The Fabric of Reality
Encyclopedia
The Fabric of Reality is a book by physicist David Deutsch
written in 1997. It expands upon his views of quantum mechanics
and its implications for understanding reality
.
This interpretation, which he calls the multiverse
hypothesis, is one of a four-strand Theory of Everything
(TOE). The four strands are:
His theory of everything is (weakly) emergentist
rather than reductive
. It aims not at the reduction of everything to particle physics
, but rather mutual support among multiverse, computational, epistemological, and evolutionary principles.
, the total of physical reality is actually a multiverse
, consisting of an infinity of universes. Deutsch argues for it on four main grounds:
However:
. In the strong form he favors it implies that a universal quantum computer, capable of rendering any physically possible environment, actually exists near the end of spacetime
in every universe and is maintained by sentient beings with the knowledge required to increase its memory, computing cycles, and energy supply. In this he follows Frank Tipler in The Physics of Immortality, though he emphasizes the scientific component of Tipler's Omega Point
hypothesis, the component that is justified by Popperian epistemology as implied by our best science. He is much less sympathetic to the non-scientific component, which provides rational reconstructions for traditional theological categories such as God
, omniscience
, omnipresence
, benevolence, creation, and so on.
The strong form of the Turing principle rests on a delicate mathematical argument favoring a universal quantum computer in all universes over such a computer in one universe, and either of these over such a computer in, say, 17 universes. So a weaker form of the Turing principle would commit to one universe. Also the reasoning for 'either one or all' presupposes a Big Crunch
cosmology in order to generate the energy for the required computing cycles. If the universe instead expands forever, the Turing principle would have to take a weaker form, implying the existence of a more or less remote approximation to a universal quantum computer.
Deutsch's Turing principle is also sometimes called the Church-Turing-Deutsch principle by those who question whether Turing's work on the foundations of computing was aiming to disclose what could be computed tractably "in nature". More conservative readings of Turing view him as concerned with what could be computed "by human computers", i.e. human mathematicians. On this reading Turing didn't aim at foundations for computing that provided tractability, because an algorithm might be calculable "by human computers" but without the speed to compute tractably what happens "in nature". (The humans might take a very long time to finish their computations.) There are tractability
issues when, for instance, factoring
and decryption problems are attacked with Turing-machine
or classical-computation methods, problems that seem to be resolved by quantum-computing techniques such as Shor's algorithm
, which takes advantage of the superposition of states in qubit
s to calculate "all at once" what a Turing machine would calculate serially. Turing universality isn't universal enough, Deutsch thinks. Turing's abstract computer needs to be replaced by the actual, physical, universal quantum computer derived from the Church-Turing (-Deutsch) principle.
That principle is also sometimes called the Matrix
principle, because Deutsch's conception of virtual reality
figures in its statement: "It is possible to build a virtual reality generator whose repertoire includes every physically possible environment." Some cognitive psychologist
s think that Deutsch's view of the brain as a virtual-reality generating computer, adequate to rendering a humanly experienced environment, affords a sufficiently robust account of subjective experience or qualia
, one consistent with a view of the mind/brain as a computer, to break down the impasse between qualophobes and qualophiles. (However, it is arguable that his approach confuses qualia with mental representations). With considerable thrust concentrated position it is theorised that this can alter the fabric of a questioned reality. A virtual-reality generator consists of an image generator to provide the subject with perceptual content from the several sensory modalities
, perhaps in the forms of transducer
s connected directly to afferent nerves by use of neural implants, and a program to handle interaction between the subject's choices and the virtual environment. Nearer to the Omega Point
this transhuman
enhanced-biology scenario gives way to a posthuman
condition, because biology becomes untenable. Gravitational shearing and other extreme forces call for more durable substrates for human psychology. The brain is replaced by sturdy computational equivalents in virtual realities, protected from the Big Crunch
and pushed in the final moments by unlimited computational cycles affording their posthuman residents the subjective experience of immortality.
farms out computing problems to other universes in order to achieve tractability for solutions that otherwise get bogged down by exponentially increasing demands for more time and other computational resources. The apparent need on a realist
conception of science to posit such collaboration inspires a pugnacious comment from Deutsch: "To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor's algorithm works." The challenge is meant to imply that a Turing machine is incapable in principle of doing what a quantum computer can do, since the latter's operations in executing Shor's algorithm
require computational resources from other worlds. And generally, a quantum computer's operations include computational steps in other worlds that are not present in any Turing-machine's tape (in this world). Deutsch thinks this has implications for proof theory
, which must abandon the Cartesian model of an inspectable list of premises leading to a conclusion, in favor of a model of a process in which the relationship between premises and conclusion may be mediated by computations that are not inspectable (in this world).
hypothesis. For instance, counterfactual conditional
s refer to nearby parallel worlds when they stipulate what a thing would do under conditions that do not actually obtain. One-worlders implicitly collapse what things can do into what they actually do (in this world). Consider a coin toss. The identical worlds in which I (copies of me) see it spinning become branched; in fifty-percent of those worlds versions of me see 'heads', and in fifty percent they see 'tails'. This actual distribution of worlds is what licenses the inference, about this world, that if the coin hadn't turned up 'heads' it would have turned up 'tails'. Instead of its being a basic fact that my observing 'heads' collapses probabilities into an actual outcome of the coin toss, those probabilities are grounded in actual universes in which both outcomes are represented. This is sometimes referred to as the difference between "collapse
theories" (e.g., the Copenhagen interpretation
) and no-collapse theories (e.g., many-worlds or the multiverse interpretation).
Deutsch acknowledges a kindred spirit in the philosopher David Lewis, whose modal realism
handles counterfactuals in a similar fashion. He takes Lewis to have "postulated the existence of a multiverse for philosophical reasons alone." This is a contentious claim, since Lewis's realism about parallel worlds
extends to worlds that are not physically possible, such as the world where Harry Potter
was schooled at Hogwarts
, whereas the parallel worlds in Deutsch's multiverse comprise all and only physically possible world
s. The worlds of the multiverse are governed by the same natural laws. Also Lewis's possible worlds are disjoint, whereas Deutsch's parallel worlds interact through interference
. On the other hand, Lewis recognizes overlapping worlds as a theoretical possibility, and Deutsch's universal quantum computer can render Harry Potter worlds to any desired degree of accuracy.
is a trans-universe structure, as one might expect because knowledge supports counterfactual implications, as revealed for instance in Robert Nozick's
tracking account of knowledge. Nearby parallel worlds are united by a common history of knowledge acquisition, spelled out in broadly Popperian terms. The resulting epistemological niche lends stability and reliability to knowledge in each universe. Life
is a similar trans-universe structure, molded by natural selection rather than rational criticism. What distinguishes genuine replicating DNA
from junk DNA is that the former but not the latter is representative of a niche of replicators that extends across worlds. Indeed personal identity is inseparable from such a niche, which Deutsch picks out with the word "copies". A person is a set of copies in nearby parallel worlds. This comes out in his analysis of free will
: I could have chosen otherwise is analysed as Other copies of me chose otherwise. And in the dénouement to a dramatic chapter that rehearses interference experiments from a multiverse viewpoint, he writes of his copies, "Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea."
through time in any given world. Time is a series of moments, and a person who exists at a moment exists there forever
in four-dimensional spacetime
, rather than being transformed continuously through the flow of time
. Such change and flow are mythical, Deutsch argues. The argument doesn't strictly require the multiverse hypothesis, because deterministic physics since Newton
has implied that the openness of the future is an illusion, and consequently that free will
is an illusion. (This conclusion could be avoided by adopting compatibilism
. Also, collapse interpretations
of quantum mechanics imply both indeterminism
and an open future).
What the multiverse adds to a block time theory is an attenuated account of common sense's ideas of causation
and free will. Although an effect can't be changed by its cause, the counterfactual
s that causal statements support are true. If the cause hadn't occurred, the effect would not have occurred. For the multiverse, which is "to a first approximation" a very large number of co-existing and slightly interacting spacetimes, includes universes in which the cause doesn't occur and its effect doesn't occur. And although the "me-copy" in this spacetime could not have done otherwise, there are me-copies in other worlds that actually do otherwise (thus, the common-sense idea that, in choosing one course of action, one refrains from another, is not retained). There is a branching of these me-copies that validates my sense that my future is open, in contrast to spacetime physics. However, the open future of common sense is a myth. As defined by the Darwinist framework, there is no flow of time dividing the actualities of the past from the unactualized potentialities of the future.
An intellectual descendant of David Hume
via the paternity of Popper, Deutsch is not only a critic of induction but also a Humean about causation, to the degree that he rejects the idea of a causal power effecting a change, in favor of construing it as a multiverse regularity. So A causes B means something like After A-copies occur in many nearby parallel worlds, including the one in this world, B-copies occur. This regularity supports counterfactuals that accompany true causal claims, such as If A hadn't happened, B would not have taken place. There are affinities to Hume's constant-conjunction understanding of causation and Popper's deductive-nomological
account.
), the temporal granularity of personhood through time is a special case of being spread out through worlds. In addition to one's identically time-stamped copies at a moment across parallel worlds transversely, there are the differently time-stamped copies across parallel worlds longitudinally, linked by natural law so as to give the individual's experience of one world and a continuous self. The implications for the theory of personal identity are not yet clear, but Derek Parfit's
Reductionist view seems to be favored: The concept of personal identity ceases to apply when branching is taken into account, but branching maintains what's important about personal identity, such as psychological continuities having to do with memory, desire, character, and so forth. Another possibility is that Robert Nozick's
Closest-Continuer theory could be modified so as to track closeness transversely as well as longitudinally. The tracked slices of "me-copies" would be the continuing person. Deutsch would seem to favor some such approach. There are "multiple identical copies" of me in the multiverse. Which one am I? Deutsch answers, "I am, of course, all of them." (The Parfitian answer would be, "The concept of personal identity doesn't apply.") Copies need not be strictly identical in the sense of the identity of indiscernibles
relativized to universes: All of my copies see a coin spinning in a coin toss, but an instant later half my copies see 'heads' come up, the other half see 'tails'. A distinction between copies, versions, and variants is at work here. Variants of me need not see the spinning coin. Versions of me see it though some of them see 'heads' and some 'tails'. The multiple identical copies of me all see the spinning coin.
is assured by Einstein's special theory of relativity, which says that an observer who accelerates or decelerates will experience less time than an observer who is at rest or in uniform motion. This time dilation
could make an astronaut's flight very short and the duration on Earth very long, but such a trip to Earth's future would be irreversible as "no amount of time dilation can allow a spaceship to return from a flight before it took off". As for past-directed time travel, it is possible as a sort of sidestep from one universe to another, requiring a path between the two universes that is "hard-wired" into the structure of the multiverse. Whether such paths exist or not is an unresolved empirical question. If they were to exist and were to allow macro-objects like human beings to traverse them, time travel could occur without the grandfather paradox
, because the time traveler would go to a point prior to the branching between his "home" universe and the universe in which he (the copy of him in the "away" universe) kills his grandfather (grandfather-copy).
The multiverse hypothesis alone doesn't avoid the knowledge paradox, in which the time traveler goes to a point where he gives the collected works of Shakespeare to a hack writer, who seems to get "knowledge for free" that he uses to become the celebrated Shakespeare in that world. The Popperian epistemological strand of Deutsch's four-strand theory is invoked at this point. Just as life is understood as a trans-universe structure that is physically possible only through processes of natural selection, so too knowledge is understood as such a structure that is physically possible only through processes of rational problem-solving. Knowledge for free isn't possible in the multiverse when the multiverse is understood through Deutsch's emergentist Theory of Everything.
David Deutsch
David Elieser Deutsch, FRS is an Israeli-British physicist at the University of Oxford. He is a non-stipendiary Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation in the Clarendon Laboratory of the University of Oxford...
written in 1997. It expands upon his views of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
and its implications for understanding reality
Reality
In philosophy, reality is the state of things as they actually exist, rather than as they may appear or might be imagined. In a wider definition, reality includes everything that is and has been, whether or not it is observable or comprehensible...
.
This interpretation, which he calls the multiverse
Multiverse (science)
The multiverse is the hypothetical set of multiple possible universes that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them...
hypothesis, is one of a four-strand Theory of Everything
Theory of everything
A theory of everything is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle....
(TOE). The four strands are:
- Hugh EverettHugh EverettHugh Everett III was an American physicist who first proposed the many-worlds interpretation of quantum physics, which he termed his "relative state" formulation....
's many-worlds interpretationMany-worlds interpretationThe many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...
of quantum physicsQuantum mechanicsQuantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, "The first and most important of the four strands". - Karl PopperKarl PopperSir Karl Raimund Popper, CH FRS FBA was an Austro-British philosopher and a professor at the London School of Economics...
's epistemology, especially its anti-inductivismInductivismIn the philosophy of science inductivism exists both in a classical naive version, which has been highly influential, and in various more sophisticated versions...
and its requiring a realistScientific realismScientific realism is, at the most general level, the view that the world described by science is the real world, as it is, independent of what we might take it to be...
(non-instrumental) interpretation of scientific theories, and its emphasis on taking seriously those bold conjectures that resist falsificationFalsifiabilityFalsifiability or refutability of an assertion, hypothesis or theory is the logical possibility that it can be contradicted by an observation or the outcome of a physical experiment...
. - Alan TuringAlan TuringAlan Mathison Turing, OBE, FRS , was an English mathematician, logician, cryptanalyst, and computer scientist. He was highly influential in the development of computer science, providing a formalisation of the concepts of "algorithm" and "computation" with the Turing machine, which played a...
's theory of computationTheory of computationIn theoretical computer science, the theory of computation is the branch that deals with whether and how efficiently problems can be solved on a model of computation, using an algorithm...
especially as developed in Deutsch's "Turing principle", Turing's Universal Turing machineUniversal Turing machineIn computer science, a universal Turing machine is a Turing machine that can simulate an arbitrary Turing machine on arbitrary input. The universal machine essentially achieves this by reading both the description of the machine to be simulated as well as the input thereof from its own tape. Alan...
being replaced by Deutsch's universal quantum computerQuantum Turing machineA quantum Turing machine , also a universal quantum computer, is an abstract machine used to model the effect of a quantum computer. It provides a very simple model which captures all of the power of quantum computation. Any quantum algorithm can be expressed formally as a particular quantum...
. ("The theory of computation is now the quantum theory of computation.") - Richard DawkinsRichard DawkinsClinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
's refinement of DarwinianDarwinismDarwinism is a set of movements and concepts related to ideas of transmutation of species or of evolution, including some ideas with no connection to the work of Charles Darwin....
evolutionary theory and the modern evolutionary synthesisModern evolutionary synthesisThe modern evolutionary synthesis is a union of ideas from several biological specialties which provides a widely accepted account of evolution...
, especially the ideas of replicator and memeMemeA meme is "an idea, behaviour or style that spreads from person to person within a culture."A meme acts as a unit for carrying cultural ideas, symbols or practices, which can be transmitted from one mind to another through writing, speech, gestures, rituals or other imitable phenomena...
as they integrate with PopperianKarl PopperSir Karl Raimund Popper, CH FRS FBA was an Austro-British philosopher and a professor at the London School of Economics...
problem-solving (the epistemological strand).
His theory of everything is (weakly) emergentist
Emergence
In philosophy, systems theory, science, and art, emergence is the way complex systems and patterns arise out of a multiplicity of relatively simple interactions. Emergence is central to the theories of integrative levels and of complex systems....
rather than reductive
Reductionism
Reductionism can mean either an approach to understanding the nature of complex things by reducing them to the interactions of their parts, or to simpler or more fundamental things or a philosophical position that a complex system is nothing but the sum of its parts, and that an account of it can...
. It aims not at the reduction of everything to particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, but rather mutual support among multiverse, computational, epistemological, and evolutionary principles.
The Many Worlds interpretation of quantum mechanics
According to the many worlds interpretation of quantum mechanicsQuantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, the total of physical reality is actually a multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...
, consisting of an infinity of universes. Deutsch argues for it on four main grounds:
- The situation of having multiple interpretations of quantum mechanics is anomalousAnomalyAnomaly may refer to:-Astronomy and celestial mechanics :* In astronomy, an anomaly is a quantity measured with respect to an apsis, usually the periapsis...
. People should settle for the most straightforward (realistic) interpretation, which is, according to Deutsch, Many Worlds. - Only a Many Worlds approach can explain the additional power of quantum computerQuantum computerA quantum computer is a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. Quantum computers are different from traditional computers based on transistors...
s compared with classical computers. - Only Many Worlds can explain quantum superpositionQuantum superpositionQuantum superposition is a fundamental principle of quantum mechanics. It holds that a physical system exists in all its particular, theoretically possible states simultaneously; but, when measured, it gives a result corresponding to only one of the possible configurations.Mathematically, it...
effects, such as those observed in single-particle double slit experiments. - Most other approaches — the Bohm interpretationBohm interpretationThe de Broglie–Bohm theory, also called the pilot-wave theory, Bohmian mechanics, and the causal interpretation, is an interpretation of quantum theory. In addition to a wavefunction on the space of all possible configurations, it also includes an actual configuration, even in situations where...
, the Transactional interpretationTransactional interpretationThe transactional interpretation of quantum mechanics describes quantum interactions in terms of a standing wave formed by retarded and advanced waves. It was first proposed in 1986 by John G...
— are Many Worlds in disguise or denial.
However:
- The interpretational debate is not unprecedented. There was a debate about the nature of gravity and action-at-a-distanceAction at a distance (physics)In physics, action at a distance is the interaction of two objects which are separated in space with no known mediator of the interaction. This term was used most often in the context of early theories of gravity and electromagnetism to describe how an object responds to the influence of distant...
in NewtonIsaac NewtonSir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
ian physics. There is a debate about the ontology of spacePhilosophy of space and timePhilosophy of space and time is the branch of philosophy concerned with the issues surrounding the ontology, epistemology, and character of space and time. While such ideas have been central to philosophy from its inception, the philosophy of space and time was both an inspiration for and a...
in relativity theory. There is even a debate about the ontology of mathematicsPhilosophy of mathematicsThe philosophy of mathematics is the branch of philosophy that studies the philosophical assumptions, foundations, and implications of mathematics. The aim of the philosophy of mathematics is to provide an account of the nature and methodology of mathematics and to understand the place of...
. - Quantum computers are still Turing-complete machines. Given a "black boxBlack boxA black box is a device, object, or system whose inner workings are unknown; only the input, transfer, and output are known characteristics.The term black box can also refer to:-In science and technology:*Black box theory, a philosophical theory...
" computer, it is impossible to determine whether the inner workings are quantum or classical. Quantum computing allows more computational power to be extracted from a given set of physical resources. It is debatable whether multiple quasi-classical worlds are a better ontological model of quantum computing than a single quantum world. (A possible riposte is at p. 217 of Fabric, where Deutsch derives the limitations of a Turing machine from a consideration about the resources available to execute Shor's algorithmShor's algorithmShor's algorithm, named after mathematician Peter Shor, is a quantum algorithm for integer factorization formulated in 1994...
. Given his thoroughgoing physicalism, he is apparently "within his argument" to dismiss the powers of an abstract black-box Turing machine.) - To interpret a superposed photon as being in two different "worlds" requires an idiosyncratic interpretation of "world". The superposition is only detectable through interferenceInterferenceIn physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have...
effects, which in turn means the two photonPhotonIn physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s (or two states of the photon, depending on interpretation) are coherent. Most Many Worlds take the view that a quantum state does not count as a "world" until it decoheres. (But compare Lev VaidmanLev VaidmanLev Vaidman is an Israeli physicist working at Tel Aviv University, Israel. He is noted for the Elitzur–Vaidman bomb-testing problem in quantum mechanics.- External links :* * *...
's discussion of worlds in MWI at Stanford Encyclopedia of Philosophy. which implies that differences among MWI theorists about the use of the term 'world' are "only semantic".) - An interesting point which perhaps relies on the "coherent worlds" approach mentioned above.
The Church–Turing thesis
This emergentist posture allows Deutsch to do some serious work with the Church-Turing thesis (or "Turing principle" as he calls it), which is fundamental in theoretical computer scienceTheoretical computer science
Theoretical computer science is a division or subset of general computer science and mathematics which focuses on more abstract or mathematical aspects of computing....
. In the strong form he favors it implies that a universal quantum computer, capable of rendering any physically possible environment, actually exists near the end of spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
in every universe and is maintained by sentient beings with the knowledge required to increase its memory, computing cycles, and energy supply. In this he follows Frank Tipler in The Physics of Immortality, though he emphasizes the scientific component of Tipler's Omega Point
Omega point
Omega Point is a term coined by the French Jesuit Pierre Teilhard de Chardin to describe a maximum level of complexity and consciousness towards which he believed the universe was evolving....
hypothesis, the component that is justified by Popperian epistemology as implied by our best science. He is much less sympathetic to the non-scientific component, which provides rational reconstructions for traditional theological categories such as God
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....
, omniscience
Omniscience
Omniscience omniscient point-of-view in writing) is the capacity to know everything infinitely, or at least everything that can be known about a character including thoughts, feelings, life and the universe, etc. In Latin, omnis means "all" and sciens means "knowing"...
, omnipresence
Omnipresence
Omnipresence or ubiquity is the property of being present everywhere. According to eastern theism, God is present everywhere. Divine omnipresence is thus one of the divine attributes, although in western theism it has attracted less philosophical attention than such attributes as omnipotence,...
, benevolence, creation, and so on.
The strong form of the Turing principle rests on a delicate mathematical argument favoring a universal quantum computer in all universes over such a computer in one universe, and either of these over such a computer in, say, 17 universes. So a weaker form of the Turing principle would commit to one universe. Also the reasoning for 'either one or all' presupposes a Big Crunch
Big Crunch
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.- Overview :...
cosmology in order to generate the energy for the required computing cycles. If the universe instead expands forever, the Turing principle would have to take a weaker form, implying the existence of a more or less remote approximation to a universal quantum computer.
Deutsch's Turing principle is also sometimes called the Church-Turing-Deutsch principle by those who question whether Turing's work on the foundations of computing was aiming to disclose what could be computed tractably "in nature". More conservative readings of Turing view him as concerned with what could be computed "by human computers", i.e. human mathematicians. On this reading Turing didn't aim at foundations for computing that provided tractability, because an algorithm might be calculable "by human computers" but without the speed to compute tractably what happens "in nature". (The humans might take a very long time to finish their computations.) There are tractability
Tractable
Tractable may refer to:*Operation Tractable, a military operation in Normandy 1944*Tractability concerning how easily something can be done...
issues when, for instance, factoring
Factoring
Factoring can refer to the following:* A form of commercial finance - see factoring ; structured settlement factoring transaction* Factorization, a mathematical concept* Factoring a design, as in code refactoring...
and decryption problems are attacked with Turing-machine
Turing machine
A Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...
or classical-computation methods, problems that seem to be resolved by quantum-computing techniques such as Shor's algorithm
Shor's algorithm
Shor's algorithm, named after mathematician Peter Shor, is a quantum algorithm for integer factorization formulated in 1994...
, which takes advantage of the superposition of states in qubit
Qubit
In quantum computing, a qubit or quantum bit is a unit of quantum information—the quantum analogue of the classical bit—with additional dimensions associated to the quantum properties of a physical atom....
s to calculate "all at once" what a Turing machine would calculate serially. Turing universality isn't universal enough, Deutsch thinks. Turing's abstract computer needs to be replaced by the actual, physical, universal quantum computer derived from the Church-Turing (-Deutsch) principle.
That principle is also sometimes called the Matrix
The Matrix
The Matrix is a 1999 science fiction-action film written and directed by Larry and Andy Wachowski, starring Keanu Reeves, Laurence Fishburne, Carrie-Anne Moss, Joe Pantoliano, and Hugo Weaving...
principle, because Deutsch's conception of virtual reality
Virtual reality
Virtual reality , also known as virtuality, is a term that applies to computer-simulated environments that can simulate physical presence in places in the real world, as well as in imaginary worlds...
figures in its statement: "It is possible to build a virtual reality generator whose repertoire includes every physically possible environment." Some cognitive psychologist
Cognitive psychology
Cognitive psychology is a subdiscipline of psychology exploring internal mental processes.It is the study of how people perceive, remember, think, speak, and solve problems.Cognitive psychology differs from previous psychological approaches in two key ways....
s think that Deutsch's view of the brain as a virtual-reality generating computer, adequate to rendering a humanly experienced environment, affords a sufficiently robust account of subjective experience or qualia
Qualia
Qualia , singular "quale" , from a Latin word meaning for "what sort" or "what kind," is a term used in philosophy to refer to subjective conscious experiences as 'raw feels'. Examples of qualia are the pain of a headache, the taste of wine, the experience of taking a recreational drug, or the...
, one consistent with a view of the mind/brain as a computer, to break down the impasse between qualophobes and qualophiles. (However, it is arguable that his approach confuses qualia with mental representations). With considerable thrust concentrated position it is theorised that this can alter the fabric of a questioned reality. A virtual-reality generator consists of an image generator to provide the subject with perceptual content from the several sensory modalities
Stimulus modality
Stimulus modality also sensory modality is one aspect of a stimulus or what we percieve after a stimulus. For example the temperature modality is registered after heat or cold stimulate a receptor. There are many modalities: temperature, taste, pressure...
, perhaps in the forms of transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s connected directly to afferent nerves by use of neural implants, and a program to handle interaction between the subject's choices and the virtual environment. Nearer to the Omega Point
Omega point
Omega Point is a term coined by the French Jesuit Pierre Teilhard de Chardin to describe a maximum level of complexity and consciousness towards which he believed the universe was evolving....
this transhuman
Transhumanism
Transhumanism, often abbreviated as H+ or h+, is an international intellectual and cultural movement that affirms the possibility and desirability of fundamentally transforming the human condition by developing and making widely available technologies to eliminate aging and to greatly enhance human...
enhanced-biology scenario gives way to a posthuman
Posthumanism
Posthumanism or post-humanism is a term with five definitions:#Antihumanism: a term applied to a number of thinkers opposed to the project of philosophical anthropology....
condition, because biology becomes untenable. Gravitational shearing and other extreme forces call for more durable substrates for human psychology. The brain is replaced by sturdy computational equivalents in virtual realities, protected from the Big Crunch
Big Crunch
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.- Overview :...
and pushed in the final moments by unlimited computational cycles affording their posthuman residents the subjective experience of immortality.
Quantum computers and proof theory
A quantum computerQuantum computer
A quantum computer is a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. Quantum computers are different from traditional computers based on transistors...
farms out computing problems to other universes in order to achieve tractability for solutions that otherwise get bogged down by exponentially increasing demands for more time and other computational resources. The apparent need on a realist
Scientific realism
Scientific realism is, at the most general level, the view that the world described by science is the real world, as it is, independent of what we might take it to be...
conception of science to posit such collaboration inspires a pugnacious comment from Deutsch: "To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor's algorithm works." The challenge is meant to imply that a Turing machine is incapable in principle of doing what a quantum computer can do, since the latter's operations in executing Shor's algorithm
Shor's algorithm
Shor's algorithm, named after mathematician Peter Shor, is a quantum algorithm for integer factorization formulated in 1994...
require computational resources from other worlds. And generally, a quantum computer's operations include computational steps in other worlds that are not present in any Turing-machine's tape (in this world). Deutsch thinks this has implications for proof theory
Proof theory
Proof theory is a branch of mathematical logic that represents proofs as formal mathematical objects, facilitating their analysis by mathematical techniques. Proofs are typically presented as inductively-defined data structures such as plain lists, boxed lists, or trees, which are constructed...
, which must abandon the Cartesian model of an inspectable list of premises leading to a conclusion, in favor of a model of a process in which the relationship between premises and conclusion may be mediated by computations that are not inspectable (in this world).
Counterfactual inferences and modal realism
Another important theme in the book is that basic ideas about the universe are either vindicated or undermined by the multiverseMultiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...
hypothesis. For instance, counterfactual conditional
Counterfactual conditional
A counterfactual conditional, subjunctive conditional, or remote conditional, abbreviated , is a conditional statement indicating what would be the case if its antecedent were true...
s refer to nearby parallel worlds when they stipulate what a thing would do under conditions that do not actually obtain. One-worlders implicitly collapse what things can do into what they actually do (in this world). Consider a coin toss. The identical worlds in which I (copies of me) see it spinning become branched; in fifty-percent of those worlds versions of me see 'heads', and in fifty percent they see 'tails'. This actual distribution of worlds is what licenses the inference, about this world, that if the coin hadn't turned up 'heads' it would have turned up 'tails'. Instead of its being a basic fact that my observing 'heads' collapses probabilities into an actual outcome of the coin toss, those probabilities are grounded in actual universes in which both outcomes are represented. This is sometimes referred to as the difference between "collapse
Wavefunction collapse
In quantum mechanics, wave function collapse is the phenomenon in which a wave function—initially in a superposition of several different possible eigenstates—appears to reduce to a single one of those states after interaction with an observer...
theories" (e.g., the Copenhagen interpretation
Copenhagen interpretation
The Copenhagen interpretation is one of the earliest and most commonly taught interpretations of quantum mechanics. It holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta,...
) and no-collapse theories (e.g., many-worlds or the multiverse interpretation).
Deutsch acknowledges a kindred spirit in the philosopher David Lewis, whose modal realism
Modal realism
Modal realism is the view, notably propounded by David Kellogg Lewis, that all possible worlds are as real as the actual world. It is based on the following tenets: possible worlds exist; possible worlds are not different in kind from the actual world; possible worlds are irreducible entities; the...
handles counterfactuals in a similar fashion. He takes Lewis to have "postulated the existence of a multiverse for philosophical reasons alone." This is a contentious claim, since Lewis's realism about parallel worlds
Parallel universe (fiction)
A parallel universe or alternative reality is a hypothetical self-contained separate reality coexisting with one's own. A specific group of parallel universes is called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute reality...
extends to worlds that are not physically possible, such as the world where Harry Potter
Harry Potter
Harry Potter is a series of seven fantasy novels written by the British author J. K. Rowling. The books chronicle the adventures of the adolescent wizard Harry Potter and his best friends Ron Weasley and Hermione Granger, all of whom are students at Hogwarts School of Witchcraft and Wizardry...
was schooled at Hogwarts
Hogwarts
Hogwarts School of Witchcraft and Wizardry or simply Hogwarts is the primary setting for the first six books of the Harry Potter series by J. K. Rowling, with each book lasting the equivalent of one school year. It is a fictional boarding school of magic for witches and wizards between the ages of...
, whereas the parallel worlds in Deutsch's multiverse comprise all and only physically possible world
Possible world
In philosophy and logic, the concept of a possible world is used to express modal claims. The concept of possible worlds is common in contemporary philosophical discourse and has also been disputed.- Possibility, necessity, and contingency :...
s. The worlds of the multiverse are governed by the same natural laws. Also Lewis's possible worlds are disjoint, whereas Deutsch's parallel worlds interact through interference
Interference
In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have...
. On the other hand, Lewis recognizes overlapping worlds as a theoretical possibility, and Deutsch's universal quantum computer can render Harry Potter worlds to any desired degree of accuracy.
Knowledge and life
KnowledgeKnowledge
Knowledge is a familiarity with someone or something unknown, which can include information, facts, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject...
is a trans-universe structure, as one might expect because knowledge supports counterfactual implications, as revealed for instance in Robert Nozick's
Robert Nozick
Robert Nozick was an American political philosopher, most prominent in the 1970s and 1980s. He was a professor at Harvard University. He is best known for his book Anarchy, State, and Utopia , a right-libertarian answer to John Rawls's A Theory of Justice...
tracking account of knowledge. Nearby parallel worlds are united by a common history of knowledge acquisition, spelled out in broadly Popperian terms. The resulting epistemological niche lends stability and reliability to knowledge in each universe. Life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
is a similar trans-universe structure, molded by natural selection rather than rational criticism. What distinguishes genuine replicating DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
from junk DNA is that the former but not the latter is representative of a niche of replicators that extends across worlds. Indeed personal identity is inseparable from such a niche, which Deutsch picks out with the word "copies". A person is a set of copies in nearby parallel worlds. This comes out in his analysis of free will
Free will
"To make my own decisions whether I am successful or not due to uncontrollable forces" -Troy MorrisonA pragmatic definition of free willFree will is the ability of agents to make choices free from certain kinds of constraints. The existence of free will and its exact nature and definition have long...
: I could have chosen otherwise is analysed as Other copies of me chose otherwise. And in the dénouement to a dramatic chapter that rehearses interference experiments from a multiverse viewpoint, he writes of his copies, "Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea."
Time, causation and free will
Not only are persons spread out through worlds, but they, like everything else, are quantizedQuantization
Quantization is the procedure of constraining something from a relatively large or continuous set of values to a relatively small discrete set...
through time in any given world. Time is a series of moments, and a person who exists at a moment exists there forever
Eternalism
The word eternalism has at least three meanings:* In philosophy, Eternalism is a philosophical approach to the ontological nature of time, which takes the view that all points in time are equally "real", as opposed to the presentist idea that only the present is real.*Eternalism is a position in...
in four-dimensional spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
, rather than being transformed continuously through the flow of time
Presentism
Presentism may refer to:* Presentism * Presentism...
. Such change and flow are mythical, Deutsch argues. The argument doesn't strictly require the multiverse hypothesis, because deterministic physics since Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
has implied that the openness of the future is an illusion, and consequently that free will
Free will
"To make my own decisions whether I am successful or not due to uncontrollable forces" -Troy MorrisonA pragmatic definition of free willFree will is the ability of agents to make choices free from certain kinds of constraints. The existence of free will and its exact nature and definition have long...
is an illusion. (This conclusion could be avoided by adopting compatibilism
Compatibilism
Compatibilism is the belief that free will and determinism are compatible ideas, and that it is possible to believe both without being logically inconsistent. It may, however, be more accurate to say that compatibilists define 'free will' in a way that allows it to co-exist with determinism...
. Also, collapse interpretations
Objective collapse theory
Objective collapse theories are an approach to the interpretational problems of quantum mechanics. They are realistic, indeterministic and reject hidden variables...
of quantum mechanics imply both indeterminism
Indeterminism
Indeterminism is the concept that events are not caused, or not caused deterministically by prior events. It is the opposite of determinism and related to chance...
and an open future).
What the multiverse adds to a block time theory is an attenuated account of common sense's ideas of causation
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....
and free will. Although an effect can't be changed by its cause, the counterfactual
Counterfactual
Counterfactual may refer to:* Counterfactual conditional, a grammatical form * Counterfactual subjunctive, grammatical forms which in English are known as the past and pluperfect forms of the subjunctive mood* Counterfactual thinking* Counterfactual history* Alternate history, a literary genre*...
s that causal statements support are true. If the cause hadn't occurred, the effect would not have occurred. For the multiverse, which is "to a first approximation" a very large number of co-existing and slightly interacting spacetimes, includes universes in which the cause doesn't occur and its effect doesn't occur. And although the "me-copy" in this spacetime could not have done otherwise, there are me-copies in other worlds that actually do otherwise (thus, the common-sense idea that, in choosing one course of action, one refrains from another, is not retained). There is a branching of these me-copies that validates my sense that my future is open, in contrast to spacetime physics. However, the open future of common sense is a myth. As defined by the Darwinist framework, there is no flow of time dividing the actualities of the past from the unactualized potentialities of the future.
An intellectual descendant of David Hume
David Hume
David Hume was a Scottish philosopher, historian, economist, and essayist, known especially for his philosophical empiricism and skepticism. He was one of the most important figures in the history of Western philosophy and the Scottish Enlightenment...
via the paternity of Popper, Deutsch is not only a critic of induction but also a Humean about causation, to the degree that he rejects the idea of a causal power effecting a change, in favor of construing it as a multiverse regularity. So A causes B means something like After A-copies occur in many nearby parallel worlds, including the one in this world, B-copies occur. This regularity supports counterfactuals that accompany true causal claims, such as If A hadn't happened, B would not have taken place. There are affinities to Hume's constant-conjunction understanding of causation and Popper's deductive-nomological
Deductive-nomological
The deductive-nomological model is a formalized view of scientific explanation in natural language. It characterizes scientific explanations primarily as deductive arguments with at least one natural law statement among its premises...
account.
Time and personal identity
Since "other times are just special cases of other universes" (an idea that has been much expanded by Julian BarbourJulian Barbour
Julian Barbour is a British physicist with research interests in quantum gravity and the history of science.Since receiving his Ph.D. degree on the foundations of Einstein's general theory of relativity at the University of Cologne in 1968, Barbour has supported himself and his family without an...
), the temporal granularity of personhood through time is a special case of being spread out through worlds. In addition to one's identically time-stamped copies at a moment across parallel worlds transversely, there are the differently time-stamped copies across parallel worlds longitudinally, linked by natural law so as to give the individual's experience of one world and a continuous self. The implications for the theory of personal identity are not yet clear, but Derek Parfit's
Derek Parfit
Derek Parfit is a British philosopher who specializes in problems of personal identity, rationality and ethics, and the relations between them. His 1984 book Reasons and Persons has been very influential...
Reductionist view seems to be favored: The concept of personal identity ceases to apply when branching is taken into account, but branching maintains what's important about personal identity, such as psychological continuities having to do with memory, desire, character, and so forth. Another possibility is that Robert Nozick's
Robert Nozick
Robert Nozick was an American political philosopher, most prominent in the 1970s and 1980s. He was a professor at Harvard University. He is best known for his book Anarchy, State, and Utopia , a right-libertarian answer to John Rawls's A Theory of Justice...
Closest-Continuer theory could be modified so as to track closeness transversely as well as longitudinally. The tracked slices of "me-copies" would be the continuing person. Deutsch would seem to favor some such approach. There are "multiple identical copies" of me in the multiverse. Which one am I? Deutsch answers, "I am, of course, all of them." (The Parfitian answer would be, "The concept of personal identity doesn't apply.") Copies need not be strictly identical in the sense of the identity of indiscernibles
Identity of indiscernibles
The identity of indiscernibles is an ontological principle which states that two or more objects or entities are identical if they have all their properties in common. That is, entities x and y are identical if any predicate possessed by x is also possessed by y and vice versa...
relativized to universes: All of my copies see a coin spinning in a coin toss, but an instant later half my copies see 'heads' come up, the other half see 'tails'. A distinction between copies, versions, and variants is at work here. Variants of me need not see the spinning coin. Versions of me see it though some of them see 'heads' and some 'tails'. The multiple identical copies of me all see the spinning coin.
Time travel
The possibility of future-directed time travelTime travel
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the...
is assured by Einstein's special theory of relativity, which says that an observer who accelerates or decelerates will experience less time than an observer who is at rest or in uniform motion. This time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
could make an astronaut's flight very short and the duration on Earth very long, but such a trip to Earth's future would be irreversible as "no amount of time dilation can allow a spaceship to return from a flight before it took off". As for past-directed time travel, it is possible as a sort of sidestep from one universe to another, requiring a path between the two universes that is "hard-wired" into the structure of the multiverse. Whether such paths exist or not is an unresolved empirical question. If they were to exist and were to allow macro-objects like human beings to traverse them, time travel could occur without the grandfather paradox
Grandfather paradox
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...
, because the time traveler would go to a point prior to the branching between his "home" universe and the universe in which he (the copy of him in the "away" universe) kills his grandfather (grandfather-copy).
The multiverse hypothesis alone doesn't avoid the knowledge paradox, in which the time traveler goes to a point where he gives the collected works of Shakespeare to a hack writer, who seems to get "knowledge for free" that he uses to become the celebrated Shakespeare in that world. The Popperian epistemological strand of Deutsch's four-strand theory is invoked at this point. Just as life is understood as a trans-universe structure that is physically possible only through processes of natural selection, so too knowledge is understood as such a structure that is physically possible only through processes of rational problem-solving. Knowledge for free isn't possible in the multiverse when the multiverse is understood through Deutsch's emergentist Theory of Everything.
External links
- The Fabric of Reality paperback edition: ISBN 014027541X