Could the Casimir effect explain microgravity?
Monday morning at half past eight in Bremen. Day one of the spring meeting of the German Physical Society (DPG). Specially identified as a "press tip": tutorial in theoretical physics. Headed with: "How can you explain complicated things in physics as clearly as possible?" Sounded promising. A crash course in what one has not understood all one's life and what has always been too difficult for one to have knelt deep enough to learn it. Great. So to speak, a cup full of "Physics to go", physics to take away ...
Four and a half hours later. Understanding. The first part of the announcement was true, some things in physics are "complicated", definitely. The second part, explaining these things "as understandably as possible", was already there. The three speakers, Domenico Giulini (Institute for Theoretical Physics, Leibniz University Hannover, and Center for Applied Space Technology and Microgravity), Martin Ammon (Theoretical-Physical Institute, Friedrich Schiller University Jena) and Klaus Fredenhagen (Institute II for Theoretical Physics, University of Hamburg) do not deny that you have made sufficient effort to make yourself understandable "as much as possible". However, the explanations and lengthy discussions that followed were at a very high level from the outset.
The 45-minute lectures were about "Global versus local structures of spacetime" (Giulini), "The holographic principle - from black holes & entanglement to quantum field theory" (Ammon) and "Quantum field theory in curved spacetime" (Fredenhagen). The latter had announced his lecture with the abstract:
"The conventional formulation of quantum field theory, as you can find it in most textbooks, is based very heavily on the Poincaré symmetry of the Minkowski space. In order to be able to take the influence of gravitational fields into account, the Minkowski space must be replaced by a curved space-time, which in general has no nontrivial symmetries. "
And further: "It turns out that the algebraic formulation of the quantum field theory, as it was already developed in the 1960s, is particularly well suited for a formulation on generic spacetime. The symmetry is replaced by a covariance bond, which the theory on Together with a local form of the positive energy condition, this formulation, which is called locally covariant quantum field theory, forms a suitable framework for quantum field theory under the influence of external gravitational fields. This can also serve as a starting point for perturbative quantum gravity. " 
Physics to go? A mistake. This abstract is certainly hard to digest material for laypeople. Nevertheless, the mutual endeavor to find a common language of the technical and everyday world in spite of all abstraction, technical terminology and numerous unmentioned theoretical, physical and mathematical prerequisites for this lecture announcement does not have to remain fruitless. Among other things, this is what the following interview, which Prof. Fredenhagen gave to Schattenblick after his lecture, is about.
Prof. Dr. Klaus Fredenhagen
Photo: © 2017 by Schattenblick
Schattenblick (SB): As a theoretical physicist, you also worked at the German Electron Synchrotron, DESY. How can you imagine your work there?
Prof. Dr. Klaus Fredenhagen (KF): First of all, I worked as a professor of theoretical physics at the University of Hamburg. My connection with DESY was only that my office is on its premises. That of course means that we had a lot of contact with DESY employees. The facility also has its own theory group with which we work closely. But my work as a professor of theoretical physics was essentially that I gave lectures and supervised students and the like. The research of a theorist consists in thinking about problems and trying to solve them.
SB: When theoretical physicists work together with experimental physicists, what does such a collaboration look like?
KF: Here you have to differentiate between different areas of theory. My field was more mathematical physics. I have had less contact with experimental physicists than with mathematicians. There are also fellow theorists who work more closely with the experimental physicists and then, for example, look at and interpret their data. This is what my colleagues do, the so-called phenomenologists. You deal with the phenomenology of elementary particles. Nowadays there are good theories available, and the phenomenologists do calculations within these theories and then compare them with the data of the experimental physicists. In addition, there are areas where the theory is not that good, where models are then developed that can be used to describe this data.
For example, in high energy physics you have the fields of the Standard Model with gluons, quarks and so on. You don't see them asymptotically, that is, the detector sees neither quarks nor gluons. But then there is what is known as hadronization. One imagines that in a collision process, quarks have collided to form gluons and are newly formed, but then through other processes they are converted into hadrons, which can be observed in detectors. The theory has not yet been able to describe the process of converting this elementary excitation of the gluons or quarks into hadrons. Essentially, one draws conclusions about this process from the data that one has. But, as I said, I have nothing to do with that myself, my colleagues do that.
SB: Did I understand correctly that the gluons are part of the theory?
KF: They are part of the theory, you don't see them directly, so to speak, but only indirectly. A big gap opens up, because one does not understand the process of how the initially created gluons or quarks are converted into hadrons, theoretically well enough. This has to do with the fact that the relevant parts of the standard model - that is quantum chromodynamics - are in fact only known at very short distances. And "very short" means small compared to the radius of a proton. In contrast, a proton is already huge!
With the distances, which is roughly a Fermi, the theory already fails. With a tenth of a Fermi it still works quite well, maybe even with a hundredth of a Fermi, but with a Fermi the theory fails, that is, this part is then only empirical. Little progress has been made in this area in the last 30 years or so. This is only going very slowly.
SB: When the theoretical physicists write mathematical formulas that express a new theory, how does the formula language turn into everyday language that then speaks of wormholes, dark matter and other pictorial terms, for example?
KF: That is perhaps more extreme in physics than in other sciences, but the problem is probably everywhere. In order to describe something in science, you have to develop new concepts. And for these concepts there are no words in everyday language, there is no real translation, so to speak. If you try to describe this in everyday language, you either have to explain the whole concept, which de facto means that you are dealing with the topic professionally, or you simplify it. And of course it is always falsified a bit. If you want to explain the effects of quantum theory colloquially, you sometimes use very vague terms, which then, let's say, trigger a certain lack of understanding in the audience. In the "Spiegel" I once read about ghostly or ghostly interactions.
SB: Surely that goes back to Albert Einstein, doesn't it? 
KF: Of course, the physicists were also very surprised about this, but they developed concepts with which one can understand this - but "understand" in the sense that one really needed new concepts. In my lecture I tried to make it clear that the concept of the particle is much more difficult and by no means as universal as one might naively think. In my opinion, the particle is a typical example of the development of concepts that were originally used in everyday language to explain, but which later mean something completely different.
SB: In everyday language, particles are clear: you grasp something that consists of particles and offers resistance.
KF: Yes, exactly. This is what you think of when talking about a particle. You imagine billiard balls or something like that. But the term particle is actually not what you really have in theory. There are mathematical physicists who then try to define such a term mathematically very precisely. There are other physicists who work less mathematically, but who then have a more intuitive access to it and know that certain things cannot be taken literally. It is of course a problem when the layman engages in it and hears something being explained in a slang. There is a risk that he will take it literally and thus misinterpret it.
SB: In your lecture today you spoke about particles that cannot be determined at the event horizon, but then again in relativistic theory, and concluded, among other things, that there is no known vacuum state.
Theatrical Version: Yes.
SB: How is that to be understood? From the point of view of quantum theorists, is there no vacuum?
KF: (hesitates) I tend to say yes to give a very simple answer. That would of course have to be qualified a little. In any case, the idea of vacuum as empty space is incompatible with what we know about quantum field theory. Rather, quantum field theory says that even if there is such a thing as the vacuum - what a theory in the Minkowski space  is a natural structure that is also correct in many examples - one must not interpret it as empty space.
SB: Does that mean that what you have learned in school about vacuum has to be deleted because it reflects a world view that is too simple?
KF: Basically you go back to very old philosophical discussions, we know that from classical Greek philosophy. There were philosophers like Parmenides  who rejected the concept of vacuum because they said it was contradicting itself. What is nothing cannot be, was roughly the argument. This is not a physical, but a logical conclusion.
In competition there was the idea that everything is made up of small particles. For a long time, physics went in the direction of propagating the concept of empty space in which there are individual particles. In my opinion the current development is going in a different direction. It is said that empty space does not actually exist.
I will explain it to you using an example. You have probably heard of the zero point oscillation in the oscillator. The uncertainty relations mean that when a pendulum is described by a harmonic oscillator, small deflections remain. It is never in peace, it cannot be in peace at all. The place and the impulse cannot be fixed at the same time, but it always fluctuates a little. The same is true for the quantum fields. They cannot be zero either, but always fluctuate a little. In my lectures I sometimes joke that for people who are afraid of electrosmog it must be a problem that the electromagnetic fields can never be zero. The reason for this is the same as for the zero point oscillation in the harmonic oscillator.
SB: Are the quantum fields more a matter of vibrations?
KF: Yes, if you have a simple field equation, then it behaves like an oscillation. Only then they are something like an infinite number of coupled, infinitely many such degrees of freedom of vibration. That then also leads to these divergences. You then have the zero point energy, but for each impulse, if you add it all up, you get out infinitely. One of the difficulties in quantum field theory, so to speak, is that many of these concepts lead to contradictions.
These zero point oscillations have been demonstrated in the so-called Kasimir effect. The effect can be seen on a plate capacitor. It can be seen that there is a certain attraction between the two plates of the capacitor, although there are no charges at all. This is attributed to the fluctuation of the electromagnetic field between the plates. Because the plates are conductive, this changes the zero point oscillations. An electric field would disappear in the conductor. This change can really be measured, it leads to the attraction effect called the Casimir Effect. There is a tendency to see something that is infinite as probable nonsense. But you can, so to speak, indirectly control that this effect is really there.
SB: The central task that theoretical physics is facing today is likely to be the design of a coherent formalism for particle physics and the theory of relativity. What do physicists hope for from a unified theory?
KF: If I don't have a unified theory and I investigate effects where both gravity and elementary particle physics are relevant, the problem is that I describe part with gravity and part with quantum theory. Where I make the difference is in a sense my gut feeling or it is arbitrary. That means, as soon as I have such effects where both theories become relevant, I come into contradictions. If, on the other hand, I had a uniform theory, I would just know how to resolve these contradictions.
If the phenomena are perfectly separated so that I never have to look at both at the same time, I can of course ignore that. But as soon as I have an effect in which both play a role, which is, for example, very crucial in cosmology, I need a uniform theory.
SB: Is it also about completing the worldview?
KF: In order to have a consistent worldview at all, yes. That would be important for that. Now one could speculate about whether such a thing even exists, i.e. whether the world can really be explained without contradictions. But that is another matter. I suppose every physicist believes that there is somehow possible, but the solution to standardization has not yet been found.
SB: Towards the end of the 19th century it was thought that physics was finished, that it had already more or less explained everything. Since then, the term "end of physics" has appeared again and again, also recently with the evidence of the Higgs particles. Would you say that with the formulation of a "theory of everything" the end of physics would actually be reached?
KF: (laughs) Well, first of all, I don't think there is any physics of everything. But that's more of my gut feeling now, I might as well be wrong about that. You may still find a theory that contains everything. But even if I do, I think we are far from it. Second, it would not be the case that this theory could really be applied to every problem. Earlier I mentioned an example from elementary particle physics, hadronization. For this we know the theory in principle. There is no suggestion that it is wrong. But we are not able to use it. Then you can turn to other dimensions and you get into problems that we do not understand sufficiently. To give a somewhat mundane example to illustrate: The weather forecasts are still not that great, although we actually understand the physics on which they are based quite well. This has little to do with misunderstood quantum theory or the like, but with the Navier-Stokes equations . In addition, you simply don't have enough data and you can't process it fast enough. This is just one example where you can see that there is still an infinite amount of work to be done.
What else you have to say: Not every problem is interesting right from the start. You may not know beforehand what will be interesting and what will not. For example, we now have many interesting phenomena in solid state physics, which fundamental physics is actually very well known. This essentially only applies to the structure, the electromagnetic interactions. Nuclear physics plays almost no role in this.
SB: Around a hundred years ago, some atomic physicists, who had laid the foundations for the construction of the atomic bomb with their theoretical work, deeply regretted this. Does the unification theory have the potential to create something that would not be desirable, i.e. a means of violence with even more destructive power?
KF: I don't see that. Such developments are difficult to predict. I mean, when Otto Hahn  made his experiments, he certainly didn't think that an atom bomb could be built a few years later. That was certainly not planned. You never know beforehand what you can do with such inventions afterwards.Whether you do something sensible with it or not is unfortunately no longer up to the person who found it. That is a certain risk, so to speak. Personally, I believe the risks we face today are much greater in biology. I would be surprised if something similar to the atom bomb were discovered in physics. There is no sign of that. Although there are always people who speculate about gravity bombs or the like.
SB: When the Large Hadron Collider was put into operation at CERN a few years ago, the media feared that a black hole could accidentally arise. Apart from the public, physics was also involved in this idea. It certainly can't be wrong to worry about it.
KF: Of course you have to think about it. With every experiment you have to consider whether it will directly harm someone. That's what this case was about. But there is no guarantee that the knowledge that is gained from it can be used for any harmful purpose afterwards. I think this is, in a sense, our destiny as humans. We have the ability to invent things and the ability to make something good out of it is unfortunately not quite as developed. But you can make an effort.
The advances in technology, in principle, allow many human problems to be solved. It is actually nonsensical to take a different direction instead. Whether this is about environmental problems or hunger in the world, solving that is actually not a technical problem today, or in my opinion they are at least solvable problems.
SB: In other areas of science the importance of interdisciplinarity and transdisciplinarity is emphasized and clusters are formed in which many research directions work together. Is there also a bridge to other sciences from theoretical physics?
KF: Of course there is a bridge to mathematics, which is very extensive. In my own research there are no direct connections to other sciences, apart from philosophy as a separate science. Of course, there are always contacts and there is mutual influence. Indirectly, but this affects less my own work than, for example, that of our experimental colleagues, physicists naturally make tools available to other sciences, such as biology and chemistry. They benefit very much from the fact that physicists now have instruments with which one can make completely different observations.
SB: Mr. Fredenhagen, thank you very much for the interview.
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