The gravitational constant describes the intrinsic strength of gravity and can be used to calculate the attraction between two objects.
Also known as “Big G” or Gthe gravitational constant was first defined by Isaac Newton in his 1680 law of universal gravitation. It is one of the fundamental natural constants with a value of (6.6743 ± 0.00015) x10^-11 m^3 kg^-1 s^-2 (opens in new tab).
The gravitational force between two objects can be calculated using the gravitational constant using an equation most of us know in high school: The gravitational force between two objects is found by multiplying the mass of those two objects (m1 and m2) and Gand then divide by the square of the distance between the two objects (F = [G x m1 x m2]/r^2).
Related: Why is gravity so weak? The answer may lie in the nature of space-time
Keith Cooper is a freelance science journalist and editor based in the UK and has a degree in Physics and Astrophysics from the University of Manchester. He is the author of The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics, and astrobiology for a variety of journals and websites.
The gravitational constant
The gravitational constant is the key to measuring the mass of everything in the universe.
For example, if the gravitational constant is known, then one couples it with the gravitational acceleration Earth, the mass of our planet can be calculated. Once we know the mass of our planet, if we know the magnitude and period of Earth’s orbit, we can measure the mass of the planet Sun. And if we know the mass of the sun, we can measure the mass of everything in the sun Milky Way Interior of the Sun’s orbit.
Measurement of the gravitational constant
The measurement of G was one of the first high-precision scientific experiments, and scientists are looking to see if it can vary at different times and places in space, which could have major implications for cosmology.
Achieving a value of 6.67408 x 10^-11 m^3 kg^-1 s^-2 for the gravitational constant relied on a rather clever 18th-century experiment prompted by trials by surveyors Map of the border between the states of Pennsylvania and Maryland (opens in new tab).
In England the scientist Henry Cavendish (opens in new tab) (1731–1810), who was interested in calculating the density of the earth, realized (opens in new tab) that the efforts of the surveyor would be doomed to fail (opens in new tab) 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, causing their readings to be thrown off. If you knew the size Gthey were able to calculate the gravitational pull of the mountains and correct their results.
So Cavendish set about making the measurement, the most accurate scientific measurement made up to that point in history.
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His experiment was dubbed the “torsional balance technique.” It was two dumbbells that could rotate around the same axis. One of the dumbbells had two smaller lead balls connected by a rod and hanging delicately by a thread. The other dumbbell featured two larger 158-kilogram weights that swung to either side of the smaller dumbbell.
When the larger weights were positioned near the smaller spheres, the gravity of the larger spheres attracted the smaller spheres, causing the fiber to twist. The degree of twisting allowed Cavendish to measure the torque (rotational force) of the twisting system. He then used this value for the torque instead of the ‘f‘ in the equation described above, and together with the masses of the weights and their distances, he could rearrange the equation to be calculated G.
Can the gravitational constant change?
It is a source of frustration among physicists that “Big G” is not known with as many decimal places as the other physical constants. For example, the charge of a electron is known to nine decimal places (1.602176634 x 10^–19 Coulomb), but G was only measured to five decimal places. Frustratingly, efforts to measure it with greater accuracy do not match (opens in new tab).
Part of the reason for this is that the gravity of things around the experimental apparatus will disturb the experiment. However, there is also a nagging suspicion that the problem is not simply experimental, but that there might be one some new physics at work (opens in new tab). It’s even possible that the gravitational constant isn’t quite as constant as scientists thought.
In the 1960s, the physicist Robert Dicke – whose team discovered 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. A scalar field describes a property that can potentially vary at different points in space (an The Earth analogy is a temperature map, where the temperature is not constant but varies with location). If gravity were a scalar field, then G may have different values over space and time. This differs from the more accepted version of general relativity, which assumes that gravity is constant throughout the universe.
Motohiko Yoshimura of Okayama University in Japan suggested that a scalar tensor theory of gravity could be linked cosmic inflation with dark energy. Inflation occurred fractions of a second after the universe was born, driving a brief but rapid expansion of space that lasted between 10^–36 and 10^–33 seconds after birth Big Banginflating the cosmos from microscopic to macroscopic in size before mysteriously shutting itself down.
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Dark energy is the mysterious force accelerating the expansion of the universe today. Many physicists have wondered if there might be a connection between the two expansion forces. Yoshimura suggests that there is – that they are both manifestations of a gravitational scalar field that was a much stronger in the early universethen weakened but has come back strong as the universe expands and matter continues to spread.
However, try to try to spot significant variations in G So far nothing has been found in other parts of the universe. For example, in 2015, the results of a 21-year study of the regular pulsations of the pulsar PSR J1713+0747 no evidence found (opens in new tab) because gravity has a different strength than here in the solar system. Both Green Bank Observatory and the Arecibo radio telescope was followed by PSR J1713+0747, located 3,750 light-years away in a binary system with a white dwarf. The pulsar is one of the most regular known, and any deviation from “Big G” would have quickly become apparent in the period of its orbital dance with the white dwarf and the timing of its pulsations.
in one expression (opens in new tab)Weiwei Zhu of the University of British Columbia, who led the study of PSR J1713+0747, said: “The gravitational constant is a fundamental constant in physics, so it is important to test this basic assumption using objects in different places and times . and gravitational conditions. The fact that we see that gravity behaves the same in our solar system as it does in a distant star system helps confirm that the gravitational constant is truly universal.”
Additional Resources
A review of laboratory tests on gravity (opens in new tab) conducted by the Eöt-Wash group at the University of Washington.
A look back at attempts to measure “Big G”. (opens in new tab) and what the results might mean.
Britannica’s definition of the gravitational constant (opens in new tab).
bibliography
“Precision measurement of Newton’s gravitational constant (opens in new tab).” Xue, Chao, et al. National Science Review (2020).
“The curious case of the gravitational constant (opens in new tab).” Proceedings of the National Academy of Sciences (2022).
“Henry Cavendish (opens in new tab).” Britannica (2022).
Follow Keith Cooper on Twitter @21stCenturySETI (opens in new tab). Follow us on Twitter @spacedotcom (opens in new tab) and on Facebook (opens in new tab).
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