Undetected past contacts with technological species: implications for technosignature science

1. Problem Statement

The Search for Extraterrestrial Intelligence (SETI) has operated for over 65 years without a confirmed detection of technosignatures (remotely detectable indicators of technology). While the "Great Silence" is often attributed to the vastness of the parameter space (the "needle in a haystack" problem), this paper investigates an alternative hypothesis: What if technosignatures have already reached Earth but remained undetected or misidentified?

The core question is: If $n \ge 1$ contacts with technosignatures occurred in the past 65 years but went undetected, what does this imply about the current detectability of such signals? Specifically, how many undetected past contacts are required to make a high-probability detection today, and are those numbers physically plausible given the population of habitable planets in the Milky Way?

2. Methodology

The author employs a Bayesian inference framework to model the relationship between past undetected contacts and present-day detectability.

• Model of Technoemissions:

  • Emitters: Technological species or artifacts distributed within the Milky Way.
  • Geometry: Signals are modeled as spherical shells expanding at the speed of light ($c$).
    • Isotropic: Omnidirectional signals (e.g., Dyson spheres, leakage).
    • Directional: Beamed signals (e.g., intentional communication), modeled with an anisotropy prefactor $\chi \le 1$.
  • Contact Condition: A "contact" occurs if the expanding signal shell intersects Earth within the observational window ($\tau = 65$ years).
  • Statistical Distribution: The number of contacts ($n$) follows a Poisson distribution based on the emission rate ($\Gamma$), the average longevity of signals ($\bar{L}$), and the search radius ($R$).

• Bayesian Framework:

  • Hypothesis ($U_{n,\tau}$): $n$ contacts occurred in the past $\tau$ years but were undetected.
  • Likelihood: The probability of observing $n$ undetected contacts given an emission rate $\Gamma$.
  • Prior: A log-uniform prior ($p(\Gamma) \propto 1/\Gamma$) is adopted to represent complete prior ignorance regarding the emission rate.
  • Posterior: The posterior distribution of $\Gamma$ is derived, leading to a Gamma distribution. This allows the calculation of the expected number of currently detectable signals ($k$) given the history of $n$ undetected contacts.

• Key Parameters:

  • $\tau$: 65 years (time since the first SETI experiment).
  • $R$: Search radius (distance from Earth).
  • $\bar{L}$: Average longevity of the technosignature.
  • $P$: Present-day detectability (probability of at least one detectable signal).

3. Key Contributions

1. Quantitative Link Between Past and Present: The paper establishes a rigorous mathematical relationship showing that a high probability of current detection requires a specific, often massive, number of past undetected contacts.
2. Independence from Anisotropy: The analysis demonstrates that the results for the expected number of detectable signals are independent of whether signals are isotropic or directional. The reduced probability of a directional beam hitting Earth is perfectly compensated by the inferred increase in the total emission rate required to produce the same number of undetected contacts.
3. Plausibility Constraints: The study introduces a "plausibility check" by comparing the required number of past contacts ($n$) against the estimated number of habitable planets in the Milky Way ($N_p \approx 10^{10}$).
4. Sensitivity Analysis: The paper tests non-stationary scenarios, such as emitter clustering near Earth or a sudden recent increase in the birthrate of technological species, to see if they alleviate the tension between high detectability and low contact counts.

4. Key Results

A. The "Unrealistic" Requirement for Nearby Detection

To achieve a high present-day detectability ($P \ge 95\%$) within a relatively short distance (e.g., $R < 1,000$ light-years), the model requires an implausibly large number of past undetected contacts ($n$).

• For long-lived signals, achieving $P \ge 95\%$ at $R \approx 1,000$ ly requires $n \approx 10^4$ undetected contacts in the last 65 years (roughly one every two days).
• For short-lived signals, the requirement jumps to $n \approx 10^6$ (roughly 40 per day).
• As $R$ decreases, the required $n$ increases rapidly, eventually exceeding the total number of habitable planets in that volume. This suggests that if contacts have occurred, they likely originated from much farther away.

B. The Trade-off: Distance vs. Number of Signals

• Large Search Radius: Extending the search to the entire Milky Way ($R \sim 60$ kly) reduces the required $n$ to plausible levels (e.g., $n \approx 5$ for long-lived signals).
• Low Expected Detections: However, even with a high probability of detectability ($P \to 1$), the expected number of currently detectable signals ($\langle k \rangle$) remains very low (less than 10, often just a few).
• Logarithmic Scaling: The expected number of detections scales logarithmically with the detection probability. Even if we are "almost certain" a signal exists, we only expect to find a handful in the entire galaxy.

C. Alternative Scenarios (Clustering and Sudden Booms)

The paper explores if specific distributions could allow for high detectability at short distances without massive $n$:

• Spatial Clustering: If emitters are tightly clustered around Earth (within $\sim 1,000$ ly), $n$ can be reduced. However, this requires almost all emitters to be within this tiny volume (0.02% of the galactic disk), which is a highly specific and unlikely scenario.
• Temporal Sudden Increase: If the birthrate of emitters increased very recently (e.g., within the last 1,000 years), $n$ could be reduced. However, this implies a "boom" in civilization emergence coinciding with human history, which is speculative. Slower growth rates revert to the stationary model's constraints.

5. Significance and Implications

• Reframing the "Silence": The lack of detection is likely not just due to poor sensitivity or wrong wavelengths, but potentially because detectable signals are intrinsically rare in the galaxy. Even if many civilizations exist, the probability of their signals intersecting Earth and being detectable today is low.
• Search Strategy: The findings suggest that SETI searches should not focus solely on the immediate solar neighborhood if the goal is to find signals from a "typical" distribution of emitters. The optimal search radius is likely several thousand light-years, where the balance between the number of potential emitters and the required number of past contacts is most favorable.
• Expectation Management: The paper argues that even if undetected past contacts have occurred, we should not expect a "flood" of detections. The most optimistic scenarios still predict only a few detectable technoemissions across the entire Milky Way.
• Robustness: These conclusions hold regardless of whether signals are isotropic (Dyson spheres) or directional (beacons), and they are robust against different prior assumptions about the emission rate.

Conclusion: The paper concludes that assuming undetected past contacts does not automatically imply a high probability of imminent discovery. Unless emitters are extremely long-lived and located thousands of light-years away, or unless civilizations are clustered unnaturally close to Earth, the number of detectable signals today remains vanishingly small.