Arno Allan Penzias (1933–2024): A visionary explorer of the Universe

Arno Penzias together with Robert Wilson detected the faint three-degree cosmic microwave radiation from the big-bang creation of the Universe starting a revolution in cosmology (1). Fifteen years later, Penzias and Wilson joined by Keith Jefferts made the first detection of the millimeter-wave radiation from interstellar carbon monoxide, thus opening another new field of research—radio spectroscopy and interstellar chemistry (2).

Arno Penzias. Image credit: American Institute of Physics Emilio Segrè Visual Archives, Physics Today Collection.

Arno Allan Penzias was born in Munich Germany on April 26, 1933, and died in San Francisco, California at the age of 90 from complications of Alzheimer’s disease on January 22, 2024. When only six years old, after narrowly avoiding deportation to a camp in Poland, Arno, together with his younger brother, Gunther, were sent alone on a train to England. A short time later, they were joined in the UK by their parents, and in 1939, the whole family emigrated to the United States. He married Anne Barras in 1954. They had three children, David, Mindy, and Laurie. Following their divorce, Arno married Sherry Levit in 1996.

After graduating from Brooklyn Technical High School, he received his BS in Physics from the City College of New York in 1954. Following two years in the Army Signal Corps developing radar systems, Arno went to Columbia University where he worked under Charles Townes to build a sensitive tunable maser amplifier as part of his dissertation research. Using the maser amplifier in an unsuccessful attempt to measure the 21-centimeter line emission from hydrogen in the Perseus cluster of galaxies, he received his PhD in physics from Columbia University in 1962.

Arno began his 37-year career at Bell Labs in 1961. Teaming up with Bell Labs colleague Robert Wilson, Arno began a radio astronomy program using the 20-foot horn reflector antenna, that was no longer needed for testing the AT&T satellite-linked intercontinental telephone network. He built a reference load cooled by liquid helium to 4.2 K that was compared with the output of the horn antenna by rapidly switching between the horn and the reference load using a switch built by Wilson (3). However, their results in May 1964 were perplexing. The horn was warmer than the 4.2 K degrees load although it should have been colder corresponding to the 2.3 K degrees expected from the atmosphere. Careful experiments confirmed that the contribution from the atmosphere was only 2.3 ± 0.3 K and that the noise due to losses in the antenna system was expected to be less than 0.9 degrees. Penzias and Wilson considered that the excess noise in their antenna might be interference from the heavily populated New York City only 20 miles away, but the additional noise was independent of antenna orientation. There was no dependence on the time of the day or the time of the year, thus eliminating any solar system or Galactic effect. They ruled out any significant contribution of the artificial radiation belts formed by high altitude thermonuclear tests conducted a few years earlier as the noise level remained constant as the radiation belts decayed with time. They also suspected that pigeon droppings inside the horn might be contributing to their anomalous results. They “evicted” the pigeons and cleaned out their droppings, but this too gave no improvement.

Arno Penzias (Right) and Robert Wilson near their Holmdel horn antenna used to discover the cosmic microwave background radiation. Image credit: Nokia Corporation and AT&T Archives.

When Arno became aware of a planned experiment at nearby Princeton University to detect microwave radio noise originating from the early Universe, he and Wilson finally understood the source of the excess noise. The very short letter by Penzias and Wilson published in the Astrophysical Journal, was one of the most highly cited astronomy papers of the 20th century (1). Their discovery of the cosmic background radiation at the Bell Telephone Laboratories, for which they received the 1978 Nobel Prize in Physics, started a new field of precision cosmology.

Despite an early unsuccessful attempt to detect interstellar hydroxyl ions (OH), Arno remained excited about the prospects of studying interstellar molecules by means of their microwave transitions, which occur primarily at millimeter wavelengths. He proposed searching for interstellar molecular transitions near 3 millimeters wavelength using the NRAO 36-foot millimeter-wave radio telescope located on Kitt Peak near Tucson, Arizona. Working with Wilson and Keith Jefferts, a Bell Labs atomic physicist, Arno and his colleagues found an unexpectedly strong signal from carbon monoxide (CO) in the direction of the Orion Nebula (2). This opened the door for the millimeter-wave observations of a wide variety of interstellar molecules which became the hottest topic in radio astronomy.

Between times spent studying the cosmic microwave background and interstellar molecules, Penzias worked with Wilson on problems more directly concerning the telephone company including the atmospheric attenuation of microwave and infrared transmissions (4).

Starting with his 1972 appointment as Head of the Radio Physics Research Department, Arno steadily rose through the AT&T Bell Labs management ranks. In 1976, he became the Director of the Radio Research Laboratory; three years later, the Executive Director of Research and Communication; and in 1984, Chief Scientist and Vice President for Research. After retiring from Bell Labs in 1998, Arno moved to California where he began a new career as a Venture Partner at New Enterprise Associates.

In addition to being elected to the NAS in 1975 and receiving the 1978 Nobel Physics Prize, Arno was recognized by many other awards and recognitions including (with Robert Wilson) the NAS Henry Draper Medal in astronomical physics “for their discovery of the cosmic microwave radiation (a remnant of the very early universe), and their leading role in the discovery of interstellar molecules.” He received honorary doctorates from more than 15 universities in the United States and abroad, held several patents, served on multiple editorial boards; and for more than 20 years, was the Vice President of the Committee of Concerned Scientists which he helped to found. In 1989, he authored the remarkably prescient book Ideas and Information: Managing in a High-Tech World about the future societal impact of computers (5). He will be greatly missed by his many friends and colleagues from both academia and industry.