Is the universe a hologram? The honest short answer is that nobody knows, and no experiment has shown it. What physicists have shown is that, in certain idealised model universes, a theory with gravity in many dimensions can be exactly equivalent to a simpler theory without gravity in fewer dimensions. In December 2013 a team in Japan used computer simulations to check that equivalence in a hard case, and the results matched. Headlines turned this into "clearest evidence yet that the universe is a hologram", which went much further than the work itself.
This article explains what the holographic principle means, where it came from, what the 2013 simulations actually did, why it matters for joining Einstein's gravity with quantum physics, and which popular claims about it are wrong.
This artist's impression shows the surroundings of the supermassive black hole at the centre of the active galaxy NGC 3783, in the southern constellation of Centaurus. It was widely used to illustrate the 2013 hologram story, but it is unrelated to the research: the Japanese calculations dealt with a theoretical black hole in a ten-dimensional model, not with any real object in the sky.
What the holographic principle says
An ordinary hologram is a flat sheet of film that stores enough information to recreate a three-dimensional image. The holographic principle borrows that picture. It proposes that everything happening inside a region of space can be fully described by information written on the boundary of that region, a surface with one dimension fewer.
The idea grew out of black-hole physics. In the 1970s Jacob Bekenstein argued that a black hole has entropy, a measure of hidden information, and that it is proportional to the area of its event horizon rather than its volume. Stephen Hawking then showed that black holes emit radiation, which fixed the entropy at one quarter of the horizon area measured in tiny Planck units. If the most information you can pack into a region depends on its surface, then in a deep sense the surface may be where the information lives.
Gerard 't Hooft turned this into a general principle in 1993, and Leonard Susskind gave it a precise form within string theory in 1995. Neither claimed that we live inside a projection like a cinema image. The claim is about bookkeeping: the amount of information a region can hold, and how it can be described.
Maldacena's 1997 breakthrough
In late 1997 the physicist Juan Maldacena found a concrete example. He showed that a version of string theory with gravity, living in a curved space called anti-de Sitter space, appears to be completely equivalent to a quantum theory without gravity living on that space's boundary. This "AdS/CFT correspondence" became one of the most cited results in modern physics, with more than 10,000 citations by 2015.
Its power is that it lets physicists translate a hard problem on one side into an easier one on the other. A question about a black hole, where gravity is overwhelming, can be turned into a question about hot particles interacting on the boundary. The catch is that the correspondence has never been proved mathematically, and checking it directly is very hard, because the interesting cases are exactly the ones where the equations become impossible to solve by hand.
What the 2013 simulations did
Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues posted two papers on the arXiv preprint server in late 2013, and the journal Nature reported on them in a news article on 10 December 2013. The work was not a discovery of black holes, real or in parallel universes, and it was not published as a research paper in Nature, as some early reports said.
In the first paper, the team calculated properties of a black hole in a ten-dimensional universe described by string theory, including its internal energy, the position of its event horizon and its entropy, while also taking account of quantum effects from virtual particles. In the second, they used computer simulations to calculate the internal energy of a corresponding one-dimensional universe with no gravity at all, whose particles behave like coupled springs. The two numbers matched.
That match is exactly what Maldacena's correspondence predicts, and it was tested in a regime where no exact pencil-and-paper check exists. Maldacena, who was not involved, told Nature that it seemed to be a correct computation. Susskind said the team had "numerically confirmed, perhaps for the first time, something we were fairly sure had to be true". It was a careful confirmation of a mathematical duality, which is valuable, but it was not news about our own universe.
Why relativity and quantum theory clash
The reason the holographic idea excites physicists is the long-standing gap between our two best theories. Einstein's general relativity describes gravity as the bending of space and time, and it is extremely accurate for planets, stars and galaxies. Quantum mechanics describes particles and forces at tiny scales, and it is just as well tested: it underpins lasers, transistors and modern chemistry.
A popular claim is that quantum physics is too abstract to be tested; that is not correct. The real problem is that the two theories are built in incompatible ways. Relativity treats space and time as flexible and dynamic, while standard quantum theory assumes a fixed background. When physicists try to treat gravity as just another quantum force, the mathematics produces infinities that cannot be removed. Both theories are needed at once only in extreme places, such as the centre of a black hole or the first instants after the Big Bang, which are far beyond the reach of any experiment.
Holography offers a possible bridge. If a gravitational universe can be rewritten exactly as a quantum system without gravity, then quantum gravity might be understood through the better-understood boundary theory. String theory and loop quantum gravity remain the best-known candidate frameworks, and neither has yet produced a confirmed experimental prediction.
Common misunderstandings
- "Scientists proved the universe is a hologram." They did not. The 2013 models had ten and one dimensions, and the ten-dimensional space had a different shape from ours. Our universe appears to be expanding at an accelerating rate, which is not the anti-de Sitter geometry where the correspondence is best understood.
- "Hologram means it is all an illusion." In physics the word describes how information can be encoded, not whether things are real. Tables, people and stars would be no less real if the deepest description of them turned out to be holographic.
- "We are watching a projection, like a cinema screen." There is no projector and no screen in the theory. The boundary description and the interior description would be two equivalent ways of writing the same physics.
- "Experiments have already detected it." Searches for direct signs have so far found nothing. The Fermilab physicist Craig Hogan predicted a kind of holographic "noise" in space, but analyses of a distant gamma-ray burst published in 2011 found no such effect at the scale he proposed.
Why it still matters
Even without proof that our own cosmos is holographic, the idea has changed how physicists think. It has given new tools for studying black holes, the information they seem to swallow, and even materials on Earth, because the same mathematics can describe some strongly interacting systems of particles. The 2013 simulations were one more sign that the duality works where it is supposed to, and that numerical methods can test it in places where theory alone gets stuck.
If you see a headline that announces the universe is a hologram, the useful questions are simple: which model universe was studied, was it a calculation or an observation, and did the researchers themselves claim anything about our world? In the 2013 case, the answers are an idealised string-theory model, a calculation, and no.
Sources: Scientific American / Nature News: Simulations back up theory that Universe is a hologram (10 December 2013); Wikipedia: Holographic principle; Wikipedia: Quantum gravity; original report: RT News. Image: artist's impression of NGC 3783 (AFP).
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