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Ask Dr. SETI ®

Chapter 2: Biochemistry

What is the Hydrogen Line?

Dear Dr. SETI:
What is the hydrogen line, and why are radio or RF transmissions prohibited at this frequency? Thanks for any info.

Chris, Florida

The Doctor Responds:
The hydrogen line (1420.40575 MHz) is the precession frequency of neutral hydrogen atoms, the most abundant substance in space. It happens to fall in the quietest part of the radio spectrum, what's known as the Microwave Window. Although there may not seem to be a lot of loose hydrogen atoms about (there's perhaps one per cubic centimeter of interstellar space), the interstellar medium contains a lot of cubic centimeters. So these individual atoms chirping away at 1420 MHz make a powerful chorus, which is readily detected by even small radio telescopes.

Hydrogen line radiation was first detected by Harvard grad student Harold Ewen and his professor, Edward Purcell, in 1951. Their instrumentation, a simple waveguide horn antenna about a meter across driving a crude diode mixer, is now on display at the National Radio Astronomy Observatory (NRAO), Green Bank WV. It's also been memorialized in song.

Why is the hydrogen line protected spectrum? It's a great frequency for observing the structure of the universe, and some of the best and most detailed Milky Way radio maps have been made on the hydrogen line. It is probably the world's most popular radio astronomy frequency, and the International Telecommunications Union (ITU) has the good sense to protect it.

In 1959 two scholars (Philip Morrison at Cornell University and Frank Drake at NRAO) independently recognized that the hydrogen line would be a likely frequency for interstellar beacons. They reasoned that more advanced civilizations would reason that young civilizations (like ours) might already be listening there. Based upon that circular reasoning, Morrison went on to co-author the world's first modern SETI article ("Searching for Interstellar Communications," Nature 184(4690):844-846, September 19, 1959), and Drake conducted a the first modern SETI study, "Project Ozma," a hydrogen line search of two nearby Sun-like stars for possible artificial signals.

Over the past forty years, about three dozen other hydrogen line searches have been conducted. It was on the hydrogen line that in 1977 the Big Ear radio telescope at the Ohio State Radio Observatory detected the so-called "Wow!" signal, the most promising, intriguing and beguiling SETI candidate signal to date. The "Wow!" is also the best known SETI signal, having been featured in the "X-files." After about 100 follow-up attempts over a twenty year period, that signal has never repeated and remains unexplaned.

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