What is Modern Physics?

Elbert Ainestine

The term "modern physics," taken literally, means of course, the sum total of knowledge under the head of present-day physics. In this sense, the physics of 1890 is still modern; very few statements made in a good physics text of 1890 would need to be deleted today as untrue...

On the other hand... there have been enormous advances in physics, and some of these advances have brought into question, or have directly contradicted, certain theories that had seemed to be strongly supported by the experimental evidence.

For example, few, if any physicists in 1890 questioned the wave theory of light. Its triumphs over the old corpuscular theory seemed to be final and complete, particularly after the brilliant experiments of Hertz, in 1887, which demonstrated, beyond doubt, the fundamental soundness of Maxwell's electromagnetic theory of light. And yet... these very experiments of Hertz brought to light a new phenomenon—thephotoelectric effect—which played an important part in establishing thequantum theory. The latter theory... is diametrically opposed to the wave theory of light; indeed, the reconciliation of these two theories... was one of the great problems of the first quarter of the twentieth century.