Gravity

What Is Gravity?

Gravity is the fundamental interaction that attracts masses toward one another, governing the motion of planets, stars, and galaxies as well as the behavior of objects near Earth's surface. Isaac Newton formalized gravity as a long-range attractive force between any two masses in his 1687 work Philosophiae Naturalis Principia Mathematica, expressing it as F = Gm₁m₂/r², where G is the Newtonian gravitational constant and r is the separation between the masses. Albert Einstein's general theory of relativity (1915) recast gravity as a geometric property of curved spacetime: mass and energy deform the metric of four-dimensional spacetime, and other masses move along the resulting geodesics. Both descriptions are in use in engineering: Newton's formulation suffices for terrestrial and near-Earth applications, while general relativity is required for precision navigation, gravitational wave physics, and strong-field astrophysics.

Gravity is the weakest of the four fundamental forces but has infinite range and affects all matter, making it the dominant force at the scales of planets, stars, and the large-scale structure of the universe. Its weakness relative to the other forces also makes the gravitational constant G the most poorly known of the fundamental constants, with a relative uncertainty of approximately 4.7 × 10⁻⁵, orders of magnitude larger than the uncertainties of constants governing electromagnetism and the nuclear forces.

Newtonian Gravitation and the Gravitational Constant

Newton's inverse-square law describes the gravitational attraction between point masses to excellent approximation across the range of everyday engineering problems, including orbital mechanics, geodesy, and geophysical surveying. The proportionality constant G, approximately 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻², was first estimated by Henry Cavendish in 1798 using a torsion balance to measure the attraction between lead spheres. NIST continues active efforts to refine the value: as detailed in NIST publications on measurements of the Newtonian constant of gravitation G, more than a dozen independent precision measurements conducted over three decades still yield results that disagree beyond their stated uncertainties, indicating either unresolved systematic errors or, less likely, new physics. The unresolved discrepancies motivate an international consortium of metrology laboratories to coordinate experimental methods.

General Relativistic Effects

Where Newtonian gravity suffices for most engineering calculations, general relativity becomes necessary when velocities approach the speed of light or when fields are strong. The Global Positioning System (GPS) provides a practical example: GPS satellite clocks run faster by approximately 38 microseconds per day due to a combination of gravitational time dilation (clocks in weaker gravity run faster) and special relativistic time dilation (clocks in motion run slower), and the system's software must correct for both effects to maintain meter-level positioning accuracy. Gravitational lensing, the bending of light paths by massive objects predicted by general relativity and confirmed during the 1919 solar eclipse, now serves as a tool for mapping dark matter distributions and detecting exoplanets through microlensing.

Gravimetric Sensing and Geophysical Applications

Measuring spatial variations in the gravitational field, a practice known as gravimetry, reveals subsurface mass distributions without drilling or excavation. NIST guidance on how to measure the strength of gravity describes the instruments and methods used, from classical pendulum clocks to modern superconducting gravimeters and atom interferometers capable of resolving variations in g at the parts-per-billion level. Airborne and satellite gravimetry, exemplified by the GRACE satellite pair that tracked gravitational changes due to melting ice sheets and groundwater depletion, extend these measurements to global scales. A PMC review of precision measurements of the Newtonian gravitational constant surveys the experimental techniques and remaining uncertainties that connect laboratory physics to geophysical and cosmological applications.

Applications

Gravity has applications in a wide range of fields, including: