SETI: Search Strategies & Current PlansJim 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 ConceptsCurrent SKA ConceptsChina KARSTChina KARST

Canadian Canadian aerostataerostat

US Large NUS Large N

Australian Australian Luneburg Luneburg LensesLenses

Dutch fixed Dutch fixed planar arrayplanar 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

Radio Optical / IR

Narrowband Pulsed

Large N Small N

High Luminosity Low Luminosity

What do we look for?Reciprocity: what do we radiate?

Radio typical: detectable to ~ few pcstrongest: planetary radar ~1

kpc

Optical/IR typical: nilpulsed 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. 1.42 GHz H 1.67 GHz OH

x 1.42 GHz etc.• 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 (1992-2001, but cancelled in 1993)

targeted: ~1000 nearest G-type stars, single, age > 3 Gyr

sky survey: full sky, 1- 10 GHz

• Phoenix = privately funded version of NASA targeted survey (SETI Institute; uses Arecibo)

Notable Radio SETI Programs• META blind survey (Harvard) 1985-1993

magic frequencies (1.42 GHz & x2)~ 106 channels

• META II Argentina

• BETA blind survey (Harvard), presentL band (H, OH), ~ 109 channels

• Serendip IV blind survey, (Berkeley), presentL band, Arecibo, ~ 3x108 channels

• SETI@Home blind survey, ongoing,L band, baseband sampled data (Arecibo),

software data reduction

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)Nsky > 1013 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 velocities500 km/s average (100 to 1700 km/s)

• Isoplanatic angle ~ 10-6 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 DISPERSIONINTERSTELLAR DISPERSION

DM = 0D ds ne(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

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 (1015watts) 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• 1.23-1.53 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 Surveys with Parkes, with Parkes, Arecibo & Arecibo & GBT.GBT.

Simulated & Simulated & actualactual

Yield ~ 1000 Yield ~ 1000 pulsars.pulsars.

SKA advantages:SKA advantages: Multibeaming, multiple Multibeaming, multiple sitessites

One station of many in SKAOne 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