A FEW DAYS ago, I warned my wife that the experiment I was about to engage in was entirely non-sexual, lest she glance over my shoulder at my iPhone. Then I installed the gay hookup app Grindr. I set my profile photo as a cat, and carefully turned off the ‘show distance’ feature in the app’s privacy settings, an option meant to hide my location. A minute later I called Nguyen Phong Hoang, a computer security researcher in Kyoto, Japan, and told him the general neighborhood where I live in Brooklyn. For anyone in that neighborhood, my cat photo would appear on their Grindr screen as one among hundreds of avatars for men in my area seeking a date or a casual encounter. Within fifteen minutes, Hoang had identified the intersection where I live. Ten minutes after that, he sent me a screenshot from Google Maps, showing a thin arc shape on top of my building, just a couple of yards wide. ‘I think this is your location?’ he asked. In fact, the outline fell directly on the part of my apartment where I sat on the couch talking to him.