Who is John Galt? This phrase is uttered as a sign of despair and hopelessness. The question lacks specific meaning and cannot be answered. Its use in everyday language is a sign that people believe answers don't exist to the problems that plague American society. Dagny and (to a lesser extent) Eddie are dynamic thinkers and people of action who believe that answers are possible and that positive steps can be taken to save American society. Consequently, they reject the pessimism that this question embodies.
Rearden Metal This is the new substance created by steel industrialist Hank Rearden after ten years of demanding effort. The new metal is lighter, stronger, and cheaper than steel. Although its innovative nature frightens people and it hasn't been accepted yet, Rearden and Dagny both know that this product will revolutionize industrial production.
