The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 6: Technology

Transmitting Beyond Hydrogen

Dear Dr. SETI:
I'm a pre-final year mechanical engineering student at BCET,Punjab, and I attended your lecture at IIT, Kanpur last week. In your presentation you said that we have been sending signals to space on the hydrogen spin-flip frequency, as we predict the aliens will recognise those signals. As hydrogen is the most abundant element in the universe, they will surely identify our urge to make contact. But, isn't such single-frequency selection limiting?

Navdeep, India

The Doctor Responds:
I think you may have misunderstood my point, Navdeep. Most SETI experiments involve reception, not transmission. (There are, indeed, a few projects that transmit messages into space -- they are classified as the companion activity METI: messaging to extraterrestrial intelligence - but those are not what I was talking about in my comments about frequency selection. In fact, METI relies on existing equipment, so transmits on any convenient frequency for which that particular equipment happens to be optimized.)

The very earliest SETI experiments, starting 50 years ago, involved equipment capable of monitoring but a single channel. These experiments started off monitoring at or near the neutral hydrogen frequency for several reasons:

  1. We already had equipment that operated there - it was being used for astrophysical research, and was available.
  2. Hydrogen is the strongest interstellar emission line, in the quietest part of the spectrum. Thus, it was the first frequency area widely exploited by radio astronomers.
  3. Another civilisation would know this, and deduce that other civilizations start out listening in the hydrogen-line region. If trying deliberately to attract the attention of emerging technological societies, it seemed logical that they would transmit here, where they reasoned we were already receiving.

Of course, with the advent of modern computers and the development of multi-channel spectrum analyzers (MCSAs), these arguments became moot. Today's receivers can monitor millions or billions of channels at once, so the idea of using "magic frequencies" is less attractive.

Still, the electromagnetic spectrum is nearly infinite in scope. It makes the task easier if we can limit our search space to something more manageable. Just a little way up from the Hydrogen emission line is another cluster of radiation lines, emanating from interstellar hydroxyl (OH) ions. These two lines, hydrogen and hydroxyl, are separated by about 240 MHz, and are in the very quietest part of the electromagnetic spectrum.

Additionally, H and OH are the disassociation products of water, the solvent which is essential for the development of life as we know it. Other civilizations that are water-based and know chemistry will recognize the significance of these two emission lines, and perhaps consider them gateposts, allowing nature to define for us an obvious interstellar communications band. In 1971, Dr. Bernard M. Oliver (then vice president of engineering at Hewlett Packard) named the band of frequencies between H and OH the "water hole". "Where shall we find our cosmic neighbors?" asked Barney? "At the water hole, where species have always gathered."

A modern MCSA can monitor the entire water hole region, in real time, dividing it up into 240 million channels, each 1 Hz in bandwidth. Although there are other interesting parts of the electromagnetic spectrum as well (and various SETI experiments do exploit them), I would say that the water hole region is still the most popular frequency range among SETI League members.

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