Puerto Rico

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Arecibo Observatory, Arecibo, Puerto Rico

18° 20′ 39″ N, 66° 45′ 10″ W

The Biggest Dish

The radio telescope at Arecibo Observatory in Puerto Rico looks like the set for a James Bond film—the 305-meter dish set into the hilly countryside near the town of Arecibo is the largest and most sensitive radio telescope ever built. Surrounded by lush vegetation and set into an ancient sinkhole, it has been listening to radio signals from the stars since 1963, with an 810-tonne platform suspended above the dish to receive signals bouncing from its surface.

The surface of the dish is made of 40,000 perforated aluminum sheets formed into a spherical shape. Since the dish itself cannot move, the receivers suspended above it can shift to listen to different parts of the sky. Hanging from the platform is a dome-like structure that can be moved so that it can focus on signals received from particular areas of the sky. Since the dish is spherical, not parabolic, its shape can be used to study different parts of the sky without moving the dish itself (see also Chapter 67 on the Sound Mirrors, which used a similar technique).

And the Arecibo Observatory can do more than just listen—the dome incorporates a 1-megawatt radar that can be used to bounce a signal off a spacecraft or planet to make measurements of their movements or surface.

Early in its history (in 1965), the dish was used to determine the length of a day on Mercury (now known to take 59 Earth days); then, in 1972, it discovered the first pulsar in a binary system (B1913+16). In 1992, the dish observed the first planets outside the Solar System, orbiting the pulsar B1257+12 (which is about 980 light years from the Sun).

The scientists who discovered of B1913+16 went on to examine the orbits of the two stars involved over a period of many years (work for which they received the Nobel Prize in Physics in 1993). Observation of the orbits showed that they had changed in accordance with Einstein’s theory of general relativity, which predicted the existence of gravitational waves.

In 1974, the dish transmitted a message containing 1,679 bits intended to explain to any alien civilization listening that we could count from 1 to 10, that we knew the atomic numbers of carbon, hydrogen, nitrogen, oxygen, and phosphorous (all the elements needed for DNA), and that we were made from DNA (the four nucleotides of DNA were transmitted along with a graphic of the structure of DNA). The transmission also included a figure of a human with its height measured, the current population of the Earth, a graphic of our Solar System, and a graphic of the Arecibo dish itself

Arecibo is also used by the Search for Extraterrestrial Intelligence (SETI) project, which looks for intelligent life anywhere in the Universe.

And in fact, it really was the set for a James Bond film—the dish was featured in the climax of 1995’s GoldenEye.

If you want to play James Bond (or just a radio astronomer), you can pay a visit to the observatory, starting at the visitor center, where the work done at Arecibo is explained. There are fixed exhibits and a documentary film about radio astronomy. Then, step outside onto the observation platform to see the telescope itself.

Practical Information

The Arecibo telescope is about a 90-minute drive from San Juan airport. Information about visiting can be found at http://www.naic.edu/.

SETI and the Wow! Signal

The SETI project works by listening to radio signals received by radio telescopes (in particular the Arecibo dish) and looking for narrowband radio signals (i.e., radio signals that only occupy a small part of the radio spectrum, just like a normal Earth radio). Since SETI doesn’t know the frequency on which extraterrestrials might be transmitting (or when), it is forced to sample enormous numbers of radio signals, break them up into narrow bands of frequencies, and examine each for potential communications.

Since the 1960s, SETI has been using radio telescopes and antennas around the world to look for narrowband signals. Since 1999, anyone with a computer has been able to assist by running the SETI@home software. This software uses spare computer cycles (such as when the screen saver is running) to search for narrowband signals in chunks of received radio signals sent to home computers over the Internet.

On August 15, 1977, SETI detected what could have been a genuine radio signal coming from an alien source. The radio transmission, sometimes referred to as 6EQUJ5, is commonly called the Wow! signal after the handwritten note Dr. Jerry R. Ehman inscribed on the computer printout after spotting the anomaly (see Figure 119-1).

Figure 119-1. The Wow! signal

The printout showed the radio signals received by the Big Ear radio telescope at Ohio State University, with numbers and letters representing the intensity of the signals. In Figure 119-1, a space indicates that no signal was received; numbers 1 through 9 indicates increasing intensities. To represent numbers starting from 10, letters were substituted (with A meaning 10, B meaning 11, etc.).

Each column of the printout represented a 10 kHz band of the received signal. An intense signal received on a single narrow band would seem to indicate an alien transmission on that particular frequency. Most of the time, the printout was filled with 1s and spaces, indicating nothing more than background noise from the cosmos.

But the Wow! signal was something else—a clear strong signal on one frequency that increased in intensity and then faded away. Each row on the printout represented 12 seconds on listening. Plotting the seconds before and after the signal shows how it grew, faded, and stood out from the noise (see Figure 119-2).

Figure 119-2. Plot of the Wow! signal

The signal lasted for a full 72 seconds—the most tantalizing part of the entire Wow! signal story. Since the Big Ear telescope was not moving, it swept the sky due to the rotation of the Earth. If a signal was being received from a distant, alien radio source, it would have appeared faintly on the printout, increased in intensity, and then faded away over a period of…72 seconds (due to the speed of the Earth’s rotation).

Unfortunately, the signal was never detected again. Even worse, Big Ear actually had two antennas that would have listened to the same spot of sky three minutes apart, but the Wow! signal was only heard once.