Fun Fact - Set the Atmosphere on Fire

Could we, deliberately or accidentally, extinguish all life on earth? Is that within our capabilities? Certainly we can wipe out species at an alarming rate, and we might even drive ourselves into extinction, but what about insects, bacteria, algae, and those tube worms down by the ocean floor? Bacteria can survive under the Antarctic ice, in subterranean chambers, in bubbling hot springs, in sulfuric acid, in radioactive waste, and in the vacuum of space for at least a year. We could never wipe away all life on earth, could we?

On July 16, 1945, a team of dedicated scientists detonated the first atomic bomb at Los Alamos, New Mexico. Soon thereafter, two such bombs were dropped on Japan in an effort to end World War II. Fourteen years later, Arthur Comton gave an interview where he suggested that the physicists of the day had not adequately considered the possibility that the bomb would trigger runaway fusion in the atmosphere and incinerate all life on earth. Or perhaps he was misinterpreted by the media - it's hard to say. He did say that Enrico Fermi jokingly took bets on whether the atmosphere would ignite just minutes before the Trinity test took place. These stories spread throughout the literature, supporting the common view that geniuses are idiots in other realms, (like Sheldon Cooper in The Big Bang Theory), and they just might be reckless enough to obliterate life on earth in the blind pursuit of technology. This is false at several levels. Nobel prize winners are often brilliant in several fields, and competent in many more, including the social sciences. They are no more likely to jeopardize the future of life on earth than you or me. Thus these reports are largely apocryphal, but there is a story behind the story.

In 1942 it was not clear that the United States, or any other country, could develop the technology needed to build a nuclear bomb, but just in case, Edward Teller asked what would happen if such a bomb exploded in the air. Would it trigger a runaway fusion reaction? Would it literally set the atmosphere on fire and end all life on earth? Certainly this was a question worth asking.

Here is the nuclear reaction Teller was worried about.

N + N = C + O

Each side of the equation contains 14 protons and 14 neutrons, but look at the atomic weights. Nitrogen 14 has an atomic weight of 14.003, carbon 12 weighs 12.000, and oxygen 16 weighs 15.994. The left side is 0.012 heavier than the right, hence energy is released. If two nitrogen nuclei are pressed together by heat and pressure, they will fuse into carbon and oxygen, converting 0.043% of their rest mass into energy. There is approximately one kilogram of air over each square centimeter of earth, containing 0.8 kg of nitrogen. If fused as above, this would release 0.8 times 0.043% times 3E82 = 3E13 joules, as per Einstein's equation e = mc2. This is almost half of a Hiroshima bomb, exploding over each square centimeter of the earth. All terrestrial life is obliterated in a matter of minutes. Furthermore, the heat and pressure might cause the hydrogen in the oceans to fuse into helium, burning off all the water. On the other hand, if water and transmuted air remained, the resulting carbon dioxide atmosphere could produce a runaway greenhouse effect, as seen on Venus. Any remaining water would boil away before deep sea life could evolve into new terrestrial forms, assuming such life somehow survived the heat and the blast in the first place. The Earth becomes completely sterile, like Venus, though possessing a much thinner CO2 atmosphere. As Compton declared, "Better to accept the slavery of the Nazis than to run the chance of drawing the final curtain on mankind!"

When Teller posed the question in 1942, Hans Bethe quickly proved it was impossible. Others, including Konopinski, the premier authority in the field, confirmed Bethe's calculations, and the Manhattan project continued on schedule. Fermi joked about atmospheric ignition just before the Trinity test to relieve the tension, but he too was absolutely sure there was no risk to mankind.

It is startling to think that these men performed a calculation that determined the fate of the world, and they were, in Bethe's words, "absolutely certain" of the science behind these calculations, confident enough to proceed with the test. Those were men of faith!

Here is some justification, without all the math. The sun's core attains a temperature of 15 million degrees, under a pressure of 250 billion atmospheres. This is sufficient to fuse hydrogen into helium, but further nuclear reactions remain out of reach. Protons can come close enough to merge, but two nitrogen nuclei possess 7 times the electric charge, and 49 times the repulsive force. More pressure, or a higher temperature, is required to press these nuclei together. When a bomb goes off in our atmosphere, million degree temperatures are produced, but there is no containing pressure. Fusion does not occur, and even if it did, it would quickly spread out and fizzle. The atmosphere will not ignite in a chain reaction. Cities are destroyed, but life on earth continues.

There is another way we might destroy all life on earth, nay, the very planet itself - or so it seemed until some calculations were performed. Particle accelerators attain ever higher energies; what if we accidentally create a microscopic black hole. Could this black hole accrete the surrounding matter, atom by atom, grain by grain, kilogram by kilogram, ton by ton, until it swallowed the entire earth? The third object from the sun possesses the mass of the earth, but it has become an invisible black hole with the moon in tow, and there is of course nobody left to see it. Maybe we should put away our toys.

Physicists quickly pointed out that nature runs these experiments every day in the upper atmosphere, and the earth has never been consumed. Cosmic rays have more energy than we will ever muster in our accelerators. They slam into molecules of air and produce all sorts of particles, but none of these events leads to the destruction of the planet, or even the atmosphere.

If a microscopic black hole formed in the wake of a subatomic collision, it would evaporate away before it had time to attract another particle. This process was explained by Stephen Hawking in 1974. Within a vacuum, pairs of particles wink in and out of existence in a quantum foam. If one photon appears inside the event horizon and the other photon lies outside the event horizon, the first remains inside the black hole forever, while the second escapes in the form of radiation. These exiting photons give the black hole a temperature, which is inversely related to mass. The black hole at the center of our galaxy, with a mass of 4 million suns, emits barely a trickle of photons, as though it had a temperature of 1.5E-14 degrees kelvin. In contrast, a black hole with a microscopic mass is blazing hot, spewing out gamma rays and evaporating away in a flash. The black hole is gone before it can accrete any nearby matter, and the earth is safe. Indeed, the Large Hadron Collider at Cern may try to create these micro black holes and detect their gamma ray bursts as they evaporate away.

Micro black holes are not the only consideration. A particle collision could, theoretically, create a tiny fragment of strange matter, comprising approximately equal numbers of up, down, and strange quarks. A strangelet could then convert all the matter in the earth into strange matter, somewhat like ice-nine, a fictional form of ice in Kurt Vonnegut's novel Cat's Cradle that exists at room temperature and converts surrounding water into ice-nine on contact, including, perhaps, all the water in the oceans if a cube is casually tossed into the waves. Once again, calculations show that strangelets will not be produced, and if they could be produced then cosmic rays would do so in our upper atmosphere.

It is fortunate that the laws of nature prevent us from destroying ourselves and our world through a single, catastrophic experiment. It's hard to believe we could refrain from such activities forever. Of course we can still harm ourselves and our biosphere over time, through short-sighted politics and unbridled greed.

Further Reading

Bacteria in hot springs, sulfuric acid, and the vacuum of space
Interview with Hans Bethe regarding the bomb and the atmosphere
Isotopes and atomic weights
The Hiroshima bomb
Hawking radiation from a black hole
Particle accelerators, risks and doomsday scenarios
An experiment gone awry