6,069 research outputs found
Evaporation induced traversability of the Einstein--Rosen wormhole
Suppose, the Universe comes into existence (as classical spacetime) already
with an empty spherically symmetric macroscopic wormhole present in it.
Classically the wormhole would evolve into a part of the Schwarzschild space
and thus would not allow any signal to traverse it. I consider semiclassical
corrections to that picture and build a model of an evaporating wormhole. The
model is based on the assumption that the vacuum polarization and its
backreaction on the geometry of the wormhole are weak. The lack of information
about the era preceding the emergence of the wormhole results in appearance of
three parameters which -- along with the initial mass -- determine the
evolution of the wormhole. For some values of these parameters the wormhole
turns out to be long-lived enough to be traversed and to transform into a time
machine.Comment: v.2 A bit of discussion has been added and a few references v.3
Insignificant changes to match the published versio
The time travel paradox
We define the time travel paradox in physical terms and prove its existence
by constructing an explicit example. We argue further that in theories -- such
as general relativity -- where the spacetime geometry is subject to nothing but
differential equations and initial data no paradoxes arise.Comment: Minor changes + an explanatory note concerning the lions with the
same world line
The exact tree-level calculation of the dark photon production in high-energy electron scattering at the CERN SPS
Dark photon () that couples to the standard model fermions via the
kinetic mixing with photons and serves as a mediator of dark matter production
could be observed in the high-energy electron scattering off nuclei followed by the decay. We have
performed the exact, tree-level calculations of the production cross
sections and implemented them in the program for the full simulation of such
events in the experiment NA64 at the CERN SPS. Using simulations results, we
study the missing energy signature for the bremsstrahlung
invisible decay that permits the determination of the mixing
strength in a wide, from sub-MeV to sub-GeV, mass range. We refine and
expand our earlier studies of this signature for discovering by including
corrections to the previously used calculations based on the improved
Weizsaker-Williams approximation, which turn out to be significant. We compare
our cross sections values with the results from other calculations and find a
good agreement between them. The possibility of future measurements with
high-energy electron beams and the sensitivity to are briefly discussed.Comment: 11 pages, 6 figures, revised version, improved cross-section
integrator is used, comparison with bremsstrahlung spectrum is added, final
conclusions remain unchange
Missing energy signature from invisible decays of dark photons at the CERN SPS
The dark photon () production through the mixing with the bremsstrahlung
photon from the electron scattering off nuclei can be accompanied by the
dominant invisible decay into dark-sector particles. In this work we
discuss the missing energy signature of this process in the experiment NA64
aiming at the search for decays with a high-energy electron
beam at the CERN SPS. We show the distinctive distributions of variables that
can be used to distinguish the signal from background. The
results of the detailed simulation of the detector response for the events with
and without emission are presented. The efficiency of the signal event
selection is estimated. It is used to evaluate the sensitivity of the
experiment and show that it allows to probe the still unexplored area of the
mixing strength and masses up to
GeV. The results obtained are compared with the results
from other calculations. In the case of the signal observation, a possibility
of extraction of the parameters and by using the missing
energy spectrum shape is discussed. We consider as an example the with the
mass 16.7 MeV and mixing , which can explain an
excess of events recently observed in nuclear transitions of an excited state
of Be. We show that if such exists its invisible decay can be observed
in NA64 within a month of running, while data accumulated during a few months
would allow also to determine the and parameters.Comment: 12 pages, 15 figures. Revised versio
The quantum inequalities do not forbid spacetime shortcuts
A class of spacetimes (comprising the Alcubierre bubble, Krasnikov tube, and
a certain type of wormholes) is considered that admits `superluminal travel' in
a strictly defined sense. Such spacetimes (they are called `shortcuts' in this
paper) were suspected to be impossible because calculations based on `quantum
inequalities' suggest that their existence would involve Planck-scale energy
densities and hence unphysically large values of the `total amount of negative
energy' E_tot. I argue that the spacetimes of this type may not be unphysical
at all. By explicit examples I prove that: 1) the relevant quantum inequality
does not (always) imply large energy densities; 2) large densities may not lead
to large values of E_tot; 3) large E_tot, being physically meaningless in some
relevant situations, does not necessarily exclude shortcuts.Comment: Minor corrections and addition
What does the Letelier-Gal'tsov metric describe?
Recently the structure of the Letelier-Gal'tsov spacetime has become a matter
of some controversy. I show that the metric proposed in \cite{letgal} is
defined only on a dense subset of the whole manifold. In the case when it can
be defined on the remainder by continuity, the resulting spacetime corresponds
to a system of parallel cosmic strings at rest w.r.t. each other.Comment: 4pages, 1 figure. v2 A few words are changed in accordance with the
published versio
Quantum field theory and time machines
We analyze the "F-locality condition" (proposed by Kay to be a mathematical
implementation of a philosophical bias related to the equivalence principle, we
call it the "GH-equivalence principle"), which is often used to build a
generalization of quantum field theory to non-globally hyperbolic spacetimes.
In particular we argue that the theorem proved by Kay, Radzikowski, and Wald to
the effect that time machines with compactly generated Cauchy horizons are
incompatible with the F-locality condition actually does not support the
"chronology protection conjecture", but rather testifies that the F-locality
condition must be modified or abandoned. We also show that this condition
imposes a severe restriction on the geometry of the world (it is just this
restriction that comes into conflict with the existence of a time machine),
which does not follow from the above mentioned philosophical bias. So, one need
not sacrifice the GH-equivalence principle to "emend" the F-locality condition.
As an example we consider a particular modification, the "MF-locality
condition". The theory obtained by replacing the F-locality condition with the
MF-locality condition possesses a few attractive features. One of them is that
it is consistent with both locality and the existence of time machines.Comment: Revtex, 14 pages, 1 .ps figure. To appear in Phys. Rev. D More
detailed discussion is given on the MF-locality condition. Minor corrections
in terminolog
Gravitational strings. Do we see one?
I present a class of objects called gravitational strings (GS) for their
similarity to the conventional cosmic strings: even though the former are just
singularities in flat spacetime, both varieties are equally "realistic", they
may play equally important cosmological r\^ole and their lensing properties are
akin. I argue that the enigmatic object CSL-1 is an evidence in favor of the
existence of GS.Comment: The published version. Minor correction
Lepton flavor violation at linear collider experiments in supersymmetric grand unified theories
Lepton flavor violation at linear collider experiments is discussed. We show
that detectable lepton flavor violation could occur through scalar lepton pair
production and decay in the supersymmetric SU(5) grand unified theory in spite
of the stringent present experimental constraints by rare process searches.
Possible cross sections about 40fb for an e+e- collider and 280fb for an e-e-
collider are illustrated.Comment: 12 pages, including 3 figures, REVTeX, eps
Closed Timelike Curves in Relativistic Computation
In this paper, we investigate the possibility of using closed timelike curves
(CTCs) in relativistic hypercomputation. We introduce a wormhole based
hypercomputation scenario which is free from the common worries, such as the
blueshift problem. We also discuss the physical reasonability of our scenario,
and why we cannot simply ignore the possibility of the existence of spacetimes
containing CTCs.Comment: 17 pages, 5 figure
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