Here's what I know about gravity:
- In physics, gravity (from Latin gravitas 'weight'[1]), also known as gravitation or a gravitational interaction,[2] is a fundamental interaction, which may be described as the effect of a field that is generated by a gravitational source such as mass (seen 20x, conf 1)
- At larger scales this resulted in galaxies and clusters, so gravity is a primary driver for the large-scale structures in the universe (seen 20x, conf 1)
- Gravity has an infinite range, although its effects become weaker as objects get farther away (seen 20x, conf 1)
- Gravity is described by the general theory of relativity, proposed by Albert Einstein in 1915, which describes gravity in terms of the curvature of spacetime, caused by the uneven distribution of mass (seen 20x, conf 1)
- [3] However, for most applications, gravity is sufficiently well approximated by Newton's law of universal gravitation, which describes gravity as an attractive force between any two bodies that is proportional to the product of their masses and inversely proportional to the square of the distance between them (seen 20x, conf 1)
- Scientists are looking for a theory that describes gravity in the framework of quantum mechanics (quantum gravity),[4] which would unify gravity and the other known fundamental interactions of physics in a single mathematical framework (a theory of everything) (seen 20x, conf 1)
- [6] In this context, gravity gives weight to physical objects and is essential to understanding the mechanisms that are responsible for surface water waves, lunar tides and substantially contributes to weather patterns (seen 20x, conf 1)
- Gravity is the word used to describe a physical law, a fundamental physical interaction that derives primarily from mass, and the observed consequences of that interaction on objects (seen 20x, conf 1)
- Gravity is the law that every object with mass attracts every other object in the universe in proportion to each mass and inversely proportional to the square of the distance between them (seen 20x, conf 1)
- The force of gravity, F is written using the gravitational constant, G, as[7]
F
=
G
m
m
′
r
2
{\displaystyle F=G{\frac {mm'}{r^{2}}}}
for two masses, m, and m′ separated by a distance r (seen 20x, conf 1)
- Gravity is considered to be one of four fundamental interactions (seen 20x, conf 1)
- The electromagnetic force law is similar to the force law for gravity: both depend upon the square of the inverse distance between objects in typical interactions (seen 20x, conf 1)
- [7] As a result, gravity can generally be neglected at the level of subatomic particles (seen 20x, conf 1)
- [8] Gravity becomes the most significant interaction between objects at the scale of astronomical bodies, and it determines the motion of satellites, planets, stars, galaxies, and even light (seen 20x, conf 1)
- Gravity is also fundamental in another sense: the inertial mass that appears in Newton's second law is the same as the gravitational mass (seen 20x, conf 1)
- [9]
The nature and mechanism of gravity were explored by a wide range of ancient scholars (seen 20x, conf 1)
- [10] While Aristotle's view was widely accepted throughout Ancient Greece, there were other thinkers such as Plutarch who correctly predicted that the attraction of gravity was not unique to the Earth (seen 20x, conf 1)
- [11]
Although he did not understand gravity as a force, the ancient Greek philosopher Archimedes discovered the center of gravity of a triangle (seen 20x, conf 1)
- [12] He postulated that if two equal weights did not have the same center of gravity, the center of gravity of the two weights together would be in the middle of the line that joins their centers of gravity (seen 20x, conf 1)
- [13] Two centuries later, the Roman engineer and architect Vitruvius contended in his De architectura that gravity is not dependent on a substance's weight but rather on its "nature" (seen 20x, conf 1)
- [15]
In 628 CE, the Indian mathematician and astronomer Brahmagupta proposed the idea that gravity is an attractive force that draws objects to the Earth and used the term gurutvākarṣaṇ to describe it (seen 20x, conf 1)
- [16]: 105 [17][18]
In the ancient Middle East, gravity was a topic of fierce debate (seen 20x, conf 1)
- The Persian intellectual Al-Biruni believed that the force of gravity was not unique to the Earth, and he correctly assumed that other heavenly bodies should exert a gravitational attraction as well (seen 20x, conf 1)
- [21]
The mid-16th century Italian physicist Giambattista Benedetti published papers claiming that, due to specific gravity, objects made of the same material but with different masses would fall at the same speed (seen 20x, conf 1)
- They also calculated the magnitude of the Earth's gravity by measuring the oscillations of a pendulum (seen 20x, conf 1)
- [28]: 5 These concepts would become central to Newton's mechanics, only to be transformed in Einstein's theory of gravity, the general theory of relativity (seen 20x, conf 1)
- These laws were central to the development of a theory of gravity a hundred years later (seen 20x, conf 1)
- [30]
In his 1609 book Astronomia nova Kepler described gravity as a mutual attraction, claiming that if the Earth and Moon were not held apart by some force they would come together (seen 20x, conf 1)
- Borelli developed the idea of mechanical equilibrium, a balance between inertia and gravity (seen 20x, conf 1)
- [32]: 848
In 1657, Robert Hooke published his Micrographia, in which he hypothesized that the Moon must have its own gravity (seen 20x, conf 1)
- [38] However his valuable insights remained hypotheses since he was unable to convert them in to a mathematical theory of gravity and work out the consequences (seen 20x, conf 1)
- This letter likely turned Newton's thinking in a new direction leading to his revolutionary work on gravity (seen 20x, conf 1)
- The revolutionary aspect of Newton's theory of gravity was the unification of Earth-bound observations of acceleration with celestial mechanics (seen 20x, conf 1)
- Newton did not publish these results at the time because he could not prove that the Earth's gravity acts as if all its mass were concentrated at its center (seen 20x, conf 1)
- But this allowed him to come to an astounding conclusion we take for granted today: the gravity of the Earth on the Moon is the same as the gravity of the Earth on an apple:
M
earth
∝
a
apple
R
radius of earth
2
=
a
moon
R
lunar orbit
2
{\displaystyle M_{\text{earth}}\propto a_{\text{apple}}R_{\text{radius of earth}}^{2}=a_{\text{moon}}R_{\text{lunar orbit}}^{2}}
Using the values known at the time, Newton was able to verify this form of his law (seen 20x, conf 1)
- [47]
Einstein's theory brought two other ideas with independent histories into the physical theories of gravity: the principle of relativity and non-Euclidean geometry (seen 20x, conf 1)
- Special relativity, as in special case, specifically did not cover gravity (seen 20x, conf 1)
- [29]: 4
While relativity was associated with mechanics and thus gravity, the idea of altering geometry only joined the story of gravity once mechanics required the Lorentz transformations (seen 20x, conf 1)
- The m in Newton's first law,
F
=
m
a
{\displaystyle F=ma}
, has the same value as the m in Newton's law of gravity on Earth,
F
=
G
M
m
/
r
2
{\displaystyle F=GMm/r^{2}} (seen 20x, conf 1)
- [48] Every description of gravity in any other coordinate system must transform to give no field in the free-fall case, a powerful invariance constraint on all theories of gravity (seen 20x, conf 1)
- [29]: 20
Einstein's description of gravity was accepted by the majority of physicists for two reasons (seen 20x, conf 1)
- By sending the rays down a 74-foot tower and measuring their frequency at the bottom, the scientists confirmed that light is Doppler shifted as it moves towards a source of gravity (seen 20x, conf 1)
- If light moves outward from a strong source of gravity it will be observed with a redshift (seen 20x, conf 1)
- [56]
Frame dragging, the idea that a rotating massive object should twist spacetime around it, was confirmed by Gravity Probe B results in 2011 (seen 20x, conf 1)
- [61] For purposes of weights and measures, a standard gravity value is defined by the International Bureau of Weights and Measures, under the International System of Units (SI) (seen 20x, conf 1)
- The force of gravity experienced by objects on Earth's surface is the vector sum of two forces:[6] (a) The gravitational attraction in accordance with Newton's universal law of gravitation, and (b) the centrifugal force, which results from the choice of an earthbound, rotating frame of reference (seen 20x, conf 1)
- The force of gravity is weakest at the equator because of the centrifugal force caused by the Earth's rotation and because points on the equator are farthest from the center of the Earth (seen 20x, conf 1)
- The force of gravity varies with latitude, and the resultant acceleration increases from about 9 (seen 20x, conf 1)
- [65]
Planets orbit the Sun in an ellipse as a consequence of the law of gravity (seen 20x, conf 1)
- Conceptually two objects in orbit are both falling off of the curve they would travel in if the force of gravity were not pulling them together (seen 20x, conf 1)
- Since the force of gravity is universal, all planets attract each other with the most massive and closest pair have the most mutual affect (seen 20x, conf 1)
- The result can be a neutron star where gravitational attraction balances neutron degeneracy pressure or, for even higher masses, a black hole where gravity operates alone with such intensity that even light cannot escape (seen 20x, conf 1)
- It also opens the way for practical observation and understanding of the nature of gravity and events in the Universe including the Big Bang (seen 20x, conf 1)
- [72]
At the cosmological scale, gravity is a dominant player (seen 20x, conf 1)
- About 5/6 of the total mass in the universe consists of dark matter which interacts through gravity but not through electromagnetic interactions (seen 20x, conf 1)
- [73]
Gravity acts on light and matter equally, meaning that a sufficiently massive object could warp light around it and create a gravitational lens (seen 20x, conf 1)
- [76]
There are some observations that are not adequately accounted for, which may point to the need for better theories of gravity or perhaps be explained in other ways (seen 20x, conf 1)
- The physical models of gravity, like all physical models, are expressed mathematically (seen 20x, conf 1)
- [83]: 44
Newton's inverse square law models gravity as a force F between two objects proportional to their mass, m:
F
12
=
G
m
1
m
2
r
12
2
{\displaystyle F_{12}=G{\frac {m_{1}m_{2}}{{r_{12}}^{2}}}}
This gravitational force causes the objects to accelerate towards each other unless balanced by other forces (seen 20x, conf 1)
- [83]: 4 However, combining the concept of relativity with gravity is enormously complex using this Newtonian model (seen 20x, conf 1)
- [83]: 48
A second equivalent approach to model gravity uses fields (seen 20x, conf 1)
- For classical field theories of gravity, the entities can be vectors associated with points in a 3-dimensional space (seen 20x, conf 1)
- [dubious – discuss] Field models are local: the field values on a sphere completely determine the effects of gravity with the sphere (seen 20x, conf 1)
- [dubious – discuss]
A third completely different way to derive a model of gravity is based on action principles (seen 20x, conf 1)
- This formulation represents the effects of gravity on a system in a mathematically abstract way (seen 20x, conf 1)
- 9
Any theory of gravity must conform to the requirements of special relativity and experimental observations (seen 20x, conf 1)
- Newton's theory of gravity assumes action at a distance and therefore cannot be reconciled with special relativity (seen 20x, conf 1)
- Consequently the effect of gravity can be described as curving spacetime (seen 20x, conf 1)
- [40]: 227
Testing the predictions of general relativity has historically been difficult, because they are almost identical to the predictions of Newtonian gravity for small energies and masses (seen 20x, conf 1)
- [88]: 79 [98]
Despite its success in predicting the effects of gravity at large scales, general relativity is ultimately incompatible with quantum mechanics (seen 20x, conf 1)
- This is because general relativity describes gravity as a smooth, continuous distortion of spacetime, while quantum mechanics holds that all forces arise from the exchange of discrete particles known as quanta (seen 20x, conf 1)
- [99] As a result, researchers have begun to search for a theory that could unite both gravity and quantum mechanics under a more general framework (seen 20x, conf 1)
- [100]
One path is to describe gravity in the framework of quantum field theory (QFT), which has been successful to accurately describe the other fundamental interactions (seen 20x, conf 1)
- The electromagnetic force arises from an exchange of virtual photons, where the QFT description of gravity is that there is an exchange of virtual gravitons (seen 20x, conf 1)
- However, this approach fails at short distances of the order of the Planck length,[103] where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required (seen 20x, conf 1)
- [110] These issues have led to the study of alternative theories of gravity (seen 20x, conf 1)