String Theory For Dummies. Andrew Zimmerman Jones

the needle of a compass.

Later experimentation by Michael Faraday and others showed that this works the other way as well — a magnetic force can influence an electrical current. As demonstrated in Figure 5-5, moving a magnet toward a conducting loop of wire causes a current to run through the wire.

      FIGURE 5-5: A magnet moving toward a metal ring creates a current in the ring.

      Faraday proposes force fields to explain these forces

      In the 1840s, Michael Faraday proposed the idea that invisible lines of force are at work in electrical currents and magnetism. These hypothetical lines make up a force field that has a certain value and direction at any given point and can be used to calculate the total force acting on a particle at that point. This concept was quickly adapted to apply to gravity in the form of a gravitational field.

According to Faraday, these invisible lines of force are responsible for electrical force (as shown in Figure 5-6) and magnetic force (as shown in Figure 5-7). They result in an electric field and a magnetic field that can be measured.

      FIGURE 5-6: Positive and negative charges are connected by invisible lines of force.

      FIGURE 5-7: The north and south poles of a bar magnet are connected by invisible lines of force.

      Faraday proposed the invisible lines of force, but he wasn’t nearly as clear on how the force is transmitted, which drew ridicule from his peers. Keep in mind, though, that Newton also couldn’t fully explain how gravity is transmitted, so there was precedent to this. Action at a distance was already an established part of physics, and Faraday, at least, was proposing a physical model of how it could take place.

      

The fields proposed by Faraday turned out to have applications beyond electricity and magnetism. Gravity, too, can be written in a field form. The benefit of a force field is that every point in space has a value and direction associated with it. If you can calculate the value of the field at a point, you know exactly how the force will act on an object placed at that point. Today, every law of physics can be written in the form of fields.

      Maxwell’s equations bring it all together: Electromagnetic waves

      Physicists now know that electricity and magnetism are both aspects of the same electromagnetic force. This force travels in the form of electromagnetic waves. We see a certain range of this electromagnetic energy in the form of visible light, but there are other forms, such as X-rays and microwaves, that we don’t see.

In the mid-1800s, James Clerk Maxwell took the work of Faraday and others and created a set of equations, known as Maxwell’s equations, that describe the forces of electricity and magnetism in terms of electromagnetic waves. An electromagnetic wave is shown in Figure 5-8.

      FIGURE 5-8: The electric field and magnetic field are in step in an electromagnetic wave.

      Maxwell’s equations allowed him to calculate the exact speed that an electromagnetic wave traveled. When Maxwell performed this calculation, he was amazed to find that he recognized the value. Electromagnetic waves move at exactly the speed of light!

Maxwell’s equations showed that visible light and electromagnetic waves are different manifestations of the same underlying phenomena. In other words, we see only a small range of the entire spectrum of electromagnetic waves that exist in our universe. Extending this unification to include all the forces of nature, including gravity, would ultimately lead to theories of quantum gravity such as string theory.

      Two dark clouds and the birth of modern physics

      Two significant unanswered questions about the electromagnetic theory remained. The first problem was that the ether hadn’t been detected, while the second involved an obscure problem about energy radiation, called the blackbody problem (described in Chapter 7). What’s amazing, in retrospect, is that physicists didn’t see these problems (or dark clouds, as British scientist Lord Kelvin called them in a 1900 speech) as especially significant, but instead believed they were minor issues that would soon be resolved. As you see in Chapters 6 and 7, resolving these two problems would introduce the great revolutions of modern physics: relativity and quantum physics.