The Physics of Superheroes: Spectacular Second Edition (35 page)

Fig. 27.
Superboy demonstrates a practical knowledge of electromagnetic theory, as he constructs a portable electromagnet (from
Superboy # 1
)

Fig. 28.
A continuation of the scene from
Superboy # 1,
where the Teen of Steel uses his brains, in addition to his brawn, to capture the “Smash and Grab Gang.”

This is all perfectly correct, from a physics perspective. The dynamo is the source of the electrical current that passes through the few miles of wire around the engine. The current in the wire creates a magnetic field that projects from the center of the wire loop. If a magnetic material such as an iron locomotive engine is placed within the loop, it enhances the generated magnetic field. However, why the strong magnetic fields generated by Superboy’s homemade electromagnet do not also attract the nearby parked cars, any loose iron in any of the buildings, or cause the steel wheels of the locomotive to seize up, thereby preventing them from rotating, remains a mystery.

19

HOW MAGNETO BECOMES ELECTRO WHEN HE RUNS—
MAGNETISM AND FARADAY’S LAW

THE VERY FIRST VILLAIN
the X-Men encounter in
X-Men # 1
is Magneto, the mutant master of magnetism, whose superpower consists of the ability to generate and control magnetic fields. Magneto could hurl missiles at our heroes and deflect magnetic objects, yet would be powerless against a wooden baseball bat. In fact, even some metallic objects are immune to Magneto’s power: He is able to pick up an automobile easily enough, but not a silver spoon or gold bracelet. What determines whether some materials are magnetic, even without an electrical current passing through them, and others not? Where does magnetism come from?

The Special Theory of Relativity may be ultimately responsible for the magnetic field created by an electrical current that involves the motion of electrical charges, but what about magnets made of iron? The magnets we use to hold grocery lists to the front of our refrigerators don’t seem to have any moving parts, yet they still create magnetic fields. It turns out that relativity is ultimately responsible for the magnetism of a stationary hunk of iron, too.

Every proton, electron, and neutron in the universe has a tiny magnetic field associated with it. This field is barely noticeable compared with the Earth’s magnetic field, or fields created by an electrical current. The electrons orbiting the nucleus can be (very roughly) considered tiny current loops that generate magnetic fields. But even without this “orbital” effect, there is still an internal magnetic field within atoms. Where does this minuscule intrinsic magnetic field of subatomic particles come from? The answer involves quantum mechanics, which we will discuss in the next section. A principle of the Special Theory of Relativity is that space and time should properly be considered on an equal footing as a single entity, called space-time. When this relativistic adjustment is made to the basic equation of quantum physics, the theory predicts that electrons should have a very small internal magnetic field, the theoretically predicted magnitude of which agrees precisely with the measured value. The internal magnetic field of electrons, protons, and neutrons is understood mathematically only in the relativistic version of quantum mechanics. Even for stationary matter, relativity turns out to be crucial for understanding magnetism. So, no Einstein, no relativity, and, hence, no magnetism. No magnetism, no magnetic iron, and, most importantly, no refrigerator magnets! Therefore, without relativity there is no way to keep our shopping lists from falling to the floor and lying there, discarded and unread. Without Einstein’s towering achievement in theoretical physics, a slow and lingering death of starvation would await us all.

Normally, the small magnetic fields of electrons inside an atom like to pair up, just as when you bring two magnets together, they orient themselves to align at separate poles. When the magnetic fields inside an atom pair, there is no net magnetic field associated with the atom, just as an ordinary atom will have no net electric field, because the number of positive protons in the nucleus is balanced by an equal number of negatively charged electrons. Most materials, such as paper and plastic, are not magnetic, and even most metals, such as silver and gold, have all of their magnetic moments paired up.
⁶²

If most materials do not have net magnetic fields because their atomic magnetic moments are paired up, then how is Magneto able to levitate himself and other people, as shown in fig. 29? The physical basis behind this trick is that Magneto is able to generate such a large magnetic field that he essentially polarizes the internal magnetic fields of our atoms, turning us, or any other object, into a magnet.

Before we begin this discussion of magnetic levitation, I first must stress that Magneto does not lift people through his influence on the iron in their blood. Let’s leave aside the question of the effect of an inhomogeneous pressure on the veins and arteries in a person’s body (this would get messy), and focus instead on the blood’s magnetism. A few metals, such as iron and cobalt, have just the right configuration of non-paired electrons’ internal magnetic fields, so that the atom has a net magnetic field. However, the iron in your blood occurs primarily in the form of hemoglobin, a protein used to collect and transport oxygen and carbon dioxide as you breathe. Hemoglobin is a very large molecule that consists of four large proteins (called globins, which look like folded worms) bonded together. Each of these proteins contains a large molecule termed a “heme” group, composed of carbon, nitrogen, oxygen, hydrogen, and iron. The iron atoms in the center of each heme molecule are chemically bonded to their neighboring atoms. There’s another technical term for an iron atom chemically bonded to oxygen atoms: rust. As anyone who has dealt with scrap metal can confirm, rust makes for a very weak magnet. The common form of rust has three oxygen atoms bonded to two iron atoms (called “hematite”) and is nonmagnetic, though four oxygen atoms bonded to three iron atoms (termed “magnetite”) is magnetic. The magnetic field of the iron in hematite disappears when it combines with oxygen atoms, because the iron and oxygen chemically sharing their electrons pair up the remaining uncanceled electronic magnets in the iron. Depending on whether the hemoglobin has picked up an extra oxygen molecule to bring to the cells, or is carrying a carbon-dioxide molecule to be exhaled, the iron can either have an uncanceled magnetic field or not. But at any given moment only a fraction of your blood is even capable of being affected by an external magnetic field.
⁶³

Fig. 29.
Scene from
X-Men # 6
(above) and
X-Men #1
(left), as Magneto attacks (above) or escapes (left) from the X-Man the Angel (the one with the wings), illustrating Magneto’s ability to levitate nonmagnetic objects, such as a boulder or himself, through the principle of diamagnetic levitation.

Even when iron is not chemically bonded to oxygen atoms, it is possible that it will be nonmagnetic, if all of the individual atoms are not properly aligned. Ordinarily the atoms inside a piece of iron or cobalt will line up, forming small regions termed “domains,” where all the iron atom’s magnetic fields point in the same direction. However, entropy considerations lead to the domains pointing in different directions, so their combined magnetic fields cancel out. Heat up a bar of iron so that the atoms have a lot of thermal energy and are free to rotate, and then place it in a strong external magnetic field. The external field induces the majority of the domains to all point in the same direction, so that the piece of iron, when cooled back to room temperature, has a large net magnetic field. If you hit the magnetized iron bar with a hammer or heat it in an oven, you will cause the magnetic domains to reorient themselves randomly, with the effect being that the magnet will lose nearly all of its field strength. Some flexible, credit card-size refrigerator magnets have their magnetic domains aligned in little strips along their length. Rather than have all of their domains point in the same direction, it is easier to line them up so that one strip has its north pole pointing toward the refrigerator, while the adjacent strip has its north pole heading away from the fridge, and so on.
⁶⁴

Materials that form magnetic domains with magnetic fields pointing in the same direction are called “ferromagnetic” (so named after iron, the most famous example). Many atoms in solids have a very weak magnetic interaction with their neighbors, so if placed in a strong external magnetic field, they will align in the direction of the field but will randomize again at room temperature once the field is removed. These materials (such as molecular oxygen, gaseous nitric oxide, and aluminum) are termed “paramagnetic.” And there is a third class of materials in which, due to the nature of the interactions between adjacent atoms and the chemical ordering of the atoms, their atomic magnets (generated by electron orbits within the atoms) line up opposite an external magnetic field. If an external magnetic field is applied to these materials and the field’s north pole points up, the atomic magnet’s north pole rotates to point down. These materials are called “diamagnetic,” and they try to cancel out any external magnetic field. Gold and silver are diamagnetic—if you are able to pick up your jewelry using a refrigerator magnet—someone has some explaining to do! Water molecules are also diamagnetic, and since we are primarily composed of water, so are we.

It is through our diamagnetism that Magneto is able to levitate himself and other people, as shown in fig. 29. In moderate-strength magnetic fields, the atoms in your body are not susceptible to being polarized. The diamagnetic interaction is weak, such that at room temperature, the normal vibrations of the atoms overwhelm the attempt to magnetically align them. In a very strong field, roughly two hundred thousand times greater than the Earth’s magnetic field (and more than one hundred times larger than the field of a refrigerator magnet), the diamagnetic atoms in your body can be induced to all point in the same direction—opposite to the direction of the applied field. Just as two magnets repel if they are brought together so that their north poles are facing each other, the now magnetically polarized person will be repelled by the external magnetic field Magneto is creating—the very field that magnetically aligned the atoms in the first place. As Magneto increases the magnetic field he generates, the magnetic repulsion can become strong enough to counteract the downward pull of gravity. (That is, the upward force of the magnetic repulsion can be equal to or larger than the downward force of the person’s weight, and there is then a net upward force on the person, lifting him off the ground.) It takes a very big magnetic field requiring a great deal of power to accomplish this, and the heavier the person, the larger the effort. But it can be done, and the High Field Magnetic Laboratory at the University of Nijmegen in the Netherlands has amusing images and movies on their website of floating frogs, grasshoppers, tomatoes, and strawberries, demonstrating the reality of diamagnetic levitation.