The Drake Equation

1
The Drake Equation

• Number of civilizations with whom we could
  potentially communicate N ? fHP ? flife ?
  fciv ? fnow
• N total of stars in Galaxy with habitable
  zones
• fHP average number of habitable planets per
  star
• flife fraction of habitable planets with life
• fciv fraction of life-bearing planets w/
  civilization at any time in past 10 billion years
• fnow fraction of civilizations around now
  lifetime of typ. technological civilization / 10
  billion years

2
The Drake Equation - Optimistic

• Number of civilizations with whom we could
  potentially communicate N ? fHP ? flife ? fciv
  ? fnow
• N 100 billion 100,000,000,000
• fHP 1
• flife 1
• fciv 0.5
• fnow 1/1000 10 million years / 10 billion
  years
• Optimistic of civilizations 50 million

3
The Drake Equation - Pessimistic

• Number of civilizations with whom we could
  potentially communicate N ? fHP ? flife ? fciv
  ? fnow
• N 100 billion 100,000,000,000
• fHP 1/10
• flife 1/100
• fciv 1/1000
• fnow 10-8 100 years / 10 billion years
• Pessimistic of civilizations 0.001
• (1 every 100,000 years, lasting only 100 years)

4
Do you think there is other intelligent life in
the Milky Way?

1. Yes.
2. No.
3. Dont know.
4. Yes. We have proof that Earth has been visited
   by aliens in the past, so we know theyre out
   there.
5. No way. Earth is the most important planet, and
   there should not be other intelligent life.
6. I cant answer that until further scientific data
   is available to evaluate the Drake Equation.

5
At what wavelength should we listen to hear
signals from aliens?Optical blocked by dust
6
only other wavelengths that reach Earths
surface without being absorbed are radio
wavelengths
7
But at what radio wavelength should we listen?
Most astronomers agree on the best radio
wavelength near the wavelength of the electron
spin-flip transition in the hydrogen atom
(electron is red above nucleus is blue like two
magnets, opposite orientation is lower-energy).
8
Figure 18.13Water Hole
9
How does SETI (Search for Extra-Terrestrial
Intelligence) work?
Looking for deliberate signals from E.T. now
unintentional later?
10
Your computer can help! SETI _at_ Home a
screensaver with a purpose.
11
Were even sending a few signals ourselves
Symbolic message sent (twice) from Earth
to globular cluster M13, for a few minutes each
time.
12
Figure 18.12Earths Radio Leakage
13
In the year 2750, we receive a signal from a
civilization around a nearby star telling us that
the Voyager 2 spacecraft recently crash-landed on
their planet. This is

1. fantasy because there are no stars within 750
   light years of the Sun.
2. fantasy because Voyager 2 will take tens of
   thousands of years to reach the distance of even
   the nearest stars.
3. fantasy because astronomers have shown that there
   are no other civilizations in the universe.
4. possible because Voyager 2 was accelerated by
   flying past giant planets as it left the solar
   system.

14
If we find other intelligent life in the
universe, should we attempt to contact it and
make our presence known?

1. Yes, it could be beneficial
2. No, it could be hostile
3. Yes, any other civilization is likely much more
   advanced than us they would have a lot to teach
   us
4. No, any other civilization is likely much more
   advanced than us they might think of us like we
   think of ants
5. I think we should listen but not talk

15
18.5 Interstellar Travel and Its Implications to
Civilization

• Our goals for learning
• How difficult is interstellar travel?
• Where are the aliens?

16
In 2030, a way is discovered to build a rocket
that burns coal as its fuel and can travel at
half the speed of light. This is

1. possible because rocket technology is constantly
   improving.
2. fantasy because purely chemical burning cannot
   release enough energy to achieve such speeds.
3. fantasy because Einstein showed that it is
   impossible to travel faster than a fraction of
   the speed of light.
4. possible because new power generators using coal
   are becoming increasingly more efficient.

17
How difficult is interstellar travel?

• Very!
• Current spacecraft travel at lt1/10,000 c 100,000
  years to the nearest stars.

Pioneer plaque
Voyager record
18

• Real interstellar travel faces huge hurdles
• Incredible energy requirements (antimatter
  fuel?)
• Ordinary particles become dangerous cosmic rays
  (minimum mass needed for shielding)
• Time dilation affects crew upon return to Earth,
  but does allow round trip to be made in a crew
  members lifetime
• This is key reason
• why UFO sightings are
• suspect (the other is
• that astronomers watch
• the sky every night, and
• dont see UFOs).

19
Where are the aliens?

• Fermis Paradox
• Plausible arguments suggest that civilizations
  should be common
• Even if only 1 in a million stars has a
  civilization at any given time ? 100,000
  civilizations today
• Interstellar travel difficult but not impossible
  even if it takes a long time, in 10 billion years
  the Galaxy should be full of signs of alien
  civilizations
• For example, can build Von Neumann machines
  robot probes that mine resources in one solar
  system and make copies of themselves to send to
  others
• So why we havent we detected such signs?

20
Possible solutions to the paradox

• We are alone life/civilizations much rarer than
  we might have guessed.
• Our own planet/civilization looks all the more
  precious

21
Possible solutions to the paradox

• Civilizations are common but interstellar travel
  is not. Perhaps because
• Interstellar travel more difficult than we think.
• Desire to explore is rare.
• Civilizations destroy themselves before achieving
  interstellar travel

These are all possibilities, but not very
appealing
22
Possible solutions to the paradox

• There IS a galactic civilization
• and some day well detect their signals

23
What have we learned?

• How many civilizations are out there?
• We dont know, but the Drake equation gives us a
  way to organize our thinking about the question.
  The equation (in a modified form) says that the
  number of civilizations in the Milky Way Galaxy
  with whom we could potentially communicate is
  where is the number of habitable planets in the
  galaxy, is the fraction of habitable planets that
  actually have life on them, is the fraction of
  life-bearing planets upon which a civilization
  capable of interstellar communication has at some
  time arisen, and is the fraction of all these
  civilizations that exist now.

24
What have we learned?

• How does SETI work?
• SETI, the search for extraterrestrial
  intelligence, generally refers to efforts to
  detect signalssuch as radio or laser
  communicationscoming from civilizations on other
  worlds.

25
What have we learned?

• How difficult is interstellar travel?
• Convenient interstellar travel remains well
  beyond our technological capabilities, because of
  the technological requirements for engines, the
  enormous energy needed to accelerate spacecraft
  to speeds near the speed of light, and the
  difficulties of shielding the crew from
  radiation. Nevertheless, it seems reasonable to
  think that we will someday achieve interstellar
  travel if we survive long enough.

26
What have we learned?

• Where are the aliens?
• It seems that we should be capable of colonizing
  the galaxy in a few million years or less, and
  the galaxy was around for at least 7 billion
  years before Earth was even born. Thus, it seems
  that someone should have colonized the galaxy
  long agoyet we have no evidence of other
  civilizations. Every possible category of
  explanation for this surprising fact has
  astonishing implications for our species and our
  place in the universe.