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تسجيل مستخدم جديدA Shiny New Method for SETI: Specular Reflections from Interplanetary Artifacts
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Brian C. Lacki
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Glints of light from specular reflection of the Sun are a technosignature of artificial satellites. If extraterrestrial intelligences have left artifacts in the Solar System, these may include flat mirror-like surfaces that also can glint. I describe the characteristics of the resulting flashes. An interplanetary mirror will appear illuminated for several hours, but if it is rotating, its glint may appear as a train of optical pulses. The resulting glints can be very bright, but they will be seen only if the mirror happens to reflect sunlight to the Earth. The detection of large mirrors is limited mainly by the fraction oriented to reflect sunlight toward Earth. I give rough calculations for the expected reach of each exposure of Pan-STARRS1, LSST, and Evryscope for mirror glints. A single exposure of Pan-STARRS1 has an effective reach of 10^-9 - 10^-7 AU^3 for interplanetary mirrors with effective areas of 10 m^2, depending on rotation rate. Over several years, Pan-STARRS1 might accumulate a reach ~10^5 times greater than this, as it tiles the sky and different mirrors enter and exit a favorable geometry.
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As our ability to undertake more powerful Searches for Extraterrestrial Intelligence (SETI) grows, so does interest in the more controversial endeavour of Messaging Extraterrestrial Intelligence (METI). METI proponents point to the SETI Paradox - if
all civilisations refrain from METI then SETI is futile. I introduce Mutual Detectability as a game-theoretic strategy aimed at increasing the success potential of targeted SETI. Mutual detectability is embodied by four laws: mutuality, symmetry, opportunity and superiority. These laws establish how SETI participants can engage each other using game theory principles applied to mutual evidence of mutual existence. The law of superiority establishes an onus to transmit on the party whom both SETI participants can judge to have better quality evidence, or common denominator information (CDI), thus avoiding the SETI Paradox. I argue that transiting exoplanets within the Earth Transit Zone form a target subset that satisfies mutual detectability requirements. I identify the intrinsic time-integrated transit signal strength as suitable CDI. Civilisations on habitable-zone planets of radius $R_{rm p}/R_{oplus} lesssim (L_*/L_{odot})^{-1/7}$ have superior CDI on us, so have game-theory incentive (onus) to transmit. Whilst this implies that the onus to transmit falls on us for habitable planets around $L_* > L_{odot}$ stars, considerations of relative stellar frequency, main-sequence lifetime and planet occurrence mean such systems are likely a small minority. Surveys of the Earth Transit Zone for Earth-analogue transits around sub-solar luminosity hosts, followed up by targeted SETI monitoring of them, represent an efficient strategy compliant with mutual detectability. A choice to remain silent, by not engaging in METI towards such systems, does not in this case fuel concerns of a SETI Paradox.
In the spirit of Trimbles ``Astrophysics in XXXX series, I very briefly and subjectively review developments in SETI in 2020. My primary focus is 74 papers and books published or made public in 2020, which I sort into six broad categories: results fr
om actual searches, new search methods and instrumentation, target and frequency seleciton, the development of technosignatures, theory of ETIs, and social aspects of SETI.
Electrons incident from a normal metal onto a superconductor are reflected back as holes - a process called Andreev reflection. In a normal metal where the Fermi energy is much larger than a typical superconducting gap, the reflected hole retraces th
e path taken by the incident electron. In graphene with ultra low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor-graphene interface due to the onset of interband Andreev processes, where the effective mass of the reflected holes change sign. Here we present measurements of gate modulated Andreev reflections across the low disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene-superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro- and interband specular- Andreev reflections.
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