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Equation 2 · Part 1 · Fine-Tuning Without a Tuner: Selection Effects in Physics

Symbol P

P(λ∣observed)  ∝  P(λ)×nobs(λ)P(\lambda \mid \text{observed}) \;\propto\; P(\lambda) \times n_{\text{obs}}(\lambda)
PP

What this part means

P is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Its job in the formula

P is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

The passage around this formula

Done correctly, the weak principle is pure statistics: it is a conditioning operation, identical in structure to correcting a biased sample in any other empirical science. A concrete instrument in this article’s own world makes the logic exact rather than analogical. A magneto-optical trap holds a cloud of atoms by continuously discarding the ones moving too fast to be captured by the cooling beams; every atom visible in the trap already survived a velocity cut. Someone who measured only the trapped cloud and concluded “the atoms in this room are unusually cold” would be committing exactly Eddington’s error — the room is not unusually cold, the trap is unusually selective, and the correction…

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A function assigns an output to each allowed input. The expression f(x) means “apply f to x”.

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Sources cited in the surrounding passage

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