They are born from the death of massive stars, consume matter, disrupt the flow of time, and, according to Stephen Hawking, even emit heat. We invite you to discover black holes, these cosmic objects where the laws of physics reach their limits.
They are born from the death of massive stars, consume matter, disrupt the flow of time, and, according to Stephen Hawking, even emit heat. We invite you to discover black holes, these cosmic objects where the laws of physics reach their limits.
A black hole is what remains of a large star that, after burning all its fuel, has collapsed in on itself to the point of creating a true hole in space. The black hole fascinates because it is a place in the universe where the great theories of physics (general relativity, quantum mechanics, and thermodynamics) reach their limits and are forced to interact.
For a long time, black holes were merely a theoretical consequence of Albert Einstein's equations, a curiosity that few people truly believed in. Today, astronomers observe thousands of them in the sky and study them in detail. The matter from the stars that formed them has rushed inside and disappeared from our view... but where does it go? This is one of the great questions still unanswered.
The black hole holds other surprises. English physicist Stephen Hawking calculated that a black hole emits heat, somewhat like a radiator. And time does not pass at the same rate inside as it does outside, to the point that a very brief event inside can seem to last a very long time when viewed from the outside.
Note: To understand a black hole, one must first accept a somewhat counterintuitive idea. Space is not an empty, motionless box in which objects move; it is a kind of immense flexible mollusk that undulates, curves, twists, and can, in extreme cases, be hollowed out.
A black hole forms from a large star that has burned all its fuel, hydrogen, and eventually goes out. Deprived of the heat from the combustion that supported it, what remains of the star collapses under its own weight, curving space to the point of creating a hole.
The formation of a black hole can be summarized in five steps:
the large star burns its hydrogen, and the heat from this combustion prevents it from collapsing,the fuel is exhausted and the star goes out,what remains of the star is no longer supported by heat and collapses under its own weight,the matter becomes increasingly dense and curves space more and more strongly,space eventually digs a hole with no exit: the black hole is born.
To measure the violence of this collapse, let's take the example of our Sun. If the Sun were to form a black hole when it stops burning, this black hole would have a diameter of only about one and a half kilometers. All the matter of our star, which is gigantic, would be compressed into a sphere one and a half kilometers wide!
The birth of a black hole may not always involve the death of a star. Some physicists consider that black holes of different sizes may have formed in the "crucible" of the very first moments of the universe, and that some of them may be exploding today.
General relativity explains black holes with a simple idea: matter curves space, and extremely concentrated matter can curve it to the point of creating a hole. In the theory that Albert Einstein completed in November 1915, the gravitational field is not spread out in space; it is space itself.
Before Einstein, Newton explained the fall of bodies and the motion of planets through a "force of gravity" acting at a distance, in a space viewed as a vast empty container. General relativity replaces this mysterious force with curvature. The most illustrative image is that of a marble rolling in a funnel: nothing pulls it toward the center; it is the slope of the walls that makes it turn. Similarly, the Earth orbits the Sun because it moves straight in a space that the Sun curves around it.
To describe this curvature, Einstein used the mathematics of curved surfaces invented by Carl Friedrich Gauss, which were then extended to three-dimensional spaces and beyond by his student Bernhard Riemann. Einstein's equation states that Riemann curvature, denoted R, is proportional to the energy of matter. In simple terms: space curves where there is matter (and the equation can be expressed in half a line, which did not prevent the Russian physicist Lev Landau from calling general relativity "the most beautiful of scientific theories").
Time also curves: it passes more quickly at higher altitudes and more slowly near the Earth, a tiny but measurable difference. The black hole is the extreme case of general relativity, where the curvature becomes such that space digs a hole with no exit and time inside does not flow at the same rate as outside.
Astronomers today observe thousands of black holes in the sky, and they study them in detail. These observed black holes were born from stars that collapsed under their own gravity, with their matter rushing inside and disappearing from our view.
This figure of several thousand is surprising when you consider where we started. Not so long ago, physics students were taught that black holes were merely implausible predictions of an esoteric theory, general relativity. Very few imagined that one day there would be so many of them!
The story of black holes resembles that of other predictions of general relativity, all of which initially seemed like the ramblings of a mad scientist before being verified one by one:
the bending of light by the curvature of space around a star, observed in 1919,time passing faster at altitude than at sea level, measured and confirmed,the expansion of the universe, observed in 1930,gravitational waves, these ripples in space whose effects on binary stars confirm the theory with an accuracy of one in a hundred billion,and black holes, which have transitioned from the status of a dubious hypothesis to that of objects observed by the thousands.
One limitation, however: seeing thousands of black holes does not mean we know what is happening inside them. The interior of a black hole escapes our view, and the fate of the matter that has fallen into it remains an open question in physics.
Einstein's equation predicts that space cannot remain still: the universe must be expanding, and this expansion of the universe was indeed observed in 1930. The same equation indicates that the expansion must have been triggered by the explosion of a young, very small, and very hot universe: this is the Big Bang.
Like black holes, the prediction of the Big Bang by Einstein's equation initially left everyone skeptical. No one really believed it. Then evidence accumulated, culminating in the observation in the sky of cosmic radiation, this diffuse glow that is the remnant of the heat from the initial explosion.
Why talk about the Big Bang in relation to black holes? Because both phenomena arise from the same equation of general relativity, and because they pose the same puzzle: in both cases, matter is compressed into a tiny space, where general relativity alone is no longer sufficient and where quantum mechanics must come into play.
Nota Bene: loop quantum gravity, a research avenue attempting to marry general relativity and quantum mechanics, suggests that the Big Bang may have actually been a "Big Bounce," meaning a great rebound. Our universe would have emerged from a previous universe that was contracting under its own weight, until it reached the size of a walnut shell, before rebounding and starting to expand again. A rebound mechanism that some physicists also imagine at the heart of black holes.
Stephen Hawking showed, based on quantum mechanics, that black holes are always "hot": they emit heat, a radiation comparable to that of a radiator. This result from the English physicist is the first concrete indication of what a "hot space" is.
To measure the significance of Hawking's calculation, one must understand what heat is. Until the mid-19th century, physicists imagined it as a kind of fluid, "caloric." Maxwell and Ludwig Boltzmann understood that this is not the case: a hot body is simply a body whose atoms and molecules move faster. Hot air is air where the molecules are moving quickly, cold air is air where they are moving more slowly; it's as simple as that.
Physicists can describe a hot electromagnetic field, like the radiation present in an oven: it can be visualized as a gas of photons vibrating randomly. But according to general relativity, the gravitational field is space itself, and even spacetime. A hot gravitational field, therefore, is a space and time that vibrate... and no one yet has the equations to describe such a hot spacetime.
Hawking's hot black hole is valuable precisely for this reason: it is a quantum effect occurring on an object of gravitational nature. The calculation has been revisited in many different ways, and many physicists today consider the heat of black holes to be real. Note: Stephen Hawking also remained famous for continuing his work despite severe medical issues that left him confined to a wheelchair and deprived of speech.
No, the heat of black holes has never been observed. For the real black holes that astronomers see in the sky, the heat predicted by Stephen Hawking is far too weak to be detected.
This is the major weakness of the case: the heat of black holes relies on a calculation, not on a measurement. The calculation is considered convincing, it has been redone through other methods, and many physicists regard the phenomenon as real. But a prediction, no matter how elegant, remains a prediction until the experiment has decided.
Caution is particularly warranted since physics has already experienced alluring theories that were disproven by facts. A theory from the 1970s called SU(5) predicted that the proton would eventually decay: physicists surrounded tons of water, which contain billions of protons, with detectors, and some dedicated their lives to it... without ever seeing a single proton decay. The so-called "supersymmetric" particles, confidently awaited for decades, have also failed to appear.
In the absence of being able to measure the heat of black holes, physicists are seeking other ways to test the quantum physics of these objects, for example by looking for signals of potential small black holes exploding in high-energy cosmic rays. Nothing is certain: the question remains open, and that is precisely what makes it exciting.
The atoms of space are, according to loop quantum gravity, the elementary grains that make up space: space would be neither continuous nor infinitely divisible, but formed of tiny quanta. These atoms of space would also be the ones that, by vibrating, heat the surface of a black hole.
Loop quantum gravity starts from two very simple observations. General relativity teaches that space is a field, something dynamic that can be compressed and twisted. Quantum mechanics teaches that every field is made of quanta, with a fine structure in grains. If we take both seriously at the same time, without adding anything, space itself must be "made of quanta."
These grains of space are so small that they are dizzying:
a billion billion times smaller than the smallest atomic nuclei,millions of billions of times smaller than the smallest distance that our most powerful instruments can probe, such as the Large Hadron Collider in Geneva.
We speak of "loops" because each atom of space is connected to others and forms a network with them, like the iron rings of a chainmail. Where are these atoms of space? Nowhere: they are not in space; they are space. Note: loop quantum gravity, developed by teams from several countries (France plays a leading role), has not yet received experimental verification, and other research avenues exist alongside it.
No one yet knows for sure what happens to matter that falls into a black hole, but a bold hypothesis offers an answer: this matter would eventually form a "Planck star." According to loop quantum gravity, the matter in a black hole cannot collapse into an infinitely small point, simply because space does not have infinitely small points, only regions of finite size.
The scenario of the Planck star unfolds in four stages:
the matter of the star collapses under its own weight and becomes increasingly dense,upon reaching a certain degree of compression, quantum mechanics generates an opposing pressure,this pressure, born from the quantum fluctuations of spacetime, eventually balances the weight of the matter: this is the Planck star,the Planck star is not stable; once maximally compressed, it rebounds, leading to the explosion of the black hole.
Let's take an example. If the Sun were to someday form a black hole about one and a half kilometers in diameter, its matter would continue to collapse inside until it formed a Planck star the size of an atom: the entire Sun would fit into the space of an atom! And if black holes seem to last so long, it's because time does not flow at the same speed inside and outside (a bit like how it passes more quickly in the mountains than by the sea, but with a huge difference). The rebound, very brief for an imaginary observer sitting on the Planck star, appears extremely slow from the outside.
A black hole would thus be a star that rebounds on itself in slow motion. If small black holes born at the beginning of the universe are exploding today, their signals could appear in high-energy cosmic rays, or even in fast radio bursts (FRBs) already detected by radio telescopes. The idea might also not work: perhaps not enough black holes formed in the primordial universe, and these radio bursts may have another origin.
The link between quantum gravity and the thermodynamics of black holes lies in the heat of their surface: it is the elementary grains of space, the "atoms of space," that, by vibrating, heat the surface of a black hole. The heat of black holes thus brings together three fundamental areas of knowledge that physics has yet to successfully merge into a single theory: quanta, gravity, and thermodynamics.
Thermodynamics is the science of heat and temperature. Its probabilistic origin was understood thanks to the ideas of Austrian physicist Ludwig Boltzmann, ideas deemed absurd during his lifetime (Boltzmann, who was not taken seriously by anyone, committed suicide on September 5, 1906, in Duino, near Trieste, without seeing his work recognized). Thermodynamics and statistical mechanics were later extended to electromagnetic fields and quantum phenomena. However, their extension to the gravitational field still encounters an unresolved problem.
The crux of the issue is that the two pillars of modern physics contradict each other. A student who attended a general relativity class in the morning and a quantum mechanics class in the afternoon would hear descriptions of two incompatible worlds: in the morning, a curved space where everything is continuous; in the afternoon, a flat space where energy quanta bounce around. Yet, both theories work remarkably well. Physicists see this as an opportunity, as it was by reconciling two theories that worked that Newton discovered universal gravitation (by combining Galileo's parabolas and Kepler's ellipses) and that Maxwell unified electricity and magnetism.
This is why the heat of black holes can be compared to a Rosetta Stone: a text written in three languages (Quanta, Gravity, and Thermodynamics) waiting to be deciphered. Quantum gravity is beginning to connect two of these languages, but no one yet has even a rough draft of a theory capable of linking all three, nor does anyone truly understand why black holes heat up. The day this text is read, it could tell us what the flow of time really is.
Black holes could help us understand the nature of the flowing time because their heat connects space, quanta, and heat, and it is precisely heat that gives time its direction. In physics, the difference between past and future only appears when there is heat.
Imagine a pendulum in thought. Picture it without any friction, film it, and play the film backward: the motion seems just as plausible. Add friction: the pendulum heats its supports, loses energy, slows down, and you immediately distinguish the forward film from the backward film (no one has ever seen a stopped pendulum start to swing again by itself). According to Ludwig Boltzmann, heat flows from hot to cold not by an absolute law but because it is by far the most probable: it is not impossible for hot tea to warm up further when in contact with a cold spoon, it is just terribly unlikely.
So why does time seem to "flow" to us? Relativity has shown that a common present for the entire universe is an illusion (Einstein, upon the death of his friend Michele Besso, wrote to his family that the distinction between past, present, and future is just a stubborn illusion), and the equations of loop quantum gravity no longer even contain the time variable. One possible answer: the flow of time arises from our blurry vision of the world, from the fact that we only interact with a handful of variables, like temperature, among the countless ones that describe reality. Some philosophers, following Martin Heidegger, who believed we "inhabit time," argue that physics misses the essential, while physicists remind them that our immediate intuitions have often deceived us, starting with the notion of a flat Earth.
To summarize the essentials about black holes: they arise from the collapse of massive stars, general relativity describes them as holes carved in space, astronomers observe thousands of them, Stephen Hawking showed that they are hot, and quantum gravity imagines a Planck star at their core that bounces in slow motion. The next time you read that a mysterious radio burst has been detected, or that a calculation about black holes is under debate, you will know what questions lie behind it... and why physicists are far from finished searching.
Author Audrey on 30 November 2021
Education and Training : Association and Organization
Author Loïc on 06 May 2026
Education and Training : School