Context
Quantum mechanics is a revolution in physics and also in metaphysics. QM has planted several basic questions in traditional physics:
- Determinacy/Indeterminacy. Before quantum physics, it was held that a physical system had one state that uniquely determined all the values of its measurable properties, and vice versa: the values of its measurable properties uniquely determined the state.
However, Heisenberg's indeterminacy principle states that there is a limit to the precision with which certain pairs of physical properties, such as position and momentum, can be known simultaneously. This means that the more accurately one property is measured, the less accurately the other property can be known.
- Superposition. In classical physics superposition describes when two physical quantities are added together to make another third physical quantity that is entirely different from the original two. For example two pulses on a string which pass through each other will interfere, following the principle of superposition.
In explaining quantum superposition a coin analogy is useful. A coin has a 50∕50 probability of landing as either heads or tails. When we flip a coin then its state in the air is in superposition; when it lands its state is definitive since we can measure it as either heads or tails.
- Multiple dimensions were an integral part of string theory, first introduced in the 1960s, then developed into five versions before it was thought, in the 1990s, that they all formed cases of a single M-theory, which has eleven dimensions. As well as the three recognised spatial dimensions (length, width, and height), quantum physics suggests extra dimensions which may be hidden from our normal experiences. String theory proposes to unify these 11 dimensions.
- The Many Worlds theory is related to multiple dimensions. It was the brainchild of Hugh Everett who, in 1957, suggested that every time a quantum event happens the universe divides into multiple non-communicating branches. If you flipped a quantum coin, there would be a universe where it landed heads and another where it landed tails.
There are so many alternative theories of how QM functions physically and metaphysically that predicting its implications has become a hard task. Lewis puts it this way:
"we can say quite confidently that quantum mechanics is metaphysically revisionary even if it is not clear what form the revisions should take."
The existing theories disagree on what exists and the laws that govern it. Lewis's book Quantum Ontology is an attempt to understand the metaphysical implications of the differing theories and provide a guide to the present philosophy of QM.
Summary
Quantum Ontology: A Guide to the Metaphysics of Quantum Mechanics by Peter J. Lewis was published in 2016.
Chapter 1
Lewis describes what he believes are revolutionary aspects of quantum mechanics: interference (particles of matter exhibiting wave-particle duality) and entanglement (non-local dependency of particle properties). The author also presents key aspects of maths formalisms in QM: matrix mechanics and wave mechanics.
Chapter 2
Given that science seeks true descriptive theories Lewis asks how interference and entanglement can be explained. The problem is that in QM there are no recognised definite properties or entities unless they are measured. For example in the double-slit experiment a beam of light might be either a particle or a wave. But when it is not measured, what is its nature? Lewis argues that it is not sufficient to point to hidden variables to explain this paradox and concludes that QM is thus incomplete in its theoretical accounts. He adds that once we accept that the universe is local and properties do not depend on physical measurements, then defining physical properties of systems will inevitably lead to erratic predictions. Which should we abandon: description of reality in a QM context, or the assumption of locality and interdependence? Lewis's response is that a realistic view could accept different shades of meaning depending on the interpretation we accept.
Chapter 3
The author proposes the development of basic ideas in each interpretation of the QM measurement problem: Bohm's hidden-variable program; GRW theories and the Many Worlds interpretation.
In Bohm's approach the theory is non-local and deterministic since there are particles that have definite positions in each moment of time. Indeterminacy is confined to certain properties and is non-local which conflicts with Einstein's theory of relativity.
Ghirardi–Rimini–Weber (GRW) is a spontaneous collapse theory which is indeterminate and non-local and argues for a law which generates the collapse of the wave function without an observer.
The many worlds theory is deterministic since each measurement is located in some world. However, this hypothesis needs to explain the probabilistic results obtained in each branch.
Chapter 4
Indeterminacy refers to the fact that some properties of physical systems have a determinable property, for example spin, yet the value of that property remains indeterminate (up or down spin). QM thus sheds light on a new form of indeterminacy.
The chapter also deals with Albert's bare theory. This approach espouses a radical defence of indeterminacy at all levels of reality, microscopic and macroscopic. The argument is that this indeterminacy could generate the illusion of determinate experiences in our perception.
Chapter 5
Bohm's theory proposed determinism but no locality. However this is also questionable, if we accepted a counterintuitive retrocausality, the idea that future measurements could exert a causal influence on the past.
Chapter 6
Lewis focuses here on how to recover probability in the many worlds theory. Even if every possible measurement occurs in some world, the observer would remain uncertain as to which result would be determinate in each case.
In this chapter the author also analyses how QM can affect themes such as free will, personal identity, retrocausality, the nature of individual particles and how you could infer your immortality according to the many worlds interpretation.
Chapter 7
The dimensionality of the universe is discussed through the issues of the wave function and the configuration space it occupies. A realist view of the wave function sees that the 4-dimensional space we perceive we inhabit is an illusion because the configuration of space is multi-dimensional. If this is so, how do we make sense of our 3D perception?
Chapter 8
Holism in QM refers to the relational properties that exist between particles, but do not depend on their internal properties. Two electrons may show anti-correlation in their spins.
Particle relationships that are defined as holistic possess property interactions that do not depend on their intrinsic properties. Two electrons that show anti-correlated spins possess a relationship which does not depend on their spin properties.
Humean supervenience argues philosophically that in our world the fundamental properties are local qualities: intrinsic properties of points. Lewis also questions the relevance of the privileged position of science as a source of metaphysical knowledge and thus the right of scientific knowledge to challenge any description of reality from a metaphysical viewpoint. The author inclines towards accepting arguments from a naturalistic metaphysics.
Themes
Pros and cons
It is clear that QM is revisionary in all of its interpretations and retreating to classical explanations is not an option. Lewis is right to investigate the possible metaphysical consequences of this radical change in our conception of reality.
However, his book hardly mentions the relevance of String theory in QM. Some basic questions are also left unanswered: do underdetermination, indeterminacy and non-locality receive a different treatment in those research fields? Should we be optimistic about realism in QM as science advances?
Philosophical Implications
The QM theory of parallel realities challenges our conceptions about the nature of reality. If there exist multiple versions of ourselves in different universes, then that raises challenges to the ideas of free will, identity, consciousness and the meaning of life. The suggestion is that we are just one of many versions of ourselves, each one living different possibilities.
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