Wirebook › Bonus lesson
How does a photo fly to the phone next to you?
You tap Share, pick the phone next to you, and a second later your photo is on it. No cable, no email, and it works even in a field with no Wi-Fi router anywhere. Here's the trick: the phones use Bluetooth to find each other, then build their own private Wi-Fi lane straight between them, and the photo flies across as lots of small numbered pieces that click back together at the other end, like a jigsaw. This is how AirDrop on iPhones and Quick Share (it used to be called Nearby Share) on Android phones work.
Step 1: a tiny Bluetooth shout
When you open the share sheet, your phone starts sending very short Bluetooth messages, a bit like whispering anyone there who takes photos? many times a second. Nearby phones that are willing to receive answer back, and they pop up in your list. Bluetooth is great for this because it sips very little battery and can keep listening all day. But it's slow, roughly one megabit a second for files, so it's only used to find each other, not to carry the photo. Those shouts fade quickly, which is why sharing nearby is meant for the same room, about 9 metres or less.
Your friend's settings decide whether their phone answers. Receiving off means it stays silent. Contacts only means it only answers phones whose owner is saved in its contacts (the phones check this with secret codes, so nobody's phone number is shouted out loud). Everyone for 10 minutes lets anyone nearby see it, then switches back on its own, so strangers on a bus can't keep sending you pictures.
Step 2: a private Wi-Fi lane, no router needed
Once your friend taps Accept, the two phones switch on a special kind of Wi-Fi that goes straight from one phone to the other. There's no router in the middle and nothing goes out to the internet, which is why it works on a mountain or a plane. The lane is encrypted, so another phone nearby that hears the radio waves just hears scrambled noise. Wi-Fi is about a hundred times faster than Bluetooth here, which is the whole reason for the switch: a video that would take most of an hour over Bluetooth takes under a minute.
Phones can use two Wi-Fi lanes. The 5 GHz lane is faster but fades sooner and through walls. The 2.4 GHz lane reaches further but is slower and more crowded, because Bluetooth, baby monitors, wireless mice and microwave ovens all use the same part of the radio. A microwave is a metal box that keeps its 2.4 GHz waves inside to heat the food, but a little leaks out, enough to make that lane jiggly for a few metres. Try it with the soup on.
Step 3: numbered pieces, put back like a jigsaw
A photo is far too big to send as one blob. The phone cuts it into thousands of small packets, each about 1,500 bytes, and writes a number on each one. They fly across very fast and might land in any order. The receiving phone uses the numbers to put them back in the right place, a bit like doing a jigsaw by the numbers on the back. Each piece also carries a small check number worked out from its contents; if the check doesn't match when it lands, the piece was damaged by noise and is thrown away. Then the receiver says please send 4 and 9 again, and only those pieces fly over a second time.
Why doesn't it show up?
The same three steps explain almost every it's not working. If your friend isn't in the list, the Bluetooth step failed: Bluetooth is off, their phone is set to receive from contacts only or not at all, the screen is locked and asleep, or you're too far apart. If they show up but the send fails or crawls, the Wi-Fi lane is the problem: Wi-Fi is switched off, the phone is busy being a hotspot (it can't share its Wi-Fi radio for both), or there are walls and other radios in the way. And iPhones and Android phones use different versions of this trick, so most of them can't send to each other this way (a few newer phones have started to). A share link or a QR code works across both.
Related lessons: how Wi-Fi radio waves get around your house · why a download can pick up where it left off · how a TV remote sends invisible flashes · how a game controller talks to a console.
Grown-up twin: Tooltern's help page on fixing AirDrop and Quick Share when they won't find or send.