The most serious problem raised by the law of conservation of energy
involved the sun. Until 1847, scientists did not question sunlight. The
sun radiated vast quantities of energy but that apparently was its
nature and was no more to be puzzled over than the fact that the earth
rotated on its axis.
Once Helmholtz had stated that energy could neither be created nor
destroyed, however, he was bound to ask where the sun’s energy came
from. It had, to man’s best knowledge, been radiating heat and light,
with no perceptible change, throughout the history of civilization and,
from what biologists and geologists could deduce, for countless ages
earlier. Where, then, did that energy come from?
The sun gave the appearance of being a huge globe of fire. Could it
actually be that—a large heap of burning fuel, turning chemical energy
into heat and light?
The sun’s mass was known and its rate of energy production was known.
Suppose the sun’s mass were a mixture of hydrogen and oxygen and it were
burning at a rate sufficient to produce the energy at the rate it was
giving it off. If that were so, all the hydrogen and oxygen in its mass
would be consumed in 1500 years. No chemical reaction in the sun could
account for its having given us heat and light since the days of the
pyramids, let alone since the days of the dinosaurs.
Was there some source of energy greater than chemical energy? What about
the energy of motion? Helmholtz suggested that meteors might be falling
into the sun at a steady rate. The energy of their collisions might then
be converted into heat and light and this could keep the sun shining for
as long as the supply of meteors held out—even millions of years.
This, however, would mean that the sun’s mass would be increasing
steadily, and so would the force of its gravitational pull. With the
sun’s gravitational field increasing steadily, the length of earth’s
year would be decreasing at a measurable rate—but it wasn’t.
In 1854 Helmholtz came up with something better. He suggested that the
sun was contracting. Its outermost layers were falling inward, and the
energy of this fall was converted into heat and light. What’s more, this
energy would be obtained without any change in the mass of the sun
whatever.
Helmholtz calculated that the sun’s contraction over the 6000 years of
recorded history would have reduced its diameter only 560 miles—a change
that would not have been noticeable to the unaided eye. Since the
development of the telescope, two and a half centuries earlier, the
decrease in diameter would have been only 23 miles and that was not
measurable by the best techniques of Helmholtz’s day.
Working backward, however, it seemed that 25 million years ago, the sun
must have been so large as to fill the earth’s orbit. Clearly the earth
could not then have existed. In that case, the maximum age of the earth
was only 25 million years.
Geologists and biologists found themselves disturbed by this. The slow
changes in the earth’s crust and in the evolution of life made it seem
very likely that the earth must have been in existence—with the sun
delivering heat and light very much in the present fashion—for many
hundreds of millions of years.
Yet there seemed absolutely no other way of accounting for the sun’s
energy supply. Either the law of conservation of energy was wrong (which
seemed unlikely), or the painfully collected evidence of geologists and
biologists was wrong (which seemed unlikely),—or there was some source
of energy greater than any known in the 19th century, whose existence
had somehow escaped mankind (which also seemed unlikely).
Yet one of those unlikely alternatives would have to be true. And then
in 1896 came the discovery of radioactivity.