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arXiv2023

On equilibria of tetrahedra

The monostatic property of polyhedra (i.e. the property of having just one stable or unstable static equilibrium point) has been in a focus of research ever since Conway and Guy \cite{Conway} published the proof of the existence of the first such object. In the same article they also proved that a homogeneous tetrahedron has at least two stable equilibrium points. By using polar duality, the same idea has been used \cite{balancing} to prove that a homogeneous tetrahedron has at least two unstable equilibria. Conway \cite{Dawson} also claimed that among inhomogeneous tetrahedra one can find monostable ones. Here we not only give a formal proof of this statement and show that monostatic tetrahedra have exactly 4 equilibria, but also demonstrate a startling new aspect of this problem: being monostatic implies certain \emph{visible} features of the shape and vice versa. Our results also imply that mono-monostatic tetrahedra (having just one stable and just one unstable equilibrium point) do not exist. In contrast, we show that for any other legal number of faces, edges, and vertices there is a mono-monostatic polyhedron with that face vector.

math.MG↗
arXiv2023

Deep Learning Framework for the Design of Orbital Angular Momentum Generators Enabled by Leaky-wave Holograms

In this paper, we present a novel approach for the design of leaky-wave holographic antennas that generates OAM-carrying electromagnetic waves by combining Flat Optics (FO) and machine learning (ML) techniques. To improve the performance of our system, we use a machine learning technique to discover a mathematical function that can effectively control the entire radiation pattern, i.e., decrease the side lobe level (SLL) while simultaneously increasing the central null depth of the radiation pattern. Precise tuning of the parameters of the impedance equation based on holographic theory is necessary to achieve optimal results in a variety of scenarios. In this research, we applied machine learning to determine the approximate values of the parameters. We can determine the optimal values for each parameter, resulting in the desired radiation pattern, using a total of 77,000 generated datasets. Furthermore, the use of ML not only saves time, but also yields more precise and accurate results than manual parameter tuning and conventional optimization methods.

physics.optics↗
arXiv2023

Ordering the universe with naked-eye observations

The geocentric universe, in its most developed form as set out by Ptolemy, was a remarkably successful and coherent theory. It did not, however, specify the order of the planets, that is, which was closer to Earth and which farther away. One would naively think that seeing one planet pass in front of another would settle the matter. In practice such mutual phenomena happen too rarely for them to have been useful. Even in principle, it turns out that most naked-eye observations of a central event would show nothing conclusive, with the exception of some occultations by Venus that would demonstrate it to be the lowest (nearest) planet. However, if one's theory were good enough to allow conclusions from {\it not} seeing changes, one could find that Mars is probably lower than Jupiter and Saturn, and possibly that the overall order is Venus-Mars-Jupiter-Saturn-Mercury.

physics.pop-ph↗
arXiv2022

German to Spanish translation of Einstein's work on the formation of meanders in rivers

In 1926 Albert Einstein gave a clear explanation of the physical processes involved in the meander formation and evolution in open channels (Einstein, 1926). Although this work is far from being recognized as one of his greatest achievements, such as his annus mirabilis papers in 1905, he shows a truly remarkable didactic skills that make it easy to understand even to the non-specialist. In particular, a brilliant explanation of the tea leaf paradox can be found in this paper of 1926, presented as a simple experiment for clarifying the role of Earth rotation and flow curvature in the differential river banks erosion. This work deserves to be considered as a pioneering work that has laid a basic knowledge in currently very active research fields in fluvial geomorphology, estuarine physics, and hydraulic engineering. In response to the curiosity aroused and transmitted to the authors over the years by undergraduates and MSc. students, and also due to its historical and scientific significance, we present here the Spanish translation of Einstein's original work published in German in 1926 in Die Naturwissenschaften (Einstein, 1926). Einstein's drawings have not been interpreted, but just updated preserving their original spirit.

physics.hist-ph↗
arXiv2023

A farewell to particles

We outline the course of affairs in the experimental and theoretical fields of nuclear and particle physics which determined its finale, and give several fragmentary remarks on its present state. The essay tells about events and their participants, known from the literature, but presented here from the perspective of a person whose 50-year labor activity, 1972--2022, proceeded at the All-Union Scientific Research Institute of Experimental Physics, Sarov. In order to predict the fate of particle physics and related astrophysics and cosmology, it is useful to become aware of the facts about another branch of physics that has already gone through its decline $\sim 30$ years ago, the physics of nuclear weapons. These facts are important not in themselves, but as evidence of the growing problems of science and social life, which are not only far from a satisfactory solution, but have not even been the subject of any serious discussion.

physics.pop-ph↗
arXiv2023

Gravitational Machines

A gravitational machine is defined as an arrangement of gravitating masses from which useful energy can be extracted. It is shown that such machines may exist if the masses are of normal astronomical size. A simple example of a gravitational machine, consisting of a double star with smaller masses orbiting around it, is described. It is shown that an efficient gravitational machine will also be an emitter of gravitational radiation. The emitted radiation sets a limit on the possible performance of gravitational machines, and also provides us with a possible means for detecting such machines if they exist.

gr-qc↗
arXiv2023

Global warming in figures and the question of its treatment: some historical and epistemological views

We first recall fundamentals of elementary climate physics: solar constant, radiative balance, greenhouse effect, astronomical parameters of the climate (theory of Milankovitch). Without disputing the analyzes of climatologists and the famous Keeling curve revealing in an indisputable way the increase in CO$_{2}$ in the atmosphere since the industrial revolution, we nevertheless insist on the main contributor to the greenhouse effect which is, as we know, water vapor. Faced with the difficulties that there will be in imposing zero-carbon policies everywhere in the world (and especially in developing countries), we show that it would perhaps be in our interest to act on soil drought, which amounts, in fact, to being interested in the clouds. The decrease in cloud cover, due to a lack of water fixation in the soil, in fact increases the general temperature and therefore the greenhouse effect. Acting on CO$_{2}$ will always have, in this context, much less effect than acting on water vapor, even indirectly. Despite the difficulty of making this action sustainable, due to the balance of atmospheric water vapor and the oceans, it would be in our interest not to neglect this path and also possibly increase forest cover for this purpose, given the problems of setting up zero-carbon policy on a global scale. In desperation, one can also consider protecting the Earth with an artificial dust cloud.

physics.soc-ph↗
arXiv2023

A Search for Extraterrestrial Technosignatures in Archival FAST Survey Data Using a New Procedure

The "search for extraterrestrial intelligence" (SETI) commensal surveys aim to scan the sky to find possible technosignatures from the extraterrestrial intelligence (ETI). The mitigation of radio frequency interference (RFI) is an important step, especially for the most sensitive Five-hundred-meter Aperture Spherical radio Telescope (FAST), which can detect more weak RFI. In this paper, we propose several new techniques for RFI mitigation, and use our procedure to search for ETI signals from the archival data of FAST's first SETI commensal survey. We detect the persistent narrowband RFI by setting a threshold of the signals' sky separation, and detect the drifting RFI (and potentially other types of RFI) using the Hough transform. We also use the clustering algorithms to remove more RFI and select candidates. The results of our procedure are compared to the earlier work on the same FAST data. We find that our methods, though relatively simpler in computation, remove more RFI (99.9912% compared to 99.9063% in the earlier work), but preserve the simulated ETI signals except those (5.1%) severely affected by the RFI. We also report more interesting candidate signals, about a dozen of which are new candidates that are not previously reported. In addition, we find that the proposed Hough transform method, with suitable parameters, also has the potential to remove the broadband RFI. We conclude that our methods can effectively remove the vast majority of the RFI while preserving and finding the candidate signals that we are interested in.

astro-ph.IM↗
arXiv2023

Geomechanics in unconventional resource development

To economically produce from very low permeability shale formations, hydraulic fracturing stimulation is typically used to improve their conductivity. This process deforms and breaks the rock, hence requires the geomechanics data and calculation. The development of unconventional reservoirs requires large geomechanical data, and geomechanics has involved in all calculations of the unconventional reservoir projects. Geomechanics has numerous contributions to the development of unconventional reservoirs from reservoir characterization and well construction to hydraulic fracturing and reservoir modeling as well as environmental aspect. This paper reviews and highlights some important aspects of geomechanics on the successful development of unconventional reservoirs as well as outlines the recent development in unconventional reservoir geomechanics. The main objective is to emphasize the importance of geomechanical data and geomechanics and how they are being used in in all aspects of unconventional reservoir projects.

physics.geo-ph↗
arXiv2023

Dalle leggi di Keplero alla materia oscura attraverso la storia di donne scienziate

The universe is composed of ordinary matter (for example, stars, planets, ourselves), dark matter and dark energy. The estimated percentage of dark matter is about $27\%$ of the entire universe, while $5\%$ is ordinary matter and the remaining part is dark energy. This article introduces the fundamental ingredients that led to conjecture the existence of dark matter and to study its properties. To this result have contributed, directly or indirectly, numerous female mathematicians and astronomers, who we quote to give them the right credit.

physics.pop-ph↗
arXiv2023

Real-Time Technosignature Strategies with SN 2023ixf

Several technosignature techniques focus on historic events such as SN 1987A as the basis to search for coordinated signal broadcasts from extraterrestrial agents. The recently discovered SN 2023ixf in the spiral galaxy M101 is the nearest Type II supernova in over a decade, and will serve as an important benchmark event. Here we review the potential for SN 2023ixf to advance ongoing techonsignature searches, particularly signal-synchronization techniques such as the "SETI Ellipsoid". We find that more than 100 stars within 100 pc are already close to intersecting this SETI Ellipsoid, providing numerous targets for real-time monitoring within ~3$^\circ$ of SN 2023ixf. We are commencing a radio technosignature monitoring campaign of these targets with the Allen Telescope Array and the Green Bank Telescope.

astro-ph.IM↗
arXiv2023

Specular reflections from artificial surfaces as Technosignature

Direct imaging of exoplanets will allow us to directly observe the planet in reflected light. Such a scenario may eventually allow for the possibility to scan the planetary surface for the presence of artificial structures made by alien civilizations. Detectability of planetary scale structures, called megastructures, has been previously explored. In this work, we show that it is possible to detect structures of much smaller scale on exoplanetary surfaces by searching for the specular reflection of host starlight from the corresponding structures. As the planet rotates, these reflections can manifest as an optical transient riding atop the rotational light curve of the planet. Due to the directional nature of specular reflection, the reflected signal is very strong, and it is comparable to the planetary flux for surfaces covering only few ppm (parts per million) of the total planet surface area. By tracking the planet around its orbit, it should be possible to scan the planetary surface for any such structures covering a size larger than a few ppm of planetary surface. The proposed method will aid in the search for extra-terrestrial intelligence in the era of direct imaging of exoplanets.

astro-ph.EP↗
arXiv2023

Block definition design for stretchable metamaterials: enabling configurable sensitivity to deformation

The sensitivity to deformation plays a key role in determining the applicability of stretchable metamaterials (MMs) to be used for conformal integration or mechanical reconfiguration. Typically, different unit designs are required to achieve the desired sensitivity, but this article proposes a block definition design for stretchable MMs that enables regulation of the MMs' response to deformation by defining various block arrangements with the same precursor structure. The article demonstrates a stretchable MM that employs the block definition design to show the mechanical reconfigurability of resonant frequency. Different block definitions result in modulation ranges of resonant frequency ranging from 39\% to 85\% when applying a 20\% tensile strain. Additionally, the proposed design is also used to realize another MM with contradictory sensitivity to the deformation and electromagnetically induced transparency (EIT) MMs with configurable transmission bandwidth to the deformation, indicating its potential for broader applications.

physics.app-ph↗
arXiv2023

The Celestial Vault is Not a Dome: Implications for the Moon Illusion

The moon illusion, in which the moon appears larger at the horizon than at higher altitudes, has been investigated since antiquity, yet it remains not fully explained. Our method of investigating the phenomenon is based on that attributed to Martin Folkes. Folkes suggests investigators identify landmarks on the ground and request human subjects indicate a place in the sky that they perceive to be above that location by raising their arm and pointing. The intersection of a vertical line rising out of the landmark and the ray extending from the subject's finger then identifies a point on the subject's model of the celestial vault. When repeated for landmarks covering a range of distances, the subject's entire celestial vault can be traced. We asked 30 subjects to perform such an identification on a series of points between a horizontal distance of 3 and 12600 meters across featureless open water in Verplanck and Ithaca, New York. The resulting data are in disagreement with the widely presumed flattened dome model, and, in particular, the Size-Distance Invariance Hypothesis. In addition, we find that the vault does not intersect the ground and leaves an indeterminate space of about 20 degrees altitude above the horizon with no correspondence to the ground. It is in this region that the Moon Illusion is most pronounced.

physics.pop-ph↗
arXiv2023

Unidentified aerial phenomena. Observations of variable objects

NASA commissioned a research team to study Unidentified Aerial Phenomena (UAP). The Main Astronomical Observatory of NAS of Ukraine conducts an independent study of UAP. A research team from San Diego also decided to conduct a study of UAP. Observations of events that cannot scientifically be identified as known natural phenomena established the existence of the UAP. However, their nature remains unclear. For UAP observations, we used a meteor station installed in Kyiv. We also present a new methodology for detecting UAPs using the latest smartphone technology in San Diego. We inform about discovering a new type of UAP.

physics.pop-ph↗
arXiv2024

Niel's Chess -- The Battle of the Quantum Age

In this paper, a quantum variant of chess is introduced, which can be played on a traditional board without the need of using computers or other electronic devices. The rules of the game arise naturally by combining the rules of conventional chess with key quantum-physical effects such as superposition and entanglement. Niel's Chess is recommended for ages 10 and above, to everyone who wishes to play a creative game with historical roots and at the same time gain intuition about the foundational quantum effects that power cutting-edge technologies like quantum computing and quantum communication, which are poised to revolutionise our society in the coming decades.

physics.pop-ph↗
arXiv2023

Disentangling Hype from Practicality: On Realistically Achieving Quantum Advantage

Quantum computers offer a new paradigm of computing with the potential to vastly outperform any imagineable classical computer. This has caused a gold rush towards new quantum algorithms and hardware. In light of the growing expectations and hype surrounding quantum computing we ask the question which are the promising applications to realize quantum advantage. We argue that small data problems and quantum algorithms with super-quadratic speedups are essential to make quantum computers useful in practice. With these guidelines one can separate promising applications for quantum computing from those where classical solutions should be pursued. While most of the proposed quantum algorithms and applications do not achieve the necessary speedups to be considered practical, we already see a huge potential in material science and chemistry. We expect further applications to be developed based on our guidelines.

quant-ph↗