Astropulse:电磁辐射

Electromagnetic Radiation

Electromagnetic radiation includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. All of these types of radiation can be thought of as waves, like the ripples that spread out when you drop a pebble into a calm pool of water. An electromagnetic wave has a few important properties:

1. Speed: how fast is each ripple moving?
2. Frequency: if you point at the water with your finger, how many ripples pass by your finger every second?
3. Wavelength: the distance between two adjacent ripples.

The speed is easy. It turns out that all electromagnetic waves have the same speed, which scientists represent with the letter c. This speed, the speed of light, is equal to 670 million miles per hour.

The frequency can be any number. It is measured in Hertz, which means "One ripple per second." If two ripples pass by your finger every second, that's 2 Hertz. Most electromagnetic radiation has frequencies much larger than 1 Hertz. So we use larger units to measure the frequency:

kHz	kiloHertz, one thousand Hertz. (1,000)
MHz	MegaHertz, one million Hertz. (1,000,000)
GHz	GigaHertz, one billion Hertz. (1,000,000,000)

For numbers that are too large to express this way, we use "scientific notation", which is just a way of saying how many zeroes are after a number. For instance, 1 MegaHertz could be written 1E6 Hertz, meaning 1 with 6 zeroes after it. 2 GigaHertz is 2E9 Hertz. And 8,000,000,000,000 Hertz is 8E12 Hertz.

Here are some examples of frequencies of electromagnetic waves: *

long-wave AM radio	200 kHz
medium-wave AM radio	1 MHz
short-wave AM radio	10 MHz
FM Radio waves	100 MHz
Microwaves in a microwave oven	2.4 GHz
Infrared light	3E12 Hz
Red light	4E14 Hz
Green light	6E14 Hz
Blue light	7E14 Hz
Ultraviolet light	1E15 Hz
X-rays	3E18 Hz
Gamma rays	3E20 Hz

"Frequency" means the same thing for electromagnetic waves that it does for sound waves. In a sound wave, the frequency is the number of sound ripples that pass by in one second. For instance, the "A 440" note, which orchestras use to tune up, has a frequency of 440 Hz. The difference is that in an electromagnetic wave, the ripples are made of electric and magnetic fields, whereas in a sound wave, the ripples are made of moving air.

Both Astropulse and the original SETI@home use frequencies around 1,420 MHz, ranging from 1,417.5 MHz from 1422.5 MHz.

• In fact, each of these waves actually has a range of frequencies. For instance, FM radio waves actually range from 87.5 to 108.0 MHz.