Why the Meaning of Quantum Mechanics Really Matters
In a New York Times Opinion, physicist Sean Carroll stated that “quantum mechanics is the most fundamental theory we have, sitting squarely at the center of every serious attempt to formulate deep laws of nature.” Yet, he lamented, “Physicists don’t understand their own theory any better than a typical smartphone user understands what’s going on inside the device” and that physicists seem to be O.K. with this. This attitude is, perhaps, understandable. More than 90 years after Einstein, Bohr, and other physicists first started debating the meaning of quantum mechanics, there is still no consensus.
The true nature of physical reality may be unresolved, but the quantum wavefunction’s description of physical reality is perfectly clear. It describes a physical state as a superposition of all potential measurement results. Quantum measurements are intrinsically statistical. An individual measurement of a particle’s position reveals only one of multiple measurable locations. Quantum mechanics describes the particle prior to measurement as a simultaneous superposition of all measurable positions.
Erwin Schrödinger tried to highlight the absurdity of equating the wavefunction’s description of reality with the physical reality, itself. Schrödinger imagined a radioactive particle and a cat in an isolated box, along with an apparatus that releases cyanide gas if the particle decays. As long as the box’s contents remain isolated and unobserved, we cannot know if the cat is dead or alive, and the quantum description of state remains a superposed wavefunction describing the cat as simultaneously dead and alive. As a description of the box’s unseen contents, this is good and fine. Equating the wavefunction and the physical state, however, would mean that the contents exist as a physical superposition of live cat and dead cat, up until it is observed, when the wavefunction collapses and observation reveals a definite state of dead or alive. Schrödinger considered the idea of a physically superposed cat to be absurd, and he proposed his thought experiment to argue that the wavefunction does not define physical reality. Yet, the still-prevailing interpretation of quantum mechanics, the Copenhagen Interpretation, does just that. The debate on how, or whether, observation causes collapse of a superposed physical state to a definite state is the widely debated and unresolved measurement problem.
Whether or not a macroscopic system, isolated and unobserved, can exist in a physically superposed state until observed is not simply a philosophical musing. Quantum computing, which is widely hailed as the next technological revolution, is based on a superposed state representing all of the possible answers to a problem. By properly controlling the wavefunction, the idea is that the correct answer will be actualized upon its collapse.
Even though physics acknowledges that entropy increases irreversibly, it does not fundamentally distinguish between past and future. Even though quantum mechanics acknowledges the randomness of quantum observations, the quantum state for an isolated, unperturbed and unobserved system is deterministic. Physics wiggles around irreversibility by defining entropy as a statistical measure of disorder, reflecting an observer’s ignorance of the actual state. Quantum mechanics wiggles around the randomness by describing observation as an ultimately deterministic but unknown perturbation from the system’s surroundings.
We may perceive irreversible change and believe we have the free will to make choices, but according to physics, there is no arrow of time and the future, as well as the past is set in stone. When we observe friction irreversibly dissipating kinetic energy to waste heat, physics tells us that we perceive heat as randomized energy of diminished usefulness, because our lack of knowledge of its microscopic details precludes us from fully utilizing heat to do work. The dispersal of energy by friction could, in principle, be reversed, and there is no fundamental change in an isolated system’s usefulness, or value. Like entropy, physics describes usefulness and value as properties of an observer’s imperfect perception.
Physics tells us that there is no arrow of time, that the future is already determined, and that the universe is fundamentally valueless. Most physicists do not dwell on philosophical implications; they simply get on with the business of doing physics. When it comes to most other sciences and the humanities, however, a timeless and valueless reality leaves many important questions unanswered and unanswerable. We observe the evolution of the universe, of matter, and of life, and we perceive evolution as leading to an increase in value over time. We observe that economic and human activities can create wealth and value. But with no fundamental framework for assigning values, there is no motivation to search for fundamental principles that might help us understand how and why systems evolve or create value. Yet, these are critical issues that we need to address the most challenging issues facing us.
Physics is regarded as the ultimate foundation of all science. Chemistry is founded on physics; biology is founded on chemistry; and human behavior and consciousness are founded on biology. Physics’ status as the most fundamental of science carries with it a large responsibility. What physics says about reality matters far beyond the confines of the physics classroom or laboratory. Physics needs to move beyond its focus on isolated states, timeless and valueless; it needs to focus on the nature of reality as it actually exists, irreversibly evolving and creating value over time. In another post, Reinventing Time, I describe an interpretation and firm conceptual foundation of physics that does just that.
