For 72 seconds on Aug. 15, 1977, the Big Ear radio telescope picked up a signal that was 30 times as loud as the normal background noise. But what makes this signal worthy of Jerry Ehman's famous "Wow"? Why does it look to many astronomers like a message from an alien planet?
First, it has to do with the hydrogen line. The frequency of the "Wow!" signal was recorded as 1420.4556 MHz, almost exactly the electromagnetic wavelength of hydrogen [source: Krulwich]. If an alien species were to choose a single frequency to broadcast a long-range message, SETI scientists concluded, that's the one.
The second striking characteristic of the "Wow!" signal is its "shape." The shape of a radio signal describes how it would look if graphed over time.
When the "Wow!" signal was first detected by Big Ear, it registered as a 6 on the telescope's "loudness" scale. A few seconds later, it jumped to an "E" (the computer could only report single digits, so when a number exceeded 9, it switched to letters). The signal peaked at "U" (the equivalent of the number 30), then it slowly decreased back to 5. Plotting the signal on a graph, you get a nearly symmetrical pyramid shape.
Why does the signal's shape matter? Because it matches the shape you would expect from a deep space source. Here's why [source: Andersen]:
- A radio telescope is located on Earth's surface
- As Earth rotates, the telescope's focal range slowly drifts across the sky
- If the origin of a radio signal is a fixed point in deep space, the signal will appear weak as it first enters the telescope's range
- When the telescope points straight at the source, the signal will be the loudest
- As the telescope drifts out of range of the signal, it decreases again, hence the pyramid shape
Another tantalizing characteristic of the "Wow!" signal was the sharpness of the transmission. When a radio telescope receives electromagnetic waves from a natural cosmic source, like a quasar, the radio waves are diffused over a band of frequencies.
Not the "Wow!" signal. The Big Ear telescope was listening on 50 different channels, not just 1420 MHz, and none of those other radio channels registered a blip [source: Andersen]. To many SETI scientists, this is a clear sign of an intentional radio transmission from a distant world, not an accidental cosmic event.
Next we'll hear what the skeptics say, and what we've discovered in the 35 years since we first heard the "Wow!"