Read Online The Physics of Superheroes: Spectacular Second Edition by James Kakalios - Free Book Online Page A

of positively charged protons in an atom is balanced by an equal number of negatively charged electrons. Electrons are very light compared to protons or neutrons, which are electrically uncharged particles that weigh slightly more than protons and reside in a nucleus. (We’ll discuss what the neutrons are doing in the nucleus in Chapter 16.) Nearly all the mass of an atom is determined by the protons and neutrons in its nucleus, because electrons are nearly two thousand times lighter than protons.
    The size of an atom, on the other hand, is determined by the electrons or, more specifically, their quantum mechanical orbits. The diameter of a nucleus is about one trillionth of a centimeter, while the radius of an atom is calculated by how far from the nucleus one is likely to find an electron, and is about ten thousand times bigger than the nucleus. If the nucleus of an atom were the size of a child’s marble (a diameter of 1cm) and placed in the end zone of a football field, the radius of the electron’s orbit would extend to the opposite end zone, 100 yards away. The spacing between atoms in a solid is governed essentially by the size of the atoms themselves (you can’t normally pack them any closer than their size).
    Thus, if quantum mechanics is the same on Krypton as on Earth, the space taken up by a given number of atoms in a rock (for example) will not depend significantly upon which planet the rock resides on. The rock will weigh more on a planet with a larger gravity, but the number of atoms it contains—as well as the spacing between the atoms, both of which determine its mass density—will be independent of which planet the rock finds itself on. Because the number of atoms also determines the mass of the rock, it follows that the density of any given object will be the same, regardless of the planet of origin.
    Most solid objects have roughly the same density, at least within a factor of ten. For example, the density of water is 1 gram/cm 3 while the density of lead is 11 gram/cm 3 (a gram is one thousandth of a kilogram). In other words, a cube that measures 1cm on each side would have a mass of 1 gram if composed of water and 11 grams if composed of lead. The higher density of lead is due almost entirely to the fact that a lead atom is ten times more massive than a water molecule. While there is a lot of water on the surface of the Earth, there’s even more solid rock within the planet, so that Earth’s average density is 5 gram/cm 3 . In fact, Earth is the densest planet in our solar system, with Mercury and Venus close behind. Even if Krypton were solid uranium, it would have an average density of 19 gram/cm 3 , which is not even four times larger than Earth’s. In order for Krypton to have a gravity fifteen times greater than Earth’s due to a larger density alone, it would have to have a density fifteen times larger than Earth’s 5 gm/cm 3 —that is, 75 gram/cm 3 —and no normal matter is this dense.
    If the density of planet Krypton couldn’t be much greater than Earth’s, perhaps the heavier gravity on Krypton is due to it being a larger planet—one with a radius fifteen times larger than Earth’s. While planets in our own solar system come in all sizes—from Pluto, with a radius one fifth as large as Earth’s, making it just barely bigger than some moons, to Jupiter, with a radius of more than eleven times Earth’s—the geology of the planet is a sensitive function of its size. Planets bigger than Uranus, with a radius four times larger than Earth’s, include Neptune, Saturn, and Jupiter. These planets are gas giants, lacking a solid mantle upon which buildings and cities may be constructed, let alone supporting humanoid life. In fact, if Jupiter were ten times larger, it would be the size of our own sun. In this case, the gravitational pressure at Jupiter’s core would initiate nuclear fusion, the process that causes our sun to shine. So, if Jupiter were just a bit larger, it would no