If Astronomers Are Going to "Listen" for Electromagnetic Radiation at 21

question 56

Multiple Choice

If astronomers are going to "listen" for electromagnetic radiation at 21 cm from extraterrestrial sources, then Earth had better not generate too many waves at this wavelength from terrestrial sources. The standard FM broadcast band (as observed on a radio dial) spans frequencies from 88 MHz (million cycles per second) to 108 MHz. How does the frequency of the 21-cm signal compare with the FM band? The frequency of the 21-cm line is

• A) below the low frequency end of the FM band by a factor of about 9.
• B) right in the middle of the FM band.
• C) right at the upper end of the FM band.
• D) higher than the top end of the FM band by a factor of about 13.

---

Definitions:

Characteristics

Distinctive or identifying features or qualities of a person, object, or phenomenon.

Hypotheses

Proposed explanations made on the basis of limited evidence as starting points for further investigation.

Theory

A system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.

Nondirectional Research Hypothesis

A hypothesis that predicts a relationship or effect but does not specify the direction of the relationship.