What to Listen For

Sound, radio, and light are all vibrations. Any vibration can be broken down into different frequencies - slow, medium, and fast. This breakdown is called the spectrum of the vibration.

There are two important kinds of signals:
(click to hear)
Narrow-band Broad-band
  • Contains a single frequency.
  • Contains many frequencies.
  • Spectrum is narrow.
  • Spectrum is broad.
  • Doesn't occur in nature.
  • Occurs in nature.
  • Sources: TV/Radio stations, cell phones
  • Sources: stars, planets, nebulae
    • (click and drag to change frequency)

    So if we hear a narrow-band signal, it's either an ET, or it's a man-made signal coming from Earth.

    How can we tell the different between man-made and ET signals? The radio telescope "beam" moves across the sky, and it takes about 12 seconds for it to cross a fixed point like a star. If there's a signal from the star, it gets louder, then softer, as the beam passes over it. Click for a demo.


    But man-made interference stays at about the same loudness. So, putting everything together:

    A narrow-band signal, getting louder and then softer over 12 seconds, may be a signal from an alien civilization.
    		What's vibrating?

    Sound is vibrating air. These vibrations travel at about 700 miles an hour (the speed of sound). Your ears can hear air vibrating between about 20 and 20,000 times per second.

    Light and radio waves are vibrations of electric and magnetic fields. These vibrations don't need air to travel - they can go through empty space, travelling at about 186,000 miles per second (the speed of light). Radio waves vibrate up to a billion times a second, light vibrates another million times faster than that.

    You can't hear radio waves directly. This exhibit converts radio waves to sound by slowing down their frequency and converting them into sound waves.