String Theory For Dummies. Andrew Zimmerman Jones

can cancel each other out. If the crest of wave 1 overlaps with the trough of wave 2, they cancel each other out at that point. This sort of interference plays a key role in one of the quantum physics problems we discuss in Chapter 7 — the double slit experiment.

      Getting some good vibrations

      String theory depicts strings of energy that vibrate, but the strings are so tiny that you never perceive the vibrations directly, only their consequences. To understand these vibrations, you have to understand a classical type of wave called a standing wave — a wave that doesn’t appear to be moving.

      In a standing wave, certain points, called nodes, don’t appear to move at all. Other points, called antinodes, have the maximum displacement. The arrangement of nodes and antinodes determines the properties of various types of standing waves.

The simplest example of a standing wave is one with a node on each end, such as a string that’s fixed in place on the ends and plucked. When there is a node on each end and only one antinode in between them, the wave is said to vibrate at the fundamental frequency.

Consider a jump rope that is held at each end by a child. The ends of the rope represent nodes because they don’t move much. The center of the rope is the antinode, where the displacement is the greatest and where another child will attempt to jump in. This is vibration at the fundamental frequency, as demonstrated in Figure 5-3a.

      FIGURE 5-3: Examples of standing waves, demonstrating the first three normal modes of a string fixed at both ends. The top wave represents the fundamental frequency.

      If the children get ambitious, however, and begin putting more energy into the wave motion of their jump rope, a curious thing happens. Eventually, the children will pump enough energy into the rope that instead of one large antinode, two smaller antinodes are created, and the center of the rope seems to be at rest, as shown in Figure 5-3b. It’s almost as if someone grabbed the middle of the rope and is gingerly, but firmly, holding it in place! (If you are a musician, you may recognize this as the second harmonic or first overtone of the rope.)

Another type of standing wave can be considered if instead of a child holding each end of the rope, one end is mounted on a ring around a pole. The child holding one end begins the wave motion, but the end on the pole is now unconstrained and moves up and down. Instead of having a node on each end, one end is a node (held by the child), and the other is an antinode (moving up and down on the pole).

      A similar situation happens when a musician uses a pipe that’s closed at one end and open at the other, like pipes in an organ. A node forms at the closed end of the pipe, but the open end of the pipe is always an antinode.

      A third type of standing wave has an antinode at each end. This would be represented by either a pipe that’s open on both ends or a rope that’s free to move on both ends.

      The more energy that’s pumped into the standing wave, the more nodes form (see Figure 5-3c). The series of frequencies that cause new nodes to form are called harmonics or, in music, overtones. The waves that correspond to harmonics are called normal modes or vibrational modes.

      Music works because of the manipulation and superposition of harmonic overtones created by these normal modes of vibration. The first three normal modes are shown in Figure 5-3, where a string is fixed on both ends.

      

In string theory, the vibrational modes of strings (and other objects) are similar to the vibrating waves that we are talking about in this chapter. In fact, matter itself is seen as the manifestation of standing waves on strings. Different vibrational modes give rise to different particles! We perceive the particles from the lowest vibrational modes, but with higher energies, we may be able to detect other, higher-energy particles.

Newton’s Revolution: How Physics Was Born

      Many see Sir Isaac Newton’s discoveries as the start of modern physics (along with a bit of help from his predecessor Galileo Galilei). Newton’s discoveries dominated two centuries of physics, until Albert Einstein took his place at the apex of scientific greatness.

Newton’s accomplishments are diverse, but he’s known largely for four crucial discoveries that define the realm of physics even today:

       Three laws of motion

       Law of universal gravitation

       Optics

       Calculus

      Each of these discoveries has elements that will prove important as you attempt to understand the later discoveries of string theory.

      Force, mass, and acceleration: Putting objects into motion

      Newton formulated three laws of motion, which showed his understanding of the real meaning of motion and how it relates to force. Under his laws of motion, a force creates a proportional acceleration on an object.

      This understanding was a necessary foundation upon which his law of gravity was built (see the next section). In fact, both were introduced in his 1686 book, Philosophiae Naturalis Principia Mathematica, a title that translates to Mathematical Principles of Natural Philosophy. This book has become known in physics circles by the shorter title Principia.

      The second law of motion says that the force required to accelerate an object is the product of the mass and acceleration, expressed by the equation F = ma, where F is the total force, m is the object’s mass, and a is the acceleration. To figure out the total acceleration on an object, you figure out the total forces acting on it and then divide by the mass.

      

Strictly speaking, Newton said that force is equal to the change in momentum of an object. In calculus, this is the derivative of momentum with respect to time. Momentum is equal to mass times velocity. Because mass is assumed to be constant and the derivative of velocity with respect to time yields the acceleration, the popular F = ma equation is a simplified way of looking at this situation.

      

This equation can also be used to define inertial mass. If we take a force and divide it by the acceleration it causes on an object, we can determine the mass of the object. One question string theorists hope to answer is why some objects have mass and others (such as the photon) do not.

NEWTON MAKES SOME LAWS ABOUT MOTION

      Newton’s second law of motion, and the way it relates force, acceleration, and mass, is the only law of motion relevant to a string theory discussion. However, for true Newton-o-philes, here are the other two laws of motion, paraphrased for ease of understanding:

       Newton’s first law of motion: An object at rest remains at rest, or an object in motion remains in motion, unless acted upon by an external force. In other words, it takes a force to cause motion to change.

       Newton’s third law of motion: When two objects interact through a force, each object exerts a force on the other object that is equal and opposite. In other words, if we exert a force on a wall with a hand, the wall exerts an equal force back on the hand.