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Q & A: Explosions

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Most recent answer: 10/22/2007
Q:
After a nuclear explosion, a void at the center is created for a short period of time. Is this correct? The force of the explosion escaping the center would create a sort of nothingness or a void different from the other parts of the explosion. The Big Bang must have produced a similar effect on the universe but with no gravity squeezing on the void to return might there be void approaching or are we in the so-called void that exists other than what might have preceded the arrival of this void.
- Vincent Romelo (age 18)
Las Vegas H.S., Las Vegas Nevada
A:
Hi Vincent,

Not really. In an explosion, a material either burns very rapidly, as is the case with conventional explosives, or undergoes nuclear reactions, as you mention. Both kinds of reactions make the material of the bomb very very hot. At high temperatures, materials vaporize -- they turn into gases which take up more space than their solid versions. Hot gases take up even more space than cold ones. The "Ideal Gas Law" says that

Pressure*Volume = (number of molecules)*Temperature*(a universal constant).

If the number of molecules doen't change but they suddenly get really hot, then the pressure times the volume goes way up. Instead of a void, one has in the center of an explosion, a region with very hot, high-pressure gas, which isn't very dense. It is this gas which presses on other stuff nearby and which causes damage. Damage also comes from fires ignited by the explosion, and in the case of nuclear explosions, also by radiation and contamination.

As the explosion progresses, the hot, expanding gases push on the surrounding gases, piling up a shock front in front of it. The expansion slows as the expanding gas cools off, and the shock front will separate and travel on its own as a wave when the speed of the expanding gases falls below the speed of sound in the air. Supersonic shock fronts are possible in very strong explosions, such as nucear explosions.

The hot ball of gas released by the explosion is not dense but takes up a lot of space, and so it floats upwards. Explosions often result in a fireball floating upwards over the explosion site. There may be an inward draft of cooler air after the initial shock front has passed as air replaces the hot gases that float away.

In the Big Bang, we think that no part of space was much different from any other part, since the left-over radiation seems about the same in every direction. So there would have been no particular place to have a 'void'. It's not to picture the Big Bang as a bunch of stuff exploding into a pre-existing space, but rather as the space itself blowing up carrying stuff with it.

Tom and Mike

(published on 10/22/2007)

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