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General Relativity Theory of Gravitation
by Ron Kurtus (2 February 2010)
In 1915, Albert Einstein formulated the Theory of General Relativity as an extension to his Theory of Special Relativity and as a new way to explain gravitation.
Newton's Law of Universal Gravitation had defined gravitation as a property of matter that is force of attraction acting at a distance. The theory works well for ordinary gravitational fields but is inaccurate when the gravitational intensity is high. Discrepancies were seen when measuring the orbit of the planet Mercury and the effect of gravitation on light.
Einstein's theory states that matter curves space and distorts time, causing objects to move toward each other. The Theory of General Relativity became accepted after it predicted the orientation of Mercury's orbit. However, some scientists see flaws in the Theory of General Relativity, due to phenomena that the theory does not explain, especially at the quantum level.
Questions you may have include:
- What is the relativity theory of gravitation?
- What are predictions proving the concept?
- What are some problems with the relativity explanation?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
Einstein's theory of gravitation
In 1905, Albert Einstein published the Theory of Special Relativity, a theory about space and time. In the following years, Einstein had noticed that acceleration produced the same effect as gravitation. For example, if you were in an accelerating spaceship—or even an elevator—you could not tell if the force on you was from inertia or gravitation. This led him to look at the mathematics of relative motion and gravitation.
(See Artificial Gravity for more on gravitation from acceleration.)
Then in 1915, he unified the Theory of Special Relativity with Newton's Law of Universal Gravitation to establish the Theory of General Relativity, which is a geometric explanation of gravitation.
Curvature of spacetime
While the classical explanation of gravitation is that it is some sort of force-at-a-distance, Einstein took a different approach, stating that matter curves space and distorts time, causing objects to move toward each other. His theory states that time is a fourth dimension, adding to the three dimensions of space. He called this four-dimensional geometry spacetime.
In ordinary situations, particles travel along straight lines in this geometry of space, following Newton's laws of motion. However, according to Einstein's theory, matter affects the geometry of spacetime causing it to be curved toward the matter. A particle that is freely moving at a constant velocity will follow such a line toward the object, as if it was attracted by some force.
Mathematics complex
Mathematical expression describing the properties of a gravitational field surrounding a given mass is stated in a set of formulas called the Einstein Field Equations. They are highly complex a system of partial differential equations, which are beyond the scope of our material. However, the equations reduce to Newton's Universal Gravitation Equation under simple conditions.
Tests verifying Relativity explanation
The Theory of General Relativity gained acceptance in the scientific community after a several predictions and tests proved correct, including an accurate prediction of the orbit of Mercury and of the deflection of light by a strong gravitational field. However, verification of the theory still remains difficult.
Orbit of planet Mercury
The orbit of the Mercury—the closest planet to the Sun—exhibits perturbations and a precession that could not be fully explained by the Law of Universal Gravitation.
Application of the General Relativity equations predicted the motion of Mercury to a high degree. This proof caused most scientists to accept the General Relativity's explanation of gravitation.
Deflection of light
Both Universal Gravitation and General Relativity predict that light can be deflected by gravitation. However, the calculation of the amount of deflection from Newton's theory was only half of what Einstein predicted.
Several years after the General Theory of Relativity was proposed, scientists measured the deflection of light from a star as it passed by the Sun during a solar eclipse. Measurements agreed with Einstein's predictions.
Other predictions
The General Theory of Relativity also predicted light coming from a strong gravitational field would have its wavelength shifted toward longer wavelengths, which is called a redshift. The theory also predicted the existence of Black Holes.
Both gravitational redshift and Black Holes were also considered possible in the Universal Gravitation theory, but measurements corresponded better with Einstein's theory.
Problems with General Relativity theory
Although General Relativity does a good job of explaining gravitation at very high levels, it does run into some problems indicating it may not be a complete theory.
Gravitation not like other forces
Gravitation, nuclear, magnetism and electrical forces are fundamental entities in the classical, as well as Quantum Mechanics theories. However, General Relativity does not look at gravitation as a force but instead a property of spacetime. This disconnect of gravitation and the various forces is a concern to some scientists that the relativity theory is not complete.
Dark matter
Measurements on the rate of expansion of the Universe indicate there is some force slowing it down. Some scientists proposed there exist what they called dark matter, which seemed to have the property of anti-gravitation. The possibility of an anti-gravitation force is not explained in the Theory of General Relativity.
Hofava splits time from matter
A very recent theory called Hofava gravitation discounts the spacetime connection and states the space and time are separate entities. The Hofava concept looks at gravitation during the Big Bang and works well with Quantum Gravitation.
This theory has had good results in predicting gravitational phenomenon, but it is new and does not yet have universal acceptance from the scientific community.
Summary
Albert Einstein formulated the Theory of General Relativity as a new way to explain gravitation. The theory states that matter curves space and distorts time, causing objects to move toward each other.
While Newton's Law of Universal Gravitation works well for ordinary gravitational fields, it is inaccurate when the gravitational intensity is high. The Theory of General Relativity became accepted by scientists after it correctly predicted the planet Mercury's orbit. However, the theory has areas it does not explain, such as at the quantum level.
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Resources
The following resources provide information on this subject:
Websites
Understanding General Relativity - Rafi Moor, Israel
Einstein's Theory of General Relativity - Andrew Zimmerman Jones, About.com
General Relativity - Wikipedia
The Meaning of Einstein's Equation - John C. Baez, University of California-Riverside and Emory F. Bunn, University of Richmond (Virginia)
The Einstein Field Equations - Larry Smarr, University of Illinois
Gravity and Gravitation Resources
Books
Top-rated
books on Simple Gravity Science
Top-rated
books on Advanced Gravity Physics
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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