There’s a little Jansky in all of us

Posted by Daniel Pendick
on Monday, June 18, 2007

This week, scientists from around the world are gathering in Charlottesville, Virginia, to celebrate the 50th anniversary of the National Radio Astronomy Observatory (NRAO). The NRAO, headquartered on the campus of the University of Virginia in Charlottesville, designs, builds, and operates radio telescopes. Scientists all over the world use NRAO instruments to explore the universe in wavelengths not visible to optical telescopes. Pulsars were first detected by a radio telescope, for example, in 1967. In a typical year, more than 1,100 scientists use 20,000-plus hours of observing time on NRAO telescopes.
 
NRAO helped usher in the age of giant radio telescopes, but the origin of the field traces back more than 50 years, to 1932, when a radio engineer named Karl Jansky (1905–1950) became the first human to hear radio broadcasts from the Milky Way Galaxy.
  
In 1928, with a physics degree from the University of Wisconsin, Jansky went to work for Bell Telephone Laboratories in New Jersey. Bell was investigating the use of shortwave radio for communicating across the Atlantic Ocean. Jansky was assigned to identify sources of static that might interfere with shortwave transmissions. So he built what could be called a primitive radio telescope.

The antenna consisted of a long armature, 100 feet (30m) long, vaguely resembling a biplane wing. It supported five metal frames about 20 feet (6m) high. The armature could rotate on a circular track, carried on four car tires. By rotating the antenna and recording the radio signal as it strengthened or weakened, Jansky could figure out the source of the signal. It's the same way you locate the source of a noise by turning your head in different directions to see where the sound seems loudest.

Working in his potato-field listening post, Jansky identified radio static coming from nearby thunderstorms. He also traced static to distant thunderstorms. And then there was this hiss. A low-level, steady hiss of uncertain origin.

Jansky studied the hiss for more than a year. Then he noticed that the signal peaked every 23 hours and 56 minutes. This is, of course, how long it takes Earth to rotate through one sidereal day. Was the radio source the Sun? Nope. It turned out to be the Milky Way. The signal was strongest in the direction of the galactic center, which lies in the constellation Sagittarius.

Now here comes one of those fascinating "what ifs" of history. Jansky wanted to build a better instrument to investigate the radio signal from space. But Bell labs wasn't interested, so he moved on to other projects. And although his discovery made The New York Times, it didn't go anywhere for awhile. The reaction must have been something like, "They discovered radio waves coming from space? How interesting! Oh, yes, please - I'll take that to go, and easy on the mustard."

In the late 1930s, a handful of scientists started seriously investigating the use of antennae to study the stars. How deliciously ironic that two Bell Labs researchers, Arno Penzias and Robert Wilson, much later won the Nobel Prize for discovering (in 1964) the cosmic microwave background radiation (CMB), the radio remnant of the Big Bang.

Jansky died very young — just 44. He had a kidney disease that ultimately prevented him from seeing the birth and triumph of radio astronomy.

I have an odd little history with Professor Jansky. In 1991, I was digging around in the archives of the State Historical Society in Madison, Wisconsin, where I was studying the history of science and medicine. A document box caught my eye (I don't remember why). It turned out to be a box of letters and papers from, you guessed it, Karl Jansky. It was pretty hum-drum stuff, but I wondered if there was a story in it I could write freelance that summer to earn desperately needed cash to stay in graduate school. I made it through — without having to freelance that story — and here I am, 16 years later, finally writing about Jansky. Good story ideas never die.

Jansky is remembered in the most enduring way possible for a scientist: as a standard unit of measurement. May I introduce you to the Jansky, which keeps good company with the Watt, Ampere, Volt, Ohm, Coulomb, Hertz, and Farad. Radio astronomers use the Jansky to express the flux density (strength) of radio sources in the sky. The Jansky is equal to one-hundreth of one trillionth of a trillionth of a watt per square meter per Hertz.

Next time you look up at the Milky Way, remember that you've got a little bit of Jansky flowing through you.

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