At the Sackler Gallery, Cai Guo-Qiang’s “Traveler” sits: the weathered wooden frame of an old Japanese boat, flooded with broken shards of porcelain plates and statues of the Buddhist goddess Guanyin. It’s a fascinating exhibit: the sheer quantity of shattered porcelain flows over the decayed timber like sea foam, speaking to the viewer of a spirit broken and a faith lost to time. Or something.