Yukawa Hideki and the meson

In short

What holds a nucleus together?

Atomic nuclei are made of protons and neutrons, held together by a strong force.

Yukawa thought that this force is carried by particles, and that the range of the force is related to the mass of the particle that carries it.

In 1934 he predicted that this particle should have a mass about 200 times that of an electron. He called it a meson, because its mass is between that of a proton and that of an electron.

Proof of the meson

Later experiments showed that mesons exist. In 1947, the British physicist Powell found tracks of mesons in cosmic rays, using thick photographic plates left at high-altitude observatories.

The Nobel Prize in Physics 1949 went to Yukawa alone, “for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces”.

Japan’s first Nobel Prize

The prize came in 1949, four years after the end of the Pacific War. Yukawa, then a professor at Kyoto University, was the first Japanese person to win a Nobel Prize.

Among Asian people, he was the third Nobel laureate, after the Indian writer Rabindranath Tagore and the physicist Chandrasekhara Raman.

Think about it

• Q1Yukawa predicted a particle that no one had seen. How could a theory point to something before it is found?
• Q2Why do you think the news of the prize meant so much to people in Japan in 1949?
• Q3Who was the first Nobel laureate from your country?

There is no single right answer. Talking it over with someone at home is a good way to learn.