The Initial Observations: Størmer and van der Pol (1927–1928)
The phenomenon now referred to as Long Delayed Echoes (LDE) was first systematically documented in 1927. Norwegian physicist Carl Størmer, working with radio engineer Jørgen Hals, recorded signals at frequencies around 31.4 metres that returned to the receiver with anomalous delays — in some instances as long as several seconds after transmission. The observations were made in Oslo, Norway.
Størmer reported the findings in Nature in 1928 alongside the Dutch engineer Balthasar van der Pol, who had independently documented similar anomalies. Both papers appeared in the same volume of Nature (Vol. 122, 1928). The delays recorded varied considerably — from approximately 3 seconds in the most common cases to isolated instances exceeding 15 seconds — and followed no predictable pattern that matched known ionospheric or atmospheric propagation physics.
Subsequent attempts to reproduce or explain the observations within standard radio propagation theory did not yield a settled conclusion. A 1951 review by Budden and Yates in the Journal of Atmospheric and Terrestrial Physics examined the earlier reports and acknowledged the absence of a satisfactory conventional explanation for the longest observed delays.
The Bracewell Hypothesis (1960)
In May 1960, Professor Ronald N. Bracewell of Stanford University's Radioscience Laboratory published a two-page paper in Nature titled Communications from Superior Galactic Communities (Vol. 186, 28 May 1960, pp. 670–671). The paper's primary argument concerned the optimal strategy for interstellar contact: Bracewell contended that direct radio transmission between star systems was an inefficient method of communication, and that a more effective approach would be to dispatch automated probes to candidate stellar systems and position them in stable orbits within the habitable zone.
The paper addressed the Long Delayed Echoes directly. Bracewell proposed that a probe of this kind, if present in our solar system, would need to attract attention by means of a radio transmitter. To ensure it operated on a frequency certain to be detected, the probe could first listen for transmissions from the target civilisation and then retransmit them back. Bracewell wrote:
— R. N. Bracewell, Nature, Vol. 186, 28 May 1960, p. 671
The paper explicitly noted that such a probe "may be here now, in our solar system, trying to make its presence known to us." Bracewell cited the 1928 Størmer and van der Pol reports as the specific empirical precedent for his hypothesis. His paper concluded that the most productive near-term strategy for detecting non-human intelligence would be to scrutinise the solar system for such probes rather than to listen for transmissions originating from distant stellar systems.
The hypothesis has since been referred to in the academic literature as the "Bracewell probe" concept. It represents the first formally published scientific proposal to connect the Long Delayed Echo phenomenon to the possible presence of an artefact of non-terrestrial origin within the solar system.
Subsequent Scientific Treatment
The LDE phenomenon continued to be observed and investigated through the latter half of the twentieth century. A number of amateur and professional radio operators documented echoes, including instances with delays measured in tens of seconds. The scientific literature did not converge on a single explanatory mechanism.
A 2016 analysis by Sverre Holm of the Department of Physics at the University of Oslo revisited the historical record of LDE observations, including the original Størmer–Hals data. Holm's work, presented at an astrophysics symposium and subsequently circulated in academic form, examined whether the delay distributions reported across multiple independent observation campaigns were consistent with natural propagation phenomena. The analysis noted that the longest delays — particularly those in the range of 15 to 40 seconds — remain without a well-established physical explanation within standard ionospheric or magnetospheric models.
1–3 sec Can be partially attributed to ionospheric scatter and plasma propagation effects, though the mechanism is not fully established for all observed cases.
3–15 sec Documented repeatedly since 1927. Several proposed physical explanations exist, including magnetospheric waveguide propagation, but none accounts for the full observed range without significant assumptions.
15–40+ sec The most anomalous category. Delays of this duration are inconsistent with known round-trip propagation paths within Earth's magnetosphere or the inner solar system at radio frequencies. No established natural mechanism accounts for these observations.
What the Scientific Record Establishes
The following points represent what the peer-reviewed and on-record scientific literature supports directly:
The LDE phenomenon is real and was documented by multiple independent observers beginning in 1927. The observations were reported in Nature by researchers at established institutions. The anomaly was not resolved by twentieth-century radio physics. A formally published hypothesis, appearing in Nature in 1960 and authored by a Stanford University professor, proposed a specific non-terrestrial mechanism — an automated probe — as a candidate explanation, citing the Størmer–van der Pol observations as its empirical basis. Subsequent academic analysis confirmed that the longest delays in the historical record do not have a settled conventional explanation.
No peer-reviewed study has confirmed the Bracewell probe hypothesis. No peer-reviewed study has definitively refuted it. The phenomenon and the hypothesis remain open questions in the scientific record.
Source Notes
All factual claims in this article are drawn from the primary sources listed in the sidebar. The Bracewell quotation is reproduced from the original 1960 Nature paper (Vol. 186, pp. 670–671) as cited directly in the text. References to the Størmer and van der Pol 1928 papers are drawn from Bracewell's own citations within that publication. The Holm analysis is referenced from the 2016 academic presentation from the University of Oslo Department of Physics. This article contains no claims that go beyond what these sources state.