Monthly Archives: March 2020

How not to make an atom bomb

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There are many books on how the atom bomb was made. They are histories of the great men who succeeded at site Y, Los Alamos, usually with a sidelight of the economics and politics in the US at the time. It’s sometimes noted that there was an equally great German group working too, and one in Japan and in Russia, that they didn’t succeed, but it’s rarely discussed what they did wrong. Nor does anyone make clear why so many US scholars were needed. What did all those great US minds to do? The design seems sort-of obvious; it appears in the note Einstein sent to Roosevelt, so what were all these people thinking about all that time, and why did the Germans fail? By way of answer, let me follow the German approach to this problem, an approach that won’t get you anywhere, or anywhere that I’ve seen.

It seems that everyone knew that making a bomb was possible, that it would be fearsomely powerful, and that it would be made using a chain reaction in uranium or plutonium. Everyone seems to have understood that there must be a critical mass: use less and there is no explosion, use more and there is one. The trick was how to bring enough uranium together make the thing go off, and as a beginning to that, there is the concept of “a barn.” A barn is a very small unit of area = 10−24 cm², and a typical atom has a cross-section of a few barns. Despite this, it is generally thought to be very easy to hit an atom at the nucleus, that is, at the right spot, as easy as hitting the board side of a barn (hence the name). The cross section of a uranium atom is 600 barnes at room temperature, or 6×10−22 cm². But each cubic centimeter of uranium holds .5 x 1023 atoms. Based on this, it comes out that a thermal neutron that enters a 1 cm cube of uranium has a virtual certainty of hitting an atom — there are 3 cm² of atoms in a 1 cm² box. You could hardly miss.

Each uranium atom gives off a lot of energy when hit with a neutron, but neutrons are hard to come by, so a practical bomb would have to involve a seed neutron that hits a uranium atom and releases two or more neutrons along with energy. The next neutron has to hit another nucleus, and it has to releases two or more. As it happens uranium atoms, when hit release on average 2.5 neutrons, so building a bomb seems awfully easy.

But things get more difficult as the neutron speeds get greater, and as the atoms of uranium get hotter. The cross-section of the uranium atom goes down as the temperature goes up. What’s more the uranium atoms start to move apart fast. The net result is that the bomb can blow itself apart before most of the uranium atoms are split. At high speed, the cross -section of a uranium atom decreases to about 5 barnes you thus need a fairly large ball of uranium if you expect that each neutron will hit something. So how do you deal with this. For their first bomb, the American scientists made a 5 kg (about) sphere of plutonium, a man-made uranium substitute, and compressed it with explosives. The explosion had to be symmetrical and very fast. Deciding how fast, and if the design would work required a room full of human “computers”. The German scientists, instead made flat plates of uranium and slowed the neutrons down using heavy water. The heavy water slowed the neutrons, and thus, increased the effective size of the uranium atoms. Though this design seems reasonable, I’m happy to say, it can not ever work well; long before the majority of the reaction takes place, the neutrons get hot, and the uranium atoms fly apart, and you get only a small fraction of the promised bang for your bomb.

How fast do you need to go to get things right? Assume you want to fusion 4 kg of uranium, or 1 x 1025 atoms. In that case, hitting atoms has to be repeated some 83 times. In tech terms, that will take 83 shakes (83 shakes of a lamb’s tail, as it were). This requires getting the ball compressed in the time it takes for a high speed neutron to go 83 x 3 cm= 250 cm. That would seem to require 1 x 10-7 seconds, impossibly fast, but it turns out, you can go somewhat slower. How much slower? It depends, and thus the need for the computers. And how much power do you get? Gram for gram, uranium releases about 10 million times more energy than TNT, but costs hardly more. That’s a lot of bang for the buck.

Robert Buxbaum, Mar 29, 2020.

The power of men’s hats

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Here’s a joke from 3rd grade: why do Indians wear feather headdresses? …… To keep their wig warm. One of the main reasons to wear a hat is to keep your head warm. Men generally wear hats outside only, and mainly to keep warm, or to keep the sun off your eyes. We thus show below a delivery boy in a knitted cap (called at torque in Canada), and a boss is a stylish fedora. The two hats keep the head warm, but the fedora protects the eyes too, and the different styles establish who you are in the social chain. It is a good thing when fashion works this way, and uncool, in my mind, when messages are reversed or unclear. It’s equally uncool to see a delivery boy in a fedora as an executive in a wool cap. Either one looks pretentious to me. One is dressing up, the other dressing down or confused. Women’s hats generally look confused to me, in part because there is no such thing as a real business-woman’s hat.

Photo by Andy Barnham.(previously spelled wrong)

Nowadays, many business men don’t wear hats, even outdoors in the sun and cold. This seems like a bad idea, but what would I know? Perhaps the problem is what to do with the hat when you come indoors. You can take it off, but then what. Emily post claims that leaving the hat on indoors is usually considered rude, though not always, and traces this back to medieval knights and to the flag code. Indoors, the delivery boy can stuff his knit hat into his pocket, or roll it into a smaller version on his head, a beanie. The fedora wearer must look for a hat rack, or accept looking rude.

Of course the lack of a hat presents problems too. Without one, you leave your hair to signal your social status and political cultural associations. For a man without a hat there are only three styles of hair: short, medium, or long. Short hair says you are a conventionalist drone, long hair, that you’re a hippy or artist, and with middle-length hair you’re …. uncertain? trans? androgynous? No matter how you slice it, it’s not a good look. Adding a mustache or beard makes it even more awkward, in my opinion, see below. I have previously written about the power of mustaches — that they send a message that you are warrior, and beards — that you are a man of fervor, — or of religious or aristocratic sympathies. But combine a mustache with middle-length hair and you begin to look like another Hitler or Stalin.

Wearing a hat allows for a great variety of social messaging, whether worn with or without facial hair. Some hats are expensive, others cheap; some signal religious affiliation, others are strongly secular, or hip. Some folks wear hats that are suitable only for work or sports, like a hard-hat, bicycle helmet, or a straw boater. They tell folks you’re busy with an activity right now. But most people who wear hats, choose one that’s multidimensional, suitable for sport and work. There is the classic Kangol cap that suggests a certain artsy vibe, or the peak cap or newsboy — that suggests (I imagine) a higher level of worker.

working man in cap

Perhaps the most popular flex-hats in the US are the baseball hat, and its relative the trucker’s hat (you adjust the size on a truckers hat using a band int he back). In the US, you can wear these on the job, or off. I think they work indoors too, but what do I know. The baseball and peaked cap suggests you are higher on the social ladder than the truckers cap, but all of them suggest you draw a paycheck. And they often say a lot more. If your trucker’s hat says, NRA, or John Deere, or Oakland As, there enough information given to start a conversation. Depending on what your cap says, you will be welcome in some societies, not welcome in others. Don’t wear your MAGA hat to a Biden rally.

There is power in hats too. A man in a policeman’s cap is a cop, even if he’s without the rest of his police gear. With no hat, the same man in uniform is a mall security guard. The postal person or UPS delivery person is on the job if wearing his USPS baseball cap or knit. An expensive visor cap, like the kangol suggest artistic status, and an expensive newsboy, or peaked cap. suggests a sort of work-life balance. It was worn by Prince Charles in the 1980s, and by me in 2020.

Although a fedora is a boss-man’s hat, I never wear one since I associate them with mobsters, hipsters, lounge singers, and Jimmy Hoffa. For more formal occasions, when not wearing a peaked cap, I wear a Homburg. Churchill wore a Homburg. In England, there is a level above this, the top-hat, and one slightly between the Homberg and fedora, the derby. In the US, none of these really caught on. The derby is sort of comic, sort of social climber. Chaplin wore one, as did Laurel and Hardy. Derby hats tend to get punched through in old-time comedies. It’s the same with most middle of the road approaches — they appeal to no one.

Robert Buxbaum, March 5, 2020.