Elliott H. Lieb
The Bose gas: A subtle many-body problem
(292K, postscript)
ABSTRACT. Now that the properties of the ground state of quantum-mechanical
many-body systems (bosons) at low density, $\rho$, can be examined
experimentally it is appropriate to revisit some of the formulas deduced
by many authors 4-5 decades ago. One of these is that the leading term in
the energy/particle is $4\pi a \rho$ where $a$ is the scattering length
of the 2-body potential. Owing to the delicate and peculiar nature of
bosonic correlations (such as the strange $N^{7/5}$ law for charged
bosons), four decades of research failed to establish
this plausible formula rigorously. The only previous lower bound for
the energy was found by Dyson in 1957, but it was 14 times too small.
The correct asymptotic formula has recently been obtained jointly
with J. Yngvason and this work will be presented. The reason behind
the mathematical difficulties will be emphasized. A different formula,
postulated as late as 1971 by Schick, holds in two-dimensions and this,
too, will be shown to be correct. Another problem of great interest
is the existence of Bose-Einstein condensation, and what little is
known about this rigorously will also be discussed. With the aid of
the methodology developed to prove the lower bound for the homogeneous
gas, two other problems have been successfully addressed. One is the
proof (with Yngvason and Seiringer) that the Gross-Pitaevskii equation
correctly describes the ground state in the `traps' actually used in
the experiments. The other is a very recent proof (with Solovej) that
Foldy's 1961 theory of a high density gas of charged particles correctly
describes its ground state energy.