An Aside - Quantum Darwinism and Decoherence
- Corey Rose
- Feb 4
- 2 min read
Updated: Feb 26
While not an interpretation of quantum mechanics itself, Quantum Darwinism explains how a classical, objective reality emerges from sub-atomic, quantum phenomena.
In the quantum world, particles can exist in a superposition of states. However, in everyday life, macroscopic objects are only ever found in a single state; Schrödinger's cat is not really both dead and alive! This is due to the process of decoherence, and it results from the fact that particles are never truly in isolated systems; instead, they are constantly surrounded by the many other particles that make up our universe.
The moment the particle interacts with the environment in any way, whether it be a dust particle, an air molecule, or a stray photon, that particle becomes entangled with its environment. That is to say, the particle and the environment no longer have independent descriptions. If the environment ends up one way and not another after the interaction, then the particle must be in a particular state. This entanglement spreads rapidly outwards as more interactions occur, a process known as decoherence that makes macroscopic objects appear classical. It is important to state that decoherence does not involve a physical collapse of the wave function describing the particle. All possibilities remain as predicted in the quantum description; they just stop interfering with one another.
This, however, does not address a key problem: why do different observers agree on what they see, for example, why two people can look at a table and agree on its position, despite the universe being composed of quantum particles? Quantum Darwinism provides an answer to this. When the environment interacts with a system, information about that system's state is copied millions of times into the environment. For example, when light reflects off the table and is scattered, information about its position is encoded in and carried by many different photons in many different directions.
Quantum Darwinism claims that only certain states are stable and robust enough to survive these constant interactions, and that most states quickly vanish under this constant monitoring. These states are known as ‘pointer’ states, and it is the information about these particular states which is copied into the environment. This process could be analogised to natural selection, where only certain genes of a species survive evolution, hence the reference to Darwinism.
The information about these states can then be accessed independently by different observers without disturbing the system itself - for example, through the reception of different photons. Out of this emerges an objective, classical reality where observers agree on a system’s properties without interfering with what they are observing. of different photons. Out of this emerges an objective, classical reality where observers agree on a system’s properties without interfering with what they are observing.
Quantum Darwinism is...
unrealistic
too 'handy-wavey'
a fascinating insight that could inspire new theories
Recommendations for Decoherent and Darwininian discoverers:
The original paper proposing Quantum Darwinism as written by Zurek is a strikingly accessible introduction to the idea
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