Currently, we rely mainly on two theories to describe our universe: the general theory of relativity and the Standard Model of particle physics. General relativity describes gravity in terms of curved space-time, while the Standard Model uses quantum fields to describe the other three fundamental interactions, where the excitation of the field can be treated as a particle. The two theories are somewhat similar in that they both employ the concept of a field – the space-time curvature field of general relativity and the quantum field of the Standard Model.
However, the two theories are fundamentally different. General relativity is a classical theory that predicts that systems will evolve deterministically over time. In contrast, the Standard Model is based on quantum theory, and at its core is the principle of superposition and probabilistic explanations. Thus, these two theories describe two very different universes at the macro and micro levels.
Although general relativity provides an excellent macroscopic description of our universe, we must now consider quantum physical phenomena. This is not a problem, and we can describe the perturbation of the curvature of space-time as a quantum field and interpret its fluctuations as gravitons, the quantum carriers of gravity, an approach that provides a framework for the quantum behavior of gravity. But when we get to a very small scale, quantum gravity no longer works on this fundamental scale, and gravity seems incompatible with the quantum world.
Physicists are trying to reconcile general relativity and quantum theory to understand the behavior of gravity at these very small scales. This requires us to delve into the abstract realm of theoretical physics in search of a promising new framework that can unify the two theories.
In the 70s of the 20th century, physicists began to take a keen interest in a theory called hypergravity. Our space-time mathematically exhibits four fundamental symmetries of translation, rotation, reflection, and temporal symmetry, which are embodied in the general theory of relativity. However, theoretical physicists have proposed a new hypothetical symmetry – supersymmetry. When supersymmetry is taken into account, we get the theory of supergravity.
Supergravity is very similar to the theory of relativity, space-time can bend to produce interesting structures, and even singularities can be created without the formation of black holes. In addition, physicists have explored the possibility of supergravity in a high-dimensional universe with additional dimensions. This has led to the generalized study of black holes with additional spatial dimensions that make up extended objects called "membranes" that can possess mass and charge.
Although the theory of supergravity is supersymmetrical in some ways, it still collapses when we explore scales close to Planck's length. In the 1980s, physicists came up with a revolutionary new theory that postulated that all elementary particles in the universe are actually made up of tiny vibrational bands of energy, known as "strings". Because it also makes use of the concept of supersymmetry, we call it superstring theory.
In superstring theory, strings exhibit their diversity by interacting, merging, or splitting. Vibrations similar to guitar strings produce different notes, and on a macroscopic scale, the vibrational patterns of these strings behave as particles as we know them. The most exciting discovery is that a vibrational mode in superstring theory is exactly the same as the behavior of gravitons, and for the first time there is a theory that allows us to describe quantum gravity in a fundamental way.
Superstring theory is promising, but it imposes some limitations on the universe: space-time is not four dimensions, but ten, and there are six additional dimensions of space that we have not yet detected. This theory further refines the types of strings that can be studied, including open and closed strings, the latter of which is capable of curling itself to form a ring-like structure. Based on these premises, the researchers found that superstring theory actually allows only five different theoretical models to exist.
If we want to describe our universe in terms of superstrings, and these five models are the only options, which version of these five models really describes our universe? Let's put this issue aside for a moment and return to supergravity. One might think that superstring theory and supergravity are two completely separate models, but in reality they are closely related on a large scale, and both describe a supersymmetric universe with gravitational pull. In fact, when applied to a universe with 10 dimensions, supergravity turns out to be an approximation of superstring theory, which also means that supergravitational membranes also exist in superstring worlds.
Another striking feature of the theory of supergravity is that it can accommodate up to 11 dimensions, one more dimension than the theory of superstrings. In this 11-dimensional model of supergravity, all the constants of the universe are mathematically determined. Unlike the five possible options of superstring theory, supergravity proposes a new independent theory that can describe the world in at least 11 dimensions.
So by the early 1990s, we had six possible mathematical models to describe the universe, five of which came from the 10-dimensional superstring theory mentioned above, and the remaining one from 11-dimensional supergravity. During this period, some researchers, notably Edward Witten, began to reveal the existence of a subtle and complex network of duality between these theories.
These dualities not only link the individual models, but also allow for the conversion between theories, making computations that are difficult to handle in one model can be converted into simplified versions in another. These discoveries of duality have greatly enriched our understanding of the universe and enabled the study of previously incomprehensible physical phenomena. A large number of studies have also led to the discovery of the AdS/CFT correspondence, according to which certain universes can be described as holograms of them.
In 1995, Witten proposed an exciting idea: there was a unified basic theory, which he called M-theory, which could be a deeper formulation of the six models mentioned above. The complexity of M-theory, which predicts an 11-dimensional supersymmetric universe containing membranes, remains a great challenge in physics today, as are other theories that try to unify gravity. While Theory M is not yet directly supported by experimental evidence, it provides a framework that allows the universe to have a variety of different space-time and geometries of space. Physicists are still exploring which configuration best describes the universe we live in, an exploration that may reveal the universe's most fundamental secrets.
Source: Vientiane Experience
编辑:ArtistET
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