[gpm]

> Or it might have ignited a fusion reaction in the atmosphere and destroyed the world

Someone's got to explain to me how this was even remotely plausible.

We've had orders of magnitude more energetic events in earth's history that they would be aware of (dinosaur slaying asteroid for instance). These didn't manage to destroy the earth by turning the atmosphere into a fusion reactor. Surely they were aware of this.

So was the theory that neutrons are somehow special in a non-thermal way for causing fusion (not fission). And specifically that a concentrated neutron burst could somehow set off a chain reaction? And I guess that [edit: solar] neutrons weren't concentrated enough to cause this even at a detectable level?

[wnissen]

You might be surprised at how little we know about fusion. We can observe the sun, but the sun is already very hot, millions of degrees, so any unknown fusion reactions would have already happened. Nowadays we have high-powered lasers that can create laboratory-scale fusion reactions.

E. O. Lawrence's 1930 cyclotron could generate protons at roughly a million degrees Celsius. But that's a single proton stream. Good for splitting atoms but not for fusing them. You really don't know what the cross section of a fusion reaction is until you do it. The properties of matter at that temperature are just super weird. If it had turned out that there was, e.g., a carbon-carbon fusion reaction with a lower initiation, that might be enough to "go critical" and kick off more fusions, and propagate around the world. According to estimates, the Chicxulub crater was 1-10,000 degrees C. Not even the same ballpark.

https://www2.lbl.gov/abc/wallchart/chapters/11/4.html

[queuebert]

We didn't know as much about possible nuclear reactions back then, so I think they thought there was a possibility that there was an exothermic chain involving N or O that could be ignited by the bomb and would be self sustaining. While an asteroid impact is very powerful over large scales, it doesn't create nuclear reactions, so Trinity was indeed a first at that scale.

But, and I'm not sure how much of this they knew back then, we do get bombarded by high-energy cosmic rays, so chances are one of these hypothetical N or O reactions should've already randomly occurred at least in isolated events over the last few billion years if possible.

[jeremyjh]

> Someone's got to explain to me how this was even remotely plausible.

You need to understand what a nuclear chain reaction is. https://en.wikipedia.org/wiki/Nuclear_chain_reaction

> We've had orders of magnitude more energetic events in earth's history

It isn't about energy. There was never an unbounded nuclear chain reaction of anywhere near this magnitude on the planet before Trinity. A large asteroid impact doesn't cause a nuclear chain reaction at all. The moon impact melted the entire crust but didn't cause a nuclear chain reaction.

In fact, the only chain reaction that happened at all before Fermi's experiment in 1942 - that we know of - was in Oklo (now Gabon) about 1.8 billion years ago. We didn't learn about that until 1972, and anyway that was more like a controlled reactor pile and it only happened because there was so much more Uranium 235 so early in the Earth's history.

The event at Trinity was completely different because so many neutrons were released at exactly the same instant. They had good reasons to be very confident in their models and calculations, but they were not 100% sure, and as TFA points out, the blast was several times more powerful than most models predicted.

[dredmorbius]

Worth noting that the along with other such events, the Giant Impact Hypothesis (a/k/a the Theia Impact) now thought to have created the Earth-Moon system had little traction until the 1970s, wasn't seriously discussed until the 1980s, and so far as I recall wasn't widely accepted until more recently, possibly the 2000s / 2010s, when I'd first encountered it.

As with other aspects of Earth's history, what's possibly surprising to younger readers here is just how much of Earth's history and the evolution of the Solar System has been uncovered recently. The far side of the Moon was unseen by humans (save for a few percent through lunar wobble / liberation) until the mid-1960s, and wasn't accurately mapped until 1969 --- a version of that map decorated my own bedroom wall as a child, and the story of that map's creation is amazing in itself, see "Race To The Moon with Richard Furno" <https://web.archive.org/web/20090129220141/https://kelsocart...> and <https://web.archive.org/web/20090130150935/https://kelsocart...>.

Close observations of other planets only began in the 1970s and is still fairly thin, though long-duration missions to Jupiter and Saturn have been impressive, though recent. The Jupiter Juno probe mission is still ongoing, having begun in 2016, and the Saturn Cassini-Huygens mission reached its destination in 2004 and culiminated (with a "grand finale" plunge into Saturn's atmosphere) in 2017.

The age of the Earth itself wasn't established until 1956, and at the time of the Trinity tests was still estimated as between 1.6 to 3.0 billion years (the presently-accepted age is 4.54 +/- 0.05 billion years). Plate tectonics wasn't accepted as the principle theory of terrestrial geology until the late 1960s. I've recently seen that this occurred between Neal Armstrong's flights on the Gemini and Apollo manned space programmes, the latter of course being his famous first footsteps (by man, that we know of) on the Moon.

But yes, as others are quite correctly noting here, there's been a huge advance in human knowledge within the span of living memory on all matter of related questions: nuclear reactions, fission, fusion, age of the Earth, major events within Earth's history, its own formation, the forces driving its evolution, and more. Looking back with the lens of the state of knowledge in 2026 is highly deceptive.

[libraryofbabel]

People that know more about nuclear physics than I do already answered, but I’ll just say that:

1) It’s easy to think about the past in terms of what we now know, and it involves a real effort to put yourself in the shoes of the people living at the time and to imagine the “fog of war” in what they knew. In 1945 nobody had ever tested a nuclear explosion before and there was still all sorts of uncertainty about it. And as one of the other commenters pointed out, in particular there was a lot of uncertainty about how fusion worked.

2) The center of the Trinity fireball did in fact produce hotter temperatures than had ever existed on Earth before. Temperature and energy being different things.

In some sense the final experimental proof that a nuclear explosion would not set off some unanticipated new chain reaction that would destroy the earth - unlikely, but hard to completely disprove - was Trinity itself. Only after Trinity is it obvious and completely proven how the physics actually worked and obvious that there were no additional reaction pathways that got missed. That is a disturbing thought.

[generuso]

There are different versions of the story. In one of them, somebody asked the question whether the atmosphere could ignite, and that was very quickly answered in the negative, but then Oppenheimer mentioned it to the people in Washington, and after that the question recurred periodically because the higher ups got unduly alarmed.

And then of course there are versions making it into a much more dramatic story.

When they were working on the fusion bomb (and Edward Teller was working on fusion full time already during the Manhattan project), it took some years to establish that even the "easy" to fuse deuterium cannot be set of by simply blowing up a fission bomb. The reaction simply did not propagate for any reasonable dimensions of the system. For any other material the energy balance would have been orders of magnitude short of what was required for a propagating fusion burn.

[twoodfin]

I think some people miss the Nolan film’s central irony:

By the conclusion, the joke about the “near zero” physical possibility of the end of the world has transmuted in Oppenheimer’s mind into certainty that Trinity ignited social forces leading to the same doom.

[P1h3r1e3d13]

You made me curious. It seems the theory is thermal, but it was never taken that seriously.

Bethe called it “absolute nonsense”—interview: https://johnhorgan.org/cross-check/oppenheimer-bethe-and-the...

Here's a student paper giving a pretty thorough analysis (AFAICT): http://large.stanford.edu/courses/2015/ph241/chung1/

Here's more discussion of it: https://history.stackexchange.com/questions/52802/where-did-...

And here (drumroll) is the original paper: https://web.archive.org/web/20200331041344/https://fas.org/s...

[voidmain]

I think the tiny size of a nuclear weapon and very short interval of nuclear reaction before "disassembly" mean that even though the energy release is small compared to an asteroid impact the temperatures are probably much higher.

(I'm not an expert, though, this is a guess)

[kkylin]

Slightly off topic (since there's already been so many better answers than I can write), but our knowledge of the dino-slaying asteroid came much later: https://en.wikipedia.org/wiki/Alvarez_hypothesis

[fluidcruft]

Dinosaur-slaying asteroid theory comes from 1980s so it probably wouldn't have crossed their minds.