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
- 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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