Roughly 60 years ago, the discovery of nuclear power promised the ultimate solution to the world’s energy problems. Today, this promise remains unfulfilled. Development of nuclear technology has stagnated, with nuclear power plants still using technology developed a half-century ago.
Nuclear energy is produced in the decay processes of heavy elements, like Uranium or Thorium. The nuclei of their atoms usually decay into two smaller nuclei and a couple of neutrons, releasing many million times more energy than any chemical process ever could. Heavy elements contain so much energy because they stored a fraction of the energy released in the supernova explosion that created our Earth and the solar system around five billion years ago.
Today’s power plants use as a fuel a special kind of Uranium, U-235, which is burned in a chain process whereby the neutrons from one nuclear decay induce the next decay. Neither Uranium-238, which is roughly 100 times more abundant than U-235, nor Thorium, which is even more accessible, is used to produce energy on a large scale.
Indeed, in principle, every heavy element, even lead, is a potential source of nuclear energy. Everybody who has studied nuclear decay processes has been struck by the myriad possible ways in which energy can be produced.
Progress in gaining access to this resource has, however, been very slow. From the scientific point of view, the main problem is a lack of knowledge. Despite many hundreds of well-functioning nuclear power plants, our understanding of nuclear forces is only empirical, and empirical knowledge is always imperfect.
For example, in producing nuclear energy, the decay reactions repeat many times, with the imperfections of every repetition resulting in a loss of predictive power of computations. This hampers the optimization process, and is one of the main reasons why several large projects investigating energy production using more abundant Uranium-238 or Thorium (fast breeder reactors) were closed in Europe and the United States before they achieved the expected level of performance.
Another problem is the nuclear waste that emerges when energy is produced in the decay process. The waste can be substantially, or even completely, reduced if we could use an alternative form of nuclear decay that is triggered by externally accelerated particles. Here, too, however, we need more precise knowledge of the properties of nuclear processes.
The force binding atomic nuclei is a special case of the “strong force,” one of the four fundamental forces in nature, and is extremely difficult to investigate, because it acts very quickly and violently. Around 50 years ago, it was proposed to study the strong forces by firing protons at each other at very high energies.
The acceleration of particles at high energy slows down all physical processes, because, according to Einstein’s theory of relativity, time runs more slowly for fast-moving objects. Since protons are the simplest nuclei, it was hoped that, at high energy, the forces acting during the scattering of protons could be observed and analyzed like in a slow-motion film, providing a precise understanding of the strong force.
Several large accelerator research centers were built, and the scattering of particles at high energies revealed a fascinating structure of matter. New particles, called gluons, were found to mediate the strong force. Their discovery should provide a clue to precise knowledge of the strong force.
At short distances, gluons create an attractive force that is pretty weak and well understood. But, at larger distances, comparable to the proton radius, the force becomes really strong, and a very large number of gluons is involved, forming complicated structures that are not well known today. Therefore, for some time, it was not expected that the properties of the strong force could be directly derived from the properties of gluons.
In the last few years, however, experiments at the HERA accelerator in Hamburg, Germany, have observed the strong interaction effects in slow motion, which could open a way to a precise understanding of the strong force. At the HERA accelerator, the scattering of electrons on protons was studied at the highest energies ever for this type of experiment.
The HERA machine operated from 1992 until 2007. One of its most important discoveries was that several distinct phenomena observed at high energies and short distances could be clearly attributed to the emission of gluons and the emergence of gluonic structures. By observing how these structures change while increasing the distance, it should be possible to follow – and thus to understand –the action of the strong force.
The appearance of such clear gluonic structures was unexpected; the experiments at HERA were not designed to study them. But the precision experiments required to measure the strong force can be designed and built with known technology. So two large groups of physicists – one concentrated around the Brookhaven and Jefferson National Laboratories in the United States, and the other around CERN in Geneva – are proposing to restart the investigation of electron-proton interactions.
The study of these interactions should provide a precise understanding of the strong force. And, as the history of physics has shown, a better understanding of natural forces will open new, completely unexpected possibilities. For example, our understanding of electromagnetic force, developed mostly in the nineteenth and early twentieth centuries, gave rise to today’s breathtaking developments in telecommunications, computers, chemistry, and material sciences.
A precise understanding of the strong force could be just as important, opening new ways to use nuclear-energy resources while solving the problems of safety and nuclear waste.
Henri Kowalski is a senior Scientist at Deutsches Elektronen-Synchrotron, DESY, Hamburg.
Copyright: Project Syndicate, 2011.