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Old 06 February 2014, 09:33 PM
A Turtle Named Mack's Avatar
A Turtle Named Mack A Turtle Named Mack is offline
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Originally Posted by Richard W View Post
ATNM, they may be using "cross section" as a measure of size, which is a concept used in particle physics to measure (essentially) how likely things are to hit each other (well, interact with each other to be slightly more precise). It's obviously not the same as the physical cross-section that we're used to in macroscopic objects but it's kind of analagous so it wouldn't necessarily be misleading in a comparison.

I don't know whether the comparison is true based on that assumption - I might be able to check later - but neutrinos are pretty unlikely to hit things, so it's not obviously wrong either.
You may be right, but that then seems rather arbitrary in application, IMHO. It seems to exclude gravitational interaction, for instance, but of course the reaction cross-section for gravitation is the entire universe (with time limitations, and a definite attenuation of effect with distance). Generally size would be a matter of the region from which other similar items are excluded by the object's presence. Generally nuclei will not go through other nuclei; OTOH, there is some give for atoms to pass each other within the range of the electron shells - they are kind of squishy in that regard. The weak interaction - the only force besides gravity recognized by neutrinos - is not generally relevant to size in any meaningful way. But I am open to correction on this.
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