created by dripsonic on 15/05/2014 at 12:29 UTC
2 upvotes, 5 top-level comments (showing 5)
This[1] wikipedia article about the manhattan project piqued my curiosity when it mentioned that one of the members of the team was concerned that a nuclear explosion might "ignite" the atmosphere. So, please ELI5: What do they mean by "ignite" the atmosphere? Why was it a concern? Why didn't (doesn't) it happen? Thanks
1: http://en.wikipedia.org/wiki/Manhattan_Project#Bomb_design_concepts
Comment by restricteddata at 15/05/2014 at 13:03 UTC
5 upvotes, 0 direct replies
The fear that was briefly held is that it would heat the surrounding air enough for the nitrogen in it to fuse. If this happened, they thought, this might release enough energy for even more nitrogen to fuse. And so on, in a self-sustaining fusion reaction across the surface of the planet.
But it didn't take much calculation to figure out that not only is it hard to make nitrogen fuse (a lot harder than, say, isotopes of hydrogen and lithium, which are what are used in H-bombs), but the reaction would not be self-sustaining at all. Any fusion that did occur would not generate enough heat for the reaction to continue; there are lots of processes by which the heat gets sapped away. In fact making thermonuclear reactions is very very hard even under very controlled conditions.
They did calculations that showed that even if you assume the reactivity of nitrogen is several orders of magnitude higher — that is, if it were thousands of times easier to make nitrogen fuse than it actually is — the heat losses would still be so significant that no reaction of consequence would take place.
How seriously did they take it? I don't think most of them took it very seriously — it was a "flight of fancy" idea that sounded cool at first, but didn't stand up to much scrutiny. However it did stick around as an amusing rumor, sort of a black joke, until the Trinity test.
Comment by Hiddencamper at 15/05/2014 at 13:32 UTC
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The calculation for this is no longer classified and is publicly available (but a bit difficult to find).
What they found was the energy released by fusion of particles in the atmosphere was technically enough to cause more fusion to occur, when you account for the fact that energy will rapidly disperse in all directions and there will be heat losses, there isn't sufficient energy density to make a self sustaining reaction capable of igniting the atmosphere.
Comment by justthistwicenomore at 15/05/2014 at 14:29 UTC
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So, the basic idea of a nuclear bomb is based on the fact that certain materials "decay" which means that their atoms naturally break apart, and emit high energy particles. When these high energy particles hit other atoms, those atoms too can "break apart."
Imagine bunch of mouse-traps, each with a ping pong ball on the part that swings down when it's triggered. They're fairly stable just sitting there, but if one triggers, it will fling it's ball into the air, landing on another, and another, until all the traps go off.
That's sort of like the uncontrolled "chain reaction" in the material that is the "core" of the bomb. So, for a uranium bomb, you use explosives to keep pushing uranium closer and closer together, until the particles the atoms emit are so numerous and close together that a big part of that lump of uranium "triggers." Since each time one of those particles is released, a little bit of matter is converted into pure energy, you get a huge explosion.
The fear was that the explosion would be so powerful that the released heat would compress something in the atmosphere, like nitrogen, and that nitrogen would be so compressed that it would start it's own chain reaction, releasing enough energy to push more nitrogen together, and on and on. If that happened, the fear was that it would spread through the whole rest of the atmosphere, until all the nitrogen was exhausted. Fortunately, nitrogen is too far apart, and the energy spreads out too much, for that to have happened.
Comment by [deleted] at 15/05/2014 at 12:31 UTC
1 upvotes, 0 direct replies
Looking at the reaction, you might want to take it to /r/askscience...
Comment by [deleted] at 15/05/2014 at 12:39 UTC
1 upvotes, 0 direct replies
I think the concern was that all that energy would trigger a self-sustaining fusion reaction like in the sun. However, it did not happen for the same reason the Earth is not already a sun - it's far below the critical mass/density needed. Its far too easy for the energy to dissipate in our (relatively) thin atmosphere as heat and pressure waves, so the energy does not stay at a high enough level to force atomic nuclei together.
That said... Despite their worries, I doubt any of them took the possibility very seriously, or else I don't think they would have tested the bomb in the first place.