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ScienceWeek
THEORETICAL PHYSICS: ON THE AETHER AND BROKEN SYMMETRY
The following points are made by Frank Wilczek (Nature 2005 435:152):
1) The concept that what we ordinarily perceive as empty space is in fact a complicated medium is a profound and pervasive theme in modern physics. This invisible inescapable medium alters the behavior of the matter that we do see. Just as Earth's gravitational field allows us to select a unique direction as up, and thereby locally reduces the symmetry of the underlying equations of physics, so cosmic fields in "empty" space lower the symmetry of these fundamental equations everywhere. Or so theory has it. For although this concept of a symmetry-breaking aether has been extremely fruitful (and has been demonstrated indirectly in many ways), the ultimate demonstration of its validity --cleaning out the medium and restoring the pristine symmetry of the equations -- has never been achieved: that is, perhaps, until now.
2) In new work, Cramer et al.[1] claim to have found evidence that -- for very brief moments, and over a very small volume --experimentalists working at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York have vaporized one symmetry-breaking aether, and produced a more perfect emptiness. This pioneering attempt to decode the richly detailed (in other words, complicated and messy) data emerging from the RHIC experiments is intricate[2], and it remains to be seen whether the interpretation Cramer et al. propose evolves into a consensus. In any case, they've put a challenge on the agenda, and suggested some concrete ways to tackle it.
3) But what exactly is this underlying symmetry of nature that is broken by the aether? How is it broken, and how might it be restored? The symmetry in question is called chiral symmetry, and it involves the behavior of quarks, the principal constituents of the protons and neutrons in atomic nuclei (among other things). Chiral symmetry is easiest to describe if we adopt the slight idealization that the lightest quarks, the up quark (u) and down quark (d), are massless. (In reality their masses are small, on the scale of the energies in play, but not quite zero.) According to the equations of quantum chromodynamics (QCD), the theory that describes quarks and their interactions via the strong nuclear force, the possible transformations among quarks are very restricted. One rule is that u-quarks and d-quarks retain their "flavor" -- that is, a (u) never converts into a (d), nor a (d) into a (u).
4) Quarks also, like the more familiar photons, have an intrinsic spin. If the spin axis is aligned with the direction of motion, then the sense of the rotation defines a handedness, known as chirality, rather like a left- or right-handed screw. The two possible states of chirality of a quark, left and right, are essentially the same concept as left and right circular polarization for photons. The fundamental interaction between quarks and gluons, to which we ultimately trace the strong nuclear force, conserves chirality as well as flavor. Thus a u-quark with left-handed chirality (written uL) never converts into a right-handed uR, and so on. But these extra conservation laws, which follow from the symmetry of QCD's equations, are too good to be true. In reality, one finds that although the rule forbidding changes of flavor holds true, there is no additional conservation law for chirality -- chiral symmetry is broken.
5) The accepted explanation for this mismatch blames a form of aether. The idea is that there is such a powerful attractive interaction between uL-quarks and R-antiquarks (every quark has an antiquark with the opposite charge), and likewise between dL-quarks and R-antiquarks, that the energy gained from their attraction outweighs the cost of creating the particles in the first place. Thus, perfectly empty space, devoid of quarks, is unstable. One can lower the energy of the vacuum by filling it with bound uL-R and dL-R pairs (and their antiparticles, L-uR, L-dR). Physicists call this process the formation of the chiral condensate. In the stable state that finally results, the conservation of chirality is rendered ineffective, as space itself has become a reservoir containing, for example, an indefinite number of uL-quarks.[3-5]
References (abridged):
1. Cramer, J., Miller, G., Wu, J. & Yoon, J. -H. preprint at http://www.arxiv.org/nucl-th/0411031 (2004)
2. Kolb, P. F. & Heinz, U. in Quark Gluon Plasma Vol. 3 (eds Hwa, R. C. & Wang, X.-N.) (World Scientific, Singapore, 2004); preprint at http://www.arxiv.org/nucl-th/0305084 (2003)
3. Adcox, K. et al. (The PHENIX collaboration) Nucl. Phys. A (submitted); preprint at http://arxiv.org/nucl-ex/0410003 (2005)
4. Adams, J. et al. (The STAR collaboration) Nucl. Phys. A (submitted); preprint at http://arxiv.org/nucl-ex/05010095 (2005)
5. Back, B. B. et al. (The PHOBOS collaboration) Nucl. Phys. A (in the press); doi:10.1016/j.nuclphysa.2005.03.084 (2005)
Nature http://www.nature.com/nature
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Related Material:
ON THE ETHER CONCEPT IN PHYSICS
Notes by ScienceWeek:
In the late 19th century, what we now call "classical" physics incorporated the assumed existence of the "ether", a hypothetical medium believed to be necessary to support the propagation of electromagnetic radiation. The famous *Michelson-Morley experiment of 1887 was interpreted as demonstrating the nonexistence of the ether, and this experiment became a significant prelude to the subsequent formulation of Einstein's *special theory of relativity. Although it is often stated outside the physics community that the ether concept was abandoned after the Michelson-Morley experiment, this is not quite true, since the classical ether concept has been essentially reformulated into several modern *field concepts.
The following points are made by Frank Wilczek (Physics Today January 1999):
1) Isaac Newton (1642-1727) believed in a continuous medium filling all space, but his equations did not require any such medium, and by the early 19th century the generally accepted ideal for fundamental physical theory was to discover mathematical equations for forces between indestructible atoms moving through empty space.
2) It was Michael Faraday (1791-1867) who revived the idea that space was filled with a medium having physical effects in itself... To summarize Faraday's results, James Clerk Maxwell (1831-1879) adapted and developed the mathematics used to describe fluids and elastic solids, and Maxwell postulated an elaborate mechanical model of electrical and magnetic fields.
3) The achievement of Einstein (1879-1955) in his paper on special relativity was to highlight and interpret the hidden symmetry of Maxwell's equations, not to change them. The Faraday-Maxwell concept of electric and magnetic fields, as media or ethers filling all space, was retained by Einstein. Later, Einstein was dissatisfied with the particle-field dualism inherent in the early atomic theory, and Einstein sought, without success, a unified field theory in which all fundamental particles would emerge as special solutions to the field equations.
4) Following Einstein, Paul Dirac (1902-1984) then showed that photons emerged as a logical consequence of applying the rules of quantum mechanics to Maxwell's electromagnetic ether. This connection was soon generalized so that particles of any sort could be represented as the small-amplitude excitations of quantum fields. Electrons, for example, can be regarded as excitations of an electron field, an ether that pervades all space and time uniformly. Our current and extremely successful theories of the *strong, electromagnetic, and weak forces are formulated as *relativistic quantum field theories with *local interactions.
5) The author states: "Einstein first purified, and then enthroned, the ether concept. As the 20th century has progressed, its role in fundamental physics has only expanded. At present, renamed and thinly disguised, it dominates the accepted laws of physics."
Physics Today http://www.physicstoday.org
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Notes by ScienceWeek:
Michelson-Morley experiment of 1887: Conducted by Albert Michelson (1852-1931) and Edward Morley (1838-1923), the experiment attempted to measure the velocity of the Earth through the "ether" by using an interferometer to detect a difference in the speed of light in the direction of Earth's rotation from the speed perpendicular to this direction. No difference was observed, indicating the absence of an ether "wind".
special theory of relativity: Proposed by Einstein in 1905, the special theory refers to inertial (non-accelerated) frames of reference. It assumes physical laws are identical in all frames of reference and that the speed of light in a vacuum is constant throughout the Universe and is independent of the speed of the observer. In general, the special theory gives a unified account of the laws of mechanics and electromagnetism (including optics). The companion theory, the general theory of relativity (1915), deals with general relative motion between accelerated frames of reference, and it is the general theory that led to Einstein's analysis of gravitation.
field: In this context, in general, the term "field" refers to a physical quantity (e.g., electric or magnetic field) that varies from point to point in space.
strong, electromagnetic, and weak forces: The fundamental forces currently identified in physics are the gravitational force, the electromagnetic force, the nuclear strong force, and the nuclear weak force. The nuclear strong force is the dominant force that acts between hadrons (e.g., the force that binds neutrons and protons in nuclei). (A "hadron" is any object made of *quarks and/or antiquarks). The weak force occurs between leptons (particles without internal structure, e.g., electrons, neutrinos) and hadrons (particles with internal structure, e.g., neutrons and protons); In general, the weak force is responsible for radioactivity.
quarks and antiquarks: 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. The antiquark is the antimatter quark entity. In general, antiparticles are homologs of elementary particles but with opposite charge. The positron, for example, is the antimatter particle homologous to the electron. Matter composed entirely of antiparticles is called "antimatter".
relativistic quantum field theories: In general, a "quantum field theory" is any quantum mechanical theory in which particles are represented by fields whose normal modes of oscillation are quantized. The term is also used to refer to a quantum mechanical theory applied to systems having an infinite number of *degrees of freedom. Quantum electrodynamics, for example, is a particular quantum field theory describing the emission or absorption of photons by charged particles. "Relativistic quantum field theories" are used to describe fundamental interactions between elementary particles (which exhibit relativistic velocities, i.e., velocities approaching the speed of light).
local interactions: In this context, a local interaction is an interaction between particles whose quantum mechanical wave functions are confined to a small region of a large system rather than being extended throughout the system.
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Related Material:
ON FIELD THEORY IN PHYSICS
Notes by ScienceWeek:
In physics, a field is an entity that acts as intermediary in interactions between particles, and which is distributed over part or all of space, and whose properties are functions of space coordinates, and except for static fields, also functions of time. There is also a quantum-mechanical analog of this entity, in which the function of space and time is replaced by an operator at each point in space-time.
The following points are made by Roman Jackiw (Proc. Natl. Acad. Sci. 1998 95:12776):
1) Present-day theory for fundamental processes (i.e., descriptions of elementary particles and forces) is phenomenally successful. Experimental data confirms theoretical prediction, and where accurate calculation and experiments are attainable, agreement is achieved to 6 or 7 figures. Two examples: a) The helium atom ground state energy (*Rydbergs) is experimentally measured as -5.8071394 and theoretically calculated as -5.8071380. b) The muon magnetic dipole moment is experimentally measured as 2.00233184600 and theoretically calculated as 2.00233183478.
2) The theoretical structure within which this success has been achieved is *local field theory, which offers a wide variety of applications, and which provides a model for fundamental physical reality as described by our theories of *strong, electroweak, and gravitational processes. No other framework exists in which one can calculate so many phenomena with such ease and accuracy.
3) But is spite of these successes, today there is little confidence that field theory will advance our understanding of nature at its fundamental workings beyond what has already been achieved. Although in principle all observed phenomena can be explained by present-day field theory, these accounts are still imperfect, requiring ad hoc inputs. Moreover, because of conceptual and technical obstacles, classical gravity theory has not been integrated into the *quantum field description of nongravitational forces: *quantizing the *metric tensor of Einstein's theory produces a quantum field theory beset by infinities that apparently cannot be controlled.
4) These shortcomings are actually symptoms of a deeper lack of understanding concerning *symmetry and symmetry breaking... Physicists are happy in the belief that Nature in its fundamental workings is essentially simple, but observed physical phenomena rarely exhibit overwhelming regularity. Therefore, at the very same time that we construct a physical theory with intrinsic symmetry, we must find a way to break the symmetry in physical consequences of the model.
5) These problems have produced a theoretical impasse for over two decades, and in the absence of new experiments to channel theoretical speculation, some physicists have concluded that it will not be possible to make progress on these questions within field theory, and they have turned to a new structure, "*string theory". In field theory, the quantized excitations are point particles with point interactions, and this gives rise to the infinities. In string theory, the excitations are extended objects -- strings -- with nonlocal interactions; there are no infinities in string theory, and that enormous defect of field theory is absent.
6) Yet in spite of its positive features, until now string theory has provided a framework rather than a definite structure, and a precise derivation of the *Standard Model has yet to be given. The author concludes: "On previous occasions when it appeared that quantum field theory was incapable of advancing our understanding of fundamental physics, new ideas and new approaches to the subject dispelled the pessimism. Today we do not know whether the impasse within field theory is due to a failure of imagination or whether indeed we have to present fundamental physical laws in a new framework, thereby replacing the field theoretic one, which has served us well for over 100 years."
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes by ScienceWeek:
Rydbergs: A unit of energy used in atomic physics, value = 13.605698 electronvolts.
local field theory: In this context, "locality" is the condition that two events at spatially separated locations are entirely independent of each other, provided that the time interval between the events is less than that required for a light signal to travel from one location to the other. For example, the quantum mechanical wave function is a "local" field.
strong, electroweak, and gravitational processes: The fundamental forces comprise the gravitational force, the electromagnetic force, the nuclear strong force, and the nuclear weak force. The "electroweak" interactions are a unification of the electromagnetic and nuclear weak interactions, and are described by the Weinberg-Salam theory (sometimes called "quantum flavordynamics"; also called the Glashow-Weinberg-Salam theory).
quantum field description: In general, a quantum field theory is a quantum mechanical theory applied to systems having an infinite number of *degrees of freedom. The term is also used to refer to any quantum mechanical theory in which particles are represented by fields whose normal modes of oscillation are quantized (see below).
degrees of freedom: In general, the number of independent parameters required to specify the configuration of a system.
quantizing: In experimental physics, a quantized variable is a variable taking only discrete multiple values of a quantum mechanical constant. In theoretical physics, "quantizing" means the consistent application of certain rules that lead from classical to quantum mechanics. In general, "quantization" is a transition from a classical theory or a classical quantity to a quantum theory or the corresponding quantity in quantum mechanics.
metric tensor: The mathematical statement (involving a set of quantities) that describes the deviation of the Pythagoras theorem in a curved space.
symmetry and symmetry breaking: If a theory or process does not change when certain operations are performed on it, the theory or process is said to possess a symmetry with respect to those operations. For example, a circle remains unchanged under rotation or reflection, and a circle therefore has rotational and reflection symmetry. The term "symmetry breaking" refers to the deviation from exact symmetry exhibited by many physical systems, and in general, symmetry breaking encompasses both "explicit" symmetry breaking and "spontaneous" symmetry breaking. Explicit symmetry breaking is a phenomenon in which a system is not quite, but almost, the same for two configurations related by exact symmetry. Spontaneous symmetry breaking refers to a situation in which the solution of a set of physical equations fails to exhibit a symmetry possessed by the equations themselves.
string theory: In particle physics, string theory is a theory of elementary particles based on the idea that the fundamental entities are not point-like particles but finite lines (strings), or closed loops formed by strings, the strings one-dimensional curves with zero thickness and lengths (or loop diameters) of the order of the Planck length of 10^(-35) meters.
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. Leptons are a class of point-like fundamental particles showing no internal structure and no involvement with the strong forces. 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.
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