dioxide, and nutrients into new wood. Not long in the future the furniture will rot and return to the soil, or perhaps burn and waft into the atmosphere as soot. Earth will outlive these belongings, but 5 billion years from now it too will incinerate as the Sun runs out of fuel and bloats into a giant ball that engulfs the inner planets. The stars themselves live and die, converting matter into energy along the way. Their deaths seed space with fresh bursts of matter--the raw material needed for the next generation of stars.

When matter undergoes a dramatic change, we usually can trace the transformation to changes in temperature, pressure, or density. On Earth we are accustomed to a narrow range of these properties. For instance, we live mostly at room temperature. Our notions of "hot" and "cold" span just a few dozen degrees on the thermometer. We have also adapted to the pressure of air and water near sea level. When we fly at high altitudes or dive deep into the sea, we need to enclose ourselves in vessels with similar pressures for our bodies to function. Similarly, we experience gases with densities typical of air, liquids with densities close to that of water, and solids with densities like the ground under our feet.

These natural biases make conditions elsewhere in the universe seem exotic. But in fact our Earthly conditions are rare. Temperatures in space plummet to near absolute zero (minus 273 degrees Celsius), the point at which all motion ceases except for ever-present quantum vibrations. Things are more extreme on the hot end of the scale, where temperatures soar to tens of millions of degrees in the cores of stars. Our sister planet Venus has an atmospheric pressure nearly 100 times greater than Earth's, enough to squash a nuclear submarine. The thick atmospheres of the planet Jupiter and the other gas giants create pressures thousands of times higher still. Deep inside the planets, this overwhelming force converts gases into bizarre states with liquid or even metallic properties. The range of densities in our universe is staggeringly large as well, from the emptiness of intergalactic space to the crushed interiors of dead stars.

We can gain some sense of how matter responds to these alien settings by re-creating extreme conditions in laboratories on Earth. For example, modern vacuum pumps, such as those used to insulate cryogenic components or to evacuate particle accelerators for astronomy and physics research, easily remove 99.99999 percent of all air from a sealed container. The best scientific pumps in the world can make small vacuums thousands of times better than that. Experiments in such vacuums offer clues about how matter may behave in the cold emptiness of space, where interactions between atoms and molecules are rare.

Even so, our vacuum chambers are crude approximations of the cosmos. Concentrations of gas between stars, such as the colorful Orion Nebula, contain up to a billion atoms and molecules of gas per cubic yard of space. Amazingly, that is 10 million times less dense than the best laboratory vacuums ever produced. Most of the rest of our galaxy is a thousand times more rarefied still. The space between galaxies is the most desolate void of all: less than one atom drifting in each cubic yard. Imagine a volume of space measuring 125,000 miles on a side--a box big enough to stretch halfway to the Moon. In intergalactic space such a box would contain about as many atoms as the air in your refrigerator.

Here's another way to think about the rarity of matter. If we spread all of the universe's matter evenly throughout space, we would see just a few atoms in each cubic yard. To achieve that same density with the number of atoms in half a grain of rice, we'd have to expand the grain to the size of Earth. Needless to say, our local environment is one of the rare spots in space that contain matter in abundance. An average cubic yard of air at Earth's surface holds about 50 trillion trillion atoms, making our planet a place where matter matters.

Other clever tools help us explore the realm of hot and dense matter. One such apparatus is the diamond anvil cell, a vise equipped with two flat-tipped diamonds. Physicists place tiny mineral samples between the diamonds, crush them at pressures mimicking those near Earth's core, and heat them with lasers to thousands of degrees. This opens an experimental window on the interior of our planet, which otherwise is shielded by thousands of miles of rock. The results help us understand how minerals behave at key boundaries within the planet. For example, Earth's solid rocky mantle lies atop a core of liquid iron, with an abrupt and active transition between these layers. At the center of that molten core sits a ball of solid iron. Geophysicists can determine its depth with laboratory experiments that gauge how iron reacts to the hellish temperatures and pressures of inner Earth. Other rocky planets and moons appear to have similar internal layers, although only Jupiter's moon Io is as dynamic as Earth.

To delve into even more extreme conditions, we must call upon the most powerful lasers yet invented. Physicists use pulses of light that carry as much energy as the entire (continued)