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In many ways, Maxwell was extraordinarily successful. He did formulate a mechanical model of the electromagnetic field

Figure 1.22 A part of Maxwell's mechanical model of the electromagnetic field. The model has been described as 'the most ingenious but least credible ever invented'. Click here for larger image (9.95kb)

and used it as a guide in writing down his now famous equations. Amongst many other things, Maxwell's equations implied that light is a wave phenomenon in which electric and magnetic fields oscillate in space and time. In an astonishing demonstration of the power of these ideas, Maxwell took the fundamental constants of electricity and magnetism, entered them in his equations, and derived an accurate value for the speed of light. In this way, the subjects of electricity, magnetism and optics, which had seemed quite distinct at the beginning of the nineteenth century, were unified into a single branch of physics. The equations even led Maxwell to predict the existence of a wider family of electromagnetic waves, most with wavelengths beyond the range of human sight. In 1888 Heinrich Hertz completed a series of experiments which confirmed the existence of electromagnetic waves with wavelengths much greater than those of visible light. These were the radio waves which, within a few decades would transform both communication and entertainment. In 1895 Wilhelm Röntgen discovered X-rays, which proved to be electromagnetic waves with wavelengths much smaller than those of visible light. Yet, in spite of these successes, Maxwell's mechanical model of the electromagnetic field remained unconvincing. From about 1865, Maxwell himself drew a clear distinction between his equations, which described the behaviour of electric and magnetic fields, and the underlying ether mechanism that was supposed to account for them. Maxwell firmly believed that he had discovered the correct equations, but did not try to defend the model that had led to them.

If Maxwell had succeeded in accounting for the electromagnetic field in terms of motion in the ether, the mechanical world-view would have reigned supreme; but it was not to be. As investigations continued, particularly after Maxwell's untimely death, it became increasingly clear that it would be impossible to find a convincing mechanical basis for the electromagnetic field. On the other hand it also became clear that Maxwell's field theory of electromagnetism, as embodied in his equations, was stunningly successful.

Nowadays, with the scaffolding stripped away, we can recognize that the true achievement of Faraday and Maxwell was in establishing the importance of fields, arguably the most radical concept in physics since the time of Newton. We now know that there are many types of field: magnetic fields, electric fields, gravitational fields and so on. Each of these fields has a particular value at each point in space. The key idea is that a particle passing through a given point will experience forces that depend on the fields at that point, or in its immediate vicinity. This means that forces are determined locally - there is no action at a distance. When two particles interact they do so because one particle creates a field in the space around itself and the other particle then responds to this field. What is more, as Faraday anticipated and Maxwellís equations established, the fields have dynamics of their own, allowing disturbances of electric and magnetic fields to spread out as waves. Crucially, this means that fields should be thought of as part of the fabric of the world - more intangible than matter, but just as real. The electromagnetic field on Earth is incredibly complex. While you are reading this, electromagnetic waves from all the channels that your radio and television could possibly receive are passing straight through your head. Added to this are signals from power lines, domestic appliances, cars and the Big Bang; tiny electromagnetic signals are even reaching you from the brains of those around you. One of the attractions of physics is its ability to reveal a much richer world than is immediately apparent to our senses. And much stranger things are yet to come.

Question 1.4 Answer
Describe one way in which Maxwell's theory satisfied Faraday's desire to find evidence that disturbances at one point in the electromagnetic field would take a finite time to reach other points.
Continue on to 4.2 Relativity, space, time and gravity, part 1 of 4