A clue, a scratch, an imprint, a glimpse, a shadow, remains of something already gone, fragments of something which is or was, indirect pictures of something which cannot be seen by the naked eye...... Can traces be unique? Isn't there always room for more than one interpretation? How is one to know which knowledge is required to interpret the traces, isn't the interpretation always coloured by preconceived notions, your frame of mind, your fears...? Is everything always the trace of something else? A trace; a beautiful fragment. Two or more traces, you start to conclude, distinguish a context, see the shadow of a pattern, start to play the sleuth and throw yourself out into the landscape where the traces were made. This landscape often appears like a snow scape, where animals and man have left their tracks, traces which the wind erased or deformed long ago, so that whoever comes after can never read the name, age, size of shoes or direction of the passer-by. There are short-lived traces in water, air, snow and sand, and there are lasting traces in rocks and the crust of the earth. How difficult the interpretation is when it comes to traces in time and space, it does not depend so much on the extent of the trace, but rather the breadth of mind of the interpreter. Who can interpret the traces of particles that exist for 10-23 seconds, or the expansion of the universe over billions of years? To register traces of particles one uses materials which trap, guide and shift the wave-length of light. This materials were originally produced for specific experiments, the shape was determined by functional and mathematical aspects. The materials are made not to be shown, but to be used! I've taken them out of their context to show them in the shape they were given because of their earlier function, and the shape I have given them in their new function as room creating elements. Thus I want to try to describe the landscape where light left its traces and made it possible to enter the passage it created.. Monica Sand

Elementary particle physics is the branch of physics which tries to understand matter in terms of its smallest constituents and how those interact with each other through different forces. This effort has been very succesful. Matter can be understood as built from only a score of fundamental building blocks which influence each other through only three types of forces. This toolkit is in principle enough to explain everything that can be observed around us, and even phenomena which lie far outside our common experiences, like for example the conditions in the early universe a millionth of a second after the Big Bang. This knowledge of first principles is how-ever far from adequate when describing our complicated everyday life. Already a large atomic nucleus is in some respects too complicated to be described in terms of interacting elementary particles without introducing simplifying assumptions. If we then want to understand things like dandelions, red wine and tiny dogs we need a completely different set of concepts. The most common question you encounter when talking about elementary particle is: "Now, these particles are so small that one cannot see them, even with a microscope, how can you then possibly learn anything about them?". The fact is that noone ever saw, and noone ever will see an elementary particle on e.g. a micrograph. Instead the physicist has to perform experiments where the elementary particles leave traces in some kind of detector, and then interpret these traces. These traces can be more or less concrete.If you for example look at a bubble chamber photograph you almost think that you can 'see' how the particles have moved through the liquid. Modern detection techniques are often more abstract, but all rely on the elementary particles toinfluence their local surrounding in a measurable way. The task is often to detect ionization, to utilize the fact that a charged particle 'disturbes' the atoms when it moves through a material in such a way that some of them loses one or more of its electrons. One therefore finds free electrons and positively charged ions in the wake of a charged particle. Another type of 'disturbed' atoms return to their normal state by sending out a small flash of light, this is what happens in a scintillator. So what an elementary particle physicist 'sees', and interprets, is never the particles proper, but rather the traces in the form of disturbed atoms which they leave behind in the apparatus. These traces are then interpreted in terms of particles and their properties. The results obtained are compared with theoretical predictions to refine those. For an experimentalist much of the effort is spent trying to interpret the traces of particles registred in the apparatus, but at least as much effort goes into giving the physicists 'eyes', detectors which are sensitive to these traces. One has to 'catch' the traces the particles leave on the atomic level and amplify and record these effects in order to make them available for interpretation. Sten Hellman Fysikum Stockholm