Is the Universe Really A Hologram?

For the past century, physicists have struggled to reconcile two major theories. The theory of general relativity, Einstein’s macroscopic theory of gravitation and space-time, and modern quantum mechanics, the microscopic theory of fundamental particles. While each theory works accurately within its own domain, attempts to unify them have repeatedly failed. There have been few theories that show promise in this grand unification effort to connect the large and tiny and make a theory of everything. One of them is the holographic principle: The belief that our Universe is a hologram.

If you go to the Massachusetts Institute of Technology Museum today, you will see more than 2000 holograms. Shining a light through a screen encoded with information produces 3D images called holograms. The holographic principle says that the entire Universe is the three-dimensional image of a two-dimensional surface encoded with information.

The holographic principle is a principle rooted in theoretical physics and math. Understanding why scientists want to ask this seemingly ridiculous question requires us to understand some theoretical concepts such as entropy and the information paradox.

The Information Paradox

Subatomic particles do not move the same way everyday objects do. Everything we can see, such as rocks, planets, and other humans, are governed primarily by the same laws of physics, known as classical mechanics. Subatomic particles are primarily governed by different laws, called quantum mechanics.

Quantum field theory, sometimes known as conformal field theory (CFT), is the most successful theory that uses quantum mechanics to describe the interactions between subatomic particles. Quantum field theory suggests that many quantum fields span space-time, permeating equally everywhere throughout the Universe. That means quantum fields are omnipresent. To interact with other fields and the Universe, quantum fields produce particles, and create all the fundamental particles that make up everything.

The electromagnetic field (EM field), the combination of the electric and magnetic field, is a quantum field. These fields form particles when provided with energy, similar to how plucking particular guitar strings held at distinct lengths produces particular notes. Specific quantum fields vibrate at specific frequencies to produce certain specific particles. The electromagnetic field makes photons (the particles of light) when provided with the right amount of energy. Similarly, all the different quantum fields create each of the fundamental particles that make up everything we see (and some of what we don’t see), from electrons and protons to forces like magnetism.

 Einstein’s general relativity is another highly successful theory that describes gravity and the behavior of planets and other massive entities. The good news is that we don’t yet really need to connect these two theories to advance technology. There are only a few places where both theories are necessary to explain a phenomenon. One of these places is in the event horizon of a black hole.

Black holes are huge concentrations of matter packed into very tiny spaces, making them infinitely dense. This is what results in their extremely strong gravitational pull, which pulls in everything including light. There is a surface around the black hole beyond which nothing can escape. This surface, known as the event horizon, is where physics stops working as expected.

Due to the immense gravity, space-time warps and quantum fields behave unusually at a black hole’s event horizon. The quantum fields start creating particles by forcefully drawing the energy from the black hole's mass, causing the black hole to evaporate into radiation from these particles. This radiation, known as Hawking radiation, was first proposed by the famous physicist Stephen Hawking in 1947. This radiation causes a phenomenon known as the Information Paradox.

Information in this case refers to quantum information, which defines the exact state of any system. The same fundamental particles form the building blocks of all things in the Universe, which implies that the only difference between the Sun and a pencil is the arrangement and quantity of these particles, also known as quantum information.Knowing the quantum information of a system would allow for the perfect recreation of the entire system. So, if we knew the quantum information of the entire Universe, we would theoretically be able to recreate everything. One of the fundamental laws of quantum physics is that quantum information cannot be destroyed.

Since a black hole sucks in everything that goes past its event horizon, all of the information that went inside the black hole was assumed to stay there. Even though this information would not be observable it would still exist. But when Hawking radiation was discovered, physicists realized that all of this information evaporates along with the black hole and disappears. The black hole fades into radiation containing no information about the objects that go into it, which means that information has been destroyed. The law of quantum information no longer holds true, creating what’s known as the Information Paradox.

Dutch Physicist Gerard T'Hooft proposed a solution to this paradox in 1985. He suggested that all the information of the objects passing through the event horizon gets encoded onto its surface. So, the surface of the black hole, the event horizon, retains all the information of the objects entering it. The information then gets encoded onto the Hawking radiation and radiates back into the Universe, thus never getting destroyed and not creating a paradox.

Stephen Hawking initially disagreed with T’Hooft and in 1997 made a bet with another contemporary physicist John Preskill, a strong supporter of T’hooft’s idea. However, Hawking later conceded the bet in 2004. He gave Preskill an encyclopedia of baseball for winning the bet and joked that he should have instead given him the ashes of the book as a symbol that information cannot be destroyed.

T'Hooft's theory suggests that the 2D surface of a black hole can describe the information of every 3D object that enters the black hole. Essentially all the information of everything inside the volume of the black hole is described on its surface. This theory is the foundation of the holographic principle, the idea that our entire Universe could be encoded onto a 2D surface

The Entropy Conundrum

From an informational standpoint, entropy is the amount of statistical information needed to describe a system. At the beginning of a game of pool, the balls are arranged in a triangle, which is ordered and easy to describe, so, the system has low entropy. As the game progresses, the balls' positions become increasingly scattered. Now, the system is harder to describe, and the placement of the balls is random, making it a high entropy system. The Second Law of Thermodynamics states that the entropy of a system never decreases.

Jacob Beckenstein, a graduate student at Princeton, proposed the No-hair conjecture, which shows us that we shouldn't let graduate students name anything. The assumption was that black holes retain no information on the outside (so it’s as if it has “no hair”…) except for the charge mass and angular momentum (the direction and speed of a body’s rotation multiplied by the mass) of any object entering the event horizon. (So technically the black hole does have 3 “hairs”—or 3 pieces of information retained—so even if you stuck with the hair analogy, it would be more accurate to call it the “Three-hair conjecture”).

Let us say a young ballet dancer saw a black hole in the distance, and the black hole had a mass of about four suns, a positive charge, and wasn't spinning. The dancer then decides to pirouette into the black hole with an angular momentum of 40 Kg m^2 per second. The black hole now has a mass of 4 suns plus a ballet dancer, the same positive charge since the dancer has no charge, and the black hole starts to spin with the angular momentum of the ballet dancer as per the No-hair conjecture. All the other information about the dancer, such as the color of her tutu, or how upset she was that she was not allowed to pursue astrophysics instead of ballet, is no longer observable.

Since the amount of information required to describe the dancer went from a lot to just 3 things, black holes seem to go against the entropy law. To solve this, Beckenstein suggested that entropy, much like quantum information, was stored on the surface of the event horizon and then radiated back into the Universe as Hawking radiation.

Beckenstein saw the correspondence between the always-increasing nature of the surface area of the event horizon and always-increasing entropy. As a theoretical experiment, he used a simulation model to build a black hole out of elementary particles that each contained a single bit, the smallest unit of information a computer can store. He found that a black hole's information content (and thus, the entropy) is proportional to its surface area. In fact, he found that the information content was very close to the surface area divided by the number of Planck areas are obtained by multiplying together two Planck lengths, which are fundamental units in physics.

A Planck length is the minimum possible length, and no natural measurement in physics can be smaller than this. To put it into perspective, if an atom was the size of the Universe, then a single tree would approximately be a Planck length tall. Since nothing can be smaller than this length, we say that space is quantized, which is a fancy way of saying that it is made of discontinuous intervals. So, an object can be 1, 2, or any integer value of Planck lengths long, but it can't be in between. Basically, the Universe is pixelated—or made up of indivisible 3D Planck length units—in the same way that your TV screen is made up of millions of 2D pixels

Beckenstein's theoretical experiment meant that each minimum possible area holds a single bit of information. This makes it impossible to fit any more information on a surface without increasing the surface area. This maximum amount of information that a specific volume can contain is known as the Beckenstein bound and is directly proportional to the surface area enclosing the volume. This was unexpected since it seems like common sense that a cube can contain more information inside it than on its surface. But instead, all the information inside a body, no matter how much, can be encoded on its surface area, This further strengthens the holographic principle, the idea that the entire volume of our Universe can be encoded on its surface.

From a thermodynamic perspective, entropy is the amount of a system's thermal energy per unit temperature. Stephen Hawking performed thermodynamic entropy calculations showing that these equations were consistent with Beckenstein's informational ones, adding further credence to Beckenstien’s Ideas.

The Leap from Black Holes to the Entire Universe

The theoretical and mathematical evidence behind taking a leap from "3D information can be encoded on 2D surfaces" to "our entire Universe is a hologram" is through string theory, where all the fundamental particles are replaced by one-dimensional strings. Although this theory has elegant mathematics, it also has a lot of loose strings (pun intended) and has become stagnant and hard to develop.

According to physicists, the Universe could exist in three possible shapes. It could be a space of positive curvature, negative curvature, or flat curvature. This means, our Universe either looks like a ball, a piece of paper or like a Pringles chip.

Argentinian Physicist Juan Maldacena proved that the Universe could be a hologram for a particular type of Universe. Maldacena first observed a correspondence between a Conformal Field Theory (CFT) in a Flat space to a fully formed string theory in a negative curvature space (known as AdS Space). This is known as AdS/CFT Duality, with which he mathematically proved that a Universe in this negative space would be a hologram. Although our Universe is presumed to be either flat or have positive curvature, there are many efforts to realize the same conclusion here as well.

Basically, Maldacena proved that a Pringle-shaped Universe would be a hologram using a theory called AdS/CFT Correspondence. However, physicists think our Universe is either ball-shaped or flat. So, they are working to develop a new theory based on AdS/CFT that would prove that our Universe is a hologram.

AdS/CFT correspondence soon became synonymous with the holographic principle. Physicists successfully used this theory to solve problems in many areas where quantum mechanics and general relativity are combined. The holographic principle promises to be the theory that will finally join quantum mechanics with general relativity, combining the large and the tiny to form one grand unified theory of everything.

So, are we and everything around us simply a projection of interactions happening on a distant surface? If so, we may have cracked one the biggest mysteries of physics.