CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 601
Discussion Questions
- Do you expect that hypothetical alien recipients of the Arecibo
message will be able to decode it? Why or why not? - How do you think the detection of extraterrestrial intelligence would
be received by the public? Would it be likelier to upset, or confi rm,
human’s beliefs about themselves and the world? - What do you think it would mean if decades of careful searches for
radio signals for extraterrestrial intelligence turn up nothing?
Problems
- A single human cell encloses about 1.5 m of DNA, containing 4.5
billion base pairs. What is the spacing between these base pairs in
nanometers? That is, how far apart are the rungs on the DNA ladder? - If you represent Earth’s history by a line 1 m long, how long a segment
would represent the 400 million years since life moved onto the land?
How long a segment would represent the 4-million-year history of
human life? - If a human generation, the average time from birth to childbearing,
has been 20 years long, how many generations have passed in the last
1 million years? - If a star must remain on the main sequence for at least 4 billion years
for life to evolve to intelligence, what is the most massive a star can
be and still possibly harbor intelligent life on one of its planets?
(Hints: See Chapter 12 and Appendix Table A-7.) - If there are about 1.4 10 −4 stars like the sun per cubic light-year,
how many lie within 100 light-years of Earth? (Hint: The volume of a
sphere is 34 __ πr^3 .) - Mathematician Karl Gauss suggested planting forests and fi elds
in gigantic geometric fi gures as signals to possible Martians that
intelligent life exists on Earth. If Martians had telescopes that could
resolve details no smaller than 1 arc second, how large would the
smallest element of Gauss’s signal have to be for it to be visible at
Mars’s closest approach to Earth? (Hints: See Appendix Table A-10 and
use the small-angle formula, Chapter 3.)
7. If you detected radio signals with an average wavelength of 20 cm
and suspected that they came from a civilization on a distant Earth-
like planet, roughly how much of a change in wavelength should you
expect to see because of the orbital motion of the distant planet?
(Hints: See the equation for the Doppler effect in Chapter 7; the
Earth’s orbital velocity is 30 km/s.)
8. Calculate the number of communicative civilizations per galaxy using
your own estimates of the factors in Table 26-1.
Learning to Look
- The star cluster shown in the
image to the right contains cool
red giants and main-sequence
stars from hot blue stars all the
way down to red dwarfs. Discuss
the likelihood that planets
orbiting any of these stars might
be home to life. (Hint: Estimate
the age of the cluster.) - If you could search for life in the
galaxy shown in the image to
the right, would you look among
disk stars or halo stars? Discuss
the factors that infl uence your
decision.
Visual NASAVisual ESO