A centrepiece of Prof Stephen Hawking's theory of black holes states that black holes can only grow. Thanks to precision observations made with the Laser Interferometer Gravitational-Wave Observatory (LIGO), this theory has now been verified for the first time.
This marks a new milestone exactly 10 years after LIGO’s first detection of gravitational waves.
Above all, it demonstrates that advanced observations of gravitational ripples can reach into the deepest roots of physics. In the future, they may even reveal that the Universe is a hologram.
Time without time
The theoretical discovery that there might be a hologram hiding underneath our familiar reality of space and time ranks among the most important, baffling and far-reaching physics discoveries of the late-20th century.
Physicists still don’t agree on what form this hologram might take, but just the mere idea of a hologram has opened up implications that have already changed theoretical physics beyond recognition.
For decades, physicists had struggled to seal the marriage of General Relativity and quantum theory. The discovery of holography did exactly that.
It showed that gravity and quantum theory need not be water and fire, but can be like yin and yang: two very different yet complementary descriptions of the same physical reality.
Physical systems can be gravitational and quantum at the same time, holography says, albeit in different dimensions. This is the shift in perspective brought about by holography.
Perhaps our entire expanding Universe might be a hologram – this is Hawking’s final theory of the Universe. In the holograms we’re familiar with, a third dimension of space emerges from the light projected on a surface.
In the cosmos-as-a-hologram idea, however, it would be the dimension of time that can be holographically encoded. That is, the evolution of the Universe might be a holographic projection.
Hawking’s final thoughts on time emerging as a holographic projection are encapsulated in a disc-like image (see diagram, above).
The outer circle represents a timeless hologram made up of countless entangled quantum bits, or qubits, like the spin state of particles. (A qubit is the basic unit of quantum information that can exist in a combination of states at once.)
From this, the evolution of an expanding Universe is thought to emerge. At the centre of the disc lies the origin of the Universe, which expands outward in the so-called ‘radial direction’.
It’s as if there’s a code operating on the entangled qubits that brings about the Universe, and this emergent process is what we perceive as the flow of time.
We can venture back in time, towards the interior of the disc, by taking a fuzzier view of the outer hologram – like zooming out of a picture. Eventually, however, one runs out of bits. This would be the origin of time in the holographic vision of the cosmos.
If true, there could be nothing before the Big Bang, because the past that holographically emerges simply doesn’t extend further back.
These theoretical insights, if proven, yield a radically new twist on the Universe’s origins. Ever since the discovery of the Big Bang in the early 1930s, it has been somewhat of an enigma how time could pop into existence.
Physicists, including Hawking, traditionally strove to give a fundamentally causal explanation of the beginning of the world.
But the cosmos-as-a-hologram idea offers a rather different view. Near the Big Bang, classical physics breaks down. In holography, the Big Bang marks not only the beginning of time but also the emergence of the familiar laws of physics.
Are black holes the ultimate holograms?
In the 1970s, Jacob Bekenstein and Stephen Hawking discovered, with an ingenious thought experiment, that black holes aren’t empty bottomless pits, as Einstein’s theory of General Relativity suggests.
Rather, they store a vast amount of information about their history in a mysterious microscopic structure. Black holes would actually be by far the most efficient hard drives in the Universe, Hawking suggested.
For example, Sagittarius A*, the huge black hole lurking in the centre of the Milky Way, can store the equivalent of no less than 1080 gigabytes (that’s a 1 followed by 80 zeros – an unimaginably huge number).
All the data in the Google storage banks could easily fit into a black hole smaller than the size of a proton.
Hawking even derived a precise mathematical formula for the amount of information, or entropy, that black holes contain. That said, Hawking’s formula is rather bewildering.
For one, it implies that the entropy of black holes grows like the surface area of their event horizon and not like their interior volume. This is surprising.
The information storage capacity of all familiar systems, like libraries, is tied to their interior volume, rather than the area of their bounding surface.
If you wanted to estimate the amount of information in a library, for example, you’d need to count the number of books on all the shelves, not just those lining the walls.
This isn’t the case with black holes, it seems.
To compute the quantum information content of a black hole, Bekenstein and Hawking invite us to consider the event horizon’s surface area and to cover this with a grid-like pattern of minuscule cells, the sides of each measuring one quantum in length, a mere 10-33 cm (that’s 0.000…0001, with 32 zeros after the decimal before the 1).
The monumental insight from this entropy equation is that each quantum on the event horizon carries one bit of information.
Each of these bits can potentially provide the answer to a single yes-or-no question about the evolution of the black hole and its microstructure, and the collection of all those bits would be all there is to know about the black hole.
This was the first glimmer of holography in modern physics: the storage capacity of black holes isn’t determined by their interior volume, but by the area of their horizon surface. It’s as if, from a quantum viewpoint, black holes don’t quite have an interior, but are actually holograms.
Now, entropy is also a measure of the amount of disorder in a physical system. High entropy means that a system is in a highly disordered state, and low entropy is one that is highly ordered.
This should also be the case when two black holes collide and merge. When this happens, their masses combine, increasing their surface area. However, they also lose energy in the form of a burst of gravitational waves.
What’s more, the merger can boost the spin of the new black hole, which, Einstein’s theory says, reduces its surface area.
Hawking’s theory states that the combination of all these conflicting factors should increase the total surface area under all circumstances, producing a bigger black hole.
In mid-2025, precision analysis of an exceptionally powerful gravitational wave from two merging black holes made it possible to verify Hawking’s theory for the first time.
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Gravitational waves
Gravitational waves are tiny vibrations in the fabric of space. Their existence was first predicted by Albert Einstein in 1916. Einstein doubted, though, that gravitational waves could ever be measured, let alone be of much significance.
Today, more than a century on, the very first detections of these elusive ripples are racing like a tsunami through physics and astronomy.
Why? Because the observational discovery of gravitational waves heralds a Galilean revolution of sorts: it marks the birth of an entirely new astronomy.
Ever since Galileo Galilei’s first telescopic observations in 1609, nearly everything we’ve learnt about the Universe has come to us through astronomical observations of light of various frequencies, from radio waves to gamma radiation.
The 21st century will be the era in which we will journey through the cosmos guided by gravitational wave observations too – a completely new way of exploring the Universe.
It’s as if humankind is developing a new sense, as if we’re learning to listen to the sounds of the cosmos, rather than merely looking up and looking out.
Bursts of gravitational waves are constantly generated in violent cosmic events, when black holes collide, for example, and even during the earliest moments of the Big Bang.
Once generated, a pulse of gravitational waves spreads out at the speed of light and passes unhindered through everything, through planets, stars and even clouds of dark matter.
When gravitational waves pass through Earth, distances and duration are briefly stretched and squeezed, ever so slightly, as if the Universe’s gentle breathing whisks through the planet.
These are the faint echoes of some of the most powerful and mysterious cosmic events.
Crucially, through reverse engineering the observed patterns of gravitational waves using Einstein’s theory of General Relativity, we can uncover the cosmic sources of gravitational wave bursts passing through Earth.
Reverse engineering the exceptionally powerful detection made last year – registered as GW250114 (the numbers indicate the date the gravitational waves reached Earth, starting with the year, 2025, then the month, January, then the day, 14), – showed that it came from two black holes, approximately 1.3 billion light-years away with masses 30–40 times that of the Sun, which collided and combined to form a new black hole.
This is a rather similar source to LIGO’s very first detection (GW150914). However, thanks to a decade of technological advances that have reduced instrumental noise, the GW250114 signal is significantly louder than GW150914.
This allowed gravitational wave researchers to determine that the surface area of the event horizon of the new black hole is indeed larger than the sum of the event horizons of the two initial black holes.
The original black holes had a total surface area of 240,000km2 – each of them with 30 times the mass of the Sun, but compressed into a sphere with a surface area roughly the size of Britain.
The new one has a surface area of about 400,000km2, a significant increase that seemingly confirms Hawking’s theory.
The most challenging part of this type of analysis involves determining the surface area of the new black hole.
The size of the two black holes before the merger can be deduced relatively easily from the gravitational wave pattern generated as they orbit each other. But when the black holes merge, the signal quickly fades away, much like the ringing of a struck bell.
However, just like a bell, the timbre of the ringing of a black hole – the characteristic frequencies at which it vibrates – depends on its size, and can be calculated very precisely from Einstein’s theory of General Relativity.
The loud GW250114 signal allowed the researchers to precisely determine the timbre, which allowed them to calculate the mass and spin of the new black hole using Einstein’s theory and subsequently determine its surface area, thereby testing Hawking’s theory.
This shows that advanced observations of gravitational waves can truly reach into the deepest foundations of physics.
But, as is often the case in fundamental research, for now, this new discovery raises more questions than it answers.
Even the loudest gravitational wave signals tell us absolutely nothing about the mysterious microstructure in which black holes supposedly store gigabytes of information.
It’s as if black holes are simultaneously simple and enormously complex.
This duality is a manifestation of the tension between Einstein’s theory of General Relativity and quantum mechanics.
What are a black hole’s qubits? Are they really confined to their horizon surface, as Hawking’s formula suggests, holographically projecting whatever interior exists? And, above all, what happens to the vast amount of information when the black hole grows old?
Fresh evidence
How a black hole explosion could explain everything
While radiation from black holes born from the collapse of dying stars is cold and undetectable, Hawking also speculated that much smaller black holes could have formed shortly after the Big Bang.
Such primordial black holes, if they exist, could be far hotter and may even explode.
In fact, in February 2026, researchers at UMass Amherst, in the US, suggested that the detection of a neutrino particle with an extraordinarily high energy, by the KM3NeT Collaboration in 2023, might be a remnant of such a primordial black hole explosion.
If confirmed, future observations of neutrinos would provide a tantalising window into the quantum dynamics of nature’s most mysterious objects.
These are uncertain, speculative findings now but – if correct – would offer a glimpse of Hawking radiation and provide a route to prove the theory of the Universe as a hologram.
Black hole radiation
Entropy goes hand in hand with temperature, even for black holes. Indeed, on the capstone of Hawking’s grave in Westminster Abbey, as if it were his ticket to immortality, you can find Hawking’s formula for the temperature of a black hole.
The letter T in Hawking’s formula stands for the temperature of a black hole and M stands for its mass.
The remaining quantities are all basic constants of nature: c is the speed of light, G is Newton’s gravitational constant, ħ is Planck’s quantum constant, and k is Boltzmann’s constant for thermodynamics – the study of energy, heat and work.
The sheer beauty of Hawking’s formula is that it brings together all these constants in a single equation.
Unlike other celebrated equations of 20th-century physics, such as the Einstein or Schrödinger equations, which describe separate domains of physics, this formula exhibits the interplay of different areas.
By combining principles from quantum theory and General Relativity, Hawking had taken a mathematical risk, but he was rewarded with an insight that neither relativity nor quantum theory alone could ever have provided: black holes send out a tiny trickle of thermal radiation.
Though cold and tiny, and utterly undetectable, the mere existence of Hawking radiation escaping from black holes meant they slowly lose mass and eventually disappear.
What is the fate of the information locked inside a black hole when it evaporates? To Hawking’s delight, this became the most vexing puzzle in theoretical physics of the late-20th century, bedevilling not just one but two generations of physicists.
In many ways, the black hole information puzzle is the contemporary analogue of the Mercury anomaly in the 19th century, the wobble in Mercury’s orbit that defied Newton’s theory.
At first, it was thought that any information in black holes had to be lost forever when the hole disappears, since Hawking radiation seemed to be entirely featureless.
Black holes are the ultimate erasers, it appeared, even though quantum theory says this is impossible. “Physics is in serious trouble”, Hawking declared, “quantum theory must be revised.”
At the dawn of the 21st century, however, holography came to the rescue.
Physicists finally got a better grip on the holographic nature of black holes – albeit when envisaged in a pristine mathematical environment – and a whole new array of thoughts and thought experiments revealed that Hawking radiation isn’t as featureless as it first seemed.
It now appears to carry a vast amount of information encrypted in a complex web of subtle entangling correlations between the radiated particles.
Physics was saved. For now.
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