October 27, 2026
Nearly a century after the first discovery, the phenomenon of “long delayed echoes” still eludes precise scientific explanation, according to Space.com. The roots of the story go back to 1927, when Norwegian civil engineer and radio amateur Jorgen Hals was following the broadcast of a shortwave radio station in the Dutch city of Eindhoven, and noticed radio echoes arriving about three seconds later than the original signals. Hals wrote at the time confirming that he heard the echo, unable to explain its source. His discovery was later reported to the Norwegian physicist Karl Sturmer, who investigated it in cooperation with the physicist Balthasar van der Poel, and they succeeded in reproducing it without knowing its causes.
These echoes differ from normal radio echoes that bounce off objects, as they are characterized by the return of signals more than 2.7 seconds after they were transmitted. What makes the matter more complicated is its unpredictable nature and the wide variation in timing of delays, which makes it difficult to narrow it down to one reason. In the mid-1980s, researchers R. C. Vidmar and V. and. Crawford conducted experiments in Alaska in which they assumed that radio signals were trapped between ionized layers in the atmosphere and directed over long distances.
Recently, University of Oslo researcher Sverre Holm proposed a similar concept known as “magnetically guided echo,” in which waves are trapped within electrically charged regions in the upper atmosphere and travel along magnetic field lines toward the opposite hemisphere before bouncing back. Holm described his mechanism as “the best understood” of five promising explanations, with the ability to predict it with an accuracy of about five thousandths of a second. However, Holm acknowledged the limitations of his theory, noting that it does not explain historical cases observed at a frequency of approximately 9.55 MHz and with a delay of between 3 and 30 seconds, compared to the much narrower frequencies and time ranges in his theory. Other scientific explanations include the possibility of radio energy being transformed into slower modes of plasma waves, or the role of solar plasma clouds.
This mystery has inspired wilder theories, dating back to Australian physicist Ronald Braswell, who hypothesized that an advanced civilization might send autonomous probes to communicate by intercepting and rebroadcasting radio signals. In 1973, Scottish writer Duncan Lunan claimed in the magazine “Space Flight” that he had decoded a message from a space probe – sent from the “Epsilon Howling” system and settling in the “Lagrange points” near the Earth – considering the HALSE echoes to be the first observation of them, and that their arrangement reveals an image of the Howling constellation 15 thousand years ago. Later, researcher Anthony Lawton presented a natural explanation that won him an award from the British Planetary Society, and Lunan retracted his ideas in 1976 before returning to adopt a version of them in the 1990s.
Although the alien hypothesis is no longer taken seriously except by a few researchers, the phenomenon still receives widespread attention. Modern models such as the Holm model offer promising mechanisms to explain some signals, while basic questions remain unanswered after nearly a hundred years of continuous research.
