## Cause and effect A summary

It follows from the laws expressed in Maxwell's equations that if an electric charge is accelerated it will start an endless chain of cause and effect. From the instant the charge begins to move, and while it is accelerating, it creates a changing electric field which gives rise to a magnetic field. We now have a magnetic field which was not there before, but according to the fourth equation, the change in the magnetic field will produce an electric field. The fields have left their source and propagate through space perpetually maintaining themselves. As one changes it creates the other, which in turn recreates the first.

As we have seen, Maxwell was able to calculate the speed at which the fields propagate in terms of the magnetic and electric constants which had been determined independently. When he put in the numerical values, the result looked familiar — it was the same as the speed of light, which had recently been measured by Fizeau and others. In his own words: 'We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.'

The medium to which Maxwell was referring was called the ether, an invisible and odourless medium proposed to exist merely in order to conduct light. The speed of a wave depends

* See Appendix 8.2 for a summary of units and dimensions.

on the elasticity of the material it passes through (Section 4.2). If light travels so very fast, then this ether must be very stiff, stronger even than steel, in order to transmit vibration at that speed. Then how come the ether seems to let ordinary objects move through it with no resistance?

This was a real puzzle and was not solved until Albert Einstein developed his theory of special relativity, which finally buried the ether idea (Chapter 15). In the meantime, it was necessary to live with the rather embarrassing concept of an improbable material, the existence of which could not be experimentally verified.

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