Communication with Distant Civilizations

602-607

Are we unique in the universe? Is there intelligent life elsewhere? These are compelling and enduring questions.

Travel Between the Stars

The vast distances that separate us from even the nearest exoplanets make travel to planets outside our own solar system unlikely. Consider the following example:

Example 20.6The Voyager 1 spacecraft was launched in September 1977 and is currently127 AU from the Sun and traveling out of the solar system with a speed of 17 km/s. At this rate how long would it take Voyager 1 to reach the nearest known exoplanet orbiting Epsilon Eridani?

Solution: Epsilon Eridani is 10.5 light years away and has at least one exoplanet. At this distance and Voyager 1's current speed it will take

With current (and even foreseeable) technology, travel to distant stars is not feasible.

Radio Communication

With direct travel out of the question perhaps the best (only?) way to approach the question of how to communicate with "extra terrestrials is to use radio communication. In 1974 a coded transmission was sent from the Arecibo radio telescope in Puerto Rico. The message was "anticoded". This means it was sent in a way that would make its interpretation "easy" for a distant civilization advanced enough to have radio telescopes. The message was coded as a digital sequences of pulses sent as a sequence that could be arranged to produce the image shown in Figure 20.10. The 1974 transmission raises an important point for consideration - what frequency band or part of the electromagnetic spectrum would be the best choice for a radio transmission? The galaxy emits radio waves at a number of wavelengths (21 cm for example) and any signal in these regions would be swamped by the "sound of the galaxy". This means that wavelengths longer than about 30 cm would not be useful, On the other hand, wavelengths less than 1 cm are easily absorbed by Earth's atmosphere. There is a region between 21 cm and 18 cm however that is relatively "quiet" and it is likely that an advanced civilization would also know that. This is called the "water hole" - a reference to the idea that a watering hole is a spot that animals in arid countries congregate around. If we either send messages in this wavelength band or listen in this band then perhaps we can communicate with extra-terrestrial civilizations. Figure 20.11 shows the water hole in relation to other radio noise sources. Sound like science fiction? Project SETI or Search for Extra-Terrestrial Intelligence was begun in 1994 and is devoted to a searching through the frequency band of the water hole in the hope of detecting signs of extra-terrestrial intelligence.

Figure 20.10 The Arecibo message sent in 1974

Example 20.7 Two radio messages are shown below - how would you assess whether these are natural or produced by an "intelligence"?

1. 10101010101010101010...... ( a series of pulses - ones - that repeat every 33 milliseconds)
2. 10001100011100011111000111111100011111111111... ( a series of pulses of increasing length)

Solution: The first sequence could be produced by a pulsar (rapidly spinning neutron star) and would not be a likely choice as a means of communicating with us. The second sequence is more interesting as it is basically the numerical sequence 1,2,3,5,7,11 or the first 6 prime numbers. We don't know of any natural processes that generate the prime numbers so a discovery like this would be intriguing to say the least! Does it "prove" that we have been contacted? No.

Figure 20.11 The "Water hole" located between the emission wavelengths of OH and H2

What are the Odds - Setting Up the Drake Equation

Can we predict the number of "other civilizations" in our galaxy? In doing this we will construct a variant on what has been called the Drake Equation named in honor of the American astronomer Frank Drake who pioneered research concerning the possibility of extra-terrestrial civilizations. To understand how the Drake Equation works consider the following factors that could contribute to establishing intelligent life elsewhere in the galaxy

All of these combine to give the Drake Equation:

N_{civilizations} = (N*)x(f_p)x(N_HZ)x (f_L)x(f_I) (f_S)

Figure 20.12 provides an applet to estimate the number of possible civilizations currently in our galaxy.

Figure 20.12 Drake Equation applet

Example 20.8 Apply both pessimistic and optimistic values to the parameters used in the Drake equation to estimate the current number of intelligent civilizations in our galaxy.

Solution: Use the Drake Equation applet and the values in Table 20.1 to develop both pessimistic and optimistic estimates:

• Pessimistic yields the result that we are alone! There is <<1 other intelligent civilization able to communicate with us!
• Optimistic yields the result that there are 10 million other advanced civilizations in our galaxy!

So - how close is our nearest neighbour? Take a closer look to see how to estimate this.

Practice

1. What barriers prevent us from knowing if other civilizations exist elsewhere in the universe?
2. How or "where" should we look for these civilizations?
3. How likely is direct, physical contact?
4. Make your own estimates for the parameters of the Drake equation - how many civilizations does that suggest there might currently be ion our galaxy?