<h1 class="pgc-h-arrow-right" data-track="1" > general relativity </h1>
Let's start with Newton's first law of motion, which states that an object is in a uniform, linear motion at rest or never stopping, unless subjected to an unbalanced external force. One day in 1907, while working at the Swiss Patent Office, Einstein conceived of Newton's idea of gravitation and imagined himself falling off the roof of a nearby building.
I sat in an armchair at the Bern Patent Office and it occurred to me: "If a man is in free fall, he does not feel his own weight." I was taken aback. I was very impressed by this simple idea. It propelled me towards the "gravitational theory" and thus onto the path to general relativity.
This led him to recognize the fact that even if an object is not subjected to any external force in free fall, it will still accelerate toward the ground. An object that is not subjected to force should move forward at a constant speed. Objects that are accelerated by gravity do not feel any force. Einstein realized that if he solved this paradox, he might explain the origin of gravity. Einstein called it "the happiest thought of my life!" He then envisions a universe in which gravity is not a force, but a geometric property of curved space-time, twisted around massive objects. As the famous theoretical physicist John Wheeler (who also coined the words "black hole" and "wormhole") describes Einstein's general theory of relativity in just 12 words.
Space-time tells matter how to move, and matter tells space-time how to bend.
Einstein spent years studying mathematics and published his general theory of relativity in 1915, which is considered to be the most mathematically and experimentally consistent theory of gravity. The laws of general relativity primarily govern the large-scale structure of the universe.
At the beginning of the 20th century, Einstein worked as a patent office clerk.
< h1 class="pgc-h-arrow-right" data-track="6" > quantum mechanics </h1>
The theory that governs the atomic and subatomic worlds is quantum mechanics, pioneered by Max Planck and further developed by many famous physicists such as Niels Bohr, de Bergley, E-Schrödinger, W-Heisenberg, Wolfgang Pauli, and so on.
What is the difference between classical mechanics and quantum mechanics?
Classical mechanics deals with the macroscopic world, while quantum mechanics deals with elementary particles and their interactions. Classical mechanics is based on Newton's laws of motion, and one of the approaches to quantum mechanics is based on Schrödinger's one-dimensional wave equations. Classical mechanics deals with deterministic problems, while quantum mechanics deals with probabilistic problems. Quantum mechanics is based on an understanding of the wave-particle duality of light. The theoretical physicist De Broglie made this proposition in 1924. De Broglie's wave of matter hypothesis holds that any particle of matter with linear momentum is also a wave. The wavelength of a wave of matter associated with a particle is inversely proportional to the magnitude of the particle's linear momentum. Another concept that underpins quantum mechanics is the uncertainty principle, also known as the Heisenberg uncertainty principle, proposed by the German physicist Werner Heisenberg in 1927, which states that the position and velocity of objects cannot be accurately measured at the same time, even in theory.
The Schrödinger equation, another major aspect of quantum mechanics, was proposed by physicist Erwin-Schrödinger in 1925 and published in 1926, forming the basis for his Nobel Prize in Physics in 1933. This equation plays the role of Newton's law and conservation of energy in classical mechanics, which predicts the future behavior of dynamic systems. As far as the wave function is concerned, it is a wave equation that analyzes and accurately predicts the probabilities of an event.
The pioneers of quantum mechanics (from top to bottom) Nils-Bohr, Albert Einstein, Max Planck and (below) Wolfgang Pauli, Werner Heisenberg, Erwin-Schrödinger
<h1 class="pgc-h-arrow-right" data-track="12" > the Standard Model of particle physics </h1>
The Standard Model of particle physics describes most of the particles in the universe and their interactions, as well as three of nature's four fundamental forces (electromagnetic, weak, and strong interactions, ignoring gravity). The force carried by the gluon binds the nucleus together and stabilizes it. The weak force carried by the W and Z bosons causes nuclear reactions that power stars such as the Sun for billions of years. The fourth fundamental force is gravity, which the Standard Model does not adequately explain. Our best understanding of how these particles and the three forces relate to each other is "compiled" in the Standard Model of particle physics. Pioneered in the early 1970s, the model successfully explained almost all experimental results and accurately predicted a wide variety of phenomena. Over time and with the progress of many experiments, the Standard Model has been established as a well-tested theory of physics. Gravity is a very, very weak force (10^40 times weaker than the electromagnetic force).
Standard Model of Particle Physics
<h1 class="pgc-h-arrow-right" Data-track="15" what's the problem >? </h1>
This inconsistency requires a theory of quantum gravity to explain all forces. In a way similar to quantum mechanics predicting the existence of photons, it infers the existence of a massless particle called a "graviton." Gravitons are particles that mediate gravity.
A graviton is a very special kind of particle that must exist according to the prediction of quantum gravity. But the problem is that since gravity is an extremely weak force, it is impossible to create gravitons using any particle physics experiment today.
However, if we depict the physical world/dimension in a completely different way, there is a small possibility that we may find such a particle very quickly one day. If the universe had tiny dimensions beyond 3-dimensional space, it would be possible to find gravitons.
When it comes to quantum gravity, much progress has been made so far, but there are two theories that stand out in terms of mathematical consistency, and they seem promising in unifying general relativity and quantum mechanics.
Superstring theory
Cyclic quantum gravity theory (cyclic quantum gravity)
<h1 class="pgc-h-arrow-right" data-track="22" > supersymmetric string theory </h1>
The theory proposes the existence of a one-dimensional object called a string that can form a closed ring, and then the motion of this ring occurs in 10-dimensional space. The diameter of a ring is the order of magnitude of planck length.
In this theory, the oscillation of a ring can produce a graviton with no mass charge. But since there is no experimental evidence of supersymmetry or strings, we can conclude that string theory may prove to be nothing more than a mathematical guess.
The origins of the concept of replacing point-like particles with vibrating strings can be traced back to the 1960s. String theory predicts a particle with a mass of zero and a spin of 2. Two mathematical physicists, John Schwarz and Joel Scherk, discovered in 1974 that this particle is the graviton, the quantum carrier of gravity. This creates an extremely astonishing way out for reconciling gravity and quantum mechanics. Theoretical physicist Leonard Mlodinow praised Schwartz and Scheke's "astonishing discovery" that gravity is part of string theory that can "reconcile the contradictions between general relativity and quantum mechanics."
A variety of vibrating strings
Superstring theory, considered by many of the great intellectuals of modern science, including the 1990 Fields Medal winner and mathematical physicist Edward Witten, to be the most promising theory of everything. However, the theory has its own limitations. Superstring theory is based on supersymmetry, and so far no such supersymmetric particles have been found. So, basically, the lack of experimental evidence is a serious limitation on the theory, although it is theoretically accurate, extremely promising, and mathematically very consistent.
<h1 class="pgc-h-arrow-right" data-track="28" > ring quantum gravity (LQG).</h1>
LQG states that space-time is clearly quantified at a scale comparable to the Planck scale and has a recessive particle structure.
In four-dimensional space-time, these rings cause spin foam. It has successfully reformulated quantum gravity into 2 rings. But how matter will be added to the theory, and how it will be extended from the Planck scale to the low-energy scale, and then to the continuum level, is unclear. If we use these nodes and gridlines to depict a three-dimensional spin network. It looks like this:
The origins of ring quantum gravity theory can be traced back to 1986, when Indian theoretical physicist Abhay Ashtekar proposed a quantum formula for Einstein's field equations of general relativity. In 1988, physicists Lee Smolin and Carlo Rovelli expanded this line of thinking and showed in 1990 that under this approach, gravity is quantified.
The three successes of ring quantum gravity theory are:
It quantifies the 3-dimensional spatial geometry of general relativity.
It allows calculations of black hole entropy.
The way we can understand ring quantum gravity is that it incorporates general relativity, which is basically just a quantum geometric version of general relativity. It makes space-time discrete and does not require a carrier of force or any particle (graviton) to describe gravity.
So far, none of these theories have been universally accepted and confirmed experimentally. Thus, the term "quantum gravity" is actually an unsolved problem, not a theory.
Because it's an unsolved mystery, it's one of the most fascinating topics in physics. Finding a suitable "theory of quantum gravity" has become the most critical study in modern physics.