Introduction to colloidal systems

Karl Marx has said that, in history, things always happen twice: the first time as a tragedy, the second time as a farce. This comment of Marx applied to Napoleon I and III. However, if we strip the subjective interpretation (tragedy or farce) from this sentence, it could apply to many phenomena in physics. In physics, there often appears to be a similarity between phenomena on very different length and time-scales but, on closer inspection, there are important, even qualitative differences. Examples abound: in some respects, light waves resemble ripples on a pond but, in most respects, they are totally different. The Bohr model of the atom resembled a planetary system but, of course, the differences are so important that, in the end, they led to the demise of the Bohr model. These two examples illustrate an important point: in physics, analogies are very useful in formulating an approximate description of a phenomenon - but even more interesting than the analogy itself, is its breakdown.