Fokitis Emmanuel @fokitis

There is a strange and beautiful irony at the heart of general relativity: Einstein wrote the equations, but he was not the first person to solve them exactly. That may sound surprising if we imagine physics as a straight line from theory to answer, but equations in fundamental physics do not behave like simple puzzles waiting for a neat solution. Writing down the correct equations is already a monumental achievement. Solving them is another problem entirely. The Einstein field equations are not just difficult; they are nonlinear equations describing how matter, energy, space and time influence one another. In simple terms, they say that mass and energy tell spacetime how to curve, and curved spacetime tells matter how to move. But turning that statement into an exact description of a real gravitational field is brutally hard. Einstein knew this. When he completed general relativity in 1915, he had built a new theory of gravity in which gravity was no longer a force acting across space, as in Newton’s picture, but the geometry of spacetime itself. Massive objects do not simply pull on other objects. They reshape the structure through which everything moves. Planets orbit the Sun not because they are dragged by an invisible rope, but because they follow the natural paths available in curved spacetime. The theory immediately explained one of the great anomalies of astronomy: the small extra shift in Mercury’s orbit around the Sun, which Newtonian gravity could not fully account for. Einstein had used an approximate solution for that problem. But the full equations were so complex that even he did not expect simple exact solutions to appear quickly. Then came Karl Schwarzschild. Schwarzschild was not an obscure amateur. He was a brilliant German physicist and astronomer, director of the Potsdam Observatory, with deep expertise in celestial mechanics, photometry and mathematical physics. When the First World War broke out, he volunteered for military service despite being over forty. While serving on the front, in the middle of war, he found time to do something extraordinary: he derived the first exact solution to Einstein’s field equations. His solution described the gravitational field outside a perfectly spherical, non-rotating mass. That may sound like a simplified case, and it was, but in physics a clean idealization can reveal something profound. Schwarzschild showed what spacetime would look like around such an object. In the mathematics, a particular radius appeared naturally. Today we call it the Schwarzschild radius. For an object with the mass of the Sun, that radius is about three kilometres. For Earth, it is less than one centimetre. This does not mean the Sun or Earth are black holes. It means that if all their mass were compressed inside those radii, the curvature of spacetime would become so extreme that an event horizon would form. Beyond that boundary, escape would no longer be possible. Not for matter. Not for radiation. Not even for light. This was not how Schwarzschild’s result was understood at first. The mathematics contained what looked like a singularity at the Schwarzschild radius, a place where the equations seemed to behave pathologically. For decades, many physicists treated it as a mathematical oddity, a coordinate problem, or a sign that such a situation could never occur in nature. Einstein himself was deeply skeptical that real stars could collapse into such objects. The idea seemed too extreme, too pathological, almost like a warning that the theory had been pushed beyond its physical domain. And in one sense, the early skepticism was understandable. A black hole is not an intuitive object. It is not simply a very dense star. It is a region of spacetime separated from the outside universe by a causal boundary. The event horizon is not a surface made of material. If you crossed it, locally nothing magical would have to happen at that exact point, especially for a very massive black hole. But globally, your future would change completely. Every possible path forward would lead inward. Escape would no longer be a question of engine power, speed or technology. It would be forbidden by the causal structure of spacetime. The modern idea of the black hole took decades to mature. In the 1930s, Subrahmanyan Chandrasekhar showed that white dwarfs have a maximum mass. If a stellar remnant is too massive, electron degeneracy pressure cannot support it indefinitely. Later, neutron stars were understood as another possible endpoint of stellar evolution, supported by neutron degeneracy pressure. But even neutron stars have limits. Above a certain mass, no known pressure can halt collapse. In 1939, J. Robert Oppenheimer and Hartland Snyder described the gravitational collapse of a massive star in general relativity. Their work showed that collapse could continue until an event horizon formed, at least in an idealized model. But the idea still remained marginal for a long time. The term “black hole” itself did not become common until John Archibald Wheeler popularized it in the 1960s. Around the same period, Roger Penrose showed something decisive: black hole formation was not merely an artifact of perfect spherical symmetry. Under broad physical conditions, gravitational collapse could naturally lead to singularities in general relativity. Black holes were not just mathematical monsters. They were robust predictions of the theory. Then astronomy began to catch up with the mathematics. Black holes cannot be seen directly in the ordinary sense, because they emit no light from inside the event horizon. But the universe gives them away through their effects. A black hole can reveal itself by the motion of stars around an invisible massive object. It can announce its presence through X-rays produced by hot gas spiralling inward in an accretion disk. It can launch powerful relativistic jets from regions just outside the horizon. It can distort the light of background objects through gravitational lensing. And when two black holes merge, they shake spacetime itself, sending gravitational waves across the cosmos. The first strong observational candidates came from X-ray astronomy. Systems such as Cygnus X-1 showed the signature of a compact, massive object pulling material from a companion star. The gas heated to extreme temperatures before disappearing into a region too small and too massive to be a normal star. Later, observations of stars near the centre of the Milky Way revealed that they were orbiting something invisible with about four million solar masses packed into a very small volume. The best explanation is Sagittarius A*, the supermassive black hole at the centre of our galaxy. In 2015, the evidence became even more direct. @LIGO detected gravitational waves from the merger of two stellar-mass black holes. For the first time, humanity measured the ripples in spacetime produced by black holes colliding more than a billion light-years away. This was not a picture. It was something deeper in some ways: a direct measurement of dynamical spacetime, behaving almost exactly as general relativity predicted. Then, in 2019, the Event Horizon Telescope released the first image of a black hole’s shadow, produced by the supermassive black hole in the galaxy M87. The image did not show the event horizon itself. It showed the glowing plasma around the black hole and the dark central shadow caused by the capture of light. Still, it was a historic confirmation that the strange objects once hidden in equations correspond to real astrophysical structures. The black hole in M87 is about 6.5 billion times the mass of the Sun, sitting roughly 55 million light-years away. A century after Schwarzschild’s calculation, we were looking at the observational imprint of the geometry his mathematics had first revealed. There is another important correction to the popular story. Schwarzschild did not sit down and say, “I have discovered black holes.” The conceptual path was not that simple. His solution contained the mathematical seed. Later generations had to understand which parts of the solution were coordinate artifacts, which parts represented true physical boundaries, and which parts pointed toward the breakdown of classical general relativity. The singularity at the event horizon turned out not to be a real physical singularity. It was a feature of the coordinates being used. The central singularity, however, is different. In classical general relativity, curvature becomes infinite there, which likely signals that the theory is incomplete and that quantum gravity is needed. That is part of what makes black holes so important. They are not just cosmic vacuum cleaners or spectacular monsters. They are laboratories for the deepest problems in physics. They connect gravity, thermodynamics, quantum theory, information and the nature of spacetime. Hawking radiation suggests that black holes are not completely black, at least quantum mechanically. The black hole information problem asks whether information that falls into a black hole is lost forever or somehow preserved. These are not decorative questions. They strike at the foundations of modern physics. Schwarzschild did not live to see any of this. He became seriously ill with pemphigus, a rare autoimmune disease affecting the skin, and died in 1916 at only forty-two. His life ended before the scientific meaning of his solution had even begun to unfold. At the time, it would have been almost impossible to imagine that his wartime calculation would become one of the pillars of modern astrophysics. And yet that is exactly what happened. The history of black holes is one of the clearest examples of how mathematics can outrun intuition. The equations pointed toward a possibility that even many of the greatest physicists resisted. For decades, black holes lived in a strange territory between formal mathematics and physical reality. Then the universe answered. Stars moved around invisible masses. Gas screamed in X-rays before falling inward. Spacetime rang from black hole mergers. A dark shadow appeared inside a ring of light. What began as an exact solution to Einstein’s equations became one of the most powerful confirmations of Einstein’s theory, and also one of the strongest hints that the theory is not the final word. Black holes are where general relativity succeeds spectacularly, and where it may eventually fail. They are not errors in the mathematics. They are places where the mathematics forced us to expand our imagination.

@ExploreCosmos_ THANK YOU SO MUCH for this wonderful work in explaining General Relativity

O σύλλογος Συνταξιουχων του ΕΜΠ οργανώνει ομιλία του Ομοτίμου καθηγητού κ. Εμ. Φωκίτη με θέμα ‘Σύμπαν και Αστροφωτογραφία’ την ΤΕΤΑΡΤΗ 3 ΙΟΥΝΙΟΥ και ώρα 10.30 π.μ.στην αίθουσα πολυμέσων, κτίριο βιβλιοθήκης στην πολυτεχνιούπολη Ζωγράφου. Ο συνδεσμος retired.ntua.gr/?p=766

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Cleaning the sensor of Nikon D3500 dslr camera. Short hands-on experienc... youtube.com/shorts/SYSFkDY… μέσω @YouTube

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A photo of Crescent Nebula and other DSOs plus reverant of Swan permanent nova in the field of view. Post processing in cooperation with George Alexandrakis

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Απροσδοκητη συναντηση με οικογενεια αγριογουρουνων στο ορος Παρνηθα. Αναζητουσαν τροφη. Φαινεται τρεφονται κυριοως με γρασιδι Enexpected encounter with a family of wild boars (αγριογουρουνα) in Parn... youtu.be/y-z9bmZBZvg?si… μέσω @YouTube

Spectacular iridescent cloud. A diffraction phenomenon caused by small water droplets individually scattering light. They often occur on lenticular clouds on top of a cumulonimbus, called pileus.

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Home life flipped fast after these simple inventions 🏠