General Relativity

General relativity

General relativity is often considered to be a “difficult” subject. Certainly Albert Einstein found it that way, since there was an eleven year gap between the publication of the Special Theory of Relativity (1905) and the General Theory of Relativity (1916). The gap between the publication of the two theories is due (in part) to the additional complexity of the general theory.

So why is general relativity so complex? Special relativity, which came first, deals with the proposition that the laws of physics are the same in all inertial frames of reference, which in effect means frames of reference that are in a state of constant motion, i.e. are not accelerating. General relativity extends this to all frames of reference, including those that are accelerating. Special relativity then becomes a special case of general relativity (i.e. in the case where accelerations are zero).

At the heart of general relativity is a principle known as the Principle of Equivalence. This stems from the equivalence of inertial and gravitational mass, and in essence it states that any local gravitational field can be replaced by an equivalent accelerating frame of reference. This enables us to deduce a number of interesting and important properties of gravitational fields. Indeed it could be said that general relativity provides us with a new and completely different way of looking at physics in gravitational fields. At least that's how I see it.

An important feature of general relativity that is not covered by Newtonian gravitation is the curvature of space-time. This means that, in the presence of a gravitational field, the motion of bodies and radiation are not described by a “flat” geometry. Instead, they are described by a curved geometry that is mathematically complex to describe and make predictions from.

There are numerous other effects that emerge fro the theory of general relativity. One such effect is gravitational time dilation. In special relativity, it is well known that the rate at which a clock ticks depends on the relative motion of the observer and the clock. The faster the clock moves relative to the observer, the slower it appears to tick. A similar effect is observed in the presence of gravitational fields – the rate at which a clock ticks depends on the gravitational potential in which the clock is located.

My treatment in these pages is necessarily very simplified. The objective is to try and express the physical principles and reasoning that gives rise to some of these general relativistic effects. You won’t find much maths on these pages. Instead, I try to concentrate on explaining why some of the effects of general relativity come about. However, I must emphasise that a proper treatment of general relativity requires a full mathematical treatment, which is well beyond the scope of this article.

Next: Principle of Equivalence