29 March 2013

Opposites attract. via Reddit
Opposites attract.

In 1752, so the story goes, Benjamin Franklin flew a kite in a thunderstorm. The most likely outcome of such an experiment is that you would get yourself killed, so there is some debate as to whether it actually occurred. We do know that similar experiments with lightning rods did occur. What Ben Franklin and others were trying to do is to show that lightning was a kind of electricity. That seems rather obvious today, but at the time electricity was the kind of thing you could make by rubbing fur on amber. Basically an early version of rubbing a balloon on your hair and sticking it to the wall. Lightning, on the other hand, was what struck church spires and barns, causing them to burn to the ground. By connecting these two very different phenomena, scientists had begun a journey not only to understand the physical world, but to discover how different parts of the world connected together.

By the mid-1700s, we had a basic understanding of electricity. We knew, for example that there were two types of charge (positive and negative) and that charge could be transferred between objects. Similar charges seemed to push away from each other, and opposite charges seemed to be attracted to each other. Then in 1785 Charles Augustin de Coulomb published a work on charge that included the equation above. This equation is now known as Coulomb’s law. In this equation F is the force between two objects, the Qs are the charges of those objects, R is the distance between them, and K is a constant known as Coulomb’s constant.

If you remember Part 2 on Newton’s gravity, you might think this equation looks familiar, and you’d be right. Like gravity, charged objects have a mutual force between them. This force is stronger when they are close together, and weaker when far apart. The force between charges is, in fact, exactly the same as the force of gravity except for one key difference. Unlike mass, charges can be positive and negative. If the two charges are the same, then they push away from each other. If the two charges are opposite, then they will attract each other. A “black hole” of charge is impossible. Instead, charges try to average out as much as possible. So if there is a buildup of charge on an object, it will try to move to a region of neutral charge. This is how static electricity works. Build up too much charge and it may jump away quickly, creating a spark. Or it may just fluff your hair as the charge on each strand tries to stay far away from other charges. This similarity between Coulomb’s equations and Newton’s led some scientists to wonder if there was a deeper connection between things like charge and mass. It also drove some scientists to search for a similar equation describing magnetism.

In many ways magnets are very similar to charges. Magnets have two poles. We call them north and south because they were originally used as compasses. Allow a magnet to freely turn, and the north pole will point north, and the south pole will point south. Like charges, similar poles push away from each other, while opposite poles attract. You could also magnetize certain materials like iron by rubbing it against a magnet. But magnets also had some properties that were very different. In particular, if you break a magnet in half, you don’t get a separate north pole and south pole, you instead get two smaller magnets, each with their own north and south pole. Charges could be separated into positives and negatives, but magnets always came in north-south pairs. A magnetic monopole (a magnet of a single pole) just doesn’t exist. This was a big mystery. Magnets and charges seemed so similar, and yet they didn’t seem quite connected.

By the early 1800s, electricity could be controlled well enough that charges could be made to move through metal wires, creating a flow of electricity called a current. In 1819, Hans Christian Oersted happened to have a compass near one of his wires, and he noticed that an electric current would deflect a compass. It seemed a flow of charge could make a magnet. In 1831 Michael Faraday demonstrated that moving a wire near a magnet could create an electric current in the wire. Electricity and magnetism were connected. Electric currents could create magnets, and magnets could create electric currents.

Then in the 1860s James Clerk Maxwell built upon the work of Coulomb, Faraday and others, and published a set of elegant and beautifully subtle equations, of which the equation above is just the first. Taken as a whole, these equations are now called Maxwell’s equations.

Maxwell’s breakthrough was to change the way we thought about the equation above. Expressed simply, it just describes the force between two charges. But how does one charge “know” to be pushed or pulled by the other charge? How do they interact across the empty space between them. Maxwell’s idea was that each charge must reach out to each other with some kind of energy. That is, a charge is surrounded by a field of electricity, a field that other charges can detect. Charges possess electric fields, and charges interact with the electric fields of other charges. The same must be true of magnets. Magnets possess magnetic fields, and interact with magnetic fields. Maxwell built upon Coulomb’s equation and others to create not a description of charges and magnets, but a description of the electric and magnetic fields themselves.

With that change of view, Maxwell found the connection between electricity and magnetism. They were connected by their fields. A moving electric field creates a magnetic field, and a moving magnetic field creates an electric field. Not only are the two connected, but one type of field can create the other. Maxwell had created a single, unified description of electricity and magnetism. He had united two different forces into a single unified force, which we now call electromagnetism.

Maxwell’s theory had another interesting consequence. Since a moving electric field could produce a magnetic field, and a moving magnetic field could produce an electric field, it was possible for an electric field to produce a magnetic field to produce an electric field. Back and forth, over and over, creating a continuing wave of electric and magnetic fields. Maxwell calculated the speed of such a wave. It was exactly the speed of light. In combining electricity and magnetism, Maxwell proved that light is an electromagnetic wave.

Maxwell’s equations are still central to modern astronomy. We now observe a wide range electromagnetic waves from visible light, to radio waves to x-rays. These waves interact with magnets and charges, and those interactions tell us about the universe.

But the equation above is important for another reason as well. Coulomb’s equation was the first of what became Maxwell’s unified theory of charges, magnets and light. Over the years we have discovered other unified theories. The unified theories we have allow us to understand the earliest moments of the big bang, and the workings of black holes. But there are still many cosmic mysteries where we don’t yet know the connections. How does quantum mechanics connect to gravity? Is dark energy a separate field, or is it connected to the fields we know? So we continue to search for deeper and more subtle connections, hoping to find a single description of all the forces and fields and particles in the universe.

A unified theory of everything.

Tomorrow: Got something to hide? Toss your secrets into a black hole, and no one will ever know. Or will they? Find out in Part 5.