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.
[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
[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
