Relative Likelihood of Success in the Searches for Primitive versus Intelligent Extraterrestrial Life

Brief

Using a Drake-type decomposition, the authors construct a dimensionless relative likelihood δ = N_t / N_b, the ratio of worlds detectable via technosignatures to those detectable via biosignatures, and show that f_l (the probability of abiogenesis) cancels out of the expression entirely, a key analytic advantage. Under fiducial values (f_i = 0.1, L = 10^4 yr, ε_d = 10^-6, f_d = 0.1, α = 1), δ ≈ 0.02, meaning biosignature searches are roughly 50× more productive per dollar under those assumptions. The JWST biosignature survey volume is estimated at ~3×10^4 stars out to ~30 pc, while a deliberate Galactic radio or optical beacon can in principle be detected across the full ~10^11-star Milky Way. The implied federal SETI funding floor is $10 million per year, scaled against existing biosignature program expenditures.

Metadata

Category

Search

Venue

Astrobiology

Type

Peer-reviewed

Year

2019

Authors

Manasvi Lingam, Abraham Loeb

DOI

10.1089/ast.2018.1936

arXiv

1807.08879

Access

Open access

Length

333.4 K

Programs

Breakthrough Listen, Project Ozma, Kepler mission

Instruments

JWST, Arecibo Observatory, Square Kilometre Array, WFIRST

Tags

SETI, biosignature, technosignature, astrobiology, Drake equation, exoplanet, funding policy

Key points

• Core result: δ ≈ 0.02 α f_cs (f_i/0.1)(L/10^4 yr)(ε_d/10^-6)(f_d/0.1)^-1, the relative likelihood of detecting a technosignature versus a biosignature under fiducial parameters.p.6
• Simplified version after incorporating a settlement-rate model: δ ≈ 0.02 (f_i/0.1)(L/10^4 yr)[1 + (L/10^4 yr)], showing δ scales super-linearly with species lifetime L.p.8
• The probability of abiogenesis (f_l) cancels from the relative likelihood expression, removing the single most uncertain parameter from the funding-allocation argument.p.6
• JWST biosignature survey is bounded to ~3×10^4 M-dwarf stars within ~30 pc via transit spectroscopy; technosignature surveys targeting deliberate broadcasts span the full Galaxy (~10^11 stars).p.4
• Critical-step model yields f_i ~ 10^-8 if each of ~4 post-abiogenesis evolutionary transitions carries probability ~0.01; at this level N_t < 1 even under optimistic remaining parameters.p.7
• Blind optical SETI detection probability ε_d ~ 10^-6 at ~10 kpc; intelligent targeting raises it to ε_d ~ 1, a six-order-of-magnitude swing that dominates the uncertainty in δ.p.6
• If 1% of technological species have lifetimes ~10^8 yr and the rest are short-lived (<10^4 yr), the mean L ~ 10^6 yr, the tail of the lifetime distribution dominates the average and can raise δ by two orders of magnitude.p.8
• Breakthrough Listen allocates $100 million over a decade, making it the largest SETI undertaking to date; the paper's analysis benchmarks a proposed federal allocation of $10 million per year against existing NASA biosignature funding.p.2

Verbatim

• “Based on the current federal funding allocated to the search for biosignatures, our analysis suggests that the search for extraterrestrial intelligence (SETI) may merit a federal funding level of at least $10 million per year, assuming that the average lifetime of technological species exceeds a millennium.”

  p.1

• “if the real bottleneck is the emergence of technological intelligence, the search for biosignatures would be heavily favored (by many orders of magnitude) relative to technosignatures.”

  p.7

• “it seems reasonable to suppose that the survey volume can be taken to encompass most of the Galaxy if the technological level of the transmitting species is more advanced than, or comparable to, that of present-day humans.”

  p.4

• “In the future, interstellar objects passing through the Solar system, like 'Oumuamua, can be searched for signs of electromagnetic signals (Enriquez et al., 2018; Tingay et al., 2018) or be subjected to detailed in situ exploration (Seligman and Laughlin, 2018).”

  p.8

Most interesting

• The probability of abiogenesis (f_l) appears in both the biosignature and technosignature count expressions and cancels exactly in the ratio δ, meaning the funding argument is entirely independent of whether life is common or rare.
• With ~10^7 eukaryotic species on Earth and only one technological, the authors note this empirically 'lends some credence to the idea that f_i ≪ 1 is possible', though they treat f_i = 0.1 as the fiducial optimistic value.
• Present-day pulsed lasers are estimated to outshine the visible light of the Sun by 4 orders of magnitude, making optical technosignatures theoretically detectable at interstellar distances with existing technology.
• A conservative interstellar settlement rate of Λ ~ 10^-4 sites/yr implies that a civilization surviving 10^8 yr could occupy α ~ 10^4 outposts, each potentially broadcasting, super-linearly amplifying δ.
• The paper explicitly brackets the prior decade of SETI as financially starved: after NASA canceled its SETI program in 1993, the field 'had to survive with relatively limited private funding' for roughly twenty years before Breakthrough Listen launched.
• For oxygen as a biosignature, false-negative risk is high: for roughly half of Earth's evolutionary history atmospheric O₂ was below 1% present atmospheric level even while life was abundant, an inhabited Earth would have been misclassified as sterile.