Silicon Valley Tried to Remake the Idea Machine

Silicon Valley, where toddler-aged companies regularly sell for billions, may be the most vibrant sector of the U.S. economy, fueling a boom in markets from housing to high-end toast (how many $4-a-slice artisanal bread bars does a place really need?). But as recent innovations — apps that summon cabs, say, or algorithms that make people click on ads — have been less than world-changing, there is a fear that the idea machine is slowing down.