Today we visit Dr. Hrvoje Nikolic of the Theoretical Physics Division of the Ruđer Bošković Institute, PhD from the University of Zagreb, on the nature of quantum theory.
David: Physical reality was a fairly straightforward concept before I studied quantum physics. With superposition and entanglement, it became vague if not contentious. In your understanding of quantum theory, how would you describe physical reality? Hrvoje: Before describing it, I should first define what I mean by it; by physical reality I mean the things out there that exist irrespective of whether we measure them or not. From this definition it follows that physical reality cannot be determined by experiments, only a theory (guided, of course, by experiments) can give us some idea what a physical reality might be. Indeed, the classical theory gives us a pretty clear picture of physical reality. But the quantum theory is different, in its standard form it is not a theory of physical reality. It is only a theory of probabilities of measurement outcomes, nothing less and nothing more. For practical purposes that may be enough, but many physicists are not satisfied because they want some intuitive picture about physical reality, which quantum theory in its standard form does not provide. That's why physicists invented various interpretations of quantum theory, the purpose of which is to give us some intuition, by telling us either what the physical reality is, or why the question of physical reality is a wrong question. We need interpretations because they give us the intuition that helps in thinking. The best interpretation is the one which gives the clearest intuition, consistent with all the experimental facts. But the clearest intuition is highly subjective; what is intuitive to me may not be intuitive to you, and vice versa. That's why physicists cannot agree on the interpretation of quantum theory. For me, the most intuitive, and hence the best, existing interpretation is the Bohmian interpretation, according to which the world is made of classical-like objects, such as point particles or continuous fields, guided by the wave function or wave functional. Among all other interpretations, the Bohmian interpretation is the most similar to classical physics, which is why I find it intuitive. Even if Bohmian mechanics is not how Nature really works, it's useful as a thinking tool because it provides a very intuitive way of thinking about quantum phenomena. At least it's intuitive to me, but I must admit that I don't understand why it is not intuitive to most other physicists. David: When I asked about his view on the many-worlds version of quantum theory, a theoretical physicist laughed, then dismissed it on the basis that one needs to simply “think about it”. When I asked a many-worlds advocate about other versions of quantum theory, he said such people were a lost cause. Most recently, I was told by a philosopher that unitarity has ruined quantum physics. From an outsider’s perspective, i.e., one gets the feeling that people are becoming more entrenched in their views. Do you, instead, see progress? Hrvoje: I don't see a progress in reaching a consensus about which interpretation is "the best". But I see a progress in recognizing that foundational problems such as the question of interpretation, the question of physical reality, or the measurement problem, are relevant problems that physicists should deal with. David: Is quantum theory complete? Hrvoje: No. The theory induces some questions (such as the question of physical reality) that it doesn't answer, hence it's not complete.
David: Thank you Professor!