The Physics of Superheroes: Spectacular Second Edition (19 page)

Recall from our discussion of Ant Man punching his way out of a paper bag (in Chapter 8) that torque is defined as the ability to rotate an object. Mathematically, it is described as the product of a force and the distance between where this force is applied and the axis of rotation. When the force acts right at the axis of rotation, it will push the object, but not change how it is spinning. Any change in the angular momentum of an object is determined by the product of the external torque and the time for which it acts, similar to the corresponding equation for Impulse (a force multiplied by time) for linear momentum. What we have seen in the preceding chapters is that there can be no change in the motion of an object, whether it is moving in a line or rotating, if there is not an outside force acting on it. No force, no change. A twirling object, isolated in outer space, will never stop spinning, for if there is no outside force to change its rotation, why would it? This is the basis by which gyroscopes, as part of inertial guidance systems, keep airplanes and missiles flying at an even keel.

Roscoe Dillon may have been wrong that developing a persona revolving around tops
²⁹
would lead to a successful criminal career, but he was right about at least one thing in
Flash # 122
—a gyroscope is at its heart a spinning top. The crucial element of the gyroscope that makes it useful for determining orientation is that it is a spinning top isolated from the rest of the world. In this way, its Angular Momentum does not change. Imagine a top, rotating in a clockwise direction when looked down upon from above. Curl the fingers of your right hand in the direction of the top’s rotation and stick out your thumb; your thumb will point down toward the surface on which the top is spinning, The direction of your thumb can be considered to be the “direction of the top’s angular momentum” (for consistency’s sake you should always use the same hand—either right or left—when comparing the rotation of different objects. The convention is to use the right hand, but which hand is not as important as always using the same hand). A gyroscope consists of a spinning disc, held in “gimbals” that act to isolate the disc from outside torques. Recent innovations in gyroscope design employ electrostatically levitated micro-discs, which avoid mechanical couplings altogether.

For a disc spinning counterclockwise, the angular momentum of the disc “points” up—let’s call this direction “north.” This is the “direction” of the angular momentum, and in the absence of an outside torque, nothing can change this. So if I want to turn the disc such that the direction of its angular momentum is east, I can only do so if I apply a twisting force (a torque) to the disc. Try to turn a spinning bicycle wheel, and you’ll note that it resists moving away from the direction of its rotation. For a gyroscope, the spinning disc is housed in the gimbals, so that if the gimbals are turned, the rotating disc will keep spinning in the same orientation.

This is very useful as a guidance system. If the spinning disc of the gyroscope is rotating in such a way that the direction of its angular momentum is pointing due north (for example), then it will keep pointing due north, regardless of the orientation of the surrounding jet plane or missile. If your rocket starts to veer from a heading relative to north, adjustment thrusters can return the projectile to its proper course. With another gyroscope pointing due east and a third pointing perpendicular to the north-east plane, one can accurately position oneself in three-dimensional space, without visual information or any signals from the ground. Not bad for a child’s toy.

Whirlwind’s mutant ability to twirl about at great rotational velocities proved to be too much for the police force when he initially embarked on a life of crime. Eventually he acquired a costumed superhero nemesis, as do all costumed supervillains: Giant Man. Just two issues previously, before he shifted gears and became Giant-Man, Henry Pym was fighting crime and foiling Communist saboteurs as the astonishing Ant Man. David Cannon was therefore particularly unlucky in that he had to deal with Pym’s twelve-foot-tall alter ego, rather than his more diminutive quarter-of-an-inch persona. However, his command over angular momentum enabled Whirlwind to be able to look his supersized adversary in the eye—so to speak.

As shown in fig. 16 from
Avengers # 139
, by increasing his rotational speed, Whirlwind is able to create a cushion of air that carries him aloft so that he can fight Giant-M an face-to-face. (Technically, at this stage, Henry Pym was using the superhero identity of Yellowjacket, a character who, like Ant-M an, employed the power of miniaturization to fight evildoers. However, when Whirlwind threatened his wife, Janet Van Dyne (who also is a shrinking superhero, known as the Wasp), who was lying ill in a hospital, Pym thought it best to battle Cannon as a twenty-foot tall protective husband rather than an insect. Whirlwind boasts that by increasing his rotation speed he can “fly like a helicopter,” and thus match Pym’s height advantage.

Helicopters hover in the air over a stationary point through the same mechanism that enables planes to fly—Newton’s third law of motion, which states that forces come in pairs. The blades of a helicopter are angled so that they deflect and push air molecules downward as they rotate, providing an upward thrust on the blades themselves. It is doubtful that this is what Whirlwind was doing as he used his mutant power of “super-rotation,” for no other reason than it is not obvious in fig. 16 what part of Cannon’s anatomy is filling the role played by the helicopter’s blades (the blades on his helmet being too small for the job).

More likely he is using the same trick that the Flash employs, as described in Chapter 4, when he was able to levitate the crook Toughy Boraz
³⁰
by creating a partial vacuum in the center of a column of rotating air. In this way David Cannon is making use of the physics of tornadoes, where the fast moving air in the twister cre ates a low-pressure region in the center. The faster the rotation, the lower the pressure and the greater the lifting force. Conservation of angular momentum is also the reason that helicopters have a tail rotor that is positioned at a right angle to the main lifting blades. Without this tail stabilizer, there can be no change in the total angular momentum of the helicopter when it’s off the ground and isolated from any external torques. As the blades spin faster, their angular momentum increases. However, as an isolated system, the total angular momentum of the helicopter is a constant. Consequently the increase in the blades’ angular momentum can only be achieved if it is compensated by the body of the helicopter itself rotating in the opposite direction. This ensures that the sum of the angular momentum of the blades and the helicopter body does not change. The body of the copter is fairly massive and has a large moment of inertia, so it won’t (thankfully) spin as fast as the blades, but it will twirl around, making navigation a challenge. This is prevented by the tail rotor, whose rotation in a direction perpendicular to the spinning top blades provides a balancing torque that keeps the body of the helicopter pointing in one direction.

Providing a counter-rotation to balance and negate the angular momentum of a rotating object, such as a tornado, is an old superhero trick. Whenever the Flash needed to counteract a destructive meteorological phenomenon such as a cyclone, he would do so by running at superspeed around the cyclone in the opposite direction of the twister’s rotation, creating what is technically known as an anti-vortex. The angular momentum of the anti-vortex is equal and opposite to that of the original vortex, and the two annihilate, as if the twister did not exist.

Speaking of rotation, surprisingly enough, a signature characteristic of a magic-based hero turns out to be physically accurate, provided we allow the standard miracle exception, of course. When the Norse god Thor needed to travel quickly from one location to another, he used his great strength to twirl his hammer at high speed. Throwing it in the direction he wanted to go, he would momentarily let go of the hammer’s handle strap and then grab on to it again, flinging himself through the air as an unguided missile. This would appear to be a perfect violation of the Principle of Conservation of Momentum. In fact, in
Bartman Comics # 3
(featuring the adventures of Bart Simpson’s superhero alter ego), Radioactive Man is so angered when he spies a Thor-like character taking flight in this manner that he socks the mock-Thor, intoning, “This is for breaking the laws of physics!” And yet, such a means of transportation is physically plausible.

When Thor twirls his hammer, the mighty Mjolnir, he plants his feet firmly on the ground. This is presumably what makes the X-Men villain the Blob so difficult to move—his mutant ability enables him to plant his feet so strongly that he effectively couples his center of mass to the Earth’s, such that dislodging the Blob requires moving the entire Earth unless the connection is broken. When Thor is ready to let fly, all he must do is jump slightly (breaking his connection with the Earth) at the moment he throws his hammer in the desired direction. He doesn’t even need to go through that business of releasing and re-grabbing the handle strap. If one is as strong as a thunder god, one can use this technique to fly through the air with the greatest of ease. No wonder they named a day of the week after this guy!

The universality of angular momentum is reflected in the fact that nearly everything undergoes some form of rotation, from the electrons, protons, and neutrons within all atoms to massive clusters of galaxies. We will consider subatomic rotations in Chapter 19, when we discuss how they can account for magnetism in iron and certain other metals. Studies of galactic rotations provide evidence for phenomenon even more mysterious than magnetism—Dark Matter.

Back in 1933, when Siegel and Shuster first conceived of Superman, astronomer Fritz Zwicky proposed that most of the mass of the universe was invisible, demonstrating once again that comic-book writers have always had to struggle to keep up with scientists’ seemingly fantastic proposals. (This is the same Fritz Zwicky who, along with Walter Baade, also in 1933, reported the first observation of neutron stars.) Observations of the Coma cluster of galaxies indicated to Zwicky that they were rotating too fast to be stable. In Chapter 7 we discussed centripetal acceleration, which is present whenever any object moves in a circle or parabolic arc, such as when Spider-Man swings on his webbing through New York. As Newton’s second law of motion insists, any acceleration must result from an external force. For Spider-Man the force causing his centripetal acceleration is provided by the tension in his webbing. For the rotating Coma cluster, it is the gravitational attraction of all of the stars in the galaxies that holds it together as it spins about an axis of rotation extending from the center of the cluster.

Consider the spiral galaxy NGC 3198, in the rather nearby Ursa Major constellation. As fortune would have it, this disc is oriented so that from Earth it appears nearly edge on, so we see little more than a thin strip. It is as if this spiral galaxy were an old-style music record on a turntable, and we are able to look along the edge of the record. We know that this massive collection of stars is rotating about an axis perpendicular through the plane formed by the galactic disc, from the Doppler effect. Recall our discussion in Chapter 4 of the shift in wavelength of sound or light waves when either wave source moves relative to the observer. If the source is moving away from the observer, the wavelength of light is increased compared with a stationary source, and is decreased when the source moves toward the observer. The faster the source is moving, the greater the wavelength shift, which is the basis for radar guns that measure a moving object’s speed.

The Doppler shift for light emitted by stars at the edges of rotating spiral galaxies indicates the speed at which the disc of stars is rotating. The intensity of light provides a determination of how many stars are present in the galaxy and how massive they are. But there must be more mass in this galaxy than we can see (which basically means more stars—for everything else is too small to contribute significant mass) in order to hold the galaxy together at the measured rotational speed. A lot more mass—as in nearly ten times more mass. If Spider-Man tries to swing too fast at the end of his line, so that the tension required is more than the webbing can support, the line will snap and he will be flung away from the smooth trajectory he started. Similarly, galaxy NGC 3198 should not be a stable object in the night sky, but should have flown apart based upon its rotational speed and the gravitational pull that can be accounted for by the stars it contains. Searching for dust clouds in the infrared portion of the spectrum, or for any other astronomical objects that might be hidden, has proven insufficient to account for the “missing mass.”