system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is radio static? Why isn’t there just silence in the absence of a broadcasting signal?
gpt
First of all, your radio is tuned to certain frequencies, but all of the others are still there. Your radio is just picking up on the very strong signal and "consciously" (as much as a circuit board is "conscious") ignoring the less strong signals. You can kind of test this if you have one of those Aux-to-FM transmitters, usually used for older cars that don't have an aux cord or bluetooth. You can set your radio to pick up a local station, and it plays. If you plug in your Aux-to-FM without any input to it, the local station will go silent. The strong signal from the transmitter is making your radio pay attention to it instead of the local station, even though it's transmitting a "silence" signal.

That means in the absence of any single strong signal at that wavelength, the antenna is still picking up every other radio station around it, your radio just doesn't have a single loud station to pay attention to. The "loudest" station is all of the stations together, which doesn't give any clear signal so you get static.

But even in the absence of all radio emissions from all humans - if you went back ten thousand years with a radio and set it up, you'd still get static. A lot of things in the universe give off radio waves. See, radio is just another kind of electromagnetic radiation, just like visible light. It just has a much longer wavelength. Stars pump out EM at a *lot* of frequencies, from radio to ultraviolet light. The peak output of our Sun happens to be in the green/yellow band, but it still cranks out some radio.

Other celestial bodies crank out even more, so even though they're far away they're comparatively "loud" enough to be heard here on Earth. Black holes blast out massive amounts of EM as they strip down matter falling in - everything from radio all the way up to x-rays and super high energy gamma rays.

The expansion of the universe also changes the way EM radiation propagates through space. They get stretched as they travel through expanding space. The farther away the source of the EM is, the more it's been stretched by space, so the longer the wavelength. This is called *redshift* - visible light gets shifted towards the red end of the spectrum. But red gets shifted to infrared, and infrared to microwaves, and microwaves to longer radio waves, and so on. If you look at the very very very edge of visible space, the very earliest moments of the universe during the Big Bang, you see the very first light given off as the universe expanded enough for electrons to recouple with atomic nuclei. That very first light is so ancient that it's all shifted deep into microwaves. This is the Cosmic Microwave Background radiation.

The CMB was found because scientists were using radio antennas to look for something else entirely, and needed to account for all of the sources of radio that would interfere. They accounted for everything they could think of, but there was still a pesky source of radio that they couldn't find. They eventually looked *up* and realized it was coming from space - and not the Sun or close by stars, but deep in space.