Portal:Gravitation

From Wikipedia, the free encyclopedia

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 (i.e., launching rockets, projectiles, pendulums, etc.), Newton's formulae are generally preferred.

Steven Weinberg noted that we would still build a suspension bridge with Newton's laws. Newton's theory is much simpler than general relativity, and used very often, so it is usually taught first.

edit

The effects of gravity

Stars in the gravitational field of a globular cluster, M80.

Newton conceived of gravitation when he considered the trajectory of a projectile (say, an apple). 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.

edit

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.

Strictly speaking, this law applies only to point-like objects. If the objects have spatial extent, the true force has to be found by integrating the forces between the various points.

The law expressed as an equation:

$F = G \frac{m_1 m_2}{r^2}$

where:

$F \$ = gravitational force between two objects
$m_1 \$ = mass of first object
$m_2 \$ = mass of second object
$r \$ = distance between the centre of mass of the objects
$G \$ = universal constant of gravitation

edit

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:

$G_{ab} = 8 \pi T_{ab} \$

where $G a b$ is the curvature of spacetime and $T a b$ is the stress-energy within it. Many exact solutions of the Einstein field equations are known. The solutions to the field equations are metrics of spacetime. These metrics describe the structure of spacetime given the stress-energy and coordinate mapping used to obtain that solution.

edit

Did you know

...That black holes were first proposed in 1783?

edit

Free-fall

Astronaut in motion above Earth's limb. The atmosphere can be glimpsed as a fuzzy layer above the enormous mass of Earth. Space, a region with far less air, is the dark background behind the astronaut and the earth.

Both the astronaut and the Earth are in free-fall.

edit

Mass

We can distinguish several meanings for mass in physics:

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 $E / c 2$ , where E is total energy of the object and c is speed of light. 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.

edit

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 for a time t:

$\ d={gt^2\over 2}$

He used a wood molding, "12 cubits long, half a cubit wide and three finger-breadths thick" as a ramp with a straight, smooth, polished groove to study rolling balls ("a hard, smooth and very round bronze ball"). He lined the groove with "parchment, also smooth and polished as possible". He inclined the ramp at various angles, effectively slowing down the acceleration enough so that he could measure the elapsed time. He would let the ball roll a known distance down the ramp, and used a water clock to measure the time taken to move the known distance; this clock was

"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.".

edit

Things you can do

An experiment is a set of actions and observations, performed to verify or falsify a hypothesis or research a causal relationship between phenomena. The experiment is a cornerstone in empirical approach to knowledge. See the list of famous experiments for historically important scientific experiments.

edit

Wikiprojects

WikiProject Physics

edit

Applications

edit

Newton's reservations

It's important to understand that while Newton was able to formulate his law of gravity in his monumental work, he was deeply uncomfortable with the notion of "action at a distance" which his equations implied. He never, in his words, "assigned the cause of this power". In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity. Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science. He lamented the fact that "philosophers have hitherto attempted the search of nature in vain" for the source of the gravitational force, as he was convinced "by many reasons" that there were "causes hitherto unknown" that were fundamental to all the "phenomena of nature". Newton himself wrote in a letter to Dr Bentley dated 25 February 1693:

It is inconceivable that inanimate brute matter should, without the mediation of something else, which is not material, operate upon, and effect other matter without mutual contact, as it must be if gravitation in the sense of Epicurus be essential and inherent in it. And this is one reason why I desired you would not ascribe innate gravity to me. That gravity should be innate, inherent and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me a great absurdity, and I believe that no man who has in philosophical matters a competent faculty of thinking, can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial I have left to the consideration of my reader.

edit