Every time I see footage of a nuke going off, there are the really cool but really confusing shots of objects turning into “smoke”. Is this the paint vaporizing off? Or the actual object vaporizing? Or, could it be something else? Thanks.
In: Physics
Follow up question:
In the footage, you can see a blast wave travelling in one direction, then reversing. What’s happening there?
[https://www.youtube.com/watch?v=wgpo0qAfwDk](https://www.youtube.com/watch?v=wgpo0qAfwDk)
Burning stuff, be it wax, paint, dust, …
The energy density of the light from such a blast is so high that anything which blocks even a little of it will likely be heated beyond it’s ability to dissipate the heat.
This would be mostly the paint instantly vaporizing while the object will just start to vaporize. The heat it very intense but doesn’t have a long duration giving, say a thick piece of metal, a long enough window to completely vaporize.
In a lot of nuclear tests, you’ll see smaller rockets which have fired to create smoke trails so that the researchers can study shockwaves as they propagate through the atmosphere.
For example, [this](https://en.wikipedia.org/wiki/Operation_Upshot%E2%80%93Knothole#/media/File:Nuclear_artillery_test_Grable_Event_-_Part_of_Operation_Upshot-Knothole.jpg) image of Operation Upshot Knothole shows a row of vertical smoke trails in the foreground, in between the artillery piece and the blast.
Other smoke sources can be material vaporizing from the light flash, which precedes the shockwave. So the paint from a car, or the tar roofing shingles on a house would start smoldering due to an immense increase in heat.
There’s actually a really good video that goes through exactly what would happen (or has happened) when a nuclear bomb is detonated in a city that mentions this. It’s basically extreme heat that evaporates everything instantly. Recommend the video though, as it goes into more detail and has cool animations! https://youtu.be/5iPH-br_eJQ
There are basic “waves” of a nuclear blast, plus the fireball. The thermal and ionizing radiation waves are effectively instant, and the blast has a lower speed. But these waves have different distances of effect due to the resistance to them in the atmosphere (with inverse square law), and what goes how far also depends on many things including the height of the burst and the type of weapon used. For example, blast damage is reduced for a ground burst compared to a higher air burst.
So if you see smoke come off of a telephone pole, and then it gets immediately flattened, that’s because the thermal radiation reached it first, making it burn (or the paint on it burn), and then right after that the heavy blast wave hit it, knocking it down.
The thermal radiation distance is usually longer than the heavy blast damage distance. So if the pole is farther away you might see the smoke, and then a little later see it get buffeted around when the only moderate-level blast hit it.
The different speeds are also how you see those shadows of people on walls in Hiroshima. Thermal radiation hit, burning the person and bleaching the stone wall behind him that’s not in his shadow, and then the blast hit. There would be no person standing to make a shadow if the blast hit first.
To clarify one point, it’s very hard to vaporize a massive object. It needs to be basically inside the fireball to be hot enough to turn into a vapor. So you’re never seeing that.
As other have noted, you are seeing a layer of paint, dust, dirt, surface material, etc., being heated to burning. _That’s_ totally possible and requires a lot less heat than vaporizing, say, a car or a house.
So if I understand your question correctly you are talking about the heat wave.
When the explosion happened right at the center a huge ball of plasma appears for a short amount of time way hotter than the surface of the Sun. This quickly expands and cools off, this creates that huge pressure change we will call the shock wave. But the temperature of the thing is insane, it radiates so much heat that within a circle (nuke dependent) everything starts burning and if something cant burn it will heat up. So cars made of metal heat up plastic and rubber in them that can burn will burn, wood starts smoking and catches on fire. Skin clothes also burn, glass melts, etc. Than the shock wave arrives.
So if you stand too close to a nuke you get burned or cooked in a second, depends on how far you are. Its a mini sun appearing for a split millisecond.
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