Presentation Speech by Professor Lars Brink of the Royal Swedish Academy of Sciences, December 10, 2004.

Professor Lars Brink delivering the Presentation Speech for the 2004 Nobel Prize in Physics at the Stockholm Concert Hall. Copyright © Nobel Media AB 2004 Photo: Hans Mehlin

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

When Isaac Newton saw the apple fall he understood how gravity works and he was able to formulate a law for it. He applied this law to explain the orbit of the moon travelling around the earth. At the same time he could explain how the force of gravity between two bodies diminishes when the distance between them increases. Later, it was discovered that there is a similar decrease of the electric force between two bodies carrying electric charges as they are separated from each other. But what is a force? How do forces arise? How can they act in empty space – in a vacuum? Almost exactly a hundred years ago, Albert Einstein realized that light can be described as a bunch of particles, called photons, which move from one electrically charged body to another and mediate the electromagnetic force between the bodies. They mediate a force in the same way as the football that the goalkeeper cannot stop or as the cannon ball that can destroy a tower of a fortress. We can also remind ourselves of how Baron von Münchhausen threw himself and grabbed a cannon ball to travel over a wall.

Why does the force diminish with increasing distance? The cannon ball meets resistance from the air but the force due to the motion of photons also decreases in vacuum. This is because according to quantum theory a beam of photon particles can also be thought of as a wave (as in the standard description of light). The further away from the source you are, the smaller the section of the wave that will hit you. When the wave that appeared when Krakatoa exploded in 1883 hit the coastline of Sumatra it caused enormous damage, but when it hit the coast of Africa much further away it was just a swell in the water. We can also understand the force of gravity in the same way as the electromagnetic force. We have not been able to discover the force particles here, but we are convinced that they exist.

At the same time as the physicist wants to understand the fundamental forces that act in Nature, he or she wants to understand what the fundamental building blocks are. We have divided up matter into atoms, which have been further subdivided into electrons and the nucleus, which is made of protons and neutrons. It became clear very early on that within the nuclei there must be different interactions, the weak nuclear force responsible for the radioactive decays, and the strong nuclear force holding the nucleus together even when the repulsive electric force between the protons tries to separate them. These forces act over very short ranges, ranges as improbably small as the nucleus itself. To understand these forces and to understand the fundamental building blocks of Nature have been the great task of particle physics for the last fifty years. This year’s Nobel Prize completes the picture that the work behind several earlier prizes initiated and as a result we now know the fundamental building blocks and we have a description of the four fundamental forces.

One of these earlier discoveries was the understanding that protons and neutrons are composite objects, which are made of even more fundamental particles called quarks. It was found that quarks come in various species that have different types of charges, which came to be called colour charges. These charges are in some sense analogous to electric charges, so it was natural to believe that the quarks should behave like the electrons. However, unlike electrons, no free quarks were ever discovered. Perversely, it seemed as if the force increased as quarks were separated. Conversely, plenty of evidence accrued that when two quarks got close to each other they could hardly feel each other. This behaviour is called “asymptotic freedom.” Could a theory of the quarks behave like that? This behaviour of the force between quarks seemed to be outside the realm of any theory of the kind that had so successfully explained the electromagnetic force. So around 1970 particle physics stood in front of a great dilemma. Common sense, and all calculations that were attempted told us that the force between quarks should behave in a manner that contradicted the experimental facts. In the end the issue came down to one specific question. Does any theory predict a minus sign in the right place? All theories that were tested gave the incorrect positive sign.

In 1973, David Gross and Frank Wilczek and David Politzer considered a novel class of theories. To the surprise of the world and to their own great astonishment they found the result –11/3 that signalled that these theories are asymptotically free. Seldom has a negative result had such a positive effect! A theory for the strong force between the quarks could now quickly be formulated and a detailed comparison with experiments could be performed. During the last fifteen years experiments at large accelerators have confirmed the theory with great accuracy. The theory of Gross, Politzer and Wilczek successfully describes the physics of quarks, the matter from which we are to a very large extent built. Since the discovery, further research has shown that these theories are unique. No other theories can account for the experimental picture and it is wonderful to know that Nature has chosen the only theory that we have found to be possible.

Professor Gross, Professor Politzer, Professor Wilczek.

You have been awarded the 2004 Nobel Prize in Physics for your discovery of asymptotic freedom in the theory of the strong interactions. It is an honour for me to convey the warmest congratulations of the Royal Swedish Academy of Sciences. I now ask you to step forward to receive your Nobel Prizes from the hands of His Majesty the King.