Perhaps you have seen one of those videos demonstrating a nuclear chain reaction with a bunch of mousetraps and ping pong balls. Each mousetrap is set, with two ping pong balls balanced on it. When a mousetrap is triggered, it launches the two ping pong balls into the air. They fall onto other mousetraps, triggering them. Thus, the first triggering releases two ping pong balls, which trigger two more mousetraps, which release four ping pong balls, which trigger four mousetraps, and so on until all the mousetraps have been triggered.
This is a fairly good model for a nuclear chain reaction. Instead of mousetraps, we have uranium nuclei. When a uranium nucleus explodes, it releases two neutrons, each of which can cause another uranium nucleus to explode.
Of course, everything hinges on the likelihood of those two neutrons hitting two uranium nuclei. If it were probable, then all the uranium in the world would have exploded long ago. But there are three mitigating factors that reduce the probability of a neutron encountering another uranium nucleus. The first is the density of the uranium. If the uranium nuclei are packed together very tightly, then the odds are high that a flying neutron will hit a nucleus, triggering another reaction. But even pure uranium metal is not dense enough in most cases to allow this.
Another mitigating factor is the existence of intervening moderators. These are nuclei other than uranium that can sop up neutrons. Cadmium is one such nucleus; if you mix cadmium with the uranium, the cadmium will sop up lots of excess neutrons and reduce the chance of the reaction running away.
A third factor is size. If the chunk of uranium is small, then the odds are high that a neutron will escape the chunk without hitting another uranium nucleus. But a huge chunk of uranium is so big that a neutron is almost certain to eventually run into a uranium nucleus. The amount of uranium you need to get a self-sustaining chain reaction is called the critical mass of the uranium.
This provides the basis for the simplest type of nuclear bomb: the gun design. You make two pieces of uranium, each one about 70% the size of the critical mass. Alone, neither one will explode. You put them in a tube with high explosives at one end of the tube. When you detonate the high explosives, the explosion drives one chunk of uranium into the other and KA-BOOM!
You don’t really need any neutrons to get the chain reaction started. Uranium nuclei will spontaneously explode, but the odds of any single uranium nucleus exploding are astronomically tiny. Of course, there are an astronomically large number of uranium nuclei in a chunk of pure uranium, so in practice there are always thousands of them going off every second. That’s not enough to start a chain reaction, but there’s always that background activity.
Social violence
This appreciation of nuclear physics can be used to understand an important point about social violence. Society is rather like a chunk of uranium, with each individual acting like a uranium nucleus. The vast majority of us are peaceable, but there are always the maniacs who go violent on us, exploding into a rage and killing. My central point is that every act of social violence emits “violence neutrons” that can induce other people to violence. How many times have you felt your blood boil at some particularly atrocious crime and felt the desire for terrible punishment to fall upon the perpetrator?
Of course, there are moderating forces at work that dampen the urge to answer violence with more violence. Our society has strong irenic forces that counsel wisdom, reminding us of the futility of violence. They might be religious leaders, politicians, or opinion leaders of any kind. Our society has strong moderating influences.
Another important moderating influence is the human cost of violence. This is especially notable in the case of war. The American public was gung-ho for the war in Vietnam for the first few years. But as more and more body bags came home, people started to sober up, and the bloodlust cooled. By 1975, the USA had learned its lesson and was convinced of the value of peace. The same thing happened 28 years later: Americans were gung-ho for war against Iraq, but once they realized that war kills people, they changed their minds.
Sadly, some developments of the last few decades have increased the chances of a runaway chain reaction. The first is that the increasing efficiency of news services has increased the effective density of our “chunk of uranium”. When a single act of violence takes place anywhere in the world, we know about it. Fifty years ago, we knew nothing of murders in Sudan or Bhutan; nowadays any atrocity anywhere in the world gets our attention. In effect, “violence neutrons” from anywhere in the world can now affect individual “nuclei” (you and me). That’s like the uranium getting denser — increasing the chance of a runaway chain reaction.
The news media have amplified this effect because violence sells. Who wants to watch TV stories about stock market prices when you can see some blood and suffering instead? The news media make more money by hitting us in the gut, and violence hits the hardest. So the “violence neutrons” are magnified.
Now, there’s always a low level of violence going on in the world. Murders, civil wars, dictators, and other evil people provide us with a steady background level of violence to which we have become inured.
The Chain Reaction is Heating Up
However, I sense that our global civilization has now begun the process of rising violence. The starting point was the attacks on September 11th, 2001. These did not have to start an expanding chain reaction: the American invasion of Afghanistan was generally regarded as justified and reasonable. Had the USA stopped there, it could have pacified and stabilized Afghanistan and kept the peace.
But George W. Bush made one of the most colossal blunders of modern times: he decided to invade Iraq on fabricated charges. Because this attack was completely unjustified, it spewed huge numbers of “violence neutrons” into the global community. Sure, there were lots of other conflicts going on, but they were background noise. The US attack was an about-face for a country known for its support for peaceful resolution of international conflicts.
But that was only the beginning. Next came the incompetent occupation of Iraq, which served only to continue hostilities. Moreover, the diversion from Afghanistan insured that it, too, would fester in violence. Add to that the atrocities at the Abu Ghraib prison, then the torture of prisoners at Guantanamo, and the USA had completely blown to hell all semblance of being a force for peace.
All those violence neutrons scattered all over the globe, triggering more violence. Mr. Putin in Russia no longer saw much need to live by the rules that Mr. Bush had trampled all over, and began a campaign of petty warfare on his borders, first with Georgia, then later with Crimea and Ukraine.
China has never been enthusiastic about international comity, and perhaps it would have been just as aggressive in its efforts to annex tiny islands off its coastline regardless of American activity. However, American aggression certainly gave China plenty of moral cover.
The worst effects of American violence were in the Islamic world; George W. Bush was the best recruiting agent an Islamic terrorist organization could hope for. What was once a rag-tag collection of starving fanatics has grown into a well-funded international movement with tens of thousands of recruits, many of them eager to die for their cause.
At this point, the chain reaction has grown to such a size that nothing can stop further growth. We are at the worst point in the curve, where the violence is great enough to infuriate everybody but too weak to frighten anybody. Violence neutrons are bouncing around everywhere, and I expect that we’ll see a great deal more global violence over the next ten or twenty years.
At this point, the only moderator to stanch the violent neutrons will be the blood of innocents. When enough of that has poured forth, people will finally become frightened enough to put a stop to it. First the blood will flow — then we will have peace.