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Topic Review
Einstein's Constant
Einstein's gravitational constant, denoted κ (kappa), is the constant appearing in the Einstein field equation which can be written: where Gαγ is the Einstein tensor and Tαγ is the stress–energy tensor. This equation relates to the curvature of spacetime, indicating that stress–energy is what causes the curvature of spacetime, thus gravitation. The constant κ is directly related to Newton's gravitational constant G: The above is for the stress–energy tensor in units of mass density (i.e., mass per volume). The units of Einstein's constant depends on how the stress–energy tensor is defined, so an alternative choice for T with units of energy density (i.e., energy per volume) yields
  • 500
  • 14 Nov 2022
Topic Review
Liquid Crystal-Tuned Planar Optics in Terahertz Range
Terahertz waves of higher frequencies compared to microwave and radio frequency have shown great potential in radar detection and high-speed wireless communication. To spatially control the wavefront of terahertz beams, various novel components, such as terahertz filters, polarization converters and lenses, have been investigated. Metamaterials and metasurfaces have become the most promising technique for the free manipulation of terahertz waves. Metadevices integrated with liquid crystals have been widely used in active terahertz devices.
  • 499
  • 17 Feb 2023
Topic Review
Magnetoelectric Magnetic Field Sensors
One of the new materials that have attracted wide attention of researchers are magnetoelectric (ME) composites. Great interest in these materials is due to their properties associated with the transformation of electric polarization/magnetization under the influence of external magnetic/electric fields and the possibility of their use to create new devices. ME magnetic field sensors based on the widely used structures Terfenol—PZT/PMN-PT,Metglas—PZT/PMN-PT, and Metglas—Lithium niobate, among others, are considered as the first applications of the ME effect in technology. Estimates of the parameters of ME sensors are given, and comparative characteristics of magnetic field sensors are presented. Taking into account the high sensitivity of ME magnetic field sensors, comparable to superconducting quantum interferencedevices (SQUIDs), this entry discuss the areas of their application.
  • 498
  • 08 Feb 2023
Topic Review
(486958) 2014 MU69
(486958) 2014 MU69, previously designated PT1 and 1110113Y, and nicknamed Ultima Thule by the New Horizons team, is a trans-Neptunian object from the Kuiper belt located in the outermost regions of the Solar System. It was discovered by astronomers using the Hubble Space Telescope on 26 June 2014. The irregular shaped classical Kuiper belt object is a suspected contact binary or close binary system and measures approximately 30 kilometers (19 miles) in diameter. In August 2015, this object was selected as the next target for the New Horizons probe shortly after it had visited Pluto. The flyby will occur on 1 January 2019, which will make it the farthest object in the Solar System ever to be visited by a spacecraft. After four course changes in October and November 2015, New Horizons is on course toward 2014 MU69. On 13 March 2018, NASA announced that (486958) 2014 MU69 would receive the nickname Ultima Thule. The decision was based on the results of a public voting campaign. Ultima Thule /ˈθuːliː/, or Ultima for short, serves as an unofficial name for the object until the IAU decides on an official name at some point after the flyby.
  • 498
  • 19 Oct 2022
Topic Review
Central High-Order Aberrations and Accommodation
High-order aberrations (HOAs) are optical defects that degrade the image quality. They change with factors such as pupil diameter, age, and accommodation. The changes in optical aberrations during accommodation are mainly due to lens shape and position changes. Primary spherical aberration (Z(4.0)) is closely related to accommodation and some studies suggested that it plays an important role in the control of accommodation. Furthermore, central and peripheral HOAs vary with refractive error and seem to influence eye growth and the onset and progression of myopia. The variations of central and peripheral HOAs during accommodation also appear to be different depending on the refractive error. Central and peripheral high-order aberrations are closely related to accommodation and influence the accuracy of the accommodative response and the progression of refractive errors, especially myopia. 
  • 497
  • 31 Mar 2023
Topic Review
Nanocarbon-Iridium Oxide Nanostructured Hybrids
Nanostructuring nanocarbons with IrOx yields to material coatings with large charge capacities for neural electrostimulation, and large reproducibility in time, that carbons do not exhibit.
  • 494
  • 21 Jul 2021
Topic Review
Simulation Argument (Coding Planck Units)
Coding Planck units for deep universe (Programmer God) Simulation Hypothesis models The deep universe simulation hypothesis or simulation argument is the argument that the universe in its entirety, down to the smallest detail, could be an artificial simulation, such as a computer simulation. A deep universe simulation begins with the big bang and is programmed by an external intelligence (external to the universe), this intelligence by definition a Programmer God in the creator of the universe context. In Big Bang cosmology, the Planck epoch or Planck era is the earliest stage of the Big Bang, where cosmic time was equal to Planck time. Thus for a deep universe simulation, Planck time can be used as the reference for the simulation clock-rate, with the simulation operating at or below the Planck scale, and with the Planck units as (top-level) candidates for the base (mass, length, time, charge) units.
  • 493
  • 22 Nov 2022
Topic Review
Hercules
Hercules is a prominent constellation in the northern celestial hemisphere, named after the legendary hero from Greek mythology known for his incredible strength and courage. It is the fifth-largest of the 88 modern constellations.
  • 493
  • 15 Mar 2024
Topic Review
Long-Wavelength Vertical-Cavity Surface-Emitting Lasers
Single-mode long-wavelength (LW) vertical-cavity surface-emitting lasers (VCSELs) present an inexpensive alternative to DFB-lasers for data communication in next-generation giga data centers, where optical links with large transmission distances are required. The use of long-wavelength (1300–1550 nm) single-mode (SM) VCSELs makes it possible to reduce the modal and chromatic dispersion in an optical link and, as a result, to extend its reach. Moreover, spatial division multiplexing (SDM) transmission by multicore fibers using long-wavelength (LW or short-wavelength infrared, SWIR) VCSELs are enabling many larger-scale data center networks than presently possible. 1300 nm VCSELs are of particularly strong importance for hybrid integration with silicon photonics, providing integrated modulators and InP- and GaAs-based integrated photonic circuits.
  • 493
  • 20 Mar 2023
Topic Review Peer Reviewed
Development of the Concept of Space up to Newton
The concept of space, ubiquitous among all humans from birth, has changed profoundly in the course of the history of Western civilization, the only one to be considered here. An important contribution to this change was the theoretical elaborations of the philosophers of nature and mathematicians, started in Ancient Greece. Here, the process is considered up to Newton, when the concept of space for physicists, who then replaced the traditional philosophers of nature, took on a connotation that remained substantially undisputed for two centuries—that of absolute space. 
  • 496
  • 18 Oct 2022
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