Waiting times and longevity

Around the same time as several other papers proposing how best to conduct a search for extraterrrestrial intelligence [1,2,3,4,5], Sebastian von Hoerner published a paper in Science with estimates for the longevity and distance between civilisations [6]. Here are my notes from reading this paper. You can find my notes on other classic SETI papers in my roundup of the technosignatures literature.

Our civilisation is average

Hoerner based his paper on several assumptions:

1. Our planet and our civilisation are about average
2. Life will emerge elsewhere if it has the right environment and enough time
3. Evolution elsewhere will follow the rules of natural selection
4. Technology will take roughly the same amount of time to develop elsewhere as it has here
5. Our present state of mind (science, technology) is one of many and will be succeeded by another
6. Civilisations have limited longevity

The alternate fates of civilisations

Hoerner presented a series of equations to calculate the fraction of stars with technological civilisations, the mean distance between neighbouring civilisations, the most likely value for their technical age, and the probability that there have been other civilisations on the same planet before them.

Like Drake and his famous equation [7], Hoerner’s equations rely on estimating the average longevity of a civilisation. With the view that science and technology have advanced largely by “the fight for supremacy and the desire for an easy life”, he proposed five scenarios which might limit longevity. He also gave estimated values for the average longevity and the probability of each of these five scenarios.

Hoerner viewed his estimates as a starting point to guide searches, stating that they “tend to become a matter of personal opinion” and that his estimates are a “subjective guess” but that without estimates there is no way to design a search. He wrote that his estimates were pessimistic to “be on the safe side” but he did not discuss his reasoning for them further. His most likely scenario, with a probability of 0.6, is the destruction of all ‘higher’ life - within an average time of 30 years from the emergence of technology.

1. Complete destruction of all life: 100 yrs (range 0-200; p = 0.05)
2. Destruction of ‘higher’ life only: 30 yrs (range 0-50; p = 0.60)
3. Physical or mental degeneration or decay: 30,000 yrs (range 10,000 - 100,000; p = 0.15)
4. Loss of interest in science and technology: 10,000 yrs (range 1000 - 100,000; p = 0.20)
5. No limitation: age of oldest stars - time to develop technology (p = 0.00)

Hoerner used his estimates for each scenario to put the average longevity at 6500 years. He then estimated the fraction of stars with habitable planets as 0.06 and used the average distance between the ten nearest stars to the sun to estimate:

• The fraction of stars with technological civilisations = \(2.6 \times 10^{-7}\)
• The average distance between them = 1174 light years
• The most likely technical age of the first civilisation we receive signals from = \(1.2 \times 10^{4}\) yrs
• The chance that this civilisation is a successor of older extinct civilisations on the same planet = 75%
• The chance of meeting a civilisation in the same phase as us = 0.4%
• The average longevity of the most frequent civilisations = \(2.4 \times 10^{4}\) yrs
• The average number of conversations with other civilisations = 10.2

Recommendations for SETI

Based on his estimates, Hoerner made several conclusions that had implications for designing searches for extraterrestrial civilisations.

• Civilisations are too rare for us to rely on searching a list of stars: for the best chance of success we must perform continuous whole sky scans
• Another civilisation will not send directed signals to us but they may send beacons all directions
• Our telescopes must be able to reach at least 1300 light years
• Any civilisation we find is likely to be more advanced and much older than ours and we could learn from them
• The average waiting time for an answer to a question will be 2000 years so conversation will be unlikely and signals will probably already contain messages (most likely designed to attract our attention first and then direct us to a more complex message)

Waiting times might be longer than longevity

Hoerner’s estimates led to the discouraging conclusion that the number of conversations between civilisations will be small (10.2 on average) and there will not be the possibility for rapid exchange of ideas or cooperation to solve problems.

If longevity is shorter than the time to receive messages then no civilisation will get an answer to a call.

These are averages, however, and there will be some civilisations that are closer than others, which might trigger an expansion of communication with contact signals being sent to new civilisations by more advanced ones. Hoerner likened this to the problem of the origin of life in general but did not take this argument further.

Hoerner hypothesised that if longevity is short and distances are large the interest in making more effort to communicate will fade. If longevity is long and distances short then successful communication will increase enthusiasm. He argued that there would be no middle ground - there will be a lot of communication or none - and we should be prepared for both scenarios.

Nature of signals

Hoerner argued that any signals that exist will be defined by their purpose and the most economical way to achieve them. He suggested three general possibilities for the purpose of signals from another civilisation:

• Local communication (highest likelihood of existing for all estimates of longevity but hard for us to detect)
• Interstellar communication with distinct partners (long-distance calls: small probability of detection)
• Attracting the attention of unknown future partners (contacting signals: highest probability of detection but only if longevity is high and distance is small)

Hoerner referenced Drake’s work (they both worked at the National Radio Astronomy Observatory) for estimates of the most economical frequency to use for interstellar communications. He also recommended searching at twice the frequency of the hydrogen absorption line, arguing that we should use our best guesses for what frequencies to search, with the assumption that the other civilisation will know what we are likely to guess.

Summary

Hoerner concluded his paper with the recommendation that we assume contacting signals exist and try our hardest to find them as soon as possible but that we prepare for not finding anything and design systems that can be used for other astronomy research.

This paper highlighted longevity as the key, but completely unknown, parameter that determines the chances of us detecting other civilisations. Hoerner proposed that we must make estimates and design our search based on these, even if they are wrong. He took a pessimistic view of the longevity of a technological civilisation, with an average of 6500 years, and his results indicated that the waiting times for an answer to a call may exceed the time a technological civilisation exists. He calculated that any signal we do receive is likely to be from a much more advanced civilisation and that what we learn from their messages might be incentive enough for us to seek it since it could increase our own longevity.

References

1. 1. Bracewell RN: Communications from Superior Galactic Communities. Nature 1960, 186:670–671.
2. 2. Cocconi G, Morrison P: Searching for Interstellar Communications. Nature 1959, 184:844–846.
3. 3. Drake FD: Project Ozma. Physics Today 1961, 14:40–46.
4. 4. Dyson FJ: Search for Artificial Stellar Sources of Infrared Radiation. Science 1960, 131:1667–1668.
5. 5. Schwartz RN, Townes CH: Interstellar and Interplanetary Communication by Optical Masers. Nature 1961, 190:205–208.
6. 6. Hoerner SV: The Search for Signals from Other Civilizations. Science, New Series 1961, 134:1839–1843.
7. 7. Drake FD: THE RADIO SEARCH FOR INTELLIGENT EXTRATERRESTRIAL LIFE. In Current Aspects of Exobiology. . Elsevier; 1965:323–345.