text:
\section{Introduction}

Alcubierre\cite{Alcubierre1994} advanced a model based on a specific general
relativistic spacetime geometry in which massive particles can travel at
superluminal speeds if they are located inside a specially designed spacetime
distortion. The physics of this possible propulsion method, named as
\textit{warp drive} (WD) after science fiction literature, consists of a
special spacetime metric forming a spacetime distortion, called
\textit{warp bubble}, such that it generates an expansion behind the
distortion and a contraction in front of it. That makes it possible for a
massive particle to be in a local sub-luminal speed inside the bubble, as
required by special relativity, whereas outside the particle is propelled
at superluminal speeds.  Therefore, employing of this spacetime distortion
a massive particle travels between two points in spacetime with apparent
time less than the time a light particle with zero mass would require to
travel between the same two points.

The WD metric, as originally proposed by Alcubierre, is based on a general
metric that uses the 3+1 formalism.\cite{adm1,adm2} It consists of a boost
in the direction of one of the spatial coordinates, described by the shift
vector, a function of spacetime coordinates and the product of two functions,
the velocity of the center of the warp bubble and the shape function that
controls the shape of the bubble. Although Alcubierre did not solve the
Einstein equations with his proposed spacetime geometry, he gave an example
of a shape function in the form of a hat function.\cite{Alcubierre1994} One
of the main caveats of the Alcubierre WD spacetime is the requirement of
negative mass-energy density and the violation of the dominant and weak
energy conditions.

Ford and Roman\cite{FordRoman1996} derived quantum inequalities for free
massless quantized scalar fields, electromagnetic fields, and massive
scalar fields in four-dimensional Minkowski spacetime and concluded that
these constraints have the form of an uncertainty principle limitation on
the magnitude and duration of negative mass-energy densities and that the
exotic solutions of Einstein equations such as wormholes and WD would have
significant limitations in their viability. Pfenning and
Ford\cite{Pfenning1997} followed this last work and calculated the upper
bound limits necessary for the WD viability, concluding then that the energy
required to create a warp bubble is ten orders of magnitude greater than
the total mass of the entire visible universe, also negative.

Krasnikov\cite{Krasnikov1998} approached the hyperfast interstellar travel
problem in general relativity by discussing the possibility of whether
or not a mass particle can reach a remote point in spacetime and return
sooner than a photon would. Krasnikov argued that it is not possible for a
mass particle to win this race under reasonable assumptions for globally
hyperbolic spacetimes. He discussed in detail the specific spacetime
topologies, but with the constraint that tachyons would be a requirement
for superluminal travel to occur. He conjectured a spacetime modification
device that can be used to make superluminal travel possible without
tachyons. Such spacetime was named as \textit{Krasnikov tube} by Everet
and Roman,\cite{EveretRoman1997} and they generalized the metric proposed
by Krasnikov by hypothesizing a tube along the path of the particle connecting
Earth to a distant star.

The Krasnikov metric cannot shorten the time for a one-way trip from Earth
to a distant star, but it can make the time for a round trip arbitrarily
short for clocks on Earth. However, the Everet and Roman\cite{EveretRoman1997}
extension of the Krasnikov metric has the property that inside the tube
the spacetime is flat and the lightcones are opened in such a way that
they allow the superluminal travel in one direction. They also mentioned
that even though the Krasnikov tube does not involve closed timelike curves
it is possible to construct a time machine with a system of two
non-overlapping tubes, and demonstrated that the Krasnikov tube also
requires thin layers of negative energy density and large total negative
energies.

Lobo and Crawford\cite{Lobo2002} discussed the Krasnikov metric in detail,
addressed the violation of the weak energy condition and the viability
of superluminal travel. These authors concluded that with the imposition of
the weak energy condition the Olum theorem\cite{Olum1998} prohibits
superluminal travel and pointed out the necessity of further research on
spacetimes with closed timelike curves and the need for a precise definition
of superluminal travel. They argued that one can construct metrics that
allow superluminal travel but are flat Minkowski spacetimes.
The quantum inequalities, brought from quantum field theory,
\cite{EveretRoman1997} were also discussed. Van de Broeck\cite{Broeck1999}
showed how a minor modification of the Alcubierre WD geometry can reduce the
total energy required for the creation of a warp bubble. He presented a
modification of the original WD metric where the total negative mass would
be of the order of just a few solar masses.

Nat\'ario \cite{Natario2002} proposed a new version of the WD theory with
zero expansion and questioned the effects that occur in the WD in
opposition to his newly proposed metric, namely, the zero expansion
Nat\'ario WD metric. Nat\'ario discussed the nature of the WD spacetime
symmetries with a series of propositions and corollaries such as the proof
that the WD spacetime is flat whenever the tangent vector to the Cauchy
surfaces is a Killing vector field for the Euclidean metric despite
being time-dependent and as a particular case. In addition, it is also
flat wherever the tangent vector is spatially constant. He also showed
that nonflat WD spacetime violate both the \textit{weak energy condition}
(WEC) and the \textit{strong energy condition} (SEC), as well as that the
WD metric can be obtained from the Nat\'ario metric by a particular choice
of coordinates. The spherical coordinates were the choice of charts for the
zero expansion of the Nat\'ario WD spacetime.

Lobo and Visser\cite{LoboVisser2004} pointed out that the WD theory is an
example of reverse engineering of the solutions of the Einstein equations
where one defines a specific spacetime metric and then one finds the matter
distribution responsible for the respective geometry. These authors verified
that the class of WD spacetimes necessarily violate the classical energy
conditions even for low warp bubble velocity. Hence, this is the case of
a geometric choice and not of superluminal properties. They proposed a more
realistic WD by applying linearized gravity to the weak WD with nonrelativistic
warp bubble velocities and argued that for the Alcubierre WD and its version
proposed by Nat\'ario\cite{Natario2002} the center of the bubble must be
massless. They found that even for low velocities the negative energy stored
in the warp fields must be a significant fraction of the particle's mass at
the center of the warp bubble.

White\cite{White2003,White2011} described how a warp field interferometer
could be implemented at the Advanced Propulsion Physics Laboratory with the
help of the original\cite{Alcubierre1994} WD ideias. He also pointed out that
the expansion behind the warp bubble and contraction in front of it is due
to the nature of the WD functions, and that the distortion of spacetime may
be interpreted as a kind of Doppler effect or stress and strain on spacetime.
Lee and Cleaver\cite{LeeCleaver2016} argued that external radiation might
affect the WD, and that the warp field interferometer proposed by
White\cite{White2003,White2011} could not detect spacetime distortions.
Mattingly et al.\cite{Mattingly2020} discussed curvature invariants in the
Nat\'ario WD.

Bobrick and Martire\cite{Bobrick2021} proposed a general WD spacetime that
encloses all WD definitions removing any alleged issues with the original
Alcubierre WD. They presented a general subluminal model with spherical
symmetry and positive energy solutions that satisfies the quantum energy
inequalities such that it reduces two orders of magnitude the WD requirement
for negative energy density. They claimed that any type of WD, including
the original one, is a place of regular or exotic material moving in
inertial form with a certain speed.

Lentz\cite{Lentz} claimed that the original warp bubble proposed by
Alcubierre\cite{Alcubierre1994} can be physically interpreted as hyper-fast
gravitational solitons and presented the first solution for superluminal
solitons in general relativity satisfying the WEC and the momentum conditions
for conventional sources of stress, like energy and momentum, that do not
require large amounts of negative energy. The basis for his work is to
assume that the shift vector components obey a kind of wave equation giving
rise to a positive energy geometry. Fell and Heisenberg\cite{Fell2021} also
addressed the WD as gravitational solitons and shed light on the Eulerian
energies and their relation to the WEC, raising the possibility for
superluminal spacetimes with viable amounts of energy density.

Quarra\cite{Quarra2021} established that within the scope of general
relativity certain gravitational waveforms can result in geodesics that
arrive at distant points earlier than light signals in flat spacetime,
and presented an example waveform that can be used to manifest superluminal
behavior.

Santiago et {al.}\cite{Santiago2021a} claimed that generic WD metrics
violate the \textit{null energy conditions} (NEC) and discussed how Eulerian
observers are privileged, meaning that these observers may perceive positive
energy densities causing the impression of viable WD. They also argue that
for the WD to become a possibility it would require that all timelike
observers observe positive energy densities. They stated that any WD spacetime
will unavoidably violate the energy conditions. In a subsequent
work\cite{Santiago2021b} they claimed that exotic spacetimes, such as the
WD one will always violate energy conditions. They also provided other
examples of such spacetimes relating them to wormholes, tractor beams and
stress beams.

For a detailed discussion on the basics of WD theory, the reader is referred
to Alcubierre and Lobo.\cite{AlcubierreLobo2017} For a nice exposition on
WD theory one is referred to the lecture notes of the course given by
Shoshany.\cite{Shoshany2019} It is particularly noteworthy the recent results
concerning the possibility of WD features in negative energy density
distribution of an experimental Casimir cavity,\cite{casimir} results that
open the striking possibility of ingenious experimental avenues on how to
create the WD phenomena in a laboratory environment.

In this work, we present a summary of the results we have recently obtained
when solving the Einstein equations with the assumption of simple fluid
distributions embedded in the original Alcubierre WD spacetime geometry.
New results such as the vacuum solutions connecting the WD to shock waves
via the Burgers equation are presented.\cite{nos1} We used the perfect fluid
energy-momentum tensor\cite{nos2} (EMT) which disclosed new exact solutions
of the Einstein equations. We also demonstrated that starting with simple
forms of mass and energy sources, vacuum solutions for the WD spacetime
arise when we impose that the WEC is trivially satisfied by a choice
of gauge on the shift vector and the spacetime coordinates.\cite{nos2}
Further results of the Alcubierre WD metric endowed with a charged dust EMT
that considers an electromagnetic tensor in curved spacetime with the
cosmological constant are mentioned.\cite{nos3} Finally, adding the
cosmological constant to the Einstein equations coupled with the Alcubierre
WD spacetime having perfect fluid as a source has the effect of leading the
energy density to possibly becoming positive depending on both the value
and sign of the cosmological constant.\cite{nos4}

The paper is organized as follows. Section 2 reviews very brief the Alcubierre
WD metric. Section 3 depicts the perfect and anisotropic fluid cases\cite{nos2}
and Section 4 discusses the relationship between the WD spacetime, vacuum
solutions, and shock waves described by the Burgers equations.\cite{nos1}
Section 5 presents a discussion on the role of the cosmological constant in the
Einstein equation solutions for the WD spacetime.\cite{nos4} Section 6 presents
the conclusions.

\section{The Alcubierre Warp Drive Spacetime Metric}

Alcubierre originally used\cite{Alcubierre1994} the Arnowitt-Deser-Misner
(ADM) approach of general relativity,\cite{adm1,adm2} a formalism where the
spacetime is described by a foliation of space-like hypersurfaces of constant
time coordinate. The general form of the WD metric in this formalism is
described by the following equation,
\begin{eqnarray}
{ds}^2 = -\,d\tau^2 &=& g_{\alpha\beta} dx^{\alpha} dx^{\beta} \nonumber \\
&=& - \Big(\alpha^2 -\beta_i\beta^{i}\Big) \, dt^2
+ 2 \beta_i \, dx^i \, dt + \gamma_{ij} \, dx^i \, dx^j \,\,,
\label{awdmetric1}
\end{eqnarray}
where $d\tau$ is the lapse of proper time, $\alpha$ is the lapse function,
$\beta^i$ is the space-like shift vector, and $\gamma_{ij}$ is the spatial
metric for the hypersurfaces. The Greek indices range from 0 to 3, whereas
the Latin ones indicate the space-like hypersurfaces and range from 1 to 3.
The lapse function $\alpha$ and the shift vector $\beta^i$ are functions
to be determined, $\gamma_{ij}$ is a positive-definite metric on each of
the space-like hypersurfaces, for all values of time, a feature that makes
the spacetime globally hyperbolic. The lapse function $d\tau$ is a measure
of proper time  between two adjacent hypersurfaces by observers moving
along the normal direction to the hypersurfaces (Eulerian observers).
The shift vector is a tangent vector to the hypersurfaces that relates
the spatial coordinate systems on different hypersurfaces.

Alcubierre assumed the following ad hoc particular choices for the
parameters of the general ADM metric,\cite{Alcubierre1994}
\begin{align}
\alpha &= 1,
\\
\beta^1& = - v_s(t)f\big[r_s(t)\big], \label{betax}
\\
\beta^2 &= \beta^3 = 0,
\\
\gamma_{ij} &= \delta_{ij}.
\end{align}
Rewriting Eq.\ \eqref{awdmetric1} with these definitions yields the
\textit{Alcubierre WD spacetime metric} below,
\begin{equation}
ds^2 = - \left(1 - \beta^2\right)dt^2 - \beta \,dx\,dt
+ dx^2 + dy^2 + dz^2.
\label{awdmetric2}
\end{equation}

\section{Einstein Equations Solutions for Simple Fluids}

We to find solutions to the Einstein equations with the Alcubierre
WD metric as the underlying geometry by coupling simple matter-energy
distributions, namely the dust of particles, the perfect fluid and the
anisotropic fluid with off-diagonal metric terms to see if those EMTs
could lead to a viable WD bubble.

Starting with incoherent fluid, or dust, vacuum solutions of the Einstein
equations for the WD metric \eqref{awdmetric2} were recovered, which led to
the connection of the WD with shock waves via Burgers equation.\cite{nos1}
Vacuum solutions mean that all the energy conditions are trivially satisfied,
but the possibility of vacuum shock waves that could physically represent
warp bubbles indicated that other known matter-energy distributions could
result in more complex solutions of Einstein equations. Under this perception
we proposed two other energy-momentum tensors, the perfect and
``parametrized'' fluids, where the latter is, in fact, an anisotropic fluid
with heat flux.\cite{nos2}

The EMT for an anisotropic and dissipative fluid may be written by the
following equation,
\begin{equation}
 T^{\alpha \beta} = \mu u^\alpha u^\beta + p h^{\alpha \beta}
+ u^\alpha q^\beta + u^\beta q^\alpha  + \pi^{\alpha \beta},
\end{equation}
where
\begin{equation}
h_{\alpha \beta} = g_{\alpha \beta} + u_a\,u_b
\end{equation}
projects tensors onto hypersurfaces orthogonal to $u^\alpha$, $\mu$ is
the matter density, $p$ is the fluid static pressure, $q^\alpha$ is the
heat flux vector and $\pi^{\alpha \beta}$ is the viscous shear tensor.
The world lines of the fluid elements are the integral curves of the
four-velocity vector $u^\alpha$. The heat flux vector and the viscous shear
tensor are transverse to the world lines, that is,

\begin{equation}
q_a\,u^a = 0, \:\:\qquad  \:\: \mbox{and}\:\: \:\:\qquad  \pi_{ab}\,u^b = 0\,.
\end{equation}

For simplicity we depicted the EMT of the anisotropic fluid in the
following matrix form,
\begin{equation}
T_{\alpha \sigma} =
\begin{pmatrix}
\mu + \beta^2 p  & \quad \!\!\! - \beta D & \quad 0  & \quad 0  \\
- \beta D        & \:\: A         & \quad 0  & \quad 0  \\
0                & \:\: 0         & \quad B  & \quad 0  \\
0                & \:\: 0         & \quad 0  & \quad C
\end{pmatrix}\,,
\label{emtanisofluid}
\end{equation}
where $p$, $A$, $B$ and $C$ are anisotropic static pressures, $D$ is the
momentum density parameter that represents the heat flux for this fluid,
and $\mu$ is the particles' matter density.
Notice that if we choose all pressures and momentum density as being equal
to the static pressure $p$ we recover the perfect fluid form shown below,
\begin{equation}
T_{\alpha \sigma} =
\begin{pmatrix}
\mu + \beta^2 p & \quad \!\!\! - \beta p & \quad 0  & \quad 0  \\
- \beta p       & \:\:  p         & \quad 0  & \quad 0  \\
0               & \:\:  0         & \quad p  & \quad 0  \\
0               & \:\:  0         & \quad 0  & \quad p
\end{pmatrix}\,.
\label{emtperffluid}
\end{equation}

The EMT for a perfect fluid can be written in terms of the tensor notation
as follows,
\begin{equation}
T_{\alpha \beta} = \left(\mu + p\right) \, u_\alpha u_\beta
+ p \, g_{\alpha \beta}\,,
\end{equation}
where $\mu$ is the matter density, $p$ is the fluid static pressure,
$g_{\alpha \beta}$ is the metric tensor and $u_{\alpha}$ is the
4-velocity of an observer inside the fluid. Perfect fluids have no
shear stress, rotation, heat conduction or viscosity. To recover the
EMT for dust of particles, we have to choose the
static fluid pressure to be zero, and the matrix form for the dust
is given by the following equation,
\begin{equation}
T_{\alpha \sigma} =
\begin{pmatrix}
\mu & 0 & 0  & 0  \\
0   & 0 & 0  & 0  \\
0   & 0 & 0  & 0  \\
0   & 0 & 0  & 0
\end{pmatrix},
\label{emtdust}
\end{equation}
which can be written in tensor notation as follows,
\begin{equation}
T_{\alpha \beta} = \mu \, u_{\alpha} u_{\beta}\,,
\end{equation}
where $\mu$ is the matter density represented by a scalar function of the
spacetime coordinates, and $u^\alpha$ are the observers 4-velocity
components.

Table~1 lists the solutions of the Einstein equations for the anisotropic
fluid defined by Eq.\ \eqref{emtanisofluid},\cite{nos2}
\begin{table}[ht!]
{Table 1:} WD solutions of the Einstein equations with anisotropic fluid
as source. \vspace{3mm}
\begin{tabular}{| m{3cm} | m{3cm} | m{5.4cm} |}
\hline
Case & Conditions & Results \\
\hline
\multirow{2}{*}{$1) \
\displaystyle{\frac{\partial \beta}{\partial z} = 0}$}
&
$1a) \ \displaystyle{\frac{\partial \beta}{\partial x} = 0}$
&
$\begin{array} {ll}
\\ [-10pt]
\displaystyle{\mu = \beta^2(2D-A-p) + \frac{A}{3}} \\ [7pt]
\beta = \beta(t,y)\\ [7pt]
\displaystyle{B = - C = \frac{A}{3}}\\ [7pt]
\displaystyle{\left(\frac{\partial \beta}{\partial y}
\right)^2 = 32 \pi C},  \\ [10pt]
\displaystyle{\frac{\partial^2\beta}{\partial y^2} =
16 \pi \beta (A-D)}
\\[7pt]
\end{array}$ \\
\cline{2-3}
&
$1b) \ \displaystyle{\frac{\partial \beta}{\partial y} = 0}$
&
$\begin{array} {ll}
\\ [-10pt]
\displaystyle{\mu = - \beta^2p}\\ [7pt]
\beta = \beta(t,x)\\ [7pt]
\displaystyle{B = C}  \\ [5pt]
\displaystyle{A = D = 0} \\ [7pt]
\displaystyle{\frac{\partial}{\partial x}\left[
\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
\right] = - 64 \pi B} \\[8pt]
\rightarrow\mbox{solution \textit{dismissed} as unphysical} \\
\end{array}$ \\
\hline

\multirow{2}{*}{$2) \
\displaystyle{\frac{\partial \beta}{\partial y} = 0}$}
&
$2a) \ \displaystyle{\frac{\partial \beta}{\partial x} = 0}$
&
$\begin{array} {ll}
\\ [-10pt]
\displaystyle{\mu = \beta^2(2D-A-p) + \frac{A}{3}} \\ [7pt]
\beta = \beta(t,z)\\ [7pt]
\displaystyle{B = - C = \frac{A}{3}}\\ [7pt]
\displaystyle{\left(\frac{\partial \beta}{\partial z}
\right)^2 = 32 \pi C}  \\ [9pt]
\displaystyle{\frac{\partial^2\beta}{\partial z^2} =
16 \pi \beta (A-D)}
\\[7pt]
\end{array}$ \\
\cline{2-3}
&
$2b) \ \displaystyle{\frac{\partial \beta}{\partial z} = 0}$
&
$\begin{array} {ll}
\\ [-10pt]
\displaystyle{\mu = - \beta^2p}\\ [7pt]
\beta = \beta(t,x)\\ [7pt]
\displaystyle{B = C}  \\ [5pt]
\displaystyle{A = D = 0} \\ [7pt]
\displaystyle{\frac{\partial}{\partial x}\left[
\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
\right] = - 64 \pi B}
\\[8pt]
\rightarrow\mbox{solution \textit{dismissed} as unphysical}
\end{array}$ \\
\hline
\end{tabular}
\caption{Summary of all solutions of the Einstein equations for the
	 WD metric having the parametrized perfect
	fluid as source of energy and momentum.}
\label{tableppf}
\end{table}
where the two solutions subsets 1b and 2b are considered unphysical because
the resulting EMT leads to the anisotropic static pressures $A$ and $D$
equal to zero, whereas $B=C$. The energy density in the $T_{00}$
component of the EMT is equal to zero, resulting in the following equation
of state relating the matter density $\mu$, the shift vector $\beta$ and the
static pressure $p$,
\begin{equation}
\mu = - \beta^2 p \,.
\end{equation}
We must point out that we made a notational change on the shift vector
definition $\beta^1$ so that it has the negative sign of the original
parameter defined by Alcubierre,\cite{Alcubierre1994} yielding the
shift vector $\beta$ used in here as being given as below,
\begin{equation}
\beta = - \beta^1 = v_s(t)f\big[r_s(t)\big]\,.
\label{newshift}
\end{equation}
Finally, one should also notice that solutions 1b and 2b brought about
the following Burgers equation in dissipative form,
\begin{equation}
\frac{\partial}{\partial x}\left[
\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
\right] = - 64 \pi B \,.
\label{burgerspi}
\end{equation}

Perfect fluid solutions have equations of state given by the expression
below,\cite{EllisElst}
\begin{equation}
p = p(\mu)=(\gamma-1)\mu\,\,,
\label{eq-state}
\end{equation}
where $\gamma=$ is a constant which for ordinary fluids can be approximated
by $1\leq\gamma\leq 2$. The incoherent matter, or dust, corresponds to
$\gamma = 1$, and radiation corresponds to $\gamma=\frac{4}{3}$. For
solutions 1b and 2b in Table 1 it is clear that $\gamma$ is a function of
the shift vector given by the following equation,
\begin{equation}
\gamma = 1 - \frac{1}{\beta^2} \,\,.
\label{pfgamma1}
\end{equation}
Notice that since the shift vector is a function of the warp velocity and it
is also the regulating function, we are facing a problem with a discontinuity
when $\beta = 0$ since outside the warp bubble the regulating function
approaches to zero, but inside the warp bubble $f(r_s) = 1$. So Eq.\
\eqref{pfgamma1} takes the following form,
\begin{equation}
\gamma = 1 - \frac{1}{v_s^2} \,,
\label{pfgamma2}
\end{equation}
where $v_s = v_s(t)$ is the warp bubble velocity. Solutions 1a and 2a shown
in Table 1 are very similar, except that for solution 1a the shift vector is
a function of spacetime coordinates $(t,y)$, whereas solution 2a is a function
of $(t,z)$ coordinates. However, in both of these cases, the shift vector may
be a complex-valued function depending on the values of the static pressure
$C$. For these solutions the equations of state relating the matter density
$\mu$ and the fluid pressures are identical and given by the following
expression,
\begin{equation}
\mu = \beta^2(2D-A-p) + \frac{A}{3}\,.
\label{eqstate1}
\end{equation}
The perfect fluid solutions listed in Table~2 are a special case of the
anisotropic fluid if we chose all pressures to be equal to $p$.
Table 3 presents the special dust case ($p=0$), which leads to vacuum.
\begin{table}[ht!]
{Table 2:} WD solutions of the Einstein equations with perfect fluid as source.
\vspace{3mm}
\begin{tabular}{| m{3cm} | m{3cm} | m{5.4cm} |}
\hline
Case & Condition & Results \\
\hline
\multirow{2}{*}{$1) \
\displaystyle{\frac{\partial \beta}{\partial z} = 0}$}
&
$1a) \ \displaystyle{\frac{\partial \beta}{\partial x} = 0}$
&
$\begin{array} {ll}
p = 3 \mu \\ [6pt]
\beta = \beta(y,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial y}
=\pm\sqrt{-32 \pi \mu}} \\ [8pt]
\end{array}$ \\ [28pt]
\cline{2-3}
&
$1b) \ \displaystyle{\frac{\partial \beta}{\partial y} = 0}$
&
$\begin{array} {ll}
p = 3 \mu = 0 \\ [6pt]
\beta = \beta(x,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
= h(t)} \\ [8pt]
\end{array}$ \\ [28pt]
\hline

\multirow{2}{*}{$2) \
\displaystyle{\frac{\partial \beta}{\partial y} = 0}$}
&
$2a) \ \displaystyle{\frac{\partial \beta}{\partial x} = 0}$
&
$\begin{array} {ll}
p = 3 \mu \\ [6pt]
\beta = \beta(z,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial z}
= \pm\,\sqrt{-32 \pi \mu}}\\ [8pt]
\displaystyle{\frac{\partial \beta}{\partial z}
=\,\pm \sqrt{\pm\,96 \pi \mu}} \\ [8pt]
\end{array}$ \\ [28pt]
\cline{2-3}
&
$2b) \ \displaystyle{\frac{\partial \beta}{\partial z} = 0}$
&
$\begin{array} {ll}
p = 3 \mu = 0 \\ [6pt]
\beta = \beta(x,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
= h(t)} \\ [8pt]
\end{array}$ \\ [28pt]
\hline
\end{tabular}
\label{tablepf}
\end{table}
\begin{table}[ht!]
{Table 3:} WD solutions of the Einstein equations with dust source
(leads to vacuum).  \vspace{3mm}
\begin{tabular}{| m{3cm} | m{3cm} | m{5.4cm} |}
\hline
Case & Consequence & Results \\
\hline
$1) \ \displaystyle{\frac{\partial \beta}{\partial z} = 0} $
&
$\displaystyle{\frac{\partial \beta}{\partial y} = 0}$
&
$\begin{array} {ll}
\mu = 0 \\ [6pt]
\beta = \beta(t,x) \\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
= h(t)}
\\[10pt]
\end{array}$ \\
\hline
$2) \ \displaystyle{\frac{\partial \beta}{\partial y} = 0}$
&
$\displaystyle{\frac{\partial \beta}{\partial z} = 0}$
&
$\begin{array} {ll}
\mu = 0 \\ [6pt]
\beta = \beta(t,x)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
= h(t)}
\\[10pt]
\end{array}$ \\
\hline
\end{tabular}
\caption{Summary of results for the WD spacetime having dust
matter content.}
\label{tabledust}
\end{table}

\section{Warp Drive and Shock Waves}

In previous studies\cite{nos1,nos2} it was found an intrinsic relationship
between the WD concept and shock waves via the Burgers equation arising from
vacuum solutions of Einstein equations for the Alcubierre WD metric. Such a
result may mean that the warp bubble could be interpreted as shock waves in
vacuum. In other words, the Burgers equation can be seen as a vacuum solution
of Einstein equations connecting the Alcubierre WD metric to shock waves.

In the case of the anisotropic fluid, we considered as unphysical the solution
which led to a Burgers equation \eqref{burgerspi} because the parameter
values resulted in an EMT with only two static pressures (diagonal terms)
and no energy density. Nevertheless, for the perfect fluid and dust EMTs we
found the Burgers equation in its viscous form describing a dissipative
system with the right-hand side being a function of time coordinate only,
according to the expression below,
\begin{equation}
\frac{\partial \beta}{\partial t}
+ \frac{1}{2}\frac{\partial}{\partial x} (\beta^2)
= h(t)\,\,.
\label{burg}
\end{equation}
Here $h = h(t)$ is an arbitrary function to be determined by the boundary
conditions. If $h(t) = 0$ Eq.\ \eqref{burg} takes its homogeneous form
describing a conservative system known as inviscid Burgers equation.

The Burgers or Bateman-Burgers, equation is a well-known partial
differential equation that models several physical systems such as gas
dynamics, traffic flows, and even stock market, since, for the latter
application, it is connected to the symmetries of the Black-Scholes equation.
Its most notorious system description is, however, the phenomena arising
from conservation laws and formation of shock waves, that is, discontinuities
that appears after a finite time and then propagates in a regularly. The
physics behind the field solutions of the Burgers equation can be seen as a
current density.

The WD is depicted by the shift vector $\beta = v_s(t) f(r_s)$, where $v_s$
is the bubble velocity, $f(r_s)$ is the regulating function of the
warp bubble shape and the inviscid Burgers in Eq.\ \eqref{burg} can be
interpreted and representing a conservation law for this current density.
Analyzing each term of Eq.\,\eqref{burg}, the first one on the l.h.s.,
$\partial \beta/\partial t$ can be interpreted as a force per unit mass,
i.e., the time derivative of momentum. The second term on the l.h.s.,
$\frac{1}{2}\,\partial (\beta^2) /\partial x$, can be interpreted as the
divergence of the total energy, which is entirely kinetic. Physically the
result can be understood by considering the WD metric as conservation of
both energy and momentum in the direction of the wave propagation.
Calculating the divergence of the parametrized, or anisotropic, perfect
fluid energy-momentum tensor, and demanding that it should be zero, one
arrives at the following system,\cite{nos2}
\begin{eqnarray}
\nonumber
&-& \frac{\partial \beta}{\partial x} (D + \mu)
- \frac{\partial \mu}{\partial t}
- \beta \left[\frac{\partial D}{\partial x}
+ \frac{\partial \mu}{\partial x}
+ \frac{\partial \beta}{\partial t}(2p + A - 3D)\right] \nonumber \\
&& \mbox{} \nonumber \\
&& \qquad \qquad +\;\beta^2\left[\frac{\partial D}{\partial t}
- \frac{\partial p}{\partial t}
+ 3\frac{\partial \beta}{\partial x}(D-p)\right]
+ \beta^3 \left(\frac{\partial D}{\partial x}
- \frac{\partial p}{\partial x}\right)= 0,
\label{divT0}  \\
&& \mbox{} \nonumber \\
&&\frac{\partial A}{\partial x}
+ \frac{\partial \beta}{\partial t} (D-A)
+ \beta \left[ 3 \frac{\partial \beta}{\partial x} (D-A)
+ \frac{\partial D}{\partial t}
- \frac{\partial A}{\partial t}\right] \nonumber \\
&& \qquad \qquad \qquad + \;\;\beta^2 \left(\frac{\partial D}{\partial x}
- \frac{\partial A}{\partial x}\right) = 0,
\label{divT1}   \\
&& \mbox{} \nonumber \\
&& \frac{\partial B}{\partial y} +
\beta \frac{\partial \beta}{\partial y} (D-A) = 0,
\label{divT2} \\
&& \mbox{} \nonumber \\
&&\frac{\partial C}{\partial z} +
\beta \frac{\partial \beta}{\partial z} (D-A) = 0.
\label{divT3}
\end{eqnarray}
The perfect fluid zero divergence is recovered by letting
\begin{equation}
p = A = B = C = D\,,
\end{equation}
hence, Eqs.\ \eqref{divT0} to \eqref{divT3} become,
\begin{eqnarray}
&-& \frac{\partial \mu}{\partial t}
- \beta \left(\frac{\partial p}{\partial x}
+ \frac{\partial\mu}{\partial x}\right)= 0,
\label{pf1adivT0} \\
\mbox{} \nonumber \\
&&\frac{\partial p}{\partial x}=\frac{\partial p}{\partial y}=\frac{\partial p}{\partial z}= 0\,\,,
\end{eqnarray}
which means that the static pressure $p$ of the perfect fluid does
not depend on the spatial coordinates, and Eq.\ \eqref{pf1adivT0}
reduces to the expression below,
\begin{equation}
\frac{\partial \mu}{\partial t}
+ \beta \frac{\partial \mu}{\partial x} = 0,
\label{divpf0}
\end{equation}
which is a continuity equation, $\mu$ is the fluid density, and $\beta$ is
the flow velocity vector field. Notice that for constant density the
fluid has an incompressible flow. Therefore, all partial derivatives of
$\beta$ in terms of the spatial coordinates vanish, and the flow velocity
vector field has zero divergence, this being a classical fluid dynamics
scenario. The local bubble volume expansion rate is zero, and the WD
metric becomes the Minkowski metric in this scenario.

If we want to find these results for the dust of particles as a
particular case, we can see that the partial time derivative
of the matter-density is zero and we have
\begin{equation}
\mu \frac{\partial \beta}{\partial x} = 0,
\label{nulldiv}
\end{equation}
which is immediately satisfied since the matter density is zero for the
dust EMT and the Einstein equations solution for the WD is the vacuum
one. This is also true for the sets of solutions 1b and 2b
found for the perfect fluid, as shown in Table 2. This result for the
vacuum solutions of the Einstein equations with the WD metric as a
background suggests that the necessary energy to create the associate
shock wave is purely geometrical. It is interesting evidence that the
WD metric can be understood as a spacetime motion equivalent to a shock
wave moving in a fluid, where spacetime itself plays the role of the fluid.

Considering all these results one can speculate about vacuum energy,
quantum fluctuations and dark matter as fuel for the feasibility of the
warp bubble, but one has to overcome possible difficulties caused by the
event horizon of the bubble that would require particles with imaginary
mass like tachyons.\cite{AlcubierreMG2021}

\section{The Cosmological Constant}

Bearing in mind the results above led to the next step of adding the
cosmological constant in the Einstein equation to seek solutions
for Alcubierre WD spacetime geometry.\cite{nos4} The motivation came from
Eq.\ (19) in the original Alcubierre work,\cite{Alcubierre1994} which was
obtained by applying the WEC considering Eulerian observers, contracting
the EMT and writing the final expression $G_{\mu\nu} = \kappa T_{\mu\nu}$
in the Einstein equation, whose result reads as follows,
\begin{equation}
T^{\mu \nu} u_\mu u_\nu = \alpha^2 T^{00} = G^{00} =
- \frac{v_s^2 \rho^2}{4 r_s^2} \left(\frac{df}{dr_s}\right)^2.
\label{weakeng}
\end{equation}
Here $u^\mu$ and $u_\mu$ are respectively the contravariant and covariant
components of the four-vector velocity of the Eulerian observers given by
\begin{equation}
u^\mu = \frac{1}{\alpha}(1, \beta^1, \beta^2, \beta^3) \ \ , \ \
u_\mu = - (\alpha, 0, 0, 0)\,,
\end{equation}
where $\alpha$ is the lapse time between the constant hypersurfaces of the
ADM-formalism and $\beta^i$ are the three-components of the shift vector
$\pmb{\beta}$. Using the original parameters from the $3+1$-formalism, its
general metric and the chosen chart of coordinates we can write Eq.\
\eqref{weakeng} as below,
\begin{equation}
T_{\alpha \beta} \, u^\alpha u^\beta
= - \frac{1}{4} \left[
\left(\frac{\partial \beta}{\partial y} \right)^2  +
\left(\frac{\partial \beta}{\partial z} \right)^2
\right].
\label{edwd1}
\end{equation}

The above expression is clearly a non-positive term everywhere,
violating the WEC in a nontrivial way. We then showed that
if we begin with simple energy and momentum sources and impose Eq.\
\eqref{edwd1} to be as identically zero we arrive at vacuum solutions
for the WD spacetime.\cite{nos1,nos2} However, if we include the
cosmological constant in the Einstein equations the expression
\eqref{edwd1} is changed, yielding,
\begin{equation}
T_{\alpha \beta} \, u^\alpha u^\beta
= \Lambda - \frac{1}{4} \left[
\left(\frac{\partial \beta}{\partial y} \right)^2  +
\left(\frac{\partial \beta}{\partial z} \right)^2
\right]\,\,.
\label{edwd2}
\end{equation}
For positive and large enough values of the cosmological constant, it may
be possible to construct a WD that does not violate the WEC, because the
original negative mass-energy density necessary to create the warp bubble
could become positive.

The solutions for the WD metric which include the cosmological constant
in the Einstein equations\cite{nos4} are presented in Table 4. Cases 1b
and 2b are just the vacuum solutions connecting the WD spacetime to shock
waves via the Burgers equations, and they also require a vanishing
cosmological constant, since both the static fluid pressure $(p)$ and the
matter density $(\mu)$ also vanish. The other two solutions subsets 1a and
2a are also similar to the equally labeled ones in Table 2, but now they
include the cosmological constant and the solutions are no longer required
to be complex if the following conditions are satisfied,
\begin{equation}
\Lambda - 8\pi \mu \geq 0\,,
\end{equation}
\begin{equation}
\Lambda - 8\pi p \geq 0\,.
\end{equation}

Taking all these results into account imply that creating a warp bubble
for superluminal travel of massive particles seem to require more complex
forms of matter than the dust for stable solutions, and that the requirement
of negative mass-energy may not be as strict as originally thought for
superluminal spacetime travel with warp speeds. The shift vector in the
direction of the warp bubble movement creates a coupling in Einstein equations
that requires the off-diagonal source terms in the energy-momentum tensor that
represent momentum densities.

Previous authors investigated the WD theory from the geometrical point of
view \cite{Krasnikov1998, EveretRoman1997, Broeck1999, Natario2002,
LoboVisser2004} and came up with other types of WD mechnics, but it now seems
clear that it is also necessary to consider both the energy and momentum
tensors to propose a superluminal propelling system in the WD theory.
\begin{table}[ht!]
Table 4: WD solutions of the Einstein equations with $\Lambda$ and
perfect fluid.
\begin{tabular}{| m{3cm} | m{3cm} | m{5.4cm} |}
\hline
Case & Condition & Results \\
\hline
\multirow{2}{*}{$1) \
\displaystyle{\frac{\partial \beta}{\partial z} = 0}$}
&
$1a) \ \displaystyle{\frac{\partial \beta}{\partial x} = 0}$
&
$\begin{array} {ll}
\Lambda =  6 \pi \left(\mu - \frac{p}{3}\right) \\ [6pt]
\beta = \beta(y,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial y}
= \pm\sqrt{4 (\Lambda - 8 \pi \mu)}} \\ [8pt]
\displaystyle{\frac{\partial \beta}{\partial y}
= \pm\sqrt{\frac{4}{3} (\Lambda - 8 \pi p)}} \\ [8pt]
\displaystyle{\beta \frac{\partial \mu}{\partial x}
+ \frac{\partial \mu}{\partial t} = 0
\ \ \text{(null divergence)}} \\ [8pt]
\end{array}$ \\ [28pt]
\cline{2-3}
&
$1b) \ \displaystyle{\frac{\partial \beta}{\partial y} = 0}$
&
$\begin{array} {ll}
\Lambda = 8 \pi \mu = 8 \pi p = 0\\ [6pt]
\beta = \beta(x,t)\\ [6pt]
\displaystyle{
\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}
(\beta^2) = h(t)\, } \\ [6pt]
\text{Null divergence is trivially satisfied}\\ [2pt]
\text{This is the solution found in Ref.\cite{nos1}} \\ [8pt]
\end{array}$ \\ [28pt]
\hline

\multirow{2}{*}{$2) \
\displaystyle{\frac{\partial \beta}{\partial y} = 0}$}
&
$2a) \ \displaystyle{\frac{\partial \beta}{\partial x} = 0}$
&
$\begin{array} {ll}
\Lambda =  6 \pi \left(\mu - \frac{p}{3}\right) \\ [6pt]
\beta = \beta(y,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial z}
= \pm\sqrt{4 (\Lambda - 8 \pi \mu)}} \\ [8pt]
\displaystyle{\frac{\partial \beta}{\partial z}
= \pm\sqrt{\frac{4}{3} (\Lambda - 8 \pi p)}} \\ [8pt]
\displaystyle{\beta \frac{\partial \mu}{\partial x}
+ \frac{\partial \mu}{\partial t} = 0
\ \ \text{(null divergence)}} \\ [8pt]
\end{array}$ \\ [28pt]
\cline{2-3}
&
$2b) \ \displaystyle{\frac{\partial \beta}{\partial z} = 0}$
&
$\begin{array} {ll}
\Lambda = 8 \pi \mu = 8 \pi p = 0 \\ [6pt]
\beta = \beta(x,t)\\ [6pt]
\displaystyle{\frac{\partial \beta}{\partial t}
+ \frac{1}{2} \frac{\partial}{\partial x}(\beta^2)
= h(t)} \\ [6pt]
\text{Null divergence is trivially satisfied}\\ [2pt]
\text{This is the solution found in Ref.\cite{nos1}} \\ [8pt]
\end{array}$ \\ [28pt]
\hline
\end{tabular}
\caption{Summary of all solutions of the Einstein equation with
the cosmological constant and the WD metric
having the perfect fluid EMT as mass-energy
source. This table is also valid for the dust particle if considered
as a particular case for the perfect fluid with null pressure $p = 0$.}
\label{tabcosmo}
\end{table}

\section{Conclusions}

In this work, we described new exact solutions of the Einstein equations
endowed with the Alcubierre warp drive (WD) spacetime geometry having simple
energy-momentum tensor (EMT) distributions of matter and energy. The incoherent
matter, or dust, the simplest fluid source EMT used with the WD metric, led
to vacuum solutions and the weak energy condition being satisfied with null
energy density. In addition, we showed that the dynamics of the vacuum WD
spacetime are governed by the Burgers equation, which links the WD bubble to
shock waves.\cite{nos1} Both the perfect fluid EMT and an anisotropic fluid
recover the Burgers equation as vacuum solutions, but some of these new sets of
solutions were considered unphysical, and others showed that the shift vector
can be a complex-valued function.\cite{nos2}
The Einstein equations with the cosmological constant were used with the
WD metric having the perfect fluid as the source, whose results showed that if
the cosmological constant is big enough it would be possible to attain positive
energy density for the WD spacetime.\cite{nos4} This result is in line with
recent research activity in WD theory by other authors, who provide examples
of different WD spacetimes geometries,\cite{Lentz,Fell2021,Bobrick2021} which
suggests that it may be possible to create warp bubbles with positive energy
densities.

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Justin Martyr: The Resurrection - Chapter VI.—The Resurrection Is Consistent with the Opinions of the Philosophers.
Those, then, who are called natural philosophers, say, some of them, as Plato, that the universe is matter and God; others, as Epicurus, that it is atoms and the void; others, like the Stoics, that it is these four—fire, water, air, earth. For it is sufficient to mention the most prevalent opinions. And Plato says that all things are made from matter by God, and according to His design; but Epicures and his followers say that all things are made from the atom and the void by some kind of self-regulating action of the natural movement of the bodies; and the Stoics, that all are made of the four elements, God pervading them. But while there is such discrepancy among them, there are some doctrines acknowledged by them all in common, one of which is that neither can anything be produced from what is not in being, nor anything be destroyed or dissolved into what has not any being, and that the elements exist indestructible out of which all things are generated. And this being so, the regeneration of the flesh will, according to all these philosophers, appear to be possible. For if, according to Plato, it is matter and God, both these are indestructible and God; and God indeed occupies the position of an artificer, to wit, a potter; and matter occupies the place of clay or wax, or some such thing. That, then, which is formed of matter, be it an image or a statue, is destructible; but the matter itself is indestructible, such as clay or wax, or any other such kind of matter. Thus the artist designs in the clay or wax, and makes the form of a living animal; and again, if his handiwork be destroyed, it is not impossible for him to make the same form, by working up the same material, and fashioning it anew. So that, according to Plato, neither will it be impossible for God, who is Himself indestructible, and has also indestructible material, even after that which has been first formed of it has been destroyed, to make it anew again, and to make the same form just as it was before. But according to the Stoics even, the body being produced by the mixture of the four elementary substances, when this body has been dissolved into the four elements, these remaining indestructible, it is possible that they receive a second time the same fusion and composition, from God pervading them, and so re-make the body which they formerly made. Like as if a man shall make a composition of gold and silver, and brass and tin, and then shall wish to dissolve it again, so that each element exist separately, having again mixed them, he may, if he pleases, make the very same composition as he had formerly made. Again, according to Epicurus, the atoms and the void being indestructible, it is by a definite arrangement and adjustment of the atoms as they come together, that both all other formations are produced, and the body itself; and it being in course of time dissolved, is dissolved again into those atoms from which it was also produced. And as these remain indestructible, it is not at all impossible, that by coming together again, and receiving the same arrangement and position, they should make a body of like nature to what was formerly produced by them; as if a jeweller should make in mosaic the form of an animal, and the stones should be scattered by time or by the man himself who made them, he having still in his possession the scattered stones, may gather them together again, and having gathered, may dispose them in the same way, and make the same form of an animal. And shall not God be able to collect again the decomposed members of the flesh, and make the same body as was formerly produced by Him?
Justin Martyr: The Hortatory Address to the Greeks - Chapter XXV. - Plato's Knowledge of God's Eternity.
Justin Martyr: The Hortatory Address to the Greeks - Chapter XXVII. - Plato's Knowledge of the Judgment.
Justin Martyr: The Hortatory Address to the Greeks - Chapter VII. - Inconsistencies of Plato's Doctrine.
Justin Martyr: The Hortatory Address to the Greeks - Chapter VI. - Disagreements between Plato and Aristotle.
Justin Martyr: The Hortatory Address to the Greeks - Chapter XX. - Testimony of Plato.
Justin Martyr: The Hortatory Address to the Greeks - Chapter XXIX. - Origin of Plato's Doctrine of Form.
Justin Martyr: The Hortatory Address to the Greeks - Chapter XXII. - Ambiguity of Plato.
Justin Martyr: The Hortatory Address to the Greeks - Chapter XXIII. - Plato's Contradiction.
Justin Martyr: The Hortatory Address to the Greeks - Chapter XXVI. - Plato Indebted to the Prophets.
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text:
Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation
Nikolić, Nikolina
Spasojević, Jelena P.
Radosavljević, Aleksandra
Milošević, Milica V.
Barudžija, Tanja
Rakočević, Lazar
Kačarević-Popović, Zorica M.
A series of silver nanoparticles (AgNPs)-poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) (PVA/PVP) hydrogel nanocomposites were synthesized by gamma irradiation to examine the matrix's impact on properties of Ag nanocrystals important for strain engineering in functional materials. SEM analysis and the swelling studies revealed nanocomposite morphology and fluid transport properties. UV–Vis absorption characterization of AgNPs has enabled an understanding of the cluster size effects. The gel content indicated inter-crosslinking of the two components of the polymer matrix, especially for equivalent (1:1) PVA/PVP ratio, affecting the investigated AgNPs structural parameters. The lattice parameters and the interface stress, analyzed using XRD, have the lowest negative magnitude from the bulk lattice constant for the PVA/PVP (2:1) and (1:1) ratio. The lowest values of the lattice strain and dislocation density, as mutually dependent parameters, are obtained in AgNPs embedded in a PVA/PVP (...2:1) and (1:1) matrix. The XPS and FTIR data show a positive shift of the O1s and a redshift of the CO – Ag vibration band indicating charge transfer to the vacant orbital of Ag nanocrystals. The N atoms of the pyrrolidine ring are not directly involved in the electronic interactions, but over the p-π conjugation with carbonyl O. In addition, the tested Ag-(PVA/PVP) hydrogel nanocomposites showed significant potential as a broad-spectrum antibacterial material.
Ag-hydrogel / Charge transfer / Dislocation density / Gamma irradiation / Interface stress / Lattice parameters
Radiation Physics and Chemistry, 2023, 202, 110564-
DOI: 10.1016/j.radphyschem.2022.110564
AU  - Nikolić, Nikolina
AU  - Spasojević, Jelena P.
AU  - Radosavljević, Aleksandra
AU  - Milošević, Milica V.
AU  - Barudžija, Tanja
AU  - Rakočević, Lazar
AU  - Kačarević-Popović, Zorica M.
AB  - A series of silver nanoparticles (AgNPs)-poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) (PVA/PVP) hydrogel nanocomposites were synthesized by gamma irradiation to examine the matrix's impact on properties of Ag nanocrystals important for strain engineering in functional materials. SEM analysis and the swelling studies revealed nanocomposite morphology and fluid transport properties. UV–Vis absorption characterization of AgNPs has enabled an understanding of the cluster size effects. The gel content indicated inter-crosslinking of the two components of the polymer matrix, especially for equivalent (1:1) PVA/PVP ratio, affecting the investigated AgNPs structural parameters. The lattice parameters and the interface stress, analyzed using XRD, have the lowest negative magnitude from the bulk lattice constant for the PVA/PVP (2:1) and (1:1) ratio. The lowest values of the lattice strain and dislocation density, as mutually dependent parameters, are obtained in AgNPs embedded in a PVA/PVP (2:1) and (1:1) matrix. The XPS and FTIR data show a positive shift of the O1s and a redshift of the CO – Ag vibration band indicating charge transfer to the vacant orbital of Ag nanocrystals. The N atoms of the pyrrolidine ring are not directly involved in the electronic interactions, but over the p-π conjugation with carbonyl O. In addition, the tested Ag-(PVA/PVP) hydrogel nanocomposites showed significant potential as a broad-spectrum antibacterial material.
T2  - Radiation Physics and Chemistry
T1  - Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation
DO  - 10.1016/j.radphyschem.2022.110564
author = "Nikolić, Nikolina and Spasojević, Jelena P. and Radosavljević, Aleksandra and Milošević, Milica V. and Barudžija, Tanja and Rakočević, Lazar and Kačarević-Popović, Zorica M.",
abstract = "A series of silver nanoparticles (AgNPs)-poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) (PVA/PVP) hydrogel nanocomposites were synthesized by gamma irradiation to examine the matrix's impact on properties of Ag nanocrystals important for strain engineering in functional materials. SEM analysis and the swelling studies revealed nanocomposite morphology and fluid transport properties. UV–Vis absorption characterization of AgNPs has enabled an understanding of the cluster size effects. The gel content indicated inter-crosslinking of the two components of the polymer matrix, especially for equivalent (1:1) PVA/PVP ratio, affecting the investigated AgNPs structural parameters. The lattice parameters and the interface stress, analyzed using XRD, have the lowest negative magnitude from the bulk lattice constant for the PVA/PVP (2:1) and (1:1) ratio. The lowest values of the lattice strain and dislocation density, as mutually dependent parameters, are obtained in AgNPs embedded in a PVA/PVP (2:1) and (1:1) matrix. The XPS and FTIR data show a positive shift of the O1s and a redshift of the CO – Ag vibration band indicating charge transfer to the vacant orbital of Ag nanocrystals. The N atoms of the pyrrolidine ring are not directly involved in the electronic interactions, but over the p-π conjugation with carbonyl O. In addition, the tested Ag-(PVA/PVP) hydrogel nanocomposites showed significant potential as a broad-spectrum antibacterial material.",
journal = "Radiation Physics and Chemistry",
title = "Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation",
doi = "10.1016/j.radphyschem.2022.110564"
Nikolić, N., Spasojević, J. P., Radosavljević, A., Milošević, M. V., Barudžija, T., Rakočević, L.,& Kačarević-Popović, Z. M.. (2023). Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation. in Radiation Physics and Chemistry, 202, 110564.
https://doi.org/10.1016/j.radphyschem.2022.110564
Nikolić N, Spasojević JP, Radosavljević A, Milošević MV, Barudžija T, Rakočević L, Kačarević-Popović ZM. Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation. in Radiation Physics and Chemistry. 2023;202:110564.
doi:10.1016/j.radphyschem.2022.110564 .
Nikolić, Nikolina, Spasojević, Jelena P., Radosavljević, Aleksandra, Milošević, Milica V., Barudžija, Tanja, Rakočević, Lazar, Kačarević-Popović, Zorica M., "Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation" in Radiation Physics and Chemistry, 202 (2023):110564,
https://doi.org/10.1016/j.radphyschem.2022.110564 . .


text:
Places in Amsterdam
1. Rijksmuseum 2. Van Gogh Museum 3. Vondelpark 4. Canal Cruise 5. Red Light District 6. Beer Tasting All Tourist Places
Amsterdam Tourism
Amsterdam Packages
View 6+ photos
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#3 Among Places to visit in Amsterdam
Timings : Anytime
Entry Fee : Free
Vondelpark, Amsterdam Overview
Sprawling across a staggering 120 acres of land, the Vondelpark is a favourite of the locals and tourists alike for a little picnic, soaking up the summer sun and relaxing.
Of more than the 30 public parks to choose from in Amsterdam, Vondelpark is hands down the most popular of them all. It attracts thousands of tourists and locals every single day. The park, which was originally called Nieuwe Park, was first opened in 1865. It has an open air theatre, and many options of restaurants and bars to choose from. The beauty of the park is enhanced by statues such as the 'Joost van del Vondel' by Louis Royer and 'The Fish' by the world famous Pablo Picasso. There are several events that take place in the park including musical and dance events in the open air theatre and even a golf tournament.
Photos of Vondelpark
Read More on Vondelpark
Vondelpark History
Vondelpark has an interesting history - a group of prominent citizens of Amsterdam created a group to lobby for the creation of a park. They put together enough money to buy several acres of grassland around the edge of the city (at that time), and assigned a famous architect (Jan David Zocher) for the landscape of the park. It was initially named the Het Nieuwe Park (or "The New Park" in English).
A couple of years after the park opened, the statue of a famous Dutch poet (Joost van den Vondel) was erected in the park, and people started calling it the Vondelpark - a name which continues to this day.
Kings Day in Vondelpark
As in all of the Netherlands, King's day is celebrated with the usual grandeur in Vondelpark. There is a huge flea market in the park which is typically run by children, where they try to sell their stuff to earn some extra cash. The atmosphere is quite festive, and you would see the whole park covered in the traditional Dutch orange colors. If you happen to be in Amsterdam on the Kings day, a walk through the Vondelpark in the morning/afternoon would be a good way to start the festivities. (Kings day is currently celebrated on 27-April)
Events in Vondelpark
The most loved park in Amsterdam, Vondelpark is the centre of a lot of outdoor activities. There is a free open air theatre inside the park, which hosts free music, dance and even cabaret events during the summer days! From prominent rock bands to local dance groups, you could see a number of performances here between June and August.
Vondelpark's History
Cafe/restaurants in Vondelpark
There are a few nice restaurants and cafes inside the park, most of them beautifully located near a pond or a canal. A few of these have outdoor seating especially during the summer months. All of them boast a great collection of beers on tap and various cuisines, such as pizzas or burgers or other European cuisines.
Best Time To Visit Vondelpark
Spring to Autumn
How To Reach Vondelpark
For such a large park in a major international city, Vondelpark is surprisingly in the centre of the town and hence well-connected by tram. You can take a tram from practically anywhere in Amsterdam for either Leidseplein or Museumplein which are both a few minutes of walking distance from the park.
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Handy Handout #166: What is a Special Education Teacher?
What Is a Special Education Teacher?
by Becky L. Spivey, M.Ed. and Joel Colón, M.Ed.
Special education teachers (also called special educators) have specialized training to work with students who have learning, behavioral, emotional, and/or physical disabilities. A special education teacher primarily works with students who qualify (through a series of tests) for special education assistance.
Special educators work in a variety of settings depending on the needs of their students and the preferences of their school district. Some special educators have their own classroom (e.g., resource room), pull the students out of their regular classroom, and assist them at particular times during the school day with their individual learning needs. Others may work in the regular education classroom with the general education teacher to support the students with special needs. Some special educators have a group of students with more complex behavioral, emotional, learning, or physical disabilities in a "self-contained" classroom. These students' needs are greater and may require the assistance of additional qualified teachers and assistants.
Regardless of the setting, all of the special educator's students have an Individualized Education Plan (IEP). This is a legal document stating the diagnoses, goals for the students, accommodations the students will receive, and assessment plans to measure the students' progress towards their goals. This plan ensures that students continually progress and eventually attain the goals set for them within the school year. Special educators are in charge of scheduling meetings between teachers, parents, and other members of the IEP team, several times throughout the school year to evaluate and adjust the IEP. Parents are able to request an IEP meeting at any time during the school year.
What Do Special Educators Do?
Special educators work with students who have a variety of individual learning styles, learning levels, and physical and intellectual disabilities. These disabilities range from mild learning disabilities to severe physical needs. Therefore, the special educator must use many different teaching techniques—"individualizing" each student's instruction according to his/her IEP. Some special educators pull students from regular education classes to conduct small group or one-onone interactions. Students work directly with the teacher on specific skills in academics or social behaviors. This practice is losing popularity because of a new trend in education—the inclusion model.
The inclusion model moves special educators into the regular classroom, so that the students with special needs may take part in the instruction and activities of the regular classroom. This "inclusion" makes it possible for students with special needs to have exposure to grade-level content and grade-level peers, improving the students' overall performance in attaining IEP goals.
Where Do Special Educators Work?
The vast majority of special educators work in a public school setting. Some private schools have funds to hire special educators. If a child attends a private school that does not have special education support, he/she is still able to obtain special education services from the public school. Many special educators work as tutors, either on their own or for third party companies (Sylvan, etc.) that specialize in one-on-one instruction with students who are having trouble in regular education subject areas.
In addition, a small number of special educators work as student advocates—teachers trained to represent parents at IEP and other educational meetings. Advocates are paid representatives who help parents understand the educational jargon at an IEP meeting. The advocate's knowledge of the law, terminology, and student/parent rights, is valuable in establishing realistic goals for the student. Advocates make sure that the student is receiving all the special education services which he/she needs based on the child's current level of performance. To find an advocate near you, visit http://www.yellowpagesforkids.com/ and select your state from the drop-down menu.
What Should I Expect from Special Education?
Special educators who work with students with severe disabilities teach basic concepts of self-care and self-help skills. Special educators expand the student's learning experience in all aspects of daily life, including social skills, communication skills, and self-care skills. Special educators who work with students with severe special needs often become very involved with the students' families in order to maximize student potential.
How do I know if my child needs special education services?
How do I know if my child is eligible for special services?
Who will test my child to determine his/her eligibility for special services?
My child is eligible for special education. What next?
What happens if my child is not eligible for services?
How will you classify my child's need for special services?
What is your specialty area (learning disabilities, behavioral disabilities, mild mental impairment, etc.)?
What is a "change" in the student's IEP?
Once in special education, will my child always be in special education?
How much time will my child be out of the regular education classroom?


text:
Q: Ruby optparse odd behavior with exception handling I was working on making a CLI ruby tool a bit more robust with error handling of the CLI component of the tool.  I'm using optparse and the documentation shows that flags can have mandatory and optional arguments.  I'm just seeing some odd/annoying behavior.
For one case where the argument of the flag is mandatory (using the equal sign), I was trying to make it fail but it still worked, it just grabbed the next flag ('-u') as the argument to that flag instead which causes the rest of the parsing to barf in a less than ideal way.  I guess this is kind of OK since '-....' can be valid but since I have it using the equal sign for setting the value of the switch, I'd assume that the 'space' style assignment wouldn't work.
op = OptionParser.new do |x|
  x.on("-u", "--user=USER", "user flag") do |user|  options[:user] = user end
  x.on("-d", "--db=DATABASE", "database flag") do |db|  options[:db] = db end
end

If I pass the following CLI input:
myprog -u -d mydb positionalarg

Then during parsing, it sets options[:user] to be -d, and options[:db] is nil since it is not encountered and there are 2 positional arguments instead of 1. Obviously this is a user error but I want to catch it and display a real error and not just return an error (in the case of the tool) that only one positional argument should be passed from the following list: ...  The true issue is that the -u flag is missing it's required argument so given that optparse has a ArgumentMissing exception, I'd assume that's what .parse! would throw.
However, with a flag that has an optional argument (using the equal sign), it is in fact optional if you do:  -a -b=something  but this does not work: -a something -b=somethingelse.  The only way to force the value is to use the equal sign: -a=something -b=somethingelse.  Given the previous behavior with the mandatory argument with an equal sign, I'd wouldn't think would be the case. Example:
op = OptionParser.new do |x|
  x.on("-u", "--user[=USER]", "user flag") do |user|  options[:user] = user unless user.nil? end
  x.on("-d", "--db=DATABASE", "database flag") do |db|  options[:db] = db end
end

So with parsing:
myprog -u -d mydb positionalarg

Then options[:user] is nil (ok) and options[:db] is mydb and there is one positional arg left over.
With this parsing:
myprog -u myuser -d mydb positionalarg

Then options[:user] is nil (not ok) and options[:db] is mydb and there are two positional args left over: myuser and positionalarg (not ok).  My error checking, again, barfs with the positional arg count.  It seems that if with mandatory flag arguments, that if both space and = works, then for optional flag args, it should be the same but this is not the case.
The other issue is that the flags with optional args (using space) are fine except if they are at the end of the command and then takes the positional argument as the flags argument.
Example:
op = OptionParser.new do |x|
  x.on("-u", "--user [USER]", "user flag") do |user|  options[:user] = user unless user.nil? end
  x.on("-d", "--db=DATABASE", "database flag") do |db|  options[:db] = db end
end

So with parsing:
myprog -d mydb -u positionalarg

Then options[:db] is mydb (ok) and options[:user] is positionalarg and there are no positional arguments left over. Note that -d mydb works with the space even those I specify it with an equals sign.
Seems a lot of people do ruby CLIs by doing optparse .parse! and take the remaining entries in ARGV as the positional arguments, but I'm thinking it might be better to strip the positional arguments off the far end first before passing ARGV to optparse (except this fails in the event of variable number of positional arguments.
I'm confident I can program around all this but I'd prefer not to if there are ways to do it in optparse that I'm unaware of.
Maybe the best thing would be to avoid flags with optional arguments :), but any advice would be appreciated.

A: Why do you use the = in the definition of the options? The example in the documentation don't use them.
If I define this MWE as test.rb:
require 'optparse'

puts "\n=Call with #{ARGV.inspect}"
options = {}

op = OptionParser.new do |x|
  x.on("-u", "--user [USER]", "user flag") do |user|  options[:user] = user unless user.nil? end
  x.on("-d", "--db DATABASE", "database flag") do |db|  options[:db] = db end
end

op.parse!

puts "Options: #{options.inspect}"
puts "ARGV     #{ARGV.inspect}"

And call it with this batfile (Windows, remove the @echo off for Linux):
@echo off
test.rb -h
test.rb -u -d mydb positionalarg
test.rb -u myuser -d mydb positionalarg
test.rb --user=myuser -d mydb positionalarg

I get:
=Call with ["-h"]
Usage: test [options]
    -u, --user [USER]                user flag
    -d, --db DATABASE                database flag

=Call with ["-u", "-d", "mydb", "positionalarg"]
Options: {:db=>"mydb"}
ARGV     ["positionalarg"]

=Call with ["-u", "myuser", "-d", "mydb", "positionalarg"]
Options: {:user=>"myuser", :db=>"mydb"}
ARGV     ["positionalarg"]

=Call with ["--user=myuser", "-d", "mydb", "positionalarg"]
Options: {:user=>"myuser", :db=>"mydb"}
ARGV     ["positionalarg"]

redpajama_set_name: RedPajamaStackExchange


CESI | Where does CESI stand?
Home > What we do > Where does CESI stand?
While policies can evolve and change, there are certain values which constantly remain at the core of CESI's work. In 2013, CESI defined (and updated in 2016) these values for member organisations and interested parties to know exactly where CESI stands as a European trade union. Read about where CESI stands.
The policy areas dealt with in CESI are separated according to their cross-industry or sectoral nature. Cross-industry policies are dealt with in Commissions, while sectoral policies are discussed within Trade Councils.
Those with a sectoral focus can be distributed to the Trade Councils within CESI, with areas including Central government administration, Local and regional government, Education, Defence, Security, Justice and Health. On the policy work conducted within Trade Councils please consult the relevant webpages.
CESI also engages in wider, more cross-sectoral debates at EU level. These issues are generally discussed within CESI's Commissions and as such focus on issue such as Equal rights and Employment and Social Affairs. For more information on the policy work conducted within Commissions please consult the relevant webpages.
All news about CESI's work on horizontal EU social and employment policies is available here. News about CESI related to cross-sector economic governance and fiscal coordination in the EU can be accessed here.
To find more information on what CESI thinks, please consult our policy positions according to the sectors we represent.
Countering and preventing third-party violence at work.


Ranked as 31756 on our all-time top downloads list with 1200 downloads.
Description Designed for beginner Japanese learners. This small program will help you memorize Kana characters (Hiragana and Katakana) like you would do with flashcards. The calculator shows you a random character and you must find the translation, you can then check your answer by pressing Enter. (French and English versions included).


Aluminum is the most popular guttering material used for people buying seamless gutters in Irvine. At Irvine Rain Gutter Pros, we estimate that over 75 percent of our clients choose aluminum seamless gutters for their residential and commercial properties.Irvine Rain Gutter Pros specializes in seamless Aluminum gutter installation for several reasons.
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First, Aluminum gutters have the benefit of resisting rust. Unlike steel used for rain gutters, aluminum is a superior choice when considering corrosion and staining left by rusting metal gutters.Choosing aluminum means you will eliminate the risk of unsightly rust on the exterior of your home. Our aluminum gutters have the added benefit of a preventative corrosion coating, limiting the likelihood that your metal gutters will degrade when exposed to water and moisture.
Secondly, gutters made with aluminum are lighter weight than copper or steel, allowing less laborious installation.There are several grades of aluminum available for guttering, but Irvine Rain Gutter Pros always recommends the thickest gauge you can afford.Thicker gauge aluminum possesses longer lasting qualities for gutters and downspouts, offering our clients more return on investment. It may seem responsible to spend less on lighter gauge aluminum, but we meet with many new clients each month looking to replace their flimsy gutter systems.As with most things in life, you get what you pay for with rain gutters, and we encourage our customers to invest wisely.
Finally, rain gutters formed in continuous seamless sheets of high quality aluminum have the advantage of being paint-able.This will allow your new guttering to perfectly match your trim and fascia boards, blending into your home. Aluminum is malleable, lightweight and durable, making it an easy choice for paintable gutter systems.
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Seamless Aluminum gutters will not leak because they are fashioned from one long, continuous sheet of aluminum on site. We are on a mission to rid Irvine of eyesore guttering, one home at a time.
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Contact Irvine Rain Gutter Pros today to find out more about our Aluminum Seamless Gutters. We are more than happy to travel outside of Irvine. We have installed hundreds of aluminum gutters in cities such as Huntington Beach, Costa Mesa and Long Beach.


Umbrella for Democratic Change (UDC) might lose out on a USD 3million (P30 million) windfall for the 2019 general election campaign should it fail to resolve its internal fights and forge ahead to expel Botswana Movement for Democracy.
Democratic Alliance (DA) and African Liberal Network (ALN) are said to have offered to mediate in Umbrella for Democratic Change (UDC) impasse which has polarised the coalition movement beyond reproach. UDC National Executive Committee (NEC) has since suspended BMD and speculation has been rife that the party might be expelled.
DA has bilateral relations with BMD and both are members of ALN. According to information reaching this publication DA President Mmusi Maimane and ALN leader Stevens Mokgalapa met with BMD leader Advocate Sidney Pilane in South Africa this past Sunday where they shared their desire to break the impasse.This publication has it on good authority that both the DA and ALN are prepared to commit about USD 3 million (P30 million) to the UDC 2019 election campaign but that can only happen if BMD is part of UDC.
The relationship between UDC and ALN-DA is consequent to BMD being part of the coalition.BMD has been accusing their partners in the UDC- Botswana Congress Party (BCP) and Botswana National Front (BNF) of using Advocate Pilane as a scapegoat for the UDC problems. BMD has been suspended on claims that its president has not received warm welcome from the public and members of the UDC.
Insiders within the BMD have revealed to this publication that a faction within the BMD NEC has initiated the talks with DA and ALN to intervene. This is said to have happened a fortnight ago. Information gathered suggest that Advocate Pilane seemed at the Sunday meeting to be willing to concede as Mokgalapa and Maimane persuaded him to consider letting go of some of the constituencies and wards for the sake of progress in the UDC.
Since the split of BMD which saw the formation of Alliance for Progressives (AP), there has been calls within the UDC for BMD to release some of its constituencies since the split has affected its capability to deliver on the 13 constituencies allocated it. BMD was yesterday (Thursday 18th) scheduled to make representation regarding the suspension. The party has since indicated that the suspension is a nullity and the grounds raised for the suspension are of no basis.
Following the Sunday meeting Maimane and Mokgalapa are said to have proposed to meet UDC President Advocate Duma Boko upon his return from Geneva where he traveled on Parliament official business. The meeting is expected to discuss the outcome of the meeting with the BMD leader and what can be done going forward to resolve the UDC problems.
BMD National Spokesperson Rasina Rasina said he is constrained to discuss the matter and referred all enquiries to Advocate Pilane, who could not be reached as his mobile phone rang unanswered. However, ALN leader Mokgalapa confirmed to this publication in a telephonic interview that they did meet Advocate Pilane on the UDC matter.
"We met the BMD President and Secretary General Gilbert Mangole in Pretoria on Sunday. We are worried about the current situation in the UDC. We are going to be meeting UDC President on Friday when he returns because he has travelled abroad. We will meet him at the airport before he connects a flight to Botswana," said Mokgalapa.
He revealed that they could not meet Advocate Boko when he left for Geneva because of time constraints when he was connecting flights at OR Tambo International Airport in Johannesburg. The ALN leader meanwhile refuted claims that they have committed funding to the UDC 2019 election campaign. "That is not true we have not committed any funding to UDC.
Remember only BMD in the coalition is a member of ALN. Other members of the UDC are not. We only have a relationship with them because we strongly believe in opposition coalition. We are only assisting in capacity building and strategies," Mokgalapa pointed out.
During the February 2018 UDC Congress at Boipuso Hall Maimane told members of the media that UDC is a viable project and the DA would do all it can to help the UDC win the 2019 general elections. He stated that with a united opposition possibility of taking over power from Botswana Democratic Party (BDP) is very likely. He explained that they would assist with strategies for campaigns and other resources including funding.
DA and ALN are said to have started work on campaign strategies for the UDC and an analysis has been made on how the election should be approached. Advocate Boko also last month revealed that a team of experts is working on the campaign strategy for the UDC which will ensure the coalition wins the 2019 general election.


I suppose noodles might get a little strange (although they soften up quickly enough in the microwave with some butter). I find that homemade rice (unlike restaurant rice) stays soft much longer, especially the whole-grain rice I invariably make.
I make huge pots of both at the beginning of the week and eat one or the other for lunch and supper each night, because I hate to cook.
Fantastic. I will consider making a huge batch on sundays!


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Our plumbers operate 24 hours, 7 days a week in Croydon. So no matter what time you require a plumber in Croydon, we will be there no matter what!
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Peter Guo (郭宝峰), a self-described "troublemaker in Amoy (Xiamen)" experienced what everyone who Twitters or blogs in China is not-so-secretly afraid of – one of his blog posts got him in trouble with the police, who threw him in jail.
He was one of as many as seven bloggers who were detained after writing about a 25-year-old woman, Yan Xiaoling, who had allegedly been gang-raped and murdered by someone connected to local authorities in Fujian. Guo's crime: reposting something that had already been put on a BBS in Fujian Province, titled "Yan Xiaoling (嚴曉玲) much more miserable than Deng Yujiao (鄧玉嬌)." Deng Yujiao is a waitress turned national heroine who became famous for stabbing an official who may have sexually assaulted her. He then posted a video he had found, completely unedited, in which Yan Xiaoling's mother accused local authorities of trying to cover up the case.
Five police came to arrest him the day after, on July 15. He has the story now out on his personal blog.
Five Mawei police (Mawei is a district of Fuzhou city) came to my company in Xiamen in the afternoon July 15, and confiscated my computer and took my cell phone away, so I was unable to contact with any of my acquaintances until they took me to the office building of Mawei Police Bureau and put my phone on the desk in front of me so that I could take this phone at any time when the police who interrogated me didn't notice. No one knew that I was seized except my boss who is also a friend of mine.
And then, they asked me to lead them to my residential place from where they took away my laptop. A police saw my iPod Touch on bedside and asked what it was. I cheated them that it was just an ordinary MP4 player rather than a memory device, so he didn't take it away. The police never thought this small device would become a powerful weapon.
After hours of interrogation, the police who had detained him became tired. He seized the opportunity to grab his phone and twitter quickly "Pls help me, I grasp the phone during police sleep" and "I have been arrested by Mawei police, SOS." The police noticed and grabbed his phone from him, but the damage had been done. His tweets were translated into Chinese by a friend and retweeted across the web.
But even more importantly, his sister – an undergrad at Xiamen University – went to his residence and picked up his name card book and iPod touch, using it to send messages to all the names in his name card book. Using the info already in the touch, she notified his friends on social networking sites, sent messages to journalists and set up a donation account to hire a lawyer.
He was finally released last week – an event he figures would have never happened if it weren't for the immense public pressure on the Mawei police. Considering none of the other bloggers who talked about Yan Xiaoling have made it out yet, he's probably right.
I am Baofeng Guo aka Peter Guo or Amoiist on Twitter.
After 16 days of detention, I was free. Of course, the release of me is totally the result of efforts made by all of netizens from home and abroad. Without them, the result would have been quite different. I was most grateful to people for their attention, donations, as well as the postcards although I didn't receive any of them during the period in the cell, and special thanks must be given to my sister Guo Lihua who is a undergraduate student from Xiamen University.
I hope that the netizens can continue to pay more attention to this case, because other people involved into this case are suffering and I am not totally free of trouble. In addition, I want to let all people know when and why I was detained, and how did I successfully turned the disadvantages into favorable condition.
First of all, I don't care about what people said about me, but I must make the following clarification: unlike some rumors said, I just merely uploaded a video to a video website abroad without any edits, translation and etc. rather than copy and paste the posts related to a woman called Yan Xiaoling to forums. If not, they had no reason to release me so quickly.
Second, I was seized at company and taken to Mawei police station in the night of July 15. They took my Blackberry cellphone for cutting off connection of me with others. After arrival at the police station, they put my phone on the desk in front of me. It was early morning and they had interrogated me for consecutive 4 hours, so two police were tired.
China: Calling Guo Baofeng back home for dinner!


At times, it feels like Jay-Z and Beyoncé talk to each other through their songs. In Lemonade, Beyoncé overcame the pain and insecurity caused by infidelity. And Jay-Z's 4:44 operates almost as an apology to Queen Bey, ruminating on commitment, fatherhood, and making up for the past. So it comes as no surprise that the power couple recorded a joint album during their much discussed rocky patch. But where is it?
At the time, though, Lemonade was closer to completion than Bey and Jay's shared record, and so it beat out the joint album to a release date because, of course, Beyoncé waits for no man. "We still have a lot of that music," Jay-Z reveals.
Unfortunately, the most important question remains unanswered: Will we ever get to hear the album?!
From the outside looking in, it seems like Beyoncé and Jay-Z have always used their collaborations to provide glimpses into their relationship. "'03 Bonnie and Clyde" hinted at a burgeoning romance, "Crazy in Love" captured infatuation, and "On the Run," released over ten years after their first duet, was a soundtrack for undying devotion.
Even if this particular joint album never sees the light of day, we're pretty confident Jay and Bey will be collaborating long into the future. Here's hoping we have a track about being gangsta grandparents to look forward to in 2040.


text:
Oregon Graduation Rates Hit 80%, An All-Time High. Here's The Story In 6 Districts.
By Elizabeth Miller (OPB)
Portland, Ore. Jan. 23, 2020 2:45 p.m.
Oregon's graduation rate has hit 80%, an all-time high since the state tightened how it tracks the rate more than a decade ago. That represents an increase of 600 students in the Class of 2019 who received a diploma, compared to 2018.
Graduation rates for students of color continue to increase, with almost every race/ethnicity group increasing 2%. Other student groups with a history of struggling to graduate saw rates increase, including a 4-point improvement for migrant students.
ODE credits a dedicated effort to provide graduation specialists for students in migrant education programs across the state.
It's all one more step toward the state's goal of getting 100% of students successfully through high school, by 2025.
Related: Class Of 2025: Follow Students From 1st Grade To Graduation
And with an infusion of funds later this year via the state's new Student Success Act, districts hope to offer students more opportunities and support, where those have been lacking or missing.
The stories of each district show how important such targeted, on-the-ground supports can be for helping students learn.
Officials from nearly every district interviewed for this story mentioned Measure 98 funding as helpful in graduation efforts. Oregon voters approved Measure 98, or the "High School Success Act," in 2016 to invest in career education, dropout prevention and advanced coursework.
Under the Student Success Act, Measure 98's focus areas receive more funding.
The Student Success Act also offers districts more targeted support – the kinds of efforts that drove Oregon's graduation rate to 80%.
But the story is about more than the numbers.
Below are the stories at six districts across Oregon, listed with their 4-year cohort graduation rate. As the state looks ahead to a 2020-21 school year with more resources, the work at these districts shows that certain efforts – targeting specific student groups, carefully tracking individual students and supporting their mental and life needs beyond academics – are making an impact.
Estacada School District and Sweet Home School District, 66.1% and 84.5%
The Estacada School District saw a 4.3% increase from last year. The district credits career technical education programs that keep students engaged. At the high school level, there is a dedicated focus to track students from the moment they start high school.
But even with last year's increase, the district is still well below the state average. Officials recognize the work that needs to be done, especially for students with disabilities and those learning English. The district in rural Clackamas County has been moving to better integrate students with disabilities into classroom settings with other students.
Related: Oregon High School Recognized For Inclusivity
"I think we're on the right track and have brought in additional supports in both of those areas and the gains have really mirrored that," said Estacada School District communications director Maggie Kelly. "I would like to see a few more years of doing the good, hard work with inclusive practices."
At another rural district, sustained efforts to help students with particular needs – such as disabilities – appear to be paying off.
Linn County's Sweet Home School District had one of the largest increases in Oregon, a 13.7% improvement over last year. Superintendent Tom Yahraes said that good news is the result of a years-long effort to get more students to graduation.
The district created a high school "care team" to track student progress and started twice-weekly homework clubs. They've added winter and summer school opportunities for students.
"Instead of … a student failing and having to make up an entire class, we work with students and their teachers to target particular standards or curriculum the student needed to redo," Yahraes said.
Yahreas said it was the district's overlapping resources that led to graduation rate increases for students experiencing homelessness, economically disadvantaged students and students with disabilities.
These more dedicated efforts add to other changes, including a longer school week.
Portland Public Schools, 80.5%
Oregon's largest school district mirrors the state in its overall graduation rate. Since 2013, the district's graduation rate has increased by 10 percentage points.
The increase is evident for a school like North Portland's Jefferson High. The school's graduation rate is up 4 percentage points, with notable gains for black and Latino students, as well as students with disabilities. That increase caps a 22-point improvement over the last six years.
A banner at Jefferson High School on the first day of school on Aug. 28, 2019.
Elizabeth Miller/OPB
PPS Regional Superintendent Joe LaFontaine credits a method Jefferson has used for years – students get separated into small groups and a group of teachers tracks each group's performance over their high school career, making sure students stay on track.
"They've been employing some strategies that we as an entire system have just begun to employ," said LaFontaine.
Other PPS high school rates increased, but some fell, including Madison High School, where graduation rates fell 4 percentage points. And the same student groups showing increased rates at Jefferson showed decreased rates at Madison.
LaFontaine said packing up for a temporary school and a leadership change could have factored into the decrease. Students at Madison moved out of their Northeast Portland school last year to attend the Marshall High School campus in Southeast, while construction crews overhaul Madison. LaFontaine is hopeful a system like the one at Jefferson will help students at PPS' other high schools.
One of Portland's alternative programs, Alliance, saw a large increase in 4-year graduation rates. But two-thirds of its students still don't graduate in four years.
Bethel School District, 75.5%
In Northwest Eugene, the Bethel School District serves about 5,600 students.
District curriculum director Kee Zublin was excited to see a 9.5% increase for the district's overall 4-year graduation rate.
As a district, Zublin said there have been changes to teach more effectively – implementing trauma-informed and restorative practices and training teachers to better understand student experiences.
"We think about what their needs might be within a classroom setting," Zublin said. "If you place their needs central to your planning, you not only help those learners who have been marginalized, but you're not disadvantaging the rest of the learners in any way."
Related: Oregon Students Fight For Excused Mental Health Days
When Oregon lawmakers toured the state in 2018 to design the Student Success Act, they repeatedly heard from teachers and students about mental health and traumatic experiences presenting barriers to student learning. Those statements helped make student mental health a priority for the $1 billion annual spending headed to public schools this fall.
Bethel is also one of many districts relying more heavily on data to better reach students in the classroom, with a focus on students from underserved groups, like students experiencing homelessness, or those children from households where English isn't the primary language.
Coquille School District and Crook County School District, 61.1% and 72.9%
Often, the numbers don't tell the whole story.
That's the case with the coastal Coquille School District and central Oregon's Crook County. Both districts saw their grad rates go down.
Coquille's 4-year cohort graduation rate is 61.1%, down from last year. Of the district's two high schools, that number is closer to the 4-year graduation rate for Winter Lakes School, the district's alternative school.
That school's graduation rate is 48.1%. But that's not the number Coquille Superintendent Tim Sweeney pays attention to.
"When you tell me that our Winter Lakes 5-year completer rate is 60%, I'm pretty excited about that number," said Sweeney.
Coquille Superintendent Tim Sweeney speaking with Oregon Gov. Kate Brown at the Capitol on March 19, 2019.
The completer rate includes graduates, plus students who earned a GED or other form of high school diploma. Sweeney said Winter Lakes students tend to come to the school behind on credits. For him, an improving completer rate is a good sign.
"That's showing that they're really doing what they set out to do – which is to help students who weren't going to graduate to finally get their schooling."
The school serves students from districts around the region – some students with special needs, others who need help beyond academics.
With a new building that just opened this month, Sweeney said the district is improving its supports for students. There are laundry and shower facilities for students who might need those, and the district is trying out a "swing" schedule so the high school can stay open until 8 p.m.
"We're really working on some of the lowest levels of Maslow's hierarchy – where am I going to be fed? Where am I going to be warm?" said Sweeney.
In the Crook County schools, the graduation rate for the district's students appeared to decrease by 5 percentage points. That drop isn't reflected in the rate for the district's high school. The other school listed for the district is Pioneer Secondary Alternative High School.
According to the district, the graduation figure for the alternative high school actually reflects three programs – an in-patient treatment center, a work-education program and the actual Pioneer High School.
Michelle Jonas, principal of Crook County High School and Pioneer, said the numbers attributed to Pioneer don't reflect the progress students there have made.
"It's hard for these Pioneer students and staff that are working really hard and getting credits and graduating and walking across that stage when the data just does not reflect that because a majority of the students aren't Crook County residents," said Jonas.
Jonas said she's proud of her district's progress. And while analyzing and responding to data is a growing part of a school administrator's job, for Jonas, student success is about looking beyond the numbers.
"Every data point is a person, so we try to focus on knowing the kids, making sure they have mentoring, and just keeping them focused, taking [career-technical education] classes, things that are going to keep them plugged into school and seeing life out of high school and how graduating can help prepare them for their future."
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