Transcript 幻灯片 1
A grand summary of this course Life in Other Worlds • What is life? – A process of systems that are capable of extracting energy and reproducing – It could be much more general, and very different, than we know or think • What is the physical base for life? – A physical mechanism capable of supporting extraction and utilization of energy – In our case: carbon-based substrate • Where can we look for life as we know it? – Easy answer: in places like our own Earth – What we need is water, carbon, oxygen, silicon, calcium, phosphorus and temperature condition not too extreme – Caution: it took Earth 4.5 billion years to develop intelligent life – We have been a “communication civilization for about 100 years – Is this typical? Can life form in other worlds? The Habitable Zone (only for life like ours) Potential candidates in the Solar System Atmospheric spectra only Earth conducive to life But Mars could have had life in the past Mars Exploration Looking for Martian fossils (none found) Martian Meteorites Is this evidence of life on Mars? Life or mineral forms? Mineral formation or fossile life? Cosmos (our own Galaxy, in fact) too big for travel • Enormous problems of storage/production of energy – Travel for small-size sip to nearest star at 1/3 c needs 40,000 times the whole yearly US energy production • Full size exploration way longer than human life span. Need colonies: will we remain what we are after 100 years in space? • Communication seems more doable – SETI – Arecibo message – Voyager message Radio telescopes Radio is best for distant communication (best of all: water hole) The Arecibo Telescope The Arecibo Message Coded in binary numbers Numbers 1 to 10 Atomic numbers of H, C N, O and Ph Chemical formulas for sugars and proteins DNA molecular structure Number of units in DNA Human figure Height of human in wavelengths (12.3 cm) Population of Earth Arecibo transmitting antenna (telescope) Solar system schematic Diameter of telescope in wavelength The Voyager Message The message on the spacecraft Drake Equation: The number of “cummunication capable” civilazations in out Galaxy • NC = N* x fp x nLZ x fL x fI x FS • N*: number of stars per galaxy, • fp: fraction of stars with planets, ~ 1011 0.01 to 0.5 • nLZ: planets per star in life zone over 4 Gyr, 0.01 to 1 • fL: fraction of suitable planets where life begins, 0.01 to 1 • fI: faction of life forms that develop intelligence, 0.01 to 1 • FS: fraction of star’s life with communicative intelligence, 10-8 to 10-4 • 2x10-5 < NC < 107 Drake Equation visualized