Read Online The Amazing Story of Quantum Mechanics by James Kakalios - Free Book Online

frequently called upon as a catch-all explanation for levitating heavier-than-air ships, while “reverse magnetism” was often invoked for force beams or other offensive weaponry. Readers of the daily newspaper’s comic-strip page have known since the mid-1960s that personal flying devices would someday be a reality, thanks to magnetism. Figure 8 shows a panel from a 1960s Dick Tracy comic strip, where Tracy, in silhouette, and his partner, Detective Sam Catchem, are able to scout for criminals using magnetized flying garbage cans. (Tracy is also carrying on a conversation using a “two-way wrist radio,” an early form of the cell phone.) Magnetism was expected to usher in the world of tomorrow—as the panel reproduced in Figure 8 promised, “The nation that controls magnetism will control the universe.”

    Figure 9: Angular momentum was a frequently invoked physics principle for futuristic weapons of war, as shown in the cover of the April 1930 Air Wonder Stories.
    Similarly, spinning is also a hallmark of the flying saucers and futuristic weapons (or sometimes both, as shown on the cover of the April 1930 issue of Air Wonder Stories in Figure 9). It would have to wait for the full relativistic form of quantum mechanics, developed by Paul Adrien Maurice Dirac in 1928, to show the fundamental connection between internal rotation and magnetism.
    The third quantum principle states that everything, matter and photons, has an internal rotation about an axis that passes through the object, like a twirling figure skater. For ordinary matter, there is only one question about the rotation—clockwise or counterclockwise? In the previous chapter we discussed linear momentum defined as the product of an object’s mass and velocity. Since the objects in Chapter 3 were moving in straight lines, we could employ the linguistic shortcut and just refer to it as “momentum” rather than the more accurate term “linear momentum.” The greater an object’s momentum, the harder it is to change its motion. A baseball thrown at 100 miles per hour has more momentum than one thrown at 1 mile per hour; the latter may be arrested safely bare-handed, without a catcher’s mitt, while I wouldn’t recommend this method for the former (in fact, you’d need to stand pretty close to the pitcher in order to catch the slower ball before it fell to the ground).
    Similarly, “angular momentum” is the rotational analog of “linear momentum.” The rotation may be about an axis passing through the object, as is seen in a spinning top, or about a distant axis, exemplified by the moon orbiting the Earth. In quantum physics, the spin of electrons or protons resembles a top or a ballerina more than it does an orbiting satellite. Moreover, the spinning of the particles within an atom is not arbitrary but must correspond to particular values of angular momentum. This is like saying that the linear momentum of a car can have two values, moving forward or backward at multiples of a given speed, such as 10 miles per hour. So the car could go 30 miles per hour forward or 30 miles per hour backward, but not, say, 13 miles per hour in either direction.
    It turns out, based on experimental observation, that certain fundamental particles in the universe have an internal angular momentum that has a value of either 0 (a very special case) or Planck’s constant, h, divided by 2π. Photons, for example, have an intrinsic angular momentum of h /2π. Other fundamental particles, such as electrons, protons, and neutrons, can have an internal angular momentum of exactly one-half of this value of Planck’s constant, h, divided by 2π, that is, (1/2) × ( h /2π). That’s it. Whether an object has an internal angular momentum that is either an integer multiplied by h /2π or a half-integer multiplied by h /2π will have a profound effect on how it interacts with other identical particles.
    As 2π is just a number, if h /2π is a measure of angular momentum then