When a nuclear device is detonated at ground level, we see the familiar mushroom cloud, which is undoubtedly caused by the weight of the atmosphere suppressing the upward thrust, causing the debris and dust to be curved down back towards the Earth. The horizontally propelled mass again does not travel very far from the point of detonation.

One would expect the nuclear explosion to escape from the atmosphere, after all Gravity is thought to be the weakest force?

But in reality, gravity is the only force and both nuclear and magnetic forces would not be present without gravity. So are we in error comparing like for like, when in reality we are comparing what are currently thought to be separate forces?

Gravity is responsible for holding all particles, from sub atomic to planets and suns together and the nuclear explosion merely illustrates it perfectly?

Nuclear blast

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    $\begingroup$ You clearly overestimate the role of gravity. It is not the only force. Also, there are no magnetic or nuclear fundamental forces: those are weak, strong, electromagnetic and gravitational. They have very disjoint roles. $\endgroup$ – QuantumBrick Aug 29 '16 at 11:59
  • $\begingroup$ Could it be you that clearly underestimates the role of gravity? $\endgroup$ – Andrew K Fletcher Aug 30 '16 at 13:08
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    $\begingroup$ @AndrewKFletcher No, the 4 forces are distinct and separate. Gravity is not what holds sub-atomic particles together. $\endgroup$ – Jim Aug 30 '16 at 13:10
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    $\begingroup$ Gravity does not even act the same or look the same as the strong force. It may be true a point mass will influence other masses similarly to how a point charge will influence other opposite charges, but that doesn't mean gravity and the EM force are the same thing. Correlation does not imply causation. Look closer and you find they are very different $\endgroup$ – Jim Aug 30 '16 at 13:14
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    $\begingroup$ The origins of gravity have not been fully theorized, I'll grant you that. But we know that gravity is not the "cement that holds everything together". We may not know exactly what it is, but we know what it isn't $\endgroup$ – Jim Aug 30 '16 at 13:48

A couple things first. Gravity is not the "only" force, and also it is not responsible for holding all particles together. A single hydrogen atom with one electron in orbit would stay held together without the force of gravity due to the electromagnetic force.

The main thing slowing down the mushroom cloud is not gravity. Mostly it is the highly energetic particles running into the previously stationary air particles.

If you made a nuclear explosion on a planet with no atmosphere I would expect some particles would be launched into space.

A related way to think about this is that we can make a rail gun powerful enough to launch something at 11km/s (fast enough to escape Earth's gravity), but we have not done it, because the main obstacle is air resistance. The projectile will experience tremendous force from air resistance slowing it down immediately after leaving the rail gun muzzle.

As MasterOfMuppits has pointed out, after the explosion initially slows down, the cloud continues to rise due to the massive amounts of heated air the explosion generated.

  • $\begingroup$ Nuclear explosion being not being a rigid object like a spacecraft does certainly slow down very much at the very beginning, but I would like to add that the only reason the material reaches high altitudes as in @count_to_10:s picture is due to massive amounts of heated air that is buoyant and carries dust with its ascent. $\endgroup$ – Communisty Aug 29 '16 at 12:26
  • $\begingroup$ Thank you for the comment @MasterOfMuppets. I have added your contribution to the answer. $\endgroup$ – Andrew Aug 29 '16 at 12:32
  • $\begingroup$ If we took a glass of water and emptied it into space, what would happen to those hydrogen bonds? Those stationary particles you refer to, are stationary because of gravity! I edited my original post because it should have said escaped the Earth's atmosphere. $\endgroup$ – Andrew K Fletcher Aug 30 '16 at 13:11
  • $\begingroup$ The water in space seems like it would be its own good question. I guess it would stay mostly together in droplets because of surface tension. If there was a free molecule of water ($H_2O$) floating around in space that was protected from the suns radiation it would stay together because of the chemical bond between the oxygen and hydrogen atoms. These bonds would still exist outside Earth's gravitational field (and not because gravity). I would say its mostly Earth's electric neutrality that keeps the air molecules stationary. See what-if.xkcd.com/140 $\endgroup$ – Andrew Aug 30 '16 at 20:29

But in reality, gravity is the only force and both nuclear and magnetic forces would not be present without gravity. 

This is a non sequitur, we live in a universe that has various forces, particles and conservation laws that, although we exploit them and try to understand them, were still "given" to us with certain properties which we cannot currently explain. You can't logically say that gravity is the only force and then immediately refer to other forces.

Either it is the only force or it isn't. And it isn't.

I have never read or been told about, other than speculation about a unified force which might have existed just after the big bang, that nuclear forces or electromagnetic forces depend on gravity for their existence. Gravity is an effect of spacetime curvature, and not a force in the same sense as electromagnetism or nuclear forces.

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One would expect the nuclear explosion to escape into the atmosphere, after all Gravity is thought to be the weakest force?

Here is a chart of how extensively the nuclear tests extended into the atmosphere. A 30 megatonne explosion reaches 30 kilometres high. By most standards, that's escaping into the atmosphere.

  • $\begingroup$ I can say gravity is the only force and all other perceived forces are a result of gravity driving them. Gravity is the result of accumulated mass! It takes a tremendous reaction to split the atoms and yet those split atoms are put back together without any violent explosion. What is putting the atoms back together? $\endgroup$ – Andrew K Fletcher Aug 30 '16 at 13:26
  • $\begingroup$ Nuclear tests are conducted well below the Earth's surface and generally are considered safe because the weight of the soil and rocks above the detonation area effectively prevents the explosion from disturbing the surface. Gravity effectively contained the nuclear event and held everything together. $\endgroup$ – Andrew K Fletcher Aug 30 '16 at 19:42

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