Origins: Fourteen Billion Years of Cosmic Evolution Read Free Page B

abhängig?” or “Does the Inertia of a Body Depend on Its Energy Content?” To save you the effort of locating the original article, of designing an experiment, and of thus testing Einstein’s theory, the answer to the paper’s title is yes. As Einstein wrote,
    If a body gives off the energy E in the form of radiation, its mass diminishes by E/c 2 . . . .The mass of a body is a measure of its energy-content; if the energy changes by E, the mass changes in the same sense.
    Uncertain as to the truth of his statement, he then suggested,
    It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test. *
    There you have it: the algebraic recipe for all occasions when you want to convert matter into energy, or energy into matter. E = mc 2 —energy equals mass times the square of the speed of light—gives us a supremely powerful computational tool that extends our capacity to know and understand the universe from as it is now, all the way back to infinitesimal fractions of a second after the birth of the cosmos. With this equation, you can tell how much radiant energy a star can produce, or how much you could gain by converting the coins in your pocket into useful forms of energy.
    The most familiar form of energy—shining all around us, though often unrecognized and unnamed in our mind’s eye—is the photon, a massless, irreducible particle of visible light, or of any other form of electromagnetic radiation. We all live within a continuous bath of photons: from the Sun, the Moon, and the stars; from your stove, your chandelier, and your nightlight; from hundreds of radio and television stations; and from countless cell-phone and radar transmissions. Why, then, don’t we actually see the daily transmuting of energy into matter, or of matter into energy? The energy of common photons sits far below the mass of the least massive subatomic particles, when converted into energy by E = mc 2 . Because these photons wield too little energy to become anything else, they lead simple, relatively uneventful lives.
    Do you long for some action with E = mc 2 ? Start hanging around gamma-ray photons that have some real energy—at least 200,000 times more than visible photons. You’ll quickly get sick and die of cancer; but before that happens, you’ll see pairs of electrons, one made of matter, the other of antimatter (just one of many dynamic particle-antiparticle duos in the universe) pop into existence where photons once roamed. As you watch, you’ll also see matter-antimatter pairs of electrons collide, annihilating each other and creating gamma-ray photons once again. Increase the photons’ energy by another factor of 2,000, and you now have gamma rays with enough energy to turn susceptible people into the Hulk. Pairs of these photons wield enough energy, fully described by the power of E = mc 2 , to create particles such as neutrons, protons, and their antimatter partners, each nearly 2,000 times the mass of an electron. High-energy photons don’t hang out just anywhere, but they do exist in many a cosmic crucible. For gamma rays, almost any environment hotter than a few billion degrees will do just fine.
    The cosmological significance of particles and energy packets that transform themselves into one another is staggering. Currently, the temperature of our expanding universe, found by measuring the bath of microwave photons that pervades all of space, is a mere 2.73 degrees Kelvin. (On the Kelvin scale, all temperatures are positive: particles have the least possible energy at 0 degrees; room temperature is about 295 degrees; and water boils at 373 degrees.) Like the photons of visible light, microwave photons are too cool to have any realistic ambitions of turning themselves into particles via E = mc 2 . In other words, no known particle has a mass so low that it can be made from the meager energy of a microwave photon. The