The prediction of electromagnetic waves by Maxwell and the demonstration of their existence by Hertz led several scientists to speculate that celestial objects, such as the Sun and stars, might generate radio waves. The following scientists set the groundwork for the later discovery of radio astronomy. Click on each one for a summary of his contibutions.
James Clerk Maxwell (1831-1879)
In the 1860s and 1870s, James Clerk Maxwell developed the theory of electric and magnetic forces, summarized in his famous four equations. These equations encapsulated all that had been discovered about electricity and magnetism in the experiments done over the previous few hundred years by Faraday, Volta, and many others. They showed that electricity and magnetism were two aspects of the same force. The equations also predicted that there should be a form of radiation, which came to be known as electromagnetic radiation. Maxwell realized that light was a form of electromagnetic radiation. Around 1862 he wrote, "We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."
The equations predicted that electromagnetic radiation could exist with any wavelength. The various colors of light have wavelengths less than a thousandth of a millimeter. Much longer wavelengths are possible.
Read more about Maxwell. (from Univ of St.Andrews, Scotland)
Heinrich Hertz (1857-1894)
In 1888, Heinrich Hertz built an apparatus that could transmit and receive electromagnetic waves of about 5 meters in length. He used a coil to generate a high voltage spark between two electrodes which served as a transmitter. The detector was a loop of wire with a small gap. A spark at the transmitter produces electromagnetic waves that travel to the detector, producing a spark in the gap. He showed that the waves were polarized, and that they could interfere with each other, just as predicted by theory.
Thomas Alva Edison (1847-1931)
Once Hertz had demonstrated the existence of electromagnetic radiation, the possibility of receiving such radiation from celestial objects may have occurred to many scientists. Edison seems to be the first on record to have proposed an experiment to detect radio waves from the Sun. The evidence of this is a letter sent in 1890 to Lick Observatory by Kennelly, who worked in Edison's laboratory. It describes building a detector by winding several cables around a mass of iron ore. There is no record that the experiment was actually carried out, but it could not have been a success. In hindsight, the proposed apparatus would be very insensitive, and could only detect very long wavelengths. The ionosphere would prevent such long waves from reaching the earth's surface. (The prediction of a reflecting layer in the upper atmosphere, the ionosphere, was made by Kennelly and Heaviside in 1902).
Letter reproduced from "The Evolution of Radio Astronomy", by J.S.Hey,
Science History Publications, 1973. See also: C.D.Shane, Pub.Astron. Soc. Pacific 70,303, 1958.
Sir Oliver J. Lodge (1851-1940)
Sir Oliver Lodge made many innovations in early radio technology, inventing a better
radio detector, introducing the use of tuned circuits, and inventing the loud speaker.
Around 1897-1900, Lodge attempted to detect radio waves from the Sun. Read his
description of the experiment.
Letter reproduced from "Classics in Radio Astronomy", by W.T.Sullivan, Reidel, 1982. Original in Lodge: "Signalling across space without wires", The Electrician Publ.Co., London, 1900.
Wilsing and Scheiner (1896)
Johannes Wilsing (1856-1943) and Julius Scheiner (1858-1913) were astrophysicists who have
the distinction of being the first to properly write up and publish their attempt to detect
radio radiation from the Sun (Ann.Phys.Chem.59,782, 1896, in German). The diagram of their
experiment, on the right, is from "Classics in Radio Astronomy" by W.T. Sullivan, Reidel, 1982.
They ran their experiment for eight days, and were unable to detect any signals that could be
associated with the Sun. They speculated that they may have been foiled by absorption
of radio waves in the atmosphere (they were wrong).
Charles Nordman (1900)Charles Nordman, a French graduate student, reasoned that if radio waves were being absorbed by the atmosphere, as Wilsing and Scheiner thought, the solution would be to go to high altitude. He set up a long wire antenna on a glacier on Mont Blanc, at about 3100m (about 10,000 ft). In hindsight we know that the antenna would have been sensitive to low frequency radio bursts from the Sun and would have been capable of detecting them. These bursts occur most often during Solar maxima, but unfortunately the Sun was at Solar miniumum in 1900. Again there was no detection. Nordman's experiment was published in Comptes Rendus Acad.Sci., vol.134, page 273, 1902. (Reprinted in English in "Classics in Radio Astronomy" by W.T. Sullivan, Reidel, 1982).
These unsuccesful attempts at finding solar radio waves may have discouraged further
experiments. But it is also possible that the theoretical breakthroughs by Planck and
Heaviside may have played a role.
Max Planck (1858-1947)
The story goes that when Max Planck was a student at the University of Munich, his advisor recommended that he not bother majoring in Physics, because all the problems had been solved. Fortunately he did not take that advice. He later found an unsolved problem, namely the theoretical explanation of the "black body", or thermal radiation curves.
It was known that when dense objects are heated to high temperatures, they radiate energy, and that the graph of intensity versus wavelength followed a curve such as is illustrated here. The higher the temperature, the shorter the wavelength of the peak of the curve.
Planck worked out how to derive this thermal radiation curve from a theory of absorption and emission of radiation by matter. The theory required that energy had to be emitted or absorbed in small packets, or "quanta" of energy. This was a breakthrough in Physics and led to further development of the quantum theory to explain all electro-magnetic phenomena.
The spectrum of light from the Sun very closely resembles a thermal radiation curve.
If one applies Planck's theory to predict the amount of radiation we might receive
from the Sun in the radio part of the spectrum (wavelengths in the 10 to 100 cm range),
the radiation would be very weak: much too weak to be detected by any receiver
available around 1900. This theoretical prediction, along with the failure of
experiments to detect the Sun, may have discouraged further attempts.
Read more about Planck. (from Univ of St.Andrews, Scotland)
Oliver Heaviside (1850-1925)
Heaviside and Kennelly, in 1902, predicted that there should be an ionised layer in the upper atmosphere that would reflect radio waves. They pointed out that it would be useful for long distance communication, allowing radio signals to travel to distant parts of the earth by bouncing off the underside of this layer. The existence of the layer, now known as the Heaviside layer or the ionosphere, was demonstrated in the 1920s.
If radio waves bounce off the inside of the ionosphere, then they must also bounce off the outside. So any radio waves from outside the earth would not get through to the ground -- they would bounce back into space.
Thus the predictions by Heaviside, combined with Planck's radiation theory, probably discouraged further attempts to detect radio waves from the Sun and other celestial objects. For whatever reason, there seem to have been no attempts for 30 years, until Jansky's unexpected discovery in 1932.
Later it was learned that the reflection from the ionosphere is very dependent on the frequency (or wavelength). It reflects most of the radiation of frequency less than about 20 MHz. But the ionosphere is not a barrier to frequencies above about 50 MHz. Radio astronomy had to wait for the development of high frequency radio receivers.
Read more about Heaviside. (from Univ of St.Andrews, Scotland)
Guglielmo Marconi (1874-1937)Marconi improved radio transmission and receiver designs and developed the first practical systems for long distance communication by radio. In 1901 he was the first to send and receive signals across an ocean, from Newfoundland to Cornwall. As a result of his pioneering efforts, commercial radiotelephone service became available in later years. In the 1930s the Bell Telephone company was working on improving their transatlantic telephone service when they assigned Karl Jansky to investigate sources of radio static, leading to his discovery of radio waves from the milky way.
Some Biographies of Marconi:
Credits: Pictures of Maxwell, Planck, Heaviside: School of Mathematics and Statistics, University of St.Andrews, Scotland. [http://www-history.mcs.st-and.ac.uk/history/Mathematicians/]
Maxwell's equations: Halliday and Resnick, "Physics for Students of Science and Engineering", Wiley 1962.
Hertz: from "Astronomy" by Fred Hoyle, Crescent Books, 1962.
Edison: from K-12 Teaching and Learning Center web site: [http://tlc.ai.org/edison.htm], Also the Franklin Institute [http://sln.fi.edu/franklin/inventor/edison.html]
Planck radiation curves diagram from Chaisson and McMillan, "Astronomy, a Beginners Guide to the Universe", Prentice Hall, 1998.
Lodge: IEE/DERA, www.dera.gov.uk/iee/lodge.jpg; and das-fotoarchiv.com/lautspre/erfinder.jpg.
Compiled by F. Ghigo, National Radio Astronomy Observatory, Green Bank, West Virginia. Modified on Thursday, 27-Mar-2003 15:10:35 EST