Lecture #29, Thursday 4-25-2001

FINAL EXAM:

• Thursday, 5-3, 8pm-10pm in 118PA (here)
• The final is really optional
  

1. Search for extraterrestrial life in class
   
2. What is Life
   1. A definition:
      1. Can react to environment
              (implies the organism is separated from its environment)
      2. Can grow
      3. Can reproduce
      4. Can evolve from generation to generation
   2. Is this a good or useful definition ?
      
3. The origin of life on earth
   
   1. Origin of the building blocks: complex organic molecules
      1. Created on earth
             Urey and Miller experiment 1953 (picture)
             need:  ammonia NH₃
                        methane CH₄
                        carbon dioxide CO₂
                        water H₂O
                        energy (lightning, radioactivity, UV-light, volcanos)
             make: amino acids, other complex organic molecules
      2. From outer space
              comets ... 
   2. First organisms
      1. 3.5 billion years ago: one cell organisms (fossils)
      2. Possible sites: shallow ocean water or hydrothermal vents
         
   3. Evolution
      - variation
      - selection
      
         There is evidence for (genetic) change of biological organisms (variation) and
         for selection:
      1. Fossil record
      2. Direct evidence: for example bacteria becoming resistant against penicillin
         
   4. Timescales
      if the age of the earth would be represented by 1 day then:
      00:00 am       formation of the earth
      05:20 am       first one cell organism
      06:40 pm       first multi cell organisms in oceans
      09:50 pm       first plants and animals on land
      10:50 pm       first Dinosaurs (extinct at 11:41 pm)
      11:58:35 pm  first human like species (Ardipithecus ramidus, Ethopia 1994)
      11:59:50 pm  human history begins (stone age, 500000yr ago)
      11:59:58 pm  homo sapiens
      
4. Extraterrestrial (intelligent) life - Drake Equation
   
     number of intelligent, technological civilizations now present in our Galaxy =
              (with which we therefore might be able to communicate)

    

   1. Rate of star formation X
           ~10 per year (100 billion stars in 10 billion year old galaxy)
              (neglects died stars and higher star formation rate earlier)
      
   2. Fraction of stars having planets X
      
   3. Number of habitable planets per planetary system (in stable orbits !) X
          
   4. Fraction of habitable planets with life X
          
   5. Fraction of life-bearing planets with intelligence X
          
   6. Fraction of intelligent life bearing planets with technology X
         
   7. Average lifetime of a technological society
          
5. Life in the solar system
   1. Face on Mars ?
   2. Mars Bacteria ?
   3. Conditions for life - need water
      1. Evidence of liquid on Mars
      2. Evidence of liquid Water on Europa
              
6. Search for extrasolar intelligent life - what can we do ?
   1. Travel
      Relativity allows travel in principle. At 1g acceleration 
      • in 20 years to the next star (centuries passed on earth), 
      • in 40 years half way to the center of the galaxy (30,000 yr on earth)
      • but - too expensive
   2. Communication - a message in the bottle
      1. Written notes - Pioneer 10
      2. Multimedia - Voyager 1,2
   3. Radio communication
      1. Which frequency works best ? (picture)
         the water hole: around 21cm (H) or 18cm (OH)
         240MHz difference - already larger range than SETI (2.5 Mhz) !
      2. Message sent: the Arecibo message
      3. Other messages (1)
      4. Other messages sent - earths radio leak fig 18.9, global call
      5. Listening - for example the SETI program