The Shadow of the Invisible: Black Holes, the Event Horizon, and the Event Horizon Telescope

The Hook: A Photograph of Something That Emits No Light

On 10 April 2019, humanity displayed for the first time a photograph of an object whose defining property is that it cannot be seen. The image looked almost disappointingly plain: a fuzzy orange ring around a dark center, like a glowing donut, captured from a distance of 55 million light-years. And yet it was one of the greatest observational achievements in the history of science. For that ring encircles the shadow of a black hole at the center of the galaxy Messier 87 — a region from which nothing escapes, not even light.

The paradox is obvious. A black hole is, by definition, black: it swallows every photon that comes too close. How do you photograph something that emits no radiation at all? The answer is at once simple and profound. You do not photograph the black hole itself, but its silhouette — the dark void it cuts into the glowing gas around it. The ring is light that only just escaped the black hole; the darkness at the center is the shadow cast by the extreme curvature of spacetime.

Black holes are the point where two of the most beautiful theories in physics — Einstein's General Relativity and quantum mechanics — collide and both reach their limits. They began as a mathematical curiosity that even Einstein believed to be physically impossible. Today we know that they exist by the millions in every galaxy, that a monstrous specimen sits at the heart of virtually every galactic center, and that they carry within them one of the deepest unsolved riddles of modern physics: What happens to the information that falls into a black hole?

This article will take you from the mathematical birth of the black hole in 1916, through its bizarre anatomy and the question of how to weigh and photograph something invisible, to the images of the years 2019 through 2024 and the information paradox that has haunted physics for half a century.

Part 1: The Birth of an Impossibility

A Thought in the Trenches

The story of the black hole begins with a remarkable coincidence. In November 1915, Albert Einstein presented his General Theory of Relativity — the idea that gravity is not a force but the curvature of space and time by mass and energy. Its essence is captured in the Einstein field equations, a system of nonlinear differential equations notoriously difficult to solve. Einstein himself initially worked only with approximations.

A few weeks later, in early 1916, the German astrophysicist Karl Schwarzschild found the first exact solution — and he did so on the Eastern Front of the First World War, where he was serving as a soldier and suffering from an autoimmune disease that would take his life just months afterward. Schwarzschild described the gravitational field of a spherically symmetric, non-rotating mass in empty space. In his solution, a strange quantity appeared: a critical radius at which the mathematical expressions became infinite. Today we call it the Schwarzschild radius.

For the Sun this radius is about 3 kilometers; for the Earth, just under 9 millimeters. As long as the mass — as with the Sun and the Earth — is spread far outside this radius, it has no physical significance. But what if one could compress a mass entirely within its own Schwarzschild radius? Then, the calculation shows, the escape velocity at that radius would reach the speed of light. Nothing could escape any longer. A boundary would form, beyond which causality itself comes to an end.

Half a Century of Doubt

For decades this idea was considered a mathematical fiction. Einstein himself published a paper in 1939 in which he tried to show that such objects could not form in nature. The English astrophysicist Arthur Eddington fought the idea just as fiercely.

The decisive impulse came from the young Indian physicist Subrahmanyan Chandrasekhar. In 1930 he calculated that a white dwarf — the burnt-out core of a Sun-like star — can remain stable only up to a certain mass, the Chandrasekhar limit of about 1.4 solar masses. Above it, the quantum-mechanical pressure of the electrons can no longer hold gravity at bay. What happens then was worked out in 1939 by Robert Oppenheimer and his collaborators: a sufficiently massive stellar core collapses unstoppably, ever further, until it vanishes within its own Schwarzschild radius.

The term "black hole" did not appear until the 1960s, popularized by the physicist John Archibald Wheeler. During this period the theory experienced a renaissance: Roy Kerr found in 1963 the solution for a rotating black hole — the realistic case, since all stars rotate — and Roger Penrose proved in 1965, using the tools of topology, that collapse to a singularity is unavoidable under very general conditions. For this work Penrose received the Nobel Prize in 2020. What had begun as an impossibility had become an inescapable consequence of relativity.

Part 2: The Anatomy of a Black Hole

A black hole is, paradoxical as it sounds, an extraordinarily simple object. To describe it completely, three numbers suffice.

The No-Hair Theorem

According to the so-called no-hair theorem (an expression coined by Wheeler), a black hole in equilibrium can be characterized completely by just three properties: its mass, its angular momentum (its rotation), and its electric charge. All other information about what fell into the black hole — whether stars, hydrogen, or encyclopedias — appears to be lost. Two black holes with the same mass, spin, and charge are indistinguishable. This radical simplicity will cost us dearly when we reach the information paradox.

In practice the charge is negligible (astrophysical black holes are essentially neutral), so two numbers suffice: mass and spin. A rotating black hole is called a Kerr black hole, a non-rotating one a Schwarzschild black hole.

The Event Horizon: A Boundary Without a Wall

The most famous structure of a black hole is the event horizon — the spherical surface at the Schwarzschild radius. It is important to understand what it is and what it is not. The event horizon is not a physical surface, not a membrane, not a wall. An astronaut crossing it would (for a sufficiently large black hole) notice locally nothing special at that moment. The horizon is rather a causal boundary: the surface beyond which every possible path — even that of a beam of light — inevitably leads inward. Seen from outside, it is the point beyond which no information can reach the outside again.

Viewed from afar, something strange happens: an object falling toward the horizon appears to slow down more and more, growing ever redder (through gravitational redshift), until it seems to freeze forever at the horizon and fade. Time itself, as seen by a distant observer, stretches toward infinity near the horizon. The infalling astronaut, by contrast, experiences the fall in finite proper time and crosses the horizon without noticing it.

The Singularity and the Photon Sphere

At the center, classical General Relativity predicts a singularity — a point (for a rotating black hole: a ring) where the curvature of spacetime becomes infinite and the theory loses its validity. Almost no one believes the singularity truly exists; it is rather a sign that here an as-yet-unknown theory of quantum gravity is needed, one that unites relativity and quantum mechanics.

Between the horizon and distant space lies another crucial structure: the photon sphere. For a non-rotating black hole it sits at 1.5 times the Schwarzschild radius. Here spacetime is so strongly curved that light can travel on closed (though unstable) circular orbits around the black hole. The photon sphere is the key to the image of a black hole: the shadow photographed by the Event Horizon Telescope is not the event horizon itself, but a projection produced by the bending of light at the photon sphere. Its diameter on the sky corresponds to about 2.6 times the diameter of the event horizon — gravity magnifies the dark region like a lens.

Three Weight Classes

Black holes come in vastly different sizes. Stellar black holes (roughly 3 to a few dozen solar masses) form in the collapse of massive stars. Supermassive black holes (millions to billions of solar masses) sit at the centers of nearly all large galaxies; how they grew so large so quickly is an open research question. In between lie the suspected intermediate-mass black holes (hundreds to thousands of solar masses), whose existence has only in recent years been reinforced by gravitational waves and individual observations.

Part 3: How Do You Weigh the Invisible?

Because black holes emit no light, they must be detected indirectly — through their effect on their surroundings. Three methods have proven especially powerful.

X-ray Light From the Feeding: Cygnus X-1

When matter — say, gas from a companion star — falls onto a black hole, it forms a rotating accretion disk. Friction heats the gas to millions of degrees, and it glows brightly in X-rays before vanishing behind the horizon. The first strong candidate for a black hole, Cygnus X-1, was identified in exactly this way in the 1960s and 70s: a bright X-ray source coupled to a blue supergiant, whose companion was too massive and too compact to be anything other than a black hole. Cygnus X-1 was also the subject of a famous bet between Stephen Hawking and Kip Thorne — Hawking bet that it was not a black hole, and conceded the wager in 1990.

Stars as Test Bodies: The Center of the Milky Way

The most elegant method is to observe stars orbiting an invisible center of mass. For more than 25 years, two teams — one led by Reinhard Genzel in Germany, one by Andrea Ghez in the United States — tracked the orbits of individual stars in the center of the Milky Way. A star named S2 circles the galactic center in only about 16 years on a tight elliptical orbit. From this orbit, Kepler's laws yield the central mass: around 4 million solar masses, concentrated in a space smaller than our solar system. Nothing other than a supermassive black hole fits the bill. Genzel and Ghez received the Nobel Prize in Physics in 2020 for this work, together with Penrose.

Ripples in Spacetime: LIGO

The third and most recent method is the direct detection of gravitational waves. When two black holes merge, they set spacetime itself vibrating, and these ripples propagate at the speed of light. On 14 September 2015, the two LIGO detectors registered such a signal for the first time, GW150914 — the merger of two black holes of about 36 and 29 solar masses into a single one of roughly 62 solar masses. The missing three solar masses were radiated away in a fraction of a second as gravitational-wave energy. This discovery, honored with the Nobel Prize in 2017, opened an entirely new window onto the cosmos and confirmed the existence of stellar black holes directly. (The related observation of merging neutron stars is discussed in the article on cosmic gold.)

Part 4: The Image — The Event Horizon Telescope

A Telescope the Size of the Earth

To image the shadow of a black hole, you need a resolving power that pushes against the boundary of the physically possible. The shadow of M87*, even though the black hole weighs 6.5 billion solar masses, appears only about 42 microarcseconds across on the sky — the angle at which an orange on the Moon would appear from the Earth. No single telescope on Earth could resolve that.

The solution is Very Long Baseline Interferometry (VLBI): you link radio telescopes across the entire globe and have them observe the same object in synchrony. By precisely combining their signals — each time-stamped with an atomic clock — you effectively create a virtual telescope the size of the Earth. That is precisely what the Event Horizon Telescope (EHT) is: a network of eight radio observatories at six sites worldwide that observed together in 2017 at a wavelength of 1.3 millimeters. The resulting volume of data was so enormous that it could not be sent over the internet but had to be transported physically on hard drives and reconstructed into an image over two years with sophisticated algorithms.

M87*: The First Image (2019)

The target M87 was cleverly chosen. The black hole at the center of the giant galaxy Messier 87, at 6.5 billion solar masses, is about 1,500 times more massive than the one in the Milky Way, but also about 2,000 times farther away (55 million light-years). On balance, its shadow appears on the sky about the same size as that of Sagittarius A — and because it is so large, its appearance changes only slowly, which makes it easier to image.

On 10 April 2019, the EHT collaboration presented the result: an asymmetric bright ring with a diameter of about 42 microarcseconds, enclosing a dark central region, with the ring appearing brighter in the south. This brightness asymmetry is no accident but a prediction of relativity: gas moving toward us is brightened by relativistic effects ("Doppler beaming"). The size and shape of the shadow matched the predictions for a Kerr black hole of the corresponding mass remarkably closely. It was the first direct, pictorial evidence that event horizons truly exist.

Two years later, in 2021, the collaboration published images of the polarized light from M87. Polarization reveals the structure of the magnetic fields at the edge of the black hole — and these fields turned out to be strong enough to partly hold back infalling gas and to explain the famous, thousands-of-light-years-long jets of matter that M87 hurls into space.

Sagittarius A*: Our Own Monster (2022)

On 12 May 2022 came the image many had been waiting for: Sagittarius A*, the black hole at the center of our own Milky Way, 27,000 light-years away. Here too a bright ring appeared — with a diameter of about 51.8 microarcseconds — around a dark center, fully consistent with a Kerr black hole of roughly 4 million solar masses. The image thus confirmed directly what Genzel and Ghez had inferred from the stellar orbits.

Photographing Sagittarius A was far harder than M87. Because it is so much smaller, the glowing gas orbits it in minutes rather than days. The image therefore changed constantly during the observation — as if trying to photograph a frantically wriggling puppy. Only with elaborate statistical averaging over thousands of plausible reconstructions could a stable image be obtained. That two black holes whose masses differ by more than a thousandfold both follow Einstein's predictions exactly is a striking testament to the universality of relativity.

In 2024, a re-analysis of M87 observations from 2018 showed that the shadow appears persistently* at the same location and the same size, while the bright region has rotated along the ring — exactly as expected if we are indeed watching turbulent gas circling a fixed horizon.

Part 5: Hawking Radiation and the Information Paradox

Black Holes Are Not Entirely Black After All

Until the 1970s the rule held: a black hole can only grow, never shrink. But in 1974 Stephen Hawking produced a result that shook this certainty. Applying quantum mechanics to the space immediately at the event horizon, it follows that a black hole radiates — albeit extremely faintly. This Hawking radiation has a thermal spectrum, so a black hole can be assigned a temperature.

Intuitively (if somewhat simplified), the effect is often described in terms of virtual particle pairs emerging from the quantum vacuum: if one falls behind the horizon while the other escapes, the black hole loses a minute amount of energy, and hence mass, in the process. Over unimaginably long timescales — for a stellar black hole far longer than the current age of the universe — a black hole would in this way evaporate completely. The smaller the black hole, the hotter and faster it radiates; a large one is colder than the space around it and, at present, tends to grow rather than shrink.

Hawking's result beautifully connected three previously separate theories: gravity, quantum mechanics, and thermodynamics. Together with Jacob Bekenstein's insight that a black hole possesses an entropy proportional to the area of its horizon, it became clear that black holes are thermodynamic objects. The Bekenstein-Hawking entropy remains to this day one of the most important clues toward the sought-after theory of quantum gravity.

The Paradox: Where Does the Information Go?

From this discovery, however, arises a deep problem that has occupied physics for more than fifty years — the information paradox. Quantum mechanics rests on an iron principle: information is never lost. The precise quantum state of a system evolves, to be sure, but it can in principle always be traced back to its initial state (the technical term is unitarity).

Now the dilemma. Throw an encyclopedia into a black hole, and by the no-hair theorem every trace of its content vanishes behind the horizon. The black hole then evaporates via Hawking radiation. But this radiation is — according to Hawking's original calculation — purely thermal, a structureless noise that depends only on the mass, spin, and charge of the black hole and carries no memory of the encyclopedia. When the black hole has finally evaporated completely, the information about its content would be irretrievably lost — a direct contradiction of the unitarity of quantum mechanics.

Two of the best theories in physics thus predict opposite things. Either quantum mechanics is violated in the presence of gravity, or Hawking's calculation is incomplete and the radiation does carry the information out after all — in some subtle way we do not yet understand.

Progress, but No Solution

As things stand today, the majority of theoretical physicists lean toward the second option: the information is preserved. A crucial hint came from string theory and so-called holography (the AdS/CFT correspondence), which suggests that everything happening within a volume can be equivalently encoded on its boundary. In recent years the "island formula" has caused a stir: a calculation showing that the information apparently trapped inside can, after all, be reconstructed from the Hawking radiation, provided one treats the quantum geometry carefully enough. This reproduces the so-called Page curve — the course of the entanglement entropy that unitarity demands.

Despite this progress, the paradox is not conclusively solved as of 2026. It is still not clear by what concrete physical mechanism the information gets into the seemingly thermal radiation. I am of the opinion that precisely herein lies the fascination: the black hole is the one object where relativity and quantum mechanics rub against each other so hard that their contradiction cannot be argued away. Whoever solves the information paradox will have come a great step closer to quantum gravity. On the occasion of the fiftieth anniversary of Hawking's discovery, several major conferences on exactly this question took place in 2025 — a sign that the debate is more alive than ever.

(The question of whether and how quantum information is preserved and can be protected in principle also touches on quantum error correction and quantum entanglement.)

The Central Takeaway

The real lesson of black holes is one about the nature of scientific certainty. A black hole is by definition invisible; it cannot be observed directly. And yet the existence of black holes is today among the best-established facts in astrophysics — not despite but because of the diversity of independent approaches. X-ray sources, orbiting stars, gravitational waves, and finally the direct image of the shadow all yield one and the same picture. No single observation would be conclusive on its own; their convergence, however, is.

For your own work, a concrete principle follows: never rely on a single measurement method or a single data source for an important conclusion. Look for evidence that arises along different, mutually independent paths — whether in physics, in a security audit, or in debugging a distributed system. When several independent methods yield the same answer even though each alone would be uncertain, a certainty emerges that none of them could bear on its own. That is exactly how humanity made the invisible visible.

A Question to Reflect On

The information paradox forces us to choose between two principles that both seemed inviolable: the causal structure of relativity and the information conservation of quantum mechanics. When two of your most trusted foundational assumptions contradict each other and the data alone cannot yet decide — by what criteria do you decide which assumption to give up and which to save?

Cross-References in the Vault

• The Cosmic Tension: Why the Universe Seems to Have Two Rates of Expansion – the expansion rate of the universe and the limits of our cosmological measurements.
• Milliseconds from the Cosmos: Fast Radio Bursts and the Universe's Missing Matter – another extreme phenomenon decoded through radio astronomy and multi-messenger methods.
• Cosmic Gold: Neutron Star Collisions, the r-Process, and the Origin of the Heavy Elements – gravitational waves and merging compact objects at the other end of the density scale.
• Order from Noise: Quantum Error Correction and the Road to a Fault-Tolerant Quantum Computer – how quantum information is protected from loss.
• Spooky Action at a Distance: Quantum Entanglement from Einstein to the Quantum Internet – entanglement as a key concept in the information paradox.

Sources

• Event Horizon Telescope Collaboration (2019): First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole. ApJ 875, L1. https://arxiv.org/abs/1906.11238
• Event Horizon Telescope Collaboration (2022): First Sagittarius A Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way.* ApJ 930, L12. https://ui.adsabs.harvard.edu/abs/2022ApJ...930L..12E/abstract
• MIT News (2022): Astronomers snap first-ever image of supermassive black hole Sagittarius A.* https://news.mit.edu/2022/first-supermassive-black-hole-sagitarrius-0512
• Event Horizon Telescope Collaboration (2024): The persistent shadow of the supermassive black hole of M 87. I. A&A 681, A79. https://www.aanda.org/articles/aa/full_html/2024/01/aa47932-23/aa47932-23.html
• The Black Hole Information Problem (review article, 2025), PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC12191829/
• The Nobel Prize in Physics 2020 (Penrose, Genzel, Ghez). https://www.nobelprize.org/prizes/physics/2020/summary/

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