Entering the strange quantum world

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Entering the strange quantum world

2018-09-25 10:25:35 309 ℃

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"New Principles Institute" (ID: newprincipia)

Second lesson (on): Quantum Mechanics History

An electronic representation of both waves and particles, a cat is in a state of life and death, we can not know the position and speed of a particle at the same time, which sounds with us The fact that everyday experience is completely different is actually only a small part of the singular quantum world.

○ Substances are composed of smaller particles. The most basic particles known to date include electrons and quarks, all of which follow the laws of quantum mechanics.

Quantization of energy, wave-particle duality, uncertainty principle, Schrödinger's cat, Feynman's historical summation, ghost-like super-distance, these are the labels of quantum mechanics. For those who are new to these concepts, the strange behavior of the quantum world is puzzling. However, this is the deepest way of working in the universe. (But its effects are usually not obvious unless we enter the microscopic scale, or the extremely low temperature world.)

When learning quantum mechanics, if we don’t go to the bottom, we believe in “shut up and calculate", you will find that it always solves the problem perfectly. It explains the structure of atoms, describes the interaction of elementary particles, and may even be the ultimate origin of all structures in the universe. In fact, it can be said to be the most successful scientific theory in history. In this class, I will try to lead you into this wonderful world. (Because of the length of the article, I will divide this class into two parts.)

Particle or wave?

At the end of the last lesson (see: "First Class: Principle of Minimum Action"), we have already seen the behavior of this little guy in electronics is very strange. In the experiment, the experimenter fired electrons toward a screen with double slits. In a world dominated by classical (non-quantum) mechanics, we obviously know what happens: electrons behave like bullets. The bullet will pass through one of the slits, then we will expect to see two bullets stacked side by side behind the screen:

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We also saw what happened in the real experiment: electrons, like bullets, hit a target at a time. Moreover, we will slowly see the establishment of a pattern. But if the behavior of the electrons is as good as bullets, then the established model will not be what we expected. Instead, the electronic pattern we get looks more like a wave pattern:

For waves, When the two slits are opened at the same time, the waves passing through the double slit interfere with each other, resulting in a series of light and dark stripes. Electronics seems to have done the same thing. But if each electron passes through a slit alone, what does it interfere with? Although each electron will only reach one place at a time, it seems that each electron seems to pass two slits at the same time. Recalling what we mentioned in the first lesson, electronics sniffed all possible paths. In the double seam experiment, we saw this behavior.

Atomic Structure

The behavior of an electron determines the structure of an atom. In classical physics, there is a simple (but wrong) image of a hydrogen atom in our mind:

< p>Power makes electrons turn around protons, just like gravity makes the earth turn around the sun. From this perspective, electrons follow a single path, independent of the size or eccentricity of the orbit.

However, in quantum mechanics, the answer is very different. The electron "sniffs" "many possible orbits, its position is smeared into the quantum orbit":


○ The atomic orbital of electrons at different energy levels in a hydrogen atom. The different colors in the figure indicate that the probability of finding electrons is different. | Image Source: Wikipedia

In addition, the allowed quantum orbitals are only a limited number and have specific energies. The orbits and their energy are "quantized". In this way, quantum mechanics explains the periodic table of elements, thus laying the foundation for all chemistry.

A brief history of quantum mechanics

The importance of quantum mechanics cannot be overemphasized, maybe you will I think it is too far away from us, but the fact is that semiconductors from computer chips to MRI technology in hospitals, etc., are all made possible by quantum mechanics. Before discussing the mathematics behind quantum mechanics, let's briefly review the history of quantum mechanics:

After a brief understanding of the history of quantum mechanics, we will focus on the mathematics behind quantum mechanics in the next section.