Gravitation portal
The first mathematical formulation of gravity was published in 1687 by Sir Isaac Newton. His law of universal gravitation was the standard theory of gravity until work by Albert Einstein
and others on general relativity. Since calculations in general
relativity are complicated, and Newtonian gravity is sufficiently
accurate for most applications, when dealing with weak gravitational
fields (e.g., when launching rockets, projectiles, or swinging pendulums, etc.), Newton's formulas are generally preferred.
Steven Weinberg noted that we could still build a suspension bridge with Newton's laws. Newton's theory is much simpler in mathematical structure than general relativity, and is used very often. Newton's Laws are generally taught at the high school level while general relativity is taught only to students opting for physics in undergraduate and graduate courses.
Quantum gravity is the attempt to unify the theories of Gravitation and Quantum mechanics in a self-consistent manner, or more precisely, to formulate a self-consistent theory which reduces to ordinary quantum mechanics in the limit of weak gravity (potentials much less than c2) and which reduces to Einsteinian general relativity in the limit of large actions.
Steven Weinberg noted that we could still build a suspension bridge with Newton's laws. Newton's theory is much simpler in mathematical structure than general relativity, and is used very often. Newton's Laws are generally taught at the high school level while general relativity is taught only to students opting for physics in undergraduate and graduate courses.
Quantum gravity is the attempt to unify the theories of Gravitation and Quantum mechanics in a self-consistent manner, or more precisely, to formulate a self-consistent theory which reduces to ordinary quantum mechanics in the limit of weak gravity (potentials much less than c2) and which reduces to Einsteinian general relativity in the limit of large actions.
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The effects of gravity
Newton conceived of gravitation when he considered the trajectory of a
projectile . A projectile under the influence of gravity travels along a
trajectory that is a conic section. The projectile follows either an elliptical path or a hyperbolic path, depending on whether its total mechanical energy is less than or greater than that necessary for escape velocity,
respectively. In the pathological case where the projectile's total
mechanic energy is exactly equal to that necessary for escape velocity,
the projectile follows a parabolic
trajectory. At low speeds and over small distances (small enough that
the surface of the Earth can be considered flat), the elliptical
trajectory of a projectile can be more easily approximated as a
parabolic trajectory.
When Newton heard the sound of an apple falling on the ground, he asked himself might the same cause (which he called gravitation) for the motion of the apple, also explain the motion of the moon?. This was the first statement of the universal law of gravitation.
When Newton heard the sound of an apple falling on the ground, he asked himself might the same cause (which he called gravitation) for the motion of the apple, also explain the motion of the moon?. This was the first statement of the universal law of gravitation.
Newton's concept
- Every object in the Universe attracts every other object with a force directed along the line of centers for the two objects that is proportional to the product of their masses and inversely proportional to the square of the separation between the two objects. (See also inverse-square law.)
- Two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The law expressed as an equation:
- = gravitational force between two objects
- = mass of first object
- = mass of second object
- = distance between the center of mass of the objects
- = universal constant of gravitation
The Gravitational equation that is given above is similar to that of Coulomb's Law which deals with the electrostatic force between two charged particles.
Einstein's concept
Einstein's theory of general relativity (1915) stated that the presence of mass, energy, and momentum causes spacetime to become curved. Because of this curvature, the paths that objects in inertial motion
follow can "deviate" or change direction over time. This deviation
appears to us as an acceleration towards massive objects, which Newton
characterized as being gravity. In general relativity however, this
acceleration or free fall
is actually inertial motion. So objects in a gravitational field appear
to fall at the same rate due to their being in inertial motion while
the observer is the one being accelerated. (This identification of free
fall and inertia is known as the Equivalence principle.)
The Einstein field equations can be written briefly in abstract index notation as rank 2 symmetric tensors:
The Einstein field equations can be written briefly in abstract index notation as rank 2 symmetric tensors:
Did you know
...That black holes were first proposed in 1783?
...That the ergosphere is a region located outside of a rotating black hole?
...That in theoretical physics, a kugelblitz is a concentration of light so intense that it forms an event horizon and becomes self-trapped?
...That the concept of anti-gravity was first introduced in H. G. Wells' The First Men in the Moon?
...That the mass of the black hole in GRO J0422+32 is the smallest yet found for any black hole?
...That the ergosphere is a region located outside of a rotating black hole?
...That in theoretical physics, a kugelblitz is a concentration of light so intense that it forms an event horizon and becomes self-trapped?
...That the concept of anti-gravity was first introduced in H. G. Wells' The First Men in the Moon?
...That the mass of the black hole in GRO J0422+32 is the smallest yet found for any black hole?
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Where to start
The Law of Falling Bodies
Galileo was the first to demonstrate and then formulate the equation for the distance d traveled by a falling object under the influence of gravity g for a time t:
Galileo was the first to demonstrate and then formulate the equation for the distance d traveled by a falling object under the influence of gravity g for a time t:
- "a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent, whether for the whole length of the channel or for a part of its length; the water thus collected was weighed, after each descent, on a very accurate balance; the differences and ratios of these weights gave us the differences and ratios of the times, and this with such accuracy that although the operation was repeated many, many times, there was no appreciable discrepancy in the results.".
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Key Topics
Main concepts |
General Relativity Related |
Mass
- Inertial mass is a measure of an object's inertia: its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily. Now we know that this mass is actually a measure of the total energy of the object and is equal , where E is total energy of the object and c is speed of light in a vacuum. Not only inertial but also all gravitational effects regarding this object are proportional to the object's inertial mass and that's why it is exactly the same as "gravitational mass", "passive" and "active". Before discovery of Einstein's theory of gravitation the "gravitational masses" were considered separate physical entities.
- Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller gravitational force than an object with a larger passive gravitational mass. This force is called the weight of the object. In informal usage, the word "weight" is often used synonymously with "mass", because the strength of the gravitational field is roughly constant everywhere on the surface of the Earth. In physics, the two terms are distinct: an object will have a larger weight if it is placed in a stronger gravitational field, but its passive "gravitational mass" (inertial mass) remains unchanged.
- Active gravitational mass is a measure of the strength of the
gravitational field due to a particular object. For example, the
gravitational field that one experiences on the Moon is weaker than that
of the Earth because the Moon has less "active gravitational mass" than
the Earth.
- Note that Einstein wrote, in his paper on General Relativity that inertial mass and gravitational mass must be equivalent. "We see that our extension of the principle of relativity implies the necessity of the law of the equality of inertial and gravitational mass."-Einstein (1916)
- The amount of matter in an object.
- A side-effect of the interaction between Higgs' Bosons.
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