Lecture 21

Is There Other Intelligent Life in the Universe?

To put the cosmos in perspective, click here and follow the slide show.

Historical Perspective (thanks to Jorge Piekarewicz)

• in 1802, William Paley publishes Natural Theology or Evidences of the Existence and Attributes of the Deity Collected from the Appearances of Nature

  

  • argument from design thesis suggests that living systems are so well-adapted to their purpose, and so incredibly complex, that they couldn't have arisen by chance

  • instead, they appear to have been designed by Supreme Designer

  • take example (not available in Paley's day) of hemoglobin molecule, which transports oxygen in blood
    • consists of 4 chains of amino acids twisted together

    • each chain contains 146 amino acids

    • life on Earth is based on 20 different kinds of amino acid

    • thus, number of different ways of making a chain of 146 amino acids from 20 different building blocks is 20 multiplied by itself 146 times, because one can place any one of 20 amino acids at each position in the chain ---> 20¹⁴⁶ or about 10¹⁹⁰!

    • if you take fastest ever built Teraflop (trillion operations per second) computer and start constructing combinations at random for as long as Universe has existed, which is 10¹⁸ seconds, you get only 10³⁰ combinations ---> chance alone cannot lead to LIFE

  • argument seems utterly compelling and was never questioned for more than 50 years

  • after that though, purely natural way to achieve adapted complexity was discovered...

• in 1859, Charles Darwin publishes The Origin of Species

  

• most revolutionary work in the history of science?

• basic idea is very simple -- so simple it is frequently misunderstood

• recognized a purely natural way to obtain highly adapted complex organisms

  

  • complexity can arise as result of cumulative effect of small random mutations

  • adaptation can arise as result of selection effect
    • if mutation confers advantage to chance that organism will survive, mutation will be adopted and passed on to next generation

    • if, however, mutation reduces chance of survival, then that mutation is less likely to be passed on to next generation because organism may perish before it reproduces

    • thus, over many generations, survivors will be those organisms that are best able to adapt and survive -- this selection effect is called Natural Selection

    

  • unlike evolution by artificial selection, evolution by natural selection, is undirected and goal-free

  • there is no aim -- what survives is what works best at that moment in that environment

  • if evolution by natural selection has occurred on Earth, then can it occur elsewhere, but result may be different due to different conditions

So, Are We Alone?

• Milky Way contains about 100 billion stars, and known Universe contains about as many galaxies

• given such huge numbers, it is reasonable to ask if we are alone

• and, if we believe that other life is likely, to ask whether some of it could be intelligent

Carbon-Based Life

• basic building blocks of carbon-based life as we know it are organic molecules

• such molecules are spread throughout the Universe

• how easy is it to go from these simple organic molecules to building blocks of life?
  • in 1952, question answered in classic experiment performed by the chemists Stanley Miller and Harold Urey

  • showed that simple molecules can combine to form molecular building blocks of carbon-based life forms under conditions that are believed to have existed on Earth in its early history

    

  • but this success is only first step towards life

  • we still have to get from amino acids to self-replicating molecules

  • natural selection proves crucial

    • although chance of producing right combination of amino acids to create viable replicating molecule is essentially zero, it is possible, through natural selection, to obtain such a molecule by cumulative effect of small random changes in a large pool of randomly-created molecules

    • over time, population of the more stable, better-adapted molecules grows at the expense of less stable, less well-adapted molecules

Evidence for Life on Mars?

• in 1984, meteorite ALH 84001 was found on Antarctic ice

  

  

• only recently recognized as one of 12 meteorites believed to have come from Mars -- how do we know?

  • contains gases trapped in small glassy ``bubbles'' (carbonate mineral globules)

  • careful analysis revealed that gases have same composition and abundance as gases in Martian atmosphere as measured by two Viking landers in 1976

  • now seems likely that ALH 84001 was blasted off surface of Mars when comet or asteroid struck planet (and killed all the martian dinosaurs, ha! ha!)

• chemical, mineral, and fossil evidence suggest that ALH 84001 once contained bacteria
  • carbonate mineral globules are martian and formed by liquid water

    • globules are much older than meteorite impact event that launched ALH 84001, and oxygen and carbon in the carbonate minerals have isotopic compositions that are characteristic of Mars

    • most researchers think that carbonate-rich globules were deposited by liquid water, despite the lack of water-bearing minerals like clays or rust

    • if globules are 3.6 billion years old as claimed, they formed when Mars' atmosphere may have been much thicker than it is now, when temperatures were much warmer, and when liquid water could have been abundant

  • McKay and his co-workers describe three kinds of features in ALH 84001 that they interpret as evidence for ancient martian life (features are all in and near carbonate mineral globules)

    • microscopic shapes that resemble living and fossil bacteria on Earth

      

      • elliptical, rope-like, and tubular structures in fractures in the carbonate mineral globules

      • structures are so small, each only 20 to 100 billionths of a meter (or nanometer) across, that they could only be seen with an advanced electron microscope

      • to show that shapes could really be fossil bacteria, McKay and co-workers have tried to show

        • that the bacteria shapes are actually the sizes and shapes of known living organisms

        • that the bacteria shapes are really part of the rock and were not produced accidentally while they prepared the sample for study

        • that the bacteria shapes are not Earth bacteria that somehow wiggled into ALH 84001 while it was in Antarctica

    • microscopic mineral grains like some produced by living and fossil bacteria on Earth

      

      • McKay and co-workers suggest that these mineral grains in ALH 84001 may have been produced by martian bacteria by arguing that

        • the mineral grains formed on Mars

        • the mineral grains have chemical compositions, crystal structures, sizes, and shapes like biologically produced grains on Earth

        • the different microscopic mineral grains, and the larger mineral crystals they occur in, are not likely to have formed by inorganic processes, without assistance from life

    • organic chemical compounds, or polycyclic aromatic hydrocarbons (PAHs), that resemble the decay products of bacteria on Earth

      • PAHs in ALH 84001 include phenanthrene, pyrene, chrysene, perylene or benzopyrene, and anthanthrene

      • even more complicated PAH molecules are common on the Earth, but are very rare in ALH 84001

      • McKay and co-workers argue that PAHs in ALH 84001 are derived from ancient martian bacteria by trying to show that

        • they are not contamination from laboratory procedures

        • they are not Earth PAHs that entered the meteorite while it was in Antarctica

        • they are not like PAHs in other meteorites (which have nothing to do with life)

        • they are consistent with decomposition of simple bacteria

  • doesn't prove, however, that there was past life on Mars and it doesn't prove that there was not either

How Do We Find Other Intelligent Life?

• one way is to listen to possible radio emission that they might be sending, but where in frequency do we look?

• if we look at background radio noise from Universe, noise level is lowest at Water Hole (radio frequency range from 1000 to 10,000 MHz that is close to the microwave emission lines of H and OH) -- hole includes the 21-cm emission line (1,429 MHz) from hydrogen, of obvious universal importance

• in 1960, Frank Drake launched first modern search for alien radio signals

• using radio telescope at the Radio Astronomy Observatory in West Virginia, Drake analyzed signal coming from two stars -- Tau Ceti and Epsilon Eridani -- for beacons at 21-cm

• his effort, Project Ozma, marked the birth of a new scientific field, the search for extraterrestrial intelligence (SETI)

• since then many people have been scanning the Universe for signs of intelligent life, so far without success

• recent discovery of extrasolar planets enhances probability that we are not alone, because if solar systems are commonplace, then it is more likely that a planet somewhat like Earth can exist elsewhere

  

• we should not expect species on these other worlds, if they exist, to be necessarily C-based life forms like those on Earth

• YOU can participate in the SETI Project

  • SETI research consists primarily of radio astronomers searching for narrow-bandwidth radio signals (radio waves are able to penetrate interstellar medium)

  • SETI@home is a scientific experiment that will harness the power of hundreds of thousands of Internet-connected computers

  • you can run a program that downloads and analyzes radio telescope data

  • there's a small but captivating possibility that your computer will detect the faint murmur of a civilization beyond Earth

    

    Click here to learn more from a Discover Magazine article.

• Arecibo message

  

  • on November 16, 1974, message sent from radio telescope in Puerto Rico

  • 1,679 bits of information, which is only divisible by two prime numbers (73 and 23), suggesting that message should be laid out in those dimensions to reveal image

    

  • picture shows our chemical makeup, our population, our height, our planetary system, and the telescope transmitting the message

  • message was aimed at the M13 globular cluster -- why? -- and will reach its destination in about 25,000 years

To read some more about extraterrestrial life, go to the Hand-Outs and Reference Materials page.

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