{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,16]],"date-time":"2026-01-16T00:19:16Z","timestamp":1768522756002,"version":"3.49.0"},"reference-count":44,"publisher":"MDPI AG","issue":"11","license":[{"start":{"date-parts":[[2022,10,30]],"date-time":"2022-10-30T00:00:00Z","timestamp":1667088000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>The observation of supermassive black holes by the Event Horizon Telescope Collaboration and the detection of gravitational waves emitted during the merging phase of compact binary objects to stellar-mass black holes by the LIGO\u2013Virgo\u2013KAGRA collaboration constitute major achievements of modern science. Gravitational wave signals emitted by stellar-mass black holes are being used to test general relativity in an unprecedented way in the regime of strong gravitational fields, as well as to address other physics questions such as the formation of heavy elements or the Hawking Area Theorem. These discoveries require further research in order to answer critical questions about the population density and the formation processes of binary systems. The detection of supermassive black holes considerably extends the range of scientific investigation by making it possible to probe the structure of spacetime around the horizon of the central mass of our galaxy as well as other galaxies. The huge amount of information collected by the VLBI worldwide network will be used to investigate general relativity in a further range of physical conditions. These investigations hold the potential to pave the way for the detection of quantum-mechanical effects such as a possible graviton mass. In this paper we will review, in a cursory way, some of the results of both the LIGO\u2013Virgo\u2013KAGRA and the EHT collaborations.<\/jats:p>","DOI":"10.3390\/sym14112276","type":"journal-article","created":{"date-parts":[[2022,10,30]],"date-time":"2022-10-30T10:47:57Z","timestamp":1667126877000},"page":"2276","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Gravitational Waves, Event Horizons and Black Hole Observation: A New Frontier in Fundamental Physics"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-6509-6467","authenticated-orcid":false,"given":"Marco","family":"Giammarchi","sequence":"first","affiliation":[{"name":"Istituto Nazionale di Fisica Nucleare\u2014Sezione di Milano, 20133 Milano, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5475-4447","authenticated-orcid":false,"given":"Fulvio","family":"Ricci","sequence":"additional","affiliation":[{"name":"Istituto Nazionale di Fisica Nucleare di Roma, I-00185 Roma, Italy"},{"name":"Dipartimento di Fisica, Universit\u00e0 di Roma \u201cLa Sapienza\u201d, I-00185 Roma, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2022,10,30]]},"reference":[{"key":"ref_1","unstructured":"Einstein, A. (1915). Die Feldgleichungen der Gravitation, K\u00f6niglich Preussische Akademie der Wissenschaften. Sitzungsbericht."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"231101","DOI":"10.1103\/PhysRevLett.119.231101","article-title":"The MICROSCOPE mission: First results of a space test of the equivalence principle","volume":"119","author":"Touboul","year":"2017","journal-title":"Phys. Rev. Lett."},{"key":"ref_3","unstructured":"Einstein, A. (1918). Gravitationswellen (On Gravitational Waves), K\u00f6niglich Preussische Akademie der Wissenschaften. Erster Halbband."},{"key":"ref_4","first-page":"20","article-title":"\u00dcber das Gravitationsfeld einer Kugel aus inkompressibler Fl\u00fcssigkeit; Reimer, Berlin 1916, S. 424-434 (Sitzungsberichte der K\u00f6niglich-Preussischen Akademie der Wissenschaften; 1916)\u2014On the Gravitational Field of a Sphere of Incompressible Liquid, According to Einstein\u2019s Theory","volume":"1","author":"Schwarzschild","year":"2008","journal-title":"Abraham Zelmanov J."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"1776","DOI":"10.1103\/PhysRev.164.1776","article-title":"Event Horizons in Static Vacuum Space-Times","volume":"164","author":"Israel","year":"1967","journal-title":"Phys. Rev."},{"key":"ref_6","first-page":"1","article-title":"Uber Die Allgemeinen Kugelsymmetrschen Losungen Der Einstei\u2019Schen GavitationsgleiChungen Im Vakuum","volume":"15","author":"Jebsen","year":"1921","journal-title":"Ark. Mat. Astron. Fys."},{"key":"ref_7","first-page":"9","article-title":"The implausible history of triple star models for Cygnus X-1 Evidence for a black hole","volume":"16","author":"Shipman","year":"1975","journal-title":"Astrophys. Lett."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"237","DOI":"10.1103\/PhysRevLett.11.237","article-title":"Gravitational Field of a Spinning Mass as an Example of Algebraically Special Metrics","volume":"11","author":"Kerr","year":"1963","journal-title":"Phys. Rev. Lett."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"3436","DOI":"10.1103\/PhysRevLett.80.3436","article-title":"Hawking Radiation without Black Hole Entropy","volume":"80","author":"Visser","year":"1998","journal-title":"Phys. Rev. Lett."},{"key":"ref_10","first-page":"191","article-title":"Black holes and thermodynamics","volume":"D13","author":"Hawking","year":"1976","journal-title":"Phys. Rev."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"737","DOI":"10.1007\/BF02757029","article-title":"Black Holes and the Second Law","volume":"4","author":"Bekenstein","year":"1972","journal-title":"Lett. Nuovo Cim."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"2333","DOI":"10.1103\/PhysRevD.7.2333","article-title":"Black Holes and Entropy","volume":"7","author":"Bekenstein","year":"1973","journal-title":"Phys. Rev. D"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"498","DOI":"10.1103\/PhysRevLett.18.498","article-title":"Gravitational Radiation","volume":"18","author":"Weber","year":"1967","journal-title":"Phys. Rev. Lett."},{"key":"ref_14","first-page":"325","article-title":"First Gravity Wave Coincidence Experiment Between Three Cryogenic Resonant-Mass Detectors: Lousiana-Rome-Stanford","volume":"216","author":"Amaldi","year":"1989","journal-title":"Astron. Astrophys."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"755","DOI":"10.1103\/RevModPhys.68.755","article-title":"On the measurement of a weak classical force coupled to a harmonic oscillator: Experimental progress","volume":"68","author":"Bocko","year":"1996","journal-title":"Rev. Mod. Phys."},{"key":"ref_16","unstructured":"Weiss, R. (1972). Electromagnetically Coupled Broadband Gravitational Antenna, MIT. Quaternary Progress Report."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"115012","DOI":"10.1088\/0264-9381\/32\/11\/115012","article-title":"Advanced LIGO","volume":"32","author":"Aasi","year":"2015","journal-title":"Class. Quantum Gravity"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"024001","DOI":"10.1088\/0264-9381\/32\/2\/024001","article-title":"Advanced Virgo: A Second-Generation of Interferometric Gravitational Wave Detector","volume":"32","author":"Acernese","year":"2015","journal-title":"Class. Quantum Gravity"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"061102","DOI":"10.1103\/PhysRevLett.116.061102","article-title":"Observation of Gravitational Waves from a Binary Black Hole merger","volume":"116","author":"Abbott","year":"2016","journal-title":"Phys. Rev. Lett."},{"key":"ref_20","unstructured":"Abbott, B.P., Abott, D.T., Acernese, F., Ackley, K., Adams, C., Adhikari, N., Adhikari, R.X., Adya, V.B., Affeldt, C., and Agarwal, D. (2021). GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run. arXiv."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"05A101","DOI":"10.1093\/ptep\/ptaa125","article-title":"Overview of KAGRA: Detector design and construction history","volume":"2021","author":"Akutsu","year":"2021","journal-title":"Prog. Theor. Exp. Phys."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"161101","DOI":"10.1103\/PhysRevLett.119.161101","article-title":"GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral","volume":"119","author":"Abbott","year":"2017","journal-title":"Phys. Rev. Lett."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"124069","DOI":"10.1103\/PhysRevD.97.124069","article-title":"Observational tests of the black hole area increase law","volume":"97","author":"Cabero","year":"2018","journal-title":"Phys. Rev. D"},{"key":"ref_24","unstructured":"Bianchi, E., Gupta, A., Haggard, H.M., and Sathyaprakash, B.S. (2018). Small Spins of Primordial Black Holes from Random Geometries: Bekenstein-Hawking Entropy and Gravitational Wave Observations. arXiv."},{"key":"ref_25","unstructured":"Salerno, V. (2022). Il segnale di onde gravitazionali GW150914 e la variazione di entropia associata. [Bachelor\u2019s Thesis, University of Rome]."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"697","DOI":"10.1086\/304535","article-title":"The MACHO Project Large Magellanic Cloud Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo","volume":"486","author":"Alcock","year":"1997","journal-title":"Astrophys. J."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"387","DOI":"10.1051\/0004-6361:20066017","article-title":"Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds","volume":"469","author":"Tisserand","year":"2007","journal-title":"Astron. Astrophys."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"103024","DOI":"10.1103\/PhysRevD.98.103024","article-title":"Methods for the detection of gravitational waves from subsolar mass ultracompact binaries","volume":"98","author":"Magee","year":"2018","journal-title":"Phys. Rev. D"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"121106","DOI":"10.1103\/PhysRevLett.125.121106","article-title":"Features of the Energy Spectrum of Cosmic Rays above 2.5 \u00d7 1018 eV Using the Pierre Auger Observatory","volume":"125","author":"Aab","year":"2020","journal-title":"Phys. Rev. Lett."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"246","DOI":"10.1086\/430667","article-title":"Young Stars and Infrared Flares in the Central Light-Month","volume":"628","author":"Eisenhower","year":"2005","journal-title":"Astrophys. J."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"L9","DOI":"10.1086\/312838","article-title":"The M-\u03c3 Relation for Supermassive Black Holes","volume":"539","author":"Merritt","year":"2000","journal-title":"Astrophys. J."},{"key":"ref_32","unstructured":"Doeleman, S., Agol, E., Backer, D., Baganoff, F., Bower, G.C., Broderick, A., Fabian, A., Fish, V., Gammie, C., and Ho, P. (2010). Astro2010: The Astronomy and Astrophysics Decadal Survey, Science White Papers. arXiv."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"L6","DOI":"10.3847\/2041-8213\/ab1141","article-title":"First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole","volume":"875","author":"Akiyama","year":"2019","journal-title":"Astroph. J. Lett."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"104","DOI":"10.3847\/1538-4357\/aadcff","article-title":"The Scattering and Intrinsic Structure of Sagittarius A* at Radio Wavelengths","volume":"865","author":"Johnson","year":"2018","journal-title":"Astroph. J."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"051","DOI":"10.1088\/1475-7516\/2020\/05\/051","article-title":"Constraining fundamental physics with the event horizon telescope","volume":"05","author":"Rummel","year":"2020","journal-title":"JCAP"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"044057","DOI":"10.1103\/PhysRevD.100.044057","article-title":"Testing the rotational nature of the supermassive object M87* from the circularity and size of its first image","volume":"100","author":"Bambi","year":"2019","journal-title":"Phys. Rev. D"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"199","DOI":"10.1007\/BF02345020","article-title":"Particle creation by black holes","volume":"43","author":"Hawking","year":"1975","journal-title":"Commun. Math. Phys."},{"key":"ref_38","doi-asserted-by":"crossref","unstructured":"Giddings, S.B. (2019). Searching for Quantum Black Hole Structure with the Event Horizon Telescope. Universe, 5.","DOI":"10.3390\/universe5090201"},{"key":"ref_39","doi-asserted-by":"crossref","unstructured":"Daas, J., Kuijpers, K., Saueressig, F., Wondrak, M.F., and Falcke, H. (2022). Probing Quadratic Gravity with the Event Horizon Telescope. arXiv.","DOI":"10.1051\/0004-6361\/202244080"},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"L12","DOI":"10.3847\/2041-8213\/abe71d","article-title":"First M87 Event Horizon Telescope Results. VII. Polarization of the Ring","volume":"910","author":"Akiyama","year":"2021","journal-title":"Astroph. J. Lett."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"L13","DOI":"10.3847\/2041-8213\/abe4de","article-title":"First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon","volume":"910","author":"Akiyama","year":"2021","journal-title":"Astroph. J. Lett."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"592","DOI":"10.1038\/s41550-022-01620-3","article-title":"Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87","volume":"6","author":"Chen","year":"2022","journal-title":"Nat. Astron."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"61","DOI":"10.3847\/1538-4357\/ac7c1d","article-title":"The Photon Ring in M87","volume":"935","author":"Broderick","year":"2022","journal-title":"Astroph. J."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"2462","DOI":"10.1093\/mnras\/stac2743","article-title":"How narrow is the M87* ring? II. A new geometric model","volume":"517","author":"Lockhart","year":"2022","journal-title":"Mon. Not. R. Astron. Soc."}],"container-title":["Symmetry"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2073-8994\/14\/11\/2276\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T01:05:59Z","timestamp":1760144759000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2073-8994\/14\/11\/2276"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2022,10,30]]},"references-count":44,"journal-issue":{"issue":"11","published-online":{"date-parts":[[2022,11]]}},"alternative-id":["sym14112276"],"URL":"https:\/\/doi.org\/10.3390\/sym14112276","relation":{},"ISSN":["2073-8994"],"issn-type":[{"value":"2073-8994","type":"electronic"}],"subject":[],"published":{"date-parts":[[2022,10,30]]}}}