SETI - The Search for Extraterrestrial Intelligence

Cosmic Evolution

• Cosmic evolution: – Includes seven major evolutionary phases

in the history of the universe: particulate, galactic, stellar, planetary, chemical, biological, and cultural evolution.

– The continuous transformation of matter and energy that has led to the appearance of life and civilization on Earth.

What are some characteristics of life? In other words, how do you decide if something is

alive?

What do we mean by life?

• Hard to define what we mean by life.• Characteristics of living organisms:

– They can react to their environment and can often heal themselves when damaged.

– They can grow by taking in nourishment from their surroundings.

– They can reproduce, passing along some of their own characteristics to their offspring.

– They have the capacity for genetic change and can therefore evolve from generation to generation to adapt to a changing environment.

Life in the Universe

• The general case in favor of extraterrestrial life is summed up in what are sometimes called the assumptions of mediocrity:– Because life on Earth depends on just a few basic

molecules.– Because the elements that make up these molecules are (to

a greater or lesser extent) common to all stars.– If the laws of science we know apply to the entire universe

(which we assume), then, given sufficient time, life must have originated elsewhere in the cosmos.

• The opposing view maintains that intelligent life on Earth is the product of a series of extremely fortunate accidents (astronomical, geological, chemical, and biological events unlikely to have happened anywhere else in the universe).

Life on Earth

• Building blocks of life as we know it - amino acids and nucleotide bases (organic, carbon-based, molecules).– Amino acids build proteins and nucleotide bases form

genes.• In 1953, the first scientist proved that you could make

amino acids and nucleotide bases from simpler ingredients that would have existed on a young Earth (water, methane, carbon dioxide, and ammonia).– Can synthesize biological molecules through

nonbiological means.– However, these experiments have yet to create a living

organism.

An Interstellar Origin

• Suggested that there wasn’t enough raw material on Earth for the reactions to occur at a significant rate to form organic material.

• An alternate possibility - the organic material was produced in interstellar space and arrived on Earth in the form of interplanetary dust and meteors that didn’t burn up during their descent through the atmosphere.

• Large amounts of organic material were detected on comets Halley and Hale-Bopp.

How do you decide if something alive is intelligent?

Diversity and Culture

• However it got here, we know life did appear.• Anthropologists believe that intelligence is

strongly favored by natural selection.• Perhaps most important was the development

of language. This allowed for cultural evolution (the changes in the ideas and behavior of society).

Life as We Know It

• Generally taken to mean carbon-based life that originated in a liquid-water environment, or life as it is on Earth.

• In our solar system, Europa and Titan both hold the possibility of harboring life.

• Most likely planet to harbor life (or to have had it in the past) is Mars.

• Need to keep in mind that life as we know it can exist in extremely hostile environments (and does on Earth as well).

The Drake Equation

• Statistical equation that gives the probability of intelligent life existing elsewhere in the universe.

• Several of the factors are a matter of opinion.• Important as it divides a huge problem into

more workable chunks.• We’ll go through each of the factors

individually.

Which of the following factors would you expect to see in the

Drake equation?A. Fraction of stars with planetary

systemsB. Fraction of planets on which life arisesC. Average lifetime of a technologically

competent civilizationD. All of the aboveE. A and B only

Rate of Star Formation

• Milky Way has roughly 100 billion stars now shining and is 10 billion years old.

• Using the above figures, we have a star formation rate of 10 stars per year.

• This is probably a fairly good average rate, even though we know it’s varied over time.

Fraction of Stars Having Planetary Systems

• If condensation theory is correct, then planetary systems are a natural result of the star formation process.

• We assign a value near 1 to this factor - we think essentially all stars have planetary systems.

• We already have proof of other planetary systems, and the number known will just increase as technology improves, so we’re not simply being overly optimistic here.

Number of Habitable Planets per Planetary System

• Habitable zone - three-dimensional zone of comfortable temperatures that surrounds every star.

• Have to exclude the majority of binary star systems.

• Have to exclude all but 10% of the systems we’ve found so far as large Jupiter-like planets with interior orbits would destabilize any terrestrial planets’ motion.

• We assign a value of 1/10 to this factor (or 10%).

Fraction of Habitable Planets on which Life Arises

• If the chemical reactions that led to the complex molecules that make up living organisms are completely random, then this factor is probably close to 0.

• Lab experiments indicate that these reactions aren’t completely random (some are more favored than others), so maybe life isn’t so rare.

• We’ll be optimistic and go with a value near 1 for this factor.

Fraction of Life-Bearing Planets on which Intelligence Arises

• One school of thought sees natural selection as a universal process and the development of intelligence as inevitable, making this factor nearly 1.

• The opposing side says that intelligent life has existed on Earth a relatively short period of time compared to simple life, so it’s probably rare, making this factor very small.

• We’ll be optimistic and assume the value is nearly 1.

Fraction of Planets on which Intelligent Life Develops and

Uses Technology• If the rise of technology is inevitable, given enough

time, then this factor is close to 1.• If it is not inevitable, then this factor could be much

less than 1.• Based on the fact that several tool-using societies

arose independently at several places on Earth, we’ll go with technology being inevitable and take this factor to be close to 1.

Average Lifetime of a Technological Civilization

• Combining our factors thus far in the Drake equation (10 x 1 x 1/10 x 1 x1 x 1 = 1), we can say that:– The number of technologically intelligent

civilizations now present in the Milky Way galaxy = lifetime of a technologically competent civilization in years.

• So if average lifetime is 1000 years, then there are 1000 civilizations present.

• If the average age is less than a few thousand years, then civilizations are unlikely to have the time to communicate with even their nearest neighbors.

Dr. Frank Drake is the Director of the SETI Institute's Center for the Study of Life in the Universe and also serves on the Board of Trustees of the SETI Institute as Chairman Emeritus.

In 1960, as a staff member of the National Radio Astronomy Observatory, he conducted the first radio search for extraterrestrial intelligence.

He is a member of the National Academy of Sciences where he chaired the Board of Physics and Astronomy of the National Research Council (1989-92). Frank also served as President of the Astronomical Society of the Pacific. He was a Professor of Astronomy at Cornell University (1964-84) and served as the Director of the Arecibo Observatory.

He is Emeritus Professor of Astronomy and Astrophysics at the University of California at Santa Cruz where he also served as Dean of Natural Sciences (1984-88).

In his spare time Frank enjoys cutting gem stones and growing orchids.Frank has three grown sons and two daughters in college. Both daughters are superb ballet dancers.

http://www.seti.org

Meeting Our Neighbors

• Let’s assume the average lifetime of a technological civilization is 1 million years.

• The Drake equation tells us that there are 1 million such civilizations and we estimate their distances to be about 100 light-years apart from one another.

• Two-way communication would then take 200 years.• Could we ever meet them? The speed of the fastest space

probes is 50 km/s. It would take us 50,000 years just to reach Alpha Centauri (one of the closest stars). A distance of 100 light-years would take a million years to travel.

Radio Searches

• A cheap way to make contact across interstellar space is to use electromagnetic radiation - fastest means of transferring information from one place to another.

• Radio is the best bet as its least affected by interstellar dust, etc.

• Possible to detect “waste” radio emissions as well (like the TV and radio emissions from Earth).

The Water Hole• Suppose that a civilization has decided to assist searchers by

actively broadcasting its presence to the rest of the galaxy. At what frequency should we listen for such an extraterrestrial beacon?

• The constituents of water (which is thought to be necessary for life), H atoms and OH molecules, radiate near 18 and 20 cm (radio wavelengths). Radio wavelengths interact the least with gas and dust, making the galaxy largely transparent to them. This region of wavelengths is called the “water hole.”

• In the same region of wavelengths, there’s the least “static” or noise from other sources (stars and interstellar clouds).

• Commonly known by the acronym SETI (search for extraterrestrial intelligence), radio searches have been underway since the late 1990s.

• No signals of extraterrestrial life have been detected.