The new particle discovered by Chadwick was destined to play a totally unexpected role, not only in the history of atomic science but also in the fate of nations. It immediately outmoded a previous concept of the nucleus that pictured it as a cluster of protons approximately half of which were neutralized by electrons crowded into the nucleus. A nucleus is now thought of as containing just protons and neutrons. The neutron was also greeted by nuclear workers as a practically perfect kind of bullet. Unlike charged alpha particles, uncharged neutrons can approach a charged nucleus completely unopposed. It is physically impossible for any kind of container to hold a swarm of free neutrons; they seep right through its walls. Matter Is Energy; Energy Is Matter So far, in the story about man’s curiosity concerning the fundamental nature and structure of matter, the development of ideas about _structure_ has been emphasized. We will now take a brief look at a development which strongly influenced our ideas about the fundamental _nature_ of matter. In 1887 reports appeared on a famous study, often referred to as the Michelson-Morley experiment, which was aimed at determining the earth’s speed through absolute space. The entirely unexpected results of the experiment had a great impact on the concepts of space and time. We will here concern ourselves with just one outcome of the experiment. In 1905, a young German-born physics student named Albert Einstein, who was working as a patent examiner in Switzerland, published three papers, each of which had a profound effect on a different field of physics. One of the papers dealt with some peculiar speculations about space and time which began to interest him when he was studying the Michelson-Morley experiment. The contents of the paper are now referred to as the Special Theory of Relativity. This paper contains several predictions that seemed incredible to the average physicist of that day. These predictions have, however, long since been proved valid. [Illustration: _Albert Einstein in 1905._ Courtesy Lotte Jacobi, Hillsboro, New Hampshire] One of Einstein’s predictions had to do with the equivalence of matter and energy. Until 1905 _matter_ had been considered as something that has mass or inertia; _energy_, on the other hand, had been regarded as the ability to do work. It was believed that the two were as different from each other as, say, a square yard is different from an hour. Einstein’s theory, however, implies that matter and energy are merely two different manifestations of the same fundamental physical reality, and that each may be converted into the other according to the famous equation: E = MC² where E = quantity of energy, M = quantity of matter, and C = speed of light in a vacuum. Nuclei Contain Energy