You’re Leaving a Digital Trail. What About Privacy? About 100 students at M.I.T. are trading privacy for a smartphone that tracks their calls, messages and movements.Credit...Jodi Hilton for The New York Times Cambridge, Mass. HARRISON BROWN, an 18-year-old freshman majoring in mathematics at M.I.T., didn’t need to do complex calculations to figure out he liked this deal: in exchange for letting r