The history of the universe can be divided into seven phases: particulate, galactic, stellar, planetary, chemical, biological and cultural. Cosmic evolution is the continuous process that has led to the appearance of galaxies, stars, planets, and life on Earth.
Living organisms may be characterized by their ability to react to their environment, to grow by taking in nutrition from their surroundings, and to reproduce, passing along some of their own characteristics to their offspring.
Powered by natural energy sources, reactions between simple molecules in the oceans of the primitive Earth are believed to have led to the formation of amino acids and nucleotide bases, the basic molecules of life. Alternatively, some complex molecules may have been formed in interstellar space and then delivered to Earth by meteors or comets.
Organisms that can best take advantage of their new surroundings succeed at the expense of those organisms that cannot make the necessary adjustments. Intelligence is strongly favored by natural selection.
The best hope for life beyond Earth in the solar system is the planet Mars, although no evidence for living organisms has been found. Saturn's Titan may also be a possibility, but conditions there are harsh by terrestrial standards.
The Drake equation provides a means of estimating the probability of intelligent life in the Galaxy. The astronomical terms in the equation are the Galactic star-formation rate, the likelihood of planets, and the number of habitable planets. Chemical and biological terms are the probability that life appears and the probability that it subsequently develops intelligence. Cultural and political terms are the probability that intelligence leads to technology, and the lifetime of a civilization in the technological state.
Taking an optimistic view of the development of life and intelligence leads to the conclusion that the total number of technologically competent civilizations in the Galaxy is approximately equal to the lifetime of a typical civilization, expressed in years.
Even with optimistic assumptions, the distance to our nearest intelligent neighbor is likely to be many hundreds of parsecs. Space travel is not presently a feasible means of searching for intelligent life. Current programs to discover extraterrestrial intelligence involve scanning the electromagnetic spectrum for signals. So far, no intelligent broadcasts have been received.
A technological civilization would probably "announce" itself to the universe by the radio and television signals it emits into space. Observed from afar, our planet would appear as a radio source with a 24-hour period, as different regions of the planet rise and set.
The water hole is a region in the radio range of the electromagnetic spectrum, near the 21-cm line of hydrogen and the 18-cm line of hydroxyl, where natural emissions from the Galaxy happen to be minimized. Many researchers regard this as the best part of the spectrum for communications purposes.
1. The assumptions of mediocrity favor the existence of extraterrestrial life. (Hint)
2. The definition of life requires that to be considered "alive" you only must be able to reproduce. (Hint)
3. Miller and Urey conducted experiments in which they produced biological molecules from nonbiological molecules. (Hint)
4. Organic molecules important for life could have reached Earth's surface via comets. (Hint)
5. Organic molecules are known to exist only on Earth. (Hint)
6. Laboratory experiments have succeeded in creating living cells from nonbiological molecules. (Hint)
7. For most of the history of Earth, life consisted of only single-celled life-forms. (Hint)
8. The Viking landers on Mars discovered microscopic evidence of life but found no large fossil evidence. (Hint)
9. The rate of star formation in the Galaxy is reasonably well known. (Hint)
10. As yet there is no direct evidence that Earthlike planets orbit other stars.(Hint)
11. In estimating whether intelligence arises and develops technology, we have only life on Earth as an example. (Hint)
12. Dinosaurs existed on Earth for 10,000 times longer than human civilization has existed to date. (Hint)
13. Our civilization has launched probes into interstellar space. (Hint)
14. One disadvantage of interstellar radio communication is that we can do it only with another civilization that has a technology equal to or greater than our own. (Hint)
15. We have already broadcast our presence to our interstellar neighbors, if any. (Hint)
1. Amino acids are the building blocks of _______ (Hint)
2. The naturally occurring molecules present on the young Earth included water, carbon dioxide, ___________, and ____________. (Hint)
3. Two sources of energy for chemical reactions available on the young Earth were ________ and _________. (Hint)
4. The fossil record clearly shows evidence of life dating back _______ years.
5. Multicellular organisms did not appear on Earth until about ________ years ago. (Hint)
6. The Murchison meteorite was discovered to contain relatively large amounts of ___________. (Hint)
7. The Drake equation estimates the number of _________ in the Milky Way Galaxy. (Hint)
8. Planets in binary star systems are not considered habitable because the planetary orbits are often _______________. (Hint)
9. The probabilities of the development of life and intelligence on Earth are both extremely ___________ if random chance is the only evolutionary factor involved. (Hint)
10. Direct contact between extraterrestrial life-forms may be impractical because of the large ___________ between civilizations. (Hint)
11. Radio communications over interstellar distances is practical because the signals travel at the speed of ______________. (Hint)
12. Radio waves can travel throughout the Galaxy because they are not blocked by interstellar _______________. (Hint)
13. Radio waves leaking away from Earth have now traveled out a distance of __________ light years.
14. Radio wavelengths between 18 and 21 cm are referred to as the _________. (Hint)
15. A two-way communication with another civilization at a distance of 100 light years will require ___________ years. (Hint)
1. Why is life difficult to define? (Hint)
2. What is chemical evolution? (Hint)
3. What is the Urey—Miller experiment? What important organic molecules were produced in this experiment? (Hint)
4. What are the basic ingredients from which biological molecules formed on Earth? (Hint)
5. How do we know anything at all about the early episodes of life on Earth? (Hint)
6. What is the role of language in cultural evolution? (Hint)
7. Where else, besides Earth, have organic molecules been found? (Hint)
8. Where—besides the planet Mars—might we find signs of life in our solar system? (Hint)
9. Do we know whether Mars ever had life at any time during its past? What argues in favor of the position that it may once have harbored life? (Hint)
10. What is generally meant by "life as we know it"— What other forms of life might be possible? (Hint)
11. How many of the terms in the Drake equation are known with any degree of certainty? Which factor is least well known? (Hint)
12. What is the relationship between the average lifetime of Galactic civilizations and the possibility of our someday communicating with them? (Hint)
13. How would Earth appear at radio wavelengths to extraterrestrial astronomers? (Hint)
14. What are the advantages in using radio waves for communication over interstellar distances?
15. What is the water hole? What advantage does it have over other parts of the radio spectrum? (Hint)
1. If Earth's 4.6-billion-year age were compressed to 46 years, as described in the text, what would be your age, in seconds? How long ago was the end of World War II? The Declaration of Independence? Columbus's discovery of the New World? (Hint)
2. According to the inverse-square law, a planet receives energy from its parent star at a rate proportional to the star's luminosity and inversely proportional to the square of the planet's distance from the star (Section 17.4). According to Stefan's law, the rate at which the planet radiates energy into space is proportional to the fourth power of its surface temperature (Section 3.4). In equilibrium, the two rates are equal. Based on this information and the data presented in Figure 28.8, estimate the extent of the habitable zone surrounding a K star of luminosity 1/10 the luminosity of the Sun. (Hint)
3. Based on the numbers presented in the text, and assuming an average lifetime of 5 billion years for suitable stars, estimate the total number of habitable planets in the galaxy.
4. A planet orbits one component of a binary star system at a distance of 1 A.U. (see Figure 28.9a). If both stars have the same mass, and their orbit is circular, estimate the minimum distance between the stars for the tidal force due to the companion not to exceed a "safe" 0.01 percent of the gravitational force between the planet and its parent star. (Hint)
5. Suppose that each of the "fraction" terms in the Drake equation turns out to have a value of 1/10, that stars form at an average rate of 20 per year, and that each star has exactly 1 habitable planet orbiting it. Estimate the present number of technological civilizations in the Milky Way Galaxy if the average lifetime of a civilization is (a) 100 years (b) 10,000 years (c) one million years. (Hint)
6. Adopting the estimate from the text that the number of technological civilizations in the Milky Way Galaxy is equal to the average lifetime of a civilization, in years, it follows that the distance to our nearest neighbor decreases as the average lifetime increases. Assuming that civilizations are uniformly spread over a two-dimensional Galactic disk of radius 15 kpc, and all have the same lifetime, calculate the minimum lifetime for which a two-way radio communication with our nearest neighbor would be possible before our civilization ends. Repeat the calculation for a round-trip personal visit, using current-technology spacecraft that travel at 50 km/s. (Hint)
7. How fast would a spacecraft have to travel in order to complete the trip from Earth to Alpha Centauri (a distance of 1.3 pc) and back in less than an average human lifetime (80 years, say)? (Hint)
8. Assuming that there are 10,000 FM radio stations on Earth, each transmitting at a power level of 50 kW calculate the total radio luminosity of Earth in the FM band. Compare this value with the roughly 106 W radiated by the Sun in the same frequency range. (Hint)
9. Convert the water hole's wavelengths to frequencies. For practical reasons, any search of the water hole must be broken up into channels, much like you find on a television, except these channels are very narrow in radio frequency, about 100 Hz wide. How many channels must astronomers search in the water hole? (Hint)
10. There are 20,000 stars within 100 light years that are to be searched for radio communications. How long will the search take if 1 hour is spent looking at each star? What if 1 day is spent per star? (Hint)
1. Some people suggest that if extraterrestrial life is discovered, it will have a profound effect on people. Interview as many people as you can and ask the following two questions: (1) Do you believe that extraterrestrial life exists? (2) Why? From your results, try to decide whether there will be a profound effect on people if extraterrestrial life is discovered.
2. Conduct another poll, or do it at the same time you do the first one. Ask the following question: What one question would you like to ask an extraterrestrial life-form in a radio communication? How many responses do you receive that indicate the person is very "Earth-centered" in thinking? How many responses suggest a lack of understanding of how alien an extraterrestrial life-form might be? Is your conclusion from the first project different or changed in any way?
3. The Drake equation should be able to "predict" at least one civilization in our Galaxy: us. Try changing the values of various factors so that you end up with at least one. What do these various combinations of factors imply about how life arises and develops? Are there some combinations that just don't make any sense—
