1960 05 28 Nature - Vol 186 No 4726 - R N Bracewell

Magazine Overview

Title: NATURE
Issue Date: May 28, 1960
Volume: 186
Issue: 4726
Publisher: Nature Publishing Group
Country: United Kingdom
Language: English

This issue of NATURE features a significant article by Professor R. N. Bracewell of the Radioscience Laboratory, Stanford University, California, titled "COMMUNICATIONS FROM SUPERIOR GALACTIC COMMUNITIES." The article delves into the speculative yet reasoned possibility of advanced extraterrestrial civilizations and proposes novel approaches to detecting them.

Communications from Superior Galactic Communities

Professor Bracewell begins by referencing the work of Morrison and Cocconi, who suggested searching for extraterrestrial transmissions at a frequency of 1,420 Mc./s., and Drake, who was developing equipment for this purpose. Bracewell's central thesis is that the assumption that planets are common byproducts of star formation, coupled with the vast number of stars in the galaxy, makes it probable that some planets host civilizations far superior to our own in technological development.

He acknowledges the ongoing efforts to detect signals, such as Drake's planned observations of stars like τ Ceti and ε Eridani. However, Bracewell questions whether the nearest superior communities would be the most likely to attempt contact. He posits that due to the vast distances involved (potentially hundreds of light-years), it might be more logical for such civilizations to focus their efforts on a larger number of stars within a certain range, rather than concentrating on a few nearby ones.

The Probe Hypothesis

A key proposition of Bracewell's article is the idea that advanced societies might be sending probes into nearby planetary systems. These probes, he suggests, would be designed to attract the attention of emerging technologies like ours. Such probes, armored against the harshness of space and powered by stellar energy, could orbit within the habitable zones of stars. They would transmit signals, potentially using a wave-length that could penetrate Earth's ionosphere and be in common use. Bracewell theorizes that these probes might listen for our signals and then repeat them back, creating echoes with delays that could be interpreted as evidence of intelligent communication, similar to unexplained phenomena reported decades earlier.

This strategy, Bracewell argues, would be more effective than a home-based transmitter handicapped by inverse-square attenuation over interstellar distances. It would also bypass the difficulty of selecting the correct star and wave-length for communication.

Searching Our Solar System

Based on this probe hypothesis, Bracewell strongly advocates for scrutinizing our own solar system for signs of such extraterrestrial probes. This approach, he believes, would allow us to effectively monitor all stars capable of reaching us. He notes that any community attempting contact is likely to be the nearest superior one, as others might already be part of a galaxy-wide communication network and would avoid duplication in their search efforts.

Bracewell suggests that our first contact might involve being "inducted" into this existing chain of communication, rather than being the first instance of contact.

Detecting and Communicating with Probes

He draws parallels between how superior communities might detect us and how we might detect them. A potential project for humanity would be to send probes outside the solar system to listen for monochromatic radio-frequency line emission spectra, similar to what Earth currently emits. If such signals are detected, it's possible that the hypothetical probes sent by our nearest superior community did no more than listen for this radiation, and a response could be on its way back.

Bracewell emphasizes that interstellar transfer of material is time-consuming, making the transfer of information more critical. Therefore, he suggests that the first probes sent to our solar system would likely contain elaborate information and complex computers, capable not only of detecting our presence but also of conversing with us.

Statistical Considerations and Lifespans of Civilizations

To survey the possibilities of superior communities within reaching distance, Bracewell presents Figure 1. This graph plots $N_c$, the number of superior communities in the galaxy, against $d$, the distance to the nearest superior community. It also shows $N_L$, the number of likely stars within a distance $d$, and $\Delta$, the average life-time of a superior community. The graph is based on a galactic mass distribution model and is accurate to within an order of magnitude.

He explores scenarios based on different values for $N_c$. If $N_c = 10^6$, then $d = 100$ light years, and the number of likely stars within this range is $10^3$. This implies a frequency of occurrence ($p$) of superior communities among likely stars of $10^{-3}$. Assuming it takes $5 \times 10^9$ years for a star to produce one superior community, and that this community endures for an average life-time $\Delta$, then $p = 10^{-3}$ implies $\Delta = 5 \times 10^6$ years. This duration would offer ample time for exploration and establishing communication chains.

However, if technology is less abundant, and $N_c = 10^8$, then $p = 10^{-7}$, $d = 2,000$ light years, and $\Delta = 500$ years. In this case, the duration of communities capable of maintaining a frequency of $10^{-7}$ is too short for interstellar traffic. Bracewell notes that even if intelligent life develops at the same tempo as ours, the mortality rate for advanced civilizations might be too high for them to become abundant.

Even with a rare technology, the possibility of a communication chain existing is considered. In a galaxy with only $10^3$ superior communities with brief life expectancies, some might achieve durability and quasi-permanence. These ancient associations, technically advanced, might seek us out using unknown methods. Bracewell speculates whether they would be interested in rudimentary societies that usually self-destruct before they can be located. However, the prospect of observing a technology near its peak might be a strong incentive for them to reach us.

Recurring Themes and Editorial Stance

The recurring theme throughout this article is the scientific exploration of extraterrestrial intelligence (SETI) and the practical considerations for detecting and communicating with it. Professor Bracewell's stance is one of reasoned speculation, grounded in astronomical observations and statistical probabilities. He advocates for innovative approaches, such as searching for probes within our solar system, and emphasizes the importance of considering the lifespan and distribution of advanced civilizations in the galaxy. The article reflects a forward-looking and inquisitive editorial stance by NATURE, publishing speculative yet scientifically informed discussions on profound questions about humanity's place in the cosmos.