One way that sensors can track your position without using an array of satellites is by measuring your acceleration as you move around -- but unless you're piloting a jumbo jet, current devices aren't very accurate. Researchers at Caltech hope to change all that with a new, ultra-sensitive accelerometer they developed, which uses laser light to detect motion changes. The scientists managed to shrink a so-called large-scale interferometer down to micro-scale sizes, creating a device "thousands of times faster than the most sensitive sensors used today." That could allow a smartphone with such a micro-sensor to detect your exact position even while inside a grocery store, and flash "ads and coupons for hot dog buns" while you're in the bread aisle, according to Caltech. All that sounds good, but we can perhaps think of more inspiring uses for the new tech.