electrical grid of the United States, but only for a trillionth of a second or so. These beams eradicate small targets placed in their paths. The temperatures and pressures within the tiny blasts approach those inside our Sun or planets like Jupiter . One of the goals of this research is to harness the energy of thermonuclear fusion that powers the Sun. That would be a much cleaner source of energy than nuclear power from the fission of uranium . However, it may take decades to learn how to sustain the fierce fusion reactions in a controlled and profitable way.

In the meantime, the experiments have shown us that hydrogen--the main component of Jupiter , Saturn , Uranus , and Neptune --takes on distinctly ungaslike properties as pressures and temperatures rise within those planets. For example, lasers have compressed and heated hydrogen into a form that appears to conduct electricity as efficiently as a metal. This odd transformation may in fact occur near Jupiter 's core, helping to produce a powerful magnetic field around the planet.

We clearly still have much to learn about how matter behaves as we move from one extreme in the universe to the other. Even so, our understanding of the nature of matter has evolved considerably since the Greek philosopher Leucippus and his student Democritus first proposed the idea of the atom in about 440 B.C. Leucippus and Democritus pondered how long a piece of iron would retain the basic properties of iron if one broke it in half again and again. They theorized that there was a basic particle, a corpuscle of matter, beyond which one could go no smaller. All matter in the universe, they reasoned, was made of these "atoms ," from the Greek word for "indivisible."

Not until the early twentieth century did we learn that atoms weren't simply an idea of convenience. The New Zealand­born physicist Ernest Rutherford did the most to prove their existence and discern their structure. Prior to his work, physicists envisioned atoms as diffuse blobs. This model held that negatively charged electrons were embedded in the blobs like raisins in a positively charged plum pudding . Rutherford and his colleagues tested that notion by firing particles at a thin gold foil . The particles, which themselves carried a positive charge, came from the radioactive decay of a small amount of uranium and moved at 5 percent of the speed of light. Most particles streamed through the gold foil, as expected. However, a tiny fraction bounced at sharp angles or even reflected back toward the gun. This result amazed Rutherford. As he said later, "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

It immediately became clear that the "plum pudding " model of the atom didn't work. A diffuse spread of positive charge within the atoms couldn't possibly make any incoming particles ricochet backward. Instead, it seemed, the particles encountered hard nuggets of positive charge in the atoms and were repelled, just as the north poles of two magnets repel each other. When Rutherford calculated how concentrated those charges had to be, he determined that each gold atom contained a nucleus measuring just 1/100,000th the diameter--and 1 million-billionth the volume--of the entire atom. The electrons darting around this nucleus carried the atom's negative charge but virtually no mass. It was no exaggeration for Rutherford to conclude that atoms were almost entirely empty space.

To put his shock into visual terms, imagine enlarging an atom until its cloud of electrons fills the volume of the Louisiana Superdome in New Orleans. The site of many football Super Bowls, the Superdome is nearly 700 feet across. Now picture a single ball bearing, one-twelfth of an inch wide, suspended in the center of the dome's cavernous volume. The ball bearing represents the atom's nucleus, surrounded by electrons flitting about within an enormous void. That's an accurate scale model of an atom as implied by Rutherford's work. Indeed, if we could somehow remove the spaces from within the atoms that make up our planet, the entire Earth would fit easily under the Superdome's roof.

Given the seemingly porous nature of every atom on Earth, what keeps us from walking through walls or sinking into the ground? The answer is electrostatic repulsion on an atomic scale. Clouds of electrons around every nucleus create what amounts to a ball of negative charge. Since negative charges repel each other, the electron clouds set up impenetrable force fields around atoms . Only the catastrophic crushing power of a dying star can overcome that barrier (page 93). Two other forces operate on the tiny scale of a nucleus. One is the aptly named strong nuclear force , which binds together protons (positive charges) and neutrons (neutral or no charge) within the nucleus. The second is the weak nuclear force , which mediates the radioactive decay of unstable elements. Don't let the name fool you, however, because the weak nuclear force is still vastly stronger than gravity on these minuscule scales.

Physicists probe the precise workings of these forces by smashing together particles in powerful accelerators . Their results apply not only to matter on Earth but (continued)