Reduction or Emergence

Earnest Rutherford used to say that all science is either physics or stamp collecting. This could have been a dig at the biologists of his time, who were still collecting samples and classifying species. He probably would have thought more highly of modern molecular biology, which is a lot like his physics in outlook: everything is determined by the DNA. It is said that Rutherford’s worst insult for a student who had done something stupid was–Chemist. The chemists had the last laugh though: Rutherford was awarded the Nobel Prize not in Physics but in Chemistry for having achieved the transmutation of elements.

Should we understand the world bottom up or top down? Which is the proper scientific view?

The Reductionist View

It is the view of most physicists that the properties of big things can be deduced from those of its constituents. Thus we believe we can reduce biology to chemistry, chemistry to atomic physics,resolve atoms into nuclei and electrons, split the nucleus into neutrons and protons, and then finally ( as far as we know now) into quarks. Rutherford’s work can be thought of as reducing chemistry to physics. Among other things, he solved the age-old problem of transmutation of elements. You cannot convert an element into another using chemical reactions, since they only rearrange electrons. You can however, achieve this by nuclear reactions. This occurs naturally in the fission of heavy elements, or the fusion of light ones. You can also induce it by bombarding a nucleus with an alpha particle or colliding nuclei in an accelerator.

With the availability of large scale computation, the reductionist view that any property of a large object can be deduced from that of its constituents has become very powerful. It is now possible to calculate from first principles ( knowing only the masses and charges of the electrons and of the nuclei) the properties of atoms and molecules with hundreds of electrons. Even further, we can predict properties of materials ab initio. Computers can predict the flow of fluids based on the motion of individual particles so well that the latest airliners are designed without testing in wind-tunnels. This view of the world, which can be said to have started with Newton, gives a special status to the study of elementary particles. In a sense, it is the most fundamental of all material knowledge.

Much of the research in high energy physics today is about trying to guess what may lie beyond the standard model of elementary particles, the most fundamental theory we know now. The last three decades have not been kind to experimental high energy physics, as repeated attempts to probe beyond have not shown any smaller structure. Each new generation of accelerators could have opened up new possibilities but only confirmed the standard model. But they say it gets darkest just before the dawn. Perhaps the Large Hadron Collider, about to start operation at CERN will be that light. Then again, it also gets darkest just before it goes completely black. The truth is, we won’t know what is there until we get there. There is no substitute for experimental research.

Emergent Phenomena

There is another view, also held by prominent physicists: that it may not be necessary to know the ultimate structure of matter to understand what happens at larger levels. Thus,even without knowing the masses and sizes of the particles that make up a fluid we can predict how they will behave collectively. Also, important phenomena like second order phase transitions depend on the constituents only some details. A highly successful principle known as universality has emerged from the work of K. G. Wilson: the effect of small scale structure on large objects is limited to the variation of a finite number of parameters. This is implemented mathematically using a technique called renormalization.

Moreover, the fundamental framework of elementary particle physics, quantum field theory, is remarkably like the theory of critical phenomena and shares with it renormalization and universality. Ideas from one area were useful in the other. It looks as though it really doesn’t matter what the next level of structure is, to understand what goes on at energies accessible to us now. This approach, pushed most notably by Laughlin and Anderson, is that the theories of elementary particles are also `emergent phenomena’ no more fundamental than the theory of superconductivity or of magnetism.

This creative tension between opposing viewpoints is what makes physics of our time especially interesting. I myself swing between the two points of view,although I am trained as a high energy physicist.

The Mystery of Confinement

Especially significant in this debate is the remarkable fact that quarks, the most fundamental constituents of matter, cannnot be isolated as free particles. This is very different from Rutherford’s day. Not only could you see that the atoms are made of nuclei and electrons, you could create free electrons ( beta rays) and nuclei ( alpha particles are Helium nuclei). Today, we can see that protons and neutrons are made of point-like constituents-quarks- using scattering experiments very much like Rutherford’s. But it is impossible to isolate quarks. This is known as quark confinement. Attempts to create free quarks simply produce more particles composed of them. The attractive force between quarks does not decrease with distance and keeps them bound to each other.

This is first time that a fundamental force of nature is found not to decrease with distance. If quarks cannot be isolated, is it possible to have a description of elementary particles without them? May be each elementary particle is made of the others, an idea known as particle democracy.

Such ideas were considered back in the sixties (most notably by Chew) but were discarded with the discovery of point-like constituents inside the proton. But now that quarks are found to be confined, there is reason to give it a second look. One of the surprises of the eighties was that another old strange idea, that the protons could be thought of as solitons, ( a kind of collective excitation) of mesons was indeed correct. So it may well be that there is a quarkless theory of nature at least in principle. More likely, there are two equivalent points of view. Just as particles behave like and waves and vice versa in quantum mechanics, it could be that certain phenomena ( weak decays) are easier to understand in terms of quarks and others (pion scattering) simpler in the soliton picture.

It appears that even if no new level of structure is discovered in the next generation of experiments, there are fundamental questions about the particles we already know about that remain unanswered. Ideas from other branches of physics, notably the theory of phase transitions, are needed along with new mathematical techniques.

Endnotes

B. Devine and F. Wilczek
“Longing for the Harmonies: Themes and Variations in Modern Physics” W. W. Norton & Company; (1989) ISBN-13: 978-0393305968

R. Teresi and L. Lederman “The God Particle: If the Universe Is the Answer, What Is the Question?” Mariner Books ISBN-13/EAN: 9780618711680

S. Weinberg Physica 96A, 327 (1979)

Anderson, P.W. (1972), “More is Different: Broken Symmetry and the Nature of the Hierarchical Structure of Science”, Science 177: 393-396

Laughlin, Robert (2005), A Different Universe: Reinventing Physics from the Bottom Down, Basic Books, ISBN 0-465-03828-X

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