135 research outputs found
Searching for Cost Optimized Interstellar Beacons
What would SETI Beacon transmitters be like if built by civilizations with a
variety of motivations, but who cared about cost? We studied in a companion
paper how, for fixed power density in the far field, we could build a
cost-optimum interstellar Beacon system. Here we consider, if someone like us
were to produce a Beacon, how should we look for it? High-power transmitters
might be built for wide variety of motives other than twoway communication;
Beacons built to be seen over thousands of light years are such. Altruistic
Beacon builders will have to contend with other altruistic causes, just as
humans do, so may select for economy of effort. Cost, spectral lines near 1 GHz
and interstellar scintillation favor radiating frequencies substantially above
the classic water hole. Therefore the transmission strategy for a distant,
cost-conscious Beacon will be a rapid scan of the galactic plane, to cover the
angular space. Such pulses will be infrequent events for the receiver. Such
Beacons built by distant advanced, wealthy societies will have very different
characteristics from what SETI researchers seek. Future searches should pay
special attention to areas along the galactic disk where SETI searches have
seen coherent signals that have not recurred on the limited listening time
intervals we have used. We will need to wait for recurring events that may
arrive in intermittent bursts. Several new SETI search strategies emerge from
these ideas. We propose a new test for SETI Beacons, based on the Life Plane
hypotheses.Comment: 19 pages, 1 Figur
What does Newcomb's paradox teach us?
In Newcomb's paradox you choose to receive either the contents of a
particular closed box, or the contents of both that closed box and another one.
Before you choose, a prediction algorithm deduces your choice, and fills the
two boxes based on that deduction. Newcomb's paradox is that game theory
appears to provide two conflicting recommendations for what choice you should
make in this scenario. We analyze Newcomb's paradox using a recent extension of
game theory in which the players set conditional probability distributions in a
Bayes net. We show that the two game theory recommendations in Newcomb's
scenario have different presumptions for what Bayes net relates your choice and
the algorithm's prediction. We resolve the paradox by proving that these two
Bayes nets are incompatible. We also show that the accuracy of the algorithm's
prediction, the focus of much previous work, is irrelevant. In addition we show
that Newcomb's scenario only provides a contradiction between game theory's
expected utility and dominance principles if one is sloppy in specifying the
underlying Bayes net. We also show that Newcomb's paradox is time-reversal
invariant; both the paradox and its resolution are unchanged if the algorithm
makes its `prediction' after you make your choice rather than before
Stability of Magnetic Equilibria in Radio Balloons
Current-carrying flows, in the laboratory and in astrophysical jets, can form
remarkably stable magnetic structures. Decades of experience shows that such
flows often build equilibria that reverse field directions, evolving to an MHD
Taylor state, which has remarkable stability properties. We model jets and the
magnetic bubbles they build as reversed field pinch equilibria by assuming the
driver current to be stiff in the MHD sense. Taking the jet current as rigid
and a fixed function of position, we prove a theorem: that the same, simple MHD
stability conditions guarantee stability, even after the jet turns off. This
means that magnetic structures harboring a massive inventory of magnetic energy
can persist long after the building jet current has died away. These may be the
relic radio "fossils," "ghost bubbles" or "magnetic balloons" found in
clusters. These equilibria under magnetic tension will evolve, retaining the
stability properties from that state. The remaining fossil is not a disordered
ball of magnetic fields, but a stable structure under tension, able to respond
to the slings and arrows of outside forces. Typically their Alfven speeds
greatly exceed the cluster sound speed, and so can keep out hot cluster
plasmas, leading to x-ray "ghosts." Passing shocks cannot easily destroy them,
but can energize and light them up anew at radio frequencies. Bubbles can rise
in the hot cluster plasma, perhaps detaching from the parent radio galaxy, yet
stable against Rayleigh-Taylor and other modes.Comment: 2 figure
Fossil AGN jets as ultra high energy particle accelerators
Remnants of AGN jets and their surrounding cocoons leave colossal
magnetohydrodynamic (MHD) fossil structures storing total energies ~10^{60}
erg. The original active galacic nucleus (AGN) may be dead but the fossil will
retain its stable configuration resembling the reversed-field pinch (RFP)
encountered in laboratory MHD experiments.
In an RFP the longitudinal magnetic field changes direction at a critical
distance from the axis, leading to magnetic re-connection there, and to slow
decay of the large-scale RFP field. We show that this field decay induces
large-scale electric fields which can accelerate cosmic rays with an E^{-2}
power-law up to ultra-high energies with a cut-off depending on the fossil
parameters. The cut-off is expected to be rigidity dependent, implying the
observed composition would change from light to heavy close to the cut-off if
one or two nearby AGN fossils dominate. Given that several percent of the
universe's volume may house such slowly decaying structures, these fossils may
even re-energize ultra-high energy cosmic rays from distant/old sources,
offsetting the ``GZK-losses'' due to interactions with photons of the cosmic
microwave background radiation and giving evidence of otherwise undetectable
fossils. In this case the composition would remain light to the highest
energies if distant sources or fossils dominated, but otherwise would be mixed.
It is hoped the new generation of cosmic ray experiments such as the Pierre
Auger Observatory and ultra-high energy neutrino telescopes such as ANITA and
lunar Cherenkov experiments will clarify this.Comment: 11 pages, 6 figures, additional references and explanations. Accepted
for publication in MNRA
Natural Wormholes as Gravitational Lenses
Visser has suggested traversable 3-dimensional wormholes that could plausibly
form naturally during Big Bang inflation. A wormhole mouth embedded in high
mass density might accrete mass, giving the other mouth a net *negative* mass
of unusual gravitational properties. The lensing of such a gravitationally
negative anomalous compact halo object (GNACHO) will enhance background stars
with a time profile that is observable and qualitatively different from that
recently observed for massive compact halo objects (MACHOs) of positive mass.
We recommend that MACHO search data be analyzed for GNACHOs.Comment: 4 pages; plus 4 figures; ReV_TeX 3.0; DOE/ER/40537-001/NPL94-07-01
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