[Appendix] Quantum Mechanics for General Knowledge (1)

Quantum Mechanics for General Knowledge

Quantum mechanics is one of the greatest laws that physicists have discovered. Quantum mechanics has enabled us to build every IT device today. But still, quantum mechanics couldn't answer the basic questions that Albert Einstein raised in the 1920s and 1930s. Question involving probability and measurement, the act of observation.

^{Bohr and Einstein}

Bohr accepted that the nature of the real world was inherently uncertain and thought that measurment and the act of observation changed everything. Bohr's idea was:

• Before you measure or observe a particle, its characteristics is uncertain.
• The act of measurement specifies characteristics of a particle, eliminating uncertainties.
• When you measure a particle, the act of measurement forces the particle to relinquish all of the possible places it could have been and select one definite location where you find it
• The act of measurement is what forces the particle to make the choice.
  
  

Einstein believed that certainty always determines the nature of the real world, not the observation. Einstein's idea was:

• The idea that the reality of the universe is determined by what we see is absurd.
• I like to think the moon is there even when I'm not looking at it
• All the detailed features of particles like their location should be able to be described even when you are not observing.
• He thought quantum mechanics was giving up on the job of the physicist
• Quantum mechanics is not a wrong theory, but it is an incomplete theory.

And in 1935, Einstein thought he'd finally found evidence that quantum mechanics was an incomplete theory. It was a quantum entanglement. Quantum entanglement is a theoretical prediction derived from the equation of quantum mechanics. When two particles are at a close distance and interact with each other, the two particles can be entangled like the left picture above. However, the two entangled particles can still be entangled, no matter how far away they are. Like the picture on the right.

Before measurement, it is in a uncertain state)	After measurement, it is in a certain state

To understand how bizarre quantum entanglement is, you can consider a property of electrons called spin for example. The spin of electrons is not known at all until they are observed like other quantum properties. It can be only described by probability. when you measure it, you will find whether the electron is spinning clockwise or counterclockwise.

The two electrons are entangled	Even if you send one electron to the moon.

According to quantum mechanics, the two entangled electrons can remain entangled, even if you leave one of them on Earth and you send one to the moon. Since the two are entangled, if you confirm the spin of an electron on the earth by measurement, the spin of the electron sent to the moon is also confirmed. For example, if the electron on the earth is measured to spin clockwise, it is determined that the electron on the moon is spinning counterclockwise. In other words, no matter how far away, without wires or transmitters connecting them, the act of measurement on one particle affects the other particle instantly.

For Einstein that kind of weird long range connection between particles was so ludicrous, he called it "spooky action at a distance" And Einstein was convinced that the mathematical equation was wrong, not the real world.

1. Putting a pair of gloves one by one in each box	2. One box is for you
3. One box for a penguin	4. And you discover the left hand glove

Einstein agreed that particles could be entangled with each other, but thought that they could be explained briefly without any mysterious remote action. For example, imagine putting a pair of gloves one by one in each box , delivering one to you, and the other to Antarctica. When you open the box and find the left hand glove, you immediately know which side hand glove is in the box in Antarctica. It is a right hand glove. Which hand glove is determined not by your observation, but by someone who first put them in the box. Einstein thought quantum entanglement was something like this.

To sum up Bohr and Einstein's position on quantum entanglement,

• Bohr said that even if the particles are far apart, the probabilistic outcome of the measurment immediately affects the other,
• Einstein saids that there is no such spooky connection, and everything is determined before we observe it.

It was not easy to judge who was right. There was no way to verify whether the physical quantities are already determined before the observation (\ * Einstein's view), or whether the observation determines the physical quantities (\ * Bohr's view). Thus, the problem of quantum entanglement was regarded as philosophy rather than science.

But in 1967, the situation changed. Columbia University graduate student John Clauser found a paper by a little known Irish physicist named John Bell. Amazingly, John Bell's paper seemed to have found a solution to Bohr and Einstein's argument. John Bell proved that if someone could build a machine that could generate and compare large amounts of entangled particles, he could determine who was right. John Bell's paper had turned the philosophical problem of quantum mechanics into an experimental problem. Claude experimented with quantum entanglement by constructing the machine, and soon the French physicist Alain Aspect developed more sophisticated experiments and tested with the heart of the debate between Bohr and Einstein. The result of the experiment meant Bohr's victory.

Now scientists are using this quantum entanglement to study teleportation. The following illustration shows the experiment of Anton Zeilinger in the Canary Islands in Africa.

1. Generating a pair of photons entangled in La Palma Island	2. Leaving one photon in La Palma island,
	Sending one photon to Tenerife Island
3. Bringing the third photon to be transmitted,	4. This makes the photon in Tenerife island set in the
and having it interact with the photon on La Palma	same quantum state as the third photon in the distance.

Anton used this technique to teleport tens of thousands of particles. In the future, scientists will be able to teleport a lot more particles. Humans are also made up of particles, so if the quantum status of all the particles that make up the body can be scanned, the teleportation that you see in science fiction will become reality.

Bit	Qubit

And another study that quantum mechanics is applied currently is quantum computers. Existing computers use a language of 0 and 1 called bit. 0s and 1s are arranged sequentially to process the information. Quantum computers also use a language of 0 and 1. However, if the bit of the existing computer is set to 0 or 1, the bit of the quantum computer is not settled like the spin of the electron. The quantum bit, or qubit, is an uncertainly probabilistic 0 and 1. Qubit, which can be 0 or 1, allows multiple tasks. Let's take a look at this in comparison with the task of getting out of the maze.

Computer of bit	Computer of qubit

When existing computers go out of the maze, they find the way through trial and error. If they come to a dead end, they go back to another road. On the other hand, a quantum computer can go through all the intersections at the same time. Because in the quantum world, particles can be anywhere. Quantum computers can be understood in the same way. Therefore, a quantum computer can perform operations that are currently impossible with only a few hundred electrons, and its size of brain can be smaller than a grain of sand.

* For example, to verify the binary digits of 1100011010 that is encrypted and unknown, existing computer starts to compare it with from * 0000000000 *, to * 0000000001 *, * 0000000010 *, * 0000000010 *, * 0000000011 *... until computer finds it. , But the quantum computer can verify all the numbers of binary digits at the same time because the qubit of each digit can be 0 or 1.

The main story of the documentary ends like this.

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The host of the documentary, Brian Greene, is a physicist who led the popularization of physics, writing books such as 'The Elegant Universe', 'The Fabric of The Cosmos', and 'The Hidden Reality'. It's quite thick, but if you're interested in physics, I'd also recommend books. And I'd like to recommend books by a physicist named Michio Kaku. They're quite interesting. Michio Kaku's books uses more imagination than Brian Green's books, dealing with little more than science fiction.

I hope your knowledge of physics has increase by this post.