Wednesday, September 08, 2004

Quantum Mechanics, Experience and Reality

Over time I’ve read with interest a number of descriptions of quantum mechanics. While my level of understanding of the topic as a layperson is limited, I have always thought it is important to try to grapple with what the theory implies about reality. Recently, I’ve come to believe that QM provides strong evidence for my view of the role subjective experience plays as a fundamental element of nature.

Brief Sketch of QM

In classical mechanics, specifying the attributes of an object or system at one moment tells you everything you need to know (in principle, anyway) about how the system will evolve in time. It is an objective process, with no need to reference an observer’s experience. Unfortunately, in analyzing atoms and sub-atomic particles and systems, classical mechanics failed, and QM was discovered as the theory which accurately described experimental outcomes.

In QM, after the initial measurement/observation the system does not evolve in a deterministic way. The system spreads out to encompass a wide range of possibilities. A new measurement/observation will cause the system to reveal a particular value, and the quantum mechanical formula which described the evolution prior to the measurement can be seen as giving the probabilities for particular values to be measured.
Between measurements, we cannot specify precise values for attributes such as position, spin, and velocity. (Further, even when a measurement is made, it turns out that certain attribute pairs cannot both be measured with precision).

QM Interpretations

Ever since the early days of the theory, physicists (along with some philosophers) have grappled with interpreting what QM means for how we view reality. Do particles and other objects we thought were familiar denizens of the world objectively exist independent of observation? If they are dependent on observation, does it need to be a conscious human doing the observing?

Different interpretations of QM often can be distinguished by the weight given to the objective “real-ness” of the Shrödinger wave function which mathematically describes the evolution of the quantum system between measurements. The early “Copenhagen” interpretation denied the reality of the wave function and instead stressed its role as a calculator for explaining experimental results. Efforts to interpret QM by assuming the wave function is objectively real have led to the “many-worlds” interpretation, where all of the possible values of a quantum system described by the wave function actually exist, but one observer cannot see them because they take place in different worlds and/or different minds. Of course other efforts to revise or enhance the theory have been made to provide for an objective trajectory for quantum variables while avoiding many-worlds. These efforts have been limited in acceptance since the theory works so well in its current form for experimental applications without need of any change.


Efforts have been made to enhance the interpretation of QM by formally using information theory (one recent example was from physicist Anton Zielinger). Specifying the number of bits of information available in the system and applying information theory can help explain the limits on the observer’s knowledge about the state of the system which arise in the theory. Interpretational difficulties remain: these arise from the fact that the concept of information still embeds in it the question of “information for whom?” as I discussed in an earlier post. (Zeilinger’s underlying perspective remains the Copenhagen interpretation)


I’ve recently started to read about the progress made in the problem of what plays the role of the “observer” in QM. In the original experimental context, it is the human observer, or perhaps his or her macroscopic measuring equipment, which plays the role of the observer. Physicists have presented increasingly effective arguments that there is nothing special about the human being involved: there is a threshold beyond which the interaction with any other system or with the “environment” is sufficient for the quantum system to take on particular values from the set of possibilities described by the wave function. This phenomenon is known as decoherence.

The Implication of QM for Reality

The solution to the interpretation of QM lies in accepting that subjective experience is a fundamental part of nature. It doesn’t make sense to say that objects or systems exist as complete entities independently of their being experienced. (I take experience to be the best term for what in QM is couched in terms of measurement, observation, information exchange, etc). Full description of an entity in the world requires the reality of its being experienced. Decoherence shows that we are not just talking about experience in the form of human consciousness. All systems in the world manifest some form of experience in their evolving interaction with the rest of reality. Conceiving that this is the case is very difficult (what does it mean for an electron to have experience?), but no other conclusion makes sense. For me, this conclusion is buttressed by the fact that I came to believe in the irreducibility and ubiquity of subjective experience as the solution to the traditional mind-body problem independent of QM considerations. I believe this convergence of scientific and philosophical inquiries bolsters the prospects for the success of this worldview.

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