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Earth Detecting Earth: At what distance could Earth's constellation of technosignatures be detected with present-day technology?

T0 review · 3 major / 8 minor · reviewed 2026-08-09 · deepseek-v4-flash

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Pith's one-line read Earth's planetary radar would be detectable from 12,000 light-years away by today's best radio telescopes.

desk verdict A useful and readable first cut at putting all of Earth's technosignatures on one distance scale, but the headline radar number is optimistic and several of the Table 2 distances don't survive contact with the printed equations. read the letter →

arxiv 2502.02614 v1 pith:OODULRPM submitted 2025-02-03 astro-ph.IM astro-ph.EPphysics.pop-ph

Sofia Z. Sheikh , Macy J. Huston , Pinchen Fan , Jason T. Wright , Thomas Beatty , Connor Martini , Ravi Kopparapu , Adam Frank This is my paper · ORCID
classification astro-ph.IMastro-ph.EPphysics.pop-ph
keywords SETItechnosignaturesplanetaryradarradioastronomyichnoscaleexoplanetatmospheresEarth-2024detectabilitydistances
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper asks how far away a copy of present-day Earth could be spotted by a civilization using instruments no more advanced than our own. It calculates the maximum detection distance, at signal-to-noise 5, for twelve of humanity's current technosignatures — radio leakage, planetary radar, atmospheric pollutants, city lights, heat islands, lasers, spacecraft, and surface artifacts — and ranks them on one distance scale. The headline result: intermittent, celestially targeted planetary radar (Arecibo-class, 20 TW EIRP) would be detectable by the coming SKA1-Mid array out to 12,000 light-years, a thousand times farther than the best non-radio signature. This establishes a quantitative 'Earth-2024' benchmark against which null SETI surveys can set upper limits, and it renews the case that radio remains the most promising window for finding a civilization at our own technological level.

What carries the argument

The paper's organizing device is the ichnoscale, the ratio of a technosignature's size to the same signature produced by current Earth technology, pinned to iota = 1 for both transmitters and detectors. The distance calculations are carried by two standard equations: the radiometer equation, $d = \sqrt{ \mathrm{EIRP} / (4\pi \, \mathrm{SNR} \, \mathrm{SEFD}) } \sqrt{ \tau_{\mathrm{obs}} / \Delta \nu }$, which converts transmitter power, receiver sensitivity, and integration time into a detection range for narrowband radio, and the $R^{-4}$ radar range equation for reflected signals. Non-radio channels use photon-counting statistics and pixel-dilution models at the same SNR = 5 threshold, so all twelve signatures are placed on a single comparable distance scale.

What would settle it

Compute the actual time-averaged EIRP of Earth's planetary radar toward a specific target star, using the historical duty cycle and beam positions of Arecibo-class systems: if that average is more than roughly three orders of magnitude below the 20 TW peak used in the paper, the claimed 12,000 ly range would fall below the 65 ly distance of the Deep Space Network uplink, overturning the paper's rank ordering.

Watch

Extended reading notes

Core claim

The central claim is that, at the present-day technological level of Earth (what the paper calls ichnoscale iota = 1, for both transmitter and receiver), Earth's detectable technosignatures span 13 orders of magnitude in range, and the most distant signature is not deliberate METI but the planetary radar used for asteroid and planetary science. Treating the Arecibo-class S-band transmitter (EIRP 20 TW) and the SKA1-Mid receiver with 1-hour integration and matched frequency resolution, the radiometer equation gives a detection distance of about 12,000 light-years at SNR = 5, beating its nearest non-radio competitor (atmospheric NO2 at 5.7 ly; resolved lasers at 5.9 ly) by a factor of $10^{3}$. The authors take this as evidence that radio remains the dominant observable signature of an Earth-2024-level civilization and as a benchmark for interpreting non-detections in SETI surveys.

Load-bearing premise

The calculation assumes that the planetary radar beam is pointed directly at the observer for the entire one-hour observation, with the receiver's frequency channelization matched to the signal; real planetary radar is intermittent and aimed at Solar System targets, so a single line of sight would carry a much smaller time-averaged signal, shrinking the headline 12,000-light-year distance.

Editorial extensions

If this is right

  • A null result in a radio SETI survey that is sensitive to Arecibo-class EIRP at distance d would imply that no Earth-2024-level civilization with such radar exists within d, after accounting for pointing and duty cycle.
  • The factor-of-10^3 radio advantage over non-radio signatures means that for the foreseeable future, the most efficient search for Earth-twin civilizations is a targeted radio search, not an infrared or optical one.
  • The multi-wavelength catalog implies that a civilization's visibility grows with proximity in a predictable order, so observational programs that combine radio, spectroscopic, and photometric channels can prioritize nearby systems and cross-confirm a detection.
  • The short detection distances of most non-radio signatures (city lights at 0.036 ly, heat islands at ~30 AU, surface artifacts at ~8600 km) set concrete limits on how far away a present-day Earth could be found with those methods alone.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A duty-cycle-corrected version of the same calculation would likely move the practical SETI benchmark for 'radio loud' Earth twins much closer, possibly to the ~65 ly scale of the Deep Space Network; that is an implicit caveat in the paper's own choice of the 'intermittent, celestially-targeted' class.
  • The method can be run backward in time: applying the same sensitivity formulas to historical transmitter strengths and atmospheric compositions would yield a 'first detectability date' for each signature, telling us when Earth became visible in each channel to a hypothetical nearby observer.
  • The same distance-ranking technique could be applied to hypothetical civilizations at other ichnoscales (iota = 0.1, 10, 100) to map how the optimal search wavelength shifts as a civilization grows, which the paper only gestures at in its discussion.
  • If the SKA1-Mid array actually observes a 20 TW narrowband source at ~10^4 ly, the paper's constellation prediction implies that a closer Earth twin should also show lower-level radio leakage and atmospheric NO2, providing a built-in confirmation strategy.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 8 minor

Summary. This paper introduces an 'Earth Detecting Earth' framework for SETI, fixing both the transmitter and receiver at the ichnoscale value ι=1 (Earth-2024 technology) and computing the maximum distance at which each of a set of present-day Earth technosignatures could be detected with present-day or near-future instruments. The technosignatures considered are radio transmissions (planetary radar, DSN uplinks, LTE leakage, Voyager-class downlinks), atmospheric NO2, city lights, deep-space optical communication lasers, urban heat islands, radar detection of space objects, surface artifacts on the Moon, and satellites in transit. The main quantitative conclusions are that Earth's detectable signatures span roughly 13 orders of magnitude in distance, and that intermittent, celestially-targeted planetary radar (Arecibo-class, 20 TW EIRP) would be detectable by SKA1-Mid out to about 12,000 light-years, a factor of 10^3 farther than the nearest non-radio competitor (atmospheric NO2 at 5.7 ly). The paper also discusses the interpretation of these results for SETI survey strategy and for the 'SETI as a mirror' program.

Significance. If the headline numbers are robust, this paper would provide a valuable quantitative benchmark for SETI: it places disparate technosignature searches on a common distance scale without extrapolating beyond current terrestrial technology, and it makes falsifiable predictions that can be updated as instruments improve. The cross-wavelength comparison and the explicit use of published instrument parameters are genuine strengths. The central qualitative message — that radio/radar remains the longest-range signature of an Earth-2024-level civilization — is likely to survive corrections, but the specific factor-of-10^3 advantage over atmospheric NO2 rests on a narrow-bandwidth coherence assumption that the paper itself flags but does not quantify. Because the main table contains at least one internally inconsistent entry and the headline distance is sensitive to interstellar scattering, the quantitative claims need revision before the paper can serve as a reliable reference for survey upper limits.

major comments (3)
  1. [Section 2, Eq. (3) and Table 2 (Case 1)] The headline 12,000 ly distance assumes that the full 20 TW Arecibo-class EIRP is detectable in a 0.01 Hz channel over a 1 hr integration. The paper itself states (Section 2) that spectral broadening 'could be significant for the narrowband signals past a few kpc,' but Table 2 applies no such correction. At a distance of 3.7 kpc (12,000 ly), interstellar scattering with even a modest scattering time of τ_sc = 1 μs gives a decorrelation bandwidth of roughly 160 kHz; re-evaluating Eq. (3) with Δν = 160 kHz reduces the detection distance to about 190 ly, and τ_sc = 1 ms (Δν_sc ~ 160 Hz) gives about 1,100 ly. These are factors of 60 and 10 below the quoted value, respectively, and they materially affect the claim that planetary radar beats NO2 by a factor of 10^3 in distance. The authors should either model scattering along representative lines of sight or restrict the claim and show how the cross-wavelength ranking changes.
  2. [Section 2, Case 3 (LTE) and Table 2] The 4.0 ly entry for LTE radio leakage does not follow from the stated inputs. Using Eq. (3) with EIRP = 4 GW, SEFD = 1.5 Jy, SNR = 5, τ_obs = 3600 s, and Δν = 20 MHz gives d ≈ 0.8 ly, not 4.0 ly. To obtain 4.0 ly from Eq. (3) one would need either Δν ≈ 20 kHz or EIRP ≈ 100 GW. Since the footnote to the 4 GW value explicitly says the quantity is not intended to be interpreted as an EIRP, the manuscript must clarify which quantity is actually being used and recompute the Table 2 entry so that the quoted distances are reproducible from the stated equations.
  3. [Section 2 and Section 6] The 'intermittent, celestially-targeted' character of Case 1 is described in words but not propagated into the headline comparison. The 12,000 ly figure is an instantaneous detection distance under the assumption that the 20 TW beam is pointed directly at the observer for the full hour. Real planetary radar is aimed at solar-system bodies and has a small duty cycle, so the expected detection distance for any particular line of sight is much smaller, and the factor-of-10^3 advantage over non-radio signatures is not the appropriate number for survey planning. The authors should state this explicitly and, if the paper is intended to guide SETI survey upper limits, provide a duty-cycle- and pointing-probability-corrected estimate for Case 1.
minor comments (8)
  1. [Section 2, Eq. (1)] The typeset form of Eq. (1) is ambiguous: the placement of Δν_t relative to the radical is unclear. Please rewrite it so that the numerator/denominator structure is explicit.
  2. [Section 2, Eq. (3)] As printed, Eq. (3) is dimensionally inconsistent if SEFD is in Jy; the conversion 1 Jy = 1e-26 W m^-2 Hz^-1 must be stated and applied before insertion. This is easy to fix but is essential for reproducibility.
  3. [Abstract and Section 6] The abstract says the detectable signatures span 13 orders of magnitude, while Takeaway #3 in Section 6 and the conclusion say 12 orders of magnitude. Table 2 spans from 9.1e-10 ly to 1.2e4 ly, which is about 13 decades; please reconcile the wording.
  4. [Section 2, Case 2 (DSN)] With the stated DSN EIRP of 965 MW and Eq. (3), the DSN detection distance is about 82 ly rather than the 65 ly listed in Table 2. Please give the exact SEFD, number of polarizations, bandwidth, and integration time used for each radio case so that the table can be reproduced.
  5. [Section 4.2] The text reports the resolved-laser detection distance as 1.8 parsecs, which is consistent with the 5.9 ly in Table 2, but for readability please use one unit throughout the section.
  6. [Section 2] '3 5-minute integrations' should be '3 × 5-minute integrations' or 'three 5-minute integrations,' and the same formatting issue appears elsewhere in the paper.
  7. [Section 7.5] The phrase 'worth nothing' should be 'worth noting.'
  8. [References] The in-text citation 'Critzen & Stoermer 2000' and the corresponding reference entry appear to be a typo for 'Crutzen & Stoermer 2000'; please correct both spelling variants.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the detectability distances are direct sensitivity calculations from externally referenced parameters, with self-citations serving as independent published support rather than as inputs recycled as outputs.

full rationale

The derivation chain is self-contained in the sense that matters for circularity. Section 2's radio distances follow from the radiometer equation (Eq. 3) with externally referenced inputs: Arecibo's 20 TW EIRP (Ekers et al. 2002), SKA1-Mid SEFD from Braun et al. (2017), SNR = 5, tau_obs = 1 hr, and Delta_nu = 0.01 Hz. None of these inputs is fitted to the Table 2 distances, and the paper does not rename a fitted parameter as a prediction. The atmospheric NO2 distance scales the published Kopparapu et al. (2021) photochemical and synthetic-spectrum model, which is external to this work and does not already contain the 5.71 ly result; the paper's contribution is the scaling. The city-light and radar distances similarly use the formalisms of Beatty (2022) and Ostro (1993) with stated instrument parameters. The ichnoscale from Socas-Navarro et al. (2021) is a definitional unit, not a derived result. Self-citations with overlapping authorship (Socas-Navarro et al. 2021; Sheikh 2020; Kopparapu et al. 2021; Beatty 2022) provide definitions, merit axes, and independently published models, but none is invoked as an unverified premise that forces a target conclusion. The paper explicitly flags the spectral-broadening and light-travel-time limitations in Section 2, which are physical caveats rather than circular reasoning. I therefore find no circular step; the 12,000 ly headline and the 13-orders-of-magnitude span are sensitivity calculations, not inputs recycled as outputs.

Assumptions & free parameters 6 free parameters · 6 assumptions · 0 invented entities

No new physical entities are introduced. The central distances are sensitivity limits from standard receiver/radar equations using published transmitter and instrument parameters. The main 'free' choices are observational thresholds, integration times, bandwidths, and the selection of extreme historical values for each signature.

free parameters (6)
  • Detection SNR threshold = 5
    All distances scale as 1/sqrt(SNR); chosen as the standard SETI detection threshold, but any other choice rescales every entry in Table 2.
  • Radio integration time = 1 hour
    All radio and radar distances scale as sqrt(tau); 1 hour is feasible for ground observatories, but longer campaigns would increase distances.
  • Radar/DSN frequency resolution = 0.01 Hz
    Adopted from finest radar science resolution; distance scales as 1/sqrt(Delta nu). This is optimistic for DSN uplink, whose true bandwidth is wider.
  • LTE matched bandwidth = 20 MHz
    Stated in Section 2 as the LTE bandwidth. Plugging EIRP=4 GW and Delta nu=20 MHz into Eq. 3 gives about 0.8 ly, not the text's 4.0 ly; the listed value implies an effective bandwidth near 30 kHz, so the calculation is internally inconsistent.
  • NO2 mixing ratio = 113 ppb
    1980 EPA peak value used for the atmospheric case. Current global NO2 is far lower; this is a best-case extreme, and the detection distance scales with the assumed column abundance.
  • Urban heat island temperature excess = 10.5 C
    Most extreme published Hong Kong urban heat island estimate; other estimates are 2.8 to 8 C. The heat-island distance scales roughly linearly with Delta T.
assumptions (6)
  • standard math Radiometer equation (Enriquez et al. 2017)
    Used for all radio cases; assumes Gaussian noise and matched filtering.
  • standard math Radar range equation (Ostro 1993)
    Used in Appendix A for radar detection of objects; assumes known target position and perfect beam alignment.
  • domain assumption Kopparapu et al. (2021) NO2 spectral model
    The NO2 distance is scaled from this photochemical/spectral model; assumes a cloud-free atmosphere and a specific vertical profile.
  • domain assumption HWO-class 6m coronagraph performance
    Assumed for atmospheric and city-light detectability; instrument is not yet built.
  • standard math Moffat PSF model for solar background suppression
    Used in the resolved-laser case to estimate background at Earth's position; choice of FWHM and power index affects distance.
  • ad hoc to paper Ichnoscale definition (Socas-Navarro et al. 2021)
    Frames the comparison but is a definition, not an empirical law; using 'most extreme historical instance' for each signature is a modeling choice.

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Cite this review

Pith. "Pith review of Earth Detecting Earth: At what distance could Earth's constellation of technosignatures be detected with present-day technology?." pith.science (2026). https://pith.science/paper/OODULRPM

@misc{pith2026250202614,
  author       = {Pith},
  title        = {Pith review of: Earth Detecting Earth: At what distance could Earth's constellation of technosignatures be detected with present-day technology?},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OODULRPM}},
  note         = {Machine review of arXiv:2502.02614}
}
abstract

The field of the Search for Extraterrestrial Intelligence (SETI) searches for ``technosignatures'' that could provide the first detection of life beyond Earth through the technology that an extraterrestrial intelligence (ETI) may have created. Any given SETI survey, if no technosignatures are detected, should set upper limits based on the kinds of technosignatures it should have been able to detect; the sensitivity of many SETI searches requires that their target sources (e.g., Dyson spheres or Kardashev II/III level radio transmitters) emit with power far exceeding the kinds of technology humans have developed. In this paper, we instead turn our gaze Earthward, minimizing the axis of extrapolation by only considering transmission and detection methods commensurate with an Earth-2024 level. We evaluate the maximum distance of detectability for various present-day Earth technosignatures -- radio transmissions, atmospheric technosignatures, optical and infrared signatures, and objects in space or on planetary surfaces -- using only present-day Earth instruments, providing one of the first fully cross-wavelength comparisons of the growing toolbox of SETI techniques. In this framework, we find that Earth's space-detectable signatures span 13 orders of magnitude in detectability, with intermittent, celestially-targeted radio transmission (i.e., planetary radar) beating out its nearest non-radio competitor by a factor of $10^3$ in detection distance. This work highlights the growing range of ways that exoplanet technosignatures may be expressed, the growing complexity and visibility of the human impact upon our planet, and the continued importance of the radio frequencies in SETI.

Figures

Figures reproduced from arXiv: 2502.02614 by the authors.

Figure 1
Figure 1. The maximum distances that each of Earth’s modern-day technosignatures could be detected at using modern-day receiving technology, in visual form. Also marked are various astronomical objects of interest. 7. DISCUSSION 7.1. On the use of ι=1 This work focuses on ι = 1, which strictly limits the kinds of technosignatures we consider, their scale, and the detection technologies that we can use to find them. It is like… view at source ↗

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