Sunday, October 16, 2005

What didn't happen

Albert Einstein once said: "The most incomprehensible thing about the world is that it is comprehensible."

And when you think about it, it is indeed remarkable that a mass of nerve fibers the size of a cantaloupe can comprehend the cosmos -- a vast geography of hundreds of billions of galaxies -- at least! -- with a 14 billion year history.

Some cantaloupe-sized masses of nerve fibers are more remarkable than others. This year we celebrate the 100th anniversary of Einstein's "year of miracles," when he published five papers on physics any one of which might have won him a Nobel Prize. One of the papers helped lay the foundations of quantum physics; two confirmed the nature and dimensions of molecules; one established his theory of special relativity; and one debuted his famous equation E=mc2, the equivalence of matter and energy.

Einstein's genius was being able to imagine what no one had imagined before -- or saw any reason to imagine. He was able to imagine, for example, that a cantaloupe-sized ball of uranium or plutonium might yield enough energy to blow up a city. And if that wasn't incomprehensible, what is?

The idea was given the ultimate test in the New Mexico desert on the morning of July 16, 1945. We've heard plenty about the test called Trinity, but few people know that what what was most important about the explosion was not what happened, but what didn't happen.

The chain reaction did not get out of control. The atmosphere and oceans did not ignite. The world did not end in a planet-enveloping blaze of light. A few dozen physicists bet everything -- all life on the planet! -- on their ability to calculate in advance the result of an experiment that had never been tried before.

Even Hitler blanched at the stakes.

Early in the war -- the spring of 1942 -- German physicists apprised Hitler, through his Minister for Armaments and War Production, Albert Speer, of the possibility of constructing a nuclear bomb. Speer asked Werner Heisenberg, spokesman for the German nuclear scientists, whether a successful nuclear explosion could be kept under control with absolute certainty, or whether it might continue through the atmosphere as a chain reaction. According to Speer, Heisenberg hedged.

Speer wrote in his memoirs: "Hitler was plainly not delighted with the possibility that the earth under his rule might be transformed into a glowing star."

A few months later, in the summer of 1942, American physicists considered the same awesome possibility. By then they knew that a fission bomb was possible. Atoms of uranium 235 are inherently unstable. When one breaks apart (fissions) spontaneously, a tiny amount of energy is released -- along with an average of two subatomic particles called neutrons. The neutrons can collide with other uranium 235 atoms and cause them to break apart. With the right amount of uranium -- a cantaloupe-sized "critical mass" -- a chain reaction of fissioning atoms will release a staggering amount of energy in a tiny fraction of a second.

Physicist Edward Teller considered another possibility. The huge temperature of a fission explosion -- tens of millions of degrees -- could fuse together nuclei of light elements, such as hydrogen, a process that also releases energy (later, this insight would be the basis for hydrogen bombs). If the temperature of a detonation was high enough, nitrogen atoms in the atmosphere would fuse, releasing energy. Ignition of atmospheric nitrogen might cause hydrogen in the oceans to fuse. The Trinity experiment might inadvertently turn the entire planet into a chain-reaction fusion bomb.

Robert Oppenheimer, chief of the American atomic scientists, took Teller's suggestion seriously. He discussed it with Arthur Compton, another leading physicist. "This would be the ultimate catastrophe," wrote Compton. "Better to accept the slavery of the Nazis than run a chance of drawing the final curtain on mankind!"

Oppenheimer asked Hans Bethe and other physicists to check their calculations of the ignition temperature of nitrogen and the cooling effects expected in the fireball of a nuclear bomb. The new calculations indicated that an atmospheric conflagration was impossible.

Later, Teller wrote of those heady days: "The discussions were fascinating and intense. Facts were questioned and the questions were answered by still more facts...A spirit of spontaneity, adventure, and surprise prevailed during those weeks...and each member of the group helped move the discussion toward a positive conclusion."

Three years later, in the New Mexico desert, there was enough uncertainty about the outcome of the experiment to make a betting pool interesting. Senior scientists each put a dollar in the kitty. Edward Teller bet the bomb would pack the explosive equivalent of 45,000 tons of TNT. Hans Bethe picked 8,000 tons. Oppenheimer chose a modest 300 tons. All of the scientists were utterly convinced that even the most optimistic estimate of the bomb's power would not produce temperatures high enough to ignite the atmosphere.

Nevertheless, the terrible possibility was on people's minds. Enrico Fermi, one of the most brilliant of the atomic scientists, offered to take bets on whether or not the bomb would ignite the atmosphere, and if so, whether it would merely destroy New Mexico or the entire world. His macabre humor was not appreciated.

When the bomb exploded, the confidence of at least one physicist was briefly tested. Emilio Segre, an eyewitness and nuclear scientist, wrote: "We saw the whole sky flash with unbelievable brightness in spite of the very dark glasses we wore...I believe that for a moment I thought the explosion might set fire to the atmosphere and thus finish the earth, even though I knew that this was not possible."

Not possible. Why?

Because the calculations said so. Calculations scribbled on blackboards and countless pads of paper. Calculations based on mathematical theories of the atom, of gas mechanics, of thermodynamics, of electromagnetism. Calculations based on three hundred years of experiments. Calculations based on the incomprehensible premise that the human mind can comprehend the fundamental architecture of the universe.

Further Reading

To my mind, far and away the best book on the development and testing of the atomic bomb is Richard Rhodes' The Making of the Atomic Bomb. His Dark Sun: The Making of the Hydrogen Bomb is powerful reading too.

Discuss this essay and more over on the Science Musings Blog.