What's on the Radio?

12/3/06 – 12/9/06
by C. Zaitz

We’ve all seen the dish. They have popped up like mushrooms all over the place. Even colleges and universities have them. However, some dishes that look like TV satellite dishes may actually be radio telescopes. They collect radio waves from the universe. I know that sounds funny, as if you could hear the universe on your radio, but I should clarify. Radio waves are very different from the sounds coming from your radio.

When we point big dishes up to the sky, even a satellite TV dish, we are collecting electromagnetic radiation in the form of radio waves. These are very long waves that can pass through our bodies without as much as a tingle. One wave can be as long as a city block, or as short as your shoe size. The radio station you listen to has to change electronic signals into carrier waves, which travel through the air. When these waves, which are flying all over the place harmlessly, get to the tuner in your radio, they are changed back into a signal that is amplified and modified. These signals produce vibrations on a speaker in your radio. It’s the sound pressure waves from the speaker going in and out quickly that reach your ear and your brain interprets as Mozart or Green Day. Pretty cool.

Sound waves cannot travel in space, since they need a medium (air) to vibrate in order to travel. The beauty of radio waves is that all they need is the original energy source that set them off. They can travel from one end of the universe to the other. Sure, they’ll lose some energy if they travel a long way, but we can still collect them if we have a big enough dish. That’s why we build giant radio dishes. One of the most famous ones was built in an old meteor crater in Arecibo, Puerto Rico. Nature hollowed out a big hole for us, and we stuck a dish in it and collect radio waves from all sorts of interesting objects.

Another cool thing about radio waves is that they can penetrate clouds and dust. Many other wavelengths, like much of the infrared and ultraviolet waves, as well as gamma and X-rays, are blocked by our atmosphere. But radio waves can go through clouds, even clouds of dust in space. Light can’t do that, so our picture of the universe in radio waves can sometimes be clearer than optical images. So what do we look at in radio waves?

The sun is a great producer of radio waves. In radio wavelengths, we see sunspots and solar flares that we might not see in visible light. Beyond the sun, there are stars and galaxies that produce radio waves. In fact, some objects emit more energy in radio waves than in light, so radio astronomy has opened up a new window on the universe. Galaxies that emitted strongly in the radio spectrum were the first clues to finding black holes and quasars. The Cosmic Background Radiation, the lynchpin in the theory of the Big Bang, was discovered by radio technology back in the 1960’s. Radio astronomy is still going strong today, and since radio dishes are relatively inexpensive and easy to build, even some schools and amateur astronomy clubs have them. So the next time your hear about a radio telescope, you’ll know that they are looking at, not listening to, the universe!

Until next week, my friends, enjoy the view.