2.2 Waves in What?
Waves of radiation differ in one fundamental respect from water waves, sound waves, or any other waves that travel through a material medium—radiation needs no such medium. When light travels from a distant cosmic object, it moves through the virtual vacuum of space. Sound waves, by contrast, cannot do this, despite what you have probably heard in almost every sci-fi movie ever made! If we were to remove all the air from a room, conversation would be impossible (even with suitable breathing apparatus to keep our test subjects alive) because sound waves cannot exist without air or some other physical medium to support them. 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
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 2.4c). If we measure the changes in the forces on these other charges, we learn about our original particle. Thus, information about our particle’s motion is transmitted through space via a changing electric field. This disturbance in the particle’s electric field travels through space as a wave.
Electric and magnetic fields are inextricably linked to one another. A change in either one necessarily creates the other. For this reason, the disturbance produced by our moving charge actually consists of oscillating electric and magnetic fields, always oriented perpendicular to one another and moving together through space (Figure 2.6). 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 distant cosmic object—a star. It is made up of charged particles, mainly protons and electrons, in constant motion. As these charged contents move around, their electric fields change, and electromagnetic waves are produced. These waves travel outward into space, and eventually some reach Earth. Other charged particles, either in our eyes or in our experimental apparatus, respond to the electromagnetic field changes by vibrating in tune with the received radiation. This response is how we “see” the radiation—with our eyes or with our detectors.
All electromagnetic waves move at a very specific speed—the speed of light (always denoted by the letter c). Its value is 299,792.458 km/s in a vacuum (and somewhat less in material substances, such as air or water). In this text, we round this value off to This is an extremely high speed. In the time needed to snap a finger—about a tenth of a second—light can travel three quarters of the way around our planet! According to the Theory of Relativity (see More Precisely 13-1), the speed of light is the fastest speed possible.