# Introduction to Physics 314

Welcome to Physics 314, "Introduction to Modern Physics." The material we will cover in this course spans the time period from 1887 to about 1930. That's at least seventy years ago. Why is it called Modern Physics if it's so old?

In the nineteenth century, the discipline of physics had reached a high point. Scientists could describe and predict many phenomena, both in the everyday world and in the far reaches of space, using only a few rules. The use of mathematics to explain the universe had several great triumphs:

• The prediction of Neptune In the early part of the 1800s, astronomers realized that the planet Uranus was deviating very slightly from the path indicated by Newton's Law of Gravitation. Adding perturbations due to the influences of the known planets to the main force of the Sun reduced the residuals, but still couldn't remove them entirely. In the 1840s, two young astronomers independently calculated the position a hypothetical eighth planet should have in order to explain the observations. In 1846, the planet Neptune was found in almost exactly the predicted position.

You can read a bit more about the discovery of Neptune if you like.

• Maxwell's equations During the early nineteenth century, many scientists investigated the mysterious phenomenon of electricity. The names of some of them -- Ampere, Ohm, Volta -- are probably more familiar to you as units than as persons. Experiments by Hans Christian Oersted pointed to a connection between electricity and magnetism. There were a number of seemingly independent empirical relationships between the various aspects of these two phenomena. But in the 1870s, James Clerk Maxwell brought them all together with a set of four differential equations, showing not only that the two were aspects of a single entity -- electromagnetism -- but that electromagnetic fields travelled at the speed of light. In fact, electromagnetic disturbances were light.

By the turn of the century, electricity and magnetism had been harnessed to serve both the home (electric lights) and industry (motors); a large hydroelectric plant at Niagara Falls began operating in 1895, for example. New inventions were springing up all the time -- Marconi first demonstrated wireless radio communications in 1895, and the first trans-Atlantic message was sent (and received) in 1901.

• Applied Thermodynamics Curious people had noticed for millenia that hot gases tend to expand and rise. In the eighteenth century, some adventurous souls rose into the air in the first hot-air balloons. Others gradually figured out how to harness the power of steam and put it to work to drive pumps and engines. In the nineteenth century, the relationships between gas pressure, volume, temperature, and heat content were put on a solid theoretical basis, allowing scientists and engineers to design ever more efficient engines. In the 1880s and 1890s, internal combustion engines fueled by gasoline began to send the first automobiles careening down city streets....

In short, scientists were rightly proud of their understanding of the world.

However, during this period of great progress, there were some things which didn't quite fit into the successful theories. During the period that we will study in this course, experiments showed again and again that the existing laws failed in certain circumstances. We will look in detail at some of these experiments, so that you can see for yourself what puzzled the scientists at the turn of the twentieth century. Over the course of several decades, physics underwent a profound revolution, as researchers gradually realized that their fundamental rules weren't so fundamental after all. In fact, their basic notions of time, space, and existence had to be discarded in certain situations. The old rules weren't wrong, really; they just didn't apply when one examined things in new and out-of-the ordinary ways.

This revolution in the early twentieth century separates classical physics from the so-called modern physics. Let me write it again: the classical theories weren't "wrong;" after all, they did uncover Neptune, and they still do underlie most of our modern civilization's infrastructure: electricity, engineering and the internal combustion engine. They just weren't the whole story ....