Читать книгу Quantum Physics is not Weird. On the Contrary. - Paul J. van Leeuwen - Страница 27

From the introduction of Maxwell's equations for electromagnetic waves, three major different types of waves ^[1] were distinguished in classical physics. For understanding the quantum physical wave phenomena that will be discussed in the coming chapters it is a good idea to delve a little bit deeper into the subject of waves at this point as wave behavior plays an extremely important role in quantum physics.

Surface waves propagate in liquids. The movement of the particles in waves traveling along the liquid surface is more or less perpendicular to the surface. The particles move only slightly back and forth in the propagation direction of the wave. The wave transports no liquid in the direction of propagation. This type of wave is easy to recognize visually. It is also easy to generate them, throwing a pebble in a pond is sufficient. In figure 4.1 the movement of the liquid particles is indicated by ellipses. The deeper under the surface, the smaller the ellipses, the less the liquid moves. A diver will notice this when swimming a little below the troubled surface. Every fluid particle - including the ones at the top - moves actually not far from its equilibrium position. Surface waves that intersect with each other exhibit superposition and interference, they reinforce or weaken each other for a moment and then simply roll on again.

Figure 4.1: The movement of particles in a liquid transporting a surface wave.

Source: Mpasternak on Wikimedia Commons.

Sound waves are propagations ^[2] of pressure fluctuations occurring in gases, liquids and solids. Their constituent particles - atoms or molecules - oscillate in the same direction as the propagation of the wave. The superposition principle also applies to sound waves, so they will also show interference. Interference of sound waves with slightly differing frequencies, will be experienced as overtones. Sound waves are not experienced as oscillations but as tones. Healthy, young ears can experience sound frequencies between 20 Hz and 20,000 Hz (Hertz: the number of complete oscillations per second).

Figure 4.2: A sound wave. Propagating compression variations.

Source: Pluke on Wikimedia Commons

Electromagnetic waves consist of varying electric and magnetic fields aligned perpendicular to the wave direction and also perpendicular to each other. See figure 3.7. We experience electromagnetic waves between 400 THz and 790 THz (Terahertz: 1 THz = 1,000,000,000,000 Hz) as light. We do not experience the wave behavior of light as oscillations but as colors. EM-waves also exhibit interference and superposition.

Modern physics has also identified other types of waves, e.g. gravity waves and quantum waves; we will deal extensively with those quantum waves in the following chapters. For gravitational waves I have to refer you to other literature.

All these types of waves exhibit refraction, which occurs when a wave propagates into another, slower, medium at an oblique angle with the boundary. Note that the level of abstraction increases in the above-mentioned order - liquid, sound and EM-waves. It is easy for us to see the everyday occurrence of waves on water. We hear sound waves, but we do not see them neither do we experience the pressure variations as oscillations. But we are still able to visualize the propagating pressure variations in our imagination. However, trying to visualize oscillating magnetic and electric fields becomes rather difficult because those waves do not need matter for their propagation. We will see that EM-waves are more like clouds of unimaginable small particles of pure energy traveling through empty space with light speed.

Note: The quantum wave is something that we never observe as a material wave. This will become important in the unmasking of the often-quoted particle-wave duality.