Did the non-zero risk of the atmosphere igniting increase as nuclear weapon yields got larger and larger?
Obviously a result of watching Oppenheimer.
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The atmosphere is full of nitrogen which could fuse during very very high temperature. Fusion of nitrogen would release energy, which means that if the energy generation from fusion would surpass the energy lost as heat radiation then the atmosphere would basically keep fusing and creating more and more energy eventually destroying the earth.
That energy balance between energy gain from fusion and energy lost from radiation was what the manhatten scientists calculated. There’s a video going into more details about the subject: [https://www.youtube.com/watch?v=nD-Dco7xSSU](https://www.youtube.com/watch?v=nD-Dco7xSSU)
The only way that the atmosphere could “ignite” (it’s not actually igniting that they were worried about) would be if the gases in the atmosphere had the right properties to undergo a self-sustained nuclear reaction that would keep propagating until the atmosphere ran out of fuel if given a triggering event.
The calculation on whether a substance is capable of self-sustained nuclear reactions upon receiving neutron inputs is fairly straightforward in concept (albeit using complicated math); basically, for a self-sustained reaction to happen, the two follow conditions must be true:
1: When an energetic neutron from a nuclear bomb strikes an atom in the atmosphere, can it induce a nuclear reaction in that atom?
2: If so, are the chances high enough and the number of energetic neutrons emitted by the induced reaction high enough to, on average, induce a reaction in at least one other atom (which would then itself trigger other reactions, and so on)
If these conditions are true, then nuclear reactions would propagate endlessly throughout the atmosphere until the atmosphere is sufficiently depleted so that each nuclear reaction causes less reactions than what caused it.
If either are false, then the reaction will peter out very quickly and not really do anything outside of the fission in the nuclear bomb. They calculated that while point 1 is sometimes true, point 2 is always far, far, far lower than what is needed to self-sustain a nuclear reaction in the context of the atmosphere.
If you pumped enough energy into air (or really, pretty much anything) the atoms that make up the air will fuse together to make heavier elements (Nitrogen combining to make either magnesium and helium or oxygen and carbon). This would release a lot of energy, the same mechanism that makes a hydrogen bomb work. This energy could (potentially) fuel further nitrogen-nitrogen reactions, releasing more energy and so on into an every increasing explosion. This is what they were worried about.
However reaction requires a LOT of energy. As the explosion happens, it expands outward in a sphere, and as it expands the energy spreads out so that the “local” energy in any particular spot drops quickly. Moreover, only a portion (a tiny portion typically) will actually go into making the nitrogen atoms collide and collisions would actually be pretty rare, the actual atoms in air are pretty spread out.
The Mahhatan project scientists calculated the very upper limit for the energies that a bomb would create, along with the very lower bound for when fusion would occur in the atmosphere. From those calculations, they determine that the atomic bomb would not have nearly enough energy to fuse nitrogen. Furthermore, they also calculated that even if nitrogen fusion occurred, the reaction would fizzle out and it would not be enough to fuse more nitrogen. The world was safe.
Kyle Hill just did a video on this and goes through science if you want more background on it. https://youtu.be/2QI88aLyaOs
Earth-like atmospheres don’t ignite, or wildfires would wreak havoc on a global scale every time they happened.
And on a nuclear-level, any nuclear explosion that could cause a chain reaction in Earth-like atmospheres would be so powerful you wouldn’t be able to use it even on your enemy on the other side of the world without direct damage to yourself (and the atmosphere would be quite a minor issue at that point).
They invented the thing using maths/physics, and to work out the possibilities they used maths/physics, like people do every single day somewhere in the world. They also tested small-scale dozens of times to confirm their numbers were correct and any major difference would have sent them back to their chalkboards.
These people were designing never-before-seen weapons. They weren’t idiots. Quite the opposite. Whether you agree with their purpose or not, they were geniuses to even work out it was possible in the first place, so running some fanciful numbers on a par with “if you move on a train over 30mph, you’ll suffocate because the oxygen will be stripped away” (another “belief” held by many people until someone did the maths/physics/engineering and then laughed at the entire principle and built a machine capable of that speed to prove it) was basically a doodle in the margin in comparison.
They found out their atomic bomb wouldn’t ignite the atmosphere, after the first atomic explosion didn’t ignite the Earth’s atmosphere
Up until then, no one was absolutely sure it wouldn’t happen
Conclusion: Energy not sufficient enough to initiate a sustained fusion reaction in Earths atmosphere.
During the early stages of developing atomic weapons, scientists, including Edward Teller, who later played a key role in developing the hydrogen bomb, raised concerns about the potential for a nuclear explosion to ignite the Earth’s atmosphere. The concern stemmed from the idea that the extremely high temperatures and energy released during a nuclear detonation might trigger a self-sustaining fusion reaction in the atmosphere, similar to what happens in a hydrogen bomb. This could potentially lead to a catastrophic chain reaction engulfing the Earth. To address this concern, scientists like Hans Bethe and Enrico Fermi performed theoretical calculations to understand the conditions necessary for a runaway nuclear reaction. **They concluded that while a nuclear explosion could indeed produce extremely high temperatures and pressures, the energy released in such an event would not be sufficient to initiate a self-sustaining fusion reaction in the Earth’s atmosphere. The energy requirements for such a reaction far exceeded what was achievable with the available nuclear weapons.**
Theoretical calculations were supported by experimental evidence from early nuclear tests. These tests showed that the energy released in atomic explosions was orders of magnitude less than what would be needed to ignite the atmosphere. In later years, as computer simulations and modeling became available, more sophisticated analyses were conducted to confirm the safety of nuclear testing. These simulations reaffirmed the earlier conclusions that a runaway atmospheric reaction was not a credible threat.
They never predicted the possibility so much as initially, they couldn’t 100% rule it out. The thought was that the temperatures from the detonation could start to fuse nitrogen in the atmosphere, but they very quickly realized A) that wasn’t possible and B) even if it did, the reaction wouldn’t be self sustaining and overtake the whole atmosphere because nitrogen doesn’t work that way.
The actual Manhattan project scientists in the years after the project have said that this story has been blown way out of proportion. There was never any *real* concern that this could happen. It mas more of a “theoretical scientists being theoretical scientists” and doing math on wild unrealistic scenarios.
When the bomb go boom boom the tiny itty bitty parts are too itty bitty, so its really unlikely they hit other atoms to make more boom boom.
You know how a log can be hard to make it catch on fire? Now imagine the atmosphere is a nice tinder pile of logs. Normally nothing would happen but what if someone lit a match and then made one spot hot enough that it made enough heat from burning that the spot next to it get hot enough to burn and now you get more and more of the logs on fire.
The nuclear reactions in an atomic bomb are similar but instead of a match you have a neutron. Why a neutron? Because they are heavy and are not electrically charged. So if you shoot one at an atom it’s like a heavy bullet. It can make it through the cloud of electrons (negative charged almost mass less particles) that form a shield around the tiny little core (nucleus) of the atom. If that neutron hits the core which is a mix of protons and neutrons in a tight ball, then it can shatter it into pieces and depending on how it shatters it can release some neutrons at high velocity. If more than two are ejected then they can repeat that process with two other nucleuses and now there are 4 neutrons. This can keep going and now you have a runaway reaction and if the atmosphere can do this it will burn until it can’t anymore.
Just like with the pile of tinder as anyone that’s lit a fire, sometimes the fire is not concentrated enough and the amount of heat lost to the air is enough that the fire stops and some logs don’t burn.
The same can happen with a nuclear reaction. If some of those neutrons start to miss other nucleuses because things are too far apart or because they get too slow and can’t break them or because they are too fast and don’t break them right or hit nucleuses that don’t break or break different then the reaction fizzes out and it stops.
So it looks plausible when we didn’t have enough data that a very big nuclear reaction might light up the atmosphere. In practice though they knew that the atmosphere is hit by lots of very energetic (really fast bullets) all the time from outer space and from experiments they had done with devices designed to create very fast particles so they really didn’t think it was a concern but it makes for a good story lol.
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