Welcome to Module 1
Lesson 2: Interpretations of Quantum Mechanics
Buddhism & Quantum Physics
Lesson 2: Interpretations of Quantum Mechanics
During the course of this lesson, you will:
- Learn the difference between realist and non-realist interpretations of QM;
- Investigate the role of consciousness in shaping reality;
- Learn about the most important interpretations of QM.
Galileo for one pioneered the use of the scientific method to deduce “facts” about an “objective” external world. This approach works well for classical physics but seemingly breaks down at the microscopic quantum level where the distinction between “objective” reality and the process of observation is not so clear cut. Whilst the foundational equations that describe quantum systems and events are not disputed (and indeed have proven to be the most powerful and accurate equations we have for any scientific theory) there remains much debate about how these equations should be interpreted. What is it really that they describe? What is the actual “ontological status” (or way of existing) of quantum “particles”? Do they “really” exist, or do the equations simply offer a way of describing certain indefinable processes? Does an “observer” really affect the outcome of quantum experiments?
Interpretations of Quantum Mechanics
The Copenhagen Interpretation
According to this interpretation, devised by Werner Heisenberg (1901-1976), Niels Bohr (1885-1962) and others, measurement plays a key role in changing quantum states. It utilizes the concept of the “Heisenberg cut” which divides quantum systems into the “observed system” (described in terms of quantum mechanics) and the “observing system” (which consists of the measuring device and the agent that acquires knowledge about the measurement outcome). The observing system impacts the observed system through the process of measurement. In the Conventional Copenhagen Interpretation, the “cut” is objective and the same for all observers: all observers are placed on the “classical side” of the cut. But, as we’ve seen, it is unclear where the “cut” is to be placed, which leads to problems like Wigner’s paradox. In the Neo-Copenhagen Interpretation, the positioning of the “cut” is subjective and depends on the observer; different observers can, in principle, partition the world in different quantum and classical regions.
Realist or Non-Realist Interpretations?
One important way of distinguishing different interpretations of Quantum Mechanics is whether they are “realist” or not. There is an old philosophical question that asks: if a tree collapses in a forest, but nobody is around to observe it, does it make a sound? A realist would say yes because there is a “real” world “out there” that exists independent of any observation; whereas a non-realist would say that there is no sound if nobody can hear it and therefore that reality is in some sense dependent on the people, or minds, that observe it.
In Quantum Mechanics, Einstein was a realist who saw the quantum world as operating deterministically; Bohr was comfortable seeing the world as operating in an anti-realist, indeterministic way and such “Copenhagen” interpretations were named after his hometown. The most popular version of the Copenhagen interpretation has both realist and anti-realist elements: the observer makes a difference, but there is a real world for them to make a difference to. A more extreme version, called the Van Neuman-Wigner interpretation, goes further, and explicitly posits that it is consciousness itself that causes the collapse of the wave-function.
Whilst Copenhagen interpretations of Quantum Mechanics remain popular, many scientists don’t like the idea that the mind plays an active role in the outcome of experiments and so, like Einstein, prefer alternative “realist” interpretations. It’s important to realize that such other interpretations exist which say quite different things about the nature of reality. The next three examples described below offer different kinds of “realist” interpretations.
Bohm’s interpretation doesn’t accord such an important status to the observer or the process of measurement. It sees no boundary between the quantum and classical system. Instead, the whole universe is considered to be a single quantum state, of which the observer is a part. His conception is “deterministic” but relies on the existence of hidden, unseen variables or “quantum potential”. Rather than viewing the equations of quantum mechanics as describing a set of probabilities, Bohm interprets them as describing an actually existing potential impacting actually existing particles. However, each particle in the universe is influenced by, and in turn influences, every other particle, via this “quantum potential”. His conception is therefore highly “non-local” and represents a “theory of the universe”, rather than just a theory of any particular system.
This interpretation is also deterministic and describes the entire universe as a single quantum system, of which observers are a part, and in which everything is “below the cut”. However, observers have a role in that their observations are the basis for defining the branching structure of the global quantum state. Whereas under other interpretations a quantum wave function “collapses” into one or other possibility, which then becomes the “actual” state of that system, with the “Many Worlds” interpretation, there is no collapse. Instead, every single possibility actually exists in a different universe. Each observation or measurement triggers many whole new universes which “branch out”.
This interpretation was developed by Dr. Carlo Rovelli among others and takes a different stance with respect to observers. As in Einstein’s Special Theory of Relativity, the properties of a system are not seen in absolute terms, but rather vary according to a chosen “frame of reference”. Similarly, here there is nothing special about the observer; this can be any physical system, rather than a conscious observer. No properties are absolute, including the “outcome” of a measurement or observation, which depends on which system is taken as a frame of reference and its relationship to the measured system.
This stands for “Quantum Bayesianism”, referring to the statistical interpretation known as Bayesian probability. It doesn’t concern itself with the ontological status of quantum entities or other “objective” features of reality, but is rather a “normative” interpretation, that is, a framework to be used by individual observers to help them make decisions and navigate the world. According to this view, the equations of quantum mechanics only reflect the knowledge and beliefs that a certain observer has about reality, given a certain amount of limited information available to them, rather than saying anything inherent about entities in the “external” world. From the perspective of any observer, only they themselves are “above the cut”; all other agents are below it.