ScienceWeek

THEORETICAL PHYSICS: ON PHYSICS AND THE REAL WORLD

The following points are made by George F.R. Ellis (Physics Today 2005 July):

1) Physics is the model of what a successful science should be. It provides the basis for the other physical sciences and biology because everything in our world, including ourselves, is made of the same fundamental particles, whose interactions are governed by the same fundamental forces. It's no surprise then, as Princeton University's Philip Anderson has noted, that physics represents the ultimate reductionist subject: Physicists reduce matter first to molecules, then to atoms, then to nuclei and electrons, and so on, the goal being always to reduce complexity to simplicity. The extraordinary success of that approach is based on the concept of an isolated system. Experiments carried out on systems isolated from external interference are designed to identify the essential causal elements underlying physical reality.

2) The problem is that no real physical or biological system is truly isolated, physically or historically. Consequently, reductionism tends to ignore the kinds of interactions that can trigger the emergence of order, patterns, or properties that do not preexist in the underlying physical substratum. Biological complexity and consciousness -- as products of evolutionary adaptation -- are just two examples. Physics might provide the necessary conditions for such phenomena to exist, but not the sufficient conditions for specifying the behaviors that emerge at those higher levels of complexity. Indeed, the laws of behavior in complex systems emerge from, but are to a large degree independent of, the underlying low-level physics. That independence explains why biologists don't need to study quantum field theory or the standard model of particle physics to do their jobs.

3) Moreover, causes at those higher levels in the hierarchy of complexity have real effects at lower levels, not just the reverse as often thought. Consequently, physics cannot predict much of what we see in the world around us. If it could predict all, then free will would be illusory, the inevitable outcome of the underlying physics.

4) True complexity, with the emergence of higher levels of order and meaning, including life, occurs in modular, hierarchical structures.[1,2] Consider the precise ordering in large intricate networks -- microconnections in an integrated chip or human brain, for example. Such systems are complex not merely because they are complicated; order here implies organization, in contrast to randomness or disorder. They are hierarchical in that layers of order and complexity build upon each other, with physics underlying chemistry, chemistry underlying biochemistry, and so forth. Each level can be described in terms of concepts relevant to its own particular structure -- particle physics deals with behaviors of quarks and gluons, chemistry with atoms and molecules -- so a different descriptive language applies at each level. Thus we can talk of different levels of meaning embodied in the same complex structure.

5) The phenomenon of emergent order refers to this kind of organization, with the higher levels displaying new properties not evident at the lower levels. Unique properties of organized matter arise from how the parts are arranged and interact, properties that cannot be fully explained by breaking that order down into its component parts.[3,4] You can't even describe the higher levels in terms of lower-level language.[5]

References (abridged):

1. G. F. R. Ellis, in The Re-Emergence of Emergence, P. Clayton, P. C. W. Davies, eds., Oxford U. Press, New York (in press); also available at http://www.mth.uct.ac.za/~ellis/emerge.doc

2. G. Booch, Object Oriented Analysis and Design with Applications, 2nd ed., Benjamin Cummings, Redwood City, CA (1994)

3. N. A. Campbell, Biology, Benjamin Cummings, Menlo Park, CA (1996)

4. R. B. Laughlin, A Different Universe: Reinventing Physics from the Bottom Down, Basic Books, New York (2005)

5. S. Hartmann, Stud. Hist. Philos. Mod. Phys. 32, 267 (2001)

Physics Today http://www.physicstoday.org

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Related Material:

PARTICLE PHYSICS: AN EXCHANGE CONCERNING RELEVANCE

Notes by ScienceWeek:

In general, "reductionism" is the idea that macroscopic phenomena can be explained in terms of microscopic entities and/or events, but the specific meaning of the term depends upon context and the conceptual identification within a particular science of levels of understanding. In biology in general, for example, "reductionism" is the term applied to attempts to explain biological phenomena in the language of physics and chemistry. In neurobiology, the term "reductionism" may be applied to attempts to explain human cognitive behavior in terms of the behavior of nerve cells and their connections. In evolutionary biology, the term "reductionism" may be applied to attempts to explain the dynamics of evolution in terms of molecular genetics. In physics and chemistry, the term "reductionism" may be applied to attempts to explain the macroscopic behavior of physical or chemical systems in terms of events at the level of atomic phenomena. Also in physics, the term "reductionism" may be applied to attempts to explain both the macroscopic behavior of a physical system and/or the microscopic atomic behavior of the entities of the system in terms of events at the still more microscopic level of fundamental particles and fundamental forces.

The various sciences are split by scientists (not by nature) into various levels of explanation, with researchers working at the various levels using various techniques and concepts. Ordinarily, in the practice of science, the working scientist does not spend much time cogitating about whether a general reductionist approach is useful or not useful, philosophically valid or not valid, or whatever. The attitude essentially is that here is a house, I choose to study in detail the nature of the bricks, you choose to study in detail the nature of the construction of the house, I enjoy what I'm doing, you enjoy what you're doing, and each of us is making some contribution to a general understanding of the nature of the entity "house". This division of labor has been quite fruitful in science, and there is never much of a problem concerning the existence of various levels of investigation until the person who studies bricks says that what he or she is doing is more important than what the person who studies the construction of the house does, or when the person studying the construction of the house says it is the study of the construction of the house that is more important than the study of bricks. From the standpoint of "nature", from the perspective of the giant star *Betelguese, for example, a relatively nearby stupendous and violent supergiant star apparently 400 to 500 times the diameter of our Sun, any serious bickering on the planet Earth about the relative merits of various levels of understanding in science begins to smack of farce. But science is a human enterprise, and occasionally the bickering about reductionism and levels of understanding does get serious and does occupy attention.

In 1996, in a most prestigious physics journal (_Reviews of Modern Physics_), the physicist Robert Cahn stated that particle physics is essential to the understanding of our everyday world, that "particle physicists construct accelerators kilometers in circumference and detectors the size of basketball pavilions not ultimately to find the *t-quark or the *Higgs boson, but because that is the only way to learn why our everyday world is the way it is... Given the masses of the quarks and *leptons, and nine other closely related quantities, [the current theory of particle interaction] can account in principle for all the phenomena in our daily lives."

In July 1998, in the journal _Physics Today_, Pablo Jensen, a condensed matter physicist, took issue with Cahn's views and suggested that Cahn's "reductionist vision seems to be shared by many other particle physicists." Stating that he wished to "reopen a debate in the physics community," Jensen made the following points: 1) The reductionist ideas of Cahn and other reductionist particle physicists are wrong: even if we knew all the "fundamental" laws, we could not say anything useful about our everyday world. Our everyday world is irremediably macroscopic, and macroscopic concepts are needed to understand it. 2) Contrary to the pretensions of particle physicists, science is organized in decoupled layers, each with its own elementary entities or concepts, which generally are not simply derived from those of the lower level but constructed in creative efforts... Particle physics is practically irrelevant to understanding our everyday world... "If we learned tomorrow that previous results and analysis had overlooked certain systematic errors, and that the t-quark mass is near 195 *GeV and not 175 GeV, it is particle physics that would have to adjust to remain in agreement with the rest of physics, and not vice versa." 3) Considering, for example, the property of *chirality of large molecules (e.g., a sugar or any biological molecule), for all practical purposes, such molecules do not show the symmetry expected from the fundamental laws -- in this case, quantum mechanics. 4) In the study of phase transitions, there are characteristics of such transitions that apparently depend on the collective behavior of the system and are not determined by the microscopic interactions. 5) Each level of complexity must be studied with its own instruments, and requires the invention of new concepts adapted to describe and understand its behavior... Intermediate concepts such as *entropy, *dissipative structures, cells, genes, etc., cannot be simply "deduced" from the fundamental laws: such concepts are said to be "emergent" because they arise at high levels of complexity and must be invented at those levels to deal with specific situations... These emergent concepts are as real and as fundamental as the concepts and particles introduced by particle physicists. The author concludes: "By all means let us each study our chosen "layer" of reality, whether it involves quarks or convective cells. But let us also remember that each layer is just one part of the greater whole. Accounting for all the phenomena in our daily lives *in principle* is entirely different from accounting for them in actuality."

In the November 1998 issue of _Physics Today_, Robert Cahn presents a rebuttal to the critique of Pablo Jensen, the author making the following points: 1) The empirical parameters of the *Standard Model of particle physics shape the most familiar aspects of our physical surroundings... Given *these parameters, the Standard Model, which subsumes the Maxwell and Schroedinger equations, determines all the fundamental processes of *electroweak and strong interactions. Changes in the basic parameters would produce worlds quite different from our own. 2) The stuff of daily life is made just of electrons and the lightest quarks. However, we cannot understand these particles by themselves, because they are intimately connected to others accessible only in high energy collisions. 3) Concerning the supposed irrelevance of particle physics, constructs that embody the essential physical features of complex systems are indispensable, but their success is not a reason for abandoning the search for basic physical laws. 4) Nature is not neatly partitioned into autonomous layers, as Jensen suggests. On the contrary, the macroscopic makes manifest the microscopic... The gross properties of the materials around us, their color, conductivity, and strength, reflect the details of their quantum mechanical states. Likewise the structure of atoms reflects divisions in the subatomic world... "Only by willfully closing our eyes can we miss the connection between the fundamental interactions and their manifestations that surround us." The author concludes: "We particle physicists share with all physicists the goal of explaining the world. We differ by asking ever more basic questions. Like young children who relentlessly insist, Why?, particle physicists ask, Why is there light? Why are electrons light and protons heavy? Why are there electrons or protons, anyway? 'Just because' and 'Who cares?' will not satisfy the curious child, nor should they satisfy us."

The same issue of the journal includes a number of letters on the subject from other physicists, and in one of these letters Paul Roman suggests that perhaps the motivation for the debate is that the physics research "grant pie is shrinking while the number of pie-hungry individuals is still increasing." Perhaps that is so, and perhaps that is also the motivation behind debates concerning the reductionist approach in other sciences. But perhaps such motivations are also part of science as a human enterprise. Meanwhile, the enormous furnace of Betelguese continues to roar.

References (abridged):

R.N. Cahn (Lawrence Berkeley Natl. Lab., US) (Rev. Mod. Phys. 1996 68:951) QY: Robert N. Cahn, Lawrence Berkeley National Laboratory, Berkeley, CA US

P. Jensen (Claude Bernard University, FR) Particle physics and our everyday world. (Physics Today July 1998) QY: Pablo Jensen, Claude Bernard University, Villeurbanne FR)

R.N. Cahn (Lawrence Berkeley Natl. Lab., US) "Particle physics and our everyday world": A reply (Physics Today November 1998) QY: Robert N. Cahn, Lawrence Berkeley National Laboratory, Berkeley, CA US

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Notes by ScienceWeek:

Betelguese: Also known as Alpha Orionis. It is the 10th brightest star in the sky, with a luminosity 5000 times that of the Sun, with an estimated distance of 400 light years. Some astronomers believe its distance is 1400 light years, which would make its luminosity 50,000 times that of the Sun. The star is a variable, its size swelling and contracting with a period of several years.

t-quark: (top-quark) A quark is a hypothetical fundamental particle, having charges whose magnitudes are one-third or two-thirds of the electron charge, and from which the elementary particles may in theory be constructed. A t-quark is one of the types of quarks and has an electrical charge of +2/3.

Higgs boson: Higgs fields (named after Peter W. Higgs, University of Edinburgh, UK) constitute a set of fundamental theoretical fields that induce spontaneous symmetry breaking. In general, spontaneous symmetry breaking occurs in systems whose underlying symmetry state is unstable. A Higgs particle is associated with a Higgs field in the same way that a photon is associated with the electromagnetic field. Higgs bosons are massive mesons whose existence is predicted by certain theories. Mesons are apparently composed of quark and anti-quark pairs; they are produced by various high-energy interactions and decay into stable particles.

leptons: Leptons are a class of point-like fundamental particles showing no internal structure and no involvement with the strong forces. There are 6 leptons: the electron, the muon, the massive tau lepton, and a specific neutrino associated with each of the former (3 neutrino "flavors").

GeV: (Gev) Also written as Bev, a billion electronvolts. An electronvolt is defined as the energy acquired by an electron falling freely through a potential difference of one volt, and is equal to 1.6022 x 10^(-19) joule.

chirality: In chemistry, chirality is a property of certain asymmetric molecules, the property being that the mirror images of the molecules cannot be superimposed one on the other while facing in the same direction.

entropy: A measure of disorder in a system.

dissipative structures: In general, a dissipative system is a system that loses energy by conversion of energy into heat.

Standard Model: In particle physics, the *Standard Model is a theoretical framework whose basic idea is that all the visible matter in the universe can be described in terms of the elementary particles leptons and quarks and the forces acting between them.

these parameters: The parameters referred to here are the masses of the quarks, the masses of the charged leptons, the strength of 3 forces, 4 numbers that describe the weak transformations of one quark type into another, the mass of the *W boson, and the mass of the Higgs boson.

W boson: Very massive charged particles (+ or -) that convey part of the weak force between leptons and *hadrons. Bose-Einstein statistics is the statistical mechanics of a system of indistinguishable particles for which there is no restriction on the number of particles that may simultaneously exist in the same quantum energy state. Bosons are particles that obey Bose-Einstein statistics, and they include photons, *pi mesons, all nuclei having an even number of particles, and all particles with integer *spin.

pi mesons: (pions) Pi mesons are subatomic particles with masses approximately 270 times the mass of the electron.

spin: In quantum mechanics, "spin" is the intrinsic angular momentum of a subatomic particle.

hadrons: Hadrons are particles with internal structure, e.g., neutrons and protons.

electroweak and strong interactions: The fundamental forces comprise the gravitational force, the electromagnetic force, the nuclear strong force, and the nuclear weak force. The electroweak interactions comprise the electromagnetic and nuclear weak interactions, the latter involved in radioactive decay processes.

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