The famous Russian physicist Andrei Sakharov left a rich heritage. He outlined an explanation for the disparity between matter and antimatter in the universe. His idea of gravitation as the quantum elasticity of the spacetime continues to inspire the search for a unified theory. He was a pure theorist who invented the Soviet H-bomb and, 20 years later, transformed into the foremost human-rights advocate and opponent of the Soviet regime, for which he received a Nobel peace prize. He put forward a powerful political principle: "Never trust a government that doesn't trust its own people."

Sakharov left also quite a few riddles. Most of them were solved with help of archival documents declassified after his death, encouraged by Gorbachev glasnost and Yeltsin openness. In March 1999 I was happy to explain to Scientific American readers what made the secret father of the Soviet H-bomb transform into an open human-rights advocate in 1968.

But one riddle stays unsolved. More than one solution has been suggested, but we lack declassified information that would lead to an unambiguous solution. All we have is circumstantial evidence. I'll lay out this fascinating detective story here, and you can cast your vote in our online poll.

You can find this riddle in Sakharov's memoirs, written during his inner exile from 1980 to 1986. At the time, information on the history of Soviet nuclear weapon development was severely restricted. Even the fact that Sakharov had played a vital role in making the Soviet H-bomb was not officially acknowledged until his death in 1989. Sakharov himself, despite his political dissent, observed his commitment not to reveal state secrets. In his memoirs he used only very general descriptions, such as the "First and Second Ideas" that led to the first Soviet thermonuclear bomb test in 1953, and the "Third Idea" that led to the full-blown bomb in 1955. Here what he wrote about the emergence of the unspecified Third Idea:

Apparently, several people in our two theoretical departments came up with the "Third Idea" simultaneously. I was one of them. I think that I understood the basic aspects of the "Third Idea" at a very early stage. As a result, and also due to the respect I had earned by then, my role in the acceptance and implementation of the "Third Idea" was perhaps a decisive one. But the role of Zeldovich, Trutnev, and several others was undoubtedly very great and perhaps they understood and foresaw the prospects and difficulties of the "Third Idea" as well as I. At the time, we (I at any rate) had not time to think about questions of author's priority, especially since that would be "dividing up the skin of a bear we had yet to kill," and restoring all the details of our discussion in hindsight is impossible, and perhaps unnecessary.

The American H-bomb project had a Third Idea of its own. Hans Bethe characterized it as "a very brilliant discovery made by Dr. Teller" in 1951, "a stroke of genius, which does not occur in the normal development of ideas." The idea was radiation implosion, which uses a fission bomb to set off a much more powerful fusion bomb. But if this discovery was so brilliant and so rare, how did it occur to several Russian physicists simultaneously -- three years after Teller's stroke of genius? Did Sakharov truly come up with the idea on his own? If so, why did he use such strange wording? Hence the riddle of the Third Idea.

It is well-established fact that the Soviet nuclear program received enormous input from espionage. The first Soviet A-bomb (a fission device) was a copy of the U.S. A-bomb. This fact was acknowledged by Yuly Khariton, the scientific head of the nuclear weaponry project from its very beginning. But he insisted that the H-bomb was indigenous. There is no serious doubt that the first Soviet thermonuclear bomb, the one tested in 1953, was an original Soviet invention based on ideas of Sakharov and Vitaly Ginzburg. But the second Soviet H-bomb, based on the Third Idea, remains puzzling.

There are three quite different solutions to this riddle. Their authors are:

  1. a Russian H-bomb veteran turned historian, German Goncharov, who worked in Sakharov's group in the spring of 1954
  2. an American H-bomb veteran turned historian, Thomas Reed, who used to work under Teller
  3. a Russian-American historian of science, who is writing this blog post

All three of us met in 1996 in Russia at a big international conference on the history of the Soviet nuclear project. A year later we took part in a semi-closed conference at the Livermore Lab. In my biography of Sakharov, I thanked both Goncharov and Reed for their knowledge and insights they shared with me. But as to the riddle of the Third Idea, we disagreed with one another. So I leave it up to you to evaluate such evidence as there is.

The Soviet H-bomb project began with the first bits of intelligence on American H-bomb research that came to the Soviet Union in 1945. It spurred a small-scale H-bomb research project under renowned theorist Yakov Zeldovich. The Soviet version of the American design (Classical Super) was named Tube (Truba) because of its cylindrical shape. But Zeldovich was still mainly focused on the A-bomb, which would be tested in August 1949. In the spring of 1948, the main Soviet atomic spy, Klaus Fuchs, provided still more intelligence on the H-bomb. The most surprising feature of this report was the idea of radiation compression, or implosion, which eventually made possible the first American thermonuclear explosion in 1952.

Fuchs's information seemed so elaborate that the Soviet leaders perceived it as proof of an intensive American H-bomb effort. The order came down to strengthen the H-bomb project by establishing an additional theoretical team to assist Zeldovich's.

That new team included Sakharov. Within a few months, he invented a brand-new design for a thermonuclear bomb that employed a special way of compressing a spherically layered configuration, dubbed Sloyka (Layercake). Ginzburg then added his idea of a specific thermonuclear explosive, LiD, dubbed Lidochka (a Russian female pet name). Soviet leaders decided to pursue both designs separately in two theoretical groups.

Sakharov, in his memoirs, straightforwardly asserted his and Ginzburg's authorships of the First and Second Ideas. So his ambiguous description of the emergence of the Third Idea is really puzzling.

The very fact that Fuchs was somehow related to H-bomb project was no big surprise for historians, but declassified Soviet documents made clear how strong his relation was. It made also more understandable why Bethe, after Fuchs's arrest, described him "as extremely brilliant and as one of the top men in the world on atomic energy" and why Teller was so concerned that Fuchs's information might have given the Soviet Union a head start. But Teller's "stroke of genius" came after Fuchs's arrest. Bethe was sure that Fuchs's information could not have helped the Soviets to reproduce it. Teller worried it might well have, since, to his mind, the principle of radiation implosion "was a relatively slight modification of ideas generally known in 1946."

One Soviet document seems to support Bethe. Evidently, Zeldovich studied Fuchs's report in the spring of 1948, but failed to appreciate its idea of radiation as a tool of compression. But another document seems to support Teller. In January 1949, Sakharov -- within his report on Layercake -- proposed the "use of an additional plutonium charge [i.e. A-bomb] for a preliminary compression of Layercake." The problem, however, was how to make atomic compression highly symmetrical. If Sakharov had had access to Fuchs's report, he would surely have appreciated its idea of radiation implosion.

For the next five years, Zeldovich's team pursued the Tube, while Sakharov's team developed Layercake. The latter's test in August 1953 was a great triumph for Sakharov, at the age of 32. However, according to a document dated January 14, 1954, by that time the Tube hit a dead end, the Layercake turned to be unimprovable, and Sakharov and Zeldovich returned to the old idea of atomic compression. It took a further few months for them to discover the Third Idea.

There are no documents shedding light on this crucial period. There are only three fragments of oral history:

  1. Sakharov's close associate Yury Romanov says: "The Third Idea emerged in the spring of 1954. It began when Sakharov brought the theorists together and set forth his idea about the high coefficient of reflection of impulse radiation from the walls made of heavy material."
  2. An associate of Zeldovich, Viktor Adamskii, recollects that one day Zeldovich burst into his study and joyously exclaimed: "We should do it differently, we'll release radiation from a spherical device!"
  3. The most intriguing is the testimony of another associate of Zeldovich, Lev Feoktistov, who questioned the Soviet originality of the Third Idea. To him the Third Idea was too sudden to be truly indigenous. He says there were "neither drawings nor accurate data from the outside," but that Sakharov and Zeldovich were bright and experienced enough to be enlightened by just a few words' hint from a well-placed spy.

Goncharov, the key figure in the project to declassify and publish the Soviet nuclear archives, believed that the clue to the riddle of the Third Idea was in the Fuchs's report of 1948. His problem was how this clue could reemerge six years later. Goncharov decided the triggering fact was the 40-fold gap in yields of Layercake (~400 kilotons) and the U.S. H-bomb tested on March 1, 1954 (~15 megatons). But there is no single evidence that this gap was realized by anybody in the Soviet nuclear project in 1954.

Reed pointed out that Fuchs's knowledge didn't inspire American physicists until Teller's discovery of 1951, so why would it have helped the Soviets? Reed argued there was an additional act of espionage in 1954. But this, too, is undocumented.

My own reading of Sakharov's writings suggests that he never had access to Fuchs's report of 1948. Consider the following words in Sakharov’s memoirs, with a note he added toward the end of life:

Now I think that the main idea of the H-bomb design developed by the Zeldovich group was based on intelligence information. However, I can't prove this conjecture. It occurred to me quite recently, but at the time I just gave it no thought. (Note added July 1987. David Holloway writes in "Soviet Thermonuclear Development," International Security 4:3 (1979/80), p. 193: "The Soviet Union had been informed by Klaus Fuchs of the studies of thermonuclear weapons at Los Alamos up to 1946. … His information would have been misleading rather than helpful, because the early ideas were later shown not to work." Therefore my conjecture is confirmed!)

I think the following happened. In January 1954, Sakharov was thinking about how to realize the idea of atomic compression. By May, he had realized that the flash of light from an exploding A-bomb -- as opposed to the neutrons and other material products -- would be the best tool to compress the thermonuclear charge. He realized that the flash of light could be confined within a metal casing for a microsecond or so, long enough to compress the charge symmetrically. He discussed this idea with Zeldovich, who recognized this as the idea he had failed to appreciate in Fuchs's report six years earlier. As soon as he grasped Sakharov's arguments, he (with his close associate Trutnev) brought in some details from Fuchs's report, but never mentioned the actual report to Sakharov, since the latter was not granted access to it. So, Sakharov drew the conclusion that Zeldovich and Trutnev had also been very close to the Third Idea.

In other words, Sakharov had the same stroke of genius as Teller. Both men were faced with the same problem and possessed the same level of scientific expertise, so it is entirely plausible they arrived at the same solution independently. Teller himself said that his colleagues, such as Bethe or Enrico Fermi, were eminently capable of coming up with the idea themselves, had they applied themselves to the problem.

Of course, my argument relies on circumstantial evidence. So riddle of the Third Idea remains open. Did it come from:

  • Fuchs report of 1948, which reemerged in the spring of 1954
  • a Soviet sleeper agent reactivated by the KGB in the spring of 1954
  • Andrei Sakharov's head
  • none of the above

Cast your vote here.

For more details, see the following publications: