Abstract
Particle physics took off after World War II. Big machines, particle accelerators, were constructed, starting with cyclotrons (a few meters diameter) and evolving to the real big machines such as LEP (Large Electron Positron collider, diameter 8.5 km) at CERN, Geneva and the Tevatron at Fermilab near Chicago (2 km), a proton collider. Given these big sizes it is not hard to imagine that we are reaching here the end of the line. Also the character of particle physics experiments changed drastically. In the beginning experimental groups contained just a few physicist while present day experiments are done by groups of typically 1500 physicists. Yet there is still some way to go, the future on the experimental side can be guessed till about 2025.
The theory is in a very remarkable state. In a sense it is suffering from its own success. Present day theory is ahead from experiment at this time theoretical predictions are more precise than experimentalists can measure. In other words, theorists have no idea what is going on beyond existing theories, no experiment is offering any clue that goes beyond our present knowledge. That does not mean that there are no problems; when looking at the known particles, showing a very remarkable structure, it is clear that there are structures that we do not understand. But till today no one has any idea about that.
The theory, without a clear direction from experiments, tends to develop on its own in a rather mathematical way. Lack of experiments facts makes present day theories indeed not more than that: exercises in mathematics. Moreover, once a theory has been developed we have no way of knowing if it is correct. They do not predict experimentally verifiable facts. This creates a definitely unhealthy situation, "theoretical physics" unchecked by experiment. We are referring here among others to string theory.
How to go on from here? There are definitely theoretical challenges, but it seems that we will have to wait till experiment catches up with theory. This may be when the LHC (Large Hadron Collider) comes into operation at CERN in 2005, or later as TESLA in Hamburg or CLIC at CERN (electron-positron machines) come into operation around 2010 or 2020 respectively. But that is far from sure.
The theory is in a very remarkable state. In a sense it is suffering from its own success. Present day theory is ahead from experiment at this time theoretical predictions are more precise than experimentalists can measure. In other words, theorists have no idea what is going on beyond existing theories, no experiment is offering any clue that goes beyond our present knowledge. That does not mean that there are no problems; when looking at the known particles, showing a very remarkable structure, it is clear that there are structures that we do not understand. But till today no one has any idea about that.
The theory, without a clear direction from experiments, tends to develop on its own in a rather mathematical way. Lack of experiments facts makes present day theories indeed not more than that: exercises in mathematics. Moreover, once a theory has been developed we have no way of knowing if it is correct. They do not predict experimentally verifiable facts. This creates a definitely unhealthy situation, "theoretical physics" unchecked by experiment. We are referring here among others to string theory.
How to go on from here? There are definitely theoretical challenges, but it seems that we will have to wait till experiment catches up with theory. This may be when the LHC (Large Hadron Collider) comes into operation at CERN in 2005, or later as TESLA in Hamburg or CLIC at CERN (electron-positron machines) come into operation around 2010 or 2020 respectively. But that is far from sure.