Quantum Darwinism

Quantum Darwinism is a new theory recently developed by physicist Wojciech Zurek and his collaborators that may herald an immensely important addition to the field of Universal Darwinism. Universal Darwinism teaches us that emergent phenomenon, where complex organization arises from chaotic building blocks, may be indicative of the unexpected existence of a new Darwinian process.

 Such is Universal Darwinism’s interpretation of the evolution from biology to culture. Culture is produced by biological agents but its added complexities are best explained in terms of the operation of additional Darwinian processes. Although the details of these processes are only now becoming clear the spawning of areas of study in the Social Sciences and Humanities such as memetics, evolutionary archeology and evolutionary epistemology testify to the power of Darwinian explanations within the cultural realm.

 The new theory of Quantum Darwinism may provide such explanatory power at a deeper, pre-biological phase of our emergent reality. It may provide a key explanation of how the classical macroscopic world containing all objects with which we are familiar may have arisen from the weird quantum world of particle physics underlying it.

The weird picture of reality portrayed by Quantum physics has made it uninviting for inclusion in most people’s worldview. Our inability to easily digest this theory is regretful as it is one of the most accurate physical theories ever devised and is the most widely accepted scientific theory for a broad range of physical phenomena. In fact quantum theory may be a unique scientific theory in encompassing all forces and matter

It is so hard to understand that even the experts seem to take pride in its obtuseness and say things like ‘quantum theory is so weird that if you think you understand it, you don’t’. We’re talking about an explanation here because science is explanation. An explanation so weird that if you think you understand it you don’t? What kind of an explanation is that? And yet the theory works, to as many decimal places as measurement has been capable.

Those troubled with integrating quantum theory into their worldview are not confined to the scientifically challenged. Einstein, too, could not accept such weird scientific explanations as complete. To him science was enlightenment not bewilderment as to the meaning of reality. Einstein mixed it up with the quantum guys, especially Bohr. Einstein got off some great lines like ‘I don’t believe God plays dice with the universe’, but in the end it looked like he lost. He grew estranged from the larger scientific community which seemed contented  with cataloguing the details of quantum weirdness. By the time of his death very few researchers were active in relativity or in Einstein’s project of unification. Einstein was very much on the sidelines and considered by many as simply wrong about the important issues facing physics.

Einstein’s theory of General Relativity is very geometrical and elegant, unlike the quantum explanation. The most widely accepted interpretation of quantum phenomena was first articulated by Bohr. It is most succinctly understood as the implications of a number of axioms:

1)   For every physical system there is a corresponding mathematical object called a state vector that has no objective physical existence. This state vector is the most complete source of information that exists concerning the physical system.

2)      The outcome of any measurement on a physical system can be predicted by performing a specific mathematical operation on its state vector.

3)      The outcome of any measurement process on a physical system can only be predicted as a probability for obtaining that result.

4)      Once a measurement is made the state vector assumes a state such that the same measurement immediately reapplied to this state has 100% probability of achieving the previous measured result.

5)      The state vector evolves in time according to a continuous, deterministic formula except when a measurement occurs and then it jumps to the state described in 4) above.

These axioms are all about mathematical manipulation of mathematical objects and are not a vision of physical reality; in fact the first axiom explicitly states that the mathematical objects of the theory have no physical embodiment.

Until now no one has found an explanation of quantum phenomena consistent with the every day world we experience. Quantum Darwinism is such a theory and attempts to explain the mechanisms responsible for transforming quantum reality from its weird abstract mathematical realm into the common sense classical reality experienced in our every day lives.

 Allowed Quantum states are vastly more numerous then those we experience in classical physics and are the cause of much of quantum theory’s weirdness. This is famously illustrated by the ‘Schrödinger’s Cat’ paradox. It poses a thought experiment where a cat is placed in a box along with some random quantum device, such as a sample of uranium in the process of nuclear decay, acting as a trigger on some other mechanism such as a vial of poison that will kill the cat if activated. According to quantum theory as time goes by there is a growing probability that the uranium will disintegrate causing the death of the cat. The quantum state describing the cat will be a superposition of both ‘cat alive’ and ‘cat dead’ states. Traditional quantum theory claims that only a measurement conducted by a human observer can resolve the matter and actually compel the cat to exist in either a live or dead state.

 

 This is clearly nonsense as such superpositions of states combining life and death are never encountered. Although complicated superpositions are by far the most numerous types of quantum states and are widely observed in experiments involving microscopic quantum phenomena they are rarely detected in our classical macroscopic reality. We don’t experience things such as being in numerous places at the same time or having other, what we think of as, mutually contradictory properties. Einstein pointed out that our experience of localized objects (being in one place at a time) is actually contradictory to the most widely accepted interpretation of quantum theory.

 As Einstein noted localization ….is not just independent but incompatible with quantum theory. [i]

 Even objectivity, a concept at the core or scientific understanding, is foreign to quantum understanding:

The most obvious feature that distinguishes the classical realm from the quantum is objectivity. Unknown classical states can be found out without being disturbed, so they are said to “exist objectively.” This is, of course, not the case for quantum states. Thus, as it has often been emphasized by Asher Peres [6, 7], the nature of a quantum state is a much more elusive thing. In quantum systems (at least isolated ones) such states cannot be regarded as existing objectively. [ii]

The key phrase here for resolving our confusion may be 'at least isolated ones' because the 'Schrödinger's Cat' paradox can only arise if the cat-box system is an isolated quantum system. That is to say not interacting with its environment in any way. Quantum interactions are extremely pervasive and it is a technical challenge, so far not met by the best labs in the world, to keep the simplest quantum systems of only a few qbits isolated for any appreciable time. The environment is rife with forces, including a sea of photons, having a high probability of interacting with any matter in their vicinity. Zurek estimates that a gram of matter at normal conditions of temperature and density will undergo an interaction with its environment within 10-40 seconds.[iii]  A system with the mass of cat, box and apparatus would interact several orders of magnitude quicker.

Misconceptions around the idea of isolation may be at the basis of the 'measurement problem'. Standard quantum theory supporting the paradox assume that the cat system would remain an isolated non-interacting quantum system until a human measurement (looking in the box) was performed. Some even ventured so far as to speculate that human consciousness was required to have the state of the cat resolved into an 'alive' or 'dead' outcome.  This view now seems hopelessly anthropomorphic and one wonders how it could have been seriously entertained.

As the work of Zurek and others on decoherence makes clear the quantum superposition of states such as 'alive' and 'dead' tend to rapidly decohere into particular classical states such as 'alive' or 'dead'. This process is now understood to be analogous with axiom 4 above but rather than a wave function 'collapse' we should view decoherence as the preferential extinction of quantum superpositions and the survival of classical states due to interactions that may be human measurements buy are typically environmental interactions.

Basically any measurement can only result in some special values associated with 'pointer states'. Pointer states are thus associated with the type of measurement being performed. For instance in the case of a measurement designed to determine the survival of the cat there are two pointer states: 'alive' and 'dead'. The isolated quantum system consists of a superposition or combination of these pointer states (as well as superpositions of all other pointer states associated with other possible measurements) typical values for these superpositions are that the cat is both alive and dead and that it is here as well as over there. Environmental interaction with the system results in the extinction of the pointer state superpositions and the survival of only 'pure' pointer states.

The weird quantum states are filtered out in the emergence of our classical reality from its quantum roots. Zurek’s Quantum Darwinism theory offers an explanation of how this is done. As is common with successful scientific theories explaining how a few possibilities allowing vast increases in complexity are selected from a multitude of fruitless alternatives he posits a Darwinian mechanism at its heart. 

 The resulting theory of Quantum Darwinism is relatively straightforward:

1)      Human measurements are only one, rather unusual, means of forcing decoherence of a superposed or entangled quantum state into simpler states. The primary mechanism causing decoherence is the many types of interactions that the quantum system has with its environment. Typically quantum systems experience a vast number of such environmental interactions selectively destroying entangled quantum states.

2)     As a result these environmental interactions, or environmental monitoring, only a small minority of quantum states, called pointer observables, are able to survive and evolve for any sustained period of time in the deterministic, classical manner of axiom 5 above. Their prolonged survival is due to the peculiar property of these pointer states that interactions with the environment and the subsequent decoherence leave them largely unchanged. They alone are able to survive in the face of environmental monitoring.

3)     As the pointer states are the only ones able to survive decoherence, and as interactions with the environment pass information concerning the quantum state to the environment, a quantum system’s environment becomes heavily imprinted with redundant copies of information concerning the quantum system’s pointer states. It is these environmental copies that we actually experience and from which we gain information concerning quantum systems in almost all cases. For instance quantum systems are in continual interaction with the vast number of photons in their immediate environment. When we observe an object visually we are actually accessing information that has been imprinted on photons during previous interactions with the quantum system under observation.

4)     The redundant imprinting of information in the environment makes this information available to multiple observers and provides the basis for our classical concept of objectivity or the ability of numerous observers to access and confirm the same information.

While this process may explain the emergence of classical physics from quantum physics it may not be clear where the Darwinian part comes in. Zurek explains his motivation in naming Quantum Darwinism:

 Using Darwinian analogy, one might say that pointer states are most ‘fit’. They survive monitoring by the environment to leave ‘descendants’ that inherit their properties. Classical domain of pointer states offers a static summary of the result of quantum decoherence. Save for classical dynamics, (almost) nothing happens to these einselected states, even though they are immersed in the environment. [iv]

Still we might quibble and demand a more formal comparison of Quantum Darwinism to the defining mechanisms of a Darwinian process. A Darwinian process is any that utilizes the following algorithm:

1)      Reproduction with variation

2)     Selective survival of individuals due to their inherited variations

 In what sense can Zurek’s theory be seen to be a Darwinian process?

1)      Reproduction. Information concerning the state of a quantum system is copied with variations. The variations most directly concerning the information’s survival concerns the degree to which it is entangled or in superposition with its environment (i.e. non-classical).

2)     Information concerning superposition of point states does not survive decoherence as a result of interactions with the environment and only information concerning non-entangled, classical pointer states survive as imprints on the environment.

Thus we can see, at least from one point of view, that Quantum Darwinism is a true Darwinian process and meets the criterion for inclusion within the field of Universal Darwinism. Such a view is cautioned against by some researchers but this may be due to the unfamiliarity of many physical scientists with Darwinian concepts. For instance Serge Haroche of the Universite de Pierre & Marie Curie in his excellent lecture notes on Quantum Darwinism[v] says:

It looks somewhat like evolution theory in biology: the dissemination of information in environment is like reproduction (but reproduction of information, not of material bodies). There is, as in biology, an environment exercising a «pressure» on system via a specific coupling, which favors some states with respect to others. There is however no direct competition between the states for a limited supply of resources, as in real life, and of course no sexual reproduction of states!

An amusing metaphor which should be used with caution

Haroche’s reasons for caution in considering Quantum Darwinism as a true Darwinian process may be summarized:

1)      reproduction of information not material.

2)     no competition for supply of resources

3)     no sexual reproduction

These objections are all based on Quantum Darwinism’s lack of exact similarity with Biological Darwinism. Darwin was the first to note that the Darwinian process is an algorithm (given above) that if followed will result in evolution and is not limited to a biological implementation. None of Haroche's objections oppose this algorithm. His first point is somewhat confusing as it is now well accepted that information always has a physical representation. A reading of research in Universal Darwinism makes clear how the Darwinian algorithm is implemented in a wide variety of fields outside of biology [vi],[vii].

Given that Quantum Darwinism is a Darwinian mechanism we should expect to see products of its evolutionary design in our world. What might these be? The answer may be both surprising and revolutionary as the surviving physical states produced by Quantum Darwinism are the totality of the classical word in which we live and experience the rest of the universe. In other words the evolutionary products of Quantum Darwinism may be the classical reality in which all other Darwinian processes operate and produce their own evolutionary products.

 

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[i] Zurek, Wojciech. (2003). DECOHERENCE, EINSELECTION, AND THE QUANTUM ORIGINS OF THE CLASSICAL. arXiv:quant-ph/0105127 v3

[ii] Blume-Kohou Robin, Zurek Wojciech. (2004). A simple example of “Quantum Darwinism”: Redundant information storage in many-spin environments. arXiv:quant-ph/0408147 v1 23 Aug 2004

[iii] Zurek, Wojciech. (2003). Decoherence and the transition from quantum to classical -- REVISITED. arXiv: quant-ph/0306072

[iv] Zurek, Wojciech. (2003). DECOHERENCE, EINSELECTION, AND THE QUANTUM ORIGINS OF THE CLASSICAL. arXiv:quant-ph/0105127 v3

[v] Haroche, Serge. (2004). Lecture 6 - Decoherence, measurement and quantum classical boundary. As view on December 31, 2005 at http://www.lkb.ens.fr/recherche/qedcav/houches/haroche/Haroche6.pdf

[vi] Dennet D. (1995). Darwin’s Dangerous Idea. Touchstone Publishing, New York

[vii] Blackmore S. (1999). The Meme Machine. Oxford University Press

[viii] Zurek, Wojciech. (2003). Quantum Darwinism: Entanglement, branches, and the emergent classicality of redundantly stored quantum information. arXiv:quant-ph/0505031