Contact

by Carl Sagan

She was working on the ruby maser. A ruby is made mainly of alumina, which is almost perfectly transparent. The red color derives from a small chromium impurity distributed through the alumina crystal. When a strong magnetic field is impressed on the ruby, the chromium atoms increase their energy or, as physicists like to say, are raised to an excited state. She loved the image of all the little chromium atoms called to feverish activity in each amplifier, frenzied in a good practical cause—amplifying a weak radio signal. The stronger the magnetic field, the more excited the chromium atoms became. Thus the maser could be tuned so that it was particularly sensitive to a selected radio frequency. She found a way to make rubies with lanthanide impurities in addition to the chromium atoms, so a maser could be tuned to a narrower frequency range and could detect a much weaker signal than previous masers. Her detector had to be immersed in liquid helium. She then installed her new instrument on one of Cal Tech’s radio telescopes in Owens Valley and detected, at entirely new frequencies, what astronomers call the three-degree black-body background radiation—the remnant in the radio spectrum of the immense explosion that began this universe, the Big Bang.