by Virginia Greco. Published: 06 February 2018

We talked with Johann Rafelski about his recent book on relativity, the way this theory is taught, and heavy-ion physics.

Johann Rafelski is a theoretical physicist and professor of physics at the University of Arizona (Tucson). After receiving in 1973 his PhD degree with Walter Greiner at the Johann Wolfgang Goethe University in Frankfurt, Germany, he started a successful research career which led him to work at top universities and research centres all around the world. He arrived at CERN in 1977 where he worked with Rolf Hagedorn and John S. Bell. Rafelski played an important role in the development of relativistic heavy-ion physics as a domain of research in its own right and proposed strangeness enhancement as a signature of quark-gluon plasma (QGP).

He has recently published a new book, called “Relativity Matters: from Einstein’s EMC2 to Laser Particle Acceleration and Quark-Gluon Plasma”, edited by Springer. We talked with him about this new endeavour, about relativity and heavy-ion physics. 


Professor Rafelski, a new book of yours, “Relativity Matters”, has been published recently. Could you tell us why you felt there was need for a new book on relativity? 

Relativity – excluding its sub-branch Gravity Relativity (GR) – is today a deeply neglected discipline. Even though it appears as a word in the expression “relativistic particle collisions”, only a few students arriving at CERN to work really understand the meaning of "relativistic". Many of them have done their exploration of relativity online. However, the web is an open domain where anybody can write whatever they want, so there is no guarantee that the concepts are explained correctly. Even the much-trusted Wikipedia has been taken over by "relativity gangs" – as I join a colleague from Lausanne in calling them  ­– spreading at best half-truths about this delicate field. 

Why is it neglected?

Relativity has fallen between the cracks of "more important" educational objectives at universities, preparing students for many things but not for working at CERN. Relativity is now often offered in fragments, taught by non-experts and using general physics texts that describe the subject in an incomplete and sometimes misleading way. For a subtle topic like relativity, this inadequate background spells doom.

In addition, when US publishers reissue new editions of their books, forcing students out of used book market, the rewrites are often done by ghost writers who know how to attract cognitive attention but have no deep comprehension of relativity. The outcome often is that, the higher the edition number, the less accurate becomes the presentation of relativity.

Despite what is presented in textbooks, it is important to keep up with the evolution of the field of relativity. It is dynamical, as for example John Bell's article “How to teach relativity” shows, (article available in John's reprint volume: “Speakable and Unspeakable...”) so one cannot simply read about it in Einstein's 1905 paper(s). 

This book is written having the student in mind and you state that professors often do not do a great job teaching special relativity at the University. Why?

There are two reasons. The first is explained in the preamble of my book, where I have reproduced a letter that my friend and mentor John Bell wrote in reply to a question of mine about which English language book to use to teach relativity. In this letter, we read: "The Einstein approach is perfectly sound, and very elegant and powerful (but pedagogically dangerous, in my opinion)". Practically, all teaching of relativity relies on Einstein pedagogy, which is really subtle, and even if the lecturer is excellent, the student can walk out of the classroom in a state of gross confusion. The only available book (before "Relativity Matters") that actively uses what I today call Lorentz-Bell pedagogy is the one I published in German in 1983.  It is through this early book that I discovered that I inadvertently hit on John Bell’s preoccupation with relativity.  Luckily, my book was using many ideas I must have picked up in conversations with Bell (I was a CERN fellow and worked with Bell between 1977 and 1979). The current volume, though, is very different from this early effort and is more insistent on applying Lorentz-Bell pedagogy to relativity. Besides, it addresses contemporary topics.

Secondly, as commented before, many elementary physics class texts are, in regard to relativity, prepared in a wanting manner. In addition, I think that non-expert (physical chemistry, biophysics, etc.) lecturers are teaching introductory relativity verbatim from such texts, since they never really studied this topic in depth. Some such lecturers will evenskip these book chapters, hoping relativity will be taught in another context, and this then leaves student education a bit haphazard but not necessarily wanting. 

What is Lorentz-Bell’s approach? How is that different from Einstein’s?

Consider for example the Lorentz-Fitzgerald body contraction and the example of a train that accelerates with respect to the laboratory reference system and enters a tunnel that is shorter than the train. But the train will fit in the tunnel, because it is Lorentz-contracted.

In the Lorentz-Bell pedagogy, you look at a slowly accelerating body; thus, you do not make a Lorentz coordinate transformation into the moving frame, but you actually transit from one inertial frame of reference to another by applying a small acceleration.  As you follow this particular process, you can show to students who is accelerating with respect to whom and which body is shortened. In the context of the Einstein pedagogy, it is possible to define a measurement method such that you obtain exactly the same outcome. Of course, the principles of relativity are satisfied in both approaches, but the Lorentz-Bell one helps creating a “reality” context. With it, you easily distinguish between changes to a body and changes to space-time, so you do not fall into the traps of “paradoxes”, which are actually inadvertently introduced by people who have not deeply comprehended Einstein’s way of teaching relativity.

At the end of many chapters, you introduce “discussions”, which actually are conversations with hypothetical students....

…Not entirely hypothetical. These are heavily edited transcripts of some real conversations with my students. The people behind the roles – which are three: “Professor”, “Student” and “Simplicius” ­– evolved and changed over the past 15 years (I started writing "Relativity Matters" just after I finished the book on "Hadrons and QGP" published by CUP in 2002). In the preamble of the book, I mention a few names of the students who contributed to these discussions. Many of these were actually the conversation group behind "Simplicius", some of them later took the role of "Student". Aside those mentioned, there were many others as well, but they did not have a large role. Discussions like these happen often when I teach, as I invite students once a week to conversation meetings and, in case the class is about relativity, invariably lots of in-principle discussions ensue. The only "invented" – or really hypothetical conversation – is at the closing of the book: I was looking for a way to frame the last words and, after a few other attempts, this seemed to be in the spirit of the book.

Who is Simplicius?

Simplicius is a truth-searching young student introduced in the spirit of Galileo Galilei's Dialogue personality. The book by Galileo is presented as a series of discussions among two philosophers and Simplicius, a lay defender of the Aristotelian geocentric view of astronomy.

What’s the aim of these discussions? 

These conversations reflect on real world challenges of teaching and explaining relativity at an introductory level. Showing how other students think about relativity helps to engage the complete beginner: in this way, I wanted to create not another textbook but a companion text for the student, from the very beginning physics class to their work at CERN and beyond. Once students reach graduate level, they can follow the mathematical details and there is little discussion needed about why something is done in one or another way. That is why, as you proceed reading “Relativity Matters”, you see fewer and fewer conversational inputs. 

Coming to your research activity, what are the topics you are working on at the moment?

Very close to my heart is the continued development of flavour dynamics in relativistic heavy-ion collisions, what is often called "strangeness signature of quark-gluon plasma". In this context, I work with students on interpreting the experimental results from the ALICE and NA61 experiments; maybe in the future a suitable set of RHIC-BES results will become available as well. I also seek to reconcile the theory of strangeness and charm production with lattice results.

With another group of students, I address the physics of strong fields and critical acceleration – as introduced in the last pages of "Relativity Matters".  I have recently become interested in understanding the force and spin dynamics of (neutral) particles in strong magnetic fields, a topic also of immediate interest to the relativistic heavy-ion collisions community. We just published a related paper (EPJ-C January 3, 2017, see also

You played an important role in developing the theory of quark deconfinement and in establishing relativistic ion-physics as a field of research. Which of your achievements are you most proud of?

I was lucky to meet, in 1975 and 1976, both Rolf Hagedorn and Léon Van Hove, who were on individual lecture tours. At that time, I was embedded in the heavy-ion nuclear physics community in the US and – to a lesser extent – in Europe, working on quark structure, a topic which clearly was beyond the horizon of nuclear science at that time. My most decisive step was to take a long leave of absence and later quit a US laboratory staff position, so that I could step back into the role of a postdoc (fellow) to work at CERN, raising the awareness of the particle and nuclear physics community about the opportunities that a relativistic heavy-ion programme would offer. In Hagedorn I also found at CERN the teacher I needed to widen my related expertise.

Let me also say that one day, it must have been late 1979, Rudolph Bock from GSI invited me to give a plenary lecture at a conference, now called Quark Matter 1 (Oct 7-10, 1980 at GSI), but under the condition that I would talk about how quark-gluon plasma can be observed, not what it is. I am really proud that I ended up presenting the strangeness signature of QGP, which I invented to satisfy the condition of my invitation.

What is your vision of the future of heavy-ion physics?

I believe that relativistic heavy-ion physics is a unique tool allowing us to address several riddles that we all know about but have not come to size up experimentally. I am not speaking about confinement, vacuum structure, origin of mass, all topics that we already work on. I think that we should really take advantage of the fact that in the QGP that we are able to produce at the LHC we find particles of all three flavour families. This may permit us to investigate “what” flavour is.  I believe that, if we are ever to make progress in understanding it, we need to look at the flavour fireball of QGP through the magnifying glasses of the next generation experimental instruments.

Then, there is the unexpected challenge of the so-easy-as-it-seems quark-gluon plasma formation that we see in collisions of light nuclei as well as of heavy, at high energy as well as at relatively low energy.  Common to all these contexts is the question: what exactly happens when we stop within an ultrashort increment in time, with critical acceleration, the energy that makes this QGP fireball? This QGP formation experimental condition is without doubt one of two potential paths to address physics of critical acceleration (seen in the last pages of "Relativity Matters"), which I believe is one of the future physics frontier.

What is your opinion about the strangeness measurements in pp and p-nucleus collisions recently published by the ALICE experiment?
In collisions at the Tera-electron-Volt (TeV) scale, an enormous reservoir of energy is available. This suggests that in some (triggered) collision events characterized by high multiplicity, enough energy is stopped such that, even in the study of pp and p-nucleus high multiplicity collisions, we achieve nucleus-nucleus like conditions. The recent results of ALICE about strangeness show that this is the case, since we observe a smooth and steady growth of multi-strange (anti)baryon signature of QGP, with reaction volume characterized by multiplicity.

Such behaviour was long (25 years!) predicted within the thermal gluon-fusion strangeness production model, since the size of this volume determines the lifespan of the plasma phase. Time is needed for strangeness to approach chemical equilibrium within the deconfined QGP phase, reaching maximum yield only for large enough volumes, characterized by high particle multiplicity.  Once this is achieved, the universal QGP hadronization conditions govern (multi-)strange particle production yields.

These results of ALICE show a rise of strange hadron yields that is gradual with multiplicity. While agreeing with our theoretical models, this smooth and gradual rise also decisively excludes canonical phase-space effects from further consideration (also excluded by phi/Xi ratio being a constant). Moreover, these measurements establish the opportunity for ALICE to pursue exploration of the QGP phase by triggering on high multiplicity pp and p-nucleus events.

To sum up, we can say that these strangeness results are very important: they confirm our understanding of the reaction mechanism of thermal strangeness production in QGP and allow us to gain a better theoretical understanding about the nature and properties of the QGP fireball as a function of volume size, as well as to characterize precisely the QGP hadronization process.

Could you explain the book subtitle “From Einstein’s EMC2 to Laser Particle Acceleration and QGP”?

This subtitle allows the reader to recognize that the book contains introductory material, but also – as its core mission – prepares the more advanced student in the two contemporary fields that require relativity the most: laser particle acceleration processes and the vast domain of quark-gluon plasma physics. The word "and" before QGP means that these novel fields of physics share a common denominator, represented by the fact that strong acceleration phenomena appear in them.