* Posts by Serguei Patchkovskii

2 posts • joined 17 Jan 2008

Boffins build super-accurate atomic clock

Serguei Patchkovskii


Actually, the original explanation in the article was quite accurate as popular science goes.

A "fermion" is any particle - elementary or otherwise - with a half-integer total angular momentum. (For an elementary particle, its intrinsic angular momentum is "spin"). Turning a fermion around any axis by 360 degrees changes the sign of it's wavefunction. If particle's wavefunction does not change upon a 360-degree rotation, it is called a "boson", and has an integer spin. The total intrinsic angular momentum of a particle has important consequences at sufficiently low temperatures: two (or more) bosons can occupy the same quantum state (or be at the same place if you will). Two fermions cannot.

Whether a compound particle, such as an atom or a molecule, is a fermion or a boson depends on two things: the spins of it's constituent parts, and the way these spins are added together, or "coupled". The way these rules work is that a compound particle containing an even number of fermions (and an arbitrary number of bosons) will be a boson. A particle with an odd number of fermion constituents (and again any number of bosons) will be a fermion.

Strontium has 38 electrons (spin-1/2 particles). In its ground state, all its electrons are "paired", so that their spins cancel out, and total electron wavefunction has integer (zero) angular momentum. Most of the isotopes of its nucleus also have an even number of particles of each type (protons and neutrons), and thus integer spin as well. However, Strontium-87, which has natural abundance of about 7% has spin 9/2. As a result, the Sr-87 atoms have half-integer total angular momentum, and are fermions.


Everex follows Asus Eee to announce 9in sub-notebook

Serguei Patchkovskii
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Eee PC CPU is NOT underclocked

In all your eee-PC articles, you keep saying that eee uses a 900 MHz Celeron M underclocked to 630 MHz. It does not - like every other mobile CPU, it is dynamically clocked, between 112 and 900 MHz depending on the current load. Just double-checked it on my 4G ...