1,082 research outputs found
Bostonia Magazine. Volume 56
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Unimodular conformal and projective relativity
We outline unimodular conformal and projective relativity (UCPR), an
extension of unimodular relativity in which the conformal and projective
structures play central roles. Under symmetry group, the
pseudo-Riemannian metric naturally decomposes into a four-volume element and a
conformal metric; and the affine connection decomposes into a one-form and a
trace-free projective connection. In UCPR, these four space-time structures are
treated as independent fields that have clear physical interpretations. A
Palatini-type variational principle for the usual general relativity Lagrangian
leads to a breakup of the Einstein field equations and the compatibility
conditions between the metric and connection. We indicate how new gravitational
theories may be generated by modifications of this Lagrangian and discuss two
such cases. Finally, we discuss possible physical consequences of our results
for quantum gravity.Comment: 6 pages. To appear in EPL. Abstract shortened. Equation typos fixed.
Two references added since v
O manuscrito de Einstein de 1912 como pista para o desenvolvimento da teoria da relatividade restrita
One examines the unpublished manuscript written by Einstein in 1912, which reviews the theory of special relativity. Some clues are given in the manuscript for how Einstein developed his ideas before 1905. In particular, the explanation given for Fizeau's experiment indicates how Einstein abandoned all resistance to the principle of the constancy of the speed of light, in spite of its apparent incompatibility with the relativity principle.Examina-se o manuscrito escrito por Einstein em 1912, mas nĂŁo publicado, que faz uma revisĂŁo da teoria da relatividade restrita. Nele, identificam-se algumas pistas sobre como Einstein desenvolveu suas idĂ©ias antes de 1905. Em particular, a explicação dada para o experimento de Fizeau indica como Einstein pĂ´de manter o princĂpio de constância da velocidade da luz, mesmo parecendo ser incompatĂvel com o princĂpio de relatividade
Measurement of the space-time interval between two events using the retarded and advanced times of each event with respect to a time-like world-line
Several recent studies have been devoted to investigating the limitations
that ordinary quantum mechanics and/or quantum gravity might impose on the
measurability of space-time observables. These analyses are often confined to
the simplified context of two-dimensional flat space-time and rely on a simple
procedure for the measurement of space-like distances based on the exchange of
light signals. We present a generalization of this measurement procedure
applicable to all three types of space-time intervals between two events in
space-times of any number of dimensions. We also present some preliminary
observations on an alternative measurement procedure that can be applied taking
into account the gravitational field of the measuring apparatus, and briefly
discuss quantum limitations of measurability in this context.Comment: 17 page
Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?
Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4–16 wt%) of garnets and the Mg# (69–92) and jadeite contents (14–48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39–40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere–transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine
Thoughts on heavy-ion physics in the high luminosity era: the soft sector
This document summarizes thoughts on opportunities in the soft-QCD sector
from high-energy nuclear collisions at high luminosities.Comment: 19 page
Thoughts on opportunities from high-energy nuclear collisions
This document summarizes thoughts on opportunities from high-energy nuclear
collisions.Comment: 10 pages, pd
- …