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ScienceWeek
THEORETICAL PHYSICS: ON STRING THEORY
The following points are made by Edward Witten (Nature 2005 438:1085):
1) Albert Einstein (1879-1955) devoted the later part of his life to seeking a theory that would offer, at least in principle, a comprehensive description of the laws of nature. This "unified field theory", Einstein believed, would endow all of nature's laws with the beauty of general relativity. Ultimately, Einstein left us with plenty of inspiration, but not many ideas about how to proceed. In fact, there are ample reasons why one might doubt whether Einstein's vision is achievable, or at least achievable in the foreseeable future. Crucial clues may be hopelessly out of reach. When looking back at Einstein's own work, most physicists would say that many of the most important clues for a unified field theory -- involving strong and weak nuclear interactions, the role of gauge theory and the world of elementary particles --were simply not known in Einstein's day.
2) Moreover, even if we could somehow find the unified field theory, it is not at all clear whether we could determine that it is right. From a simple combination of Planck's constant, the speed of light, and Newton's gravitational constant, one can construct a natural unit of length -- the Planck length. First defined by Max Planck a century ago, this length is so fantastically small that if it, or something close to it, is fundamental in physics, then some of the most important phenomena may be permanently beyond our experimental reach.
4) A second reason has to do with what physicists have learned in developing string theory. String theory forces general relativity upon us, whereas standard quantum field theory apparently makes it impossible to incorporate general relativity. And string theory leads in a remarkably simple way to a reasonable rough draft of particle physics unified with gravity.
5) And finally, string theory has proved to be remarkably rich, more so than even the enthusiasts tend to realize. It has led to penetrating insights on topics from quark confinement to quantum mechanics of black holes, to numerous problems in pure geometry. All this suggests that string theory is on the right track; otherwise, why would it generate so many unexpected ideas? And where critics have had good ideas, they have tended to be absorbed as part of string theory, whether it was black-hole entropy, the holographic principle of quantum gravity, noncommutative geometry, or twistor theory.
References:
1. Ramsey, N. F. Phys. Today 34, 26 34 (November 1981)
2. Zwiebach, B. A First Course in String Theory (Cambridge Univ. Press, 2004)
3. Greene, B. The Elegant Universe (W. W. Norton, 1999)
Nature http://www.nature.com/nature
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PARTICLE PHYSICS: ON FUNDAMENTAL PARTICLES AND STRING THEORY
The following points are made by Juan Maldacena (Nature 2003 423:695):
1) In fundamental physics, our description of nature involves four forces: gravitational, electromagnetic, weak and strong. The strong force is responsible for binding protons and neutrons inside the atomic nucleus. Two different theoretical approaches have been taken in describing the workings of the strong force and the structure of particles such as the proton and neutron. The theories are seemingly at odds with each other, but steps are gradually being taken to reconcile the two.
2) In the 1960s, experiments on high-energy collisions between protons revealed a plethora of other short-lived, strongly interacting particles. Shortly afterwards, a theory emerged that proposed that all of these different particles were particular excitation modes of a string: as a violin string can vibrate with different frequencies, these strings could oscillate in different ways, corresponding to the "zoo" of particles that was observed. This "string theory" proved useful in explaining some aspects of the masses and spins of the particles.
3) But further experiments carried out through the 1970s showed that protons are not fundamental particles. In the same way that, much earlier in the century, Rutherford had shown that the atomic nucleus was much smaller than an atom, experimenters showed that protons, and neutrons, have small point-like constituents. This did not fit with the theory of protons as strings, which are extended objects. In fact, these experiments led to a new description of the strong interaction in terms of point-like quarks and gluons, through a theory called quantum chromodynamics (QCD).
4) As the electron carries an electric charge, quarks and gluons carry a new type of charge, called "colour" (hence "chromodynamics"). The gluons transmit the strong force between quarks in much the same way that the photon transmits the electromagnetic force between electrons and other charged particles. To describe the strong force we need three "colours" -- three different types of charges, usually designated "red", "green", and "blue". The validity of QCD has been spectacularly confirmed by experiments at high energies in particle colliders. But, despite this success, it is still remarkably hard to do theoretical calculations with QCD at low energies. And that is exactly where things should get interesting: at low energies, the colour flux lines form bundles of energy that should behave like a string -- a tantalizing connection from QCD to string theory. These strings, made of gluons, bind the quarks together.
Nature http://www.nature.com/nature
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ON STRING THEORY
The following points are made by B.R. Greene et al (Proc. Nat. Acad. Sci. 1998 95:11039):
1) Particle physics has spent much of this century grappling with one basic question in various forms: What are the fundamental *degrees of freedom needed to describe nature, and what are the laws that govern their dynamics.
2) The current "Standard Model" of particle physics -- which is nearly 25 years old and which has much experimental evidence in its favor -- involves 6 quarks, 6 *leptons, 4 forces, and the as yet unobserved *Higgs boson. But this model contains internal indications that it too may be just another step along the path of uncovering the truly fundamental degrees of freedom. The Standard Model is valid to distances as small as 10^(-16) cm, and there is some evidence that the next level of structure will be detected only at a distance scale of the order of 10^(-32) cm, which is far beyond our abilities to measure in the laboratory.
3) A related important issue concerns the unification of general relativity and quantum mechanics. A serious problem arises when general relativity is extrapolated to small distance scales of the order of 10^(-32) cm where quantum effects must be taken into account: the relevant theoretical equations produce uncontrollable divergences, and the history of particle physics suggests this is an indication of a new physics occurring at these distance scales.
4) String theory offers hope of addressing both of these issues. There is only one known way to cure the divergence problem in the quantum-mechanical expansion of general relativity, and that is to model the particles in the theory not as points but as one-dimensional loops of "string". Every consistent such string model necessarily contains a special kind of particle -- the "*graviton" -- whose long-distance interactions are described by general relativity. So in a sense, string theory predicts gravity.
5) An exciting new frontier was opened during the past few years with the discovery of "string duality", which predicts equivalences among various different physical systems. This discovery has its roots in the properties of "supersymmetry", a novel type of symmetry that all consistent string theories possess. Briefly, supersymmetry relates properties of two basic types of particles -- bosons and *fermions -- which cannot be related by ordinary symmetry. There is a current belief that supersymmetry will play a role in the structure of particle physics beyond the Standard Model. One of the important achievements of string duality has been the determination of the behavior of the 5 consistent string theories when interactions become strong. All the consistent string theories are apparently related to each other, and to an elaboration known as "membrane theory" (M-theory).
6) String duality has produced hope that there may be only one possible string-theoretic model of the universe, and that it may be possible to eventually predict such features as particle masses and interaction strengths directly from such a theory. The authors conclude: "Development has been rapid on many fronts since string duality was introduced. We may be seeing glimpses of the underlying principle manifested in these new results. The challenging task that lies ahead is to discover that principle and thereby find what may well be the truly fundamental degrees of freedom in our universe."
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Notes by ScienceWeek:
leptons: A class of elementary particles. Although they are affected by electromagnetic and gravitational forces, apart from that they are involved only with weak interactions, acted upon by weak forces but not by strong forces, as opposed to quarks, which are acted upon by strong forces but not by weak forces. One further difference between leptons and quarks is that leptons can be isolated as single particles, whereas quarks apparently cannot. The leptons include the electron, the muon, the tau, and their associated neutrinos. The mass of the tau is approximately 3484 times the mass of the electron; the mass of the muon is intermediate.
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.
graviton: Several quantum field theories consistent with both quantum mechanics and special relativity postulate that the gravitational force between two quantum domain particles is generated by the exchange of an intermediate particle called a graviton.
fermions: Fermions (electrons, protons, neutrons) are particles that obey the Pauli exclusion principle: i.e., no two fermions of the same kind can occupy the same quantum state.
Proc. Nat. Acad. Sci. http://www.pnas.org
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