Getting at reality

Today we chat with Dr. Valia Allori, PhD in physics from the University of Genova, PhD in philosophy from Rutgers University, and Professor of Philosophy at the University of Bergamo, on her ideas at the interface of science and philosophy.

David: Is there one aspect of quantum theory (e.g. quantization, wave-particle duality, superposition, non-locality) that truly singles it out as different from classical theories or is it a combination of concepts?

Valia: I think the most distinctive aspect of quantum theory is entanglement which is naturally linked to quantum separability and nonlocality. Entanglement can be a feature of matter, and so is separability, while locality is a property of the way in which matter interacts. A physical system whose components are deeply connected, no matter how far apart they are, is called ‘entangled.’ For an entangled state, only the composite can be said to have a given property, while the parts do not. That is, the whole is not separable into its parts. This also means that a change in one particle's state will instantaneously affect the other, regardless of their distance. This suggests that the two parts of the system interact instantaneously, or nonlocally. For instance, a pair of electrons in a single spin state is entangled, and only the pair has a definite spin property, not each electron. The pair spin property is not reducible to the properties of the individual particles. However, when someone measures the spin of one particle, which thereby acquires a definite value, the spin of the other particle also becomes defined through what seems to be a nonlocal interaction. This phenomenon was a major concern for Einstein who argued that this nonlocality was evidence of the incompleteness of quantum theory. His famous 1935 argument with Podolsky and Rosen in fact might be summarized this way: if quantum theory were complete, then there would be nonlocal interaction, which is absurd. He was therefore concluding that the only way to avoid nonlocality was to add ‘hidden variables’ to quantum mechanics to complete its description. However, he was wrong: John Stuart Bell in 1964 demonstrated that such completed theory makes different predictions which in 1982 have been falsified by Aspect and collaborators. Therefore, quantum nonlocality has to be accepted as an empirical fact. This is, in my opinion, the distinctive feature of quantum phenomena, and it is the true mystery everyone should try to understand.

David: At a basic level our picture of reality involves matter and energy moving through space over time. To what degree does quantum theory undermine that picture?

Valia:  The story one finds in many physics textbooks is that the transition from classical to quantum theory is a scientific revolution in the Kuhnian sense: we need to drop the old classical paradigm and endorse the new quantum paradigm, much like what happened in the passage from Ptolemaic and Copernican astronomy.  In the new paradigm, the microscopic reality is fundamentally inaccessible, we can only provide partial pictures in terms of our inadequate concepts of particles and waves. We can no longer intuitively describe the microscopic world in terms of matter moving in time through space. However, as it is now recognized by many, it is not necessary to accept this view. There are at least three promising quantum theories which do not require to completely change the classical paradigm, even if some of them are more radical than others. These theories are the spontaneous localization theory (also called GRW theory, from the names of Ghirardi, Rimini and Weber, who in 1986 proposed it), the pilot-wave theory (also called de Broglie-Bohm theory, as de Broglie had the idea in 1924, and Bohm developed in 1952), the many worlds theory (known also as Everettian mechanics, from Everett’s work in 1957).

David: What does the measurement problem illustrate?

Valia: The theories above are solutions of this problem. The measurement problem, also known as the problem of Schrödinger’s cat, is the problem of dealing with the unobserved quantum superpositions produced by ‘pure’ quantum mechanics, namely a theory in which there is an object called the wavefunction which completely describes the physical system, and it evolves according to an evolution called the Schrödinger equation. Imagine a cat in a box, in which a vial of poison is connected to a radioactive atom. When the atom decays, it breaks the vial, and kills the cats; otherwise, nothing happens. However, the atom’s superposition between “having-and-not-having decayed,” propagates macroscopically producing a superposition of “dead-and-alive” cat, which is never observed. This shows that pure quantum theory is empirically falsified. One can make it empirically adequate by stipulating that the act of measurement randomly collapses the superpositions into one of its terms, as von Neumann proposed. Nonetheless, it is unclear why measuring something is such a special action, and the theories mentioned above aim at solving this problem without stipulating this. The spontaneous localization theory changes the Schrödinger evolution and makes collapse part of the fundamental law of evolution of the wavefunction, so that macroscopic objects like the cat spontaneously collapse regardless of measurement. The pilot-wave theory instead assumes that matter is made of particles, whose motion is governed by a Schrödinger evolving wavefunction. So, while the wavefunction can be in superposition, it is the particle location that selects whether the cat is dead or alive.  Finally, the many worlds theory embraces that the superpositions are real at all scales, even if we do not observe them because they loose the ability to interact with one another.   

David: Is there progress in the foundations of quantum theory?

Valia: I think so. In the 1990s, people believed that one had to accept that quantum theory is a new paradigm, and that we needed to stop thinking reality was comprehensible. Now this is no longer the common opinion, at least among philosophers of physics. The three theories above were earlier a forbidden research field, while now one no longer kills their career by deciding to work in quantum foundations. Now the disagreement is about which of these theories is more plausible. Those who think making new predictions is important, favor the spontaneous localization theory, which is the only theory among the three which is not empirically equivalent to quantum theory. If one prefers to understand phenomena in terms of emerging structures or constraining principles, then they will naturally be attracted to the many-worlds theory. Instead, thinking that microscopic entities compose macroscopic objects like Lego bricks will lead to the pilot-wave theory.   

David: How much of your research direction would you estimate to be the result of subjective preferences about aspects of physical theories and/or intuitions about how to move toward better ideas?

Valia: Pretty much I base everything on my preferences about which type of explanation of the phenomena I find most satisfactory. I think that one truly explains the phenomena if one can find a microscopic picture that allows for a compositional or constructive explanation. That is, a phenomenon is explained if one can find its fundamental microscopic components and one can account for its properties in terms of their dynamics. There could be other types of explanation, for instance by constraining principles like the one in thermodynamics that says that entropy always increases: only the phenomena obeying this principle will happen. However, they are provisional because they are ultimately lacking: one would like to explain why a given principle is true, which can be done in terms of a more fundamental constructive theory. In the case of entropy, the principle can be compositionally derived from Newtonian mechanics, which can be seen as the constructive theory underlying thermodynamics. It is my preference for constructive explanation that drove me to the pilot-wave theory, which is naturally the deeper constructive theory underlying quantum mechanics: everything is made of particles, moving along quantum trajectories determined by the wavefunction.     

David: Can you describe some of the challenges you overcame in becoming a philosopher of science?

Valia: I have always wanted to study philosophy of nature, but I soon realized that I needed to know physics very well to do this successfully. So, I enrolled in physics in my home country (Italy) and also got a doctorate (this time in the US). Then, I finally went back to philosophy (also in the US, before coming back to Italy last year). In this journey my main difficulty has been to convince philosophers that what I was doing was, indeed, philosophy, and convince physicists that my work was relevant to their field as well. In short, I had trouble finding a true academic home, in which I could be myself. However, this is true also for many other philosophers of physics, so I was never truly alone, thankfully. In any case, the situation has now improved, and the gap between philosophy of science and physics has considerably reduced.