To concentrate energy in this fashion would violate something called “the second law of thermodynamics”. This was first proposed in 1850 by the German physicist Rudolf Julius Emmanuel Clausius (1822-1888) and had proved so useful that physicists did not like to abandon it unless they absolutely had to.
Another possibility was that radioactive atoms were creating energy out of nothing. This, of course, violated the law of conservation of energy (also called “the first law of thermodynamics”) and physicists preferred not to do that either.
The only thing that seemed to remain was to suppose that somewhere within the atom was a source of energy that had never made itself evident to humanity until the discovery of radioactivity. Becquerel was one of the first to suggest this.
It might have seemed at first that only radioactive elements had this supply of energy somewhere within the atom, but in 1903 Rutherford suggested that all atoms had a vast energy supply hidden within themselves. The supply in uranium and thorium leaked slightly, so to speak, and that was all that made them different.
The room in which the Curies discovered radium. Pierre Curie’s writing is on the blackboard.
But if a vast supply of energy existed in atoms, it was possible that the solution to the puzzle of the sun’s energy might rest there. As early as 1899 the American geologist Thomas Chrowder Chamberlin (1843-1928) was already speculating about a possible connection between radioactivity and the sun’s energy.
If it were some variety of this newly discovered source of energy (not necessarily ordinary radioactivity, of course) that powered the sun—millions of times as intense as chemical energy—then the sun might be pouring out energy for hundreds of millions of years without perceptible physical change—just as uranium would show scarcely any change even in so mighty a time span. The sun would not have to be contracting; it would not have had to fill the earth’s orbit 25,000,000 years ago.
This was all exciting, but in 1900 the structure of the atom had not yet been worked out and this new energy was just a vague supposition. No one had any idea of what it actually might be or where in the atom it might be located. It could only be spoken of as existing “within the atom” and was therefore called “atomic energy”. Through long habit, it is still called that much of the time. And yet “atomic energy” is not a good name. In the first couple of decades of the 20th century, it became apparent that ordinary chemical energy involved electron shifts and those electrons were certainly components of atoms. This meant that a wood fire was a kind of atomic energy.
The electrons, however, existed only in the outer regions of the atom. Once Rutherford worked out the theory of the nuclear atom, it became apparent that the energy involved in radioactivity and in solar radiation had to involve components of the atom that were more massive and more energetic than the light electrons. The energy had to come, somehow, from the atomic nucleus.