The long-term impact of scientific-technological developments on twentieth-century life was enormous. These developments were crucial, for example, in greatly increasing people's life expectancy (see below) and the global population from 1.6 billion in 1900 to 6 billion in 1999, an increase that would have major environmental consequences (see Chapter 6).

The line between technology and science, especially applied sciences such as aeronautics, agronomy, electronics, engineering, medicine, and metallurgy, is sometimes difficult to discern. For understanding twentieth-century history, however, what is most essential is to comprehend the areas of human life that were most affected by their combined impact. In general, science deals more with principles and theories, whereas technology is concerned with techniques, tools, instruments, machines, and other products that help humans to accomplish various tasks. Science itself was often poorly understood and took on significance for most people only when it led to technological developments like advancements in medical treatment or sending men to the moon. In the early twentieth century physicists like Max Planck, Albert Einstein, Niels Bohr, Ernest Rutherford, James Chadwick, Werner Heisenberg, Hans Bethe, and Enrico Fermi participated in a golden age for their science. And people heard of some of their new terms such as Quantum Mechanics, the Theory of Relativity, and the Uncertainty Principal, but few non-scientists really understood their meaning.