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Physical constant relating the gravitational force between objects to their mass and distance
The gravitational constant is a physical constant that is difficult to measure with high accuracy.
This is because the gravitational force is extremely weak as compared to other fundamental forces.[c]
In other words, in Planck units, G has the numerical value of .
Thus, in Planck units, and other natural units taking G as their basis, the value of the gravitational constant cannot be measured as this is set by definition. Depending on the choice of units, uncertainty in the value of a physical constant as expressed in one system of units shows up as uncertainty of the value of another constant in another system of units. Where there is variation in dimensionless physical constants, no matter which choice of physical "constants" is used to define the units, this variation is preserved independently of the choice of units; in the case of the gravitational constant, such a dimensionless value is the gravitational coupling constant of the electron,
The above equation is exact only within the approximation of the Earth's orbit around the Sun as a two-body problem in Newtonian mechanics, the measured quantities contain corrections from the perturbations from other bodies in the solar system and from general relativity.
From 1964 until 2012, however, it was used as the definition of the astronomical unit and thus held by definition:
Since 2012, the AU is defined as exactly, and the equation can no longer be taken as holding precisely.
The quantity GM--the product of the gravitational constant and the mass of a given astronomical body such as the Sun or Earth--is known as the standard gravitational parameter and (also denoted ?).
The standard gravitational parameter GM appears as above in Newton's law of universal gravitation, as well as in formulas for the deflection of light caused by gravitational lensing, in Kepler's laws of planetary motion, and in the formula for escape velocity.
This quantity gives a convenient simplification of various gravity-related formulas. The product GM is known much more accurately than either factor is.
Between 1640 and 1650, Grimaldi and Riccioli had discovered that the distance covered by objects in free fall was proportional to the square of the time taken, which led them to attempt a calculation of the gravitational constant by recording the oscillations of a pendulum.
The existence of the constant is implied in Newton's law of universal gravitation as published in the 1680s (although its notation as G dates to the 1890s), but is not calculated in his Philosophiæ Naturalis Principia Mathematica where it postulates the inverse-square law of gravitation. In the Principia, Newton considered the possibility of measuring gravity's strength by measuring the deflection of a pendulum in the vicinity of a large hill, but thought that the effect would be too small to be measurable.
Nevertheless, he estimated the order of magnitude of the constant when he surmised that "the mean density of the earth might be five or six times as great as the density of water", which is equivalent to a gravitational constant of the order:
The Schiehallion experiment, proposed in 1772 and completed in 1776, was the first successful measurement of the mean density of the Earth, and
thus indirectly of the gravitational constant. The result reported by Charles Hutton (1778) suggested a density of (4 times the density of water), about 20% below the modern value.
This immediately led to estimates on the densities and masses of the Sun, Moon and planets, sent by Hutton to Jérôme Lalande for inclusion in his planetary tables.
As discussed above, establishing the average density of Earth is equivalent to measuring the gravitational constant, given
Earth's mean radius and the mean gravitational acceleration at Earth's surface, by setting
Based on this, Hutton's 1778 result is equivalent to G ? .
Diagram of torsion balance used in the Cavendish experiment performed by Henry Cavendish in 1798, to measure G, with the help of a pulley, large balls hung from a frame were rotated into position next to the small balls.
The first direct measurement of gravitational attraction between two bodies in the laboratory was performed in 1798, seventy-one years after Newton's death, by Henry Cavendish.
He determined a value for G implicitly, using a torsion balance invented by the geologist Rev. John Michell (1753). He used a horizontal torsion beam with lead balls whose inertia (in relation to the torsion constant) he could tell by timing the beam's oscillation. Their faint attraction to other balls placed alongside the beam was detectable by the deflection it caused. In spite of the experimental design being due to Michell, the experiment is now known as the Cavendish experiment for its first successful execution by Cavendish.
Cavendish's stated aim was the "weighing of Earth", that is, determining the average density of Earth and the Earth's mass. His result, ρ? = , corresponds to value of G = . It is surprisingly accurate, about 1% above the modern value (comparable to the claimed standard uncertainty of 0.6%).
The accuracy of the measured value of G has increased only modestly since the original Cavendish experiment.G is quite difficult to measure because gravity is much weaker than other fundamental forces, and an experimental apparatus cannot be separated from the gravitational influence of other bodies. Furthermore, gravity has no established relation to other fundamental forces, so it does not appear possible to calculate it indirectly from other constants that can be measured more accurately, as is done in some other areas of physics.
Cavendish's experiment was first repeated by Ferdinand Reich (1838, 1842, 1853), who found a value of , which is actually worse than Cavendish's result, differing from the modern value by 1.5%.
Cornu and Baille (1873), found .
Cavendish's experiment proved to result in more reliable measurements than pendulum experiments of the "Schiehallion" (deflection) type or
"Peruvian" (period as a function of altitude) type. Pendulum experiments still continued to be performed, by
Robert von Sterneck (1883, results between 5.0 and ) and Thomas Corwin Mendenhall (1880, ).
Cavendish's result was first improved upon by John Henry Poynting (1891), who published a value of , differing from the modern value by 0.2%, but compatible with the modern value within the cited standard uncertainty of 0.55%.
In addition to Poynting, measurements were made by C. V. Boys (1895) and Carl Braun (1897), with compatible results suggesting G = .
The modern notation involving the constant G was introduced by Boys in 1894 and becomes standard by the end of the 1890s, with values usually cited in the cgs system.
Richarz and Krigar-Menzel (1898) attempted a repetition of the Cavendish experiment using 100,000 kg of lead for the attracting mass. The precision of their result of
was, however, of the same order of magnitude as the other results at the time.
Paul R. Heyl (1930) published the value of (relative uncertainty 0.1%),
improved to (relative uncertainty 0.045% = 450 ppm) in 1942.
Published values of G derived from high-precision measurements since the 1950s have remained compatible with Heyl (1930), but within the relative uncertainty of about 0.1% (or 1,000 ppm) have varied rather broadly, and it is not entirely clear if the uncertainty has been reduced at all since the 1942 measurement.
Some measurements published in the 1980s to 2000s were, in fact, mutually exclusive.
Establishing a standard value for G with a standard uncertainty better than 0.1% has therefore remained rather speculative.
By 1969, the value recommended by the National Institute of Standards and Technology (NIST) was cited with a standard uncertainty of 0.046% (460 ppm), lowered to 0.012% (120 ppm) by 1986.
But the continued publication of conflicting measurements led NIST to radically increase the standard uncertainty in the 1998 recommended value, by a factor of 12, to a standard uncertainty of 0.15%, larger than the one given by Heyl (1930).
The uncertainty was again lowered in 2002 and 2006, but once again raised, by a more conservative 20%, in 2010, matching the standard uncertainty of 120 ppm published in 1986.
For the 2014 update, CODATA reduced the uncertainty to 46 ppm, less than half the 2010 value, and one order of magnitude below the 1969 recommendation.
The following table shows the NIST recommended values published since 1969:
Timeline of measurements and recommended values for G since 1900: values recommended based on a literature review are shown in red, individual torsion balance experiments in blue, other types of experiments in green.
In the January 2007 issue of Science, Fixler et al. described a measurement of the gravitational constant by a new technique, atom interferometry, reporting a value of , 0.28% (2800 ppm) higher than the 2006 CODATA value. An improved cold atom measurement by Rosi et al. was published in 2014 of . Although much closer to the accepted value (suggesting that the Fixler et. al. measurement was erroneous), this result was 325 ppm below the recommended 2014 CODATA value, with non-overlapping standard uncertainty intervals.
As of 2018, efforts to re-evaluate the conflicting results of measurements are underway, coordinated by NIST,
notably a repetition of the experiments reported by Quinn et al. (2013).
In August 2018, a Chinese research group announced new measurements based on torsion balances,
and based on two different methods.
These are claimed as the most accurate measurements ever made, with a standard uncertainties cited as low as 12 ppm. The difference of 2.7? between the two results suggests there could be sources of error unaccounted for.
A controversial 2015 study of some previous measurements of G, by Anderson et al., suggested that most of the mutually exclusive values in high-precision measurements of G can be explained by a periodic variation. The variation was measured as having a period of 5.9 years, similar to that observed in length-of-day (LOD) measurements, hinting at a common physical cause which is not necessarily a variation in G. A response was produced by some of the original authors of the G measurements used in Anderson et al. This response notes that Anderson et al. not only omitted measurements, they also used the time of publication rather than the time the experiments were performed. A plot with estimated time of measurement from contacting original authors seriously degrades the length of day correlation. Also taking the data collected over a decade by Karagioz and Izmailov shows no correlation with length of day measurements. As such the variations in G most likely arise from systematic measurement errors which have not properly been accounted for.
Under the assumption that the physics of type Ia supernovae are universal, analysis of observations of 580 type Ia supernovae has shown that the gravitational constant has varied by less than one part in ten billion per year over the last nine billion years according to Mould et al. (2014).
^"Newtonian constant of gravitation" is the name introduced for G by Boys (1894). Use of the term by T.E. Stern (1928) was misquoted as "Newton's constant of gravitation" in Pure Science Reviewed for Profound and Unsophisticated Students (1930), in what is apparently the first use of that term. Use of "Newton's constant" (without specifying "gravitation" or "gravity") is more recent, as "Newton's constant" was also
used for the heat transfer coefficient in Newton's law of cooling, but has by now become quite common, e.g.
Calmet et al, Quantum Black Holes (2013), p. 93; P. de Aquino, Beyond Standard Model Phenomenology at the LHC (2013), p. 3.
The name "Cavendish gravitational constant", sometimes "Newton-Cavendish gravitational constant", appears to have been common in the 1970s to 1980s, especially in (translations from) Soviet-era Russian literature,
e.g. Sagitov (1970 ), Soviet Physics: Uspekhi 30 (1987), Issues 1-6, p. 342 [etc.].
"Cavendish constant" and "Cavendish gravitational constant" is also used in
Charles W. Misner, Kip S. Thorne, John Archibald Wheeler, "Gravitation", (1973), 1126f.
Colloquial use of "Big G", as opposed to "little g" for gravitational acceleration dates to the 1960s (R.W. Fairbridge, The encyclopedia of atmospheric sciences and astrogeology, 1967, p. 436;
note use of "Big G's" vs. "little g's" as early as the 1940s of the Einstein tensorG vs. the metric tensorg,
Scientific, medical, and technical books published in the United States of America: a selected list of titles in print with annotations: supplement of books published 1945-1948, Committee on American Scientific and Technical Bibliography National Research Council, 1950, p. 26).
^Depending on the choice of definition of the stress-energy tensor it can also be normalized as .
^For example, the gravitational force between an electron and proton one meter apart is approximately , whereas the electromagnetic force between the same two particles is approximately . The electromagnetic force in this example is some 39 orders of magnitude (i.e. 1039) greater than the force of gravity--roughly the same ratio as the mass of the Sun to a microgram.
^Cavendish determined the value of G indirectly, by reporting a value for the Earth's mass, or the average density of Earth, as
^Gundlach, Jens H.; Merkowitz, Stephen M. (23 December 2002). "University of Washington Big G Measurement". Astrophysics Science Division. Goddard Space Flight Center. Since Cavendish first measured Newton's Gravitational constant 200 years ago, "Big G" remains one of the most elusive constants in physics
Ries JC, Eanes RJ, Shum CK, Watkins MM (20 March 1992). "Progress in the determination of the gravitational coefficient of the Earth". Geophysical Research Letters. 19 (6): 529-531. Bibcode:1992GeoRL..19..529R. doi:10.1029/92GL00259.CS1 maint: extra punctuation (link)
^J.L. Heilbron, Electricity in the 17th and 18th Centuries: A Study of Early Modern Physics (Berkeley: University of California Press, 1979), 180.
^Davies, R.D. (1985). "A Commemoration of Maskelyne at Schiehallion". Quarterly Journal of the Royal Astronomical Society. 26 (3): 289-294. Bibcode:1985QJRAS..26..289D.
^"Sir Isaac Newton thought it probable, that the mean density of the earth might be five or six times as great as the density of water; and we have now found, by experiment, that it is very little less than what he had thought it to be: so much justness was even in the surmises of this wonderful man!" Hutton (1778), p. 783
^Published in Philosophical Transactions of the Royal Society (1798); reprint: Cavendish, Henry (1798). "Experiments to Determine the Density of the Earth". In MacKenzie, A. S., Scientific Memoirs Vol. 9: The Laws of Gravitation. American Book Co. (1900), pp. 59-105.
^C.V. Boys, Phil. Trans. Roy. Soc. A. Pt. 1. (1895).
^Carl Braun, Denkschriften der k. Akad. d. Wiss. (Wien), math. u. naturwiss. Classe, 64 (1897).
Braun (1897) quoted an optimistic standard uncertainty of 0.03%, but his result was significantly worse than the 0.2% feasible at the time.
^Sagitov, M. U., "Current Status of Determinations of the Gravitational Constant and the Mass of the Earth", Soviet Astronomy, Vol. 13 (1970), 712-718, translated from Astronomicheskii Zhurnal Vol. 46, No. 4 (July-August 1969), 907-915 (table of historical experiments p. 715).
^C. Rothleitner; S. Schlamminger (2017). "Invited Review Article: Measurements of the Newtonian constant of gravitation, G". Review of Scientific Instruments. 88. doi:10.1063/1.4994619. 111101. However, re-evaluating or repeating experiments that have already been performed may provide insights into hidden biases or dark uncertainty. NIST has the unique opportunity to repeat the experiment of Quinn et al.  with an almost identical setup. By mid-2018, NIST researchers will publish their results and assign a number as well as an uncertainty to their value. Referencing:
Standish., E. Myles (1995). "Report of the IAU WGAS Sub-group on Numerical Standards". In Appenzeller, I. (ed.). Highlights of Astronomy. Dordrecht: Kluwer Academic Publishers.(Complete report available online: PostScript; PDF. Tables from the report also available: Astrodynamic Constants and Parameters)