Online Astronomy eText: Background Physics: Motion and Forces What Is The Gravitational Constant, G? (also see A Matter Of Some Gravity) Page created Sep 27, 2017(a hurried discussion for now, but to be fact-checked and edited ASAP) What IS The Gravitational Constant, G?  In the equation that describes the gravitational force acting between two known masses separated by a known distance there is a number, G, which is called the Gravitational Constant (this number appears in both Newton's and Einstein's treatment of how gravity works). The value of this "constant" is one of the most poorly known numbers in physics, because the gravitational force between two objects of reasonable size is extremely small, and therefore extremely difficult to measure. It is usually represented in units of kilograms (a unit of mass), meters (a unit of distance) and seconds (a unit of time) or in smaller units such as grams, centimeters and seconds; but depending on the set of units used there are either seven or ten zeroes after a decimal point before you get to any actual numbers, and despite more than two hundred years of effort to measure G since the first measurement by Henry Cavendish in 1798, the best that any experiment has been able to do is to establish a value for the non-zero digits somewhere between 6.672 and 6.674 (which makes Cavendish's value of 6.754 seem surprisingly accurate, given the technology available in his day). That is, using meters, kilograms and seconds for units, the gravitational constant is between 0.00000000006672 and 0.00000000006674 m3 / kg sec2,and using centimeters, grams and seconds for units, the gravitational constant is between 0.00000006672 and 0.00000006674 cm3 / gm sec2, Supposed Changes In the Gravitational "Constant"  Given the difficulty of measuring the gravitational constant, different experiments are occasionally mistakenly touted as showing that the "constant" of gravity may not actually be constant, but varying over time. In fact, a nearly century old theory about the origin of gravity predicts that local gravitational fields are related to the distribution of mass throughout the Universe, and as the Universe expands, the gravitational constant should be decreasing (NOTE TO SELF: NEED TO FIND REFERENCE BEFORE FINALIZING PAGE). As a result, if a new experiment yields a lower value for the gravitational constant than the previously "accepted" value, there may be a flurry of news reports about how gravity is getting weaker (accompanied by some form of doom and gloom in the more wild-eyed portions of the "social media"). However, all experiments contain experimental errors of unknown origin, and in making such a difficult measurement, those errors can appear significant in comparison to the very small value of the calculated result; but in reality they are no more significant than the difference in the number of heads or tails obtained when flipping a coin a thousand times, over and over again. All such efforts would yield values somewhere near 500 heads and 500 tails, but values "off" by as much as 30 or 50 extra heads or tails would be far more common than a result of exactly 500 heads and tails, simply because of the randomness of the process. How Do We Know That The Gravitational "Constant" IS Constant?  If the gravitational constant were to change, the force compressing the interior of stars would change, and because most nuclear reactions are extremely sensitive to the temperature, density and pressure in the core of a star, the lives and deaths of stars would be significantly affected by any change in the value of the gravitational constant. In particular, a specific type of supernova (type Ia) caused by the annihilation of a white dwarf as a result of runaway nuclear reactions initiated by a binary companion dumping large amounts of mass onto the surface of the white dwarf would be significantly brighter or fainter than usual if the gravitational constant varied with time. However, a study of nearly 600 type Ia supernovae shows that their actual brightness (adjusted for certain factors which can affect their maximum brightness) implies that G has been constant to better than one part in a billion over the last ten billion years or so. This means that the gravitational constant, although its actual value is very poorly known, has been a constant for most of the history of the Universe (and more than likely, at least since the end of the Cosmic Fireball, about 14 billion years ago).