In
physics,
theories of
gravitation postulate mechanisms of interaction governing
the movements of bodies with mass. There have been numerous
theories of
gravitation since ancient
times.
Antiquity
In the
4th century BC, the
Greek philosopher
Aristotle believed that there is no
effect or
motion
without a
cause. The cause of the downward
motion of heavy bodies, such as the element
earth, was related to their
nature, which caused them to
move downward toward the center of the universe, which was their
natural place. Conversely, light bodies such as the element
fire, move by their nature
upward toward the inner surface of the
sphere of the Moon. Thus in Aristotle's
system heavy bodies are not attracted to the earth by an external
force of gravity, but tend toward the center of the universe
because of an inner
gravitas or heaviness.
Middle Ages
The
Indian astronomer
Brahmagupta, in his
Brahmasphuta Siddhanta ("
The
Opening of the Universe") (
628), recognized
gravity as a force of attraction. Brahmagupta followed the
heliocentric solar
system of gravitation, earlier developed by
Aryabhata in 499, and understood that there was a
force of attraction between the Sun and the Earth. The 11th century
Persian astronomer
Abu al-Rayhan
al-Biruni, in his
Ta'rikh al-Hind, later translated
into
Latin as
Indica, commented on
their works and wrote that critics refuting Aryabhata's
heliocentric system argued:
According to Biruni, Brahmagupta responded to these criticisms with
the following argument:
The
Sanskrit term Brahmagupta used for
gravity,
gruhtvaakarshan, phonetically similar to the
English 'gravity', had roughly the same meaning as
"attraction".
Al-Biruni himself described the Earth's
gravitation as:
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,
Ibn al-Haytham (Alhacen), a contemporary of
Biruni, 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 claimed that gravity varies with the
distance from the centre of the Earth,though he believed that the
weight of heavy bodies increased as they moved farther from the
centre of the Earth.
These early attempts at explaining gravity were largely
philosophical concepts and were neither given proper scientific
treatment nor regularly verified by experimentation. It would not
be until
Isaac Newton that the force of
gravity was given proper
scientific treatment and an accurate mathematical
expression upon which a correct description of
gravity can be deduced.
Modern Era
Before 1543 in
De
revolutionibus orbium coelestium Copernicus wrote :
"...inter centrum
gravitatis terrae, & centrum magnitudis..."
During the
17th century,
Galileo found that, counter to Aristotle's
teachings, all objects accelerated equally when falling.
In the
1660s, influenced by the ideas of
Alkindus,
Robert
Hooke explained his law of
celestial gravity:
In the late
17th century, as a result
of Robert Hooke's suggestion that there is a
gravitational force which depends on the
inverse square of the
distance,
Isaac Newton
was able to
mathematically derive
Kepler's three
kinematic laws of planetary motion,
including the
elliptical orbits for the seven known
planets:
So Newton's original formula was:
- {\rm Force\,of\,gravity} \propto \frac{\rm
mass\,of\,object\,1\,\times\,mass\,of\,object\,2}{\rm
distance\,from\,centers^2}
where the symbol \propto means "is proportional to".
To make this into an equal-sided formula or equation, there needed
to be a multiplying factor or constant that would give the correct
force of gravity no matter the value of the masses or distance
between them. This
gravitational
constant was first measured in 1797 by
Henry Cavendish.
In
1907 Albert
Einstein, in what was described by him as "
the happiest
thought of my life", realized that an observer who is falling
from the roof of a house experiences no gravitational field. In
other words, gravitation was exactly equivalent to
acceleration. Between
1911
and
1915 this idea, initially stated as the
Equivalence principle, was
formally developed into Einstein's theory of
general relativity.
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 could not 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 doom Newton's theory. By the end of the 19th century, it
was known that the orbit of
Mercury
could not be accounted for entirely under Newton's theory, and all
searches for another perturbing body (such as a planet orbiting the
Sun even closer than Mercury) have been
fruitless. This issue was resolved in 1915 by
Albert Einstein's new
general relativity theory. This theory
accounted for the discrepancy in Mercury's orbit.
Although Newton's theory has been superseded, most modern
non-relativistic gravitational calculations are based on Newton's
work because it is a much easier theory to work with and sufficient
for most applications.
Mechanical explanations of gravitation
The
mechanical theories or explanations of
the
gravitation are attempts to explain
the law of gravity by aid of basic mechanical processes, such as
pushes, and without the use of any
action at a distance. These
theories were developed from the 16th until the 19th century in
connection with the
aether
theories.
René Descartes (1644) and
Christiaan Huygens (1690) used
vortices to explain gravitation.
Robert Hooke (1671) and
James Challis (1869) assumed, that every body
emits waves which lead to an attraction of other bodies.
Nicolas Fatio de Duillier (1690)
and
Georges-Louis Le Sage
(1748) proposed a
corpuscular model, using
some sort of screening or shadowing mechanism. Later a similar
model was created by
Hendrik
Lorentz, who used
electromagnetic radiation instead
of the corpuscles.
Isaac Newton (1675)
and
Bernhard Riemann (1853) argued
that aether streams carry all bodies to each other.Newton (1717)
and
Leonhard Euler (1760) proposed a
model, in which the aether loses density near the masses, leading
to a net force directing to the bodies.
Lord
Kelvin (1871) proposed that every body pulsates, which might be
an explanations of gravitation and the
electric charges.
However, those models were overthrown because most of them lead to
an unacceptable amount of
drag, which
is not observed. Other models are violating the
energy conservation law and are
incompatible with modern
thermodynamics.
General relativity
In
general
relativity, the effects of gravitation are ascribed to
spacetime curvature instead of to 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 is not 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 and Hilbert 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:
Gravity and quantum mechanics
Several decades after the discovery of general relativity it was
realized that it cannot be the complete theory of gravity because
it is incompatible with
quantum
mechanics. Later it was understood that it is possible to
describe gravity in the framework of
quantum field theory like the other
fundamental forces. In this
framework 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.
It is notable that in general relativity, gravitational radiation,
which under the rules of quantum mechanics must be composed of
gravitons, is created only in situations where the curvature of
spacetime is oscillating, such as is the case with co-orbiting
objects. The amount of 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.
References
- Edward Grant, The Foundations of Modern Science in the
Middle Ages, (Cambridge: Cambridge Univ. Pr., 1996), pp.
60-1.
- Olaf Pedersen, Early Physics and Astronomy,
(Cambridge: Cambridge Univ. Pr., 1993), p. 130
- Khwarizm, Foundation for Science Technology and
Civilisation.
- K. A. Waheed (1978). Islam and The Origins of Modern
Science, p. 27. Islamic Publication Ltd., Lahore.
- Robert
Briffault (1938). The Making of Humanity, p. 191.
- Dr. Nader El-Bizri, "Ibn al-Haytham or Alhazen", in Josef W.
Meri (2006), Medieval Islamic Civilization: An
Encyclopaedia, Vol. II, p. 343-345, Routledge, New York, London.
- Donald Routledge Hill (1993),
Islamic Science and Engineering, p. 61, Edinburgh University Press.
(cf. Salah Zaimeche PhD (2005),
Merv, p. 5, Foundation for Science Technology and
Civilization.)
- Professor Mohammed Abattouy (2002). "The Arabic Science of
weights: A Report on an Ongoing Research Project", The Bulletin
of the Royal Institute for Inter-Faith Studies
4, p. 109-130.
- Asghar Qadir (1989). Relativity: An Introduction to the
Special Theory, p. 6-11. World Scientific, Singapore.