Andrew Zimmerman Jones - String Theory For Dummies

In the 19th century, the physical understanding of the nature of light changed completely. Experiments began to show strong cases where light acts like waves instead of particles, which contradicted Newton (see the “ Optics: Shedding light on light’s properties” section for more on Newton’s findings). During the same time, experiments into electricity and magnetism began to reveal that these forces behave like light, except we can’t see them.

By the end of the 19th century, it became clear that electricity and magnetism are different manifestations of the same force: electromagnetism . One of the goals of string theory is to develop a single theory that incorporates both electromagnetism and gravity.

Newton had treated light as particles, but experiments in the 19th century began to show that light acts like a wave. The major problem with this was that waves require a medium. Something has to do the waving. Light seemed to travel through empty space, which contained no substance at all. What was the medium that light used to move through space? What was waving?

To explain the problem, physicists proposed that space is filled with a substance. When looking for a name for this hypothetical substance, they turned back to Aristotle and named it luminous aether, or ether . (No relation to the gas used in old-timey surgeries as anesthetic.)

Even with this hypothetical ether, though, there were problems. Newton’s optics still worked, and his theory described light in terms of tiny balls moving in straight lines, not as waves. It seemed that sometimes light acts like a wave and sometimes it acts as a particle.

Most physicists of the 19th century believed in the wave theory, largely because the study of electricity and magnetism helped support the idea that light is a wave, but they were unable to find solid evidence of the ether.

Electricity is the study of how charged particles affect each other. Magnetism, on the other hand, is the study of how magnetized objects affect each other. In the 19th century, research began to show that these two seemingly separate phenomena are, in fact, different aspects of the same thing. The physicist Michael Faraday proposed that invisible fields transmit the force.

An electrical force acts between two objects that contain a property called electrical charge, which can be either positive or negative. Positive charges repel other positive charges, and negative charges repel other negative charges, but positive and negative charges attract each other, as in Figure 5-4.

FIGURE 5-4:Like repels like, but opposites attract.

Coulomb’s Law, which describes the simplest behavior of the electric force between charged particles (a field called electrostatics ), is an inverse square law, similar to Newton’s law of gravity. This provided some of the first inklings that gravity and electrostatic forces (and, ultimately, electromagnetism) may have something in common.

When electrical charges move, they create an electrical current. These currents can influence each other through a magnetic force. This was discovered by Hans Christian Oersted, who found that a wire with an electrical current running through it can deflect 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.

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.

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.