Bayes’ Theorem and the Search for Supersymmetry

Interesting comments about Bayes’ theorem and the prospects for detecting supersymmetry at the Large Hadron Collider. This piece explains how a non-detection isn’t always “absence of evidence” but can indeed by “evidence of absence”. It’s also worth reading the comments if you’re wondering whether what people say about Lubos Motl is actually true…

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Here’s a puzzle. There are three cups upside down on a table. You friend tells you that a pea is hidden under one of them. Based on past experience you estimate that there is a 90% probability that this is true. You turn over two cups and don’t find the pea. What is the probability now that there is a pea underneath? You may want to think about this before reading on.

Naively you might think that two-thirds of the parameter space has been eliminated, so the probability has gone from 90% to 30%, but this is quite wrong. You can use Bayes Theorem to get the correct answer but let me give you a more intuitive frequentist answer. The situation can be models by imagining that there are thirty initial possibilities with equal probability. Nine of them have a pea under the first cup, nine more under the second and nine more under the third…

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8 Responses to “Bayes’ Theorem and the Search for Supersymmetry”

  1. I thought that blog was making the opposite point: i.e. that absence of evidence of SUSY was *not* evidence of absence.

  2. Brendan Says:

    Sort of. It was making the point that absence of evidence in a Bayesian framework is not as much an evidence of absence as you might expect.

  3. Brendan Says:

    Also, I didn’t know what people said about Lubos Motl, but I think I can now guess!

    • The sad thing is that Lubos actually writes some pretty interesting stuff about particle physics on his blog, but to get to it you need to wade through the gratuitous insults, the raving-climate-sceptic rants, and all those awful flashing ads. If you have the patience.

    • Anton Garrett Says:

      He’s a mixed bag alright, although not everybody agrees about which is the good stuff and which is the bad. (He didn’t think much of “Not Even Wrong” about string theory…)

  4. I think if you judge the actual physics content, Motl’s blog is not only brilliant but kind of peerless in communicating important ideas in advanced theoretical physics. The colorful language is irrelevant to me, since my only goal is to understand physics.

    So for the people who are in position to benefit from it, I definitely encourage them not to get distracted by the personal style. Much of it is optimized for a fairly sophisticated audience though, which Im sure is a major reason that the personality comes so often but the quality and importance of the content does not.

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