The gravitational constant describes the intrinsic force of gravity and can be used to calculate the gravitational attraction between two objects.
Also known as “Big G” or gthe gravitational constant was first defined by Isaac Newton in his law of universal gravitation formulated in 1680. It is one of the fundamental constants of nature, with a value of (6.6743 ± 0.00015) x10^–11 m^3 kg^–1 s^–2 (opens in a new tab).
The gravitational attraction between two objects can be calculated with the gravitational constant using an equation that most of us come across in high school: the gravitational force between two objects is found by multiplying the mass of these two objects (m1 and m2) and gthen dividing 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 very nature of spacetime.
Keith Cooper is a freelance science journalist and editor in the UK. He graduated 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 multitude magazines and websites.
The gravitational constant
The gravitational constant is the key to measuring the mass of everything in the universe.
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. Once we know the mass of our planet, knowing the size and period of Earth’s orbit allows us to measure the mass of the Sun. And knowing the mass of the sun allows us to measure the mass of everything in the Milky Way interior to the orbit of the sun.
Measurement of the gravitational constant
The measure of g was one of the first high-precision science experiments, and scientists are investigating whether it can vary at different times and places in space, which could have big implications for cosmology.
Arriving at a value of 6.67408 x10^–11 m^3 kg^–1 s^–2 for the gravitational constant relied on a rather clever 18th century experiment, prompted by surveyor’s attempts to map the border between the states of Pennsylvania and Maryland (opens in a new tab).
In England, the scientist Henry Cavendish (opens in a new tab) (1731-1810), who was interested in calculating the density of the Earth, realized (opens in a new tab) that the efforts of the surveyor to be doomed (opens in a new tab) because nearby mountains would subject the surveyors’ “plumb line” (a tool that provided a vertical reference line against which surveyors could make their measurements) to a slight gravitational pull, disturbing their readings. If they knew the size of gthey could calculate the gravitational pull of mountains and alter their results.
So Cavendish set out to make the measurement, the most precise scientific measurement ever made in history.
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His experiment was called the “Torsion Balance Technique”. It was two dumbbells that could rotate around the same axis. One of the dumbbells had two small lead spheres connected by a rod and delicately suspended by a fiber. The other dumbbell featured two larger 348-pound (158-kilogram) lead weights that could swing to either side of the smaller dumbbell.
When the larger weights were positioned near the smaller spheres, the gravitational pull of the larger spheres attracted the smaller spheres, causing the fiber to twist. The degree of torsion allowed Cavendish to measure the torque (the turning force) of the torsion system. He then used that value for the torque instead of the ‘F‘ in the equation described above, and with the masses of the weights and their distances, he could rearrange the equation to calculate g.
Can the gravitational constant change?
It is a source of frustration among physicists that “Big G” is not known to as many decimal places as the other fundamental constants. For example, the load of a electron is known to nine decimal places (1.602176634 x 10^–19 coulomb), but g was only measured accurately to within five decimal places. Frustrating efforts to measure it more accurately don’t agree with each other (opens in a new tab).
This is partly because the gravity of objects around the experimental device interferes with the experiment. However, there’s also the lingering suspicion that the problem isn’t just experimental, but that there might be new physics at work (opens in a new tab). It is even possible that the gravitational constant is not as constant as scientists thought.
In the 1960s, physicists Robert Dicke, whose team was led to discover 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 Einsteinit is general theory of relativity. A scalar field describes a property that can potentially vary at different points in space (a Earth analog is a temperature map, where the temperature is not constant, but varies with location). If gravity were a scalar field, then g could potentially have different values in space and time. This differs from the most accepted version of general relativity, which posits that gravity is constant throughout the universe.
Motohiko Yoshimura of Okayama University in Japan has proposed that a scalar-tensor theory of gravity could relate cosmic inflation with dark energy. Inflation occurred within fractions of a second after the birth of the universe and spurred a brief but rapid expansion of space that lasted between 10^–36 and 10^–33 seconds after the birth of the universe. universe. big Bangswelling the cosmos from microscopic to macroscopic in size, before mysteriously fading away.
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Dark energy is the mysterious force that is accelerating the expansion of the universe today. Many physicists have wondered if there could be a connection between the two expansionist forces. Yoshimura suggests that there are – that they are both manifestations of a gravitational scalar field which was a much stronger in the early universethen weakened, but came back strong as the universe expands and matter spreads out.
However, attempts to try to detect any significant variation in g in other parts of the universe have so far found nothing. For example, in 2015, the results of a 21-year study of the regular pulsations of the pulsar RPS J1713+0747 found no evidence (opens in a new tab) for gravity having a different strength compared to here in the solar system. Both Green Bank Observatory and the Arecibo Radio Telescope tracking PSR J1713+0747, which is 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 quickly become apparent in the period of its orbital dance with the white dwarf and the timing of its pulsations.
In a statement (opens in a new tab)Weiwei Zhu of the University of British Columbia, who led the study of PSR J1713+0747, said that “The gravitational constant is a fundamental constant in physics, so it is important to test this basic assumption by using objects at different places, times, and gravitational conditions.The fact that we see gravity acting the same way in our solar system as in a distant star system helps confirm that the gravitational constant is truly universal.
A review of lab tests on gravity (opens in a new tab) conducted by the Eöt-Wash group at the University of Washington.
A review of attempts to measure the “Big G” (opens in a new tab) and what the results might mean.
Britannica’s definition of the gravitational constant (opens in a new tab).
“Precision measurement of the Newtonian gravitational constant (opens in a new tab).” Xue, Chao, et al. National Scientific Review (2020).
“The curious case of the gravitational constant (opens in a new tab).” Proceedings of the National Academy of Sciences (2022).
“Henry Cavendish (opens in a new tab).” Britannica (2022).