Wednesday, May 26, 2010

The Mystery of Yukawa's "New Insight" (Revised)

Please read the Second Revision instead of this article, which is retained only as the record of the processes of my solving the mystery.

The crucial point in the development of Hideki Yukawa's meson theory came to him one night in October 1934. He writes in his autobiography Tabibito [1] as follows:
My new insight was the realization that this distance [the effective range of the nuclear force] and the mass of the new particle that I was seeking are inversely related to each other. Why had I not noticed that before?

This passage contradicts the fact that Yukawa used the above relation already in 1933. Namely, in April that year he made his first oral presentation at a meeting of an academic society. It was entitled "A consideration about the problem of electrons within nuclei," and the meeting was the one held in Sendai by Physico-Mathematical Society of Japan. The abstract of the presentation includes the following sentence [2]:
From the fact that the electron has the rest mass, we consider that the strength of the interaction decreases rapidly as the distance between the neutron and the proton becomes large compared with h/(2πmc).

In the work of the above presentation, Yukawa treated the possibility that the electron might be the mediator of the nuclear force between the neutron and the proton. This hypothesis included difficulties related to the spin and statistics of the electron. Therefore, Yukawa totally abandoned the hypothesis later together with the relation of the distance and the mass of the particle that would mediate the nuclear force [3]. Did this abandonment make it necessary for Yukawa to rediscover the relation? If so, this explains the contradictory remark of his "new insight."

We find another possible explanation of the contradictory description in the process of the making of Yukawa's autobiography. Hisao Sawano of the Assahi Shimbun Company helped the publication by editing Yukawa's manuscript [4]. If Sawano's editing had been to such an extent as to change Yukawa's original version into more dramatic one here and there, the passage that began with "My new insight" might have been Sawano's modification. To eliminate the expression different from the fact, Yukawa, perhaps, regularly checked the changes made by Sawano. However, Yukawa finished writing the last section related to the discovery of the meson theory just a few hours before his trip to Europe [5]. Thus, it is quite possible that Yukawa did not check Sawano's modification of that section.

Both Yukawa and Sawano are now in heaven. Thus, the mystery of the "new insight," i.e., which of the above two explanations was the case, does not seem to be solved easily. However, we have another clue to solve this problem.

In her autobiographical book* [6], Hideki Yukawa's wife, Sumi, wrote about the days of their visit to Stockholm for Hideki's receiving Nobel Prize. On arriving at a hotel there, they found that many journalists were waiting to interview Hideki. Sumi's passage about part of the interview is as follows:
One of the questions addressed to him was this: "We hear that the Japanese people study sitting on their legs in a straw-matted room. Did you, Dr. Yukawa, write your paper sitting on your legs or sitting on a chair at the Western-style desk?" Hideki thought a little while and said, "I did in neither of those ways. I put my thoughts together at night in bed." This is true. Hideki wrote the paper that brought him Nobel Prize after he had kept thinking many nights at the age of 27 in the year of Shōwa 9 [Note by the translator: 1934]. This story seems to have wrongly come across to Japan. Thus, in Japan they believe that the idea flashed to him in the middle of a night. [Translated by T.T. from Japanese.]
Here Sumi clearly denies the story in Ref. 1 of the "new insight" that came suddenly. Therefore, the second possibility given above, i.e., the contradiction is due to Sawano's modification, should be regarded as the case.

Hideki Yukawa's own autobiography was translated into English, French and German. Therefore, the wrong version of the story modified by Sawano has become famous world over. This is unfortunate. Sumi Yukawa's account quoted above should be informed widely.

(This article owes much to the discussion we have had at Osaka Science Museum among the members of "Citizens' Study Group on Hideki Yukawa.")


* The title Kuraku-no-Sono was taken from the street name in Nishinomiya City, where Hideki and Sumi Yukawa lived in the years shortly after their marriage. It has the meaning of "the garden (sono) of joys and sorrows (kuraku)."

  1. H. Yukawa, Tabibito (The Traveler), translated by L. Brown and R. Yoshida (World Scientific, 1982) p. 202.
  2. H. Yukawa, Sūbutu-gakkaisi, Vol. 7, No. 2 (1933) quoted in Nihon-no Buturigaku-shi (History of Physics in Japan) (Tokai University Press, 1978) p. 319 (in Japanese; English translation of the quoted passage by the present author).
  3. M. Kawabe and M. Konuma, Butsuri Vol. 37, p. 265 (1982) (in Japanese).
  4. H. Yukawa, Atogaki (Afterwords) in Tabibito, (Kadokawa, 1960) (in Japanese).
  5. H. Yukawa, Hon-no Naka-no Sekai (The World in Books) (Iwanami, 1963) p. 182 (in Japanese).
  6. S. Yukawa, Kuraku-no-Sono (Kōdansha, Tokyo, 1976) pp. 349–350 (in Japanese).


Chiara said...

I'm glad to read that the 'mistery' has finally a solution. This is probably the obvious one, but it's nice to think of Yukawa relaxing and spending the nights thinking of what would become his meson theory.
If find that often the moments of relax before sleeping are useful, as the ideas we have been thinking of so hard during the day can find their way through, and become clearer. It happened to me a lot when I was a student, and sometimes it happens now, but of course I don't think about such big ideas as Yukawa's theory (at least not in an orignal way...).

While reading I could not help smiling a little about the question asked by the journalists. I'd would certainly be curious to know if Yukawa had a special place to work, or a special way to concetrate, but such a specific question on where he was sitting seems a little too much. Did Mrs Yukawa write whether they had been surprised by the question? I like to think that maybe, when remaining silent for a while before giving his answer,
Yukawa himself made a little smile (kindly and respecfully, of course) in his mind.

Ted said...

Thanks, Chiara, for coming to read this essay so quickly and leaving a nice comment. Last evening I was revising the Japanese version of this essay and noticed that Sumi Yukawa's account denied the description in "Tabibito" only partially. This additional finding came from the quote, included in the endnote of the Japanese version, of Yukawa's words taken from Ref. 3. Therefore, it is necessary to revise the above essay again. I will tell you about the completion of the second revision by Twitter.

Ted said...

As for the question of the journalist quoted by Sumi Yukawa, I think neither that it was too much nor that Sumi and Hideki were surprised at it. In the days before about 1950, most Japanese people lived a life of old Japanese style sitting on their legs in straw-matted rooms. Hideki Yukawa also liked to wear an old Japanese style dress kimono in his home, though his study room was of western style. It is very likely that he studied sitting on his legs in his student days. The journalist might have wondered how modern theory could have come out if Yukawa's life style would have been of old Japanese style.