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I was watching a movie which is mostly a fun fantasy romp that bears no relation to science whatsoever but there is one scene towards the end that got me thinking; a character is sitting in a pod about 5 feet from a 200 megaton nuclear weapon, we see it count down, hear the capacitors charge and fade to white and cut to the pod exploding from the outside. Now in the scenario the character most likely has a moment to realise what's about to befall them, especially since they helped put the weapon there in the first place.

My question is if someone was sitting next to a nuclear weapon they didn't know was armed and it went off would they have time to realise they were going to die or would the criticality happen fast enough to spare them that knowledge.

I understand that gun type weapons take the longest to go critical from a standing start so I'd like to focus on them; if you were sitting next to such a weapon and heard the propellant charge go off would there be enough time to realise what was about to happen before the weapon went critical and vapourised you or would you be dead too fast to realise that the warhead next to you was live and had just gone off?

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  • $\begingroup$ The Wikipedia page suggests that it takes some tens of milliseconds for the bomb to go critical and explode; this is of the same order of magnitude as the time needed for a stimulus to propagate to the cortex. So, I'd say that it is possible, or at least believable. On the other hand, there would not be enough time to send a command from the brain to any muscle... again possibly with the exception of some muscles very close to the brain, such as the oculomotor muscles. $\endgroup$ – AlexP Jun 2 '18 at 19:08
  • $\begingroup$ @AlexP It also says it takes 100 shakes and 1.35milliseconds in different parts of the article, that's a big part of the problem I'm having is knowing what numbers to use to try and work this out. But based on that estimate you'd have just long enough, that's nasty. $\endgroup$ – Ash Jun 2 '18 at 19:11
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    $\begingroup$ Human reaction times are probably too slow for someone to consciously be aware a nuclear weapon was about to detonate. So Im on the side of no. For dramatic purposes, I'd suggest adding a device that plays a detonation and then counts down to the moment of its explosion. A few whirs and clicks as it arms itself would add to the drama. $\endgroup$ – a4android Jun 3 '18 at 4:58
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I Have Exploded A Few Times

Once my truck ran over an old Russian antipersonnel mine left over from the time Russia tried to invade Afghanistan. It was a bouncing type antipersonnel mine, and exploded underneath the hull of out armored truck. I heard the propellant charge fling the thing out of it's casing, I heard it clang off the bottom of the hull, then next thing I knew my ears were ringing and I had a nosebleed from the over pressure.

The second time somebody fired a rocket at our truck and it hit the engine block. I saw the RPG fire, then next thing I know I'm laying on the floor of our armored vehicle with a heavy concussion and a messed up face from headbutting the machine-gun really hard, I'm not sure if that happened due to the blast or the driver crashing into a wall at 45 miles per hour because I was unconscious already when I did it. The third time we ran over an IED and there was no warning, I just was launched out of my seat into the ceiling and ended up with a severe concussion and a whip lashed neck again (wasn't wearing my seat belt because being blown up twice already apparently didn't teach me anything.)

In the first two cases there was a microsecond between realizing I was going to explode then exploding. The third time there was zero warning, I just woke up feeling like crap. In the prior two there was juuuuuust enough time to abstractly register that something bad was going to happen but that's about it. It wasn't like a "in that moment he knew..." type moment when you wreck your bike or trip over something. Its like just a microsecond of realizing something is wrong then lights out. Its really hard to explain properly, but you don't even have time to form coherent realizations or complex thoughts, its just a microsecond of panic and then its over.

Theoretically you could hear something happen with the nuclear device's arming mechanism a split-second before it went off, especially if it were an older analog operated gun type device. I imagine quite a few things have to happen at once within a split second for them to detonate and you could theoretically have enough time to be ever so briefly startled before you died.

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    $\begingroup$ There's nothing like a good answer based on personal experience. It also gives insight into the experience of war. I'm glad you survived after all that. Plus one. $\endgroup$ – a4android Jun 3 '18 at 5:00
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The speed of sound is about 1125 ft/s (depending on temperature and humidity) At 5 feet, it takes about 5.625 ms for the sound of a trigger to reach.

Converserly, given that a nuclear explosion velocities range from 15k ft/s to greater than 50k ft/s, a delay of ms guarantees the blast outruns the trigger.

This site shows that the fireball of a nuclear explosion is already 100 ft in diameter 1ms after the operator pressed the detonate button.

In order to capture the effects of a nuclear detonation, they have to use a camera that captures an image every 10 nanoseconds.

So, absent an arbitrarily long detonation sequence with audible warnings, those within a few hundred feet will not be able to register that anything happened. Outside that, they may be able to recognize the light, but not the sound. At least until you get past the initial detonation and can hear/see the shock wave of the nuke (which is slightly slower than speed of sound).

Final note, the aforementioned explosion data was based on 10s to 100s of kiloton explosions, a 200 MT explosion would be far more energetic, possibly going out to thousands of feet before any awareness would be feasible. The majority of the explosion recognizable by humans isn't the initial nuclear explosion (which travels at very near light speed), its the shock waves that follow.

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