Inphysical science, aninverse-square law is anyscientific law stating that the observed "intensity" of a specifiedphysical quantity (being nothing more than the value of the physical quantity) isinversely proportional to thesquare of thedistance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space.
Radar energy expands during both the signal transmission and thereflected return, so the inverse square for both paths means that the radar will receive energy according to the inversefourth power of the range.
To prevent dilution ofenergy while propagating a signal, certain methods can be used such as awaveguide, which acts like a canal does for water, or how a gun barrel restricts hot gas expansion to onedimension in order to prevent loss of energy transfer to abullet.
In mathematical notation the inverse square law can be expressed as an intensity (I) varying as a function of distance (d) from some centre. The intensity is proportional (see∝) to the reciprocal of the square of the distance thus:
It can also be mathematically expressed as :
or as the formulation of a constant quantity:
Thedivergence of avector field which is the resultant of radial inverse-square law fields with respect to one or more sources is proportional to the strength of the local sources, and hence zero outside sources.Newton's law of universal gravitation follows an inverse-square law, as do the effects ofelectric,light,sound, andradiation phenomena.
The inverse-square law generally applies when some force, energy, orother conserved quantity is evenly radiated outward from apoint source inthree-dimensional space. Since thesurface area of asphere (which is 4πr2) is proportional to the square of the radius, as theemitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source.Gauss's law for gravity is similarly applicable, and can be used with any physical quantity that acts in accordance with the inverse-square relationship.
Gravitation is the attraction between objects that have mass. Newton's law states:
The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance. The force is always attractive and acts along the line joining them.[1]
If the distribution of matter in each body is spherically symmetric, then the objects can be treated as point masses without approximation, as shown in theshell theorem. Otherwise, if we want to calculate the attraction between massive bodies, we need to add all the point-point attraction forces vectorially and the net attraction might not be exact inverse square. However, if the separation between the massive bodies is much larger compared to their sizes, then to a good approximation, it is reasonable to treat the masses as a point mass located at the object'scenter of mass while calculating the gravitational force.
As the law of gravitation, thislaw was suggested in 1645 byIsmaël Bullialdus. But Bullialdus did not acceptKepler's second and third laws, nor did he appreciateChristiaan Huygens's solution for circular motion (motion in a straight line pulled aside by the central force). Indeed, Bullialdus maintained the sun's force was attractive at aphelion and repulsive at perihelion.Robert Hooke andGiovanni Alfonso Borelli both expounded gravitation in 1666 as an attractive force.[2] Hooke's lecture "On gravity" was at the Royal Society, in London, on 21 March.[3] Borelli's "Theory of the Planets" was published later in 1666.[4] Hooke's 1670 Gresham lecture explained that gravitation applied to "all celestiall bodys" and added the principles that the gravitating power decreases with distance and that in the absence of any such power bodies move in straight lines. By 1679, Hooke thought gravitation had inverse square dependence and communicated this in a letter toIsaac Newton:[5]my supposition is that the attraction always is in duplicate proportion to the distance from the center reciprocall.[6]
Hooke remained bitter about Newton claiming the invention of this principle, even though Newton's 1686Principia acknowledged that Hooke, along with Wren and Halley, had separately appreciated the inverse square law in theSolar System,[7] as well as giving some credit to Bullialdus.[8]
The force of attraction or repulsion between two electrically charged particles, in addition to being directly proportional to the product of the electric charges, is inversely proportional to the square of the distance between them; this is known asCoulomb's law. The deviation of the exponent from 2 is less than one part in 1015.[9]
Theintensity (orilluminance orirradiance) oflight or other linear waves radiating from apoint source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source, so an object (of the same size) twice as far away receives only one-quarter theenergy (in the same time period).
More generally, the irradiance,i.e., the intensity (orpower per unit area in the direction ofpropagation), of asphericalwavefront varies inversely with the square of the distance from the source (assuming there are no losses caused byabsorption orscattering).
For example, the intensity of radiation from theSun is 9126watts per square meter at the distance ofMercury (0.387AU) but only 1367 watts per square meter at the distance ofEarth (1 AU)—an approximate threefold increase in distance results in an approximate ninefold decrease in intensity of radiation.
For non-isotropic radiators such asparabolic antennas, headlights, andlasers, the effective origin is located far behind the beam aperture. If you are close to the origin, you don't have to go far to double the radius, so the signal drops quickly. When you are far from the origin and still have a strong signal, like with a laser, you have to travel very far to double the radius and reduce the signal. This means you have a stronger signal or haveantenna gain in the direction of the narrow beam relative to a wide beam in all directions of anisotropic antenna.
Inphotography andstage lighting, the inverse-square law is used to determine the “fall off” or the difference in illumination on a subject as it moves closer to or further from the light source. For quick approximations, it is enough to remember that doubling the distance reduces illumination to one quarter;[10] or similarly, to halve the illumination increase the distance by a factor of 1.4 (thesquare root of 2), and to double illumination, reduce the distance to 0.7 (square root of 1/2). When the illuminant is not a point source, the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%.[11]
The fractional reduction in electromagneticfluence (Φ) for indirectly ionizing radiation with increasing distance from a point source can be calculated using the inverse-square law. Since emissions from a point source have radial directions, they intercept at a perpendicular incidence. The area of such a shell is 4πr2 wherer is the radial distance from the center. The law is particularly important in diagnosticradiography andradiotherapy treatment planning, though this proportionality does not hold in practical situations unless source dimensions are much smaller than the distance. As stated inFourier theory of heat “as the point source is magnification by distances, its radiation is dilute proportional to the sin of the angle, of the increasing circumference arc from the point of origin”.
LetP be the total power radiated from a point source (for example, an omnidirectionalisotropic radiator). At large distances from the source (compared to the size of the source), this power is distributed over larger and larger spherical surfaces as the distance from the source increases. Since the surface area of a sphere of radiusr isA = 4πr 2, theintensityI (power per unit area) of radiation at distancer is
The energy or intensity decreases (divided by 4) as the distancer is doubled; if measured indB would decrease by 6.02 dB per doubling of distance. When referring to measurements of power quantities, a ratio can be expressed as a level in decibels by evaluating ten times the base-10 logarithm of the ratio of the measured quantity to the reference value.
Inacoustics, thesound pressure of asphericalwavefront radiating from a point source decreases by 50% as the distancer is doubled; measured indB, the decrease is still 6.02 dB, since dB represents an intensity ratio. The pressure ratio (as opposed to power ratio) is not inverse-square, but is inverse-proportional (inverse distance law):
The same is true for the component ofparticle velocity that isin-phase with the instantaneous sound pressure:
In thenear field is aquadrature component of the particle velocity that is 90° out of phase with the sound pressure and does not contribute to the time-averaged energy or the intensity of the sound. Thesound intensity is the product of theRMS sound pressure and thein-phase component of the RMS particle velocity, both of which are inverse-proportional. Accordingly, the intensity follows an inverse-square behaviour:
For anirrotational vector field in three-dimensional space, the inverse-square law corresponds to the property that thedivergence is zero outside the source. This can be generalized to higher dimensions. Generally, for an irrotational vector field inn-dimensionalEuclidean space, the intensity "I" of the vector field falls off with the distance "r" following the inverse (n − 1)th power law
given that the space outside the source is divergence free.[citation needed]
The inverse-square law, fundamental inEuclidean spaces, also applies tonon-Euclidean geometries, includinghyperbolic space. The curvature present in these spaces alters physical laws, influencing a variety of fields such ascosmology,general relativity, andstring theory.[12]
John D. Barrow, in his 2020 paper "Non-Euclidean Newtonian Cosmology," expands on the behavior of force (F) and potential (Φ) within hyperbolic 3-space (H3). He explains that F and Φ obey the relationships F ∝ 1 / R² sinh²(r/R) and Φ ∝ coth(r/R), where R represents the curvature radius and r represents the distance from the focal point.
The concept of spatial dimensionality, first proposed by Immanuel Kant, remains a topic of debate concerning the inverse-square law.[13] Dimitria Electra Gatzia and Rex D. Ramsier, in their 2021 paper, contend that the inverse-square law is more closely related to force distribution symmetry than to the dimensionality of space.
In the context of non-Euclidean geometries and general relativity, deviations from the inverse-square law do not arise from the law itself but rather from the assumption that the force between two bodies is instantaneous, which contradictsspecial relativity. General relativity reinterprets gravity as the curvature of spacetime, leading particles to move along geodesics in this curved spacetime.[14]
John Dumbleton of the 14th-centuryOxford Calculators, was one of the first to express functional relationships in graphical form. He gave a proof of themean speed theorem stating that "the latitude of a uniformly difform movement corresponds to the degree of the midpoint" and used this method to study the quantitative decrease in intensity of illumination in hisSumma logicæ et philosophiæ naturalis (ca. 1349), stating that it was not linearly proportional to the distance, but was unable to expose the Inverse-square law.[15]
In proposition 9 of Book 1 in his bookAd Vitellionem paralipomena, quibus astronomiae pars optica traditur (1604), the astronomerJohannes Kepler argued that the spreading of light from a point source obeys an inverse square law:[16][17]
Sicut se habent spharicae superificies, quibus origo lucis pro centro est, amplior ad angustiorem: ita se habet fortitudo seu densitas lucis radiorum in angustiori, ad illamin in laxiori sphaerica, hoc est, conversim. Nam per 6. 7. tantundem lucis est in angustiori sphaerica superficie, quantum in fusiore, tanto ergo illie stipatior & densior quam hic.
Just as [the ratio of] spherical surfaces, for which the source of light is the center, [is] from the wider to the narrower, so the density or fortitude of the rays of light in the narrower [space], towards the more spacious spherical surfaces, that is, inversely. For according to [propositions] 6 & 7, there is as much light in the narrower spherical surface, as in the wider, thus it is as much more compressed and dense here than there.
In 1645, in his bookAstronomia Philolaica ..., the French astronomerIsmaël Bullialdus (1605–1694) refuted Johannes Kepler's suggestion that "gravity"[18] weakens as the inverse of the distance; instead, Bullialdus argued, "gravity" weakens as the inverse square of the distance:[19][20]
Virtus autem illa, qua Sol prehendit seu harpagat planetas, corporalis quae ipsi pro manibus est, lineis rectis in omnem mundi amplitudinem emissa quasi species solis cum illius corpore rotatur: cum ergo sit corporalis imminuitur, & extenuatur in maiori spatio & intervallo, ratio autem huius imminutionis eadem est, ac luminus, in ratione nempe dupla intervallorum, sed eversa.
As for the power by which the Sun seizes or holds the planets, and which, being corporeal, functions in the manner of hands, it is emitted in straight lines throughout the whole extent of the world, and like the species of the Sun, it turns with the body of the Sun; now, seeing that it is corporeal, it becomes weaker and attenuated at a greater distance or interval, and the ratio of its decrease in strength is the same as in the case of light, namely, the duplicate proportion, but inversely, of the distances [that is, 1/d²].
In England, the Anglican bishopSeth Ward (1617–1689) publicized the ideas of Bullialdus in his critiqueIn Ismaelis Bullialdi astronomiae philolaicae fundamenta inquisitio brevis (1653) and publicized the planetary astronomy of Kepler in his bookAstronomia geometrica (1656).
In 1663–1664, the English scientistRobert Hooke was writing his bookMicrographia (1666) in which he discussed, among other things, the relation between the height of the atmosphere and the barometric pressure at the surface. Since the atmosphere surrounds the Earth, which itself is a sphere, the volume of atmosphere bearing on any unit area of the Earth's surface is a truncated cone (which extends from the Earth's center to the vacuum of space; obviously only the section of the cone from the Earth's surface to space bears on the Earth's surface). Although the volume of a cone is proportional to the cube of its height, Hooke argued that the air's pressure at the Earth's surface is instead proportional to the height of the atmosphere because gravity diminishes with altitude. Although Hooke did not explicitly state so, the relation that he proposed would be true only if gravity decreases as the inverse square of the distance from the Earth's center.[21][22]
Newton went up to independently develop and derive the inverse-square law for gravity in hisPrincipia (1686). This later led to apriority debate between Newton and Hooke through correspondence.[23]
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 22 January 2022.
I say aCylinder, not a piece of aCone, because, as I may elsewhere shew in the Explication of Gravity, thattriplicate proportion of the shels of a Sphere, to their respective diameters, I suppose to be removed in this case by the decrease of the power of Gravity.