Like the scientists first assumed in the movie ‘Oppenheimer’.
In: Physics
At the time nuclear physics was in its infancy. Scientists understood that large unstable atoms could trigger the nuclear decay of other unstable atoms in an ongoing process called **fission**, producing smaller byproducts and the release of massive amounts of energy. Similarly it was known that light atoms could be made to fuse together under massive temperatures and pressures to form larger byproducts along with obscene amounts of energy. This process was called **fusion**.
Fission could be made to occur by simply piling enough large radioactive atoms close together. In fact that is what the first nuclear reactors were called, “nuclear piles”. It could also occur naturally in certain geological formations on Earth! Fusion though required such temperatures and pressures that it only happened in stars, not on Earth.
The problem was that by building and detonating a nuclear bomb they would be creating conditions on Earth that had never occurred before. The explosion would make a blast wave that was hotter and more compressed than Earth’s atmosphere had ever encountered. Our atmosphere also contains significant amounts of relatively light atoms such as nitrogen.
What some people pondered was if that hot, super-compressed shock wave could possibly reach the conditions required to fuse nitrogen or similar components together, releasing more energy to just make the shock wave hotter and more compressed. If it could do that it might create a self-sustaining fusion reaction that would spread across the entire atmosphere!
Those scientists familiar with the required energies could dismiss the idea without even doing any math, but there were not very many such scientists and considering the stakes they actually did do the math. It turns out there was no real danger, the required conditions and energy released by such a fusion reaction would not happen and couldn’t be self-sustaining.
It was a “what if” theory like “is LHC is going to create an Earth-destroying black hole”.
Calculations quickly showed this was impossible, and the theory only remained in anti-nuke propaganda. Movie further exaggerates it for cinematic effect.
Igniting the oxygen-nitrogen atmosphere in the assumed way requires more energy than is produced from it burning, so this reaction can’t continue on its own.
The scientists never assumed that. They never predicted the possibility and no one was ever worried. This is a story that has gotten blown way out of proportion since the Manhattan project and won’t go away, and the Manhattan Project scientists have said as much. There was never any real concern that this could happen. It was simply a case of a “theoretical scientists being theoretical scientists” and doing math on wildly unrealistic but untested scenarios, and the story has taken on almost mythical proportions.
Very early on in their theoretical research, understanding of nuclear processes were still incomplete. I don’t recall who (I believe it was Hans Bethe but I might be wrong) realized that they didn’t have enough understanding to rule it out the possibility that the extreme pressures and temperatures from the nuclear explosion could start a runaway fusion process of the nitrogen in the atmosphere. Pretty quickly after that, they gained some more understanding and had the math to be able to calculate that it was actually 100% impossible.
Over the years, people have misinterpreted “they did the math on this scenario” to mean “they were worried it might happen,” which is simply not true. They simply did the math because that’s what scientists do, and quickly realized it *wasn’t* a possibility. Oppenheimer portrays this for dramatic effect.
The bomb was tiny and the atmosphere is very big.
It did explode the air around the detonation but not to any degree larger than its blast radius.
To understand why it *didn’t*, it’s helpful to understand why it *might have*.
Besides radioactive decay, there are two main types of nuclear reaction – fission (splitting a big nucleus into smaller ones) and fusion (joining small nuclei into bigger ones). The sun runs on fusion. Nuclear power and these early nuclear bombs run on fission.
Fusion requires very high temperatures and very high pressures. The scientists on the Manhattan Project were worried that an atomic bomb would briefly provide enough pressure and temperature to cause atmospheric gases to undergo fusion.
If the atomic bomb had caused fusion, and caused enough of it, then that fusion would have released more heat, expanding the zone of high temperature and pressure, creating still more fusion, and so on in a chain reaction that would encompass the whole globe.
When they calculated more carefully, they realised that the chance of this happening was miniscule, and it wasn’t a realistic worry.
That’s not to say fission bombs never cause fusion:
* Probably, in every atomic explosion, there are some fusion reactions happening nearby. Just nowhere near enough to cause a chain reaction.
* However, the way fusion bombs work is by redirecting all the energy of a fission bomb onto one specific spot, creating enough temperature and pressure to cause a large amount of fusion and therefore a bigger explosion. This doesn’t cause a runaway effect because the bomb is vaporised pretty quickly and the focusing mechanism is destroyed.
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