Quantum Gravity and Entanglement

Mark Van Raamsdonk

[Every year (since 1949) the Gravity Research Foundation honors best submitted essays in the field of Gravity. This year's prize goes to Mark Van Raamsdonk for his essay "Building Up Spacetime with Quantum Entanglement". The five award-winning essays will be published in the Journal of General Relativity and Gravitation (GRG) and subsequently, in a special issue of the International Journal of Modern Physics D (IJMPD). Today we present here an invited article from Prof. Raamsdonk on his current work.
-- 2Physics.com ]

Author: Mark Van Raamsdonk
Affiliation: Department of Physics and Astronomy,
University of British Columbia, Vancouver, Canada.

Quantum Mechanics and Entanglement :

The development of quantum mechanics in the early 20th century is surely one of the most remarkable achievements of mankind. Quantum mechanics is fundamentally different than the physical theories developed earlier to describe physics on macroscopic scales, yet is absolutely essential in understanding atomic scale physics. At the heart of quantum mechanics is the idea of quantum superposition: in quantum mechanics, objects can in some sense be in two places at once (or more generally two physical configurations at once). Mathematically speaking, every state of a physical system can be associated with a kind of vector, and if A and B are vectors representing two allowed physical configurations, then A + B is also an allowed physical state. In a simple example, A could be a state where an object is in one place, and B could be a state where the same object is in a different place; A + B then represents a state where a single object has no definite location. If a measurement of the object’s location is performed, no definite prediction for the result is possible; we will find it either in one location or the other, and quantum mechanics can at best predict the probability for each possible outcome.

2Physics articles by past winners of the Gravity Research Foundation award:
Alexander Burinskii (2009): "Beam Pulses Perforate Black Hole Horizon"
T. Padmanabhan (2008): "Gravity : An Emergent Perspective"
Steve Carlip (2007): "Symmetries, Horizons, and Black Hole Entropy"

Intimately related to the idea of quantum superposition is the notion of quantum entanglement. If we have a physical system with two parts (e.g. a ball and a box) then in a general quantum state, we cannot say with certainty what is the state of the ball (e.g. whether or not it is in the box) or what is the state of the box (e.g. whether the box is open or closed). But for certain quantum states, this uncertainty can be correlated for the two objects. For example, suppose A represents a quantum state where the ball is in the box and the box is closed, and B represents a quantum state where the ball is not in the box and the box is open. Then in the state A + B, neither the location of the ball nor the state of the box is definite, but a measurement which determines the state of the box effectively also determines the location of the ball: if we measure the state A+B and find the box closed, we can be sure that the ball is in the box; if we find it open, we can be sure the ball is not in the box. In this situation, we say that the ball and the box are entangled, since a measurement of one part of the system influences the quantum state of the other part of the system. In practice, it would be exceedingly difficult to prepare a macroscopic system such as a ball and box in such an entangled state, but such situations are commonplace at the atomic scale. The phenomenon of entanglement is an intrinsically quantum phenomenon; indeed, it can be shown that a computer making use of quantum entanglement can perform certain calculations far faster than any ordinary computer; entanglement is the basic property of quantum systems that allows quantum computation.

Strange as they may seem, the rules of quantum mechanics have now been tested beyond any reasonable doubt and allow us to understand physical processes in nature with incredible precision. For certain properties of elementary particles, predications based on quantum mechanics have been shown to be correct to one part in 100,000,000 or better. We now have a fully quantum mechanical description (known as quantum field theory) for the strong, weak, and electromagnetic forces, that allows us to understand how these interactions operate even at distance scales 100,000,000,000,000 times smaller than we can resolve with our eyes.

Quantum Gravity :

The approach that allowed physicists to develop a quantum mechanical theory for the strong, weak, and electromagnetic forces turns out not to work when applied to the remaining force, the force of gravity. In fact, it fails miserably. As a result, finding the correct quantum mechanical theory of gravity has been a prominent open question for decades; indeed it is one of the greatest challenges in theoretical physics. While Einstein’s Theory of General Relativity is almost entirely adequate for the purposes of describing the observed gravitational dynamics of planets, stars, galaxies, and even the expansion of the universe as a whole, it cannot be the whole story, since it does not incorporate the quantum mechanics principles that are believed to underlie all physics in our universe. Usually, a quantum mechanical description of nature is only necessary at very short distance scales; at macroscopic distance scales, the pre-20th century ``classical’’ physics provides an excellent approximation. But there are certain situations, such as in the interior of a black hole, in the early universe just after the big bang, or in a hypothetical scattering of particles with energies many orders of magnitude larger than we can currently produce in an accelerator, where gravitational effects would be important at distance scales small enough that a quantum mechanical description of the physics is essential. Finding the right theory of quantum gravity is essential if we want to fully understand the workings of nature.

String theory and the AdS/CFT correspondence :

One example of a theory that is fully quantum-mechanical but also includes gravitational physics is provided by string theory. Until the mid 1990s, the mathematical description of string theory was such that it allowed only relatively simple calculations; for example, one could predict the results for scattering of a fixed number of particles (including gravitons) on some fixed spacetime background (e.g. flat spacetime). This was not an entirely satisfactory situation. We recall that in Einstein’s theory of gravity, space itself is a dynamical entity that can be curved or warped by matter and energy; it is the effect of this warping on other objects that gives rise to gravitational ``forces.’’ In a complete theory of quantum gravity, different quantum states should correspond to spacetimes with different geometries (i.e. different warpings); the original formulation of string theory could most readily describe only different types of particles on a fixed geometry.

The situation for string theory changed dramatically between 1995 and 1997 in what is now known as “the Second Superstring Revolution.” (The first revolution was the period in the mid 1980s when it became clear that the original formulation of string theory was mathematically consistent.) This period culminated in a stunning proposal by Juan Maldacena known as the AdS/CFT correspondence, or gauge theory / gravity duality. (This followed an earlier proposal of the same nature by Tom Banks, Willy Fischler, Steve Shenker, and Lenny Susskind). The proposal states that there is an exact equivalence between certain examples of string theory (full-fledged theories of quantum gravity) and certain ordinary quantum mechanical systems without gravity (often quantum field theories). These much simpler ordinary quantum mechanical systems suffer none of the restrictions found in the original formulation of string theory, and thus, via the equivalence, may be used to provide a complete formulation of the corresponding string theory, able to quantum mechanically describe gravity and other forces on a spacetime which can fluctuate dynamically. Remarkably, this much better formulation of string theory turns out to be no more complicated than the quantum mechanical description of the other forces, completely understood almost half a century ago.

Geometry from Entanglement :

According to the AdS/CFT correspondence, there must be a dictionary that allows us to associate to every state of some conventional quantum mechanical system a state of the corresponding equivalent quantum gravity theory. Different states in the quantum mechanics correspond to different spacetime geometries (i.e. different distributions of matter and a different warping of space). For example, the quantum state A might correspond to completely empty space, while the state B corresponds to space with some gravitational waves, and state C corresponds to a space with orbiting black holes. While the dictionary between quantum state and corresponding spacetime is known for very simple states, more generally the correspondence is far from obvious. Ideally, one would like to know the gravity interpretation for an arbitrary quantum state of the conventional system; understanding the general dictionary is a crucial open question for the field.

The central suggestion in my essay [1] is that crucial information about what the spacetime associated to a given quantum state looks like is contained in how the various parts of the ordinary quantum are entangled with each other in the given state. While the arguments rely on some specific results in string theory, it is not difficult to give some sense of where the idea comes from.

To start, suppose that a specific quantum system has a corresponding gravity theory such that each state of the system corresponds to some spacetime. Now consider a second quantum system, which we obtain by taking two copies of the first system (with no physical interactions between the two systems). For the larger system, the simplest states are those with no entanglement between the two parts. That is, we can consider a state A = (A1,A2) in which the first system is in state A1 and the second system is in state A2. Now A1 and A2 each correspond to some particular spacetime according to the AdS/CFT correspondence. Thus, we can interpret the state A of the larger system as corresponding to two completely disconnected spacetimes (imagine our universe and some parallel universe with which there is no possible communication).

More generally, we can consider states which are quantum superpositions such as (A1,A2) + (B1,B2) . For such states, there is entanglement between the two parts. In [1], based on various earlier works, I pointed out that for states with enough entanglement (certain states which are quantum superpositions (A1,A2) + (B1,B2) + (C1,C2) + … with many states in the superposition) the resulting complicated state can be interpreted as a single connected spacetime, in which two distinct parts are connected by something like a wormhole (or a black hole/white hole). Since all the individual states in the superposition had interpretations as disconnected spacetimes, we can say that a quantum superposition of disconnected spacetimes has produced a connected spacetime. Alternately, we can say that by entangling the two parts of our original quantum system, we have managed to connect up two parts of the corresponding spacetime.

Starting from this hint of a connection between entanglement and spacetime geometry, one can argue that more quantitative measures of entanglement in states of a quantum system give direct information about quantitative geometrical quantities in the corresponding spacetimes, such as areas and geodesic distances. The complete picture for how to deduce the spacetime associated with a particular state in the AdS/CFT correspondence is certainly still beyond our reach, but I believe these connections between entanglement and geometry may be an important part of the story. If correct, they suggest a deep connection between quantum gravity and quantum information theory (the natural setting for studies of entanglement in quantum systems) that may be of fundamental importance.

References
[1] Mark Van Raamsdonk, “Building up spacetime with quantum entanglement,” arXiv:1005.3035. Link.
[2] Nielsen, M.A., Chuang, I.L., “Quantum Computation and Quantum Information” (Cambridge University Press, Cambridge, 2000).
[3] Juan Maldacena, “The Illusion of Gravity” -, Scientific American, November 2005. Link.
[4] Brian Greene, "The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory" (Vintage Series, Random House Inc, February 2000).

Cosmology: 5 Needed Breakthroughs -- Alexander Vilenkin

Alexander Vilenkin [photo courtesy: Institute of Cosmology, Tufts University]

[In our ongoing feature '5-Breakthroughs' we invited today Prof. Alexander Vilenkin, Director of Institute of Cosmology and L. and J. Bernstein Professor of Evolutionary Science at Tufts University.

Prof. Vilenkin's current research interests cover a wide range of subtopics in cosmology, quantum field theory and gravitation: cosmic inflation, dark energy, cosmic strings and monopoles, quantum cosmology, high energy cosmic rays, the multiverse, anthropic selection etc.

He received his undergraduate degree in physics in 1971 at Kharkov State University in the former Soviet Union. In 1976 he emigrated to USA and received his PhD at SUNY Buffalo in 1977. In 1978 he joined the faculty at Tufts.

During what has been a very productive and creative span of last thirty-five years, Prof. Vilenkin wrote over 200 research papers and contributed some crucial components of modern cosmology. His work on cosmic strings has been pivotal and his ideas on 'eternal inflation' and 'quantum creation of the universe from nothing' paved the path for new fields of investigation. Occasionally, he also took time to work on condensed matter physics and even topics like statistical analysis of DNA sequences. His work has been featured in numerous newspaper and magazine articles all over the world, as well as in many popular books. Here is a link to a list of his published work: Google Scholar.

Prof. Vilenkin is a Fellow of the American Physical Society. During 1984-89, he received Presidential Young Investigator award from National Science Foundation.

In 1994 he (with P. Shellard) wrote a monograph on "Cosmic Strings and Other Topological Defects" (Cambridge University Press, 1994). In 2006 he authored the well-acclaimed book "Many Worlds in One: The Search for Other Universes" (Hill & Wang, 2006) which has been translated into many languages.

It gives us lot of pleasure for having the opportunity of presenting to you this list of 5 breakthroughs that Prof. Vilenkin would like to see in the field of Cosmology.
-- 2Physics.com]

1. Cosmic superstrings. Some superstring inspired cosmological models predict the existence of fundamental strings of astronomical dimensions. Discovery of cosmic superstrings may be the only way to test superstring theory by direct observation. In fact, discovery of cosmic strings of any kind ("super" or not) would be a great breakthrough, since it will open new windows into particle physics of ultra-high energies and into the early universe cosmology.

2. Further evidence for inflation. We have substantial evidence for cosmic inflation, but the details are very uncertain and a large number of models are consistent with the data. Discovery of gravitational waves from inflation or of non-Gaussian features in the cosmic microwave background would be important breakthroughs in this area.

3. Evidence for the multiverse. Inflationary cosmology leads to the multiverse picture, with multiple "bubble universes" expanding and occasionally colliding with one another. Collisions of our bubble with others may have observational signatures in cosmic microwave background and in gravitational waves. A discovery of such a collision would provide a direct evidence for the existence of the multiverse.

4. Solution to the measure problem. This is a perplexing problem in inflationary cosmology. Inflation is generically eternal, and bubble universes like ours are constantly being produced. Anything that can happen will happen in the eternally inflating universe, and it will happen an infinite number of times. We have to learn how to compare these infinities, since otherwise we cannot distinguish probable events from highly improbable, which makes it hard to make any predictions at all.

5. Discovery of supersymmetry. Non-discovery at Large Hadron Collider (LHC) would also have important implications.

You may be wondering why "dark energy" is not on my list. This is because I believe it is cosmological constant. But if I am wrong, and the dark energy density is changing with time, the discovery of this fact would be a great breakthrough.

5 Most Important Breakthroughs That My Field of Research Needs -- Nathan Seiberg

Nathan Seiberg [photo courtesy: Institute for Advanced Study, Princeton]

[Our guest today in the feature ‘5-Breakthroughs’ is Nathan Seiberg, Professor at the Institute for Advanced Study in Princeton, NJ. Prof. Seiberg’s work has spanned a wide spectrum of research revolving around particle physics phenomenology, field theory, gauge theory, Matrix theory, string theory, and supersymmetry.

In early 1990s, he formulated the application of holomorphy to calculations in gauge theories with supersymmetry. In his famous 1994 article “Electric-Magnetic Duality in Supersymmetric Non-Abelian Gauge Theories” (Abstract link) he conjectured a new kind of Strong-Weak duality or S-duality relating two different supersymmetric QCDs which are not identical, but agree at low energies. This is now well-known as Seiberg duality.

Working with Edward Witten, he also devised a series of partial differential equations that simplified the classification of 4-dimensional manifolds. The invariants of such compact smooth 4-manifolds are now known as Seiberg–Witten invariants. Later, they analyzed the appearance of non-commutative geometry in theories containing open strings, and identified a low energy limit of open string dynamics as a noncommutative quantum field theory.

Prof. Seiberg also made pioneering contribution in Matrix Theory, M Theory and various subfields of particle physics. Here is link to his list of publications: Google Scholar.

He received his Ph.D from the Weizmann Institute of Science in Israel in 1982. Before joining the Institute for Advanced Study, he had been a Professor of Physics at the Weizmann Institute for Science and at Rutgers University.

Prof. Seiberg is a member of National Academy of Sciences and Fellow of American Academy of Arts and Sciences. He received The John D. and Catherine T. MacArthur Fellowship (Genius Grant) in 1996. In 1998, American Physical Society awarded Dannie Heineman Prize for Mathematical Physics to Nathan Seiberg and Ed Witten "for their decisive advances in elucidating the dynamics of strongly coupled supersymmetric field and string theories. The deep physical and mathematical consequences of the electric-magnetic duality they exploited have broadened the scope of Mathematical Physics (quote from the citation)."

It’s an honor and privilege on our part to present 5 most important breakthroughs that Prof. Seiberg would like to see in his fields of research.
— 2Physics.com ]

1. Origin of electroweak symmetry breaking. This will shed light on the origin of mass of elementary particles. An effective description of this phenomenon in terms of the Higgs mechanism is known. The Large Hadron Collider (LHC) will explore it in detail and perhaps will point to a deeper structure. One possibility is that the LHC will discover supersymmetry – a new kind of symmetry which extends our understanding of space and time. Alternatively, it will find new particles which might have a description in terms of new space dimensions. If only the Higgs particle is discovered, its mass might be set anthropically. Is this true?

2. Origin of the elementary particles. What determines the properties of the quarks and the leptons (their quantum numbers)? Why do they appear in 3 generations? Most of the parameters of the Standard Model of particle physics are associated with the quark and lepton masses. It is possible that the underlying structure which controls them exists at very high energies which will not be explored soon. One possible explanation of the properties of the quarks, the leptons, and their interactions is the idea of grand unification. Is this idea correct?

3. Dark matter and dark energy of the Universe. Is the dark matter weakly interacting massive particles? This question could be settled soon either by detecting these particles, or the product of their interactions, or by creating them at the LHC. Is the dark energy a cosmological constant? What sets the value of the cosmological constant today? Is it anthropic?

4. Inflation. It seems that in the past the Universe had a period of rapid expansion known as inflation, during which the cosmological constant was large. What is the detailed description of this phenomenon? The study of inflation naturally leads to the idea of a multiverse – the Universe is a lot larger than what we observe and different parts of the Universe have different physics. How should we think about physics in such a setup? What are the correct observables? What is the precise role of anthropic ideas in this context?

5. Theory of quantum gravity. The correct theory of quantum gravity appears to be string theory. At the moment we do not have a clear conceptual formulation of the theory, nor do we have clear experimentally verifiable predictions of string theory. Can we solve these problems? Presumably, a deeper understanding of string theory will show that space and time are emergent concepts which are not present in the fundamental formulation of the theory. This could have important implications for the mysteries of the Big Bang.

Cosmology: 5 Needed Breakthroughs -- Robert Brandenberger

[Today's guest in our ongoing feature '5 Breakthroughs' is Robert Brandenberger, Canada Research Chair and Professor of Physics of McGill University, where he taught and conducted research since 2004. Before that he was a professor at Brown University for about 18 years.

In his long career spanning about a quarter of a century (He received his PhD from Harvard University in 1983; Thesis "Topics in Quantum Field Theory and Cosmology'), Prof. Brandenberger made crucial contributions in various important subfields of cosmology (link to a list of publications).

His current research interests cover a wide spectrum of topics in cosmology and related fields and include
(A) Conceptual problems in inflationary universe cosmology, in particular, trans-Planckian problem for cosmology,
(B) Theory of cosmological perturbations, in particular, back reaction problems, evolution of perturbations in nonsingular cosmologies, and parametric amplification of fluctuations during reheating,
(C) Superstring cosmology, in particular, string gas cosmology and structure formation, mechanisms for obtaining inflation from string theory, resolution of cosmological singularities in string theory, dualities and brane gases in the early universe,
(D) Topological defects in cosmology, in particular, topological defects and Baryogenesis, topological defects and direct signatures, stabilization of embedded defects by plasma effects,
(E) Nonequilibrium processes, in particular, parametric resonance during reheating in inflationary cosmology, nonequilibrium production of topological defects,
(F) Particle-Astrophysics, in particular, constraining physics beyond the Standard Model using cosmology, new mechanisms for CP violation and Baryogenesis,
(G) Large-scale structure, in particular, use of topological statistics to analyze large-scale redshift surveys, studies of weak gravitational lensing maps using new statistics.
(H) Formation of structure in the early universe, in particular, coupling of adiabatic and entropy fluctuations in multi-field, and cosmological models.

In March, 2008 issue of 'Physics Today', Prof. Brandenberger presented an excellent account of current status of inflationary cosmology in his article 'Alternatives to cosmological inflation' (article link here).

Prof. Brandenberger is a Fellow of the American Physical Society. He was an Alfred P. Sloan Research Fellow in years 1988-1992 and received the Outstanding Junior Investigator award of Department of Energy in years 1988-1991.

It gives us great pleasure to present this list of 5 most important breakthroughs that Prof. Brandenberger would like to see in Cosmology.
-- 2Physics.com ]

Breakthrough 1:
Solution of the (old) cosmological constant problem: why is the cosmological constant not given by the cut off scale of relativistic quantum field theory?

Breakthrough 2:
Solution of the new cosmological constant problem: why is there an apparent cosmological constant which is beginning to dominate the evolution of the universe at the current cosmological epoch?

Breakthrough 3:
Resolution of the cosmological singularity: without resolving the cosmological singularity a cosmological model will always be incomplete. Standard Big Bang cosmology had to be replaced by a new early universe cosmology because of this problem. The current paradigm, scalar field-driven inflationary cosmology still suffers from this problem and is therefore incomplete.

Breakthrough 4:
Non-perturbative understanding of superstring theory: will lead to a new cosmological model of the very early universe which will either yield a convincing realization of inflationary cosmology or else to an alternative model.

Breakthrough 5:
An observational discovery of a cosmic superstring: this will cement the link between string theory and cosmology and will also lead to a new theory of the very early universe.

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 -- Pierre Ramond

Pierre Ramond [photo courtesy: University of Florida, Gainesville]

[ Prof. Pierre Ramond, Distinguished Professor of Physics at University of Florida in Gainesville, is today's guest in our ongoing feature '5-Breakthroughs'.

During his long career starting with the PhD work at Syracuse University in 1969, Prof. Ramond contributed in some significant developments in the study of elementary particles and fields. Notable among those is the crucial role he played in the early development of superstring theory.

Early string theory proposed by Yoichiro Nambu and others in 1970 was based on bosonic string. At that point, Pierre Ramond took the crucial step of generalizing the Virasoro algebra, the symmetry algebra of the bosonic string, to a superconformal algebra including anticommuting operators. The inclusion of a fermionic string to accompany the bosonic ones completed the theory of strings. In 1971, he generalized Dirac's equation for point-like particles to string-like ones, which laid a solid foundation for the superstring theory. A comprehensive list of the variety of work he did can be found in Google Scholar link.

Prof. Ramond is a Fellow of American Physical Society and American Academy of Arts & Sciences. In August 2004, he was awarded Oskar Klein Medal by Swedish Royal Academy of Sciences and Stockholm University.

Many of us grew up with his celebrated book "Field Theory: A Modern Primer" (Addison / Wesley, 1981) and also experienced the pleasure of "Journeys Beyond the Standard Model"(Perseus, 1999), his other book. It's thus our pleasure to present the 5 most important breakthroughs that Prof. Ramond would like to see in High Energy Physics.
-- 2Physics.com Team]

Here is my list of five:

Finding Supersymmetry with the Large Hadronic Collider, and if found, understanding Supersymmetry breaking.

Understanding why there are three chiral families of Elementary Particles (closely related to finding the organizing principle behind chiral symmetry breaking, e.g. Yukawa interactions).

Observation of Proton Decay in the Laboratory.

Determining the Character (Majorana or Dirac) of Neutrino masses.

Identifying Dark Matter.

Superstring Theory: 5 Needed Breakthroughs -- John H. Schwarz

John H. Schwarz (photo credit: Patricia Schwarz)

[In 2001, Prof. John Preskill of Caltech wrote a poem "To John Schwarz" (full text here) which started as ..

"Thirty years ago or more
John saw what physics had in store.
He had a vision of a string
And focused on that one big thing."

The name 'Schwarz' is intimately associated with the origin and evolution of Superstring theory. In 1971 John Schwarz and André Neveu developed an early version of superstring theory, which led among other things to the discovery of supersymmetry. In 1974 Joël Scherk and he proposed that string theory should be used to construct a unified quantum theory containing gravitation.

In 1984 Michael Green and he discovered an anomaly cancellation mechanism, which resulted in string theory becoming one of the hottest areas in theoretical physics. As Prof. Preskill's poem describes:

If you weren't there you couldn't know
The impact of that mightly blow:
"The Green-Schwarz theory could be true ---
It works for S-O-thirty-two!"

John Schwarz is the Harold Brown Professor of Theoretical Physics at California Institute of Technology (Caltech) where he taught and conducted research since 1972. He received the Dirac Medal of the International Centre for Theoretical Physics, Trieste in 1989, as well as the Dannie Heineman Prize for Mathematical Physics of the American Physical Society in 2002. He was a fellow of the MacArthur Foundation in 1987 and in 1997 he was elected to the National Academy of Sciences.

Prof. Schwarz coauthored a new string theory textbook entitled `String Theory and M-Theory: A Modern Introduction,' which was published earlier this year by Cambridge University Press.

We can't resist ending this note quoting again from John Preskill's poem:

Because he never would give in,
Pursued his dream with discipline,
John Schwarz has been a hero to me.
So please, don't spell it with a "t"!

Ladies and Gentlemen, it's our honor and privilege to share with you the excitement of superstring theory by presenting this list of 5 breakthroughs that John Schwarz would like to see.
-- 2Physics.com Team]

5 most important breakthroughs that I would like to see in
SUPERSTRING THEORY
by John H. Schwarz

(1) Discovery of supersymmetry at the Large Hadron Collider (LHC):

Supersymmetry is an intrinsic feature of superstring theory, and therefore I am convinced that it exists at a fundamental level. The big question is whether it is broken at a sufficiently low energy (the TeV scale) that supersymmetry partner particles can be discovered at the LHC. There are several well-known arguments for why this is likely. Discovery of supersymmetry would not prove that superstring theory is the correct fundamental theory and nondiscovery would not prove that it is wrong. Still, if it is discovered, string theory would deserve credit for spawning the study of supersymmetry in the first place.

The experimental discovery of superpartner particles (and hence supersymmetry)would be very exciting for several reasons: It would set the agenda for experimental particle physics for decades to come ensuring the vitality of high-energy physics research. It would be enormously informative, leading eventually to the formulation of a "supersymmetric standard model'' extending the current standard model to much higher energies. Such a supersymmetric standard model would provide a much better target for string theorists to try to relate to Planck scale physics, where string theory is most directly applicable, by "top-down reasoning". String theorists would like to predict all of this in advance, of course,but that does not seem to be possible.

(2) Other experimental evidence for string theory:

Aside from supersymmetry, there are a number of other possible experimental signals for string theory that have been considered, and there may be others that nobody has thought of yet. In my opinion, all of the following are unlikely to be observed, because the Planck scale (the natural energy scale of quantum gravity) is so far beyond what is experimentally accessible. However, there are scenarios in which quantum gravity phenomena can extend to much lower energies, and thereby possibly become observable, which certainly are worth exploring. The methodologies for making such a discovery fall into two broad categories: astronomical/cosmological observations and accelerator experiments. The first category can look for cosmic strings, primordial gravity waves, and certain subtle features of the cosmic microwave background. Accelerators, such as the LHC, can look for signals indicating the presence of extra dimensions, black holes, gravitons, or fundamental strings.

(3) More fundamental formulation of string theory/M-theory; emergent spacetime :

The current understanding of string theory is based on perturbation theory expansions of various symmetrical limits supplemented by a beautiful web of conjectured duality relations. What is missing is a single complete formulation of the theory that accounts for these various symmetrical limits and dualities. Such a formulation is likely to implement some deep principle that has not yet been recognized. It is also likely to be completely unique without any adjustable parameters or other features that can be altered.

There are various reasons to believe that the existence of space and time is not something built into the theory itself, but rather emerges as a property of certain classes of solutions. If this is correct, the theory will be radically different from any previous physical theory all of which describe what happens in a given spacetime. Even Einstein's theory of gravity (the general theory of relativity), in which the geometry of spacetime is determined dynamically,assumes the prior existence of a spacetime manifold.

(4) Determine whether time is emergent and clarify the status of quantum mechanics:

The previous item suggested that space and time are emergent properties of solutions to string theory rather than intrinsic features of the underlying theory. There is considerable evidence for the emergence of spatial dimensions in various settings, but there is no compelling evidence for the emergence of time. Experience with relativity makes it hard to imagine that space and time could be radically different in this regard. On the other hand, the notion of time is central in quantum mechanics, which is formulated as unitary time evolution. If time is emergent, some extension of the rules of quantum mechanics would seem to be required. The consistency of string theory requires that quantum mechanics is exactly correct. I am not questioning that this will continue to be the case in the future, only that quantum mechanics may need to be generalized somewhat to extend its domain of applicability.

(5) Determine the correct solution of the theory:

A unique equation can have many different solutions. By the same token, string theory can describe a rich variety of physical realities. We are still in the early stages of mapping out the possibilities, but the indications are that the number of possibilities is enormous. The picture that has been proposed, whose validity is not completely evident, is that there is an energy function that is a complicated function of many variables (called moduli) and that each of the minima of this function corresponds to a different solution of the theory. Assuming its validity, this picture raises a lot of questions: How is the "correct'' solution (i.e., the one that describes the Universe that we observe) determined? Is it a cosmological accident or is there some other principle? How can we determine the correct solution? How much empirical information needs to be input in order to determine it uniquely and make everything else computable (in principle)? These types of questions are very important to explore. They are stimulating a lot of serious research, as well as some spirited debate that is even spilling over into the public domain.