Interstellar Link Budget

Three channels to cross 17,900 light-years: radio, laser, and neutrinos. Compare power requirements, aperture sizes, data rates, and Kardashev tiers for a one-way signal reaching ωCen — or arriving from it.

🔬 Established Physics ⚠ ETI Application Speculative
Distance context: ωCen is at 5.49 kpc = 17,900 ly. A signal sent today arrives in 17,900 years; a round-trip takes 35,800 years. Radio is the conventional SETI choice — but ISM free-free absorption, dispersion, and scintillation degrade signals below ~1 GHz. Lasers beat radio in photons-per-bit for short distances but suffer from diffraction at galactic scales. Neutrinos have effectively zero absorption — but enormous transmitter power is required due to the tiny cross-section.
Radio Link Parameters
Power Required vs Data Rate — all three channels at 5.49 kpc

Minimum transmitter power to achieve given data rate at ωCen distance. Dots = current settings per channel. Horizontal band = Kardashev I–II–III power range.

Cross-Channel Summary — Fixed: 1 bit/s minimum detectable signal
ChannelMin Power for 1 bit/sBeam divergenceISM degradationRTT delayKardashev tier

Radio link budget (Friis 1946): received power P_r = P_t × G_t × G_r × (λ/4πd)² where G_t and G_r are transmitter and receiver antenna gains, λ is wavelength, and d is distance. Minimum detectable signal (MDS) = k_B × T_sys × Δν for bandwidth Δν and system temperature T_sys. Data rate (Shannon) = Δν × log₂(1 + SNR). ISM effects: free-free absorption (negligible above 1 GHz for |b| > 5°), dispersion measure ~50 pc/cm³ toward ωCen (estimated), scintillation bandwidth ~1 kHz at 1 GHz.

Laser link budget: beam divergence θ ≈ λ/D (diffraction limit) for aperture D. Beam area at distance d: A = π(θd/2)². Received power: P_r = P_t × (A_rx / A_beam). Photon rate = P_r / (hν). For optical SETI: detector counts require P_r > few photons per pulse. Background: sky brightness ~100 photons/ns/m² in V-band from ωCen direction.

Neutrino link budget: see the Neutrino SETI Sensitivity tool for full details. Summarised here: effective area σ_CC × ρ_ice × V_det, flux = P_tx / (4πd² × E_ν). This channel's "data rate" is effectively symbolic — individual neutrino detection is binary (detected/not detected).

Round-trip time: 17,900 ly one-way, 35,800 ly round trip. All three channels travel at c. No known shortcut.

References: Friis 1946 Proc. IRE 34:254 · Shannon 1948 Bell Syst. Tech. J. 27:379 · Anchordoqui et al. 2008 · Townes 1983 PNAS 80:1147 · Oliver & Billingham 1971 (Project Cyclops)