Final Exam

• Comprehensive– Most questions from Ch. 15-18, some from Ch. 4-14,

few from Ch. E-3

• Multiple choice plus few short answer questions• Please study:

– Midterm exams (available on homepage)– Homework– Activities– Textbook– Powerpoint slides

Green Bank (or Drake) Equation

• Estimated number of technological civilizations present in the Milky Way galaxy is given by

the average rate of star formation (R*)

fraction of stars having planetary systems (fP)

average number of planets within the habitable zone for various types of star and star system (NP)

fraction of habitable planets that develop life (fL)

fr. of life-bearing planets on which intelligence appears (fI)

fraction of intelligent life forms that develop technology (fT)

average lifetime of a technological civilization (T)

• Could be 100 to 1 billion (?)

Pessimistic Example

• Estimated number of technological civilizations present in the Milky Way galaxy:

N = (R*=10/year)

(fP=10%)

(NP=1)

(fL=10%)

(fI=10%)

(fT= 10%) (T=1000 years)= 10 0.11 0.1 0.1 0.1 1000= 1 (one civilization in the Milky Way us!)

Optimistic Example

• Estimated number of technological civilizations present in the Milky Way galaxy:

N = (R*=20/year)

(fP=50%)

(NP=4)

(fL=20%)

(fI=50%)

(fT= 50%) (T=100,000 years)= 20 0.54 0.2 0.5 0.5 100,000= 200,000

Illustration of Drake Equation

Habitable Zones

• 1 A.U. = average Earth-Sun distance

Extinction of the Dinosaurs

• Possibly caused by impact of a large meteorite– Large amount of dust thrown into atmosphere,

causing global cooling, disruption of the food chain– Evidence:

• Iridium layer found in fossil record at about time of extinction of dinosaurs

• Large numbers of species become extinct at about the same time

• Crater in Yucatan may be “the one”

• Are extinctions periodic?

Life in the Universe - The Next Step

• Assume there is life

• Now try to find it!

• How? – Depends on the distance & type of life!– Our solar system: can get there, study, probe– Around other stars:

• Simple life need to identify signs

• Intelligent life will send out signs (?)

Finding life on Exoplanets

• Problem: Exoplanets are very far away!– Can’t get there– Hard enough to discover the planets themselves– Need to discover something living on them

Green Bank (or Drake) Equation

• Estimated number of technological civilizations present in the Milky Way galaxy is given by

the average rate of star formation (R*)

fraction of stars having planetary systems (fP)

average number of planets within the habitable zone for various types of star and star system (NP)

fraction of habitable planets that develop life (fL)

fr. of life-bearing planets on which intelligence appears (fI)

fraction of intelligent life forms that develop technology (fT)

average lifetime of a technological civilization (T)

• Could be 100 to 1 billion (?)

Homework: Conversions

• Convert from hours to years by “multiplying with one” (conversion factors)

• x years = y hrs (#years/hrs)

= y hrs (#years/days)(#days/hrs) = y hrs

(1years/365days)(1day/24hrs)

• E.g. 5000 hrs = 5000/365/24 years = 0.57 yr

Fermi Paradox• If ETI exists, it must be widespread

• If it’s widespread, why aren’t they among us?

• ETI must have had plenty time to occur

• Maybe they do not exist

• Maybe we didn’t look hard/long enough?

• Maybe they are among us?

Signals

• Probably electromagnetic waves – Easy to generate– not exceedingly absorbed by interstellar

medium, planetary atmospheres– Information can be imprinted on them with

minimal energy cost

• Travels fast (but not fast enough?!)• We are detectable since 12. December 1901

Time is of the Essence

• A lot of things can go wrong in “cosmic instances” like a few thousand years

• It is “guesstimated” that a technological civilization might last about 3000 years

SETI• If average lifetime is 1 million years, then the average

distance between civilizations in the galaxy is 150 ly– Thus 300 years for messages to go back and forth

• Communications via radio signal– Earth has been broadcasting in RF range for most of this century– Earth is brighter than the Sun in radio– 18–21 cm wavelength range good for interstellar communication

• SETI search is ongoing– SETI@Home: http://setiathome.ssl.berkeley.edu

• If they exist, should we contact them?

SETI with Radio Telescopes

• Radio frequencies are used because– Civilizations are likely to use these frequencies

– We can observe them from the ground

• Biggest radio telescope is in Arecibo, Puerto Rico

CETI – Talking to Aliens

• How can we communicate? – Put up a big sign (?!)– Send a (radio) signal– Send a space probe with a message

• Should we try to communicate?

Our Messages to the Aliens• Golden plate

with essential info on humans

• On board Pioneer 10 space probe

• Started in the 70’s

• past solar system

Our EM Message to the Aliens

• In 1974 sent radio signal from Arecibo to globular cluster M13 (300,000 stars, 21,000ly away)

• Brighter than the Sun• “The signal, transmitted

at 2380 megahertz with a duration of 169 seconds, delivered an effective power of 3 trillion watts, the strongest man-made signal ever sent.”