Presentation Speech by Professor Gösta Ekspong of the Royal Academy of Sciences

Translation from the Swedish text

Your Majesty, Your Royal Highnesses, Ladies and Gentlemen.

By decision of the Royal Swedish Academy of Sciences, this year’s Nobel Prize for Physics has been awarded Professor James Cronin and Professor Val Fitch for their discovery in a joint experiment of violations of fundamental symmetry principles. The experiment was carried out in 1964 at Brookhaven National Laboratory in the United States of America and was concerned with a forbidden decay of a certain type of elementary particles, named the neutral K-meson.

Suppose the TV-news suddenly reported one evening that visitors from outer space were planning to land on Earth; that the space travellers have radioed a demand for immediate information about the composition of the Earth. Does it consist of Matter or Antimatter? The answer to this question is one of life and death. The two kinds of matter are known to annihilate each other atom by atom. The space travelers claim, furthermore, that the nature of their own kind of matter was determined before leaving. What they now want to know is, whether the same tests have been made on Earth. Thanks to Cronin’s and Fitch’s discovery it is now possible to give them a clear-cut answer, so they can avoid a disastrous landing. Let us now leave the world of science fiction, remembering, however, what a fortunate circumstance it was that no space visits occurred before 1964.

Symmetries are science’s lodestars and symmetry principles act as guiding rules to help us discover the mathematical laws of Nature. Three mirror symmetries are of immediate interest in relation to the prize-winning discovery. One of them is ordinary mirror reflection, which corresponds to switching left and right. The other two symmetries of interest concern reflection of time and of charge, which implies switching forward and backward movements and switching matter and antimatter, respectively. In the latter case it is positive and negative electric charges that are switched.

The beauty of spatial symmetries is well known in the realm of art and architecture, from the ornamental arabesques of the old Alhambra to the recent intricate woodcuts signed by Escher, from the palace of the Doges in Venice to the Town Hall in Stockholm. A master such as Johann Sebastian Bach has created music with ingenious symmetries, generated both by reflection in space of the theme and by reflection in time when the theme is played backwards. The laws of physics resemble a canon by Bach. They are symmetric in space and time. They do not distinguish between left and right, nor between forward and backward movements. For a long time everyone thought it had to be like that. A remarkable exception exists, however, in the law governing radioactive decay, which violates the left-right symmetry. Lee and Yang were awarded the Nobel prize for physics in 1957 for this revolutionary discovery.

The third mirror symmetry is not present in art. The laws for electric and magnetic phenomena contain a complete symmetry between the two kinds of electric charge. The discoveries of antimatter with plus and minus charges in exchanged roles are among the most profound of the last half-century. Nowadays microscopic amounts of antiparticles are produced with relative ease in such special laboratories as Brookhaven National Laboratory in the U.S. or CERN in Europe.

Cronin and Fitch elected to carry out tests to find out whether a certain decay of K-mesons occurred, in spite of being forbidden by symmetry. Their research team found that two out of a thousand K-mesons did in fact decay in the forbidden manner. This means that some law of Nature now must be changed or a new law invoked. In what way does this discovery concern antimatter? As early as 1955 Gell-Mann and Pais had analyzed the neutral K-mesons and found that they are strange, indeed unique in their ambivalence with respect both to matter and antimatter. If perfect symmetry were to prevail, a decaying K-meson would have to be antimatter in exactly half the cases and in the other half, matter. Lee’s and Yang’s Chinese revolution did not change the conclusions, but new arguments were required. Cronin and Fitch interpreted the results of their experiment as a small but clear lack of symmetry. Their conclusion has been confirmed in a long series of other experiments. The new symmetry violation constitutes the basic prerequisite for the claim that a definite answer can be relayed to our visitors from outer space.

The discovery also implied consequences for time reflection. At least one theme is played more slowly backwards than forwards by Nature.

Artists nearly always introduce symmetry breaking elements into their works. Perhaps, the laws of nature, too, are in the deepest sense works of art. Violations of perfect symmetry open roads to new insights, or in the words of a poet:

“A knot there is in th’entendrill’d arabesque
No mortal eye but mine has ever seen”.

Professor Fitch, Professor Cronin,

The scientific world was shocked when you first announced your discovery. Nobody, absolutely nobody, had anticipated anything like it. You had pursued your experiment with skill and determination and found the impossible to be possible.

On behalf of the Royal Swedish Academy of Sciences I have the pleasure and the honour of extending to you our warmest congratulations. I now invite you to receive your Prizes from the hands of His Majesty the King.