Why is quantum mechanics so complicated

Heisenberg's quantum mechanics

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Heisenberg's quantum mechanics

The world of quanta could also have sprung from a science fiction novel. Light behaves like a particle, matter like a wave, the boundaries between the object of investigation and the observer seem to be blurring.

Werner Heisenberg was born in Würzburg in 1901. After quickly studying physics in Munich, he soon worked with famous colleagues. As early as 1927 he became a professor at the University of Leipzig and in 1932 he received the Nobel Prize in Physics for the development of quantum mechanics, which is based on the work of Max Planck.

"Quantum" is Latin and means "a certain amount". In chemical or physical processes, energy is only given off in small packages. This property of nature is called "discrete". Half an energy package or a continuous energy flow does not occur.

What is quantum mechanics good for?

In the everyday world, physicists speak of the macrocosm (gr.macro = large), quantum phenomena are never encountered. One can describe the macrocosm without worrying about the smallest parts. But if you want to understand what the world is actually made of, then you have to penetrate into the world of the smallest things (microcosm) and examine them. And in the microcosm, the rules of the quantum game apply.

Quantum mechanics deals with atoms, their building blocks and their interaction. On such a small scale, the physical formulas that describe our everyday world are no longer correct. It is clear to us that objects are made up of tiny particles, the atoms, and light is an electromagnetic wave that propagates in space. So two completely different things.

Quantum mechanics now says that light waves can behave like particles and particles of matter like waves. Matter and light can be both: a propagated wave or a point-like particle. In the case of light particles, one speaks of photons, the matter waves are described as a wave function. This is also called the wave-particle dualism.

The beam of an atomic laser. It is about two millimeters long and consists of a huge wave of matter measured on an atomic scale.

That was in 1927 and is still today a revolutionary thought that completely changed the everyday view of the world. Mainly because waves of matter like rays of light can also overlap. See "Does Schrödinger's cat really exist?" Atomic lasers have also been built that emit a short beam of matter instead of light.

What is the "uncertainty relation"?

When Heisenberg dealt with the world of quanta, he found that you can either determine the exact location of a particle or its speed, never both together. If you know one thing very precisely, then you only roughly know the other. It's not because you just have to take a closer look: It's a fundamental limit of knowledge.

These are the so-called orbitals of the hydrogen atom. The lighter the color, the more likely it is to find an electron there. You can't say where it is exactly.

In the macrocosm, on the other hand, everything is in order: You are currently sitting at 0 km / h in front of a computer and reading this text. You can know exactly where you are and how fast. Things are different in the microcosm. This is based on the fact that matter can be particles and waves. When researchers examine very small things, the wave character becomes more and more evident. You can then no longer say that a particle is at a certain point.

That sounds very theoretical, but the uncertainty principle explains that helium is the only element that never freezes to a solid, no matter how cold it is.

Does Schrödinger's cat really exist?

No, Schrödinger's cat is just a thought experiment to illustrate the weirdness of quantum mechanics. Erwin Schrödinger was a colleague of Heisenberg. Imagine a cat locked in a dark box. Inside is a vial of poison gas. This vial is accidentally destroyed at some point and the cat dies.

As long as you don't look, according to a certain interpretation of quantum mechanics, the cat is dead and alive at the same time. It is only when you look into the box that the cat is pushed into a certain state. Observation plays an important role in this.

The aim is to make it clear that in the world of quanta several equal states can exist at the same time and side by side. And you want to show that observation and measurement have an impact on what is observed and measured.

What do quantum mechanics and quantum computers have to do with each other?

So far there is no quantum computer, but individual, important building blocks have already been built. The problem is that it is very difficult to work with "quanta". You need very low temperatures and you have to query the results quickly. Because the matter waves that a quantum computer works with evaporate very quickly.

A functioning quantum computer will revolutionize the world. A normal computer calculates step by step and has to process its commands one after the other. A quantum computer, however, processes a large number of commands at the same time. As with Schrödinger's cat, all results are available simultaneously in one memory cell. This enables him to perform math problems that today's computer takes thousands of years to solve in a matter of seconds.



What use is quantum mechanics to me?

With the help of quantum mechanics, the phenomenon of superconductivity could be explained. Current flows in an electrical conductor without resistance. This enables magnetic levitation trains such as the Transrapid or medical devices such as the magnetic resonance tomograph.

If all of this sounds very complicated, then perhaps the saying of another famous physicist, Richard Feynman, is comforting: "Anyone who thinks they have understood quantum theory has not understood it."

Quotes from Heisenberg:

"The first drink from the cup of science makes you atheistic, but God waits at the bottom of the cup."

"Few know how much you have to know in order to know how little you know."

If you want to know more ...

A WIW article about Max Planck, the father of quantum theory

All articles at WIW that deal with particle physics

You can find more information about the atomic laser here

An online experiment on wave-particle dualism

Here you will find a program with which you can carry out a famous experiment on wave-particle dualism on the computer

Have a look at WAS IST WAS Volume 79: Modern Physics

or in WAS IST WAS volume 102: Our cosmos

Text: -jj- July 28, 2005 Photo Atomlaser © and with the kind permission of the Max Planck Institute for Quantum Optics; Werner Heisenberg / Public Domain / Wikipedia; Atomic orbitals / GFDL / Wikipedia; Superconductivity / Public Domain / Wikipedia

Note: All images and links have been removed from the archive