快猫短视频

A CLOUD of lithium atoms has broken the record for the coolest atomic gas on
Earth. By chilling the atoms to less than a quarter of a millionth of a degree
above absolute zero, scientists have seen the gas develop 鈥淔ermi
pressure鈥濃�攁 strange quantum effect that stops superdense stars collapsing
completely.

Fermi pressure only occurs in a gas of fermions, the class of particles that
includes electrons and neutrons. Fermions obey a quantum rule that forbids them
from occupying exactly the same energy states. So at extremely low temperatures
or at enormous densities, the particles simply jam up all the lowest energy
states available, like filling up the bottom rungs of a ladder, forming a
鈥渄egenerate Fermi gas鈥�.

This in turn stops the particles bunching any closer together, creating the
Fermi pressure. This pressure, for instance, prevents gravity crushing dense
stars into black holes. 鈥淭his is an effect that is responsible for stabilising
dead stars like white dwarfs and neutron stars,鈥� says Randall Hulet of Rice
University in Houston, Texas.

To make a degenerate Fermi gas, Hulet and his colleagues took a cloud of
lithium-6 atoms, which are fermions, and cooled them to within a whisker of
absolute zero. Sure enough, the cloud became degenerate and started to resist
shrinking as the temperature dropped.

快猫短视频s had previously created a partially degenerate
potassium cloud at 0.3 millionths of a kelvin
(快猫短视频, 18 September 1999, p 6).
鈥淏ut we were able to take pictures of the size of
the cloud, and that demonstrates the Fermi pressure in a rather dramatic way,鈥�
says Hulet, whose results will appear in the 30 March issue of Science.