Beijing time on December 2, according tomedia reports, face the reality, quantum mechanics is really confusing. All the laws of physics we know are broken in the realm of quantum, and physicists are still trying to reconcile the two different worlds of quantum and macro.
Imagine putting a particle in a box, which, according to classical physics (and common sense), should stay in the box forever. But in quantum mechanics, this particle may jump out of the box the next time you look at it. In classical thinking, we can accurately measure the momentum and position of any object, but not in the quantum world, where the more you know about one object, the less you know about another. Is an object a wave or a particle? According to the classical point of view, you can choose only one, but if it is quantum mechanics, both are possible.
The quantum world is hard to understand, but in a way, subatomic rules give way to macro rules. What’s going on? We are not sure, it is a long and strange journey to find the answer to this question.
The first person to label the quantum world useful was physicist Niels Bohr. At the beginning of the 20th century, scientists around the world began to realize the strange and unexpected behavior of atoms and subatomic systems. After decades of hard work, they found that certain properties, such as energy, appear in the form of energy-level discrete packets known as “quantums”. Physicists began to try to explain these experiments on a mathematical basis, but no one could develop a complete, consistent framework. Bohr was the first to try to do so. Although he did not provide a complete theory of quantum mechanics, he did lay some important foundations, and he put forward some theories that later became the cornerstone of modern quantum theory.
The first theory came from his early attempts to build atomic models. In the 1920s, physicists had learned through various experiments that atoms were made up of a heavy, dense, positively charged nucleus surrounded by a group of tiny, light, negatively charged electrons. We also know that atoms can only absorb or emit radiation from certain energys.
But what does an atom look like?
Bohr puts electrons in an “orbit” around the nucleus, dancing to a waltz around a dense nucleus, like a planet in the solar system. In the real solar system, planets can exist in different orbits, but in Pol’s atoms electrons are stuck in very narrow orbits — they have only a pre-defined orbital distance.
By jumping electrons from one orbit to another, atoms can receive or emit radiation from a particular energy, and their quantum nature is thus encoded.
The Connection between Quantum Mechanics and Classical Physics
On this basis, Bohr added an interesting twist. There are many possible ways to build quantum models of atoms, so why would he do that? Bohr found that when electrons were in orbit far from the nucleus, their quantum properties disappeared, and classical electromagnetism perfectly described atoms — two charged particles.
This is known as correspondence principle and is the basis for Bohr’s support for his own atomic model. You can propose any quantum theory you want, but the correct theory is that there are certain limitations that give way to classical physics, which in Bohr’s atomic model means that electrons must be far from the nucleus.
Bohr’s atomic model was incomplete and was later replaced by the valence electron layer model, but Bohr’s correspondence was still in play and became the cornerstone of all later quantum theories – it was a beacon that allowed physicists to construct and select the right mathematical methods to describe the subatomic world.
Bohr didn’t stop there. He argues that while this correspondence allows for a link between the quantum world and the classical world, the two worlds are not the same.
While Bohr was perplexed by all this, his good friend Werner Heisenberg proposed his impending rise to fame. When you try to measure the position of a tiny particle, you lose its momentum information, and instead, when you try to determine its momentum, you know nothing about its position.
Bohr adopted the idea and put it into practice. He sees Heisenberg’s indessure principle as part of a larger dimension of the quantum world, where everything is paired. Think of the most famous pair in the quantum world: waves and particles. In a classical system, an object is either a pure wave or a pure particle, and you can sort out certain behaviors by choosing one, but in quantum mechanics these two properties exist in pairs: all objects are both particles and waves, and always exhibit some of the properties of both.
In addition, in essence, quantum rules depend on probability — on average, quantum mechanics can reproduce only classical physics. Based on these two points of view, Bohr argues that quantum theory can never explain classical physics, in other words, atoms and other particles run under one set of rules, and trains and people run under another set of rules. They can and must be linked through the principle of correspondence, but in addition, they lead parallel lives of their own.
Is Bohr right? Some physicists believe that we basically live in a quantum world, but we don’t fully understand the principles, and that we can reproduce classical physics from pure quantum rules, while others believe that Bohr has solved this problem and we don’t need to discuss it. Of course, most people are just focusing on mathematical models, and they don’t pay much attention to them. However, this is still a question worth thinking about. (Any day)