Let’s start with a problem. You think some bills have just been paid out of your account. You want to check how much money you have left. You have a computer in your hand - your mobile phone - but that computer does not currently know your balance. There is at least one computer that does know your balance, but it does not belong to you, and it sits in a large building hundreds of miles away. How does the number that represents your balance get from your bank’s computer to the mobile phone in your hand?

This is one of a series of articles which explores how we use computers to run the world by considering the round trip question: what happens when we press a button on a mobile banking app which tells it to do something with our money?

Simply by virtue of being computers, the phone in your hand and the machine in your bank have a lot of similarities. For now, though, I’d like to explore their differences, as those differences are what makes the round trip question interesting.

Something has been bothering me: I have been designing, building and running computer systems for over thirty years, and believe that the explosive growth of information technology in that time is a net good for the world. At the same time, I have observed a growing gap in understanding between the people who build and run technology for a living and the people who use it. Information technology touches more people’s lives more deeply every day - yet is increasingly difficult to understand.

I sometimes test this feeling by asking what I call ‘the round trip question’. The question is: when you hit a button on your mobile banking app which tells it to do something (get your balance, make a payment, or some other function), what do you imagine happens? (It doesn’t have to be banking, but it’s the industry I happen to have worked in for longest.)