Catch a Wave

« Hidden in Plain Sight | Home | Such a Deal! »

By Linda | December 11, 2008

While watching the NatGeo program, Journey to the Edge of the Universe, my 10-year-old asked several questions that I thought were well worth answering here on ScienceJunkies.

The program takes us on a virtual tour of the Universe, beginning in our own solar system and moving ever farther away to the outer edges of the Cosmos. As a way to illustrate distances, the narration tells us what we would see or hear at various points on our voyage were we to tune into Earth’s TV and radio broadcasts. The farther we travel away from Earth, the older those broadcasts become. At one point we hear a snippet of the 1936 Olympics. This confused my son a little bit because he took it to mean that our simulated trip took us backward in time rather than that the radio waves took so long to travel across the vast distances.

It was easier for him to grasp the concept when I explained it the other way around, citing the fact that the the actual photons that hit our eyes as we look up at a night sky began their journey millions of years ago and that we are seeing those stars as they were then and not as they are now. Even light from our own sun takes eight minutes to reach Earth! Still a big concept, but for some reason easier to grasp for him.

The two question that he had about those old radio broadcasts were very astute ones, I think.

1. Remembering that sound waves require some sort of medium (air, water, etc.) to move through (in space no one can hear you scream), he wanted to know how those sounds were able to travel through the vacuum of space.

2. He knew that sound travels faster than light (you see lightning, then hear the thunder) and wondered why the television broadcasts didn’t get to those far flung stars quicker than the radio broadcasts did.

Fortunately, I was able answer both questions at once!

It’s easy to confuse radio waves with sound waves. Sound waves are vibrations or pressure waves that do indeed need a medium in order to travel beyond their source. Radio waves on the other hand, are part of the electromagnetic spectrum (as is visible light) all of which travel fastest through a total vacuum. Electromagnetic waves are slowed down a bit when they encounter matter, but in essence, all electromagnetic energy travels at the speed of light.

Above is a nice graphic of the electromagnetic spectrum. It was interesting to my son that the visible range humans are capable of seeing is so small compared to the whole scale.

It’s also fun to know that some animals can see (sense, really) a bit beyond the human range - snakes on the infrared side, and butterflies on the ultraviolet end.

Here is a link to an excellent Q&A at NASA’s Cosmicopia that addresses this, and many other related questions.