Are we living in a giant 'cosmic hologram'?

In science fiction movies, we often see virtual objects projected from the air, and this futuristic technology has long been quietly used in our daily lives: anti-counterfeiting patterns on credit cards, virtual performances of deceased stars "recreated" in concerts, and even three-dimensional product models that appear in shopping mall displays, all rely on holographic technology.

In 1947, Hungarian physicist Denis Gerber was racking his brains to improve the resolution of electron microscopes. He wanted to find a way to capture the tiny details of objects more clearly than traditional microscopes. In an experiment, Gerber accidentally discovered a new way to record images - capturing the phase information of light waves through interference phenomena, and reproducing three-dimensional images when reading through diffraction phenomena. This unintentional discovery is "holography". Simply put, traditional photography only records the intensity of light, that is, light and dark information, while holograms record the phase information of light at the same time. The phase can be understood as the "shape" or "position" of the light wave. This allows holograms to not only reproduce the brightness of objects, but also show the spatial depth and structure of objects. It is precisely because it can record such complex information that we can present realistic three-dimensional effects through two-dimensional surfaces, just like taking a three-dimensional snapshot of light waves.

The process of making holograms relies on the phenomenon of interference. We can imagine that two calm water surfaces are suddenly dropped with a stone each. At this time, two waves will be generated, and when these two sets of waves meet, a series of phenomena will be generated, which is called "interference". The recorded interference pattern contains key light field information such as phase. In holography, when a laser beam is divided into two beams - one beam directly shines on the holographic plate, and the other beam shines on the object and reflects to the holographic plate, the interaction between the two produces an interference pattern. These interference fringes record the light wave information of every point on the surface of the object. It is in this way that holograms can capture and "save" the three-dimensional information of objects. When we want to view a hologram, we only need to illuminate it with a suitable light source, so that the phase and amplitude information of the original light wave can be "decoded" to reconstruct the three-dimensional image of the object.

Schematic diagram of the hologram production principle

The emergence of holograms provides us with a new way to understand three-dimensional objects on a two-dimensional surface, and also provides us with a revolutionary perspective on the road to understanding the nature of the universe - the holographic universe. The holographic universe theory puts forward an amazing conjecture: perhaps the three-dimensional universe we live in is just a projection of two-dimensional information. The physical information of the entire universe, including the matter, energy and space we can perceive, may come from some higher-dimensional surface information.

The holographic universe theory was not proposed out of thin air, but was built on decades of physics research on black holes. Black holes are one of the most mysterious and extreme celestial bodies in the universe. They are formed by the gravitational collapse of massive stars at the end of their lives. Due to the strong gravitational field of black holes, even light cannot escape from them, so we cannot directly observe what is happening inside black holes. In the 1970s, physicists discovered a puzzling phenomenon in their research on black holes. According to our usual understanding, the size or internal complexity of an object is usually related to its volume. For example, the larger a box is, the more information it can hold. However, the entropy of a black hole is proportional to its boundary surface area, not to its volume.

entropy

Entropy is a core concept in thermodynamics, first proposed by German physicist Rudolf Clausius in the 19th century. Entropy is essentially a measure of the degree of disorder in a system, reflecting the "amount of information" or "probability" contained in a system. The higher the entropy of a system, the greater its degree of disorder and the more information it contains.

With further scientific research, Stephen Hawking, a well-known physicist, discovered in 1974 that black holes emit thermal radiation (i.e., "Hawking radiation"), indicating that black holes are not eternal closed systems, but actually gradually "evaporate" through radiation, and this radiation itself is also determined by the information on the boundary of the black hole. This strange property of black holes provides inspiration for the holographic universe theory: since all the information in a black hole can be "stored" on its surface, then, can the information of the entire universe also be stored on a "surface" in some way? This is one of the core ideas of the holographic universe theory: the three-dimensional universe we live in may just be a projection of information on a two-dimensional surface.

If we can extend this information storage method of black holes to the entire universe, it may mean that all physical phenomena in the universe - including matter, energy and space itself - are manifestations of some higher-dimensional two-dimensional surface information. This idea breaks our intuitive understanding of three-dimensional space and proposes a completely new concept of space and information. This provides a preliminary scientific basis for the theory of holographic universe, and the holographic principle came into being.

In addition to changing our understanding of the world, the development of the holographic universe theory is also trying to solve a more fundamental physics problem: how to combine the two major physical theories of quantum mechanics and general relativity to establish a self-consistent theoretical framework. Before explaining this problem, let us quickly understand the core ideas of these two major physical theories.

General relativity

This theory, proposed by Albert Einstein in 1915, revolutionized our understanding of gravity. Traditionally, people thought of gravity as a force between objects, but Einstein's theory tells us that gravity is actually the curvature of space and time. Matter causes the spacetime around it to deform, and this is why we feel gravity. For example, the reason why the earth revolves around the sun is not because the sun is "pulling" the earth, but because the huge mass of the sun causes the space around it to "bend" and the earth moves along this curved path.

Quantum Mechanics

Quantum mechanics is a set of theories developed in the early 20th century that has revolutionized our understanding of the microscopic world. Traditional physics holds that the position and motion of objects can be described precisely, while quantum mechanics shows that the behavior of objects at the microscopic scale no longer follows classical deterministic laws, but is probabilistic.

This raises the question: when matter enters a black hole, where does its original information go? The problem lies in the ultimate fate of the black hole. According to Hawking radiation, a black hole does not exist forever, and eventually it will slowly evaporate through this quantum effect, releasing radiation until the black hole disappears completely. However, Hawking radiation is thermal radiation, which does not carry specific information about the matter inside the black hole, which means that when the black hole completely evaporates, all the information that entered the black hole will no longer exist. This conflicts with the "information conservation" principle of quantum mechanics: if no trace of the original information is left at the end of the black hole evaporation, then this information is completely lost, which violates quantum mechanics.

Therefore, the black hole information paradox is:

General relativity allows information to be hidden behind the event horizon. When a black hole eventually evaporates completely through Hawking radiation, all the matter and its information that originally entered the black hole seem to be lost along with the black hole. This situation is reasonable within the framework of general relativity because it does not require the preservation of information.

One of the basic principles of quantum mechanics is the conservation of information. The evolution of any physical system should be reversible, and even if a system evolves, its original information can still be preserved in some form. Information cannot disappear out of thin air. Therefore, when a black hole evaporates, all material information seems to disappear, which conflicts with the basic principles of quantum mechanics.

According to the holographic principle, information does not really disappear, but is encoded on the event horizon of the black hole. When the black hole evaporates through Hawking radiation, the matter and information that were originally "swallowed" do not disappear with the black hole, but are somehow preserved on this two-dimensional surface and may be decoded through some mechanism of quantum mechanics. This solves the conflict between the information conservation principle of quantum mechanics and general relativity in the process of black hole evaporation.

With the rapid development of physics, the study of the holographic universe has brought us more and more surprises. The ancient philosophical question of "where do we come from and where are we going" may be answered one day. Perhaps everything that exists in three-dimensional space - whether it is the mobile phone we are reading this article on, the earth under our feet, or a distant galaxy, can actually be represented on a two-dimensional plane in some way. It's as if we are just living in a huge "cosmic holographic projection", and the real information exists in some invisible high-dimensional space.

Planning and production

Author: Cai Wenchui, a postgraduate student at Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences

Reviewer: Liu Xi, researcher at Beijing Planetarium