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GRAVITY AND GRAVITATION |
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One of the things I like to do on a clear evening is to go outside and star gaze. Most of the objects you see in the sky are of course stars, but you can see a few planets from our Solar System too. The motion of the planets in the Solar System is governed largely by the gravitational attraction from the sun. The gravitational force, its consequences for planetary motion, and its treatment in general relativity is the subject of this series of articles. |
Up until a few hundred years ago, it was generally believed that the earth sat at the centre of the universe, and that all of the other celestial bodies revolved around the earth. However, in 1543, the Polish astronomer Copernicus proposed a theory that was completely contrary to this way of thinking. On the basis of observations of the sun and the planets that was available at that time, he proposed that the Earth rotates daily on its axis and revolves yearly around the Sun. In addition, he argued that the planets also rotate around the Sun, and that the Earth precesses on its axis (wobbles like a top) as it rotates.
Today, we know of course that this is exactly how the Solar System works. The theory and understanding of planetary motion was undertaken by a gifted German astronomer, Johannes Kepler, and it is this that we are interested in in this series of articles. To understand the motion of the planets, we need to understand something of Newton's Laws of Motion and the theory of gravity and gravitation. From these we can make predictions about how the planets move around the sun, in the form of Kepler's Laws. Following this, we will take a look at the usefulness of the concept of gravitational potential.
In the final four articles, we will see take a brief look at general relativity, the most detailed theory that there is regarding the effects of gravitation. General relativity is a very complex and technical subject when done properly (i.e. developed in a mathematical framework), but it is possible to get a glimpse of some of the predictions of general relativity by more physical means. The Principle of Equivalence is fundamental to gaining a physical understanding of the relativistic effects of gravity. Curvature of space-time and gravitational time dilation can all be deduced by making use of this principle.
Note
that the words and interpretation here are my own. I have made
no attempt at a rigorous mathematical treatment, such as would be
required to perform calculations for generalised systems of objects
under the influence of gravitation. My aim is only to give a
flavour of what the theory of gravitation and general relativity is all about.
Introduction to General Relativity