90 research outputs found

### Visualizing curved spacetime

I present a way to visualize the concept of curved spacetime. The result is a
curved surface with local coordinate systems (Minkowski Systems) living on it,
giving the local directions of space and time. Relative to these systems,
special relativity holds. The method can be used to visualize gravitational
time dilation, the horizon of black holes, and cosmological models. The idea
underlying the illustrations is first to specify a field of timelike
four-velocities. Then, at every point, one performs a coordinate transformation
to a local Minkowski system comoving with the given four-velocity. In the local
system, the sign of the spatial part of the metric is flipped to create a new
metric of Euclidean signature. The new positive definite metric, called the
absolute metric, can be covariantly related to the original Lorentzian metric.
For the special case of a 2-dimensional original metric, the absolute metric
may be embedded in 3-dimensional Euclidean space as a curved surface.Comment: 15 pages, 20 figure

### A covariant formalism of spin precession with respect to a reference congruence

We derive an effectively three-dimensional relativistic spin precession
formalism. The formalism is applicable to any spacetime where an arbitrary
timelike reference congruence of worldlines is specified. We employ what we
call a stopped spin vector which is the spin vector that we would get if we
momentarily make a pure boost of the spin vector to stop it relative to the
congruence. Starting from the Fermi transport equation for the standard spin
vector we derive a corresponding transport equation for the stopped spin
vector. Employing a spacetime transport equation for a vector along a
worldline, corresponding to spatial parallel transport with respect to the
congruence, we can write down a precession formula for a gyroscope relative to
the local spatial geometry defined by the congruence. This general approach has
already been pursued by Jantzen et. al. (see e.g. Jantzen, Carini and Bini,
Ann. Phys. 215 (1997) 1), but the algebraic form of our respective expressions
differ. We are also applying the formalism to a novel type of spatial parallel
transport introduced in Jonsson (Class. Quantum Grav. 23 (2006) 1), as well as
verifying the validity of the intuitive approach of a forthcoming paper
(Jonsson, Am. Journ. Phys. 75 (2007) 463) where gyroscope precession is
explained entirely as a double Thomas type of effect. We also present the
resulting formalism in explicit three-dimensional form (using the boldface
vector notation), and give examples of applications.Comment: 27 pages, 8 figure

### Antirasismens Andra:

The category of the “immigrant student”, which is often used in the educational setting from where the following study collects its data, is indeed an elusive one – especially so when it is employed as a label of students who have no experience of migration at all. The frequent use of the category is even more confusing, considering that Swedish publicity could be described as characterized by a hegemonic anti-racist discourse, including an often reproduced master narrative of a country which – except from a few right wing extremists – is considered to be a nation without racism. Drawing on linguistic ethnographic data from a fieldwork in a high prestigious secondary school in inner city Stockholm, this article examines how narratives of an unruly classroom behavior become associated with ”the young immigrant student” category, and furthermore, how students rhetorically manage the dilemma to tell that story without sounding disparaging. The paper is an investigation of anti-racist waysoftalkingaboutfailing students as the ethnic or racial Others

### Generalizing Optical Geometry

We show that by employing the standard projected curvature as a measure of
spatial curvature, we can make a certain generalization of optical geometry
(Abramowicz and Lasota 1997, Class. Quantum Grav. 14 (1997) A23). This
generalization applies to any spacetime that admits a hypersurface orthogonal
shearfree congruence of worldlines. This is a somewhat larger class of
spacetimes than the conformally static spacetimes assumed in standard optical
geometry. In the generalized optical geometry, which in the generic case is
time dependent, photons move with unit speed along spatial geodesics and the
sideways force experienced by a particle following a spatially straight line is
independent of the velocity. Also gyroscopes moving along spatial geodesics do
not precess (relative to the forward direction). Gyroscopes that follow a
curved spatial trajectory precess according to a very simple law of
three-rotation. We also present an inertial force formalism in coordinate
representation for this generalization. Furthermore, we show that by employing
a new sense of spatial curvature (Jonsson, Class. Quantum Grav. 23 (2006) 1)
closely connected to Fermat's principle, we can make a more extensive
generalization of optical geometry that applies to arbitrary spacetimes. In
general this optical geometry will be time dependent, but still geodesic
photons move with unit speed and follow lines that are spatially straight in
the new sense. Also, the sideways experienced (comoving) force on a test
particle following a line that is straight in the new sense will be independent
of the velocity.Comment: 19 pages, 1 figure. A more general analysis is presented than in the
former version. See also the companion papers arXiv:0708.2493,
arXiv:0708.2533 and arXiv:0708.253

### An intuitive approach to inertial forces and the centrifugal force paradox in general relativity

As the velocity of a rocket in a circular orbit near a black hole increases,
the outwardly directed rocket thrust must increase to keep the rocket in its
orbit. This feature might appear paradoxical from a Newtonian viewpoint, but we
show that it follows naturally from the equivalence principle together with
special relativity and a few general features of black holes. We also derive a
general relativistic formalism of inertial forces for reference frames with
acceleration and rotation. The resulting equation relates the real experienced
forces to the time derivative of the speed and the spatial curvature of the
particle trajectory relative to the reference frame. We show that an observer
who follows the path taken by a free (geodesic) photon will experience a force
perpendicular to the direction of motion that is independent of the observers
velocity. We apply our approach to resolve the submarine paradox, which regards
whether a submerged submarine in a balanced state of rest will sink or float
when given a horizontal velocity if we take relativistic effects into account.
We extend earlier treatments of this topic to include spherical oceans and show
that for the case of the Earth the submarine floats upward if we take the
curvature of the ocean into account.Comment: 14 pages, 21 figure

### An intuitive approach to inertial forces and the centrifugal force paradox in general relativity

Abstract. As the velocity of a rocket in a circular orbit near a black hole increases, the outwardly directed rocket thrust must increase to keep the rocket in its orbit. This feature might appear paradoxical from a Newtonian viewpoint, but we show that it follows naturally from the equivalence principle together with special relativity and a few general features of black holes. We also derive a general relativistic formalism of inertial forces for reference frames with acceleration and rotation. The resulting equation relates the real experienced forces to the time derivative of the speed and the spatial curvature of the particle trajectory relative to the reference frame. We show that an observer who follows the path taken by a free (geodesic) photon will experience a force perpendicular to the direction of motion that is independent of the observers velocity. We apply our approach to resolve the submarine paradox, which regards whether a submerged submarine in a balanced state of rest will sink or float when given a horizontal velocity if we take relativistic effects into account. We extend earlier treatments of this topic to include spherical oceans and show that for the case of the Earth the submarine floats upward if we take the curvature of the ocean into account. I Introduction Consider a rocket in a circular orbit outside the event horizon of a black hole. 1 If the orbit lies within the photon radius, the radius where free photons can move on circular orbits, 2 a greater outward rocket thrust is required to keep the rocket in orbit the faster the rocket moves. However, outside of the photon radius the outward thrust decreases as the orbital speed increases just as it would for a similar scenario in Newtonian mechanics (the thrust will be inward directed for sufficiently high speeds, see Analogous to the situation in Newtonian mechanics we can introduce in general relativity a gravitational force that is velocity independent. This force is fictitious (unlike in Newtonian mechanics). We can also introduce a velocity dependent (fictitious) centrifugal force that together with the gravitational force balances the real force from the jet engine of the rocket. By this definition, the centrifugal force is directed inward inside of the photon radius and directed outward outside of the photon radius. This reversal of the direction of the fictitious centrifugal force is described by the formalism of optical geometry (see Appendix A) in which the phenomena has been discussed. We start by illustrating how the fact that the rocket thrust increases with increasing orbital speed (sufficiently close to the black hole) follows naturally from the equivalence principle (reviewed in Appendix B). We do so by first considering an idealized special relativistic scenario of a train moving relative to an (upward) accelerating platform. We then consider a more general but still effectively two-dimensional discussion of forces perpendicular to the direction of motion for motion relative to an accelerated reference frame in special relativity. In a static spacetime, the reference frame connected to the static observers behaves locally like an accelerating reference frame in special relativity and the formalism can therefore be applied also to this case. We then illustrate how to apply the formalism of this paper to the submarine paradox. Next we generalize the formalism of forces and curvature of spatial paths to include three-dimensional cases, forces parallel to the direction of motion, and rotating reference frames. The acceleration and rotation of the reference frame will be shown (as in Newtonian mechanics) to introduce terms that can be interpreted as iner-

### Inertial forces and the foundations of optical geometry

Assuming a general timelike congruence of worldlines as a reference frame, we
derive a covariant general formalism of inertial forces in General Relativity.
Inspired by the works of Abramowicz et. al. (see e.g. Abramowicz and Lasota,
Class. Quantum Grav. 14 (1997) A23), we also study conformal rescalings of
spacetime and investigate how these affect the inertial force formalism. While
many ways of describing spatial curvature of a trajectory has been discussed in
papers prior to this, one particular prescription (which differs from the
standard projected curvature when the reference is shearing) appears novel. For
the particular case of a hypersurface-forming congruence, using a suitable
rescaling of spacetime, we show that a geodesic photon is always following a
line that is spatially straight with respect to the new curvature measure. This
fact is intimately connected to Fermat's principle, and allows for a certain
generalization of the optical geometry as will be further pursued in a
companion paper (Jonsson and Westman, Class. Quantum Grav. 23 (2006) 61). For
the particular case when the shear-tensor vanishes, we present the inertial
force equation in three-dimensional form (using the bold face vector notation),
and note how similar it is to its Newtonian counterpart. From the spatial
curvature measures that we introduce, we derive corresponding covariant
differentiations of a vector defined along a spacetime trajectory. This allows
us to connect the formalism of this paper to that of Jantzen et. al. (see e.g.
Bini et. al., Int. J. Mod. Phys. D 6 (1997) 143).Comment: 42 pages, 7 figure

### Inequalities in income and education are associated with survival differences after out-of-hospital cardiac arrest : nationwide observational study

Published online: 12 November 2021Background:
Despite the acknowledged importance of socioeconomic factors as regards cardiovascular disease onset and survival, the relationship between individual-level socioeconomic factors and survival after out-of-hospital cardiac arrest is not established. Our aim was to investigate whether socioeconomic variables are associated with 30-day survival after out-of-hospital cardiac arrest.
Methods:
We linked data from the Swedish Registry for Cardiopulmonary Resuscitation with individual-level data on socioeconomic factors (ie, educational level and disposable income) from Statistics Sweden. Confounding and mediating variables included demographic factors, comorbidity, and Utstein resuscitation variables. Outcome was 30-day survival. Multiple modified Poisson regression was used for the main analyses.
Results:
A total of 31 373 out-of-hospital cardiac arrests occurring in 2010 to 2017 were included. Crude 30-day survival rates by income quintiles were as follows: Q1 (low), 414/6277 (6.6%); Q2, 339/6276 (5.4%); Q3, 423/6275 (6.7%); Q4, 652/6273 (10.4%); and Q5 (high), 928/6272 (14.8%). In adjusted analysis, the chance of survival by income level followed a gradient-like increase, with a risk ratio of 1.86 (95% CI, 1.65–2.09) in the highest-income quintile versus the lowest. This association remained after adjusting for comorbidity, resuscitation factors, and initial rhythm. A higher educational level was associated with improved 30-day survival, with the risk ratio associated with postsecondary education ≥4 years being 1.51 (95% CI, 1.30–1.74). Survival disparities by income and educational level were observed in both men and women.
Conclusions:
In this nationwide observational study using individual-level socioeconomic data, higher income and higher educational level were associated with better 30-day survival after out-of-hospital cardiac arrest in both sexes

### Development of a Model System to Identify Differences in Spring and Winter Oat

Our long-term goal is to develop a Swedish winter oat (Avena sativa). To identify molecular differences that correlate with winter hardiness, a winter oat model comprising of both non-hardy spring lines and winter hardy lines is needed. To achieve this, we selected 294 oat breeding lines, originating from various Russian, German, and American winter oat breeding programs and tested them in the field in south- and western Sweden. By assaying for winter survival and agricultural properties during four consecutive seasons, we identified 14 breeding lines of different origins that not only survived the winter but also were agronomically better than the rest. Laboratory tests including electrolytic leakage, controlled crown freezing assay, expression analysis of the AsVrn1 gene and monitoring of flowering time suggested that the American lines had the highest freezing tolerance, although the German lines performed better in the field. Finally, six lines constituting the two most freezing tolerant lines, two intermediate lines and two spring cultivars were chosen to build a winter oat model system. Metabolic profiling of non-acclimated and cold acclimated leaf tissue samples isolated from the six selected lines revealed differential expression patterns of 245 metabolites including several sugars, amino acids, organic acids and 181 hitherto unknown metabolites. The expression patterns of 107 metabolites showed significant interactions with either a cultivar or a time-point. Further identification, characterisation and validation of these metabolites will lead to an increased understanding of the cold acclimation process in oats. Furthermore, by using the winter oat model system, differential sequencing of crown mRNA populations would lead to identification of various biomarkers to facilitate winter oat breeding

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