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Vindicating the von Neumann Interpretation of Quantum Mechanics by Unveiling a Logical Inconsistency of the Dead-Alive Physicist “Gedanken” Experiment

By Amal Pushp, Affiliate Physicist at the Resonance Science Foundation 

The foundations of quantum mechanics is a sub-discipline of quantum physics that essentially deals with questions concerning the nature of the wave function, whether the wave function represents an ontic or an epistemic state, the measurement problem and wavefunction collapse process, etc. One would come across several interpretations that have been proposed to address these problems, however, there’s a lack of substantial consensus among the physics community even after decades of work.  

Physicists and philosophers of physics continue to create novel models to explain quantum phenomena and bring about an interpretation that they believe to explain the internal dynamics of such phenomena. Recently, two physicists Carlo Roselli and Bruno Raffaele Stella proposed a thought experiment namely the Dead-Alive Physicist (DAP) and claimed that it falsifies the von Neumann interpretation of quantum mechanics [1, 2]. In a counter to their paper, I published a critical analysis wherein some of the major misunderstandings and flaws in their model were highlighted [3].  

Our principal concern was to argue that the interpretation is considered in the same category as other interpretations of quantum mechanics without essentially being biased. In this article, our aim is to discuss the thought experiment briefly and highlight the key inconsistencies discovered that help to vindicate the von Neumann interpretation under question. 

Credit: Foundations of Physics (2021) 

The above figure shows the setup of the DAP thought experiment involving a physicist inside a sealed room. On the ceiling, a photon source L has been attached and scheduled to emit a single photon at a certain time and is followed by a beam splitter BS, vertically in line with the photon source. Two photo-detectors D and D’ are fixed beyond the beam splitter and the efficiency of both is supposed to be 100%. D is located along the path of the photon and is fixed on the top of a box. D and D’ have an angular separation of 90 degrees. Now, inside the box, there is a hammer and a glass flask containing a fatal gas called LGD. The emitted photon from L would be in a state of superposition after reaching the beam splitter BS.  

The experiment is supposed to be working in the following algorithmic manner: Photon reaches detector D —> Hammer gets activated —> Flask breaks down, gas is released —> the physicist dies. If D gets activated, the physicist results in the state “Dead” and if D’ gets activated the result is the state “Alive”. The experiment has been formulated along similar lines as that of Schrodinger’s cat scenario. Where the latter relies upon the radioactive decay of the atom, the former depends on the probabilistic behavior of the photon.  

However, in the DAP thought experiment, the authors have introduced a factor that distinguishes the model from all other relatable scenarios, for example, the Schrodinger’s cat. The factor is the consumption of a drug, that acts as a conscious perceptions breaker (CPB), by the physicist performing the experiment so that he is not able to watch the experiment directly and is unconscious throughout the course of the experiment. The physicist comes back to his normal experience only at a particular moment in time. As explained in [3], the authors’ interpretational flaw associated with this strategy is one of the key reasons that leads them to claim the falsification of the von Neumann interpretation.  

Before moving further let us look at some of the salient features of the DAP analysis: 

  • The authors point out that on the occasion that the CPB were not used, P’s consciousness would cause the collapse of the wave function as soon as P has become aware of whether he was bound to die or to survive. This is in accordance with the principles of the von Neumann interpretation. 
  • The authors accept the fact that in a quantum physics experiment, the observer must perform a necessary measurement/observation to gain knowledge about the state of the system and that in this case consciousness is transitive meaning that it plays an active role in the surrounding physical environment. Again, this point correlates with the empirical observations such as those involving double slit with elementary particles like electrons. 
  • According to the model, the introduction of CPB means the physicist plays a passive role in the scenario. However, as argued in [3] this is not very accurate. 
  • The entire system works on the probabilistic behaviour of the photon which decides whether the flask containing LGD would break or stay intact. 
  • Later into the thought experiment, there is the entry of another person, say W, whose intention is to check whether P is alive or dead. W is a supporter of the von Neumann interpretation of QM, unlike P. In the original paper, two possibilities have been stated since W enters the scenario. According to the first outcome W would find P in the state “Dead” and would conjecture that it was her own act which caused the collapse of the wave function from the superposition state to the definite eigenstate “Dead”. On the other hand, the second outcome expresses that W would find P in the state “Alive” and would enquire him about the experiment, to which P’s response is that he is conscious from the past half hour. W then concludes that the wave function collapse occurred at sharp 1:00 PM, thus removing the dead state from the quantum superposition.  


A schematic representation of a Wigner’s friend thought experiment. Figure source: Fortschritte der Physik (Progress of Physics), Wiley 


Roselli and Stella have strongly argued that their model falsifies the von Neumann interpretation. The reasoning has been discussed in an easy-to-understand manner in the article by Andrew Knight, which we reproduce here. He says, 

“Essentially, it’s a Wigner’s-Friend-esque thought experiment in which a poison-containing breaks or does not break at 12pm, per a QM outcome, but the person in the room will be unconscious until 1pm.  That’s it.  If CCCH is correct, then collapse of the wave function will not occur until the person is conscious at 1pm... but if he is conscious at 1pm, how could the wave state possibly collapse to an outcome in which the person dies at noon?” 

We have already dealt with the entire dynamics of this thought experiment in our paper [3]. Essentially, we have realised that there are two key points that we discovered during the research that is crucial for noticing the inherent inconsistency in the DAP model. We would like to emphasize on the same in this article (It is important for the reader to note that we are only discussing a gist of some of the points here to keep the argument succinct. For greater details, it is advisable to refer to our paper [3]).  

First, we need to accept the fact that when two or more observers are involved in the same experiment entailing a superposition state, the same wavefunction collapse can’t be compatible for all observers in the absence of an instantaneous communication channel. Therefore, measurement outcomes significantly depend on each individual observer’s reference frame. This fact which is completely ignored in the DAP model is essentially based on known features of quantum mechanics. For instance, consider Alice and Bob, take measurements of non-commuting observables A and B on the same system, but Alice measures A then B, while Bob measures B then A. Then at the end of this, the system is an eigenfunction of B for Alice, but an eigenfunction of A for Bob. 

Secondly, Roselli and Stella have made a paradoxical conclusion at the end of section 4 of their paper according to which P’s consciousness emerges as an effect of the wave function collapse as well as the CPB reaction fading away. 

It is quite counterintuitive to see how they could both be consistently true as the logic suggests that the CPB reaction fades away which results in P becoming conscious and which ultimately collapses the wavefunction for P. So, P’s consciousness emerges essentially because the reaction has gone, and the collapse only occurs a few moments after this (specifically for P). However, it is also difficult to ignore the fact that cause and effect are intimately related in this case. The wavefunction reduction cannot happen without P returning to consciousness and equally, until the wavefunction reduces, P is in a superposition state so he cannot identify himself as truly conscious until this occurs. 

The above would be clearer through this — Consider two outcomes: a) measurement by P at 1:00 PM (of his state), b) No measurement at 1:00 PM. According to the first possibility, P returns to consciousness at 1:00 PM and confirms that he is alive. Now considering the second possibility, no measurement takes place at 1:00 PM, hence the waveform does not collapse, so P remains in a quantum superposition at 1:00 PM, and obviously continues to be in the same thereafter. However, one of the possibilities in the superposition is that the photon hits D’ at 12:00 PM. This means P returned to consciousness at 1:00 PM (after the CPB effect is terminated), thereby performing the measurement. This is in contradiction with the supposition that no measurement takes place at 1:00 PM, so the state cannot be in superposition after 1:00 PM. We clearly have a major inconsistency coming up here. 

The points discussed above suffice to display the inconsistency of the DAP model. This in turn means that the von Neumann interpretation of quantum mechanics is strictly non-trivial and has been defended in a yet another case. However, it is models like these that help refine existing theories and frameworks of physics so they should be approached constructively and regarded as a useful contribution to the subject. At the same time, interpretations like that of von Neumann should be considered in the same category as all other quantum interpretations without being biased towards a particular one. This is the key intention behind our proposal that we have tried to convey here.  



[1] C. Roselli, B.R. Stella, “The Dead-Alive Physicist experiment: a case-study against the hypothesis that consciousness causes the wave-function collapse in the quantum mechanical measurement process”, Foundations of Physics 51 Article number: 21 (2021). 

[2] C. Roselli, B.R. Stella, “Correction to: The Dead-Alive Physicist Experiment: A Case-Study Against the Hypothesis that Consciousness Causes the Wave-Function Collapse in the Quantum Mechanical Measurement Process”, Foundations of Physics 51 Article number: 51 (2021). 

[3] A. Pushp, “The Dead Alive Physicist Gedankenexperiment Debunked”, Quant. Phys. Lett. 11 (1): 1-7 (2022).  


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