Some physicists actually believe that the universe we live in could be a giant hologram. This scientific profession is becoming more and more popular. And the most interesting thing is that this idea does not quite resemble modeling like the "Matrix", but rather leads to the fact that although it seems to us that we live in a three-dimensional universe, it may have only two dimensions. This is called the holographic principle.
The idea boils down to this: some distant two-dimensional surface contains all the data necessary to fully describe our world - and, like in a hologram, this data is projected into three dimensions. Like the characters on the TV screen, we live on a flat surface that only seems deep to us.
Sounds absurd. But if physicists come to the conclusion that their calculations are correct, all the big problems of physics - like the nature of black holes and the reconciliation of gravity and quantum mechanics - will be much easier to solve. In short, the laws of physics make more sense when written in two dimensions rather than three.
“This is not a crazy idea among most theoretical physicists, ” says Leonard Susskind, the Stanford physicist who first formally formed the idea decades ago. "It has become a working day-to-day tool for solving physics problems."
However, there is an important point to note. There is no direct evidence that our universe is actually a two-dimensional hologram. These calculations are not the same as mathematical proof. Rather, they are an intriguing suggestion that our universe might be a hologram. And while not all physicists are convinced that we have a good way to test the idea experimentally.
Where did the idea come from that the universe could be a hologram?
This idea originally emerged from a couple of paradoxes associated with black holes.
1. The paradox of information loss in a black hole
In 1974, Stephen Hawking discovered that black holes, contrary to popular belief, emit small amounts of radiation over time. Ultimately, when all the energy has flowed out beyond the event horizon - the outer edge of the black hole - the black hole should completely disappear.
Nevertheless, this idea led to the emergence of the problem of information loss in a black hole. For a long time it was believed that physically information cannot be destroyed: all particles take their original form, or in the event of a change, they affect other particles, so the changes can be used to restore the original state of the particles.
As an analogy, imagine a stack of documents being fed to a shredder. Even if the documents are torn into tiny particles, the information in them will still exist. It will be broken into small parts, but it will not disappear, and within a certain time the document can be reassembled. Therefore, you will be able to find out what was written in it. Basically, the same can be applied to particles.
But there is a problem: if a black hole disappears, information about every object sucked into it also seems to have disappeared.
One solution, proposed by Susskind and Dutch physicist Gerard t'Hooft in the mid-90s, was that when an object is pulled into a black hole, it leaves behind a kind of two-dimensional imprint encoded in the event horizon. Later, when radiation exits the black hole, it picks up the fingerprints of this data. Thus, the information is not actually destroyed.
Calculations have shown that enough information can be stored on the two-dimensional surface of a black hole to fully describe all possible three-dimensional objects inside.
“The analogy that we both thought of independently is something like a hologram - a two-dimensional piece of film that can be used to encode information about a three-dimensional region of space, ” says Susskind.
2. The entropy problem
There was also a related problem of calculating the amount of entropy in a black hole - that is, the amount of disorder and randomness among its particles. In the 70s, Yaakov Bekenstein calculated that its entropy is limited and its bar is proportional to the two-dimensional region of the black hole's event horizon.
"For systems of ordinary matter, entropy is proportional to the volume, not the area, " says Juan Maldacena, an Argentine physicist who participated in the study of the holographic principle. Ultimately he and others came to the conclusion that what looks like a three-dimensional object - a black hole - could be better understood in two dimensions.
How did this idea go from black holes to the whole universe?
None of this proves that black holes are holograms. But almost immediately, Susskind says, physicists recognized that treating the universe as a two-dimensional object that only appears to be three-dimensional could help solve many of the deepest problems in theoretical physics. The mathematics of the theory works equally well whether you are talking about a black hole, a planet, or the whole universe.
In 1998, Maldacena demonstrated that a hypothetical universe could be a hologram. His private hypothetical universe was the so-called anti-de-Sitter space (in simple words, a shape curved at large distances, as opposed to our flat universe).
Moreover, when looking at this universe in two dimensions, he found a way to draw on the incredibly popular idea of string theory - a broad theoretical field in which the basic building blocks of our universe are one-dimensional strings, not particles.
And more importantly, in the process, he combined two incredibly important and separate concepts of physics into one theoretical framework. “The holographic principle connected the theory of gravity with the theories of particle physics, ” says Maldacena.
Combining these two fundamental ideas into one coherent theory (often called quantum gravity) remains one of the holy grails of physics. Of course, this also does not tell us that our universe - and not hypothetical - is a hologram.
Could our universe, in principle, be a hologram - or is this idea only applicable to a hypothetical one? This remains the subject of fierce debate.
There has been a lot of theoretical work lately that has suggested that the holographic principle may work for our universe - including high profile papers by Austrian and Indian physicists that came out in May.
Like Maldacena, they also sought to apply the principle and find similarities between the disparate fields of quantum physics and the theory of gravity. In our universe, these two theories do not converge: they predict different results regarding the behavior of any individual particle.
But in the new work, physicists have calculated how these theories can predict the degree of entanglement - a strange quantum phenomenon in which the states of two tiny particles can correlate in such a way that a change in one particle affects another, even at great distances. Scientists have found that by treating one particular model of a flat universe as holograms, they can get the same results from both theories.
However, while this is a bit closer to the universe Maldacena was working on, scientists worked with only one particular type of flat space, and their calculations did not take into account time - only three spatial dimensions. Moreover, even if it could be applied directly to our universe, it would only show that it can be a hologram.
How can we prove that our universe is a hologram?
The best type of proof should start with some kind of testable prediction inferred from holographic theory. Experimental physicists could gather evidence to see if the results match predictions. For example, the Big Bang theory predicted that we could find remnants of energy emanating from the entire universe as a result of a violent expansion 13.8 billion years ago - and in the 1960s astronomers did just that, in the form of the cosmic microwave background.
There is currently no universal test that provides solid evidence for this idea. However, some physicists believe that the holographic principle predicts a limit to how much information space-time can contain, since our apparent three-dimensional space-time is encoded in a limited amount of two-dimensional information.
Craig Hogan of Fermi Laboratories uses a tool called the Holometer to capture the evidence for the above. It relies on powerful lasers to seek a fundamental limit to the amount of information present in space-time itself - at ultra-small, sub-microscopic levels. If found, it will be proof that we live in a hologram.
Other physicists, including Susskind, do not believe in this experiment and say that it will not provide any evidence for the holographic principle.
Ok, we live in a hologram. What's next?
Strictly speaking, nothing. The laws of physics by which you live your life will remain the same. Your house, dog, car, body will continue to be three-dimensional objects, as they always seemed and were. But in a deep sense, this discovery will revolutionize our existence at a fundamental level.
For our daily life, it does not matter that 13, 8 billion years ago, in a sudden and violent explosion, from a single point of matter, our Universe was formed. But the discovery of the Big Bang remains an important tool in our understanding of the history of the universe and our understanding of our place in space.
Likewise, the strange principles of quantum mechanics - entanglement in which two distant particles somehow influence each other - do not affect our daily life in any way. You don't see atoms and you don't know what they are doing at the smallest level. But these principles allow us to discover unexpected laws of nature.
The confirmation of the holographic principle will be the same. Living our lives, we may never even know about the peculiar and contradictory fact that we live in a hologram. But this discovery will be an important step towards a full understanding of the laws of physics - which govern every action you take.