Why Does E=mc2_ - Brian Cox [9]
Advances in fundamental physics rarely come from experiments alone, however. Faraday wanted to understand the underlying mechanism behind his observations. How could it be, he asked, that a magnet not physically connected to a wire can nevertheless cause an electric current to flow? And how can a pulse of electric current wrench a compass needle away from magnetic north? Some kind of influence must pass through the empty space between magnet, wire, and compass; the coil of wire must feel the magnet passing through it, and the compass needle must feel the current. This influence is now known as the electromagnetic field. We’ve already used the word “field” in the context of the earth’s magnetic field, because the word is in everyday usage and you probably didn’t notice it. In fact, fields are one of the more abstract concepts in physics. They are also one of the most necessary and fruitful for developing a deeper understanding. The equations that best describe the behavior of the billions of subatomic particles that make up the book you are now reading, the hand with which you are holding the book in front of your eyes, and indeed your eyes, are field equations. Faraday visualized his fields as a series of lines, which he called flux lines, emanating from magnets and current-carrying wires. If you have ever placed a magnet beneath a piece of paper sprinkled with iron filings, then you will have seen these lines for yourself. A simple example of an everyday quantity that can be represented by a field is the air temperature in your room. Near the radiator, the air will be hotter. Near the window, it will be cooler. You could imagine measuring the temperature at every point in the room and writing down this vast array of numbers in a table. The table is then a representation of the temperature field in your room. In the case of the magnetic field, you could imagine noting the deflection of a little compass needle at every point, and in that way you could form a representation of the magnetic field in the room. A subatomic-particle field is even more abstract. Its value at a point in space tells you the chance that the particle will be found at that point if you look for it. We will encounter these fields again in Chapter 7.
Why, you might legitimately ask, should we bother to introduce this rather abstract notion of a field? Why not stick to the things we can measure: the electric current and the compass needle deflections? Faraday found the idea attractive because he was at heart a practical man, a trait he shared with many of the great experimental scientists and engineers of the Industrial Revolution. His instinct was to create a mechanical picture of the connection between moving magnets and coils of wire, and for him the fields bridged the space between them to forge the physical connection his experiments told him must be present. There is, however, a deeper reason why the fields are necessary, and indeed why modern physicists see the fields as being every bit as real as the electric current and compass deflections. The key to this deeper understanding of nature lies within the work of Scottish physicist James Clerk Maxwell. In 1931, on the centenary of Maxwell’s birth, Einstein described Maxwell’s work on the theory of electromagnetism as “the most profound and the most fruitful that physics