Quantum Darwinism

Quantum theory without the weirdness

This is an updated excerpt from the book Darwin Does Physics.

A recent resolution of some quantum mysteries has been achieved by considering the behaviour of quantum systems in terms of their environmental interactions as opposed to prior explanations focusing on quantum systems as isolated entities. Unfortunately, as Richard Feynman famously noted, these prior interpretations of quantum theory are understood by no one (1). This new quantum interpretation has many advantages of biological theories that rely heavily on organisms’ environments. In general, we might consider that things behave as they do largely in response to constraints and opportunities of their environments and this perspective, has shed new light on quantum systems. As described in a 2014 review article by Maximilian Schlosshauer (2):

It is a curious “historical accident" that the role of the environment in quantum mechanics was appreciated only relatively late. While one can find -for example, in Heisenberg's writings – a few early anticipatory remarks about the role of environmental interactions in the quantum-mechanical description of systems, it wasn't until the 1970s that the ubiquity and implications of environmental entanglement were realized by Zeh. It took another decade for the formalism of decoherence to be developed, chiefly by Zurek

Wojciech Zurek is a widely recognized leader in interactions between quantum systems and their environments and has described this within the theories of quantum decoherence and quantum Darwinism. Zurek’s theoretical progress may be encapsulated using three new concepts he introduced (3):

Figure 1: Wojciech Zurek

1) Einselection describes the type of information which may be transferred between a quantum system and in its environment during a quantum interaction. Zurek demonstrates that only information in the form of the system’s ‘pointer states’ can survive transfer to the environment. These pointer states depend upon both the nature of the system and the component of its environment with which it shares information. Pointer states span only a small subset of quantum information, limited to a few classical characteristics, such as the system’s position or momentum, which is all that the environment can experience of the quantum system. And as we will see, this web of connections between quantum systems defines classical reality.

2) Envariance assigns a probability to each of the possible packets of information which may be transferred. These packets correlate with measurement values and thus envariance may predict the outcome of possible measurements. It is of interest that Zurek derived these probabilistic predictions from fundamental quantum axioms rather than following the tradition of including them as a separate axiom.
 
3) Quantum Darwinism describes the relative reproductive success which the various transferred information packets achieve in their environments. Zurek has shown that classical information described by einselection and envariance achieves a high level of redundancy in the environment; other types of information have a low survival rate. Zurek considers that the high level of redundancy of classical information provides classical reality with its objective nature; numerous observers will agree on the nature of reality as they have access to a shared, highly redundant but limited pool of information.

Together these concepts describe quantum interactions in terms of a Darwinian process, and I will refer to all three as the process of quantum Darwinism. In each generation of information transfer just a small subset of information can survive and reproduce in its environment. This is deeply analogous to natural selection where in each generation of genetic information transfer, only a small subset of genetic possibilities can survive and reproduce in their environments. 

But this is an explanation of physics in terms of a biological analogy. It is perhaps shocking to many physicists that the central organizing principle of biology has been used to explain one of the most fundamental physical processes. Although Zurek’s technical description has been endorsed within consensus physics, its Darwinian implications have been shunned. If you Google ‘quantum Darwinism’ and ignore articles written by him, you will be left with only meager comment. Zurek however is unequivocal concerning the Darwinian nature of his theory (4):

The aim of this paper is to show that the emergence of the classical reality can be viewed as a result of the emergence of the preferred states from within the quantum substrate through the Darwinian paradigm, once the survival of the fittest quantum states and selective proliferation of the information about them are properly taken into account.

In my view this conjunction of fundamental physical and biological explanations provides great hope for the unification of scientific understanding within a few overarching concepts such as universal Darwinism.

Quantum Darwinism provides a detailed description of a Darwinian process in terms of a single cycle, the evolution which occurs during a single generation. While the shift in probabilities in a single generation may be the unit of evolutionary change, it is the long-term effects due to numerous repetitions of this cycle which produce the dramatic accomplishments of evolutionary systems.

To date the development of the theory of quantum Darwinism has focused on processes involved in a single cycle and has not yet paid much attention to the long-term consequences produced by continual iterations over cosmic time. We should be careful to understand that generational succession in quantum Darwinism refers to the same phenotype, such as an atom, only later and possibly under slightly different circumstances. In this regards it may be analogous to generations of single-celled asexual life where succeeding generations are not truly new individuals but may rather be considered as variations on the same individual with perhaps small changes in circumstance.

In several earlier sections I have repeated Zurek’s claim that ‘classical reality emerges from the quantum substrate’ but have not provided much explanation for that astounding assertion. The claim itself may seem somewhat metaphysical, as if classical reality arises as a kind of ephemeral vapor from a more fundamental quantum reality. With a better understanding of the mechanisms of quantum Darwinism, we are now prepared to discuss this dramatic claim.  

We might view the quantum substrate as a vast sea of quantum systems which can interact with each other only by way of the four fundamental forces – make that three if gravity is considered emergent from quantum forces. As each system can only receive information concerning any other system by way of these few forces, severe constraints are placed on the information which can be received. Zurek understands these constraints as a Darwinian selection mechanism which he describes within the theory of quantum Darwinism. ‘Classical’ information best survives the transfer to other quantum systems. Thus, the information that one system may acquire of another is largely classical information. 

We should understand that this information is physical, it describes the only forces known to operate in the universe. Although these are quantum forces, they are well described by Newton’s classical mechanics because the classical information, even though it describes only a small subset of the quantum substrate is all that is available, all that passes through the Darwinian selection mechanism. This information obeys classical logic and the mathematics of probability theory which we consider forms of common sense. While this classical information forms classical reality it is insufficient to fully describe the quantum reality. Quantum reality really is weird and seems to operate as described by other forms of mathematics, such as the uncertainty principle, that do not jibe with common sense. But networks of quantum systems contain largely classical information, and this forms our familiar classical reality.

Crucially, the information which can survive the transfer entails all that one entity can feel, understand, or react to concerning another entity. For example, information obtained through vision depends upon quantum interactions between photons and pigments in our retinas, and the information transferred in this process is classical information. In other words, the ability of any entity to detect the outside world is largely restricted to the classical characteristics of that world. Thus, classical reality emerges from the quantum substrate. 

Both the logic we are familiar with, and the physical nature of our reality are explained as due to a Darwinian selection mechanism which regulates the flow of information between entities. The universe is not static. On the reception of information entities may react, such as with an acceleration, broadly as described by the classical theories of Newtonian mechanics, Maxwell’s electro-magnetism and the theories of special and general relativity.  These are the forces and logic which defines our classical reality. They are also the forces which powered the evolution of the universe from the Big Bang onward. 

Zurek’s theory implies that this evolution should be understood as the operation of a Darwinian process. Each generation of information exchanged by quantum systems is selected based on its reproductive success and earlier physical forms are distinguished from later ones by their relative ability to survive and reproduce. In this view we can understand the physical evolution of the universe since the Big Bang as a form of Darwinian evolution.

The great power of natural selection is not obvious in changes taking place over a single generation, but rather in the cumulative effect of these changes over evolutionary history. Already in On the Origin of Species Darwin made the case that the many biological forms found in nature have evolved from a single ancestral individual through the process of natural selection. We might say that over evolutionary time genomes have successively probed the huge number of environmental circumstances in which they have found themselves for strategies of reproductive success and that new phenotypes have been successively constructed to fit those circumstances.

Zurek has described decoherence and quantum Darwinism as responsible for the transition from quantum to classical realities. In short, classical reality is composed of an extremely small subset of quantum possibilities which are selected through a Darwinian process. In this view we must see the evolution of classical reality as originating in this same Darwinian process. Thus, the history of the universe may be considered as shaped through the operation of quantum Darwinism.

In Zurek’s terminology ‘pointer states’ are those quantum states which may exist in classical reality. As he explains these states are defined by their ability to achieve reproductive success within their environments (5):

According to decoherence theory, the ability to withstand scrutiny of the environment defines pointer states. As I will discuss, the proliferation of records about those states throughout the environment is the essence of quantum Darwinism.

We often assume the physical evolution of the universe since the Big Bang to be a rather straight forward case of cause and effect; the continued expansion and cooling of the universe resulted in new forms and phenomena. The Wikipedia article on the Chronology of the Universe provides many examples of this way of thinking about the evolution of matter in a cooling universe since the big bang (6):

At about one second, neutrinos decouple; these neutrinos form the cosmic neutrino background (CνB). If primordial black holes exist, they are also formed at about one second of cosmic time. Composite subatomic particles emerge—including protons and neutrons—and from about 2 minutes, conditions are suitable for nucleosynthesis: around 25% of the protons and all the neutrons fuse into heavier elements, initially deuterium which itself quickly fuses into mainly helium-4.

By 20 minutes, the universe is no longer hot enough for nuclear fusion, but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaque plasma.

Zurek’s theory suggests that this historical process may have been more nuanced. New physical forms may have emerged as the universe changed due to their ability to discover new environmental niches in which they could survive.

Within the scope of this theory, we find a close analogy to natural selection. We might say that over evolutionary time quantum systems have successively probed the huge number of environmental circumstances in which they have found themselves for strategies of reproductive success and that new phenotypes have been successively constructed to fit those circumstances. If we understand ‘phenotype’, in this context, as the succession of physical forms, such as gluons -> fermions -> atoms ->molecules, the physical evolution of the universe since the Big Bang may be understood in terms of a Darwinian process: Quantum Darwinism.

References

1. Goodstein, David. Feynmans Lost Lecture. s.l. : WW Norton; Pap/Com edition (Jan. 18 2000), 2000. ISBN-10 ‏ : ‎ 0393319954 .

2. Schlosshauer, Maximilian. The quantum-to-classical transition and decoherence. [book auth.] M. Aspelmeyer, et al. Handbook of Quantum Information. s.l. : Springer: Berlin/Heidelberg, 2014.

3. Quantum Darwinism. Zurek, Wojciech H. s.l. : http://www.nature.com/nphys/journal/v5/n3/abs/nphys1202.html, 2009, Nature Physics, vol. 5, pp. 181-188.

4. Zurek, Wojciech. Quantum Darwinism and Envariance. [book auth.] P. C. W. Davies, and C. H. Harper, eds. J. D. Barrow. Science and Ultimate Reality: From Quantum to Cosmos. s.l. : Cambridge University Press, http://arxiv.org/pdf/quant-ph/0308163.pdf, 2004.

5. Quantum Darwinism, Classical Reality and the randomness of quantum jumps. Zurek, Wojciech. s.l. : Physics Today, 2014, Vols. 67, 10, 44.

6. Wikipedia. Chronology of the universe. Wikipedia. [Online] [Cited: February 7, 2023.] https://en.wikipedia.org/wiki/Chronology_of_the_universe.