Wi-Fi devices that people use for home networking have radios built in. And it's via radio waves that the miracle of wireless networking takes place.
A home Wi-Fi device generates radio waves that are sent out in 360 degrees, like the ripples that radiate across the surface of a pond when you throw a rock into it. These radio waves go right through the walls of the house, and out into the world at high speed.
At some point in their journey, Wi-Fi radio waves breach the private-public barrier. They wash over your privately owned lawn before continuing on over the publicly owned sidewalk and street.
A person walking or driving by is physically penetrated by these waves. (Some studies have suggested that the waves may increase the risk of cancer; they probably don't.) The radio waves enter people's bodies, are conveyed through their bodies, and then continue on their journey on the other side.
Wi-Fi radio waves also trespass onto other peoples' private property. If your laptop can see the name of your neighbor's Wi-Fi, that means he is broadcasting radio waves over your property line, through the walls of your house and into your home.
What's interesting about this broadcasting of electromagnetic radiation is that it's nothing new. People do it all the time with other parts of the electromagnetic spectrum.
Shining light on Wi-Fi 'snooping'
Let's say you took your big-screen TV and put it in your home's front window facing out so that people could see the screen from the sidewalk or the street.
Then let's say you connected your laptop to it, then made a PowerPoint slide so that in large letters, the TV displayed the following phrase: "My password is bond007."
If anyone, whether Google or your neighbor, stood on the sidewalk and took a picture of your house, they would be recording your password. And this would be, and should be, perfectly legal.
There are two reasons why this is legal. First, the recording happened from a public space. And second, the recording was made with a device in general public use -- a camera.
Any outrage expressed by the person broadcasting his password via electromagnetic radiation (light) into the public space would be ludicrous.
In fact, there are many ways to use electromagnetic radiation to broadcast personal data into the public airwaves.
One could say "My password is bond007" into a walkie-talkie, a ham radio or a CB radio.
It's perfectly legal for a passerby or anyone else to listen to and record that audio signal, as long as he's in a public space or in his own home.
Even if the broadcaster is ignorant of the fact that speaking into a radio conveys his voice into the public space, it's still not illegal for someone else to listen to it or record it.
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