The 1420 MHz (21 cm) radio line generated by neutral hydrogen atoms is a good choice for interstellar communications because
A) it is the easiest radio frequency to generate
B) it is the only radio frequency that is not blocked by interstellar matter
C) all intelligent civilizations should be aware of its universal significance and would probably also think of sending and receiving signals at this frequency
D) it is the only frequency to which radio telescopes can be tuned

Question

Quizplus · Accepted Answer

The 1420 MHz (21 cm) radio line generated by neutral hydrogen atoms is a good choice for interstellar communications because it is a universal frequency that should be recognized and used by all intelligent civilizations. This makes it a likely frequency for sending and receiving signals in interstellar communications. It is not the easiest frequency to generate, nor is it the only frequency that is not blocked by interstellar matter. Additionally, radio telescopes can be tuned to a wide range of frequencies, not just 1420 MHz.

The 1420 MHz (21 Cm) Radio Line Generated by Neutral