Misconceptions and assumptions concerning quantum mechanics
I get somewhat frustrated every time I read another blog post, book review, or journal article that claims Einstein was wrong about quantum mechanics (QM). It must make for good headlines and is almost cliché. First, these articles often give the misleading impression that Einstein was the only physicist who had concerns with quantum mechanics during its development and exposition. That simply is not true. Many physicists (Schrödinger, de Broglie, Podolsky, Rosen, and several other major figures) had concerns. Additionally, the relatively small fraction of physicists that are active today in the foundations and interpretations of quantum mechanics continue to debate the meaning, the implications, and the completeness of the theory with great vigor. There is not yet a general consensus among experts as to the answers to some of the most fundamental questions about the implications of quantum theory in its present form.
For decades, there has been a common misconception among many physicists that the conceptual problems with QM were already resolved or that any remaining questions were purely philosophical. Contributing to this state of affairs, many textbooks focused solely on the computational aspects. If interpretations or foundations were discussed at all, the focal point was on the Copenhagen interpretation. There was little or no discussion of other viable formulations, and the solutions to conceptual problems that these formulations offered. The prevailing interpretation of QM does not give a clear answer to the question “what, if anything, is objective reality”. Some alternatives, such as de Broglie-Bohm mechanics, do. According to de Broglie-Bohm mechanics, particles are objective point-like objects with deterministic trajectories. These trajectories are guided by wave functions, which also objectively exist.
Alternatives to conventional quantum mechanics
I am not at all claiming that de Broglie-Bohm mechanics in its current form is the final word. And I am not claiming that we need to immediately replace our existing paradigm with it, without further consideration or modification. However, de Broglie-Bohm mechanics has not been properly vetted by generations of physicists. I think failure to fully consider and evaluate such approaches may be blinding us to the way ahead. The prevailing, fractured conceptual understanding of QM may be holding us back from making the next theoretical and technical leap in our quest to understand the universe.
The venerable John S. Bell had this to say about de Broglie’s wave theory (see Speakable and Unspeakable in Quantum Mechanics):
“Is it not clear from the smallness of the scintillation on the screen that we have to do with a particle? And is it not clear, from the diffraction and interference patterns, that the motion of the particle is directed by a wave? De Broglie showed in detail how the motion of a particle, passing through just one of two holes in screen, could be influenced by waves propagating through both holes. And so influenced that the particle does not go where the waves cancel out, but is attracted to where they cooperate. This idea seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored”
And this about Bohmian mechanics:
“In 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into nonrelativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the “observer,” could be eliminated. … But why then had Born not told me of this “pilot wave”? If only to point out what was wrong with it? … Why is the pilot wave picture ignored in text books? Should it not be taught, not as the only way, but as an antidote to the prevailing complacency? To show us that vagueness, subjectivity, and indeterminism, are not forced on us by experimental facts, but by deliberate theoretical choice?”
EPR and quantum entanglement
The famous “EPR paper”, so-named due to its authorship: A. Einstein, B. Podolsky, and N. Rosen, “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”, laid out some of Einstein’s main concerns. These included lack of an objective physical reality in which deterministic properties of observables exist regardless of measurement. And nonlocality, in which a measurement process carried out on one of a pair of entangled particles can seemingly affect the other particle’s properties, instantaneously and without regard to distance. Einstein continued to voice his objection to this fundamental property of quantum mechanics: “because it cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance.” (Max Born, ed., The Born-Einstein Letters: Friendship, Politics and Physics in Uncertain Times (Macmillan, 1971), p. 178).
After Einstein’s death, the phenomenal John Bell figured out how to quantify the “spooky” part of the intrinsically probabilistic behavior of a pair of entangled particles. See his papers in “Speakable and Unspeakble in Quantum Mechanics”. Years later, experimentalists such as Freedman, Clauser, and Aspect, confirmed that Nature really does make use of this spooky action at a distance, or nonlocality. But to what end?
Although nonlocality has subsequently been confirmed experimentally, it is ludicrous to criticize Einstein for his concerns about a theory that included it. It would be a sad day for science if such a huge paradigm shift swept over the community without raising a few hairs. Additionally, physicists still do not understand how the nonlocality is achieved, nor its implications.
The quantum measurement problem
A related issue is wave function collapse and the “measurement” problem. The measurement problem manifests itself in the fact that there are two rules for how a quantum state evolves in time. The Schrödinger equation tells us how the wave function (or more generally, the state vector) evolves in time when a quantum system is not being “observed” or “measured”. With the Schrödinger equation, you can calculate the probabilities for possible outcomes to different measurements, and how those probabilities change over time. This evolution of the state vector while no one is looking is continuous. However, instantaneous collapse of the state vector into a particular eigenstate occurs upon measurement. Why the discontinuity in the descriptions of the two processes? What constitutes a measurement? What are the dynamics for wave function collapse? Does this mean that wave functions (or state vectors) are approximations to some more complete description of quantum systems?
The collapse postulate is ad hoc, based on the fact that we never observe superpositions of quantum states. The core of the measurement problem is the inability of QM to explain the abrupt transition from linear evolution of the wave function, to non-unitary wave function collapse. Steven Weinberg summarizes it thusly: “during measurement the state vector of the microscopic system collapses in a probabilistic way to one of a number of classical states, in a way that is unexplained and cannot be described by the time-dependent Schrödinger equation.”
So, objective reality is not understood, nonlocality is not understood, wave function collapse is not understood. We could go on. My impression, based on trends in the literature, is that more and more of the community of physicists is recognizing the holes that remain in our conceptual understanding of the quantum world. As more and more theoretical and experimental physicists struggle with these issues, perhaps we will get closer to a breakthrough.
Here is a YouTube video with a quick introduction to entanglement: Quantum Entanglement – The Weirdness Of Quantum Mechanics. And a ScienceDaily article on quantum entanglement, including links to additional background information on quantum mechanics.
Comrade on the quest
To my delight, just as I finished writing and editing this post, I found the following article on the electronic preprint archive, arXiv.org. Submitted today by Pablo Echenique-Robba, who apparently shares many of my views on the current state of QM:
Abstract: If you have a restless intellect, it is very likely that you have played at some point with the idea of investigating the meaning and conceptual foundations of quantum mechanics. It is also probable (albeit not certain) that your intentions have been stopped on their tracks by an encounter with some version of the “Shut up and calculate!” command. You may have heard that everything is already understood. That understanding is not your job. Or, if it is, it is either impossible or very difficult. Maybe somebody explained to you that physics is concerned with “hows” and not with “whys”; that whys are the business of “philosophy” — you know, that dirty word. That what you call “understanding” is just being Newtonian; which of course you cannot ask quantum mechanics to be. Perhaps they also complemented these useful advices with some norms: The important thing a theory must do is predict; a theory must only talk about measurable quantities. It may also be the case that you almost asked “OK, and why is that?”, but you finally bit your tongue. If you persisted in your intentions and the debate got a little heated up, it is even possible that it was suggested that you suffered of some type of moral or epistemic weakness that tend to disappear as you grow up. Maybe you received some job advice such as “Don’t work in that if you ever want to own a house”. I have certainly met all these objections in my short career, and I think that they are all just wrong. In this somewhat personal document, I try to defend that making sense of quantum mechanics is an exciting, challenging, important and open scientific endeavor. I do this by compulsively quoting Feynman (and others), and I provide some arguments that you might want to use the next time you confront the mentioned “opinions”. By analogy with the anti-rationalistic Copenhagen command, all the arguments are subsumed in a standard answer to it: “Shut up and let me think!”