Note: The AIP History Weekly Edition is on a summer break and will resume publication of original content on September 4. This article was first published in June in AIP’s semiannual History Newsletter. Part 1 was republished last week.

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Secrets locked in static

In 1928 Bell Laboratories was trying to find ways to suppress static on telephone calls and assigned a young physicist named Karl Jansky to track down the sources of interference plaguing transatlantic shortwave communications. Working from a field station in Holmdel, New Jersey, Jansky built a large directional antenna. Mounted on a set of Ford Model T wheels so it could rotate full circle, the contraption earned the nickname “Jansky’s merry-go-round.” He quickly identified static from nearby thunderstorms and more distant storm systems, but there was also a faint, persistent signal of unknown origin.¹

Jansky noticed that the signal rose and fell not every twenty-four hours but twenty-three hours and fifty-six minutes—the time it takes the Earth to complete one rotation relative to the distant stars rather than the Sun. The signal was keeping time with the galaxy itself. By the spring of 1933, Jansky had traced the strongest emission to the direction of Sagittarius, in the center of the Milky Way. As one former Bell Labs scientist later put it, the discovery fell squarely between the disciplines: radio waves had nothing to do with astronomy, and cosmic signals had nothing to do with telephones.²

The discovery made the front page of the New York Times, and the next day NBC’s Blue Network broadcast an interview with Jansky— including a live sample of the cosmic hiss—to a national audience. Jansky proposed building a thirty-meter dish antenna to continue the work, but Bell Labs reassigned him, seeing no way to eliminate the interference and therefore no business problem left to solve. He never returned to the subject and died in 1950 at the age of forty-four, his work largely unrecognized by astronomers, who had no framework for interpreting radio signals from space.³

The person who did follow up was not a professional astronomer but a twenty-six-year-old hobbyist from suburban Chicago. Grote Reber, an electrical engineer and avid ham radio operator, read about Jansky’s discovery and applied unsuccessfully for a job at Bell Labs to pursue it further. Taking on the project at his own expense, in 1937 Reber assembled a dish in his yard from scrap lumber, sheet metal, and salvaged Ford parts, leaving his neighbors bewildered. After confirming Jansky’s discovery, Reber turned to producing radio-frequency maps of the sky and by 1942 had found bright sources in Cygnus, Cassiopeia, and the galactic center that had no obvious optical counterparts. His results, published in the Astrophysical Journal, attracted modest attention at first, and for nearly a decade he was the world’s only radio astronomer.⁴

What transformed radio astronomy into a global enterprise was the Second World War. Radar development trained a generation of physicists and engineers in the detection and manipulation of radio-frequency signals, and many of them turned their expertise to the sky when they resumed peacetime work. Martin Ryle and his colleagues at Cambridge pioneered radio interferometry—the technique of linking networks of small antennas to achieve the resolving power of a much larger instrument—and used it to catalogue hundreds of discrete radio sources across the sky. At Jodrell Bank in the Cheshire countryside, Bernard Lovell constructed a massive steerable dish, 250 feet in diameter and the largest in the world when it was completed in 1957, just in time to track the carrier rocket of Sputnik. In Australia radio astronomers at CSIRO’s facilities made foundational contributions to understanding the structure of the Milky Way at radio wavelengths.⁵

The field grew with extraordinary speed: by the early 1960s, radio observatories on three continents were producing data that optical telescopes simply could not. Back at Holmdel, Bell Labs now saw fit to employ radio astronomers. Robert W. Wilson and Arno Penzias took over an antenna built to accompany an early communications satellite, but, like Jansky decades earlier, they encountered mysterious noise that they could not attribute to any source of interference. Unlike Jansky’s signal, this one appeared to come from every point in the sky. Eventually, they learned of Princeton University theorist Robert Dicke’s theory that a universe that had expanded from a single point would have left behind a low-level radiation background. Wilson and Penzias had serendipitously discovered what soon came to be seen as crucial evidence in favor of the “Big Bang” theory of the expanding universe.⁶

A violent universe comes into focus

In the late 1930s, physicist Hans Bethe uncovered the source of stellar energy by positing that reactions between elements in the stars are governed by the fusion of nuclei rather than chemistry. Working out the proton-proton chain and the carbon-nitrogen-oxygen cycle, he discovered how hydrogen can become helium and how mass destroyed in the process is converted into energy in accord with Einstein’s equation E=mc².⁷ Almost twenty years later, Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle harmonized astrophysical measurements, laboratory data, and Hoyle’s theoretical ideas about stellar evolution into a landmark 1957 paper describing how, during their lives and deaths, stars forge nearly every element heavier than helium.⁸

Historians Luisa Bonolis and Stefano Furlan have recently highlighted the collaboration between the Burbidges, Fowler, and Hoyle as indicative of the intense disciplinary bridge-building that began to characterize astrophysics. They further point to a symposium held in Dallas in 1963 as an “epochal” gathering of radio and optical astronomers, relativists, and theoretical astrophysicists to explore an idea put forward by Fowler and Hoyle that connected the almost unexplainable energy output of ultrabright radio sources, soon known as quasars, with the extreme gravitational collapse phenomenon originally posited by Chandrasekhar.

Soon, astrophysics would be routinely challenged by all sorts of extreme phenomena that were characteristic of what Bonolis and Furlan call “the violent universe,” borrowing from the title of a 1969 documentary in which Carl Sagan made his debut as an on-screen presenter. In the 1960s, Livermore Laboratory physicist Stirling Colgate moved fluidly between simulating thermonuclear explosions and supernovas, and his programs were soon adapted to modeling the formation of black holes, which had not yet been proven to exist. In 1967 Jocelyn Bell Burnell serendipitously discovered a pulsar using a radio array she had helped build, and these were soon identified as neutron stars, another previously hypothetical product of stellar collapse. Efforts to study cosmic rays fed into the emergence of x-ray and gamma-ray astronomy, aided by the access rockets now provided to space, further revealing the startlingly high energies produced in less quiescent corners of the cosmos.⁹

By the final decades of the 20th century, astronomers had opened observational windows across the entire electromagnetic spectrum—infrared, ultraviolet, x-ray, gamma-ray—and had even begun detecting neutrinos from the Sun and from Supernova 1987A. The culmination came in 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves, a phenomenon first predicted by Einstein’s general theory of relativity a century earlier. The first detection, and many after, recorded black holes merging. Then, in 2017, another detection, in concert with telescopes around the world, captured a neutron star collision, confirming the event as an important mechanism for the synthesis of heavy elements. A new era of “multimessenger” astronomy had arrived.¹⁰

History in progress

It was not unreasonable of Auguste Comte to suppose in 1835 that physical knowledge of the stars was unattainable. In fact, it is shocking to consider just how much knowledge has proven to be within reach. Many of the episodes related here are well known to astronomers and historians. But what we are only beginning to grasp is how astrophysicists learned to measure and reason about objects that are not just millions of miles, but sometimes billions of light years away. Astrophysicists proved extraordinarily adept in learning how to know. There are still astrophysical mysteries that challenge observational skill and the laws of physics themselves: dark matter, dark energy, the dominance of matter over antimatter. It is possible that there will be some mysteries of the universe that will never be solved. But if astrophysicists remain optimistic that they can be, the history of astrophysics offers reason to suppose they may be right.

Notes and references

1. Karl G. Jansky, “Electrical Disturbances Apparently of Extraterrestrial Origin,” Proceedings of the Institute of Radio Engineers 21, no. 10 (1933): 1387–1398, doi.org/10.1109/JRPROC.1933.22745.
2. Kenneth I. Kellermann and Ellen N. Bouton, Star Noise: Discovering the Radio Universe (Cambridge University Press, 2020), chapter 1, doi.org/10.1017/9781009023443.009.
3. Kenneth I. Kellermann, Ellen N. Bouton, and Sierra S. Brandt, Open Skies: The National Radio Astronomy Observatory and Its Impact on US Radio Astronomy (Springer, 2020), doi.org/10.1007/978-3-030-32345-5.
4. Grote Reber, “Notes: Cosmic Static,” Astrophysical Journal 91, no. 5 (1940): 621–624; on Reber’s telescope and legacy, see greenbankobservatory.org/about/telescopes/reber-telelescope/ and Keith Martin, “Grote Reber, Radio Astronomer,” April 18, 2017, www.nist.gov/blogs/taking-measure/grote-reber-radio-astronomer .
5. Malcolm Longair, “A Brief History of Astronomy, Astrophysics, and Cosmology 1945–2000,” June 8, 2022, royalsociety.org/blog/2022/06/brief-history-of-astronomy-astrophysics-and-cosmology-1945-2000/; on Jodrell Bank specifically, see Jon Agar, Science and Spectacle: The Work of Jodrell Bank in Postwar British Culture (Routledge, 1998).
6. See, for instance, chapter 6 in Kellerman and Bouton, Star Noise.
7. H. A. Bethe, “Energy Production in Stars,” Physical Review 55, no. 5 (1939): 434–456, doi.org/10.1103/PhysRev.55.434.
8. E. Margaret Burbidge, G. R. Burbidge, William A. Fowler, and F. Hoyle, “Synthesis of the Elements in Stars,” Reviews of Modern Physics 29, no. 4 (1957): 547–650, doi.org/10.1103/RevModPhys.29.547.
9. Luisa Bonolis and Stefano Furlan, “Unveiling the Violent Universe, Part 1. New Cosmic Messengers, New Astronomies: Building a Transdisciplinary Research Culture,” European Physical Journal H 50, article no. 14 (2025), doi.org/10.1140/epjh/s13129-025-00102-0; see also chapter 7 in Kellerman and Bouton, Star Noise.
10. Historians have only begun to outline the history of multimessenger astronomy; see the special issue Centaurus 67, no. 1 (2025), doi.org/10.1484/J.CNT.5.143069, edited by Luisa Bonolis, Roberto Lalli, and Adele La Rana.

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Rebecca Charbonneau
American Institute of Physics
rcharbonneau@aip.org

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