Nothing is more timely than an exhibition exploring barriers. It isn’t simply the Trump administration’s determination to build a divisive wall along the southern U.S. border. Alienation is on the rise, despite globalization. Photographs from more than 70 artists, including Banksy, JR, Marina Abramović, and John Moore look at the origins, history, and meaning of walls both real and imagined. Subjects include the obvious candidates—the Berlin Wall, Jerusalem’s Western Wall, the Great Wall of China—but also your average fence. What messages lie within their construction? —J.V.