SETI: Search Strategies & Current Plans Jim Cordes 23 September 2002 Motivation for searching we’re here life expected to be common (especially microbial life) technology common? Yes  N>>1 No  N=1 Search issues Cross section of SETI programs so far Future SETI Editorial comments

Parkes MB Feeds Arecibo Multibeam Surveys

Current SKA Concepts China KARST Canadian aerostat US Large N Australian Luneburg Lenses Dutch fixed planar array (cf. Allen Telescope Array, Extended VLA) (cf. LOFAR = Low Freqency Array)

Expand The Galactic Exploration ATA Phoenix SKA

Why search? Assessing the Odds The astrophysical case: p(habitable planets | Galaxy) The biological case: p(life | habitable planets) Complexity: p(technology | life) p(extroversion | technology)

Related Issues Copernican principle – we’re mediocre & there must be lots more like us Anthropic principle – the universe necessarily has properties that allow complex (but mediocre) beings like ourselves to have evolved. Fermi Paradox – given CP + AP, Where is everybody?

Additional SETI Issues Large N  optimism about evolutionary trends leading to technological life, its longevity, and perhaps about Galactic colonization Counterpoints: –What took hominids so long to evolve on Earth? –‘Rare Earth’ arguments (Ward & Brownlee) Our preconceptions about N have a strong influence on –how luminous ET transmissions must be for detection –beaming of ET transmissions (toward us?!) N determines how far we must look in the Galaxy How far we look determines the role of propagation effects from ISM plasma (radio) or grains (IR/optical)

SETI Conundrums Deliberate transmissions Leakage transmissions RadioOptical / IR NarrowbandPulsed Large NSmall N High LuminosityLow Luminosity

What do we look for? Reciprocity: what do we radiate? Radio typical: detectable to ~ few pc strongest: planetary radar ~1 kpc Optical/IR typical: nil pulsed IR lasers: ~ few x 10 pc  -rays 1 Mton: ~ 1 AU with CGRO

Rationale for Radio SETI No Galactic absorption No background from host stars Maximum S/N in microwave band (1-10 GHz) Magic frequency arguments e.g GHz H 1.67 GHz OH  x 1.42 GHzetc. Narrowband signals << thermal Doppler widths of natural, astrophysical sources Propagation effects (dispersion, scintillation, pulse broadening from ISM) are important

Notable Radio SETI Programs Ozma 1960 (Frank Drake) targeted (two nearby stars) Serendip I-IV (piggyback surveys at Green Bank and Arecibo) blind surveys (1970s – present) NASA targeted survey + sky survey ( , but cancelled in 1993) targeted: ~1000 nearest G-type stars, single, age > 3 Gyr sky survey: full sky, GHz Phoenix = privately funded version of NASA targeted survey (SETI Institute; uses Arecibo)

Notable Radio SETI Programs META blind survey (Harvard) magic frequencies (1.42 GHz & x2) ~ 10 6 channels META II Argentina BETAblind survey (Harvard), present L band (H, OH), ~ 10 9 channels Serendip IV blind survey, (Berkeley), present L band, Arecibo, ~ 3x10 8 channels blind survey, ongoing, L band, baseband sampled data (Arecibo), software data

Optimizing radio SETI against background noise

Anticipated radio ET Signals (by ‘strong SETI’ proponents) Narrowband (~ 1 Hz) Weakly modulated (~ 1 bit/s) Drifts in frequency (orbital + planetary motion) df/dt ~ 10 to 100 Hz/hour (some argue that deliberate transmissions to us would be Doppler corrected) Pattern recognition algorithms: search for narrowband, drifting features in the frequency-time plane (dechirping algorithms) Search space: (B/  )(T/  t)N sky > trials  need very high threshold (e.g. 30  ) to achieve small false-alarm rate

Spectra from the OH masers in W49 Power spectra calculated from baseband sampled data from Arecibo; Signal statistics = exponential

Real-world effects Terrestrial & spacecraft radio frequency interference (RFI): diverse, mimics anticipated signals (our RFI = their ETI signal and vice versa) Interstellar scintillation causes deep fading and occasional amplification

Electron density irregularities exist on scales from ~ 100’s km to ~ pc as approximately a power-law spectrum (~ Kolmogorov) Pulsar velocities >> ISM, observer velocities 500 km/s average (100 to 1700 km/s) Isoplanatic angle ~ arc sec  AGNs don’t show DISS, pulsars do Expect ETI sources to show DISS  Deep fading & amplification (100% modulation)  longer time scales than pulsars (lower velocities)

INTERSTELLAR DISPERSION DM =  0 D ds n e (s) Known for ~1200 pulsars DM ~ 2 to 1100 pc cm -3 Variable at ~10 -3 pc cm -3 Variations with d,l,b show obvious Galactic structure

Dynamic spectrum of pulsar scintillation

Narrowband signals will show deep modulation with exponential statistics

Optimizing a search: better to split total time per target into ~4 intervals so that ISS is uncorrelated between them

Rationale for Optical/IR SETI Pulsed lasers distinguishable from host star with reasonable power (nanosecond pulses) Optical/IR not susceptible to ISM plasma propagation effects … But interstellar absorption and scattering from grains important for optical and near IR (scattering  smearing of pulse)

Laser power Petawatt (10 15 watts) pulse lasers exist for laser fusion, are sufficient to produce detectable pulses from systems on planets around G-type host stars. For ns pulses, a 1-m telescope + photomultiplier is sufficient to detect sources out to ~ 30 pc. Programs at Berkeley, Harvard, amateur.

I. Arecibo Galactic-Plane Survey |b| < 5 deg, 32 deg < l < 80 deg GHz bandwidth = 300 MHz digital backends (<0.3 MHz channels) –Correlator based e.g. 7 x (2 x WAPP)? (200 MHz) –FPGA-FFT or Polyphase filter approach? (300 MHz) ~300 s integrations, 3000 hours total Can see 2.5 to 5 times further than Parkes MB –period dependent –from AO sensitivity + narrower channels (larger DM) Expect ~1000 new pulsars

Surveys with Parkes, Arecibo & GBT. Simulated & actual Yield ~ 1000 pulsars.

SKA advantages: Multibeaming, multiple sites One station of many in SKA

Comments Observational phase space is very incompletely covered to date [, F,  t, , d /dt, transients, etc.] many of the radio sources in large scale surveys remain unidentified (though many are likely to be AGNs, pulsars, microquasars, HII regions and flare stars)  empirical conclusions about N not yet possible SETI strategies that strongly leverage notions about the motivations of ETI are not robust in their ability to constrain N An economical approach is to design telescopes & surveys for astrophysical purposes & conduct SETI as a subset or spinoff of the sky coverage  requires SETI specific digital backend systems that exploit Moore’s law. [, F,  t, , d /dt, transients, etc.]  empirical conclusions about N not yet possible