With this post, I begin to layout some concerns that I have with descriptions and interpretations of quantum physics. We still do not have a conceptual understanding of what the heck is going on in quantum mechanical processes. Albert Einstein took issue with several aspects of quantum theory: the inherent randomness, the nonlocality, and the lack of realism, for example. We may need to accept these aspects of nature, but is it asking too much to be able to understand how/why/what the universe is doing in these situations?
Quantum physics is a fickle mistress
Quantum Mechanics (QM) is perhaps one of the most successful hypotheses in the history of physics. That is, if you evaluate success based on agreement with experiment and ability to make predictions that are later confirmed by experiment. And, quite frankly, that is (quite appropriately) how science judges hypotheses and theories. Thousands of experiments have been performed, verifying the accuracy and relevance of QM. These experiments include emission or absorption spectra predictions and measurements, magnetic moment predictions and measurements, and multiple variations of the double slit experiment, to name a few. Physicists, chemists, and engineers have subjected matter to all kinds of bizarre tests that have validated the theory’s un-intuitive predictions. Additionally, QM is not just a theoretical curiosity. The range of technologies based on it is staggering. Without QM, we would not have PCs, iPads, smart phones, laptops, modern TVs, modern medical imaging equipment, the microchips that control everything from our cars to our refrigerators, and so on.
Yet, after all this, we still do not understand HOW quantum physics works. Even though the theory is a century old, we are far from a proper conceptual understanding of what it actually means and HOW the universe pulls off this behavior. How does a particle manage to take every possible path? How does a wave function seem to collapse, essentially instantaneously, across arbitrary distances? How do entangled particles influence each other, seemingly without regard to time and space? Why do certain quantities have to be quantized, rather than continuous? These difficulties are related to the fact that a complex-valued state vector is used to describe a physical system. So another way to ask these questions is, “why are complex quantities and a state vector required to quantitatively describe behavior at the quantum level?”
Why is it so hard to visualize quantum physics?
We can visualize general relativity (GR). It is understood as the interplay between matter and spacetime. Apart from some warping and dilation, GR makes intuitive sense. There is a speed limit and strict enforcement of locality. With some mathematics, we can readily convince ourselves that causality is safe. Electromagnetic and nuclear interactions are described mathematically and conceptually as due to the exchange of particles called bosons. These particles (photons for electromagnetism, gluons for the strong nuclear force, W and Z bosons for the weak nuclear force) account for the transfer of momentum and energy between fermions (i.e. quarks, electrons, protons, etc.). They also account for the transfer of conserved quantum numbers. In none of these fundamental forces do we have “spooky action at a distance”, or nonlocality. We do have virtual particles, which is another story and takes some time to get used to. But at least, even then, we have a picture in our heads of what is going on.
In quantum physics, the state vector is not a physical description of the system. And the evolution of the state vector seems to occur in two distinct phases. First, a continuous evolution of the state vector occurs as the system evolves in time and space (described, for example, by the Schröedinger equation). Then, there is an abrupt and discontinuous collapse of the state vector, into a particular eigenstate; the dynamics of which are not understood. A common misconception is that this state vector collapse is caused by the interaction with the measurement device. But clever, interaction-free measurement processes have been devised. The collapse of the wave function has been verified in situations where interactions play no role in the measurement. At least no known interactions.
Why should we care about conceptualizing quantum physics?
Given that QM works so well, and (so far) no experiments have contradicted it, why should we care how it is interpreted? QM does not provide a physical description of a process. The old adage is to just “shut up and calculate” (David Mermin). However, this lack of a conceptual understanding may be what is holding physicists back from uniting the two pillars of modern physics, QM and GR. It may be the key to understanding many of the most fundamental and provocative questions physicists are struggling with:
- How do we unite the two theoretical paradigms of modern physics, QM and GR?
- What happened at (and before) the origin of the universe?
- What will be the ultimate fate of the universe?
- What is driving the apparent acceleration of the universe’s expansion (i.e. what is dark energy)?
- What happens inside a black hole?
- What is time?
It may also be necessary for the next great leap in technology, such as quantum computing. Besides all that, I just want to know. Is that asking too much?
Competing interpretations of quantum physics
At least to some extent, I think QM has been a victim of its own success. Since it has worked so well, there is little less motivation to fix it. Additionally, it is very difficult to distinguish between the predictions of some of these different interpretations. So it will be difficult to experimentally validate the correct interpretation, at least for some time.
Many different interpretations have been offered up over the years. I will dig into some of these in later posts. To name a few (Frank Laloë, “Do We Really Understand Quantum Mechanics?”):
- Statistical interpretation
- Relational interpretation
- Logical, or algebraic approach
- Veiled reality
- Additional (hidden) variables
- Modified Schröedinger dynamics
- Transactional interpretation
- History interpretation
- Everett interpretation
- Modal interpretation
One of my favorites is the de Broglie-Bohm pilot wave interpretation. In this model, a particle’s motion is determined by a wave. Hence, you can reproduce both particle and wave-like behaviors and the predictions of generic QM. However, there are some issues with de Broglie-Bohm theory. These include things like relativistic invariance and the dynamics for how the particle and wave influence each other. “We’ll talk about that later”.
Unfortunately, alternative explanations have not been given full and proper consideration over the past 86 years (since the 1927 Solvay Conference and the birth of the Copenhagen interpretation). Some alternatives have been appropriately disproven. However, others have just been over-ridden or ignored. A common theme is that someone publishes a paper “showing” that some interpretation is not workable. Later, someone else shows how that paper was in error. People remember the first paper and continue to assume that a certain idea is untenable. Various interpretations are confused with each other, or assumptions are confused with conclusions. Scientists are erroneously lead to believe that a particular approach is not valid.
These myths are perpetuated in text books and lectures. One example is de Broglie’s hidden variables theory, which was relegated to the scrap heap after the 1927 Solvay Conference. It was resurrected after David Bohm developed his theory in the 1950s, and the similarities between it and de Broglie’s earlier work were noticed. Another example: experiments confirming the violation of Bell’s inequality and hence confirming the concerns of the famous EPR paper (Einstein, Podolsky, Rosen) are often cited as confirming hidden variables theories are unworkable. They actually show that hidden variables theories cannot sidestep the apparent nonlocality, not that they are altogether un-viable.
De Broglie-Bohm mechanics at work?
Take a look at this amazing video from the Science Channel’s Through the Wormhole, on Wave/Particle dynamics with silicon droplets. It shows how the results of the double slit experiment can be reproduced by a silicon droplet (the “particle”) riding on an actual, physical “wave”. There are a lot of details that go into this experiment, including how the apparatus works and how it is filmed. So it is definitely not a proof of de Broglie-Bohm mechanics. However, it is intriguing, and offers an irresistible visualization that begs further investigation.
I think a significant factor in our failure to develop a consistent and deep conceptual understanding of quantum physics is rooted in the dogmatic presentation of the prevailing interpretation. For decades, up-and-coming physicists have been indoctrinated in the “Copenhagen interpretation”. Presented with the implicit assumption that interpretation questions are settled, many students don’t dig deeper. The development of a proper, conceptual understanding has been further hobbled by misconceptions that are perpetuated through textbooks and instructors. Students either assume the issue is resolved and look elsewhere for research opportunities, or they are discouraged by their advisors and forced to conform to availability of funding and job opportunities.
I recall being confused and frustrated, as an undergrad physics student, when the explanations in the textbook or provided by the professor, just did not make sense and did not seem to be consistent with what the mathematics implied. For example, the meaning and implication of the uncertainty principle are often explained as being due to the unavoidable transfer of momentum to the observed particle during a measurement. However, experiments have been done that show this is not the case. Moreover, it is an intrinsic property of the mathematics, in which momentum and position are Fourier transformations of each other, like time and frequency in everyday applications of Fourier theory to acoustic or electromagnetic signals.
In the weeks and months ahead, I will expand on the specific points brought up in this article. It is entirely possible that Nature really is unknowable. The Universe probably does not feel compelled to satisfy my desire for a visual, comprehensible model, unless it already intended to do that anyway.