JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Microwave window. Graph showing the variation with frequency of the radio noise (equated with Kelvin temperatures) arriving from space at the Earth's surface. Total noise level (bold line bounding the blue area) is least in the 1000-10,000 megahertz frequency microwave region. Two prominent lines, found in this region, arise from the emission of microwaves by interstellar hydrogen atoms (H) and hydroxyl molecules (OH). The range of frequencies bounded by these lines (water hole, dark blue), is thought to be a good place to broadcast and listen for messages from space. Three components of the total radio noise are shown: atmospheric effects, galactic noise and cosmic background radiation.
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