First-hand accounts
Metaphorical
Objectivity
Imagine you need to study an object that absorbs all light. An object that distorts space itself around it. One so dense that a teaspoon of its substance would weigh a billion tons. The task seems impossible, yet this is precisely what astrophysicists do every day when they study black holes.
Three years ago, I spoke with Dr. Sheperd Doeleman of the Event Horizon Telescope collaboration in his lab at the Massachusetts Institute of Technology. «We can't photograph the black hole itself», he explained, pointing to a computer screen with an orange ring. «We see its shadow.»
This «photo» of the black hole M87* circled the globe in 2019. But what does it actually show? And how is it possible to see something that emits no light?
Anatomy of an Invisible Monster
To understand the methods for studying black holes, we first need to understand their structure. Let's imagine a black hole as a layered cake, where each layer follows its own physical laws.
At the center lies the singularity – a point of infinite density where the laws of physics break down. Surrounding it is the event horizon – an invisible boundary that not even light can escape. For a black hole with the mass of ten suns, the radius of this boundary is about thirty kilometers.
Further out begins a region astrophysicists call the ergosphere. Here, spacetime is so intensely warped that it drags all matter along, forcing it to rotate with the black hole. And finally, at a distance of several event horizon radii, an accretion disk forms – a ring of gas and dust heated to millions of degrees, spiraling into the black hole.
It is this very disk that becomes our window into the world of black holes. The matter within it heats up to temperatures at which it emits X-rays and radio waves. The nature of this radiation can be used to determine the mass, size, and rotation speed of the invisible object.
«We are like detectives studying a criminal by the traces they leave behind», Professor Andrea Ghez of the University of California, who won a Nobel Prize for her research on the black hole at the center of our galaxy, told me. «The stars, the gas, the light – all of these are witnesses that tell us what is happening near the black hole.»
Gravitational Wave Detectives
The first revolutionary method for studying black holes emerged in 2015, when the LIGO detectors first registered gravitational waves. This discovery changed astrophysics as radically as the invention of the telescope four hundred years ago.
To understand how LIGO works, imagine a giant four-kilometer-long ruler. When a gravitational wave from merging black holes passes through Earth, it stretches and compresses space, changing the length of this ruler by an amount a thousand times smaller than the diameter of a proton.
How is it even possible to measure such infinitesimal changes? LIGO engineers created the most precise measuring instrument in human history – a laser interferometer. A laser beam is split in two, which travel down perpendicular four-kilometer tunnels, reflect off mirrors, and return. When the lengths of the tunnels are perfectly identical, the beams combine and cancel each other out. But as soon as a gravitational wave passes, one tunnel becomes slightly longer than the other. A light signal appears.
In nine years of operation, the LIGO, Virgo, and KAGRA detectors have registered dozens of black hole mergers. Each such event tells its own story: about the masses of the merging objects, the distance to them, and even the expansion rate of the Universe.
«Gravitational waves are like the seismograms of the cosmos», explained Barry Barish, one of LIGO's creators. «We are hearing the final seconds of the lives of black holes that circled each other for billions of years before merging in a cosmic cataclysm, releasing more energy than all the stars in the visible universe combined.»
How to Photograph a Shadow
While physicists were learning to listen to gravitational waves, astronomers were working on an even more ambitious task: to see a black hole. Or rather, its shadow.
The Event Horizon Telescope is not a single telescope but a network of radio telescopes scattered across the globe, from Greenland to Antarctica. The project's goal is to observe the immediate surroundings of the supermassive black hole Sagittarius A* at the center of the Milky Way, as well as the even larger black hole in the supergiant elliptical galaxy Messier 87.
The principle behind it is very-long-baseline interferometry. All the telescopes observe the same black hole simultaneously, recording radio signals with nanosecond precision. This data is then combined, creating a virtual telescope the size of Earth.
The resolving power of such a system is phenomenal. If you had such vision, you could read a newspaper in New York while standing in Munich. This is exactly the precision needed to see the shadow of a black hole – the region where the hot gas of the accretion disk disappears behind the event horizon.
But even with such resolution, obtaining an image of a black hole is incredibly difficult. The data from the telescopes arrives as raw radio signals, from which a picture must be reconstructed. It's like trying to assemble a mosaic with only a tenth of the pieces in hand.
«We aren't just taking a photograph», Katie Bouman, one of the developers of the EHT's image-processing algorithms, told me. «We are solving an inverse problem: reconstructing the appearance of an object that no one has ever seen from incomplete data.»
The first image of the M87* black hole in 2019 was a triumph of this technology. The orange ring with a dark region in the center is exactly what Einstein's general theory of relativity predicted a hundred years ago. Three years later, the EHT team produced an image of the Sagittarius A* black hole – at the center of our own galaxy.
Windows to the Universe
Beyond gravitational waves and radio telescopes, there is a whole arsenal of methods for studying black holes. Each one opens its own window into the physics of these enigmatic objects.
X-ray astronomy allows us to observe the most high-energy processes in the Universe. As matter falls onto a black hole, it heats up to hundreds of millions of degrees and emits X-rays. The nature of this radiation can reveal the black hole's mass, its rotation speed, and even the chemical composition of the matter being consumed.
Particularly interesting are X-ray flares – short bursts of radiation that occur when a black hole tears apart and consumes a passing star. Such events are called tidal disruption events, and they provide a unique opportunity to study the behavior of matter under the most extreme conditions.
Optical astronomy has also contributed to the study of black holes. By observing the motion of stars around the invisible center of our galaxy, astronomers have been able to precisely determine the mass and size of the black hole Sagittarius A*. Some of these stars approach it to within a distance of just a few light-hours, reaching speeds of up to seven thousand kilometers per second.
«Imagine you are observing the planets of the Solar System, but the Sun is invisible», explained Reinhard Genzel, another Nobel laureate for research on the galactic center. «From the orbits of the planets, you can calculate the mass and position of the star. We study our black hole in the same way – by the dance of the stars around it.»
In Search of Wormholes
But the most intriguing questions about black holes concern what happens beyond the event horizon. Classical physics says that a singularity lies there – a point of infinite density where all known laws of nature cease to apply.
However, quantum mechanics suggests more exotic possibilities. One of the most fascinating hypotheses is the existence of wormholes, tunnels in spacetime that could connect distant regions of the Universe or even different universes.
Theoretically, if a wormhole were stable and large enough, one could travel through it. But in practice, all known solutions to Einstein's equations require the existence of exotic matter with negative energy density – a substance that no one has ever observed.
«Wormholes are like bridges in spacetime», Kip Thorne, a theorist from Caltech, told me. «But for such a bridge not to collapse, it must be propped up by a material that may not exist in nature.»
Nevertheless, the search for wormholes continues. Some theorists suggest they might manifest through specific gravitational-wave signals or unusual properties of accretion disks. If a wormhole connects two regions of space, matter could pass through it in both directions, creating characteristic oscillations in luminosity.
The Information Paradox
One of the deepest mysteries of black holes relates to the fate of information. According to quantum mechanics, information cannot be irretrievably lost. But what happens to the information about a particle that falls into a black hole?
Stephen Hawking showed that black holes should evaporate by emitting thermal radiation. But this radiation appears to be completely random and carries no information about what fell into the black hole earlier. This creates a paradox: information seems to vanish from the Universe, which contradicts the fundamental principles of quantum mechanics.
«It's as if you burned a book and tried to reconstruct its contents from the ash», Leonard Susskind of Stanford explained to me. «Classical physics says this is impossible. But quantum mechanics insists that the information must be preserved.»
Over the past twenty years, many solutions to this paradox have been proposed. One is the holographic principle, which suggests that all information about the interior volume of a black hole is encoded on its surface, like a hologram. Another solution proposes that information does indeed return, but only after the black hole has completely evaporated over a fantastically long time – far longer than the current age of the Universe.
Time Machines and Paradoxes
If wormholes do exist, they might not only be tunnels through space but also time machines. Theoretically, if one end of a wormhole were accelerated to near the speed of light and then brought back, a time difference would arise between the two ends. A journey through such a wormhole could allow one to travel into the past.
But here, physics confronts the famous grandfather paradox: what would happen if a time traveler killed their grandfather before their father was born? Most theorists believe that nature must have built-in mechanisms to prevent such paradoxes.
One hypothesis is the Novikov self-consistency principle. According to it, any attempt to change the past would lead to events that ensure history remains unchanged. In other words, if you tried to prevent your own birth, something would inevitably go wrong, and in the end, everything would happen exactly as it was meant to.
«The Universe protects itself from paradoxes», believes Igor Novikov, the author of this hypothesis. «Time travel may be possible, but only in ways that do not violate cause-and-effect relationships.»
Black Holes as Laboratories
Modern research into black holes is not just astronomy; it is a test of the fundamental laws of physics under the most extreme conditions. Near a black hole, gravity is trillions of times stronger than on Earth, and velocities approach the speed of light. Here, the general theory of relativity reveals its full power.
One of the most striking confirmations of Einstein's theory is the observation of the orbital precession of the star S2, which orbits the black hole Sagittarius A*. Over twenty years of observation, astronomers have recorded that the star's orbit slowly rotates in space, exactly as predicted by general relativity.
Another effect is gravitational redshift. As light escapes from a strong gravitational field, it loses energy and becomes redder. This effect is observed in the spectra of stars passing near black holes.
«Black holes are laboratories of extreme physics that we could never create on Earth», Avi Loeb of Harvard told me. «They allow us to test our theories under conditions where space and time behave in completely unusual ways.»
Why We Will Never See the Bottom
The thorniest question about black holes is: what's inside? And here, physics gives an unequivocal answer: we will never know. Not because we lack the technology, but because it is fundamentally impossible.
Imagine your friend is falling into a black hole, and you are watching from a distance. From your perspective, time for them slows down as they approach the event horizon. At first, your friend moves more slowly, their voice deepens, and their image reddens. At the event horizon itself, their time, from your point of view, will stop forever.
And what does the falling person see? For them, time flows normally, and they cross the event horizon without issue. But they can no longer send a signal out – all their messages remain trapped inside the black hole.
These are not technical limitations but fundamental properties of spacetime. Information cannot leave the region beyond the event horizon – that is a law of nature.
«The event horizon is a one-way membrane for information», explained John Wheeler, who, by the way, coined the term «black hole.» «Everything can go in, but nothing can come out. This is a fundamental property of the geometry of spacetime.»
The Future of Research
Despite the fundamental impossibility of peering beyond the event horizon, research on black holes continues to advance at a breathtaking pace. The next generation of gravitational-wave detectors will be able to register black hole mergers at distances of tens of billions of light-years, effectively observing the birth of the first black holes in the Universe.
The Event Horizon Telescope is also preparing for new discoveries. The addition of new telescopes and a shift to higher-frequency observations will make it possible to obtain real-time videos of black holes, observe turbulence in their accretion disks, and perhaps capture relativistic jets – streams of particles ejected by black holes at speeds close to the speed of light.
Space missions of the next decade promise even more detailed studies. The European LISA mission – a space-based gravitational-wave detector the size of Earth's orbit – will be able to register waves from supermassive black holes that are inaccessible to ground-based facilities.
«We are in a golden age of black hole research», believes Sheperd Doeleman. «Every few months bring new discoveries that force us to reconsider our understanding of the most extreme objects in the Universe.»
Perhaps we will never manage to look beyond the event horizon. But every new observation brings us closer to understanding how reality is structured in its most incredible manifestations. And that, perhaps, is the greatest value of science – turning the impossible into the understandable, the invisible into the observable, and the mysterious into the explainable.
Black holes remain some of the most enigmatic objects in the Universe. But thanks to the ingenuity of scientists and the advancement of technology, we can study them with a precision that seemed like science fiction just thirty years ago. Who knows what other secrets will be revealed to us in the coming decades?
