The history of the two-fluid concept and a new look at the Bose-Einstein condensation.
Laszlo Tisza (LMIT, Cambridge, USA)

Jeudi 14 juin 2001

The first part of my talk is a personal account of the thought processes that led me to the two-fluid concept early 1938. This includes my indebtedness to Lev Landau and to Fritz London for what I learned from them before the discovery of superfluidity. We were the first theorists to respond in our own way to this discovery, but the perspective of half a century is sufficient to do justice to all three of us for contributions that have stood the test of time. Yet the time is still not quite ripe for a fully satisfactory treatment of the superfluidity of helium in terms of the BEC. However, the recent confirmation of BEC in weakly interacting atomic gases is free from the technical difficulties presented by liquid helium.

The clarification of the situation by these discoveries suggested to me a new line of inquiry. How did Einstein arrive at this incisive prediction about the objects of quantum mechanics (QM) in January 1925, six months before Heisenberg’s paper was to initiate this discipline ? And, having such an early insight into the nature of quantum objects, why did he reject the pragmatically successful QM from 1927 on ? My answer is that the true meaning of Einstein’s path to Bose -Einstein statistics is that it constitutes a method of quantization and belongs to the category of the foundational works of 1925-27. Unfortunately, Einstein’s concern with unified field theory kept him from recognizing this fact and from following up this potential lead. Had he done so, the result would have been a more intuitive QM than the one on our books. I consider the central legacy that Einstein bequeathed to us is making up for this omission and by the same token, by following his advice and establish QM as a "theory of principle" in the spirit of his foundation of the relativity theories.