Artículos con la etiqueta ‘High Energy Physics – Theory (hep-th)’

The Large N Limit of Superconformal Field Theories and Supergravity

Por • 7 jul, 2013 • Category: Ciencia y tecnología

We show that the large $N$ limit of certain conformal field theories in various dimensions include in their Hilbert space a sector describing supergravity on the product of Anti-deSitter spacetimes, spheres and other compact manifolds. This is shown by taking some branes in the full M/string theory and then taking a low energy limit where the field theory on the brane decouples from the bulk. We observe that, in this limit, we can still trust the near horizon geometry for large $N$. The enhanced supersymmetries of the near horizon geometry correspond to the extra supersymmetry generators present in the superconformal group (as opposed to just the super-Poincare group). The ‘t Hooft limit of 4-d ${\cal N} =4$ super-Yang-Mills at the conformal point is shown to contain strings: they are IIB strings. We conjecture that compactifications of M/string theory on various Anti-deSitter spacetimes are dual to various conformal field theories. This leads to a new proposal for a definition of M-theory which could be extended to include five non-compact

The World as a Hologram

Por • 27 jun, 2013 • Category: Ciencia y tecnología

According to ‘t Hooft the combination of quantum mechanics and gravity requires the three dimensional world to be an image of data that can be stored on a two dimensional projection much like a holographic image. The two dimensional description only requires one discrete degree of freedom per Planck area and yet it is rich enough to describe all three dimensional phenomena. After outlining ‘t Hooft’s proposal I give a preliminary informal description of how it may be implemented. One finds a basic requirement that particles must grow in size as their momenta are increased far above the Planck scale. The consequences for high energy particle collisions are described. The phenomena of particle growth with momentum was previously discussed in the context of string theory and was related to information spreading near black hole horizons. The considerations of this paper indicate that the effect is much more rapid at all but the earliest times. In fact the rate of spreading is found to saturate the bound from causality.

Why all these prejudices against a constant?

Por • 9 may, 2013 • Category: Educacion

The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the «great mystery» of the «nature of a dark energy». We argue that these arguments are unconvincing, or ill-founded.

An Acataleptic Universe

Por • 27 abr, 2013 • Category: Ciencia y tecnología

John Wheeler advocated the principle that information is the foundation of physics and asked us to reformulate physics in terms of bits. The goal is to consider what we know already and work out a new mathematical theory in which space, time and matter are secondary. An application of the converse of Noether’s second theorem to the holographic principle implies that physics must have an underlying hidden symmetry with degrees of symmetry that match physical degrees of freedom in order to account for the huge redundancy of information in the interior of a black-hole. I have been working on a theory that builds infinite dimensional symmetries using layers of quantisation from information as suggested by Wheeler’s contemporary Carl von Weizs\»acker.

The strange (hi)story of particles and waves

Por • 7 abr, 2013 • Category: Educacion

Attempt of a non-technical but conceptually consistent presentation of the state of the art in quantum theory from a historical perspective. While its first part is written for a general readership, Sect. 4 may be provocative, in particular, to quantum field theorists. I argue that the wave functions of quantum mechanics, in contrast to the wave functionals of QFT, which represent the true quantum states, have to be interpreted as field modes and their combinations that are «occupied» (excited states of their corresponding quantum oscillators).

Disappearance and emergence of space and time in quantum gravity

Por • 17 feb, 2013 • Category: Ciencia y tecnología

We discuss the hints for the disappearance of continuum space and time at microscopic scale. These include arguments for a discrete nature of them or for a fundamental non-locality, in a quantum theory of gravity. We discuss how these ideas are realized in specific quantum gravity approaches. Turning then the problem around, we consider the emergence of continuum space and time from the collective behaviour of discrete, pre-geometric atoms of quantum space, and for understanding spacetime as a kind of «condensate», and we present the case for this emergence process being the result of a phase transition, dubbed «geometrogenesis». We discuss some conceptual issues of this scenario and of the idea of emergent spacetime in general. As a concrete example, we outline the GFT framework for quantum gravity, and illustrate a tentative procedure for the emergence of spacetime in this framework. Last, we re-examine the conceptual issues raised by the emergent spacetime scenario in light of this concrete example.

Is a tabletop search for Planck scale signals feasible?

Por • 28 ene, 2013 • Category: Educacion

Quantum gravity theory is untested experimentally. Could it be tested with tabletop experiments? While the common feeling is pessimistic, a detailed inquiry shows it possible to sidestep the onerous requirement of localization of a probe on Planck length scale. I suggest a tabletop experiment which, given state of the art ultrahigh vacuum and cryogenic technology, could already be sensitive enough to detect Planck scale signals. The experiment combines a single photon’s degree of freedom with one of a macroscopic probe to test Wheeler’s conception of «quantum foam», the assertion that on length scales of the order Planck’s, spacetime is no longer a smooth manifold. The scheme makes few assumptions beyond energy and momentum conservations, and is not based on a specific quantum gravity scheme.

Constraints on the Universe as a Numerical Simulation

Por • 27 dic, 2012 • Category: Opinion

Observable consequences of the hypothesis that the observed universe is a numerical simulation performed on a cubic space-time lattice or grid are explored. The simulation scenario is first motivated by extrapolating current trends in computational resource requirements for lattice QCD into the future. Using the historical development of lattice gauge theory technology as a guide, we assume that our universe is an early numerical simulation with unimproved Wilson fermion discretization and investigate potentially-observable consequences. Among the observables that are considered are the muon g-2 and the current differences between determinations of alpha, but the most stringent bound on the inverse lattice spacing of the universe, b^(-1) >~ 10^(11) GeV, is derived from the high-energy cut off of the cosmic ray spectrum. The numerical simulation scenario could reveal itself in the distributions of the highest energy cosmic rays exhibiting a degree of rotational symmetry breaking that reflects the structure of the underlying lattice.

Effective Field Theories and the Role of Consistency in Theory Choice

Por • 13 nov, 2012 • Category: Ciencia y tecnología

Promoting a theory with a finite number of terms into an effective field theory with an infinite number of terms worsens simplicity, predictability, falsifiability, and other attributes often favored in theory choice. However, the importance of these attributes pales in comparison with consistency, both observational and mathematical consistency, which propels the effective theory to be superior to its simpler truncated version of finite terms, whether that theory be renormalizable (e.g., Standard Model of particle physics) or nonrenormalizable (e.g., gravity). Some implications for the Large Hadron Collider and beyond are discussed, including comments on how directly acknowledging the preeminence of consistency can affect future theory work.

Models of Topology Change

Por • 23 oct, 2012 • Category: Crítica

We show how changes in unitarity-preserving boundary conditions allow continuous interpolation among the Hilbert spaces of quantum mechanics on topologically distinct manifolds. We present several examples, including a computation of entanglement entropy production. We discuss approximate realization of boundary conditions through appropriate interactions, thus suggesting a route to possible experimental realization. We give a theoretical application to quantization of singular Hamiltonians, and give tangible form to the «many worlds» interpretation of wave functions.