Last modified on 9 June 2009, at 23:50

Modern Physics/Introduction to Special Relativity

At first glance, special relativity seems weird. A number of rules which we use to understand the world no longer apply. These rules include the rule that velocities can be added to each other, the rule that time and space are different things, the rule that matter and energy are separate, the rules for momentum. Because we can no longer rely on our normal intuition to figure things out, things can seem bizarre and confusing.

Fortunately, the apparent oddness of special relativity does not arise from deep complexity. Underneath the surface, there are some very beautiful simple symmetries and rules, and these symmetries and rules allow us to relate special relativity with the physics with which we are familiar, with only a few minor amendments.

Nevertheless, some of the ideas of special relativity seem so contrary to the way that we naively expect that the universe ought to behave and to our everyday experience that a perfectly natural reaction is that special relativity could be not possibly be correct.

However, numerous experiments have demonstrated that it's not special relativity that is incorrect, but rather our low speed intuition about how things should work. After all, we don't normally deal with things moving at close to the speed of light, and there is no particular reason to believe that our low speed thinking applies at high speed. And experiments clearly indicate that they don't. Furthermore, it turns out that many ordinary phenomena really are the result of relativity. Magnetism, for example, is a purely relativistic phenomenon that we see daily but which can't be properly explained without relativity.

But coming to this realization was difficult for physicists. One way of intuiting special relativity is take the journey that physicists did. We shall start with the ordinary everyday universe of classical Newtonian physics, which we studied in the previous two books, and then gradually travel out to the places which are odd. We are helped in this journey by the fact that the road has already been travelled by generations of physicists who gradually came to the realization that the world was stranger than they had imagined.

Therefore, the approach we shall take is to first go down a short historical survey which illustrates why special relativity is necessary. We shall then talk about the great conceptual breakthrough that Albert Einstein made, and then from the simple axioms that he figured out we shall see the subtle consequences that follow. We shall see too how straightforward special relativity really is, how simple at its heart.