In the beginning: back in the olden days—the first trillionth of a second after the big bang—energy was matter, matter was energy, and E=[mc.sup.2] ruled

Physics describes the behavior of matter, energy, space, and time, and how the forces of nature enable their interplay. From what scientists have been able to determine, all biological, chemical, and physical phenomena emerge from how four, and only four, forces push and pull the contents of the universe. But is that all there is?

In almost any area of scientific inquiry--but particularly in physics--the frontiers of discovery live at the extremes of measurement. At the extremes of matter, such as the neighborhood of a black hole, you find gravity (one of the four forces) badly warping the surrounding fabric of space-time. At the extremes of energy, you sustain thermonuclear fusion in the ten-million-degree cores of stars (where the attraction of the strong nuclear force overwhelms the repulsion of the electromagnetic force). And at every extreme imaginable, you get the outrageously hot, outrageously dense conditions that prevailed during the first few moments of the universe.

Daily life, I'm happy to report, is entirely devoid of extreme physics. On a normal morning, you get out of bed, wander around the house, eat something, dash out the front door. And by day's end, your loved ones fully expect you to look no different than you did when you left, and to return home in one piece. But imagine arriving at the office, walking into an overheated conference room for an important 10 A.M. meeting--and suddenly losing all your electrons. Or worse yet, having every atom of your body fly apart. Or suppose you're sitting in your office trying to get some work done by the light of your desk lamp, and somebody flicks on the overhead light, causing your body to ricochet from wall to wall until you're jack-in-the-boxed out the window. Or what if you go to a sumo wrestling match after work and see the two spherical gentlemen collide, disappear, then spontaneously become two beams of light?

If that kind of scene played itself out daily, modern physics wouldn't look so bizarre, knowledge of its foundations would flow naturally from life experience, and our loved ones probably would never let us go to work. But back in the early minutes of the universe, those antics happened all the time. To envision that era, and understand it, one has no choice but to establish a new form of common sense, an altered intuition about how physical laws apply at the extremes of temperature density, and pressure.

Enter the world of E=[mc.sup.2].

Einstein first published a version of s his famous equation in 1905, in a seminal research paper titled "On the Electrodynamics of Moving Bodies." Better known as the special theory of relativity, the concepts advanced in that paper forever changed the understanding of space and time. Einstein, then just twenty-six years old, offered further details about his tidy equation in a separate, remarkably short paper published later that year: "Does the Inertia of a Body Depend on Its Energy-Content?" To save you the effort of digging up the original article, designing an experiment, and testing the theory, the answer is "Yes." As Einstein wrote,

If a body gives off the energy E in the form of radiation, its mass diminishes by E/[c.sup.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....

Sensibly cautious about the truth of his statement (it was a theoretical prediction, after all), 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 it is: the algebraic recipe for all occasions when you want to convert matter into energy or energy into matter. In those simple sentences Einstein unwittingly gave astrophysicists a computational tool, E=[mc.sup.2], that extends their reach from the universe as it now is, all the way back to infinitesimal fractions of a second after its birth.

The most familiar form of energy is the photon, a massless, irreducible particle of light. You are forever bathed in photons: from the Sun, the Moon, and the stars, to your stove, your chandelier, and your night light. So why don't you experience E=[mc.sup.2] every day? The energy of visible-light photons is far less than the amount of energy that is equivalent to the mass of the least massive subatomic particles. There is nothing else those photons can become, and so they live happy, though boring, lives.

Want a little action? Start hanging around gamma-ray photons that have some real energy--at least 200,000 times more than that of visible photons. You'll quickly get sick and die of cancer, but before that happens, you'll-see something truly weird. Matter-antimatter pairs of electrons--one of the many dynamic duos in the particle universe--pop into existence where photons once roamed. Yes, energy turns into matter. Then, as you watch, you'll see some of the matter-antimatter pairs of electrons collide, annihilating each other and creating gamma-ray photons once again. And yes, matter turns back into energy. It all happens according to E=[mc.sup.2].