Julius Wess - a Life Concerned with Symmetry

bridges vol. 20, December 2008/ Feature Article

By Daniela Klammer




{enclose Vol20_Wess.mp3}

julius_wess_small.jpg
Prof. Julius Wess

I met Prof. Julius Wess at a conference in Alessandria, Italy, on a wonderful spring day in 2007. Sitting next to him at dinner, I found that my table neighbor - one of the most distinguished theoretical physicists of our time - happened to be a friendly and gentle man. We talked about the current situation of physics in Vienna. He had the impression that Vienna had changed a lot since the fall of the Iron Curtain. This encounter took place about half a year before his unexpected death.

Despite his outstanding contributions to theoretical physics, Julius Wess is hardly known to the Austrian public. This is a short story about a fine man and superb physics.

{access view=guest}Access to the full article is free, but requires you to register. Registration is simple and quick – all we need is your name and a valid e-mail address. We appreciate your interest in bridges.{/access} {access view=!guest} The academic career of Julius Wess began at the Institute of Theoretical Physics at the University of Vienna, where he received his Ph.D. in 1957 under the supervision of Prof. Hans Thirring. Erwin Schrödinger, one of the founders of quantum mechanics, was an examiner at his defense. After a fellowship at the European Organization for Nuclear Research (CERN) and a couple more years back in Vienna, Wess became an associate professor at the Courant Institute of New York University in 1966. Two years later, he accepted a call to a full professorship in Karlsruhe, Germany. He ended his official career in Munich, where he became director of the Max-Planck Institute for Physics and a professor at the Ludwig Maximilian University in 1990. Still active in publishing and organizing conferences, he worked after his retirement at the Deutsches Elektronen Synchrotron in Hamburg, where he died on August 8, 2007, due to a fatal stroke. He was 72.

Scientific Achievements

During his time in Vienna from 1962 until 1966 Julius Wess met Prof. Bruno Zumino, a visiting scientist at that time. Their intense and highly fruitful collaboration would continue for decades. The outstanding highlight of this strong liaison was the 1973 publication "Supergauge Transformations in Four Dimensions." Cited over 1400 times, this paper has had tremendous implications for particle physics. What is known today as supersymmetric theories and the Wess-Zumino model is founded on this work.

While Wess is mostly known for his work on supersymmetry, it is certainly not the only field to which he contributed. Dr. Harold Steinacker, a former collaborator of Julius Wess and a Ph.D. student of Bruno Zumino, points out that Wess initiated a number of new research subjects which then became some of the main themes in current theoretical physics. Julius Wess himself did not care much about scientific fashion or trends. From 1991 on, his favored subject was Noncommutative Quantum Field Theory, a theory that has the potential to describe space-time at very small distances and is thus a possible route towards an understanding of quantum gravity.

Working in the Wess group

One of his former Ph.D. students, Michael Wohlgenannt, describes working with Wess as intense - that is, being at the blackboard 2 to 3 hours per day. "You would see how fine his comprehension was, how much he could guess beforehand." Steinacker says Wess was a friendly and patient teacher who could not stand arguments that were too general or "high-in-the-sky," "Only explicit mathematical derivations and computations could catch Wess' attention. He liked to work out everything from scratch, typically collaborating with a few selected individuals while managing large research groups."

Throughout his scientific carrier, Wess was highly attached to Southeastern Europe. After the 1990s war in former Yugoslavia, he used his influence to organize meetings, financial support, and fellowships such that scientific contacts could be revived and strengthened. "He was always ready to help to improve science," says Prof. Josip Trampetic from the Ruder Boskovic Institute in Zagreb, Croatia.

bayrischzell_small.jpg
Bayrischzell from above: Bavarian Alps in winter

Born in 1934 in Oberwölz, a small village in Styria, right in the heart of the Austrian Alps, Julius Wess enjoyed the mountains. He started the tradition of annual small meetings at the end of winter in Bayrischzell, Bavaria - combining skiing and physics, two of his favorite activities.

Julius Wess certainly would not fit the stereotype of the absent-minded professor. But you could call him an absolutely devoted physicist. Prof. Harald Grosse of the University of Vienna remembers that in 2003, during a stay in Vienna for a Senior Lecture, Julius Wess suffered a serious heart attack. Grosse, a colleague and friend of Wess for years, came to visit him in the hospital. He remembers the first thing Wess gave him were two sheets of paper with formulas: his first sketches for a deformation of Einstein gravity. Wess was also a friend of rather dark humor. During his next visit in Vienna he wanted to go to the very same restaurant where he had broken down before. "How far will I come this time?" he asked.

Julius Wess was always concerned with symmetries, which were the central theme of his thought. To pay tribute to his achievements let me introduce you to the world of supersymmetry.

The Standard Model - the fundamental structure of matter that we know of today

In the 20th century the understanding of most concepts in physics changed: What are space and time? What are forces? What is matter made of?  

At the end of the 1960s, a theory now called the Standard Model of particle physics found its final shape. It describes our current understanding of the fundamental constituents of matter and how forces act on them. Next to Einstein's Theory of General Relativity, it is the second cornerstone of modern physics.

According to this theory, there are 12 fundamental matter particles: Six quarks and six leptons (see Figure 1). Moreover, for every particle there is an antiparticle - a particle with the same mass but with opposite electric charge.

standard_model_small.jpg
 Figure 1: The Standard Model of particle physics.



How do particles attract and repel each other? Matter particles interact with each other via forces. There are four fundamental forces known today: The electromagnetic force, the weak force, the strong force, and gravity. A force acts on matter particles via the exchange of corresponding force particles. Each type of force goes along with a corresponding particle. Force particles and matter particles belong to two types of particles that are very different in their behavior: fermions and bosons. Two fermions will never be found on the same spot. Bosons, on the other hand, really like to share the same place. Fermions are lone wolves, bosons are gregarious. The Standard Model tells us that matter particles belong to the fermionic group, and force particles to the bosonic.

Supersymmetry - going beyond the Standard Model

Although these two kinds of particles are so different, in the 1970s Julius Wess and Bruno Zumino showed that there exists a symmetry - called supersymmetry (SUSY) - that could interchange these particles, and that the laws of physics would not change under such an interchange. In order to make this symmetry work, you need the same number of bosons and fermions. It is very much like in a chess game, where you can interchange black and white chessmen. For there to be a symmetry, you must have the same number of chessmen of each color and of each type.
In the same spirit, the paired bosons and fermions must have the same mass and charge as each other. Each particle should have its partner, which means that in the theory of supersymmetry the number of particles is doubled with respect to the number of particles in the Standard Model.

 
sssm_small.jpg
 Figure 2: In a supersymmetric world, the number of particles is doubled. For each particle (matter and force) of the Standard Model, there is a corresponding supersymmetric partner particle.



At this point you will probably become suspicious. If the supersymmetric particles have the same mass as their partner particles, we should have already found these particles, but we have not. Where are they?
Although symmetry is one of the most powerful concepts in theoretical physics, the rather difficult idea of symmetry breaking is equally important. Imagine you are going with your friends to the movies. There are cup holders between the seats in the cinema. The cup holder on the right is just as good as the one on the left. Hence there is a symmetry. However, the moment that you or someone else chooses a holder for his or her drink the symmetry is broken. You can put this idea of symmetry breaking into mathematical language which, in return, will tell you that the superparticles we are looking for can be rather heavy. Precisely because the symmetry is not exact, i.e., it is broken, the masses of the particles do not have not match.

With the start-up of the large hadron collider (LHC), the new accelerator at CERN near Geneva, energies could be high enough to create these particles. Julius Wess was very excited about the possibility of discovering them, which would demonstrate that supersymmetry is realized in nature -  a prediction that would certainly justify a Nobel prize.

The charm of supersymmetry

Ultimately most physicists hope to find a single theory describing the universe, from its smallest to its largest scales. Supersymmetry beautifully predicts a unification of the strong, the weak, and the electromagnetic forces, and thus it could be a major step towards such a unified theory.

Supersymmetry could also tell us what the universe is mostly made of. Astronomical data show that 80 percent of the matter content of the universe consists of some strange form of matter that does not interact with light. Hence we cannot see it, we can only observe its gravitational effects. This elusive matter is called dark matter. We do not know much about it, except that it cannot be one of the particles from the Standard Model. Supersymmetry provides the most promising dark matter candidate: the lightest, electrically neutral supersymmetric particle.

Due to its mathematical beauty and due to its capacity to provide answers to many of the most urgent problems such as the unification of the forces, dark matter and others, supersymmetry has become perhaps the most studied novel theoretical concept over the last 30 years. When Frank Wilczek, 2004 Nobel Prize laureate, recently came to Washington, DC, to promote his newest book, he was talking only about supersymmetry. To him and many others, supersymmetry appears to be the natural pattern for understanding many of the riddles of the universe.

These are exciting times for particle physicists. According to Lyn Evans, LHC project director, an answer might be very close. "If SUSY is real, it should be one of the earliest results from the LHC. Now let's get the machine up and running," he says.

Dr. Evans, I totally agree. And so would Professor Wess.


***

The author, Daniela Klammer, is a Ph.D. candidate in the Working Group of Mathematical Physics at the University of Vienna. 
 

References

The standard text book on supersymmetry:
Julius Wess and Jonathan Bagger, Supersymmetry and Supergravity, Princeton Series in Physics, 1992

Popular science book on topics in modern physics such as supersymmetry and extra dimensions:
Lisa Randall, Warped Passanges, Published in Hardcover by ECCO Press of HarperCollins; 1 edition (August 30, 2005)

Obituary in New York Times
http://www.nytimes.com/2007/08/27/world/europe/27wess.html?_r=1&oref=slogin

Ian J R Aitchison, Supersymmetry and the MSSM: An Elementary Introduction
http://arxiv.org/abs/hep-ph/0505105

 

{/access}