Energy | Pages 126-127 | See Linked Version

Temperatures slowly rose, baking more carbon dioxide out of rocks near the surface. At some point a "runaway" greenhouse effect took over. It didn't stop until the surface reached a scalding 900 degrees Fahrenheit--hot enough to vaporize lead. Today, the atmosphere of Venus is almost 100 times denser than that of Earth and consists of 96 percent carbon dioxide.

Microwave radiation, slightly less energetic than infrared, is most familiar as a means to rejuvenate last night's casserole. A microwave oven heats the water molecules within all food. The molecules jiggle and vibrate, releasing their newly acquired microwave energy in the form of kinetic energy. Water is common in space as well, and it behaves the same way as it does in your kitchen. In clouds of cool gas, billions of tons of water vapor absorb microwaves emitted by nearby stars. Under certain conditions, charged gases around the clouds can stimulate the water molecules to emit microwaves as well. These microwaves are in turn absorbed and reemitted by molecules in the cloud, creating a powerful feedback loop. When this cycle becomes especially intense, beams of microwave radiation shoot into space. This curious event is called "microwave amplification by stimulated emission of radiation," or "maser" for short. Astronomers see these microwave outbursts all over the sky. If the word "maser" sounds familiar, it should. "Laser" is nearly the same acronym, with the word "microwave" replaced by "light." Your CD player and pocket laser pointer do the same thing as a maser, except they amplify beams of visible light (usually red) instead of microwaves. If you had microwave vision, you'd also see pulses of radar from your friendly highway patrol officer and blazes of light from the relay towers used to transmit cellular telephone calls.

Microwave astronomy provides the most compelling evidence of our origins. Theory suggests that the universe was born in a Big Bang. If so, then its early searing heat--with temperatures that reached trillions of degrees--should have wafted evenly through space as the universe expanded. After billions of years, space should still be awash with the residual warmth, although at very low levels--less than three degrees above absolute zero, the coldest possible temperature. In 1965 the physicists Arno Penzias and Robert Wilson found this "cosmic microwave background radiation." The radiation has its peak strength in microwaves, but it also contains radio waves. Penzias and Wilson didn't even plan their observation, but it turned into the most important single discovery in the history of cosmology. Other lines of evidence also support the Big Bang, but microwaves are the strongest link. The next time you hear static between your AM radio stations at night, keep in mind that you are picking up a few cosmic microwaves from every direction in the universe--radiation that dates from the beginning of everything we know.

Stimulating Light

Light waves with lengths ranging from the thickness of a dime to that of a dime-store novel are called microwaves. In nature, microwaves are emitted by clouds of interstellar gas and also by the gaseous shrouds of newly formed stars. These interstellar clouds are largely made up of diatomic hydrogen, two atoms of hydrogen joined in a single molecule. When the clouds are surrounded by energetic plasma or radiation fields, the water molecules in the cloud can be stimulated to emit microwaves as well. A feedback loop can result (right), producing powerful beams of microwave radiation shooting into space. This phenomenon is called "microwave amplification by stimulated emission of radiation," or "maser" for short.

In interstellar space, a diatomic molecule of hydrogen (red), collides with a water molecule, shown here with a blue oxygen atom and red hydrogen atoms.

The collision transfers energy to the water molecule, causing it to jump to a higher energy state. The hydrogen molecule loses energy in the process.

When the excited water molecule drops to a lower energy level, it emits a photon (yellow). Rather than dropping to the lowest ground state, the molecule shown here has dropped to an intermediate metastable state, where it can remain for some time.

When a passing photon strips the metastable molecule of its energy, it drops to the ground state and emits a photon of the same wavelength, traveling in the same direction as the incoming photon. On encountering other metastable molecules, the two photons set off a chain reaction of emissions.