3.2 Waves in What?

Waves in What?

Waves of radiation differ fundamentally from water waves, sound waves, or any other waves that travel through a material medium. Radiation needs no such medium. When light radiation travels from a distant galaxy, or from any other cosmic object, it moves through the virtual vacuum of space. Sound waves, by contrast, cannot do this; if we were to remove all the air from a room, conversation would be impossible. Communication by flashlight or radio, however, would be entirely feasible.

The ability of light to travel through empty space was once a great mystery. The idea that light, or any other kind of radiation, could move as a wave through nothing at all seemed to violate common sense, yet it is now a cornerstone of modern physics.

INTERACTIONS BETWEEN CHARGED PARTICLES

To understand more about the nature of light, consider for a moment an electrically charged particle, such as an electron or a proton. Like mass, electrical charge is a fundamental property of matter. Electrons and protons are elementary particles—"building blocks" of atoms and all matter—that carry the basic unit of charge. Electrons are said to carry a negative charge, whereas protons carry an equal and opposite positive charge.

Just as a massive object exerts a gravitational force on any other massive body (as we saw in Chapter 2), an electrically charged particle exerts an electrical force on every other charged particle in the universe. (Sec. 2.7) Buildup of electrical charge (a net imbalance of positive over negative, or vice versa) is what causes "static cling" on your clothes when you take them out of a hot clothes dryer, or the shock you sometimes feel when you touch a metal door frame on a particularly dry day.

Figure 3.5 Charged Particles (a) Particles carrying like electrical charges repel one another, whereas particles carrying unlike charges attract. (b) A charged particle is surrounded by an electric field, which determines the particle's influence on other charged particles. We represent the field by a series of field lines. (c) If a charged particle begins to vibrate back and forth, its electric field changes. The resulting disturbance travels through space as a wave.

Unlike the gravitational force, which is always attractive, electrical forces can be either attractive or repulsive. As illustrated in Figure 3.5(a), particles with like charges (that is, both negative or both positive—for example, two electrons or two protons) repel one another. Particles with unlike charges (that is, having opposite signs—an electron and a proton, say) attract.

How is the electrical force transmitted through space? Extending outward in all directions from any charged particle is an electric field, which determines the electrical force exerted by the particle on all other charged particles in the universe (Figure 3.5b). The strength of the electric field, like the strength of the gravitational field, decreases with increasing distance from the charge according to an inverse-square law. By means of the electric field, the particle's presence is "felt" by all other charged particles, near and far.

Now suppose our particle begins to vibrate, perhaps because it becomes heated or collides with some other particle. Its changing position causes its associated electric field to change, and this changing field in turn causes the electrical force exerted on other charges to vary (Figure 3.5c). If we measure the change in the force on these other charges, we learn about our original particle. Thus, information about the particle's state of motion is transmitted through space via a changing electric field. This disturbance in the particle's electric field travels through space as a wave.

ELECTROMAGNETIC WAVES

The laws of physics tell us that a magnetic field must accompany every changing electric field. Magnetic fields govern the influence of magnetized objects on one another, much as electric fields govern interactions between charged particles. The fact that a compass needle always points to magnetic north is the result of the interaction between the magnetized needle and Earth's magnetic field (Figure 3.6). Magnetic fields also exert forces on moving electric charges (that is, electric currents)—electric meters and motors rely on this basic fact. Conversely, moving charges createmagnetic fields (electromagnets are a familiar example). In short, electric and magnetic fields are inextricably linked to one another: a change in either one necessarily creates the other.

	Figure 3.6 Magnetism Earth's magnetic field interacts with a magnetic compass needle, causing the needle to become aligned with the field—that is, to point toward Earth's north (magnetic) pole. The north magnetic pole lies at latitude 80º N, longitude 107º W, some 1140 km from the geographic North Pole.

Thus, as illustrated in Figure 3.7, the disturbance produced by our moving charge actually consists of vibrating electric and magnetic fields, always oriented perpendicular to one another and moving together through space. These fields do not exist as independent entities; rather, they are different aspects of a single physical phenomenon:electromagnetism. Together, they constitute an electromagnetic wave that carries energy and information from one part of the universe to another.

Now consider a real cosmic object—a star, say. When some of its charged contents move around, their electric fields change, and we can detect that change. The resulting electromagnetic ripples travel outward in waves, requiring no material medium in which to travel. Small charged particles, either in our eyes or in our experimental equipment, eventually respond to the electromagnetic field changes by vibrating in tune with the received radiation. This response is how we detect the radiation—and how we see. Figure 3.8 shows a more familiar example of information being transferred by electromagnetic radiation. A television transmitter causes electric charges to oscillate up and down a metal rod near the tower's top, thereby generating electromagnetic radiation. This radiation can be detected by rooftop antennas. In the metal rods of the receiving antenna, electric charges respond by vibrating in time with the transmitted wave frequency. The information carried by the pattern of vibrations is then converted into sound and pictures by your TV set.

	Figure 3.7 Electromagnetic Wave Electric and magnetic fields vibrate perpendicular to each other. Together they form an electromagnetic wave that moves through space at the speed of light in the direction perpendicular to both the electric and the magnetic fields comprising it.

Figure 3.8 Television Signal Charged particles in an ordinary household television antenna vibrate in response to electromagnetic radiation broadcast by a distant transmitter. The radiation is produced when electric charges are made to oscillate in the transmitter's emitting antenna. The vibrations in the receiving antenna "echo" the oscillations in the transmitter, allowing the original information to be retrieved.

How quickly does one charge feel the change in the electromagnetic field when another begins to move? This is an important question, because it is equivalent to asking how fast an electromagnetic wave travels. Does it propagate at some measurable speed, or is it instantaneous? Both theory and experiment tell us that all electromagnetic waves move at a very specific speed—the speed of light (always denoted by the letter c). Its exact value is 299,792.458 km/s in a vacuum (and somewhat less in material substances such as air or water). We will round this value off to c = 3.00 x 10⁵ km/s. This is an extremely high speed. In the time needed to snap your fingers (about a tenth of a second) light can travel three-quarters of the way around our planet! If the currently known laws of physics are correct, then the speed of light is the fastest speed possible. (see More Precisely 22-1)

The speed of light is very large, but it is still finite. That is, light does not travel instantaneously from place to place. This fact has some interesting consequences for our study of distant objects. It takes time—often lots of time—for light to travel through space. The light we see from the nearest large galaxy—the Andromeda Galaxy, shown in Figure 3.1—left that object about 2.5 million years ago, around the time our first human ancestors appeared on planet Earth. We can know nothing about this galaxy as it exists today. For all we know, it may no longer even exist! Only our descendants, 2.5 million years into the future, will know if it exists now. So as we study objects in the cosmos, remember that the light we see left those objects long ago. We can never observe the universe as it is—only as it was.

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