The Search for Extraterrestrial Intelligence SETI

1
The Search for Extraterrestrial Intelligence
(SETI) Friday, February 22, 2008
2
Midterm Marks - Distribution
3
The probability of success is difficult to
estimate but if we never search, the chance of
success is zero. - Morrison and Cocconi
(1959)
4
Signatures of Life and their Detection

• the current (and forseeable future) search and
  detection of life on other worlds is dependent
  primarily upon remote sensing and robotic probes
• living systems alter their environments and
  generate signatures of their functional processes
• three types of signatures are possible
• biosignatures the direct consequences of
  biological activity
• geosignatures the effects and alterations of the
  geological environment by living systems
• geoindicators planetary characteristics that can
  be detected that are likely consistent with the
  presence of life

5
The Search for Extraterrestrial Intelligence
(SETI)

• SETI is a misnomer because there is no known way
  to detect intelligence across interstellar
  distances
• the best that we can do is to search for a
  manifestation of another technology and, having
  detected it, infer the existence of intelligent
  technologists, who may or may not still be
  associated with the detected technology
• which technology might an extraterrestrial
  civilization use, and what are the observable
  consequences using current terrestrial remote
  sensing equipment?

6
The Kardashev Classification

• Nikolai Kardashev felt that our civilization
  might progress through four different stages, and
  that other alien civilizations might be
  categorized as being in one of those four
  distinct stages
• classifies possible extraterrestrial
  civilizations according to the energy at their
  disposal (i.e., transmission power)

Type 0 This civilization has only just begun to
tap planetary resources such as solar power,
geothermal power wind power. Most of its power
generation is still based on non-renewable fossil
fuel resources, e.g., oil, coal and natural
gases. This stage of civilization is like our
own. Type 1 These civilizations can
effectively control the entire resources of their
planet they can predict weather patterns and
earthquakes very accurately, and even control
them using artificially induced greenhouse
effects or space-based lasers. A Type 1
Civilization could conceivably halt an ice-age.
Has the power capability of an entire sun.
7
Type 2 These types of civilizations have
extended their power to their entire Solar System
by harnessing the power of their suns through
Dyson spheres. Having colonized or at least
extensively explored all the planets within their
Solar System, they are a largely space- faring
race and have launched expeditions to other stars
using interstellar craft. Type 3 These
civilizations have colonized thousands if not
millions of solar systems in entire galaxies.
They can harness the power of galaxies.
Therefore, there are no type 3 civilizations in
the Milky Way galaxy.
8
Signatures of More Advanced Life

• Local Technologies
• examples of technologies employed for the sole
  use of civilizations that invented them include,
  generating energy, local transportation, etc.
• advanced civilizations might revel themselves
  through major astroengineering projects that are
  visible from great distances
• these types of engineering technologies would
  only be visible through unintended manifestations
  of their technologies
• artifacts left behind, either deliberately or
  accidentally

9
Dyson Sphere

• the Dyson sphere was originally proposed by the
  astronomer Freeman Dyson as a way for an advanced
  civilization to utilize all of the energy
  radiated by their sun
• it is an artificial sphere the size of an
  planetary orbit
• the sphere would consist of a shell of solar
  collectors or habitats around the star, so that
  all (or at least a significant amount) energy
  will hit a receiving surface where it can be used
• this would create a huge living space and gather
  enormous amounts of energy

10
Signatures of More Advanced Life

• Spacecraft
• extraterrestrial civilizations may be capable of
  interstellar travel and interstellar information
  exchange, can build spacecraft with us in mind
• spacecraft might leave behind telltale signs of
  their operation

11
Signatures of More Advanced Life

• Electromagnetic Radiation
• the majority of searches for extraterrestrial
  intelligence have concentrated on finding signals
  that are the result of exchanging information
  either unintentional leakage or directed beacons

12
Optimal Requirements for Transmitting Information

• there is little chance of discovering
  extraterrestrial life via direct contact
• the best method to communicate is by radio waves
  (electromagnetic radiation photons)

13
Optimal Requirements for Transmitting Information

• electromagnetic (EM) waves have three physical
  characteristics - wavelength, frequency and
  energy
• the velocity of all EM radiation is constant at
  'c' (the speed of light)
• wavelength and frequency are inversely
  proportional to each other, and the energy is
  directly proportional to the frequency
• in other words, as the frequency and energy
  increase, the wavelength decreases

14
Optimal Requirements for Transmitting Information

• therefore radio and microwaves, being of such
  low frequency, have very low energy
• other exotic, massless particles proposed by
  theoretical physicists may also travel at the
  speed of light, but we do not know how to
  manipulate them
• if such exotic particles are the choice of
  technologies more advanced than our own, the only
  strategy for detecting such strategies is to
  survive as a technological species until we learn
  to generate and capture them ourselves

15
Optimal Requirements for Transmitting Information

• photons are the ideal carriers of information
  because
• are massless
• travel at the speed of light
• have very small energies
• can carry information
• are easily generated and detected
• are undeflected by galactic magnetic fields
• at many frequencies have a very small probability
  of being scattered or absorbed

16
Optimal Requirements for Transmitting Information

• the search for electromagnetic signals is the
  methodology of SETI
• having decided on the methodology, it is
  necessary to decide
• where to search (three spatial dimensions)
• when to search (one temporal dimension), and
• what to search for (a frequency, two possible
  polarizations, a modulation scheme, and a signal
  strength a combination of transmitter power and
  distance)
• this is the nine-dimensional cosmic haystack

17
The Drake Equation
How likely is it that there are other
civilizations in the universe capable of
communication?
What do we need to know about to discover life
in space? - Frank Drake, 1961
18
The Drake Equation

• there are over 300 billion stars in the Milky Way
  galaxy and billions of galaxies in the universe
• how can we estimate the number of technological
  civilizations that might exist among the stars?
• the Drake Equation is an attempt to quantify
  estimates of the number of civilizations in our
  galaxy capable of interstellar radio
  communication
• identifies specific factors thought to play a
  role in the development of such civilizations
• although no unique solution to this equation, it
  is a generally accepted tool used by the
  scientific community to examine these factors
• the terms are listed in decreasing estimates of
  reliability

19
N R fp ne fl fi fc L

• N the number of communicative civilizations.
  The number of civilizations in the Milky Way
  Galaxy whose radio emissions are detectable
• R the rate of formation of suitable stars. The
  rate of formation of stars with a large enough
  "habitable zone" and long enough lifetime to be
  suitable for the development of intelligent life
• fp the fraction of those stars with planets.
  The fraction of Sun-like stars with planets is
  currently unknown, but evidence indicates that
  planetary systems may be common for stars like
  the Sun
• ne the number of "earths" per planetary system.
  All stars have a habitable zone where a planet
  would be able to maintain a temperature that
  would allow liquid water. A planet in the
  habitable zone could have the basic conditions
  for life as we know it

20
N R fp ne fl fi fc L

• fl the fraction of those planets where life
  develops. Although a planet orbits in the
  habitable zone of a suitable star, other factors
  are necessary for life to arise. Thus, only a
  fraction of suitable planets will actually
  develop life
• fi the fraction life sites where intelligence
  develops. Life on Earth began over 3.5 billion
  years ago. Intelligence took a long time to
  develop. On other life-bearing planets it may
  happen faster, it may take longer, or it may not
  develop at all
• fc the fraction of planets where technology
  develops. The fraction of planets with
  intelligent life that develop technological
  civilizations, i.e., technology that releases
  detectable signs of their existence into space
• L the "Lifetime" of communicating
  civilizations. The length of time such
  civilizations release detectable signals into
  space

21
How Reliable is the Drake Equation?

• on the basis of 300 billion stars in the Milky
  Way galaxy, Frank Drakes current estimate of the
  number of communicating civilizations is 10,000
• the Lunar Planetary Laboratory of the
  University of Arizona estimates the number at 15
• the real value of the Drake Equation is not in
  the answer itself, but the questions that are
  prompted when attempting to come up with an
  answer, and to identify areas of research
• obviously, there is a tremendous amount of guess
  work involved when filling in the variables
• as we learn more from astronomy, biology, and
  other sciences, we'll be able to better estimate
  these variables

22
Where Do We Search?

• a radio transmitter can be pointed in all
  directions and communicate simultaneously with
  many different civilizations
• interstellar dust absorbs optical radiation, but
  not radio waves, so in the plane of our Galaxy,
  radio signals can penetrate to great distances
• if you point your radio telescope at the sky
  there are all kinds of signals - the sky is very
  noisy at low frequencies because of the planets
  and comets, giant clouds of gas and dust, and
  stars and galaxies, emit light at many different
  wavelengths, and at higher frequencies due to
  atmospheric noise

23
Search Strategies

• figure displays the average natural background
  radiation from astrophysical sources
• to be detectable at a given frequency, a
  transmitted signal, or the portion of it that
  enters a particular detector, must have an
  intensity that can successfully compete with this
  background, as well as the instrumental noise of
  the receiver

• plane and pole - background radiation from Milky
  Way Galaxy
• CMB 2.7 K
• IR infrared emission from warm dust and gas
• optical (visible) emission is light from stars
  and galaxies
• UV - red-shifted alpha emission from ionized gas
  within distant galaxies
• X-ray gamma-rays distant galaxies

24
Search Strategies

• figure shows the height above sea-level to which
  radiation at any given wavelength can penetrate
• SETI observations have concentrated on microwave
  radio searches (1 to 10 GHz), where the natural
  background is low and where atmospheric
  transparency approaches 100

25
Targets or Sky Sweeps?

• two strategies to search systematically for ETC
  signals in the cosmic haystack
• sweep the sky and look in all possible directions
• focus the search on directions that seem a priori
  more likely to contain a technological
  civilization, i.e., targeted search strategy

26
Sky Sweeps

• in this method, you survey large chunks of the
  sky, one at a time, for signals
• a wide-field search allows the entire sky to be
  searched at a low resolution in a short period of
  time
• if a signal is detected, however, it would be
  difficult to pinpoint the exact source without a
  subsequent high-resolution search
• smaller telescopes, with larger beams on the sky
  and detectors that respond in the short time
  available to look at any particular direction,
  are better suited to sky surveys

27
Targeted Search

• since the only such civilization we know about
  has evolved on a planet in orbit about a G2 V
  star, solar analogues are the usual targets for
  the focused strategy
• in this method, you make intensive investigations
  of a limited number (1,000 to 2,000) of sun-like
  stars for ET signals
• the targeted-search allows for more detailed
  investigations of small areas that we think might
  be probable locations of ET, such as stars with
  planets and conditions favorable for life (as we
  know it)
• this approach, however, ignores large portions of
  the sky and might yield nothing if the guesswork
  is wrong
• large telescopes with detectors that can analyze
  data for a long time to achieve good sensitivity
  on weak signals are routinely used

28
Radio SETI

• techniques of radio SETI use a large radio dish
  almost always a radio telescope
• in a targeted search, a radio telescope is
  pointed in selected directions on the assumption
  that not all locations are equally probable for
  sites of intelligent life

Aerial view of the Arecibo radio telescope,
Puerto Rico.
29
Which Frequency Range?

• figure presents a more detailed look at the
  background radiation encountered by radio
  telescopes
• a frequency band of 1-10 GHz defines a low-noise
  terrestrial microwave window called the water
  hole ideal for sensitive observations from the
  Earths surface

• atmospheric molecules responsible for the various
  absorption bands are shown
• this noise can be eliminated by going into space
  orbits (or on the far side of the Moon)
• IGAL - galactic synchrotron radiation from cosmic
  rays

30
(No Transcript)
31
Current Radio SETI Projects
While todays radio SETI experiments are vastly
more sensitive and comprehensive than previous
experiments, they are limited by two major
constraints

• all current projects use radio telescopes that
  are used for other purposes, and thus get only
  limited observing time
• terrestrial radio interference, especially from
  radar and orbiting satellites, is an increasing
  problem

32
SETI Projects

• Five radio SETI surveys
• META II (1990-)
• Project Argus (1995-)
• SERENDIP IV (1996-)
• Southern SERENDIP (1998-)
• SETItalia (2000-)
• Seven targeted surveys, including
• Project Phoenix (1998-2004)
• Six optical SETI (OSETI) programs
• Columbus Optical SETI (1997-)
• Harvard Optical SETI (1998-)
• Princeton Optical SETI (2001-)
• Berkeley (1998-)
• Lick (2001-)
• OZ SETI (2000-)

33
Project Phoenix Targeted Search System

• 800 stars, F, G, K
• within 200 ly
• 1 3 GHz

34

• Project Phoenix was the world's most sensitive
  and comprehensive search for extraterrestrial
  intelligence between 1998 to 2004
• to detect extraterrestrial civilizations by
  listening for radio signals that are either being
  deliberately beamed our way, or are inadvertently
  transmitted from another planet
• Phoenix was privately funded and is the successor
  to the NASA SETI program that was cancelled in
  1993 by a budget-conscious US Congress
• Phoenix searched the vicinities of nearby,
  sun-like stars likely to host long-lived planets
  capable of supporting life
• 800 stars were targeted for observation by
  Project Phoenix, all within 200 light-years
  distance

35
Project Phoenix Targeted Search System

• the goal of Project Phoenix was to discover
  evidence of extraterrestrial civilizations
  through detection of microwave signals generated
  by their technology
• the Targeted Search System (TSS) is a
  transportable SETI system that is used in
  conjunction with existing radio telescopes for
  high sensitivity SETI observations
• it is composed of several subsystems, each
  responsible for one aspect of the signal
  processing and controlled by sophisticated
  software for highly automated operation

36

• computers simultaneously listen to millions of
  radio channels
• Phoenix looks for signals between 1 and 3 GHz,
  signals
• signals that are at only one spot on the radio
  dial (narrow-band signals) are the "signature" of
  an intelligent transmission
• the spectrum searched by Phoenix is broken into
  very narrow 1 Hz-wide channels, so two billion
  channels are examined for each target star

Project Phoenix used the facilities of
the Aercibo Observatory located in Puerto Rico
and Lovell Telescope.
37
Allen Telescope Array

• the Allen Telescope Array will consist of 350
  antennas of 6 m diameter each manufactured by the
  makers of satellite dishes, resulting in an
  instrument with a collecting area exceeding that
  of a 100 m telescope
• because of its ability to study many areas of the
  sky at once, with more channels and for 24 hours
  a day, the Allen Telescope Array will permit an
  expansion from Project Phoenixs stellar
  reconnaissance of 800 stars to 100,000 or even
  1,000,000 nearby stars

38
Allen Telescope Array

• 350 - 6.1m dishes
• inexpensive to make
• commercial, massproduced electronics

SETI Institute UC Berkeley RAL
39
Allen Telescope Array

• the advantage of the Allen Telescope Array is
  that it will dedicated to the search for ETCs 7
  days a week, 24 hours per day
• the radio telescope at Aericibo is only available
  for the
• search for ETCs six weeks a year

40

• in general, large radio telescopes allow you to
  detect weak signals and resolve them -- so, the
  larger the dish, the greater the resolution of
  the signal
• however, large dishes are difficult and expensive
  to construct and maintain
• to get around this problem, radio astronomers use
  a technique called interferometry
• interferometry combines the signals from several
  small radio telescopes spread out over a large
  area to achieve the equivalent of one large dish
  over the same area

41
42 Down, 308 To Go!
42
Optical SETI

• some SETI researchers have considered the
  possibility that alien civilizations might be
  using powerful lasers for interstellar
  communications at optical wavelengths
• using both dedicated and piggyback instruments,
  the aim is to detect laser pulses with time
  scales of a few nanoseconds (billionth of a
  second) emitted by extraterrestrial societies
• two main problems
• lasers are highly "monochromatic", i.e.,, emit
  light only on one frequency, making it difficult
  determine which frequency to look for
• however, emitting light in narrow pulses results
  in a broad spectrum of emission, with the spread
  in frequency becoming higher as the pulse width
  becomes narrower, making it easier to detect an
  emission
• the other problem is that while radio
  transmissions can be broadcast in all directions,
  lasers are highly directional, i.e., a laser beam
  could be easily blocked by clouds of interstellar
  dust, and Earth would have to cross its direct
  line of fire by chance to receive it

43
SETI Results

• so far, no SETI experiment has turned up a
  confirmed extraterrestrial transmission
• however, in 1977, an Ohio State astronomer found
  a signal so impressive that he wrote Wow on the
  printout margin
• the Wow signal, was never seen again despite
  repeated attempts at the same frequency and sky
  position

44
Searches for extraterrestrial intelligence have
at least partially scanned for Earth-level radio
transmitters out to 4,000 light-years away from
our planet (yellow circle) and for so-called type
I advanced civilizations out to 40,000
light-years (red circle)
45

• Using the frequency channel of 1.42 gigahertz
  (hydrogen), no signal has ever been detected,
  which means that any civilizations either are out
  of range or do not transmit with enough power to
  register on our instruments

• the null results therefore rule out certain types
  of civilizations, including primitive ones close
  to Earth and advanced ones farther away.

(Crawford 2000)
46
The dashed circle marks the galactic disk, the
cross its center and the dot marked Sun is the
position of the Earth. Solid circles indicate
distances from which travel times of radio waves
to the Earth are multiples of 10 000 years.
Possible radio-emitting societies somewhere in
the galaxy are shown as dots.
If next society is located roughly a galactic
radius, 50,000 Ly, away from us, and because of
the finite speed of light, their first radio
signals will not reach us until 47,500 years in
the future. This shows that with our present
receivers we will not be able to detect the
initial radio transmissions from our closest
temporal neighbors, but only signals from older
more advanced societies that live closer.
47
Fermi Paradox

• if intelligent extraterrestrial life is
  commonplace, where are they? Should their
  presence not be obvious?
• the Milky Way galaxy contains roughly 300-500
  billion stars, and if the Earth is not unique, if
  even a very small fraction of these have planets
  which develop technological civilizations, there
  must be a very large number of such civilizations
  
• if any of these civilizations produce cultures
  which colonize over interstellar distances, even
  at a small fraction of the speed of light, the
  galaxy should have been completely colonized in
  no more than a few million years
• since the galaxy is billions of years old, Earth
  should have been visited and colonized long ago
• the absence of any evidence for such visits is
  the Fermi paradox

48
Solutions to the Fermi Paradox

• we are alone - civilizations are so rare that we
  are the first to have arisen in the galaxy
• civilizations are common, but no one has
  colonized the galaxy
• technological difficulties, e.g., interstellar
  travel is difficult or expensive
• sociological considerations no desire to leave
  their home systems
• self-destruction
• there is a galactic civilization, but it has
  deliberately avoided revealing its existence to
  us

49
What if We Detect a Signal?
Six international space organizations have
endorsed the Declaration of Principles
Concerning Activities Following the Detection of
Extraterrestrial Intelligence Essentially, the
document states that upon confirmation of the
detection of an extraterrestrial civilization,
this information should be widely disseminated,
but that no response to a signal or other
evidence of extraterrestrial intelligence should
be sent until appropriate international
consultations have taken place. For more
information lthttp//www.seti.org/post-detection.h
tmlgt