Wonders of the Universe. Andrew Cohen

like lots of little sources of light, all slightly displaced from one another. This means that there will be places beyond the slits where the waves cancel each other out, and places where they will add up, leading to the light and dark areas seen by experimenters like Young. This was taken as clear evidence that light was some kind of wave – but waves of what?

      The results of Young’s double-slit experiment are revealed in this detailed, wide pattern. The experiment demonstrates the inseparability of the wave and particle natures of light and other quantum particles.

       GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY

      The movement of waves across the ocean can be explained by a set of equations; Maxwell discovered a similar form of equation explained waves within magnetic fields.

MESSENGERS FROM ACROSS THE OCEAN OF SPACE

      As is often the way in science, the correct explanation for the nature of light came from an unlikely source. In the mid-nineteenth century, the study of electricity and magnetism engaged many great scientific minds. At the Royal Institution in London, Michael Faraday was busy doing what scientists do best – playing around with wire and magnets. He discovered that if you push a magnet through a coil of wire, an electric current flows through the wire while the magnet is moving. This is a generator; the thing that sits in all power stations around the world today, providing us with electricity. Faraday wasn’t interested in inventing the foundation of the modern world, he just wanted to learn about electricity and magnetism. He encoded his experimental findings in mathematical form – known today as Faraday’s Law of Electromagnetic Induction. At around the same time, the French physicist and mathematician André-Marie Ampère discovered that two parallel wires carrying electric currents experience a force between them; this force is still used today to define the ampere, or amp – the unit of electric current. A single amp is defined as the current that must flow along two parallel wires of infinite length and negligible diameter to produce an attractive force of 0.0000007 Newtons between them. Next time you change a thirteen-amp fuse in your plug, you are paying a little tribute to the work of Ampère. Today, the mathematical form of this law is called Ampère’s Law.

      By 1860, a great deal was known about electricity and magnetism. Magnets could be used to make electric currents flow, and flowing electric currents could deflect compass needles in the same way that magnets could. There was clearly a link between these two phenomena, but nobody had come up with a unified description. The breakthrough was made by the Scottish physicist James Clerk Maxwell, who, in a series of papers in 1861 and 1862, developed a single theory of electricity and magnetism that was able to explain all of the experimental work of Faraday, Ampère and others. But Maxwell’s crowning glory came in 1864, when he published a paper that is undoubtedly one of the greatest achievements in the history of science. Albert Einstein later described Maxwell’s 1860s papers as ‘the most profound and the most fruitful that physics has experienced since the time of Newton.’ Maxwell discovered that by unifying electrical and magnetic phenomena together into a single mathematical theory, a startling prediction emerges.

      Electricity and magnetism can be unified by introducing two new concepts: electric and magnetic fields. The idea of a field is central to modern physics; a simple example of something that can be represented by a field is the temperature in a room. If you could measure the temperature at each point in the room and note it down, eventually you would have a vast array of numbers that described how the temperature changes from the door to the windows and from the floor to the ceiling. This array of numbers is called the temperature field. In a similar way, you could introduce the concept of a magnetic field by holding a compass at places around a wire carrying an electric current and noting down how much the needle deflects, and in what direction. The numbers and directions are the magnetic field. This might seem rather abstract and not much of a simplification, but Maxwell found that by introducing the electric and magnetic fields and placing them centre stage, he was able to write down a single set of equations that described all the known electrical and magnetic phenomena.

      These picture strips illustrate maps of the Milky Way Galaxy as they appear in different wavelength regions.

       NASA

      Maxwell’s equations had exactly the same form as the equations that describe how soundwaves move through air or how water waves move through the ocean.

      THE RELATIONSHIP BETWEEN ELECTRICITY, MAGNETISM AND THE SPEED OF LIGHT IS SUMMARIZED IN THE EQUATION:

      Where c is the speed of light and the quantities ₀ and ₀ are related to the strengths of electric and magnetic fields. The fact that the velocity of light can be measured experimentally on a bench top with wires and magnets was the key piece of evidence that light is an electromagnetic wave.

      At this point you may be wondering what all this has to do with the story of light. Well, here is something profound that provides a glimpse into the true power and beauty of modern physics. In writing down his laws of electricity and magnetism using fields, Maxwell noticed that by using a bit of simple mathematics, he could rearrange his equations into a more compact and magically revealing form. His new equations took the form of what are known as wave equations. In other words, they had exactly the same form as the equations that describe how soundwaves move through air or how water waves move through the ocean. But waves of what? The waves Maxwell discovered were waves in the electric and magnetic fields themselves. His equations showed that as an electric field changes, it creates a changing magnetic field. But in turn as the magnetic field changes, it creates a changing electric field, which creates a changing magnetic field, and so on. In other words, once you’ve wiggled a few electric charges around to create a changing electric and magnetic field, you can take the charges away and the fields will continue sloshing around – as one falls, the other will rise. And this will continue to happen forever, as long as you do nothing to them.

      This is profound in itself, but there is an extra, more profound conclusion. Maxwell’s equations also predict exactly how fast these waves must fly away from the electric charges that create them. The speed of the waves is the ratio of the strengths of the electric and magnetic fields – quantities that had been measured by Faraday, Ampère and others and were well known to Maxwell. When Maxwell did the sums, he must have fallen off his chair. He found that his equations predicted that the waves in the electric and magnetic fields travelled at the speed of light! In other words, Maxwell had discovered that light is nothing more than oscillating electric and magnetic fields, sloshing back and forth and propelling each other through space as they do so. How beautiful that the work of Faraday, Ampère and others with coils of wire and pieces of magnets could lead to such a profound conclusion through the use of a bit of mathematics and a sprinkling of Scottish genius! In modern language, we would say that light is an electromagnetic wave.

      In order to have his epiphany, Maxwell needed to know exactly what the speed of light was. Remarkably, the fact that light travels very fast, but not infinitely so, had already been known for almost two hundred years. As we will discover now, it had first been measured by Ole Romer in 1676

CHASING THE SPEED OF LIGHT

      Open your eyes and the world floods in; light seems to jump from object to retina, forming a picture of the world instantaneously. Light seems to travel infinitely fast, so it is no surprise that Aristotle and many other philosophers and scientists believed light travelled ‘without movement’. However, as the Greek philosophers gave more thought to the nature of light, a debate about its speed of travel ensued