Enrico Fermi’s battle with cancer was nearing its end in late 1954 when he received a visitor.

Fermi, a Nobel laureate in physics, had fled fascism in Europe and become a founder of the nuclear age, helping bring the world’s first reactor and first atom bomb to life.

The visitor, Richard L. Garwin, had been Fermi’s student at the University of Chicago, the laureate calling him “the only true genius I have ever met.” Now, he had done something known at the time only by Fermi and a handful of other experts. Not even his family knew. Three years earlier, the boy wonder, then 23, had designed the world’s first hydrogen bomb, which brought the fury of the stars to Earth.

In a test, it had exploded with a force nearly 1,000 times as powerful as the atomic bomb that leveled Hiroshima, its power greater than all the explosives used in World War II.

To his reverential student, Fermi confided a regret. He felt his life had involved too little participation in crucial issues of public policy. He died a few weeks later at 53.

After that visit, Dr. Garwin set out on a new path, seeing nuclear scientists as having a responsibility to speak out. His resolve, he later told a historian, came from a desire to honor the memory of the scientist he had known best and admired most.

“I modeled myself to whatever extent I could after Fermi,” he said.

Dr. Garwin, the designer of the world’s deadliest weapon, died last Tuesday at age 97, leaving behind a legacy of nuclear horrors he devoted his life to countering. But he also left a strange puzzle.