critical mass

Critical Mass is a worldwide 'organized coincidence' where large amounts of cyclists gather and ride together. It started in San Francisco in the early 90s and has spread to over a hundred cities since.

Critical Mass is an enjoyable way to be obnoxious and take the streets back from the roaving herds of dangerous cars which have highjacked our cities. We're not blocking traffic, we are traffic.

Critical mass is the key component of the network effect phenomenon. It refers to the population of consumers that creates a momentum of popularity for the product.

In reference to the bicycle critical mass, the name stems from what happens at busy intersections in large cities. Many car drivers (I'm speaking from experience) are too air headed to yield to a bicycle which might have the right of way. This causes many people to end up loitering at an intersection waiting for a chance to cross on their bikes. When the bicyclists reach "critical mass" there is enough of them to cause a large enough controlling force (or distraction from a driver wanking) to where they may cross. This term was first coined in a documentary in support of the bike (and other topics), "Return of the Scorcher" by Ted White.

In nuclear physics, the term refers to the threshold amount (note I do not say 'mass') of a given substance that will cause a chain reaction in that substance. The chain reaction effect is what makes nuclear energy economical, as otherwise splitting atoms would be as hard and expensive as fusing them currently is. (It is the lack of a chain-reaction type effect that is keeping fusion from being an economical energy source - if only a fusion reaction could be made to emit more muons than it consumes...)

Critical mass of a given element or, more usually, isotope is somewhat of a misnomer, since criticality depends upon shape as well as mass. A thin foil of any isotope, no matter how large and heavy, will not become critical, while a sphere of the same mass might instantly detonate. This is because a chain reaction depends upon the neutrons released when an atom fissions striking other atoms and causing them to fission and release more neutrons. If an otherwise critical mass is the wrong shape, too many neutrons will escape into the air (or whatever is surrounding the mass) to cause a chain reaction.

Critical mass for a substance can be reduced by enriching the percentage of isotopes or elements that release lots of neutrons or fission readily. In uranium, critical mass is reduced by enriching the percentage of the ²³⁵U isotope, which fissions more readily than the ²³⁸U isotope (which tends to absorb neutrons without fissioning). In plutonium, however, each fission releases more neutrons than in uranium, so the critical mass for plutonium is lower - about 2kg, compared to 5kg for weapons grade uranium.

Critical mass is also reduced by the use of shields or 'tamers' that reflect escaping neutrons back into the mass. This means that a subcritical mass of some substance can be triggered into criticality simply by dropping a shield over it, in some cases. Accidents have been reported where researchers have accidentally caused chain reactions by dropping shields, immersing masses in water or concentrating solutions, and in at least one case simply by physical proximity of their bodies acting as reflectors.

There is a Bicycle Critical Mass in more than a hundred cities in the United States, and hundreds worldwide in Canada, Europe, Israel, South America, and Japan. Critical Mass rides occur simultaneously throughout the world on the last Friday of every month.

Critical Mass is wonderful in so many ways. It is a spontaneous not for profit and not for sale public event without corporate sponsorship and logos for people to enjoy themselves in the streets that were supposedly made for them, a type of event that seems to have disappeared from the American experience altogether. It is a demonstration of the utility and beauty of bikes, and a statement and protest against car culture and the havoc that basing society on the automobile has wreaked on our environment and quality of life.

Many Critical Mass demonstrations attempt to tie-up or completely paralyze traffic in the city in question. This is only fair given that in the other 99% of every month cars rule the roads and motorists play lightly with Bicyclists' safety.

crippleware = C = crlf

critical mass n.

In physics, the minimum amount of fissionable material required to sustain a chain reaction. Of a software product, describes a condition of the software such that fixing one bug introduces one plus epsilon bugs. (This malady has many causes: creeping featurism, ports to too many disparate environments, poor initial design, etc.) When software achieves critical mass, it can never be fixed; it can only be discarded and rewritten.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Critical mass is the minimum amount of a substance that is needed to sustain a reaction. This amount can vary based on how it is shape and the density of the substance. When a reaction occurs it emits neutrons that are capable of producing more reactions but only if they collide with more atoms. Higher probability of a collision means more collisions which is a more efficient reaction.

When a reaction starts an atom is split and neutrons are emitted. Optimally, all neutrons hit other atoms because if they do not then the overall reaction is less efficient because time emphasizes these errors as shown in the second table. If an atom is completely surrounded by other atoms when it gets split then its neutrons have a 100% chance of hitting more atoms. A 100% chance means no waste, at least for that particular atom. If an atom is on the surface of the mass then its neutrons have a chance to go away from the mass and not produce another reaction. If it is within the mass they are forced to react with more atoms because no matter what direction they go in they cannot avoid splitting an atom. This means that we want to minimize the mass's surface area and maximize its volume which is controlled by the shape of the mass.

Some shapes have excellent surface area to volume ratios. Here are calculations for the surface area of various shapes given 1000 volume:

Plane: 2000
Cube: 600
Rectangle: 700
Sphere: ~9

As you can see a sphere is by far the best, only about 1% of the volume is exposed to the surface. This is because spheres only have one surface and no corners. Also consider that a sphere cannot be tangent to a plane at more than one point, planes being the worst for SA:V ratios. Spheres produce better reactions because they have the smallest surface area to volume ratio.

The last factor that controls critical mass is density. When the density of the mass is increased, the neutrons cannot go as far without colliding so it makes the chain reaction faster. Critical mass is just enough mass to make the reaction continue happening. If the best shape and highest density is used then the quantity of mass required for a sustained reaction or the critical mass is lessened.

Here is a table illustrating how fast the total reaction is going with different quantities of successful (colliding) neutrons:

    2    3    4
-------------------
1|  2    3    4
2|  4    9   16
3|  8   27   64
4| 16   81  256
5| 32  243 1024

The smaller the number of successful reactions the more dramatic the results are later on. In practice since it is determined by probability whether or not a collision fails to occur it is not this bad unless the probability is very low. The chain increases in total reactions exponentially over time so the longer it has been going the worse the effect of a miss.

In summary, critical mass is the quantity of a substance needed to sustain a chain reaction and it is directly dependent upon the shape and density, and when these factors are manipulated correctly (spheres and high density) critical mass is lessened.