During the development of nuclear physics one question haunted physicists: Could a nuclear chain reaction be sustained? Yes, there had been bench-top reactions but they were to study what particles were being emitted when atoms were split.
Predictions from existing theories were uncertain; either a chain reaction would exhaust itself and slow down to a halt, or else the reaction would immediately "wildcat" and surge out of control--with disastrous results.
By 1942 one successful experiment to create a chain reaction had been done at Columbia University. But that was to initiate a chain reaction on a large scale. Once initiated, it was halted.
Now with the pressure of the second world war and fears over NAZI "hard water" experiments, the question of a sustainable fission reaction was vital. Physicists lobbied President Roosevelt to create a nuclear weapons program, but Roosevelt and his advisors were not convinced a nuclear weapon could be created--nor would even work.
Enrico Fermi had a simple solution: Build a reactor and create a sustainable nuclear chain-reaction. With limited federal funding the physics department of the University of Chicago stacked a giant cube of graphite bricks interspersed with small globes of uranium fuel.
After careful manipulations of the control rods, the first self-sustaining nuclear chain-reaction was created December 2nd, 1942. It ran for 28 minutes and then was shut down by inserting the safety control rods.
All of this, the construction, the operation of the power pile, the timing of the sustainable reaction were exactly calculated by Enrico Fermi. Observers and co-workers were astonished at Fermi's ability to accurately predict every phase and step of this operation through the manipulation of his slide rule.
The formula crucial to his calculation's was the Special Theory of Relativity first proposed in 1905 by Albert Einstein. E=MC2 predicted atomic energy, demonstrated how a nuclear fission chain-reaction was possible and showed how much energy was needed to initiate this chain-reaction.
In order for this formula to work, in order for Fermi's pile to be successful, in order for a sustainable nuclear chain reaction to take place C2, the speed of light squared, had to be a constant. Were C2 a variable, Fermi's calculations would have been useless. No atomic energy, no power pile, no sustainable chain-reaction, no Manhattan Project, no atomic bomb. No Cold War, no nuclear medicine, no X-rays, no CAT scans.
With the speed of light proven a constant instead of a variable, radioactive half-life decay is also established as a reliable scale of measurement. With this the existence of the Earth is shown to be measured in the billions of years, not mere thousands.
Day after day the overwhelming evidence for the evolutionary development of the Earth and its lifeforms accumulates. The speed of light not only helped us unlock the atom, it shines a light upon the vast scale of time we developed from--and have inherited.
The re-insertion of the control rods ended the chain-reaction, proving we could both initiate and control this incredible power. We held the reins in our hands; we could stop it, shut it down. No such ability, no such predictions have arisen or will ever arise from a religious text. The assertion the whole of our universe and the power it contains can be comprehended through a set of ancient tribal laws and myths is not just delusion, but madness.
Enrico Fermi (Wikipedia)
The First Reactor (Dept of Energy)
Note: There is a link at top left of page to download a 6meg PDF file.
An excellent overview and history of atomic physics is the book The Making of the Atomic Bomb by Richard Rhodes.