Author Archives: gmcdavid

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About gmcdavid

Retired IT professional with a wide range of interests. Married. Three sons, two with autistic-spectrum disorders and the third being transgender with Ehlers-Danlos syndrome. From Chicago but now living in the Twin Cities metro, Minnesota. Episcopalian. Carleton College (BA 1972, physics) and Stanford University (MS 1974, Applied Physics; MS 1976 Statistics).

Thomas Bayes looks through the NSA’s “Prism”

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Leaked NSA slide-deck claims that NSA has “direct access” to servers at Google, Apple, Facebook, Skype, Yahoo, and many others

This story has received a lot of coverage in the last few days, even extending to Jay Leno’s monologue. Understandably, there is a lot of concern about the civil liberties implications. But there is also another question, answered by Corey Chivers: How likely is the NSA PRISM program to catch a terrorist?

We don’t really know anything about how PRISM works (NSA = Never Say Anything), but with some plausible assumptions we can estimate the answer. Suppose

  1. If a terrorist is in the system, the probability is 99% that PRISM will flag him/her.
  2. An good guy has only a 1/100 chance of being flagged as potential terrorist.
  3. The actual number of terrorists is quite small, say 1 in 1,000,000.

Using Bayes’ rule, Chivers shows that only 1 in 10,102 of the people flagged as suspects will actually be a terrorist!
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Now you see it, now you don’t

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No Planet of Alpha Centauri B?

Last October astronomers announced big news: the discovery of a rocky, scorching hot, Earth-sized planet circling our closest stellar neighbor, the orange dwarf star Alpha Centauri B just 4.3 light-years away. Exoplanet astronomer Debra Fischer (Yale) told the New York Times that the planet next door was the “story of the decade.” Almost lost in the excitement was the caveat that the planet’s detection was still iffy and required heroic efforts to extract any sign of it from the background noise of the star’s radial-velocity measurements.

Now the plot has become more muddled. A new analysis of the data by an independent researcher has failed to confirm the planet’s existence.

How to resolve this issue: Get a lot more data. Everybody involved agrees on that. This is science at work. Finding a planet in the Alpha Centauri system would be really cool (ask any science fiction fan), but we need to be sure it is really there.

Dark Age Greece

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The Lost Community of Lefkandi

I particularly noticed:

From around 1250 BC onwards, post-Mycenaean ‘refugee’ settlements began to appear, establishing a pattern that was to continue throughout the Dark Ages (Whitley 2011: 77-78). The characteristic Dark Age remote, defensible positions, often over 500m above sea level – as in evidence at Karphi (ibid: 78) – exhibited a continuity of older traditions and no obvious change in population levels (ibid.).

This pattern was noted by Robert Drews in The End of the Bronze Age as evidence that the disasters in the Eastern Mediterranean c. 1200 BC were the result of human action, not natural causes. The only reason the survivors would rebuild in such difficult locations is fear of an attack by raiders or invaders. See also my post on From Bronze to Iron.