1. N = total # of stars in Galaxy with habitable zones
The Drake Equation
Number of civilizations with whom we could potentially communicate = N  fHP  flife  fciv  fnow
N = total # of stars in Galaxy with habitable zones
fHP = average number of habitable planets per star
flife = fraction of habitable planets with life
fciv = fraction of life-bearing planets w/ civilization at any time in past 10 billion years
fnow = fraction of civilizations around now = lifetime of typ. technological civilization / 10 billion years
It’s usually worth going through this equation slowly so that students can see how it leads to the number of civilizations that we can communicate with.

2. The Drake Equation - Optimistic
Number of civilizations with whom we could potentially communicate = N  fHP  flife  fciv  fnow
N = 100 billion = 100,000,000,000
fHP = 1
flife = 1
fciv = 0.5
fnow = 1/1000 = 10 million years / 10 billion years
Optimistic # of civilizations = 50 million
It’s usually worth going through this equation slowly so that students can see how it leads to the number of civilizations that we can communicate with.

3. The Drake Equation - Pessimistic
Number of civilizations with whom we could potentially communicate = N  fHP  flife  fciv  fnow
N = 100 billion = 100,000,000,000
fHP = 1/10
flife = 1/100
fciv = 1/1000
fnow = 10-8 = 100 years / 10 billion years
Pessimistic # of civilizations = 0.001
(1 every 100,000 years, lasting only 100 years)
It’s usually worth going through this equation slowly so that students can see how it leads to the number of civilizations that we can communicate with.

4. Do you think there is other intelligent life in the Milky Way?
Yes.
No.
Don’t know.
Yes. We have proof that Earth has been visited by aliens in the past, so we know they’re out there.
No way. Earth is the most important planet, and there should not be other intelligent life.
I can’t answer that until further scientific data is available to evaluate the Drake Equation.
:00

5. At what wavelength should we listen to hear signals from aliens
At what wavelength should we listen to hear signals from aliens? Optical blocked by dust…

6. …only other wavelengths that reach Earth’s surface without being absorbed are radio wavelengths

7. But at what radio wavelength should we listen
But at what radio wavelength should we listen? Most astronomers agree on the best radio wavelength: near the wavelength of the electron ‘spin-flip’ transition in the hydrogen atom (electron is red above; nucleus is blue; like two magnets, opposite orientation is lower-energy).

8. Figure Water Hole

9. How does SETI (Search for Extra-Terrestrial Intelligence) work?
Emphasize that current SETI efforts could not detect signals as weak as our own radio/TV broadcasts. For now, at least, we are looking for deliberately broadcast signals..
Looking for deliberate signals from E.T. now; unintentional later?

10. Your computer can help! SETI @ Home: a screensaver with a purpose.
Optional:There’s a lot of data to sift though, and you can be a part of the effort… .
Your computer can help! Home: a screensaver with a purpose.

11. We’re even sending a few signals ourselves…
Message sent from Arecibo in 1974… M13 distance approx. 21,000 light-years -- but offers a lot of potential targets with its high density of stars…
Symbolic message sent (twice) from Earth to
globular cluster M13, for a few minutes each time.

12. Figure 18.12 Earth’s Radio Leakage

13. fantasy because there are no stars within 750 light years of the Sun.
In the year 2750, we receive a signal from a civilization around a nearby star telling us that the Voyager 2 spacecraft recently crash-landed on their planet. This is…
fantasy because there are no stars within 750 light years of the Sun.
fantasy because Voyager 2 will take tens of thousands of years to reach the distance of even the nearest stars.
fantasy because astronomers have shown that there are no other civilizations in the universe.
possible because Voyager 2 was accelerated by flying past giant planets as it left the solar system.
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14. If we find other intelligent life in the universe, should we attempt to contact it and make our presence known?
Yes, it could be beneficial
No, it could be hostile
Yes, any other civilization is likely much more advanced than us; they would have a lot to teach us
No, any other civilization is likely much more advanced than us: they might think of us like we think of ants
I think we should listen but not talk
:00

15. 18.5 Interstellar Travel and Its Implications to Civilization
Our goals for learning
How difficult is interstellar travel?
Where are the aliens?

16. In 2030, a way is discovered to build a rocket that burns coal as its fuel and can travel at half the speed of light. This is…
possible because rocket technology is constantly improving.
fantasy because purely chemical burning cannot release enough energy to achieve such speeds.
fantasy because Einstein showed that it is impossible to travel faster than a fraction of the speed of light.
possible because new power generators using coal are becoming increasingly more efficient.
:00

17. How difficult is interstellar travel?
Very!
Current spacecraft travel at <1/10,000 c; 100,000 years to the nearest stars.
Our first interstellar emissaries: the Pioneer and Voyager spacecraft.
Pioneer plaque
Voyager record

18. Real interstellar travel faces huge hurdles:
Incredible energy requirements (antimatter fuel?)
Ordinary particles become dangerous cosmic rays (minimum mass needed for shielding)
Time dilation affects crew upon return to Earth, but does allow round trip to be made in a crew member’s lifetime
This is key reason
why UFO sightings are
suspect (the other is
that astronomers watch
the sky every night, and
don’t see UFOs).
If you discuss the difficulty of interstellar travel in any detail, you might wish to discuss the implications to UFOs; see box on p. 483.

19. Where are the aliens? “Fermi’s Paradox”
Plausible arguments suggest that civilizations should be common
Even if only 1 in a million stars has a civilization at any given time  100,000 civilizations today
Interstellar travel difficult but not impossible; even if it takes a long time, in 10 billion years the Galaxy should be full of signs of alien civilizations
For example, can build Von Neumann machines: robot probes that mine resources in one solar system and make copies of themselves to send to others
So why we haven’t we detected such signs?
This is one of our favorite topics, and if you have time it is worth using these slides as a capstone to your course…

20. Possible solutions to the paradox
We are alone: life/civilizations much rarer than we might have guessed.
Our own planet/civilization looks all the more precious…

21. Possible solutions to the paradox
Civilizations are common but interstellar travel is not. Perhaps because:
Interstellar travel more difficult than we think.
Desire to explore is rare.
Civilizations destroy themselves before achieving interstellar travel
These are all possibilities, but not very appealing…

22. Possible solutions to the paradox
There IS a galactic civilization…
…and some day we’ll detect their signals…
A truly incredible possibility to ponder…

23. What have we learned? • How many civilizations are out there?
We don’t know, but the Drake equation gives us a way to organize our thinking about the question. The equation (in a modified form) says that the number of civilizations in the Milky Way Galaxy with whom we could potentially communicate is where is the number of habitable planets in the galaxy, is the fraction of habitable planets that actually have life on them, is the fraction of life- bearing planets upon which a civilization capable of interstellar communication has at some time arisen, and is the fraction of all these civilizations that exist now.

24. What have we learned? • How does SETI work?
SETI, the search for extraterrestrial intelligence, generally refers to efforts to detect signals—such as radio or laser communications—coming from civilizations on other worlds.

25. What have we learned? • How difficult is interstellar travel?
Convenient interstellar travel remains well beyond our technological capabilities, because of the technological requirements for engines, the enormous energy needed to accelerate spacecraft to speeds near the speed of light, and the difficulties of shielding the crew from radiation. Nevertheless, it seems reasonable to think that we will someday achieve interstellar travel if we survive long enough.

26. What have we learned? • Where are the aliens?
It seems that we should be capable of colonizing the galaxy in a few million years or less, and the galaxy was around for at least 7 billion years before Earth was even born. Thus, it seems that someone should have colonized the galaxy long ago—yet we have no evidence of other civilizations. Every possible category of explanation for this surprising fact has astonishing implications for our species and our place in the universe.