Thursday, May 07, 2015

The Accidental Birth of Radio Astronomy

Sometimes the biggest scientific discoveries arise from technicians simply trying to do their job. This was certainly the case for one of the key astronomical discoveries of the past century: the 1933 detection of radio waves from outer space. This accidental discovery gave birth to the field of radio astronomy and the subsequent detection of the most distant and most powerful phenomena in the universe: quasars, pulsars, and the cosmic microwave background.
The Milky Way and the Cosmic Microwave Background, as imaged by the Planck space telescope. Credit: European Space Agency, HFI and LFI consortia

Static on the Phone Line

Karl Jansky. Image courtesy of NRAO/AUI
No one likes a poor-quality phone call, even back in the 1920s. In 1928 Bell Telephone Laboratories hired a man called Karl Jansky fresh from his undergraduate physics degree at the University of Wisconsin. His task was to hunt down the sources of static noise that caused interference in early trans-Atlantic wireless phone calls.

Jansky set about building a basic antenna 30 meters long that could receive "short-wave" radio signals (here, short-wave means a wavelength of about 14.6 meters). He built the antenna in Holmdel, New Jersey, on a rotating platform that allowed a full-sky scan every 20 minutes. The antenna soon acquired the nickname "Jansky's merry-go-round".

With this simple scaffolding made of brass pipe and a few circuits, Jansky identified three noise culprits in 1932: nearby thunderstorms, distant thunderstorms, and a "steady hiss type static of unknown origin".

The third, unknown type of static didn't always come from the same place in the sky. Instead, a few months of observations revealed that the location of maximum static moved across the sky on roughly a 24-hour cycle. Jansky naturally guessed that this noise was extraterrestrial, or not from Earth, and initially proposed the Sun as the source.

Karl Jansky with his "merry-go-round" radio antenna that discovered radio waves from outer space.
Credit: Bell Telephone Laboratories. Rights: Fair Use via Wikimedia Commons

From the Center of the Milky Way

Within a year, however, further observations revealed a cycle that was 4 minutes less than 24 hours, and precisely equal to the length of a sidereal day, a measure of the Earth's rotation with respect to the stars instead of the Sun. This meant that the unknown static was coming from somewhere beyond the Solar System. The level of the static was never high but Jansky described its presence during otherwise quiet periods as "unmistakable".

In a 1933 paper, Jansky was cautious about suggesting an origin for this mysterious signal, but pointed out that it appeared to come from "the Milky Way in the direction of Sagittarius". Sagittarius is a teapot-shaped constellation that marks the direction of the center of the Milky Way, designated Sagittarius A*. Today we know that a supermassive black hole lurks in the center and that the high-energy material circling around the black hole emits a large radio signal.

Jansky discovered that the center of the Milky Way is a strong source of radio waves. This 1993 image shows how the galaxy center appears in radio—the bright white spot in the center is the supermassive black hole known as Sagittarius A* and the spiral arms are streams of hot gas swirling around the black hole.
Credit: NRAO/AUI

All because of a simple quest to reduce static noise in phone calls, Jansky unwittingly discovered the first source of cosmic radio waves, opening up the field of radio astronomy. Astronomers later found that most astronomical objects, including our Sun, Jupiter, and other stars and galaxies, emit radio waves. Jansky was not an astronomer but he was intrigued by his cosmic discovery and wanted to continue the work further. However, Bell Labs assigned him to another project and he never worked in astronomy again.

An artist's impression of the Square Kilometre Array at night. Credit: SKA Organisation

The Next Generation of Radio Astronomy

Today Karl Jansky is known as the "father of radio astronomy" and is recognized with an astronomical unit—the Jansky is a measure of how bright a radio source appears from Earth.

He would undoubtedly be pleased to know that the most ambitious astronomical project to date is a radio telescope soon to begin construction. The Square Kilometre Array will be the largest, most advanced telescope in the world, but many of the techniques (and issues with static noise) that it encounters will be the same as Jansky pioneered 85 years ago.

Further reading: 

By Tamela Maciel, also known as "pendulum"

1 comment:

  1. No mention of Grote Reber, the first radio astronomer? Reber's actual dish antenna has a place at the NRAO in Green Bank too, in view of a replica of the Jansky antenna.