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Gravity


Gravity is the force of nature by which all object in the universe attract all other objects in the universe. This universal force is what keeps us on the Earth, without it we would fly off into space at hundreds of  miles an hour. Gravity dominates the universe on the large scale being the binding force of planetary systems, galaxies and galactic clusters.

Gravity is the first force of nature to be discovered  the theory of which was published in 1687 by Sir Isaac Newton. However it was first scientifically studied by Galileo Galilee. While gravity is one of five fundamental forces of nature it is unique in a number of ways. Despite its dominance on large scales gravity is actually the weakest of the five forces being about 1/1036th the strength of electromagnetism which is why a small magnet can counter the Earth's gravity. It is also the only one of the five forces that shows no evidence of having a mediating particle being most successfully explained as a stretching  space-time.

While the theory of Gravity was published by Sir Isaac Newton to fully understand the history behind it requires going back before Newton. Though they did not call it gravity his predecessors laid the ground work for the Newtonian theory of gravity. These predecessors include Aristotle, Galileo Galilee and Johannes Kepler each provided peaces of the puzzle but Newton put them together.


Aristotle

Aristotle was a Greek philosopher that lived from 384 BC to 322 BC and earliest recorded person to speculated about physics. Unfortunately he never really tested his philosophy based physics yet it was accepted without question  based on his authority for about 2,000 years.

The Aristotelian explained gravity by imagining that all bodies move toward their natural place. In his physics he imagined four elements earth, water, air and fire. The natural place of  earth, was the center of the universe which was considered to be the Earth. The natural place of water was seen as a concentric shell around earth. Air's natural place was then seen as a concentric shell around the natural place of water bordering at sea level. The natural place of fire was seen as a concentric shell around  the air. 

As a result terrestrial objects would tend to move towards different parts of the universe and with a speed based on their composition of the four elements. Earth being considered  the heaviest element, objects with more earth would fall faster than objects with less Earth and even rise if the were made mainly of air and fire. As a result Aristotle reasoned heavier objects fall faster then lighter objects but he never tested it.

While Aristotle was ultimately proven wrong it took 2,000 years simply because no one bothered to test it. Aristotelian physics did however serve as a starting point that ultimately lead to the beginnings of science.


Galileo Galilee

Galileo Galilee can be considered the father of the science of physics and in many ways what we today call science. Galileo lived from1564 to 1642 and he championed both experimentation and mathematics which are foundational to modern science.

For the record his problems with the Catholic church resulted mainly from the fact that he was seen as a challenge to the authority of the Roman Catholic church rather than heliocentrism itself. The historical context of Galileo's persecution by the Roman Catholic church is often ignored but it is needed to fully understand the event. His trial by the Roman Catholic church occurred as the Protestant Reformation was in full swing having been started by Martin Luther in 1517. It occurred after he attacked Pope Urban VIII in his book Dialogue Concerning the Two Chief World Systems, the Pope saw Galileo as a threat to what was left of his power and so he was tried and found guilty of heresy by the inquisition in 1615.

While Galileo is most known for his support for heliocentrism he made the breakthrough that eventually destroyed Aristotelian physics. He did so by doing something that does not seem to have been in the nearly 2,000 years since Aristotle lived. He put the idea to the test and showed that objects of different weight fall at the same rate.

Galileo made this breakthrough by considering that two objects of the same weight would fall at the same rate. This would be the cased even according to Aristotelian physics.

Like wise three objects of the same weight would fall at the same rate. This would also be the cased according to Aristotelian physics.

Further more the three objects of the same weight would fall at the same rate even if two were attached. This would be the cased as well according to Aristotelian physics.

Galileo concluded that this would be identical to two objects of different weights falling together. He correctly concluded that objects fall at the same rate regardless of their weight.  This meant that Aristotle was wrong and gave him a way to test between the two views. To test this Galileo is commonly thought to have dropped two canon balls of different weights from the Leaning Tower of Pisa. Even if Galileo never actually did this experiment it was done on the moon during the Apollo 15 mission as shown in the following video.


Johannes Kepler

Johannes Kepler lived from 1571 to 1630. His contribution to gravity was the discovery that planets orbit the sun in elliptical rather than circular orbits. This discovery resulted in his laws of planetary motion.

#1 "The orbit of every planet is an ellipse with the Sun at one of the two foci."

This describes the relationship between the planet's orbit and the position of the sun.

#2 "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time."

This showed that the planets moved faster when closer to the sun than when they were further away.

#3 "The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit."

This describes the relationship between the size of a plane's orbit and their orbital period.

These three law played a major roll in the development of gravity. While Kepler made no apparent effort to find out why the planets follow the path's they do his discovery of these patters paved the for future development.


Sir Isaac Newton

Sir Isaac Newton was an English physicist and mathematician who lived 1642 to 1727. He was the one who is actually responsible for the term gravity and the development his theory of gravity.   He didn't provide a mechanism for how gravity worked but simply described how it works. His main break through was the realization that the same force that causes objects to fall governs the motion of the planets. The other factors that helped Newton develop his  theory of gravity was that he had already developed theories of force and motion. He had also developed the mathematics of calculus which gave him the mathematical tools to work it out.

Now known as the Law of Universal Gravitation it can be summed up in the simple Formula.


F = force
G = the Gravitational Constant
m = the mass of each body
r = the distance between the center of mass of each body.

Put in to English this means that the strength of gravity decreases by the square of the distance between the two objects increases as shown in the above graph.

Newton's Law of Universal Gravitation is so accurate that it is still used in calculating space craft flight paths. However it has turned out to be only a very close approximation of gravity. So close that it is accurate in most circumstances but it is still just an approximation. 


Albert Einstein

Albert Einstein was a German-born theoretical physicist who lived 1879 – 1955. His contribution to gravity came in the form of his General Theory of Relativity. General Relativity not only describes how the affects produced by gravity behave but what actually causes gravity.


 From Wikimedia Commons

General Relativity shows gravity to be a warping of the fabric of space-time caused by the presence of matter. This images is a two-dimensional visualization of the space-time distortion. It is this warping that is interpreted as gravity. It is the force being applied back on the mass by space-time that results in the observed force between masses.

So far the General Theory of Relativity has passed every test given it. The only real hold outs are gravity waves but they are extremely weak and difficult to detect. The one thing that General Relativity seems to have a problem with is galactic rotation rates but this suggests ether the presence of dark matter or a need for a modification to General Relativity.


Basic Concept

The basic concept of gravity is that it is a force that attracts every particle in the universe to every other particle in the universe. While General Relativity shows gravity to be a warping of the fabric of space-time caused by the presence of matter it dose not change the fact that this is the result. While not perfect Newton's Law of Universal Gravitation is accurate enough for most calculation only deviating significantly in strong gravity. The sun has a strong enough gravitational field to show these differences in a small but detectable manner but it become obvious near black holes.

F = force
G = the Gravitational Constant
m = the mass of each body
r = the distance between the center of mass of each body.

The force of gravity between two bodies gets larger as the mass of each body gets larger but it gets smaller as the two bodies get further apart by the square of the distance. As a result when an object is close to a larger object like the Earth it has a fairly strong gravitational force but it quickly drops as the object gets further away. This is why despite the fact that the sun is much larger than the Earth that the Earth's gravitational force is stronger than the sun's near or on the Earth.

The result is that as an object falls it will accelerate.

 

It also means that two objects will accelerate at the rate as they fall.

 

This is the case even when the two objects have different masses.


Proposed Expansions of Gravity

 By PhilHibbs (Own work in Inkscape 0.42) [GFDL (http://www.gnu.org/copyleft/fdl.html),
CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.0-uk  (http://creativecommons.org/licenses/by-sa/2.0/uk/deed.en)], via Wikimedia Commons

There is an anomaly in galactic rotation measurements that has lead to two theoretical modifications to gravity. Both Newtonian and General Relativistic gravity would tend to predict that the orbital velocities of stars as the get further from they center of the galaxy should follow line A in this graph but in reality they follow line B. This anomaly is called the galaxy rotation problem. The standard model for dealing this is that there exists a sphere non-luminous matter around galaxies referred to as dark matter. However two new theories have been proposed as alternatives. One is Modified Newtonian dynamics or Mod theory which is a modification to the way that gravity interacts with matter over long distances. The other is called Cosmological Relativity which is an extension of the principals of Special and General Relativity to cosmological scales. 

Modified Newtonian dynamics was developed specifically to deal with the galaxy rotation problem. This makes it as ad hoc a model as dark matter but without dark matter's simplicity. One other drawback is that it can only be tested by observing things at galactic distances. Dark matter on the other hand at least has the potential of being discovered in a lab and some observations may have already done so.

Cosmological Relativity was not developed to explain the galaxy rotation problem but dose so any way. This means that unlike MOD theory and dark matter it is not ad hoc. However it has the unique distinction of actually predicting the observed acceleration of the expansion of the universe.


Conclusion

On the large scale gravity is the main binding force of the Universe. Developing our current understanding of gravity has taken centuries but it is now fairly well under stood even though there is room for further development. This is particularly the case on the small scale. The main area where development is still needed is in the area of bringing Gravity down to quantum levels.


Aristotle

Galileo Galilee

Johannes Kepler

Sir Isaac Newton

Albert Einstein

Basic Concept

Gravity in action

Expansions Gravity

 

 

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