The term can refer to a double, a ghost, a shadow. But it can mean much more. From our online mirror world, to digital simulators, to the Earth's twin--TED speakers learn from the uncanny second self. Guests include author Naomi Klein, aerospace engineer Karen Willcox, planetary scientist Sarah T. Stewart and psychologist Nancy Segal.TED Radio Hour+ subscribers now get access to bonus episodes, with more ideas from TED speakers and a behind the scenes look with our producers. A Plus subscription also lets you listen to regular episodes (like this one!) without sponsors. Sign-up at plus.npr.org/ted.