At the start of the 21st century, the total output of the American economy weighed roughly the same as it did 100 years earlier. Yet the value of that output, in real terms, was 20 times greater. Output is increasingly weightless, produced from intellectual capital rather than physical materials. Production has shifted from steel, heavy copper wire and vacuum tubes to microprocessors, fine fiber-optic cables and transistors. Services have increased their share of GDP. This weightless or dematerialized economy, most economists agree, is not just lighter but also more efficient.