences so I quickly became very familiar with the constant throughout my degree. We always used the value of Big G to be 6.67x10^-11 but this is not actually all that accurate.
The reason Big G is so hard to measure accurately is because it is the weakest of the force fundamental natural forces: Gravity, Electromagnetism, Strong Nuclear Force, and Weak Nuclear Force. The other three of these forces have values known pretty accurately to 6 or 7 significant figures but gravity is harder to pin down and we only know it accurately to 3 or 4 sig figs. A group of scientists at the International Bureau of Weights and Measurements (BIPM) in France conducted a precision experiment in 2007 which Schlamminger and his colleagues at NIST hoped to recreate. If they got the same value in their replication that BIPM did in the original then it could be confirmed that the results of this precision experiment were accurate and we would known Big G to more accuracy. The experiment used something known as a torsion balance which is a device that can sense tiny tiny forces (and was first used by English physicist Henry Cavendish in 1798 in order to get a first value of Big G). Schlamminger went to great lengths to control every variable in his experiment, he even asked his colleague to scramble the data for him to remove any unconscious bias he might have, the actual results of the experiment on revealed to him when he opened a secret envelope at the end. Unfortunately though, it was not enough. The value determined by the experiment at NIST was 0.0235% lower than the one determined at BIPM. This may seem like a tiny difference but is notable when getting down to 6 or 7 significant figures. Schlamminger's team did not succeed in increasing the accuracy we known Big G to but they did succeed in adding another value and experiment to our understanding of the constant, and, as I read recently in Operating Manual for Spaceship Earth by Buckminster Fuller, any experiment works to advance our knowledge of science, and you can never end up knowing less than you started with. This beign said, it is safe to say, the mystery remains unsolved for now.
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