Everything about Gravitational totally explained
Gravitation is a natural
phenomenon by which all objects with
mass attract each other, and is one of the
fundamental forces of physics. In everyday life, gravitation is most commonly thought of as the agency that gives objects
weight. It is responsible for keeping the Earth and the other planets in their
orbits around the Sun; for keeping the
Moon in its orbit around the Earth, for the formation of
tides; for
convection (by which hot fluids rise); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the
Earth, the
Sun, and most macroscopic objects in the
universe; without it, matter wouldn't have coalesced into these large masses and
life, as we know it, wouldn't exist.
Modern
physics describes gravitation using the
general theory of relativity, but the much simpler
Newton's law of universal gravitation provides an excellent approximation in most cases.
The terms
gravitation and
gravity are mostly interchangeable in everyday use, but in scientific usage a distinction may be made. "Gravitation" is a general term describing the attractive influence that all objects with mass exert on each other, while "gravity" specifically refers to a
force that's supposed in some theories (such as Newton's) to be the cause of this attraction. By contrast, in
general relativity gravitation is due to spacetime curvatures that cause
inertially moving objects to accelerate towards each other.
History of gravitational theory
Early history
Efforts to understand gravity began in ancient times.
Philosophers in
ancient India explained the phenomenon from the 8th century BC. According to
Kanada, founder of the
Vaisheshika school, "
Weight causes falling; it's and known by
inference."
In the 4th century BC, the
Greek philosopher Aristotle believed that there was no
effect without a
cause, and therefore no
motion without a
force. He hypothesized that everything tried to move towards its proper place in the
crystalline spheres of the heavens, and that physical bodies fell toward the center of the
Earth in proportion to their
weight.
Brahmagupta, in the
Brahmasphuta Siddhanta (AD 628), responded to critics of the
heliocentric system of
Aryabhata (AD 476–550) stating that "all heavy things are attracted towards the center of the earth" and that "all heavy things fall down to the earth by a law of nature, for it's the nature of the earth to attract and to keep things, as it's the nature of water to flow, that of fire to burn, and that of wind to set in motion... The earth is the only low thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth."
In the 9th century, the eldest
Banū Mūsā brother,
Muhammad ibn Musa, in his
Astral Motion and
The Force of Attraction, hypothesized that there was a force of attraction between heavenly bodies, foreshadowing
Newton's law of universal gravitation. In the 1000s, the
Persian scientist Ibn al-Haytham (Alhacen), in the
Mizan al-Hikmah, discussed the theory of attraction between
masses, and it seems that he was aware of the
magnitude of
acceleration due to gravity.
In 1121,
Al-Khazini, in
The Book of the Balance of Wisdom, differentiated between
force,
mass, and
weight, and discovered that gravity varies with the distance from the centre of the Earth,
though he believed that the weight of heavy bodies increase as they're farther from the centre of the Earth. All these early attempts at trying to explain the force of gravity were philosophical in nature and it would be
Isaac Newton that gave the first correct description of
gravity.
Scientific revolution
Modern work on gravitational theory began with the work of
Galileo Galilei in the late 16th century and early 17th century. In his famous (though probably apocryphal) experiment dropping balls from the
Tower of Pisa, and later with careful measurements of balls rolling down
inclines, Galileo showed that gravitation accelerates all objects at the same rate. This was a major departure from Aristotle's belief that heavier objects are accelerated faster. (Galileo correctly postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere.) Galileo's work set the stage for the formulation of Newton's theory of gravity.
Newton's theory of gravitation
In 1687, English mathematician
Sir Isaac Newton published
Principia, which hypothesizes the
inverse-square law of universal gravitation. In his own words, “I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.”
Newton's theory enjoyed its greatest success when it was used to predict the existence of
Neptune based on motions of
Uranus that couldn't be accounted by the actions of the other planets. Calculations by
John Couch Adams and
Urbain Le Verrier both predicted the general position of the planet, and Le Verrier's calculations are what led
Johann Gottfried Galle to the discovery of Neptune.
Ironically, it was another discrepancy in a planet's orbit that helped to point out flaws in Newton's theory. By the end of the 19th century, it was known that the orbit of
Mercury couldn't be accounted for entirely under Newton's theory, but all searches for another perturbing body (such as a planet orbiting the
Sun even closer than Mercury) had been fruitless. The issue was resolved in 1915 by
Albert Einstein's new
General Theory of Relativity, which accounted for the discrepancy in Mercury's orbit.
Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are still made using Newton's theory because it's a much simpler theory to work with than
General Relativity, and gives sufficiently accurate results for most applications.
General relativity
In
general relativity, the effects of gravitation are ascribed to
spacetime curvature instead of a force. The starting point for general relativity is the
equivalence principle, which equates free fall with inertial motion. The issue that this creates is that free-falling objects can accelerate with respect to each other. In
Newtonian physics, no such acceleration can occur unless at least one of the objects is being operated on by a force (and therefore isn't moving inertially).
To deal with this difficulty, Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. (This type of path is called a
geodesic.) More specifically, Einstein discovered the
field equations of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him. The
Einstein field equations are a set of 10
simultaneous,
non-linear,
differential equations. The solutions of the field equations are the components of the
metric tensor of spacetime. A metric tensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metric tensor.
Notable solutions of the Einstein field equations include:
General relativity has enjoyed much success because of how its predictions of phenomena which are not called for by the theory of gravity have been regularly confirmed. For example:
General relativity accounts for the anomalous perihelion precession of Mercury.
The prediction that time runs slower at lower potentials has been confirmed by the Pound-Rebka experiment, the Hafele-Keating experiment, and the GPS.
The prediction of the deflection of light was first confirmed by Arthur Eddington in 1919, and has more recently been strongly confirmed through the use of a quasar which passes behind the Sun as seen from the Earth. See also gravitational lensing.
The time delay of light passing close to a massive object was first identified by Irwin Shapiro in 1964 in interplanetary spacecraft signals.
Gravitational radiation has been indirectly confirmed through studies of binary pulsars.
The expansion of the universe (predicted by Alexander Friedmann) was confirmed by Edwin Hubble in 1929.
Gravity and quantum mechanics
Several decades after the discovery of general relativity it was realized that general relativity is incompatible with quantum mechanics. It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, with the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required. Many believe the complete theory to be string theory, or more currently M Theory.
Specifics
Earth's gravity
Every planetary body (including the Earth) is surrounded by its own gravitational field, which exerts an attractive force on all objects. Assuming a spherically symmetrical planet (a reasonable approximation), the strength of this field at any given point is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.
The strength of the gravitational field is numerically equal to the acceleration of objects under its influence, and its value at the Earth's surface, denoted g, is approximately expressed below as the standard average.
, assuming negligible air resistance.
Gravity and astronomy
The discovery and application of Newton's law of gravity accounts for the detailed information we've about the planets in our solar system, the mass of the Sun, the distance to stars, quasars and even the theory of dark matter. Although we've not traveled to all the planets nor to the Sun, we know their masses. These masses are obtained by applying the laws of gravity to the measured characteristics of the orbit. In space an object maintains its orbit because of the force of gravity acting upon it. Planets orbit stars, stars orbit galactic centers, galaxies orbit a center of mass in clusters, and clusters orbit in superclusters. The force of gravity is proportional to the mass of an object and inversely proportional to the square of the distance between the objects.
Gravitational radiation
In general relativity, gravitational radiation is generated in situations where the curvature of spacetime is oscillating, such as is the case with co-orbiting objects. The gravitational radiation emitted by the solar system is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as PSR 1913+16. It is believed that neutron star mergers and black hole formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as LIGO have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.
Anomalies and discrepancies
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.
Stars on the outskirts of galaxies are moving faster than they should. Also galaxies within galaxy clusters are moving faster than they should. Dark Matter and MOND have both been proposed as explanations.
The expansion of the universe seems to be speeding up. Dark Energy has been proposed to explain this. A recent alternative explanation is that the geometry of space isn't homogeneous (due to clusters of galaxies) and that when the data is reinterpreted to take this into account, the expansion isn't speeding up after all.
The Pioneer spacecraft seem to be slowing down in a way which has yet to be explained.
Various spacecraft have experienced greater accelerations during slingshot maneuvers than expected.
An apparent frame dragging effect has been measured by Martin Tajmar and others which exceeds that predicted by General Relativity by many orders of magnitude.
Alternative theories
Historical alternative theories
Aristotelian theory of gravity
Le Sage's theory of gravitation (1784) also called LeSage gravity, proposed by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire universe.
Nordström's theory of gravitation (1912, 1913), an early competitor of general relativity.
Whitehead's theory of gravitation (1922), another early competitor of general relativity.
Recent alternative theories
Brans-Dicke theory of gravity (1961)
Induced gravity (1967), a proposal by Andrei Sakharov according to which general relativity might arise from quantum field theories of matter.
Rosen bi-metric theory of gravity
In the modified Newtonian dynamics (MOND) (1981), Mordehai Milgrom proposes a modification of Newton's Second Law of motion for small accelerations.
The new and highly controversial Process Physics theory attempts to address gravity
The self-creation cosmology theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation.
Nonsymmetric gravitational theory (NGT) (1994) by John Moffat
Tensor-vector-scalar gravity (TeVeS) (2004), a relativistic modification of MOND by Jacob Bekenstein
Further Information
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