La relatività generale, la migliore teoria di cui oggi disponiamo per descrivere la gravità, si basa su una matematica complessa, la cui piena comprensione richiede competenze professionali. Ma le geniali intuizioni che sono a fondamento della teoria pubblicata da Einstein alla fine del 1915 sono accessibili a chiunque, grazie all’aiuto illuminante di alcuni esperimenti mentali
Dalla relatività speciale alla relatività generale
La relatività speciale o ristretta, pubblicata da Einstein nel 1905, portò alla luce aspetti del mondo fisico che rivoluzionavano profondamente la visione dell’Universo su cui era basata la cosiddetta fisica classica. La teoria sviluppata da Einstein aveva a suo fondamento due soli princìpi:
- le leggi fisiche, a cominciare da quelle della meccanica, sono valide in tutti i sistemi di riferimento inerziali [1];
- la luce si propaga nel vuoto a velocità costante indipendentemente dallo stato di moto della sorgente o dell’osservatore.
Da questi due postulati derivavano conseguenze sconcertanti per il senso comune. La relatività ristretta dimostrò che non esistono uno spazio e un tempo assoluti, come invece pensava Newton. Quando sono coinvolte velocità relativistiche, cioè velocità che rappresentano una frazione significativa della velocità della luce, i corpi e le distanze si accorciano nella direzione del moto e il tempo rallenta. Il fatto che le durate e le distanze dipendano dal sistema di riferimento dell’osservatore fa perdere al concetto di simultaneità la sua validità universale. Due eventi che appaiono simultanei a un dato osservatore possono apparire sfalsati nel tempo a un altro osservatore che si trova in un diverso sistema inerziale. …
The two large spiral galaxies will eventually collide in five billion years or so, according to calculations based on data published in the second release of the Gaia astrometric satellite catalog. But it won’t be a head-on collision
Our galaxy, the Milky Way, together with about seventy other galaxies, is part of a cluster called the Local Group, which extends for about 10 million light-years. The two largest and most massive galaxies of the Local Group are Andromeda (M31) and the Milky Way, followed by another spiral, the Triangle Galaxy (M33). The fourth galaxy of the group by mass and size is the Large Magellanic Cloud, which is considered a minor galaxy, as it is a Milky Way’s satellite.
The glue that holds the Local Group together is the mutual gravitational attraction between the several galaxies that are part of it. The major centers of this attraction are obviously the three main galaxies — Andromeda, the Milky Way, and the Triangle. For many years, astronomers have been studying the motions of stars in these galaxies to try to calculate as accurately as possible the route they are following under the pull of mutual attraction. It has been known for almost a century that Andromeda has a negative radial velocity with respect to us. In other words, unlike the vast majority of galaxies in the Universe, it is not moving away from the Milky Way but rather approaching it. Not surprisingly because it is inevitable that the two most massive objects in the Local Group will attract each other. …
At the center of the Milky Way hides a four-million solar mass black hole. The confirmation of its existence, which took place thanks to the study of star orbits conditioned by its immense gravity, earned the Nobel Prize 2020 for physics to Andrea Ghez and Reinhard Genzel. But the story of the observations that led to the discovery of this supermassive black hole begins in the 1950s and deserves to be told
A powerful radio source towards the Galaxy’s darkened center
In 1968, Eric E. Becklin and Gerald Neugebauer, two Caltech astronomers, managed to scan the central parsecs of the Milky Way in four different infrared wavelengths, obtaining the best results at 2.2 µm. Overcoming 25 magnitudes of obscuration due to the dust in the interposed spiral arms, they discovered swarms of stars huddled together with an unlikely density, compared to the enormous distances that, in the galactic periphery, separate the Sun from its neighbors. An article published in Scientific American in April 1974 (R.H. Sanders and G.T. …
A new study recalculates the main parameters of Betelgeuse based on a triple series of astrophysical simulations. The result is that the red supergiant seems to be a little less gigantic than indicated by previous calculations and a little closer
Betelgeuse, the red supergiant in Orion, has always been one of the most observed and studied stars by astronomers. Despite the nearly 1,500 scientific papers that have been dedicated to it from 1850 to today, it is surprising how much our knowledge of this magnificent star is still inaccurate. In particular, Betelgeuse’s exact distance and radius have been the subject of a long series of estimates, none of which has so far managed to agree once and for all the researchers interested in the question.
A new article, published on 13 October in The Astrophysical Journal, is part of this wide and varied panorama of studies. The six authors — Meridith Joyce, Shing-Chi Leung, László Molnár, Michael Ireland, Chiaki Kobayashi, and Ken’ichi Nomoto — presented the results of one of the most in-depth and complete astrophysical simulations ever dedicated to Betelgeuse. …
Un nuovo studio ricalcola tutti i parametri di Betelgeuse sulla base di una triplice serie di simulazioni astrofisiche. Il risultato è che la supergigante rossa sembra essere un po’ meno gigantesca di quanto indicato da calcoli precedenti e anche un po’ più vicina
Betelgeuse, la supergigante rossa in Orione, è da sempre una delle stelle più osservate e studiate dagli astronomi. Nonostante i quasi 1.500 articoli scientifici che le sono stati dedicati dal 1850 ad oggi, è sorprendente quanto le nostre conoscenze su questa magnifica stella siano ancora imprecise. In particolare, la distanza esatta e il raggio di Betelgeuse sono stati oggetto di una lunga serie di stime, nessuna delle quali è riuscita finora a mettere d’accordo una volta per tutte i ricercatori interessati alla questione.
In questo panorama di studi ampio e variegato, si inserisce un nuovo lavoro, pubblicato il 13 ottobre su The Astrophysical Journal. I sei autori — Meridith Joyce, Shing-Chi Leung, László Molnár, Michael Ireland, Chiaki Kobayashi e Ken’ichi Nomoto — hanno presentato i risultati di una delle più approfondite e complete simulazioni astrofisiche mai dedicate a Betelgeuse. …
Is it possible that the moons orbiting a planet have moons in their turn? Theoretically, yes, but only in special cases. A study addresses the issue in detail, putting forward some suggestive hypotheses
One evening, before going to bed, Levi, a four-year-old kid, asked his mother one of those questions that adults can’t answer, questions that spring spontaneously from the unbridled imagination of children of that age:
“Mom, can moons have moons?”
Creatively using archival data from Chandra and XMM-Newton X-ray space telescopes, a group of researchers detected the transit of a planet in a binary system consisting of a blue supergiant and a neutron star (or black hole). All this in M51, the Whirlpool Galaxy, located 28 million light-years away from us
Are there extragalactic planets?
Until the 1990s, the only known planets were those of the Solar System. Astronomers were pretty sure that other stars have planetary systems, too, but they had no proof. The existence of planets orbiting stars other than the Sun remained a simple possibility, a probable but not proven fact. Then, in the mid-90s, starting with the discovery of 51 Pegasi b, a real flood of exoplanets came to fill the gap of knowledge that has accompanied humanity since the dawn of time. In just a quarter of a century, as many as 4,284 planets were found orbiting Milky Way stars.
But each new knowledge brings with it new questions. We know today that our galaxy is teeming with planets. What about other galaxies? Just as it was doubtful that only the Sun, in the Milky Way, had a planetary system, it is equally unlikely that planetary formation affected only our galaxy and not the others. However, science needs evidence. What evidence do we have of extragalactic planets? …
Usando in modo creativo dati di archivio dei telescopi spaziali a raggi X Chandra e XMM-Newton, un gruppo di ricercatori ha rilevato il transito di un pianeta in un sistema binario formato da una supergigante blu e da una stella di neutroni (o un buco nero). Tutto questo in M51, la galassia Vortice, situata a 28 milioni di anni luce da noi
Esistono pianeti extragalattici?
Fino agli anni ’90 del secolo scorso, gli unici pianeti noti erano quelli del Sistema Solare. Gli astronomi erano abbastanza sicuri che anche altre stelle fossero accompagnate da sistemi planetari, ma non ne avevano le prove. L’esistenza di pianeti in orbita intorno a stelle diverse dal Sole rimaneva una semplice possibilità, un dato probabile ma non dimostrato. Poi, a metà degli anni ’90, a partire dalla scoperta di 51 Pegasi b, una vera alluvione di esopianeti è venuta a colmare il vuoto di conoscenza che accompagnava l’umanità fin dalla notte dei tempi. In appena un quarto di secolo, sono stati individuati ben 4.284 …
About two billion years ago, in a region of Central Africa located in the current Gabonese Republic, an incredible and unrepeatable series of coincidences caused the ignition of at least seventeen natural nuclear reactors, which remained in operation for hundreds of thousands of years.
Phosphine and the Search for Life in the Clouds of Venus
A study published in Nature Astronomy reports the discovery of phosphine in the clouds of Venus, a compound that on Earth is exclusively associated with biological processes. This makes it plausible that microbial life forms exist floating in those clouds. …
About two billion years ago, in a region of Central Africa located in the current Gabonese Republic, an incredible and unrepeatable series of coincidences caused the ignition of at least seventeen natural nuclear reactors, which remained in operation for hundreds of thousands of years
A surprising discovery
In 1972, samples of uranium-containing minerals from various mines were collected in the uranium enrichment plant in Pierrelatte, France. Analyzing a sample of uranium hexafluoride from the Oklo mine in Gabon with a mass spectrometer, physicist Francis Perrin noticed something strange. The ratio between uranium-235 (²³⁵U) and uranium-238 (²³⁸U) present in the sample was 0.007171, a value slightly lower than the typical value of 0.007252. It was necessary to understand the origin of this difference. Numerous other samples from the same mine were analyzed. It was found that they contained a lower than average amount of ²³⁵U, in some cases much below, up to a minimum ratio of 0.00440 compared to the ²³⁸U isotope. …
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