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Cloak of invisibility

A cloak of invisibility is a hypothetical or fictional garment or device that renders its wearer undetectable to the naked eye or other senses by manipulating light or other electromagnetic waves around the body.^[1] Rooted in ancient myths and folklore, the concept has evolved from magical artifacts in literature to tangible scientific pursuits using engineered materials.^[2] In mythology and folklore, invisibility cloaks or similar items appear across cultures as tools for deception, heroism, or divine intervention. For instance, in Greek mythology, the Helm of Hades—a cap or helmet—granted invisibility to gods like Hades, Athena, Hermes, and the hero Perseus, enabling stealthy actions in battles and quests.^[2] Germanic and Norse folklore, such as in the Nibelungenlied and related Volsunga Saga, feature the Tarnkappe, a cloak of concealment owned by the dwarf Alberich that grants invisibility and strength. These early narratives often symbolize moral ambiguity, as invisibility tempts users toward unethical behavior, a theme echoed in Plato's Republic with the Ring of Gyges, a mythical artifact that similarly hides the wearer from judgment.^[2] The motif gained prominence in modern literature and popular culture, particularly through J.K. Rowling's Harry Potter series, where the protagonist inherits a rare, heirloom-quality cloak woven from Demiguise hair and infused with enchantment, one of the three Deathly Hallows.^[1] This fictional device, which provides near-perfect invisibility without the time limits of spells, has captivated global audiences and inspired real-world research by blurring the line between fantasy and feasibility. Earlier literary examples include H.G. Wells' The Invisible Man (1897), which explores the scientific and ethical perils of invisibility through chemical means, influencing subsequent speculative fiction.^[3] Scientific efforts to realize invisibility cloaks began in earnest with the advent of metamaterials—artificial structures engineered to control electromagnetic waves in ways natural materials cannot. In 2006, physicist John Pendry and colleagues at Duke University demonstrated the first such cloak, a cylindrical device using metamaterials to bend microwaves around a small two-dimensional object, effectively hiding it from radar detection. Subsequent advancements integrated metasurfaces (ultrathin patterned layers) and zero-index materials (with near-zero refractive index) to create hybrid cloaks that tunnel light around three-dimensional objects, achieving up to 87% transmittance at microwave frequencies like 10.2 GHz.^[4] These designs reduce thickness to skin-like levels and support arbitrary shapes, though current prototypes operate only at specific wavelengths (e.g., visible light at 730 nm) and scales (micrometers), limiting practical applications to military stealth or biomedical imaging.^[5] As of 2025, ongoing research has broadened applications to include AI-resistant camouflage fabrics and aeroamphibious cloaks for drones, aiming to enhance stealth in diverse environments.^[6]^[7]

Historical and Cultural Origins

Tarnkappe in Germanic Folklore

The Tarnkappe, a magical cap or cloak in medieval Germanic sagas, granted its wearer the power of invisibility, serving as a key artifact in heroic narratives.^[8] This object derives its name from Middle High German tarnkappe, where tarn relates to Old High German tarni meaning "secret" or "hidden," combined with kappe for "cap" or "cloak," emphasizing its function of concealment.^[8] Originating in 13th-century German literature, the Tarnkappe evolved from earlier Indo-European invisibility tropes, influenced by Norse mythological elements such as shape-shifting and magical aids in the Volsunga Saga.^[9] In the Nibelungenlied, an epic poem composed around 1200 CE, the Tarnkappe first appears as a prized possession of the dwarf Alberich, guardian of the Nibelung treasure hoard.^[10] Siegfried acquires it by defeating Alberich, along with other treasures like the sword Balmung, integrating it into his heroic exploits (strophes 93–94).^[9] The artifact plays a pivotal role in Siegfried's adventures, particularly in the seventh aventiure, where he uses it to assist King Gunther in overcoming Queen Brünhild's superhuman challenges during their courtship (strophe 457).^[10] Later, Siegfried dons the Tarnkappe again to subdue Brünhild on Gunther's behalf, taking her belt and ring as trophies, which sows the seeds of deception and eventual tragedy (strophe 679).^[10] The Tarnkappe renders its wearer invisible, a power that heightens its narrative utility in stealth and combat while also bestowing the strength of twelve men upon the user (strophes 337–338, 414–415).^[10] Often depicted alongside a wishing cap in folklore variants, the Tarnkappe symbolizes control over hidden realms and supernatural forces, reflecting broader Germanic themes of power dynamics in medieval society.^[9]

Invisibility Motifs in Global Mythologies

In Greek mythology, the helmet of Hades, known as the kynē or cap of invisibility, granted its wearer the power to become unseen. This artifact was lent to the hero Perseus by the gods to aid in his quest to slay the Gorgon Medusa, allowing him to approach undetected and later escape the pursuit of her immortal sisters, Stheno and Euryale, after beheading her. The motif appears in Hesiod's works, dating to approximately the 8th century BCE, where it symbolizes divine intervention in heroic endeavors against formidable foes.^[11] In Irish folklore, the féth fíada—a druidic mist or spell of concealment—enabled beings to render themselves invisible, often enveloping individuals or entire groups in an otherworldly veil. This power was attributed to the Tuatha Dé Danann and druids, used strategically in tales of the Ulster Cycle, such as the epic Táin Bó Cúailnge (composed between the 8th and 12th centuries CE), where it facilitated evasion during conflicts and protected sacred sites like the hollow hills (síde). Medieval Irish texts portray the féth fíada as a supernatural tool incompatible with Christian doctrine, highlighting its roots in pre-Christian magical traditions.^[12] African traditions feature invisibility through spiritual charms and herbal preparations, particularly among the Yoruba and Zulu peoples. In Yoruba culture, the àfẹ́ẹ̀rí serves as an invisibility charm, crafted via esoteric knowledge from Ifá divination, which consults oracles to invoke protective deities and ensure the charm's efficacy against threats. This practice, integral to precolonial survival strategies, blends spiritual invocation with practical concealment. Similarly, Zulu umuthi—traditional herbal medicines—include concoctions believed to induce temporary unseen states, often prepared by healers to shield individuals from enemies or spirits, drawing on ancestral guidance and natural elements for potency.^[13] Asian mythologies depict invisibility as a supernatural ability tied to yokai and Taoist arts. In Japanese lore, Tengu yokai possess concealment powers, such as through the magical kakuremino cloak that grants invisibility, allowing them to traverse realms undetected; early references trace to foundational texts like the Kojiki (712 CE), where precursor spirits exhibit elusive, otherworldly traits. In Chinese traditions, Taoist spells for evasion appear in the 16th-century novel Journey to the West, where characters like Sun Wukong use the power of 72 transformations, including abilities such as invisibility and vision obstruction, to outmaneuver foes through bodily concealment.^[14] Across these global mythologies, invisibility motifs consistently represent divine or magical aid rather than mechanical devices, serving purposes like trickery in heroic quests, protection from peril, or conveyance of moral lessons about humility and the unseen forces of the cosmos. Similar to the European Tarnkappe in Germanic folklore, these non-Western examples emphasize cultural variations, from oracle-guided charms in Africa to shape-shifting arts in Asia, underscoring a universal theme of the supernatural bridging human vulnerability and otherworldly power.

Depictions in Literature and Popular Fiction

Early Literary Examples

One of the earliest literary depictions of invisibility as a narrative device appears in Plato's Republic (c. 375 BCE), where the myth of the Ring of Gyges introduces a magical ring that grants its wearer the power to become invisible at will.^[15] In the dialogue, Glaucon recounts the tale of the shepherd Gyges, who discovers the ring on a corpse in a crack in the earth and uses it to seduce the queen, murder the king, and seize the throne of Lydia, thereby illustrating the philosophical question of whether justice is pursued for its own sake or merely to avoid punishment.^[16] The ring serves as a thought experiment to probe human morality, suggesting that unchecked power through invisibility would lead most individuals to commit injustice without restraint.^[17] In medieval Welsh literature, the Mabinogion (compiled in the 12th–13th centuries) features cloaks of invisibility as tools for deception and conquest, drawing from earlier Celtic folklore traditions such as the Germanic Tarnkappe. In the Second Branch, "Branwen Daughter of Llyr," the warrior Caswallawn employs a magical cloak to render himself invisible while assassinating Branwen's suitors and seizing the British throne, allowing him to strike unseen with his sword.^[18] Similarly, Arthurian legend, intertwined with Welsh and English medieval tales, includes the Mantle of King Arthur—one of the Thirteen Treasures of Britain—which cloaks its wearer in complete invisibility while permitting outward vision, aiding knights in heroic exploits or escapes during battles.^[19] By the 19th century, invisibility transitioned toward proto-scientific explanations in literature, blending mythical elements with emerging ideas of accident and invention. In Mark Twain's A Connecticut Yankee in King Arthur's Court (1889), the wizard Merlin provides the knight Sir Sagramor with a "fleecy veil" enchanted to make him invisible to his opponent during a tournament duel, though it fails to conceal him from observers, satirizing medieval superstition through the lens of modern rationality.^[20] H.G. Wells's The Invisible Man (1897) further evolves this motif, portraying the scientist Griffin who achieves invisibility through a chemical process gone awry, rendering his body transparent but leaving him isolated and prone to madness, marking a shift from artifact-based magic to unintended scientific consequences. Across these works, invisibility consistently enables themes of mischief, heroism, and ethical dilemmas, evolving from mythical artifacts that test moral boundaries in Plato to practical plot devices for concealment and conflict in medieval tales, and finally to cautionary explorations of power's corrupting influence in 19th-century fiction.^[21]

Modern Adaptations in Film and Media

In J.K. Rowling's Harry Potter series (1997–2007), the Invisibility Cloak serves as one of the three Deathly Hallows, a rare magical artifact that grants its wearer complete optical invisibility by rendering them unseen to the naked eye.^[22] Inherited by protagonist Harry Potter from his father James, the cloak originates from the Peverell family and provides enduring protection unlike temporary invisibility spells or lesser garments.^[22] However, its invisibility can be pierced by specialized magical detection, such as the enchanted eye of Auror Alastor "Mad-Eye" Moody, which sees through cloaks and invisibility charms.^[23] In film, the 1987 sci-fi action movie Predator depicts an advanced alien cloaking suit worn by the Yautja hunter, which bends light around the user to create near-perfect camouflage, making the wearer appear as a distorted silhouette against the environment.^[24] This technology malfunctions when exposed to water, as shown when the Predator emerges visible after crossing a river, highlighting vulnerabilities in its light-manipulating mechanism.^[24] Similarly, Peter Jackson's The Lord of the Rings film trilogy (2001–2003) portrays elven cloaks gifted to the Fellowship, woven from light-bending fibers that provide camouflage by blending the wearer with surrounding foliage and shadows, aiding stealth during perilous journeys.^[25] These cloaks, crafted by the Elves of Lothlórien, do not confer full invisibility but enhance disguise against watchful enemies like orcs.^[25] In comics and television, the Invisible Woman (Susan Storm Richards), debuting in Marvel's Fantastic Four #1 in 1961, employs psionic force fields to achieve invisibility by mentally redirecting light wavelengths around herself, others, or objects up to 40,000 cubic feet in volume.^[26] This superpower, resulting from cosmic ray exposure, allows her to render targets partially visible for tactical advantages, such as exposing internal structures, and she can detect externally induced invisibility.^[26] On television, Star Trek's Klingon cloaking devices, first integrated into Klingon ships in the 1984 film Star Trek III: The Search for Spock and retroactively contextualized in the series from 1966 onward, enable vessels to become undetectable by rendering them invisible to sensors and visual observation through advanced energy fields. Originating from Romulan technology shared via alliance, these devices allow Klingon warships to ambush foes stealthily, though they strain power systems and limit weapon use while active. Building briefly on early literary precedents like H.G. Wells' The Invisible Man, these modern portrayals have amplified the invisibility cloak as a versatile trope in 20th- and 21st-century media, symbolizing espionage in films like Die Another Day (2002), superhuman abilities in superhero narratives, and speculative sci-fi gadgets for tactical superiority.^[3] The post-2000 surge in such depictions, fueled by advancements in digital visual effects, has enabled more realistic renderings of light-bending and camouflage technologies, enhancing audience immersion in stories of hidden agents and unseen threats.^[3]

Scientific Principles of Invisibility

Optical Refraction and Light Manipulation

Optical refraction governs the bending of light as it passes from one medium to another, a phenomenon central to achieving invisibility by routing light around an object without distortion. Snell's law, formulated in the 17th century, quantifies this bending at interfaces between media with different refractive indices n, stating that n_1 \sin \theta_1 = n_2 \sin \theta_2, where \theta_1 and \theta_2 are the angles of incidence and refraction, respectively.^[27] In the context of cloaking, this principle extends to designing materials that gradually vary the refractive index, enabling light to curve continuously around a concealed object in a full 360-degree path, as if the object were absent from the optical field.^[27] Various techniques for manipulating light have been explored to mimic invisibility, though each has limitations. Absorption, as in a perfect blackbody that captures all incident light, prevents reflection but inevitably produces shadows by blocking light transmission to observers behind the object.^[28] Simple reflection, such as using mirrors, can redirect light but fails for three-dimensional objects because it cannot consistently bend rays around all sides without distorting the wavefront or revealing edges.^[28] Ideal optical cloaking can be achieved using materials with a negative refractive index (n < 0), which inverts the bending direction of light rays, allowing precise control to guide light smoothly around the object while maintaining phase coherence, or with positive indices in certain designs.^[29] The ray tracing concept underpins these manipulations by modeling light propagation as straight or curved paths that must seamlessly integrate with the surrounding environment. In a cloak, incoming rays are deflected around the hidden region and emerge on the opposite side following the exact trajectory they would take in free space, thereby preserving the original wavefronts and preventing any scattering or phase shift that could betray the object's presence.^[30] Scientific interest in these principles traces back to early 19th-century optics experiments, particularly those of Augustin-Jean Fresnel, whose work on the wave theory of light provided foundational insights into diffraction and interference essential for modern light control. In 1816–1819, Fresnel's diffraction studies, including predictions of bright spots in shadows and interference patterns from mirrored sources, mathematically solidified the wave nature of light, paving the way for advanced refraction-based designs.^[31]

Challenges with Opacity and Detection

One of the primary challenges in achieving true optical invisibility stems from the opacity and light-scattering properties inherent to most materials. In particular, small particles or inhomogeneities in materials cause Rayleigh scattering, where light is scattered proportionally to 1/λ⁴ (with λ as wavelength), resulting in visible distortions and haze around purportedly cloaked objects.^[32] Even highly advanced "perfect" absorbers, such as Vantablack—a carbon nanotube array developed in 2014 that absorbs up to 99.965% of visible light—fail to enable invisibility, as they still intercept and block light rays, casting detectable shadows.^[33] Cloaking effective against visible light wavelengths remains vulnerable to alternative detection modalities, exacerbating opacity limitations. Thermal imaging, for example, reveals objects through infrared emissions dictated by the Stefan-Boltzmann law, which states that the total power radiated by a blackbody is P = εσA T⁴ (where ε is emissivity, σ is the Stefan-Boltzmann constant, A is surface area, and T is temperature in Kelvin), rendering visible-light cloaks ineffective against heat signatures from any object above absolute zero.^[34] Similarly, detection via radar (electromagnetic waves in the microwave range) or acoustic waves necessitates entirely separate cloaking mechanisms, as these operate outside the visible spectrum and cannot be addressed by optical designs alone.^[34] Scaling cloaking to practical sizes and broadband operation introduces further insurmountable barriers related to wavelength bandwidth. Experimental demonstrations of metamaterial cloaks have succeeded primarily in narrowband regimes, such as microwaves (around 10 GHz), where material properties can be tuned more readily, but performance degrades rapidly outside these confines.^[35] For visible light (400–700 nm), passive cloaks face fundamental bounds derived from causality and passivity principles: the fractional bandwidth B for suppressing scattering scales inversely with object size and permittivity, yielding impractically small values (e.g., B ≈ 0.0005 for a 1 m radius object at 500 nm), requiring material responses that violate achievable dispersion limits.^[36] Environmental interactions compound these issues, preventing robust invisibility in real-world scenarios. Many optical cloak designs are polarization-sensitive, functioning only for specific linear or circular polarizations of incident light, as their anisotropic structures (e.g., in transformation optics) alter phase differently for orthogonal components, leading to partial visibility under unpolarized natural light.^[37]

Developments in Cloaking Technologies

Metamaterials and Transformation Optics

Metamaterials are artificially engineered composite materials composed of subwavelength-scale structures designed to exhibit electromagnetic properties not achievable in natural substances, such as simultaneous negative permittivity (ε < 0) and permeability (μ < 0), enabling phenomena like negative refraction. These structures, often fabricated using metallic resonators or dielectric elements, allow precise control over light propagation by manipulating the effective refractive index at scales much smaller than the wavelength of the incident radiation. Transformation optics provides the theoretical foundation for designing metamaterial-based cloaks, relying on the invariance of Maxwell's equations under coordinate transformations to guide electromagnetic waves around an object as if it were absent. Introduced by Pendry et al. in 2006, this approach involves mapping physical space to a transformed coordinate system, where the material parameters (permittivity ε and permeability μ) are derived to bend light paths accordingly. The core principle stems from the form-invariance of Maxwell's equations; for instance, the Faraday's law in differential form remains unchanged:

\nabla \times \mathbf{E} = -\frac{\partial \mathbf{B}}{\partial t}

This invariance allows the cloak's material properties to mimic a compressed space, effectively creating a "hole" in the electromagnetic field without disturbing the surrounding wavefronts. Key milestones in practical implementation include the first experimental microwave-frequency cloak developed by Schurig et al. in 2006 at Duke University, using cylindrical arrays of copper split-ring resonators and wires to achieve near-invisibility for a 3-cm copper cylinder over a 7 GHz bandwidth centered at 3.6 GHz.^[38] Efforts to extend cloaking to visible wavelengths began with attempts at UC Berkeley in 2009, where Liu et al. demonstrated a broadband carpet cloak using nanostructured silicon layers to hide objects under a textured "carpet" surface in the near-infrared range (around 1.5 μm), reducing scattering by transforming the reflection profile to mimic a flat surface. In 2017, nanoscale graphene-based metamaterials for terahertz cloaking leveraged graphene's tunable conductivity via electrostatic gating to enable dynamic light manipulation with minimal thickness. For example, designs incorporating graphene-coated nanowires have shown tunable invisibility in the THz regime by suppressing scattering cross-sections through plasmonic effects, achieving significant reduction in visibility for cylindrical objects at frequencies around 10 THz.^[39] However, challenges persist due to inherent losses in metallic and dielectric components, leading to energy dissipation and reduced efficiency, particularly at higher frequencies where ohmic losses in metals become pronounced, limiting broadband performance.^[40]

Acoustic and Multispectral Cloaking

Acoustic cloaking technologies manipulate sound waves to render objects undetectable, typically employing phononic crystals or acoustic metamaterials that bend waves around the target in a manner analogous to refraction described by Snell's law for acoustics.^[41] These structures exploit subwavelength features to control wave propagation, creating an effective "shadow" region where sound appears uninterrupted. Seminal theoretical foundations for such cloaks were laid in 2008 by researchers at Duke University, demonstrating the feasibility of a three-dimensional acoustic cloak. The first experimental realization of a practical three-dimensional acoustic cloak occurred in 2014, again at Duke University, where engineers used 3D-printed sheets of plastic perforated with repeating hole patterns, assembled into a pyramid-like lattice and tested in water.^[42] This device rerouted broadband sound waves around a concealed object, such as a metal sphere, making it sonically invisible to incoming pulses from various angles and validating the cloak's performance across a 2-3 kHz frequency range. The approach highlighted the potential for scalable fabrication via additive manufacturing, paving the way for applications in noise reduction and submarine stealth. Multispectral cloaking advances invisibility across broader segments of the electromagnetic spectrum, including radar (microwave) and infrared (IR) bands, to evade diverse sensors simultaneously. These designs often integrate dielectric layers or metamaterials to achieve low scattering over wide bandwidths without relying on lossy metals. A key 2015 contribution involved all-dielectric multilayer cylindrical structures that suppressed total scattering, enabling cloaking for plane waves incident on dielectric objects at optical frequencies while maintaining broadband operation.^[43] This work emphasized the role of high-index dielectrics in minimizing losses and extending performance to multiple wavelengths, contrasting with earlier narrowband metallic designs. Thermal cloaking masks heat signatures by redirecting diffusive heat flow around an object, preserving external temperature gradients as if the object were absent. In 2014, researchers experimentally demonstrated a bilayer thermal cloak using bulk isotropic materials like copper and stainless steel, arranged to form an exact transformation-optics-based shield that guided steady-state heat flux without perturbation.^[44] The device, validated through infrared thermography, cloaked a central region while maintaining isothermal lines outside, achieving near-perfect concealment for heat conduction rates up to several watts. Subsequent adaptations have incorporated lightweight aerogels for enhanced insulation in dynamic thermal environments, further reducing detectable signatures in IR imaging.^[45] Seismic cloaking deflects ground-borne vibrations from earthquakes or explosions, protecting structures by altering wave paths in soil via metamaterial-like arrays. A pioneering 2013 field experiment in France involved drilling parallel lines of 5-meter-deep boreholes into a 200-square-meter basin of silted clay to form a phononic barrier, successfully redirecting Rayleigh surface waves around a central "protected" zone.^[46] Generated by a vibrocompaction probe at 5 Hz, the waves were scattered and delayed, reducing amplitude in the target area by up to 50% compared to unshielded soil, as confirmed by embedded accelerometers.^[47] This large-scale test underscored the practicality of geometric inhomogeneities for low-frequency seismic shielding. Advancements in the 2020s have leveraged artificial intelligence for optimizing cloaking designs, enabling dynamic adaptation to varying conditions across spectra. Machine learning algorithms, such as generative adversarial networks and topology optimization, have accelerated the inverse design of metamaterials for thermal and acoustic cloaks, yielding structures with unprecedented efficiency and fabricability. For instance, a 2024 AI framework discovered realistic metamaterials with exotic properties by screening vast parameter spaces faster than traditional simulations.^[48] In 2025, researchers demonstrated an open, dynamic invisible space using reconfigurable metasurfaces and self-play reinforcement learning, allowing freely moving objects to remain undetectable.^[49] Additionally, a large-scale underwater acoustic cloak based on honeycomb lattice pentamode metamaterials was developed.^[50] These innovations enhance military applications, exemplified by the F-35 Lightning II's multispectral stealth suite, which significantly reduces the radar cross-section (comparable to a golf ball) in the X-band while incorporating features to minimize infrared signatures for overall low observability.^[51]

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One path to acoustic cloaking - IOPscience - Institute of Physics
We discuss the possibilities of experimentally demonstrating acoustic cloaking and analyse why this special equivalence of acoustics and electromagnetics ...<|separator|>
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World's first 3-D acoustic cloaking device hides objects from sound
Mar 11, 2014 · The new device reroutes sound waves to create the impression that both the cloak and anything beneath it are not there.
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All-Dielectric Multilayer Cylindrical Structures for Invisibility Cloaking
Apr 10, 2015 · We study optical response of all-dielectric multilayer structures and demonstrate that the total scattering of such structures can be suppressed leading to ...Missing: multispectral broad
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Experimental Demonstration of a Bilayer Thermal Cloak
Feb 3, 2014 · In this Letter, we demonstrate a bilayer thermal cloak made of bulk isotropic materials, and it has been validated as an exact cloak.
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Nanofibrous Kevlar Aerogel Films and Their Phase-Change ...
Jan 30, 2019 · Thermal cloaks which can manipulate heat diffusion are unable to realize large-scale manufacture. Some phase-change materials have been explored ...
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First Test of Seismic Invisibility Cloak | MIT Technology Review
Feb 6, 2013 · The French team created its so-called metamaterial by drilling three lines of empty boreholes 5 metres deep in a basin of silted clay up to 200 ...Missing: earthquake | Show results with:earthquake
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Experiments on Seismic Metamaterials: Molding Surface Waves
Mar 31, 2014 · A large-scale experiment shows that a periodic array of boreholes embedded in the soil can deflect the energy of an incoming seismic wave.
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New AI tool discovers realistic 'metamaterials' with unusual properties
Feb 9, 2024 · Researchers from TU Delft have now developed an AI tool that not only can discover such extraordinary materials but also makes them fabrication-ready and ...Missing: optimized | Show results with:optimized
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The F-35's advanced stealth doesn't make it invisible, just hard to kill
May 17, 2025 · Lockheed Martin's F-35 fighter jet is one of the world's top stealth aircraft. But it isn't impossible to spot.