Workshop: Causality Meets Quantum Mechanics


Date: September 2, 2015

Place: University of Salzburg, Austria

Organizer/s: Nina Retzlaff


Quantum mechanical phenomena irritate our habit to structure events in cause and effect. In everyday use we do not question that the cause occurs temporally before its effect, that a cause produces its effect, but this effect never produces its cause, and that cause and effect are spatially close to each other (cf. Hüttemann 2013, p. 7). 

In quantum mechanics, certain phenomena challenge those features of causation. The strongest discussed phenomenon is the quantum entanglement, which questions the locality, that is the feature that cause and effect are spatially close to each other. If a quantum system consists of several distinct subsystems, then local operations on each individual subsystem can be done. For example with respect to a two-photon system the polarization at each photon can be measured. If the system is in an entangled state, then a local operation at one of the subsystems has impact on the states of all other subsystems, namely immediately and independent of their distance. If the two-photon system is in an entangled state, then the measurement of the polarization of one photon determines the polarization of the other photon. (cf. Audretsch 2005, S. 110)

How do we deal with this fact? Should we call into question the features of causation or have we to rephrase features for some areas such as the microcosm, or have we to give up causality in quantum mechanics?

Audretsch J. (2005): Verschränkte Systeme: Die Quantenphysik auf neuen Wegen. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.
Hüttemann A. (2013): Ursachen. Berlin; Boston: Walter de Gruyter.

Aim of the Workshop

The goal of this workshop is to capture the theme of causality in quantum mechanics and search for answers how to deal with the facts and how to combine the two theories: Causality and Quantum Mechanics. We will start in general and become more and more specific from talk to talk. The workshop will be opened with the general issue of causality in physics by Prof Dr. Paul Weingartner. Florian Boge will then explore the issue of causality specifically with regard to quantum entanglement. After a short break Alexander Gebharter gives us the specific formal tool used in the last two talks, the causal (Bayes) nets. Nina Retzlaff will then apply this tool on certain quantum phenomena and Dr. des. Paul Näger will talk especially about the Causal Markov condition in the quantum realm.


09:00 - 09:15 Workshop Opening: Introduction

09:15 - 09:45 Paul Weingartner: The need of pluralism of Causality 

09:45 - 10:15 Florian Boge: Locality, Causality, Reality. Implications of Bell's Inequality

10:15 - 10:45 Coffee Break

10:45 - 11:15 Alexander Gebharter: Introduction to causal (Bayes) nets

11:15 - 11:45 Nina Retzlaff: Causality within Quantum Mechanics 

11:45 - 12:15 Paul Näger: The Causal Markov Condition in the Quantum Realm 

12:15 - 12:45 Concluding Discussion

Prof. Dr. Paul Weingartner (Salzburg): The need of pluralism of Causality

In this talk it will be shown that a pluralism of causality is needed. Not, as might be expected, for such different domains as natural sciences and humanities, but even within the domain of physics different causal relations are necessary. This will be illustrated with examples from Classical Mechanics and Special Relativity, Thermodynamics and Quantum Mechanics. In these domains causal relations differ in their properties. The talk will be divided into the following chapters:

1. Introduction

2. Three Main Types of Causality

3. Properties of Causality Relations

4. Causality Relations in Causal Explanations

5. The Basic Logic for the Model of Causal Relations

6. The Model RMQC of Causal Relations

Florian Boge (Cologne): Locality, Causality, Reality. Implications of Bell's Inequality

In 1935, Albert Einstein, Nathan Rosen, and Boris Podolsky (henceforth: EPR) published a paper purporting to show that quantum mechanics (QM), today's most successful physical theory, was incomplete. David Bohm (1951) later offered a significantly simplified version of the thought-experiment on which EPR based their argument: Take two systems prepared in a certain type of quantum mechanical state, e.g. two atoms resulting from molecular decay, which are then separated by a large spatial distance. Surprisingly, QM predicts that these two atoms will show a remarkably correlated behavior long after any local interaction at the source (the decaying molecule) should have ceased.

While QM predicts that these correlations exist, any more 'classical' physical theory should include some reasonable assumptions that prohibit this kind of behavior. This especially goes for the special theory of relativity which provokes a conflict with explanations that attempt to invoke a causal connection between the two separated systems (atoms), given that their separation is large enough. In 1964, John Bell found a way to make things testable by deriving an inequality that should hold based on the aforementioned reasonable assumptions, and should be violated according to QM. Experiments, notably that of Aspect et al. (1982), have since been strongly in favor of QM. But what are we to make of this?

In my talk, I want to give an overview of three central questions which 'naturally' offer themselves in this context, and how they are related: (i) what becomes of spatiotemporal constraints set up by the (special) theory of relativity, (ii) what becomes of a causal interpretation of the situation, and (iii) what becomes of our view of reality, in the light of the two aforementioned points? To establish a connection, I will show why a causal explanation must be 'non-local' in a specific sense and what difficulties arise from this and other features of causal assessments of the situation. But if, on the other hand, we give up on explaining this phenomenon causally, this has a definite impact on (certain kinds of) scientific realism. In conclusion, I will offer a glimpse at my own view to provide a constructive outlook on the situation.


Aspect, A., Dalibard, J., and Roger, G. (1982). "Experimental Test of Bell's Inequalities Using Time-Varying Analyzers". Physical Review Letters, 49(25):1804--1807.

Bell, J. S. (1987[1964]). "On the Einstein-Podolsky-Rosen paradox". In Bell, J. S., editor, Speakable and unspeakable in quantum mechanics, pages 14--21. Cambridge, New York: Cambridge University Press.

Bohm, D. (1951). Quantum Theory. New York: Dover Publications Inc.

Einstein, A., Podolsky, B., and Rosen, N. (1935). "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?". Physical Review, 47:777--780.

Alexander Gebharter (Düsseldorf): Introduction to causal (Bayes) nets

In this talk I give a brief and self-contained introduction to causal (Bayes) nets (CBNs) which were developed by researchers such as Neapolitan (1990, 2003), Pearl (1988,2009) and Spirtes, Glymour, and Scheines (2000). I introduce important basic notions and the theory's core axioms as well as alternative and philosophically more transparent formulations of these axioms. I proceed by illustrating how these axioms connect causal structures to empirical data. Then I highlight a few advantages of the theory of CBNs over more classical philosophical theories of causation. In particular, the theory seems to give us the best grasp on causation we have so far from an empirical point of view: It gives rise to a multitude of methods for uncovering causal structures on the basis of empirical data, it provides the best available explanation of certain statistical phenomena, and certain theory versions (i.e., combinations of the theory's axioms) can be independently tested on purely empirical grounds (Schurz & Gebharter, 2015). Furthermore, the theory of CBNs allows for a clear distinction between observation and manipulation: CBNs can be used for making predictions based on observations, but also for predicting the effects of possible interventions on the basis of (non-experimental) observational data. 


Neapolitan, R. E. (1990). Probabilistic reasoning in expert systems. Wiley.

Neapolitan, R. E. (2003). Learning Bayesian networks. Prentice Hall.

Pearl, J. (1988). Probabilistic reasoning in intelligent systems. Morgan Kaufmann.

Pearl, J. (2009). Causality. Cambridge University Press.

Schurz, G., & Gebharter, A. (2015). Causality as a theoretical concept: Explanatory warrant and empirical content of the theory of causal nets. Synthese. Advance online publication. doi:10.1007/s11229-014-0630-z

Spirtes, P., Glymour, C., & Scheines, R. (2000). Causation, prediction, and search. MIT Press.

Nina Retzlaff (Düsseldorf): Causality within Quantum Mechanics

To combine causality with quantum mechanics, it is useful to examine probabilistic theories of causality. Using a probability distribution, which assigns a probability to every event of an experiment, cause-effect relations are analysed in the so-called Bayes nets approach (Pearl 2009, pp. 8-14). By means of mathematical formalization, algorithms can be developed, which identify the causal connections of a complex system and output directed acyclic graphs (DAGs) representing these connections. The commonly accepted algorithms like the SGS-algorithm generate a class of related DAGs based on the conditional independence relations derived from the probability distribution, under the assumption that both the Causal Markov Condition and the Faithfulness Condition are satisfied (Spirtes, Glymour, Scheines 2009, p. 81). Certain quantum mechanical phenomena, however, violate at least one of these conditions, so the algorithms' DAG-outputs are not always adequate. One kind of these phenomena are quantum correlations, which have already been discussed with reference to causal discovery algorithms (Wood, Spekkens 2014). In this talk I focus on other quantum phenomena, which have not yet been modeled and analysed by means of causal Bayes nets.


Pearl J. (2009): Causality: models, reasoning, and inference. 2nd edition. New York: Cambridge University Press.

Spirtes P., Glymour C., Scheines R. (2000): Causation, Prediction, and Search. 2nd edition. Cambridge: MIT Press.

Wood C. J., Spekkens R. W. (2014): "The lesson of causal discovery algorithms for quantum correlations: Causal explanations of Bell-inequality violations require fine-tuning", preprint: (received 10.02.2015).

Dr. des. Paul M. Näger (Münster): The Causal Markov Condition in the Quantum Realm

The causal Markov condition, which is a generalisation of Reichenbach's principle of the common cause, is the central principle of causal explanation. In a non-technical way it says that every correlation has to be explained by a causal connection. While the principle seems to be well-founded in the deterministic macroscopic realm, van Fraassen (1980: The Scientific Image, 1982: Rational Belief and the Common Cause Principle) and Cartwright (1988: How to Tell a Common Cause, 1989: Nature's Capacities and Their Measurement) have argued that the principle fails for indeterministic quantum mechanics: there are common causes that do not screen off. This poses the dilemma that one either has to deny that the quantum world is causal (van Fraassen's horn) or one denies that the theory of causal Bayes nets adequately captures causal facts (Cartwright's horn). In this talk I shall re-investigate the alleged failure and discuss options for a via media, which upholds basic ideas of the theory of causal Bayes nets and understands the quantum world in a causal way.

About the speaker

Paul Weingartner (University of Salzburg, Austria)

Paul Weingartner is professor emeritus of philosophy (University Salzburg). 1961 Doctor of philosophy (major: philosophy, minor: physics) at the University of Innsbruck. As research fellow he was studying with Popper, Britzlmayr and Stegmüller. 1965 assistant professor of philosophy (venia legendi), University of Graz. 1966 assistant professor of philosophy (venia legendi), University of Salzburg. 1966 Kardinal Innitzer Price for Philosophy of the year. 1970 Associate Professor of philosophy at the University of Salzburg. 1971 Full Professor of philosophy at the University of Salzburg. 1995 Honorary Doctorate (Dr. h.c.) from Marie Curie Sklodowska University, Lublin (Poland). In 1997 he received a Membership of the New York Academy of Sciences. 

His research areas are philosophy of science, logic and philosophy of religion, with a particular focus on laws of nature, causality, truth, necessity and possibility - He published 10 Books, 36 editions and more than 160 articles in renowned journals like the Journal of Symbolic Logic, Journal of Philosophical Logic, Journal of Symbolic Logic, Grazer Philosophische Studien, Erkenntnis, and Philosophia Nauralis. In recent times he also published about God, theory of conscience and the natural law in the philosophy of Thomas Aquinas.

Florian Boge (University of Cologne, Germany)

Florian Boge is a PhD student in philosophy under supervision of Prof. Dr. Andreas Hüttemann at the University of Cologne. He graduated in 2012 from the M.A. study in philosophy at the Heinrich-Heine-University Düsseldorf, with a thesis on trope theory and similarity. His PHD thesis is concerned with the prospects of ontological and epistemological approaches to the interpretation of QM. He is currently also working on a degree in physics.

Alexander Gebharter (University of Düsseldorf, Germany)

Alexander Gebharter is a research fellow at the Düsseldorf center for Logic and Philosophy of Science (DCLPS) at the University of Düsseldorf and within the DFG funded research unit "Causation, Laws, Disposition, Explanation: At the Intersection of Science and Metaphysics" (FOR 1063). His research interests lie in philosophy of science and metaphysics. He is especially interested in causality and related topics such as modeling, explanation, prediction, intervention and control, mechanisms, constitution, supervenience, theoretical concepts, empirical content, etc. For a list of publications and more information, see the following webpage:

Nina Retzlaff (University of Düsseldorf, Germany)

Nina Retzlaff is a research fellow at the Düsseldorf center for Logic and Philosophy of Science (DCLPS) at the Heinrich Heine University Düsseldorf. She studied mathematics with a minor in biology at the University of Cologne and is interested in quantum mechanics. Her research interests lie in philosophy of science and metaphysics, especially in causality within quantum mechanics. In the context of her PhD thesis, she is investigating causality with regard to quantum mechanics.

Paul M. Näger (University of Münster, Germany)

Paul Näger (Dr. des. phil.), University of Münster. 2000-2008 Studies in physics and philosophy, LMU Munich. 2006 Diploma in physics (with distinction). 2008-2009 Studies in philosophy, University of Oxford. 2008-2013 Dissertation in philosophy: Entanglement and causation (summa cum laude), Supervisor: Prof. Dr. Manfred Stöckler, PD Dr. Meinard Kuhlmann. 2010-2013 Research Fellow (Wiss. Mitarbeiter), Dept. of Philosophy, University of Bremen, Prof. Dr. Manfred Stöckler. Since 2013 Research fellow (Wiss. Mitarbeiter), Dept. of Philosophy, University of Münster, Prof. Dr. Ulrich Krohs.

Recent publications:

- The Causal Problem of Entanglement. Synthese, 2015. doi:10.1007/s11229-015-0668-6

- [P. Näger, M. Stöckler] Verschränkung und Nicht-Lokalität: EPR, Bell und die Folgen. In C. Friebe et al., Philosophie der Quantenphysik, Springer Spektrum, Heidelberg 2014.

- [C. Friebe, M. Kuhlmann, H. Lyre, P. Näger, O. Passon, M. Stöckler] Philosophie der Quantenphysik. Springer Spektrum, Heidelberg 2014.

all participants
Paul Weingartner
Florian Boge
Nina Retzlaff
Paul Näger
Alexander Gebharter
Responsible for the content: E-MailProf. Dr. Gerhard Schurz (Redakteur: Alexander Christian)