74 research outputs found
The physical world as a virtual reality: a prima facie case
This paper explores the idea that the universe is a virtual reality created by information
processing, and relates this strange idea to the findings of modern physics about the physical
world. The virtual reality concept is familiar to us from online worlds, but the world as a virtual
reality is usually a subject for science fiction rather than science. Yet logically the world could be
an information simulation running on a three-dimensional space-time screen. Indeed, that the
essence of the universe is information has advantages, e.g. if matter, charge, energy and
movement are aspects of information, the many conservation laws could become a single law of
conservation of information. If the universe were a virtual reality, its creation at the big bang
would no longer be paradoxical, as every virtual system must be booted up. It is suggested that
whether the world is an objective or a virtual reality is a matter for science to resolve, and
computer science could help. If one could derive core properties like space, time, light, matter and
movement from information processing, such a model could reconcile relativity and quantum
theories, with the former being how information processing creates space-time, and the latter how
it creates energy and matter
Nova Geminorum 1912 and the Origin of the Idea of Gravitational Lensing
Einstein's early calculations of gravitational lensing, contained in a
scratch notebook and dated to the spring of 1912, are reexamined. A hitherto
unknown letter by Einstein suggests that he entertained the idea of explaining
the phenomenon of new stars by gravitational lensing in the fall of 1915 much
more seriously than was previously assumed. A reexamination of the relevant
calculations by Einstein shows that, indeed, at least some of them most likely
date from early October 1915. But in support of earlier historical
interpretation of Einstein's notes, it is argued that the appearance of Nova
Geminorum 1912 (DN Gem) in March 1912 may, in fact, provide a relevant context
and motivation for Einstein's lensing calculations on the occasion of his first
meeting with Erwin Freundlich during a visit in Berlin in April 1912. We also
comment on the significance of Einstein's consideration of gravitational
lensing in the fall of 1915 for the reconstruction of Einstein's final steps in
his path towards general relativity.Comment: 31 p
The Atacama Cosmology Telescope: Dynamical Masses and Scaling Relations for a Sample of Massive Sunyaev-Zel'dovich Effect Selected Galaxy Clusters
We present the first dynamical mass estimates and scaling relations for a
sample of Sunyaev-Zel'dovich effect (SZE) selected galaxy clusters. The sample
consists of 16 massive clusters detected with the Atacama Cosmology Telescope
(ACT) over a 455 sq. deg. area of the southern sky. Deep multi-object
spectroscopic observations were taken to secure intermediate-resolution
(R~700-800) spectra and redshifts for ~60 member galaxies on average per
cluster. The dynamical masses M_200c of the clusters have been calculated using
simulation-based scaling relations between velocity dispersion and mass. The
sample has a median redshift z=0.50 and a median mass M_200c~12e14 Msun/h70
with a lower limit M_200c~6e14 Msun/h70, consistent with the expectations for
the ACT southern sky survey. These masses are compared to the ACT SZE
properties of the sample, specifically, the match-filtered central SZE
amplitude y, the central Compton parameter y0, and the integrated Compton
signal Y_200c, which we use to derive SZE-Mass scaling relations. All SZE
estimators correlate with dynamical mass with low intrinsic scatter (<~20%), in
agreement with numerical simulations. We explore the effects of various
systematic effects on these scaling relations, including the correlation
between observables and the influence of dynamically disturbed clusters. Using
the 3-dimensional information available, we divide the sample into relaxed and
disturbed clusters and find that ~50% of the clusters are disturbed. There are
hints that disturbed systems might bias the scaling relations but given the
current sample sizes these differences are not significant; further studies
including more clusters are required to assess the impact of these clusters on
the scaling relations.Comment: 15 pages, 4 figures. Accepted for publication in The Astrophysical
Journal; matches published version. Full Table 8 with complete spectroscopic
member sample available in machine-readable form in the journal site and upon
request to C. Sif\'o
Metallicity Evolution of Damped Lyman-alpha Systems out to z~5
We present chemical abundance measurements for 47 damped Lyman-alpha systems
(DLAs), 30 at z>4, observed with the Echellette Spectrograph and Imager and the
High Resolution Echelle Spectrometer on the Keck telescopes. HI column
densities of the DLAs are measured with Voigt profile fits to the Lyman-alpha
profiles, and we find an increased number of false DLA identifications with
SDSS at z>4 due to the increased density of the Lyman-alpha forest. Ionic
column densities are determined using the apparent optical depth method, and we
combine our new metallicity measurements with 195 from previous surveys to
determine the evolution of the cosmic metallicity of neutral gas. We find the
metallicity of DLAs decreases with increasing redshift, improving the
significance of the trend and extending it to higher redshifts, with a linear
fit of -0.22+-0.03 dex per unit redshift from z=0.09-5.06. The metallicity
'floor' of ~1/600 solar continues out to z~5, despite our sensitivity for
finding DLAs with much lower metallicities. However, this floor is not
statistically different from a steep tail to the distribution. We also find
that the intrinsic scatter of metallicity among DLAs of ~0.5 dex continues out
to z~5. In addition, the metallicity distribution and the alpha/Fe ratios of
z>2 DLAs are consistent with being drawn from the same parent population with
those of halo stars. It is therefore possible that the halo stars in the Milky
Way formed out of gas that commonly exhibits DLA absorption at z>2.Comment: Accepted for publication in the Astrophysical Journal. 24 pages, 17
figure
Stability in Cosmology, from Einstein to Inflation
I investigate the role of stability in cosmology through two episodes from the recent history of cosmology: (1) Einstein’s static universe and Eddington’s demonstration of its instability, and (2) the flatness problem of the hot big bang model and its claimed solution by inflationary theory. These episodes illustrate differing reactions to instability in cosmological models, both positive ones and negative ones. To provide some context to these reactions, I also situate them in relation to perspectives on stability from dynamical systems theory and its epistemology. This reveals, for example, an insistence on stability as an extreme position in relation to the spectrum of physical systems which exhibit degrees of stability and fragility, one which has a pragmatic rationale, but not any deeper one
Stability in Cosmology, from Einstein to Inflation
I investigate the role of stability in cosmology through two episodes from the recent history of cosmology: (1) Einstein’s static universe and Eddington’s demonstration of its instability, and (2) the flatness problem of the hot big bang model and its claimed solution by inflationary theory. These episodes illustrate differing reactions to instability in cosmological models, both positive ones and negative ones. To provide some context to these reactions, I also situate them in relation to perspectives on stability from dynamical systems theory and its epistemology. This reveals, for example, an insistence on stability as an extreme position in relation to the spectrum of physical systems which exhibit degrees of stability and fragility, one which has a pragmatic rationale, but not any deeper one
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