Segnalo, nel caso a qualcuno interessi.
In fondo c'� una domanda, una curiosit� pi� che altro...
http://www.news.iastate.edu/news/2011/may/carbon14
Iowa State physicists explain the long, useful lifetime of carbon-14
====================================================================
[...] Why, exactly, does carbon-14 have a half-life of nearly 6,000
years while other light atomic nuclei have half-lives of minutes or
seconds?
[...] the underlying reason turned out to be a fairly exotic one."
The reason involves the strong three-nucleon forces (a nucleon is
either a neutron or a proton) within each carbon-14 nucleus. It's all
about the simultaneous interactions among any three nucleons and the
resulting influence on the decay of carbon-14.
The research project's findings were recently published online by the
journal Physical Review Letters
(
http://prl.aps.org/abstract/PRL/v106/i20/e202502) [where the
physicists reported] the microscopic origins of the anomalously
suppressed beta decay of 14C to 14N using the ab initio no-core shell
model with the Hamiltonian from the chiral effective field theory
including three-nucleon force terms. The three-nucleon force induces
unexpectedly large cancellations within the p shell between
contributions to beta decay, which reduce the traditionally large
contributions from the nucleon-nucleon interactions by an order of
magnitude, leading to the long lifetime of 14C.
[One of the researchers], in explaining the findings, likes to remind
people that two subatomic particles with different charges will attract
each other. Particles with the same charges repel each other. Well,
what happens when there are three particles interacting that's
different from the simple addition of their interactions as pairs?
The strong three-nucleon interactions are complicated, but it turns out
a lot happens to extend the decay of carbon 14 atoms.
"The whole story doesn't come together until you include the
three-particle forces," said Vary. "The elusive three-nucleon forces
contribute in a major way to this fact of life that carbon-14 lives so
long."
Maris said the three-particle forces work together to cancel the
effects of the pairwise forces governing the decay of carbon-14. As a
result, the carbon-14 half-life is extended by many orders of
magnitude.
To get that answer, [...] researchers needed a billion-by-billion
matrix and a computer capable of handling its 30 trillion non-zero
elements. They also needed to develop a computer code capable of
simulating the entire carbon-14 nucleus, including the roles of the
three-nucleon forces. Furthermore, they needed to perform the
corresponding simulations for nitrogen-14, the daughter nucleus of the
carbon-14 decay. And, they needed to figure out how the computer code
could be scaled up for use on the Jaguar petascale supercomputer.
It's no easy task to simulate those interactions.
In this case, it took about 30 million processor-hours on the Jaguar
supercomputer at Oak Ridge National Laboratory in Tennessee. Jaguar has
a peak performance of 2.3 quadrillion calculations per second, a speed
that topped the list of the world's top 500 supercomputers when the
carbon-14 simulations were run.
=============
La curiosit�.
Considerata la complessit� dei calcoli, i moderni calcolatori 'dal
costo umano' sembrano l'equivalente dei calcolatori meccanici degli
anni '40.
C'� in vista un rivoluzione del calcolo numerico che possa portare a un
salto di qualit� equivalente a quello che c'� stato tra i calcolatori
meccanici e quelli elettronici?
Il 'quantum computing' - del quale nulla so - pu� essere la chiave di
volta per rendere problemi di questo tipo trattabili anche dal singolo
ricercatore (che al posto del laptop avr� il quantop)?
Received on Wed Jun 01 2011 - 18:13:58 CEST