Fermi Coordinates and Penrose Limits

We propose a formulation of the Penrose plane wave limit in terms of null Fermi coordinates. This provides a physically intuitive (Fermi coordinates are direct measures of geodesic distance in space-time) and manifestly covariant description of the expansion around the plane wave metric in terms of components of the curvature tensor of the original metric, and generalises the covariant description of the lowest order Penrose limit metric itself, obtained in hep-th/0312029. We describe in some detail the construction of null Fermi coordinates and the corresponding expansion of the metric, and then study various aspects of the higher order corrections to the Penrose limit. In particular, we observe that in general the first-order corrected metric is such that it admits a light-cone gauge description in string theory. We also establish a formal analogue of the Weyl tensor peeling theorem for the Penrose limit expansion in any dimension, and we give a simple derivation of the leading (quadratic) corrections to the Penrose limit of AdS_5 x S^5.Comment: 25 page

ILR Impact Brief - The Sources of International Differences in Wage Inequality

Wage inequality in the U.S. exceeds that of Canada, Denmark, Finland, Italy, Netherlands, Norway, Sweden, and Switzerland. Some researchers have pointed to the higher relative rewards for higher cognitive skill and more education in the U.S. as an important cause of this difference; others emphasize the greater diversity of labor market skills within the American population. This paper uses recently collected international data on cognitive skills, earnings, age, and years of formal schooling to assess the relative importance of population heterogeneity and higher relative pay for more cognitive skill in explaining higher U.S. wage inequality

On the Hagedorn Behaviour of Singular Scale-Invariant Plane Waves

As a step towards understanding the properties of string theory in time-dependent and singular spacetimes, we study the divergence of density operators for string ensembles in singular scale-invariant plane waves, i.e. those plane waves that arise as the Penrose limits of generic power-law spacetime singularities. We show that the scale invariance implies that the Hagedorn behaviour of bosonic and supersymmetric strings in these backgrounds, even with the inclusion of RR or NS fields, is the same as that of strings in flat space. This is in marked contrast to the behaviour of strings in the BFHP plane wave which exhibit quantitatively and qualitatively different thermodynamic properties.Comment: 15 pages, LaTeX2e, v2: JHEP3.cls, one reference adde

Substitution Between Individual and Cultural Capital: Pre-Migration Labor Supply, Culture and US Labor Market Outcomes Among Immigrant Woman

In this paper we use New Immigrant Survey data to investigate the impact of immigrant women's own labor supply prior to migrating and female labor supply in their source country to provide evidence on the role of human capital and culture in affecting their labor supply and wages in the United States. We find, as expected, that women who migrate from countries with relatively high levels of female labor supply work more in the United States. Moreover, most of this effect remains when we further control for each woman’s own labor supply prior to migrating, which itself also strongly affects labor supply in the United States. Importantly, we find a significantly negative interaction between pre-migration labor supply and source country female labor supply. We obtain broadly similar effects analyzing the determinants of hourly earnings among the employed in the United States, although the results are not always significant. These results suggest an important role for culture and norms in affecting immigrant women's labor supply, since the effect of source country female labor supply on immigrant women's US work hours is still strong even controlling for the immigrant’s own pre-migration labor supply. The negative interaction effects between previous work experience and source country female labor supply on women's US work hours and wages suggest that cultural capital and individual job-related human capital act as substitutes in affecting preparedness for work in the US.gender, immigration, labor supply, human capital

Scalar Field Probes of Power-Law Space-Time Singularities

We analyse the effective potential of the scalar wave equation near generic space-time singularities of power-law type (Szekeres-Iyer metrics) and show that the effective potential exhibits a universal and scale invariant leading x^{-2} inverse square behaviour in the ``tortoise coordinate'' x provided that the metrics satisfy the strict Dominant Energy Condition (DEC). This result parallels that obtained in hep-th/0403252 for probes consisting of families of massless particles (null geodesic deviation, a.k.a. the Penrose Limit). The detailed properties of the scalar wave operator depend sensitively on the numerical coefficient of the x^{-2}-term, and as one application we show that timelike singularities satisfying the DEC are quantum mechanically singular in the sense of the Horowitz-Marolf (essential self-adjointness) criterion. We also comment on some related issues like the near-singularity behaviour of the scalar fields permitted by the Friedrichs extension.Comment: v2: 21 pages, JHEP3.cls, one reference adde

PP-wave and Non-supersymmetric Gauge Theory

We extend the pp-wave correspondence to a non supersymmetric example. The model is the type 0B string theory on the pp-wave R-R background. We explicitly solve the model and give the spectrum of physical states. The field theory counterpart is given by a sector of the large N SU(N) x SU(N) CFT living on a stack of N electric and N magnetic D3-branes. The relevant effective coupling constant is g_{eff}=g_sN/J^2. The string theory has a tachyon in the spectrum, whose light-cone energy can be exactly computed as a function of g_{eff}. We argue that the perturbative analysis in g_{eff} in the dual gauge theory is reliable, with corrections of non perturbative type. We find a precise state/operator map, showing that the first perturbative corrections to the anomalous dimensions of the operators have the behavior expected from the string analysis.Comment: 19 pages. Revised versio

Gender and Youth Employment Outcomes: The US and West Germany, 1984-91

This paper examines gender differences in labor market outcomes for hard-to-employ youth in the US and West Germany during the 1984-91 period. We find that young, less educated American men and especially women are far less likely to be employed than their German counterparts. Moreover, less educated young women and men in the United States have lower earnings relative to more highly educated youth in their own country, and also fare much worse than less educated German youth in absolute terms, correcting for purchasing power. The relatively high employment rates of less educated German youth combined with their relatively high wages raise the question of how they are successfully absorbed into the labor market. We present evidence that the large public sector in Germany in effect functions as an employer of last resort, absorbing some otherwise unemployable low skilled youth. Our findings also suggest that the US welfare system accounts for very little of the US-German difference in employment rates.

International Differences in Male Wage Inequality: Institutions versus Market Forces

While changes in the demand for skilled labor appear to have led to a widening of the wage structures in many countries during the 198Os,considerable differences in the level of wage inequality remain. In this paper, we examine the sources of these differences, focusing primarily on explaining the considerably higher level of wage inequality in the U.S. We find that the greater overall dispersion of the U.S. wage distribution reflects considerably more compression at the bottom of the distribution in the other countries, but relatively little difference in the degree of wage inequality at the top. While differences in the distribution of measured characteristics help to explain some aspects of the international differences, U.S. labor market prices--that is, higher rewards to labor market skills-are an important factor. Labor market institutions, chiefly the relatively decentralized wage-setting mechanisms in the U.S. compared to other countries, appear to provide the most persuasive explanation for these international differences in prices. In contrast, the pattern of cross-country differences in relative supplies of and demands for skills does not appear to be consistent with the pattern of observed differences in wage inequality.

Do Cognitive Test Scores Explain Higher U.S. Wage Inequality?

Using microdata from the 1994-8 International Adult Literacy Survey (IALS) for nine countries, we examine the role of cognitive skills in explaining higher wage inequality in the United States. We find that while the greater dispersion of cognitive test scores in the United States plays a part in explaining higher U.S. wage inequality, higher labor market prices (i.e., higher returns to measured human capital and cognitive performance) and greater residual inequality still play important roles, and are, on average, quantitatively considerably more important than differences in the distribution of test scores in explaining higher U.S. wage inequality

The Supply of Quality in Child Care Centers

We use data from a sample of day care centers to estimate the relationships between cost and the quality of the child care service provided, and between revenue and quality. We use a measure of child care quality derived from an instrument designed by developmental psychologists. This measure of quality has been found to be positively associated with child development. Taking the estimated cost-quality and revenue-quality relationships as given, we then estimate the objective functions of the firms and compute the supply function for quality. The results indicate that (1) the estimated cost function is inconsistent with the implications of cost-minimization; (2) for-profit firms operate at a positive level of marginal cost, but non-profit firms operate at zero or negative marginal cost; (3) revenue is positively but weakly associated with quality; and (4) the supply of quality is inelastic, with point estimates of the supply elasticity of .04-.05 for both for-profit and non-profit firms. Implications of the results for child care policy are discussed.