The Dead Man's Switch
I went looking for the oldest dead man's switch I could find, because I needed one for the book and I wanted to know whether the thing I was about to invent already existed.
It did, on a streetcar.
Electric trams put a great deal of power under one person's hand, and the first version in wide service rode on the Birney One-Man Safety Car, which did what the name says and took the conductor off the platform. The motorman kept pressure on the controller. Let go, because you have fainted or died, and the current cuts and the brakes come on without you.
What made the thing universal was a wreck it could not have prevented. On the evening of 1 November 1918 a Brooklyn Rapid Transit train took a curve rated for six miles an hour at somewhere above thirty, and at least ninety-three people died under Malbone Street. The man at the controls was a crew dispatcher with no experience on that line, put there because the company was on strike and short of drivers. He had not fainted and he had not died. He was wide awake and out of his depth, and a dead man's control would have done nothing for him at all.
The industry fitted them everywhere regardless, and I cannot really argue with that, because the idea underneath is hard to refuse. The machine should be at its safest in the exact moment the human being stops being there. We have been building versions of that ever since.
Software rebuilt it almost at once. A watchdog timer reboots a device that has stopped resetting it. Leases behave the same way, and so do the credentials a cloud provider issues you: they expire on a clock, whether or not anyone remembers to revoke them. All of it is built to distrust silence. Something has to keep arriving on time, and when it stops arriving the system changes state without being asked.
Cryptography makes the signal specific. Instead of any message, you require a signed one, a heartbeat that only a particular key can produce. Now silence no longer means that a machine has stopped. It means that one person has stopped, because only that person could have kept it going. The heartbeat is proof they are still there, and its absence is proof of the opposite.
A mechanism that measures a life can be pointed in either direction.
Set up in your favour, it is the best insurance an engineer can buy. The network needs a signed heartbeat every twelve hours or it halts. The key that produces it lives nowhere except in your memory. Kill you and the system stops in front of everyone who depends on it, inside half a day, and stays stopped. You have made yourself expensive to murder, which for a while feels a lot like being safe.
That holds only as long as the check does. Anyone who wants you gone has to ship a version of the software that stops asking for a heartbeat, and get enough of the network running it. That is a coordination problem, and coordination problems get solved by people who are patient and well funded.
I spent a long time looking for a way out of that for the man in my book, on the assumption that a clever enough engineer would find one. He does not, and neither did I, which is roughly when I knew the book was working.