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# Theories of Gravitation

by Ron Kurtus (revised 30 January 2010)

Since ancient times, scientists used observations about the effect of gravity on objects near the Earth (like the proverbial apple falling from a tree), as well as measurements of the movement of planets, to establish laws about the properties of gravitation and theories about possible causes of the phenomenon.

The first major concept was Newton's *Law of Universal Gravitation*, which stated that all objects of matter attract each other and provided an equation to measure the force of attraction.

Albert Einstein reformulated the gravitation laws to fit his *Theory of General Relativity*, explaining that gravitation is caused by a curvature of space and especially applies to large gravitational fields. Theories in *Quantum Mechanics* about gravitation connect it to the fundamental forces of matter and state that gravitation has wavelength and is a particle.

Questions you may have include:

- What is the classical explanation of gravitation?
- What is the relativistic view of gravitation?
- What are the quantum gravitation explanations?

This lesson will answer those questions. Useful tool: Units Conversion

## Law of Universal Gravitation

Isaac Newton combined concepts from other scientists to define the *Law of Universal Gravitation* in 1687. This scientific law states that all objects consisting of matter are attracted toward other objects, due to a force called gravitation. Universal gravitation assumes that the law holds throughout the Universe.

Note:Newton created alaw, as opposed to atheory. A law is based on observations and measurements but does not explain the reasons for something happening, as does a theory.

Newton also established the *Universal Gravitation Equation*, which can be used to calculate the force of attraction between two objects, provided you know their respective masses and the separation between their centers:

F = GMm/R^{2}

This equation allowed the calculation of orbits of the planets, as well as other motion problems.

(

See Law of Universal Gravitation for more information.)

This classical view of gravitation is that objects attract each other through some force that acts at a distance. It was uncertain what the mechanism was for that force and how it tied into other forces that acted at a distance. Although measurements could be made, the fact there was not a good explanation for gravitation differentiated the law from being a theory.

(

See Mysterious Force at a Distance for more information on that subject.)

## Theory of General Relativity

Although the Law of Universal Gravitation was able to predict many phenomena, there were some experimental measurements—notably the variations in the orbit of the planet Mercury around the Sun—where it was not accurate. This problem helped initiate Albert Einstein's *Theory of General Relativity* in 1915. This theory explained gravitation, especially in areas of high intensity fields, such as with a planet like Mercury that is relatively close to the Sun.

(

See General Relativity Theory of Gravitation for more information.)

### Curvature of space

Einstein postulated that the presence of matter changes the geometry of space, such that it curves the space around it. The straight line that at freely moving object traveled would curve toward another object of mass, resulting in the effect of gravitation.

In effect, Einstein claimed that gravitation was not really a force but simply a result of this property of space being curved. This is a pretty abstract concept, but Einstein demonstrated it with highly complex mathematical equations.

### Experimental proof

A number of experimental measurements proved very accurate using Einstein's equations, including the orbit of Mercury, the bending of light by a large gravitational field and properties of Black Holes. This brought on the acceptance of the Theory of General Relativity as an explanation of gravitation.

Sun's gravitation bends beam of light

## Quantum theories

Although the Theory of General Relativity worked fine on large scales, it seemed to fail at the subatomic level and did not follow the rules of *Quantum Mechanics* or *Quantum Physics*—the study of matter at extremely small separations.

(

See Quantum Theory of Gravitation for more information.)

One problem with the Theory of General Relativity's explanation of gravitation as being caused by the curvature of space is the question why gravitation is so different than other fundamental forces that act at a distance—such as magnetism.

### Gravitation as a waveform

Scientists then predicted that a gravitational field would exhibit wavelengths, similar to electromagnetic waves. Using highly sensitive instruments, experiments were made to verify that gravity is indeed a waveform of some sort.

### Wave-particle duality

To follow the wave-particle duality in Quantum Physics, it was also predicted that gravity consists of particles called *gravitons*. This is similar to the theory that light is not only a waveform, but also consists of particles called photons. Electrons are also viewed as both particles and waves.

This view of gravity being a wave or particle goes back to the idea that matter exhibits gravitation and that there truly is a force of gravity. (Nobody said that Physics would be easy and that everything was known or explained.)

### Quantum Mechanics

Recently there have been new theories that the force of gravity is caused by graviton particles or by gravity waves. These theories satisfy rules of Quantum Mechanics that Einstein's concepts didn't.

### Dark matter

There is also a theory that there exists some sort of "dark matter" that repels instead of attracts, resulting in anti-gravitation.

(

See Effect of Dark Matter and Dark Energy on Gravitation for more information.)

## Summary

Newton's *Law of Universal Gravitation* said that all objects of matter attract each other. He also provided an equation to measure the force of attraction. Over 200 years later, Albert Einstein reformulated the gravitation laws to fit his *Theory of General Relativity*, stating that gravitation is caused by a curvature of space. *Quantum Mechanics* theories about gravitation connect it to the fundamental forces of matter and state that gravitation has wavelength and is a particle.

Think of all the possibilities

## Resources and references

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### Books

**Top-rated books on Gravitation**

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## Theories of Gravitation