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In the past few decades, with many of us spending a lot of time online, the real world seems to have become wider. However, in one field of theoretical physics, the situation seems to be developing in the opposite direction. In the past twenty years (although this article was written in 2009, it is still worth reading), string theorists have been discussing the view that the time and space we live in, including ourselves, may be just an illusion.Holograms produced by "some reality". And this reality lacks a key feature of the world we perceive: the third dimension. Professor Juan Maldacena of Princeton Institute of Advanced Studies has played a decisive role in the development of the idea of holographic principle. In the 1990s, Marda Sina proposed the first implementation.Cosmological model of holographic principle. Recently, he was interviewed by the author during his visit to Cambridge.
The holographic principle originated from one of the biggest scientific problems in the 20th century:There is incompatibility between general relativity and quantum mechanics..
At the beginning of the twentieth century, Einstein discovered thatTime and space are inseparable.He called the structure formed by the two space-time. His general theory of relativity points out that space-time itself willDistorted by a massive objectAnd gravity is the result of this distortion. It’s like putting a billiard ball on a trampoline, which will create a depression and make the nearby marbles roll into it; Similarly, massive objects (such as planets) will bend space-time and attract nearby objects by their gravity. According to Einstein, gravity is not something that propagates through space, but is caused by the geometric structure of space-time itself.
general theory of relativityThe main description isPlanets and galaxiesThe world, and quantum mechanics mainly focuses on the subatomic scale, that is, the basic particles that make up matter. At this scale, the mass is so small that gravity can be ignored. Quantum field theory is a quantum mechanical description of particle physics, which holds that elementary particles transmit force through messenger particles called gauge bosons:One elementary particle transmits force to another particle by sending some gauge bosons..
Figure 1. Huge objects distort space-time.
In the development of the 20th century, electromagnetic force, weak nuclear force and strong nuclear force are the four basic forces.Messenger particleAre all in the experimentBe observedYes. In order to maintain theoretical consistency, Einstein’s theory should also be able to be rewritten with similar messenger particles. Physicists willGravitational messengerParticles are calledgion(graviton), but we haven’t found its trace yet. Even more frustrating, tryingUsing quantum field theory to describe graviton meetinglead toinsignificance/meaninglessThe answer. Marda Sinar said: "Simply quantizing gravity is not feasible, which will lead to mathematical inconsistency. We need something new. "
So far, a much-anticipated new quantum gravity theory has not been discovered. A strong competitor isString theory, or "string theory family"-because string theory is actually a set of logically self-consistent.A collection of theories. The core point of string theory is to put the basicParticles are regarded as tiny vibrating strings.This provides a way for us to bypass the mathematical problems in simple quantum gravity. As a mathematical theory that has attracted many theorists with its beauty, the disadvantage of string theory is that it has not been given.A complete description of the worldMany physical quantities cannot be described yet. In addition, string theory has not been verified by experiments, and it can’t be verified in fact. However, it is string theory that provides Marda Sinar with a clue to solve the mystery of quantum gravity: regard gravity as an illusion produced by quantum holograms.
Figure 2. Quantum mechanics describes the subatomic world well.
The contradiction between general relativity and quantum mechanics will not bother most practical applications. What physicists usually study isLarge-scale worldHere it is.The quantum effect is not obviousOr a small-scale world, where particles are lighter and gravity has less influence. However, in a special case, the conflict between the two theories becomes particularly obvious: when a large amount of mass is concentrated in a small area of space, a black hole will be formed.. The gravity produced by a black hole is so strong that even light cannot escape.Therefore, when studying black holes, we can’t ignore the influence of gravity. At the same time,The small scale of black holes also means the existence of quantum effects.. Therefore, in order to explain the phenomena in black holes, we really need a unified theory of quantum gravity.
Black holes are the original holographic principle.Theoretical sources. They have a boundary that cannot be returned, which is called the event horizon. Once you cross this boundary, you will be sucked into the black hole and never escape. When you fall into a black hole, a lot of information will disappear with you. This information includes not only your DNA and one or two best ideas, but also countless combinations of blood cells in your veins and all the confused thoughts in your mind. However, in the world of black holes, things look much simpler. Classical physics assumes that nothing can escape from a black hole, and thinks that a black hole can be used only.Three messagesTo fully describe:Its quality、electric chargeandspin velocity. Therefore, when you fall into a black hole, all the information needed to describe you will be sucked into these three parameters of the black hole-your fall makes the universe a little simpler.
Figure 3. A simulated black hole with the background of the Milky Way.
This reduction in complexity usually worries physicists because it violates one of the most basic laws of physics:second law of thermodynamics. The second law of thermodynamics shows that things will never get simpler. The total amount of information needed to describe a system is measured by a physical quantity called entropy. In classical physics, entropy is defined for thermodynamic systems, such as melting ice under the sun. Classical entropy measures the degree of heat (or energy) dissipation in a system.
However, energy is related to excited atoms (in ice, water molecules are arranged in a fixed lattice in order, while in liquid water, they move around), so entropy is also a standard to measure the disorder of the system. The disorder degree of the system is related to information: the periodic arrangement of water molecules in ice crystals can be described in one sentence, but for liquid water, you need to provide the exact position of each molecule, which involves a lot of information. Therefore, entropy is related to both thermodynamics and information.
The second law of thermodynamics shows that,Entropy will never decrease.. Under thermodynamic conditions, this means that the system will strive to reach the equilibrium state of complete energy dissipation. In the context of information, this means that things will not automatically become simpler. From the classical point of view, a black hole is not a hot object, and it is very simple to describe, so there should be no entropy. When you fall into a black hole, your positive entropy will become the zero entropy of the black hole, whichViolation of the second law of thermodynamics.
When this potential problem is noticed, some physicists can only accept the fact that the second law of thermodynamics may not be as basic as we thought. However, one physicist, Jacob Bekenstein, was unwilling to give up easily. In 1972, Beckenstein discovered the relationship between entropy and the properties of black holes discovered by Stephen Hawking. Hawking has been thinking about the horizon of a black hole. The event horizon wraps a certain space volume like an egg shell-it is a surface, and you can measure its area. Hawking has proved that,The horizon will never shrink.No matter what you do or put into a black hole, the area of the horizon will only increase, just like entropy.
Figure 4. The second law of thermodynamics paves the way for the holographic principle.
This analogy with thermodynamics was initially thought to be purely coincidental, but Beckenstein put forward a controversial explanation: "Beckenstein thinks you can putThe area of event horizon is regarded as a kind of entropy., "Marda Sina explained. "The idea was not clear at first, but when Hawking discovered that black holes couldquantity of radiant energy(now known as Hawking radiation), this idea became more clear. In other words, black holes are hot objects, so they must have entropy. "By combining Beckenstein’s original idea with Hawking radiation, we can calculate that the entropy of a black hole is indeed equal to the area of a black hole measured in a certain length scale-Planck unit. Because Planck unit is very small, soEntropy of black holeIs a considerable value. Although the black hole occupies the three-dimensional volume of space, its information content seems to be a feature of its two-dimensional event horizon.
You may think that black holes are very strange things, but physicists Gairal Us’ t Hofter and Leonard Susskind went one step further, and they considered those in ordinary space areas.information content. Whether it is the pages of books, neurons in the brain or photons transmitted through optical fibers on the Internet, information exists in physical form. This physical form involves energy. Since energy is equal to mass (recall Einstein’s E = MC), compressing information into a limited area of space is equivalent to compressing mass into it. If you try to put too muchQuality/information is compressed into it.You will get a black hole, so the information content in the limited space of non-black hole is limited. Hofter and Susskind calculated this limit, and found that it is measured in the same way as a black hole, which is measured by the surface area of the regional boundary.
Marda Sina said: "This may sound very simple and naive, but in all our other descriptions of the world, variables increase with the increase of volume. For example, if we want to describe the electromagnetic field in a space, we will divide the volume into many parts and describe the electromagnetic field of each part. " If you double the size of the area, you should also double the number of partitions, so the information required for your description should also be doubled. According to this intuitive idea, information should increase with the increase of volume, not with the increase of area as described by holographic principle. If the holographic principle is correct, then the three-dimensional method of our physics is wrong. We should be able to use a simpler version of physics, one that depends on area rather than volume. This leads to a puzzling question, that is, whether the third dimension really exists, or whether it is just an illusion, just like a three-dimensional image created in a hologram.
So far, no one has found an accurate formula to describe the two-dimensional version of physics in our three-dimensional world. However, in 1995, Susskind redefined string theory centering on holographic principle. Then in 1997, Juan Marda Sinar, who was only 29 years old, put forward a concrete description of the first holographic universe in history.
Marda Sina’s universe is not completely in line with the universe we actually live in: it is amodelA "toy" universe has its own complete set of physical rules. Because all the physical phenomena that occur in it can be defined by only.Physical theory on the boundaryTo describe, so this toy universe is aholograph. More importantly, in this universe, the problem between gravity and quantum mechanics has been completely solved: the theory defined on the boundary is pure quantum, which does not contain gravity, but the creatures living in it can still feel gravity. In this universe,Gravity is only part of the hologram illusion..
In order to understand the toy world of Marda Sina, we need to understand the world of map making first. In order to show the sphere on a flat piece of paper, we need to cut the sphere and flatten it, which will inevitably introduce some distortion. In the traditional Mercator projection of the earth, this distortion is the most serious near the poles. When you look at the map, Greenland looks as big as Africa, but in fact it is more than 14 times smaller. Besides, if you project the shortest path from London to Sydney on the map, you will get a curve instead of a straight line. On a map like this,A straight line does not correspond to the shortest path..
Figure 5. Mercator projection of the earth. The red disks actually have the same area, but their different sizes on the map illustrate the distortion of the map.
MC escher’s famous woodcut "Circle Limit III" shows a two-dimensional map of Marda’s Sina universe. Similar to Mercator projection, there are some distortions here. In escher’s map, the shortest route between two points is not a straight line connecting them, but an arc intersecting with the boundary circle of a disk at right angles.
Fig. 6. The Circle Limit III by MC escher. The shortest path is shown in white.
If you use this new metric to measure the size of fish, you will find that, contrary to their appearance, they don’t get smaller and smaller as they approach the boundary circle, but they are actually equal in size. Just as travelers walking on the surface of the earth will not realize the distortion shown by Mercator projection, people living in this so-called hyperboloid world will never notice any distortion in the size of fish. More importantly, in order to reach the boundary circle, the hyperbola must pass through an infinite number of copies of fish of the same size. In other words, it will have to span an infinite distance. For hyperbola, the boundary circle is infinitely far away.
Unlike a hyperboloid map, a "real" hyperboloid can hardly be drawn because it is severely distorted. Hyperbolic surfaces have what mathematicians call negative curvature. The small area of this "plane" looks like a saddle: along one direction, they look like the peak of a ridge; In the other direction, they look like the bottom of the valley.
For external observers like us, this strange two-dimensional world has an interesting feature: although according to new metrics, theirThe scope is infinite.But we can see their boundaries-this is exactly what we need to apply the holographic principle, which describes the interior of the space area according to the boundaries. In Marda Siner’s model of the universe, he used the three-dimensional simulation of hyperboloid plus the fourth dimension of time to form a model called anti-sitter space, which was named after the Dutch physicist William de Sitter.
Anti-de Sitter space is completely different from the world we actually live in.Where time and space are distorted in strange ways, but this does not prevent us from creating a set of physical rules for it. What you need is something.basic concept, such as elementary particles and forces, and the mathematical laws describing their interaction. Marda Sinar used a version of string theory to describe the physical phenomena in his model universe. As you remember, string theory includes quantum mechanics and gravity, so creatures living in Marda Sinar’s model universe will feel gravity in a way similar to ours.
Figure 7. A holographic universe
Marda Sinar’s key discovery is that the string theory describing the interior of the universe leaves a "shadow" on the boundary of the universe: you can define a quantum field theory on the boundary, so that every elementary particle in the interior has a corresponding particle on the boundary, and every interaction between elementary particles in the interior exactly corresponds to the interaction between boundary particles. Now you can completely describe it with the theory of boundary, for example, the action of throwing an apple inside. This means that you can even completely ignore the inner world without losing any information-this world is a real hologram.
From the point of view of quantum gravity, the key point is that the boundary theory is a quantum theory of particle physics that we are very familiar with, which is very similar to the theory used to describe the process of subatomic particles in nature. They only involve small scales, so they don’t include gravity. However, the quantum theory on this boundary can completely describe the mysterious quantum gravity theory in the internal world. This is the first time that we can completely describe a quantum spacetime.
So far, Marda Sina’s modelIt’s just a model. We don’t know whether the universe we live in is a hologram, and we still don’t have a consistent quantized description of gravity applicable to our world. The assumption of negative curvature in Marda’s Sinar model is very important, but our universe shows a slight positive curvature in the observation results. Marda Sina said: "We don’t know if there is a similar description in the case of positive curvature. People are exploring various ideas, but we don’t have a complete answer yet. "
But if it turns out that the holographic principle really applies to the world we live in, what will it mean? Does this mean that we and time and space are just an illusion? "Yes, you can say that we are an illusion or an emerging phenomenon," said Marda Sina. If we live in such a universe, then we are actually an approximate description in a sense. But this is not a new concept in physics. Take the lake as an example. It looks like a well-defined surface on which insects can walk. But if you look at it with a powerful enough microscope, you will find that there are molecules moving around and there is no clearly defined surface. For time and space, the situation may be similar. It is not clearly defined in absolute sense, but we are too big to realize it. Like insects on the lake, our eyes for observing the world are too rough to reveal the true nature of time and space. Ignorance is bliss. Although it is endless fun to discuss the philosophical aspects of things, from the perspective of daily practice, weWhether you live in a hologram or not may not matter.. "
However, for Marda Sina himself, does he really believe that the holographic principle is true? He responded: "Well, I regard this idea as a model, but it is a model that gives a complete mathematical description of quantum spacetime. Therefore, we should take it seriously until someone refutes it or comes up with a better idea. "
Author: Marianne Freiberger
Translation: K.Collider (
Revising: Xiao Cong
Original link: The illusory Universe
The translated content only represents the author’s point of view.
Does not represent the position of Institute of Physics, Chinese Academy of Sciences.
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