The calculations of Ulam and the excellent people he- who worked with him, was worrisome, but to my mind were not conclusive. At the same time however, Johnny von Neumann, with recently improved and greatly improved computers, repeated the calculation and his results were similarly negative. And that now, toward the end of 1950, looked really like evidence that the most radical versions of the hydrogen bomb would not work. The worries of that led me some time, a few weeks before Christmas, to a very definite new approach. You see, in the original discussions of the hydrogen bomb the problem was, you may remember, that too much energy went into radiation and not enough energy remained for making nuclei collide in a sufficiently close fashion to make them react. All this had happened before - was really planned and discussed - before we came to Los Alamos.