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Comment Re:Personal mobility (Score 1) 282

Not so simple. I live a 30-min walk from my work, through urban Philly. To bike it (I've tried) is 10 minutes on the bike, door-to-door, owing to lights and other traffic rules. I often need to change clothes (summer is very humid). I have to lock my bike up outside due to fire-code regulations preventing them in the building. Changing clothes and locking/unlocking add time to the bike commute. Walking is just simpler. In rainy weather, can you bike with an umbrella? And no way would I listen to a podcast on a bike -- you need your ears to bike in the city. I was a bicycle commuter (4 miles) in graduate school on the west coast and loved it. For a simple 1.5-mile commute, walking seems better.

Comment Galactic-to-solar-system precision (Score 1) 6

Imagine the Milky Way as a disk with a circumference expressed, as usual, as 2*pi*R, where R is the radius. Having pi precise out to 10^12 digits means that the last significant digit measures distance on the order of a billion meters, or about 1/100th the distance between the Earth and the sun (since the radius of the Milky Way is about 5x10^20 meters.

Comment Glycine is simple... (Score 5, Informative) 148

Glycine is the simplest amino acid, and it the only one that lacks a chiral center on the alpha carbon. Of the four groups attached in a tetrahedral arrangement to the alpha carbon, two are hydrogen atoms. In all other amino acids, one of the two hydrogens of glycines is replaced by a distinct functional group. The really interesting thing about biologically used amino acids is that it is always the same hydrogen of the two that is replaced -- all the 19 non-glycine amino acids are so-called "L-stereoisomers." Discovery of any one of the 19 amino acids other than glycine in a comet would be quite a story, and it would be even more surprising if there were a mixture of "L" and "D" stereoisomers other than 50/50. My bet is that if another amino acid is found in cometary debris, it is asparagine, since it can form by the reaction 2*glycine - water.

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