It eventually became clear that radioactivity involved the giving off of
energy. Uranium emitted gamma rays that we now know to be a hundred
thousand times as energetic as ordinary light rays. What’s more, alpha
particles were being emitted at velocities of perhaps 30,000 kilometers
per second, while the lighter beta particles were being shot off at
velocities of up to 250,000 kilometers per second (about 0.8 times the
velocity of light).
At first, the total energy given off by radioactive substances seemed so
small that there was no use worrying about it. The amount of energy
liberated by a gram of uranium in 1 second of radioactivity was an
insignificant fraction of the energy released by a burning candle.
In a few years, however, something became apparent. A lump of uranium
might give off very little energy in a second, but it kept on for second
after second, day after day, month after month, and year after year with
no perceptible decrease. The energy released by the uranium over a very
long time grew to be enormous. It eventually turned out that while the
rate at which uranium delivered energy did decline, it did so with such
unbelievable slowness that it took 4.5 billion years (!) for that rate
to decrease to half what it was to begin with.
If _all_ the energy delivered by a gram of uranium in the course of its
radioactivity over many billions of years was totalled, it was
enormously greater than the energy produced by the burning of a candle
with a mass equal to that of uranium.
Let’s put it another way. We might think of a single uranium atom
breaking down and shooting off an alpha particle. We might also think of
a single carbon atom combining with 2 oxygen atoms to form carbon
dioxide. The uranium atom would give off 2,000,000 times as much energy
in breaking down, as the carbon atom would in combining.
The energy of radioactivity is millions of times as intense as the
energy released by chemical reactions. The reason mankind had remained
unaware of radioactivity and very aware of chemical reactions was,
first, that the most common radioactive processes are so slow that their
great energies were stretched over such enormous blocks of time as to be
insignificant on a per second basis.
Secondly, chemical reactions are easily controlled by changing
quantities, concentrations, temperatures, pressures, states of mixtures,
and so on, and this makes them easy to take note of and to study. The
rate of radioactive changes, however, could not apparently be altered.
The early investigators quickly found that the breakdown of uranium-238,
for instance, could not be hastened by heat, pressure, changes in
chemical combination, or, indeed, anything else they could think of. It
remained incredibly slow.
But despite all this, radioactivity had at last been discovered and the
intensity of its energies was recognized and pointed out in 1902 by
Marie Curie and her husband Pierre Curie (1859-1906).
Where, then, did the energy come from? Could it come from the outside?
Could the radioactive atoms somehow collect energy from their
surroundings, concentrate it several million-fold, and then let it out
all at once?
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.
[Illustration: _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.
What is involved then in radioactivity and in the sun is “nuclear
energy”. That is the proper name for it and in the next section we will
consider the subsequent history of the nuclear energy that broke upon
the startled consciousness of scientists as the 20th century opened and
which, less than half a century later, was to face mankind with untold
consequences for good and for evil.