'Invisibility Cloaks' Could Break Sound Barriers

Steven Cummer

Contrary to earlier predictions, Duke University engineers report in Physical Review Letters that a three-dimensional sound cloak is possible, at least in theory.

Such an acoustic veil would do for sound what the "invisibility cloak" previously demonstrated by the research team does for microwaves -- allowing sound waves to travel seamlessly around it and emerge on the other side without distortion. Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering said that such a cloak might hide submarines in the ocean from detection by sonar, he said, or improve the acoustics of a concert hall by effectively flattening a structural beam.

As in the case of the microwave cloak, the properties required for a sound cloak are not found among materials in nature and would require the development of artificial, composite metamaterials. (For more about metamaterials, click this link) The engineering of acoustic metamaterials lags behind those that interact with electromagnetic waves (i.e. microwaves or light), but "the same ideas should apply," Cummer said.

In 2006, researchers at Duke and the Imperial College London used a new design theory to create a blueprint for an electromagnetic invisibility cloak. Only a few months later,the team demonstrated the first such cloak, designed to operate at microwave frequencies.

Cummer and David Schurig, a former research associate at Duke who is now at North Carolina State University, later reported in "The New Journal of Physics" a theory showing that an acoustic cloak could be built. But that theory relied on a "special equivalence" between electromagnetic and sound waves that is only true in two dimensions. A report by another team had also suggested that a 3-D acoustic cloak couldn't exist. It appeared they had reached a dead end.

This time, he started instead from a shell like the microwave cloak his team had already devised and attempted to derive the mathematical specifications required to prevent such a shell from reflecting sound waves, a key characteristic for achieving invisibility. On paper, at least, it worked.

Image Description: Analytical computation of the interaction of an acoustic wave with the ideal cloaking shell. The peaks and valleys in the color scale represent the instantaneous pressure field of a uniform plane wave traveling from left to right as it impinges on a rigid sphere surrounded by a cloaking shell whose thickness is equal to the radius of the interior sphere. The shell smoothly bends the acoustic wavefronts around the interior rigid sphere so that they smoothly pass around the object. No acoustic wave is scattered back toward the source, nor does the object cast any acoustic shadow. From the pressure field outside the shell, there's no way to know whether there is an object there or not.

Although the theory used to design such acoustic devices so far isn't as general as the one used to devise the microwave cloak, the finding nonetheless paves the way for other acoustic devices, for instance,those meant to bend or concentrate sound. The existence of an acoustic cloaking solution also indicates that cloaks might possibly be built for other wave systems, including seismic waves that travel through the earth and the waves at the surface of the ocean.

Reference
"Scattering Theory Derivation of a 3D Acoutic Cloaking Shell"
S. Cummer, B.-I. Popa, D. Schurig, D.R. Smith, J. Pendry, M. Rahm and A.Starr,
Physical Review Letters, 100, 024301 (2008), Abstract Link.

High Energy Physics: 5 Needed Breakthroughs -- Michael Dine

[Professor Michael Dine of the University of California at Santa Cruz is today's guest in our ongoing feature '5 Breakthroughs'. He is also a faculty member at the university's Santa Cruz Institute for Particle Physics (SCIPP).

In his long career spanning about 3 decades (He got his PhD from Yale University in 1978), Prof. Dine made major contributions in the areas of supersymmetry, string theory, and other efforts to develop a "new physics" beyond the standard model of particle physics.

He has been one of the principal contributors (with various collaborators) to the set of ideas associated with supersymmetry, and was among the first to propose that supersymmetry might well be broken at these energy scales. Prof. Dine developed some of the first potentially realistic models of supersymmetry phenomenology, and was among the first to explore the dynamics of supersymmetric theories, uncovering an array of surprising phenomena, some of potential relevance to experiments, and others of interest to mathematicians and more theoretically minded physicists. In recent years, he developed a proposal for the phenomenology of supersymmetry which has become a standard for both theoretical and experimental analyses. Currently, he is engaged in a number of projects exploring the experimental possibilities for the Large Hadron Collider (LHC).

Prof. Dine also made significant contributions in superstring theory. Most of his work has been motivated by the hope of making specific predictions from the theory for accelerators, but in the course of these efforts, he made several important contributions to the overall theoretical structure. Much of his current effort is involved with trying to understand whether one can make predictions from this theory relevant to the Large Hadron Collider (LHC). At the moment, he believes there is a promising (but not certain), approach, based on a popular set of ideas commonly referred to as the `landscape'.

In December, 2007 issue of 'Physics Today', Prof. Dine provided an excellent account of the relationship between string theory and particle experiments in an article entitled "String Theory in the era of the Large Hadron Collider" (p.33, Article Link).

He also authored a widely acclaimed book on this topic: "Supersymmetry and String Theory: Beyond the Standard Model" (Cambridge University, 2007).

In the field of cosmology, he made significant contributions to the theory of inflation, and to ideas about the dark energy and dark matter. Simultaneously with others, he proposed the axion as a dark matter candidate, which has remained, over the years, one of the two most plausible possibilities (the other arising in supersymmetric theories). He also proposed one of the most widely studied ideas for understanding the origin of the matter-antimatter asymmetry (known as the Affleck-Dine mechanism) explaining why there was not, initially, an equal amount of matter and antimatter, which could have simply annihilated each other.

It's our pleasure to present this list of 5 most important breakthroughs that Prof. Dine would like to see in the physics of elementary particles.
-- 2Physics.com ]

Five needed breakthroughs in elementary particle physics

1) Determination of the origin of electroweak symmetry breaking – the masses of the W and Z bosons, quarks and leptons. Is it a single Higgs field (particle), as in the simplest version of the standard model? Or is it associated with supersymmetry, large or warped extra dimensions, or something else? This question should be settled over the next three to five years by the Large Hadron Collider at CERN, due to be commissioned late this year.

2) Identifying the dark matter. There are several plausible, well-motivated candidates coming from particle physics: the lightest supersymmetric particle (LSP), the axion (a hypothetical particle seemingly required to understand features of the strong nuclear force), and others. There are ongoing, dedicated searches for both the LSP and the axion. If the LHC discovers supersymmetry, there is a good chance we will discover the dark matter particle in underground experiments, and we will be able to study in some detail how this particle was produced at the earliest stages of the big bang. The axion searches also have a real chance of finding something, if the axion is the dark matter, though detectors with a broader reach may be necessary.

3) Theoretically, one urgent question is: does string theory predict that supersymmetry, warping, or something else is responsible for electroweak symmetry breaking? Can we settle this question theoretically before the LHC? Can we make more detailed predictions? Recent developments associated with the string landscape suggest this might be possible, but the problem is challenging.

4) There are many problems of quark and lepton flavor (the occurrence of several types of quarks and leptons, and the puzzling features of their masses and couplings) which we would like to understand. What is the scale of baryon number violation? What can we understand, theoretically and experimentally, about the origin of neutrino mass? Can we develop a compelling theory, which explains the very different features of the charged fermion masses and those of the neutrinos? Can we establish experimentally the nature of the neutrino masses? Can we decide that leptogenesis, and not, say, coherent effects associated with supersymmetry, are responsible for the asymmetry between matter and antimatter in the universe?

5) Theoretically and experimentally, what more can we learn about inflation, the period of rapid expansion in the very early universe for which there is growing observational evidence, as well as strong theoretical arguments? At a microscopic level, we are far from understanding how inflation comes about. All existing models have troubling features. Can we get beyond this situation? Can supersymmetry or string theory help? If we have improved theories, they will be subject to some experimental tests; how far can we go?

High Energy Physics : 5 Needed Breakthroughs -- Mark Wise

[In the ongoing feature '5 Breakthroughs', our guest today is Mark Wise, the John A. McCone Professor of High Energy Physics at California Institute of Technology.

Prof. Wise is a fellow of the American Physical Society, and member of the American Academy of Arts and Sciences and the National Academy of Sciences. He was a fellow of the Alfred P. Sloan Foundation from 1984 to 1987.

Although Prof. Wise has done some research in cosmology and nuclear physics, his interests are primarily in theoretical elementary particle physics. Much of his research has focused on the nature and implications of the symmetries of the strong and weak interactions. He is best known for his role in the development of heavy quark effective theory (HQET), a mathematical formalism that has allowed physicists to make predictions about otherwise intractable problems in the theory of the strong nuclear interactions.

To provide a background of his current research activities, Prof. Wise said,"Currently we have a theory for the strong, weak and electromagnetic interactions of elementary particles that has been extensively tested in experiments. It is usually called the standard model. Even with this theory many features of the data are not explained. For example, the quark and lepton masses are free parameters in the standard model and are not predicted. Furthermore the theory has some unattractive aspects -- the most noteworthy of them being the extreme fine tuning needed to keep the Higgs mass small compared to the ultraviolet cutoff for the theory. This is sometimes called the hierarchy problem."

He explained,"My own research breaks into two parts. One part is using the standard model to predict experimental observables. Just because you have a theory doesn’t mean it’s straightforward to use it to compare with experiment. Usually such comparisons involve expansions in some small quantity. One area I have done considerable research on is the development of methods to make predictions for the properties of hadrons that contain a single heavy quark".

He elaborated,"The other part is research on physics that is beyond what is in the standard model. In particular I have worked on the development of several extensions of the standard model that solve the hierarchy problem: low energy supersymmetry, the Randall-Sundrum model and most recently the Lee-Wick standard model. This work is very speculative. It is possible that none of the extensions of the standard model discussed in the scientific literature are realized in nature."

Prof. Wise shared the 2001 Sakurai Prize for Theoretical Particle Physics with Nathan Isgur and Mikhail Voloshin. The citation mentioned his work on "the construction of the heavy quark mass expansion and the discovery of the heavy quark symmetry in quantum chromodynamics, which led to a quantitative theory of the decays of c and b flavored hadrons."

He obtained his PhD from Stanford University in 1980. While doing his thesis work, he also co-authored the book 'From Physical Concept to Mathematical Structure: an Introduction to Theoretical Physics' (U. Toronto Press, 1980) with Prof Lynn Trainor of the University of Toronto (where he did his B.S. in 1976 and M.S. in 1977). He also coauthored, with Aneesh Manohar, a monograph on 'Heavy Quark Physics' (Cambridge Univ Press, 2000).

We are pleased to present the list of 5 needed breakthroughs that Prof. Mark Wise would be happy to see in the field of high energy physics.
-- 2Physics.com]

"Here go five breakthroughs that would be great to see:

1) An understanding of the mechanism that breaks the weak interaction symmetry giving the W's and Z's mass. This we should know the answer to in my lifetime since it will be studied at the LHC (Large Hadron Collider) and I am trying to stay healthy.

2) Reconciling gravity with quantum mechanics. Currently the favored candidate for a quantum theory of gravity is String Theory. However, there is no evidence from experiment that this is the correct theory. Perhaps quantum mechanics itself gives way to a more fundamental theory at extremely short distances.

3) An answer to the question, why is the value of the cosmological constant so small? I am assuming here that dark energy is a cosmological constant. (Hey if it looks like a duck and quacks like a duck it's probably a duck.) A cosmological constant is a very simple term in the effective low energy Lagrangian for General Relativity. The weird thing about dark energy is not what it is but rather why it's so small.

4) An understanding of why the scale at which the weak symmetry is broken is so small compared to the scale at which quantum effects in gravity become strong. This is usually called the hierarchy problem. Breakthrough (1) might provide the solution to the hierarchy problem or it might not.

5) Discovery of the particle that makes up the dark matter of the universe and the measurement of its properties (e.g., spin, mass, ...).

There are other things I would love to know. For example, is there a way to explain the values of the quark and lepton masses? But you asked for five."