arXiv subjects

Mark D. Roberts

Publications and source records attributed to Mark D. Roberts.

At least 37 records · Page 2Linked to original sources

arXiv2007

Frame Dependence in Scalar-tensor Theory

Palatini variation of Jordan frame lagrangians gives an equation relating the dilaton to the object of non-metricity and hence the existence of the dilaton implies that the spacetime connection is more general than that given soley by the Christoffel symbol of general relativity. Transferring from Jordan to Einstein frame, which connection, lagrangian, field equations and stress conservation equations occur are discussed: it is found that the Jordan frame has more information, this can be expressed in several ways, the simplest is that the extra information corresponds to the function multiplying the Ricci scalar in the action. The Einstein frame has the advantages that stress conservation implies no currents and that the field equations are easier to work with. This is illustrated by application to Robertson-Walker spacetime.

gr-qc↗

arXiv2006

Strings and Unified Field Theory

It is argued that string theory predicts unified field theory rather than general relativity coupled to matter fields. In unified field theory all the objects are geometrical, for strings the Kalb-Ramond matter field is identical to the asymmetric part of the metric except that the fields contribute to different sides of the field equations. The dilaton is related to the object of non-metricity.

hep-th↗

arXiv2004

Interacting with the fifth dimension

Some new five dimensional minimal scalar-Einstein exact solutions are presented. These new solutions are tested against various criteria used to measure interaction with the fifth dimension.

gr-qc↗

arXiv2004

Does Meaning Evolve?

A common method of making a theory more understandable, is by comparing it to another theory which has been better developed. Radical interpretation is a theory which attempts to explain how communication has meaning. Radical interpretation is treated as another time-dependent theory and compared to the time dependent theory of biological evolution. The main reason for doing this is to find the nature of the time dependence; producing analogs between the two theories is a necessary prerequisite to this and brings up many problems. Once the nature of the time dependence is better known it might allow the underlying mechanism to be uncovered. Several similarities and differences are uncovered, there appear to be more differences than similarities.

cs.CL↗

arXiv2003

A Generalized Higgs Model

The Higgs model is generalized so that in addition to the radial Higgs field there are fields which correspond to the themasy and entropy. The model is further generalized to include state and sign parameters. A reduction to the standard Higgs model is given and how to break symmetry using a non-zero VEV (vacuum expectation value) is shown. A 'fluid rotation' can be performed on the standard Higgs model to give a model dependant on the entropy and themasy and with a constant mass.

hep-th↗

arXiv2003

Quantizing the line element field

A metric with signature (-+++) can be constructed from a metric with signature (++++) and a double-sided vector field called the line element field. Some of the classical and quantum properties of this vector field are studied.

gr-qc↗

arXiv2002

Galactic Metrics

The paths of stars in galaxies have circular velocity independent of their distance from the centre of the galaxy. Newtonian mechanics with a logarithmic potential has such paths. In relativity these paths can be taken to be geodesics and this requirement places a resticting equation on the spacetime metric. This equation has a non-unique spherically symmetric solution that in the newtonian limit has a logarithmic potential. It also can be solved in terms of a conformal factor. In particular it has solutions which are conformal to the vacuum-Einstein solutions and such spacetimes are solutions to the vacuum-Bach equations. Therefore it is suggested that the Bach equations describe dynamics on galactic lenght scales.

astro-ph↗

arXiv2002

The Rotation and Shear of a String

Whether a string has rotation and shear can be investigated by an anology with the point particle. Rotation and shear involve first covariant spacetime derivatives of a vector field and, because the metric stress tensor for both the point particle and the string have no such derivatives, the best vector fields can be identified by requiring the conservation of the metric stress. It is found that the best vector field is a non-unit accelerating field in x, rather than a unit non-accelerating vector involving the momenta; it is also found that there is an equation obeyed by the spacetime derivative of the Lagrangian. The relationship between membranes and fluids is looked at.

hep-th↗

arXiv2002

Spacetime Exterior to a Star: Against Asymptotic Flatness

In many circumstances the perfect fluid conservation equations can be directly integrated to give a Geometric-Thermodynamic equation: typically that the lapse $N$ is the reciprocal of the enthalphy $h$, ($ N=1/h$). This result is aesthetically appealing as it depends only on the fluid conservation equations and does not depend on specific field equations such as Einstein's. Here the form of the Geometric-Thermodynamic equation is derived subject to spherical symmetry and also for the shift-free ADM formalism. There at least three applications of the Geometric-Thermodynamic equation, the most important being to the notion of asympotic flatness and hence to spacetime exterior to a star. For asymptotic flatness one wants $h\to 0$ and $N\to 1$ simultaneously, but this is incompatible with the Geometric-Thermodynamic equation. Observational data and asymptotic flatness are discussed. It is argued that a version of Mach's principle does not allow asymptotic flatness.

gr-qc↗

arXiv2001

Vacuum Energy

There appears to be three, perhaps related, ways of approaching the nature of vacuum energy . The first is to say that it is just the lowest energy state of a given, usually quantum, system. The second is to equate vacuum energy with the Casimir energy. The third is to note that an energy difference from a complete vacuum might have some long range effect, typically this energy difference is interpreted as the cosmological constant. All three approaches are reviewed, with an emphasis on recent work. It is hoped that this review is comprehensive in scope. There is a discussion on whether there is a relation between vacuum energy and inertia. The solution suggested here to the nature of the vacuum is that Casimir energy can produce short range effects because of boundary conditions, but that at long range there is no overall effect of vacuum energy, unless one considers lagrangians of higher order than Einstein's as vacuum induced. No original calculations are presented in support of this position. This is not a review of the cosmological constant {\it per se}, but rather vacuum energy in general, my approach to the cosmological constant is not standard.

hep-th↗

arXiv2001

Ultrametric Distance in Syntax

Phrase structure trees have a hierarchical structure. In many subjects, most notably in Taxonomy such tree structures have been studied using ultrametrics. Here syntactical hierarchical phrase trees are subject to a similar analysis, which is much siompler as the branching structure is more readily discernible and switched. The occurence of hierarchical structure elsewhere in linguistics is mentioned. The phrase tree can be represented by a matrix and the elements of the matrix can be represented by triangles. The height at which branching occurs is not prescribed in previous syntatic models, but it is by using the ultrametric matrix. The ambiguity of which branching height to choose is resolved by postulating that branching occurs at the lowest height available. An ultrametric produces a measure of the complexity of sentences: presumably the complexity of sentence increases as a language is aquired so that this can be tested. A All ultrametric triangles are equilateral or isocles, here it is shown that X structur implies that there are no equilateral triangles. Restricting attention to simple syntax a minium ultrametric distance between lexical categories is calculatex. This ultrametric distance is shown to be different than the matrix obtasined from feaures. It is shown that the definition of c-commabnd can be replaced by an equivalent ultrametric definition. The new definition invokes a minimum distance between nodes and this is more aesthetically satisfing than previouv varieties of definitions. From the new definition of c-command follows a new definition of government.

cs.CL↗

arXiv2001

P-model Alternative to the T-model

Standard linguistic analysis of syntax uses the T-model. This model requires the ordering: D-structure $>$ S-structure $>$ LF. Between each of these representations there is movement which alters the order of the constituent words; movement is achieved using the principles and parameters of syntactic theory. Psychological serial models do not accommodate the T-model immediately so that here a new model called the P-model is introduced. Here it is argued that the LF representation should be replaced by a variant of Frege's three qualities. In the F-representation the order of elements is not necessarily the same as that in LF and it is suggested that the correct ordering is: F-representation $>$ D-structure $>$ S-structure. Within this framework movement originates as the outcome of emphasis applied to the sentence.

cs.CL↗

arXiv2000

Second Variations of String Actions

A short article describing the derivation of the string deviation equation: $\dotΠ^ν_τ+Π'^μ_σ=R^ν_{.αβγ}r^γ(P^β_τ\dot{x}^α+P^β_σx'^α)$.

hep-th↗

arXiv2000

Quantum Perturbative Approach to Discrete Redshift

On the largest scales there is evidence of discrete structure, examples of this are superclusters and voids and also by redshift taking discrete values. In this paper it is proposed that discrete redshift can be explained by using the spherical harmonic integer l; this occurs both in the metric or density perturbations and also in the solution of wave equations in Robertson-Walker spacetime. It is argued that the near conservation of energy implies that l varies regularly for wave equations in Robertson-Walker spacetime, whereas for density perturbations l cannot vary regularly. Once this is assumed then perhaps the observed value of discrete redshift provides the only observational or experimental data that directly requires an explanation using both gravitational and quantum theory. In principle a model using this data could predict the scale factor R (or equivalently the deceleration parameter q). Solutions of the Klein-Gordon equation in Robertson-Walker spacetimes are used to devise models which have redshift taking discrete values, but they predict a microscopic value for R. A model in which the stress of the Klein-Gordon equation induces a metrical perturbation of Robertson-Walker spacetime is devised. Calculations based upon this model predict that the Universe is closed with $2q_0-1=10^{-4}$.

astro-ph↗

arXiv1999

The String Deviation Equation

The relative motion of many particles can be described by the geodesic deviation equation. This can be derived from the second covariant variation of the point particle's action. It is shown that the second covariant variation of the string action leads to a string deviation equation.

gr-qc↗

arXiv1999

The World Function in Robertson-Walker Spacetime

A method for finding the world function of Robertson-Walker spacetimes is presented. It is applied to find the world function for the $k=0, \ga=2$, solution. The close point approximation for the Robertson-Walker world function is calculated upto fourth order.

gr-qc↗

arXiv1999

Imploding Scalar Fields

Static spherically symmetric uncoupled scalar space-times have no event horizon and a divergent Kretschmann singularity at the origin of the coordinates. The singularity is always present so that non-static solutions have been sought to see if the singularities can develop from an initially singular free space-time. In flat space-time the Klein-Gordon equation $\Box\ph=0$ has the non-static spherically symmetric solution $\ph=\si(v)/r$, where $\si(v)$ is a once differentiable function of the null coordinate $v$. In particular the function $\si(v)$ can be taken to be initially zero and then grow, thus producing a singularity in the scalar field. A similar situation occurs when the scalar field is coupled to gravity via Einstein's equations; the solution also develops a divergent Kretschmann invariant singularity, but it has no overall energy. To overcome this Bekenstein's theorems are applied to give two corresponding conformally coupled solutions. One of these has positive ADM mass and has the properties: i) it develops a Kretschmann invariant singularity, ii)it has no event horizon, iii)it has a well-defined source, iv)it has well-defined junction condition to Minkowski space-time, v)it is asymptotically flat with positive overall energy. This paper presents this solution and several other non-static scalar solutions. The properties of these solutions which are studied are limited to the following three: i)whether the solution can be joined to Minkowski space-time, ii)whether the solution is asymptotically flat, iii)and if so what the solutions' Bondi and ADM masses are.

gr-qc↗

arXiv1999

Symmetry Breaking using Fluids II: Velocity Potential Method

A generalization of scalar electrodynamics called fluid electrodynamics is presented. In this theory a fluid replaces the Higgs scalar field. Fluid electrodynamics might have application to the theory of low temperature Helium superfluids, but here it is argued that it provides an alternative method of approaching symmetry breaking in particle physics. The method of constructing fluid electrodynamics is to start with the velocity decomposition of a perfect fluid as in general relativity. A unit vector tangent to the flow lines of an isentropic fluid can be written in terms of scalar potentials: $V_a=h^{-1}(\ph_a+\al\bt_a-þS)$. A novel interacting charged fluid can be obtained by applying the covariant derivative: $D_a=\p_a+ieA_a$ to these scalar potentials. This fluid is no longer isentropic and there are choices for which it either obeys the second law of thermodynamics or not. A mass term of the correct sign occurs for the $A$ term in the stress, and this mass term depends on the potentials in the above vector. The charged fluid can be reduced to scalar electrodynamics and the standard approach to symmetry breaking applied; alternatively a mass can be induced by the fluid by using just the thermodynamic potentials and then fixing at a critical point, if this is taken to be the Bose condensation point then the induced mass is negligible.

hep-th↗