Human is an intelligent being. Human has learned how to predict events. Based on the present, one can say what will happen next. Say, we shoot a gun and we can easily guess which direction the bullet will go and where it will hit(obviously approximately). But there is a different place where predictions do not seem to work very well. It is the world of [**Quantum Mechanics**](https://en.wikipedia.org/wiki/Quantum_mechanics), i.e. the dimensions of microscopic scale.

<center>http://dtlon6z3v1kfl.cloudfront.net/wp-content/uploads/2017/08/23094116/quantum-physics-1024x677.jpg</center>

In 1900 **Max Planck** showed that energy is not a continuous being, rather a combination of discrete pouches of a certain energy value. In the early 19th century the physicist **Neils Bohr** used this concept to introduce his model of an atom and counter the model presented by **Earnst Rutherford**. **Rutherford** said that electrons in an atom move around a positively charged particle like the planets in the solar system. But [**Classical Electromagnetism**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwj_h_ny49nYAhUDpZQKHf_uAvYQFggoMAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FClassical_electromagnetism&usg=AOvVaw1qP4oj-9Q577Ld2S22naEG) says that an accelerating particle should emit energy in the form of electromagnetic radiation. Thus, **Rutherford**’s electrons should lose energy and spiral into the nucleus. But, that counters the idea of the ability of an atom. 

What **Bohr** said is that the electrons have only some certain orbits of a certain radius, not in any random orbit in which they can stay safe without spending any energy. It is only when they jump from one orbit to another, by absorbing energy on heating. When they jump from a higher orbit to a lower one, they lose energy in the form of electromagnetic radiation i.e. light of a specific color. For this reason, a heated gas shows stark bands of colors. Though **Bohr** was successful in describing the single electron atoms, he was unable to describe the model of higher atoms i.e. many-electron atoms.

<center>https://www.universetoday.com/wp-content/uploads/2014/12/420px-Bohr_atom_model_English_svg.png</center>
<center>**Bohr's Model of Atom**: [**Source**](https://www.universetoday.com/46886/bohrs-atomic-model/)</center>

**Bohr**’s model could not justify the [**Heisenberg’s Uncertainty Principle**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwiU25L65NnYAhUJjJQKHRlRAzsQFggwMAI&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FUncertainty_principle&usg=AOvVaw1nG8QzreNbZ1FafBA_qaSU) which says that you cannot measure the position and momentum of a microscopic particle simultaneously to arbitrary precision. When you measure the position, you change the momentum and it causes uncertainty in the value of momentum and vice versa.

Also, the **Double Slit Experiment** with streams of electron showed that electron can also form interference patterns which is a property of a wave. This raised the question, “**Does the electron also have wave nature?**”

To answer these questions an Austrian physicist, [**Erwin Schrodinger**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjSkOWl5dnYAhXEipQKHch-AeYQFggoMAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FErwin_Schr%25C3%25B6dinger&usg=AOvVaw1IIUtVymIA9hfdkYDOQyry) developed an equation known as the [**Schrodinger’s Equation**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwjD6Iy25dnYAhWCnZQKHW-sDy8QFgheMAw&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FSchr%25C3%25B6dinger_equation&usg=AOvVaw3OwJ3j-AUz4DX3KIZthfA4) which gave birth to the mysterious field of physics, **Quantum Mechanics**. This equation describes a particle as a three-dimensional wave. That is the particle is everywhere at the same time before we look for it. It is only at the moment you look for it, it appears at one certain location. That sounds crazy!! right? 

Let us know about some of the bizarre predictions of **Quantum Mechanics**.

### Tunneling
If you are sitting in front of a wall and have a rubber ball in your hand, try throwing the ball on the wall. We know from our daily experience that even if you try for eternity, the ball will never pierce through the wall to reach the other side. But, if we have to accept the laws of **Quantum Mechanics**, it is possible. If we keep trying, there is a finite probability that the ball would pass through the wall. The thinner the wall, more the probability of crossing the wall. Isn't that bizarre? You may think that it is a fuss, not possible. But let me tell you that this property has been used to design the scanning tunneling microscope, tunnel diode, and many other applications. This phenomenon is known as [**Quantum Tunneling**](https://en.wikipedia.org/wiki/Quantum_tunnelling). **Tunneling** is the reason behind the radioactivity of the radioactive elements like Uranium, Radium, Plutonium etc. **Tunneling** happens because, in **Quantum Physics** a matter is considered to be a wave. Some portion of this wave is extended to the other side of the barrier.

<center>http://www1.kcn.ne.jp/~h-uchii/Bohr/tunnel.jpg</center>
<center>**Tunneling Effect**: [**Source**](http://www1.kcn.ne.jp/~h-uchii/Bohr/tunnel.html)</center>

### Quantum Entanglement
[**Quantum Entanglement**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiT37mp6dnYAhXGjZQKHSagDqkQFggoMAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FQuantum_entanglement&usg=AOvVaw2JGRzxS3nl0yAW98eMLnz5) is another strange prediction of **Quantum Mechanics**. Even **Einstein** found it difficult to digest. If we bring two fundamental particles like electron or photon close enough to each other and let them interact. Now, according to **Quantum Mechanics**, they get correlated in such a way that if you make any action on of the particle, there will be an immediate reaction on the other particle no matter whatever distance the two particles are apart. This phenomenon is known as **Quantum Entanglement**. 

<center>https://resonance.is/wp-content/uploads/Particles_SpookyAction_Stars_2H.gif</center>
<center>**Quantum Entanglement**: [**Source**](https://resonance.is/new-results-testing-quantum-entanglement-spatially-separated-particles/)</center>

Let us consider spin of two particles. Now, let us bring the particles close enough and let them interact. Now, we separate them and take one to the south pole and the other one to the north pole. Now, until we measure both of particles are in both spin-up and spin-down state at the same time. But, as soon as we measure the particle in the south pole and find that its spin is up, then the spin of the particle in the north pole automatically turns out to be in spin down state. It is as if something carried the information of the measurement(faster than light) to the particle in the north pole. This is something **Einstein** had an issue with. According to Einstein's [**Special Theory of Relativity**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwj10sno6dnYAhWGX5QKHRKCAbgQFggoMAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FSpecial_relativity&usg=AOvVaw1zLa4b49iKP1d8OUTxPF1n), nothing can move faster than light. Then how could the information of the measurement reach the other particle? **Einstein** tried to disprove it. But experiments carried out at later times proved that **Entanglement** is indeed a reality.

### Debate over Quantum Mechanics
As I mentioned above, **Einstein** was not happy with the interpretation of Quantum Mechanics([**Copenhagen Interpretation**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwiq0qfC6tnYAhWBKpQKHRTnBbUQFggwMAI&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCopenhagen_interpretation&usg=AOvVaw1UPeJGqlDuEOZE7C0YsBTL)) and had a never-ending debate with one of the prominent Quantum Physicist of that time, **Neils Bohr**. The **Copenhagen Interpretation** of **Quantum Mechanics** says that reality is probabilistic. Everything in nature works on the laws of probability. Whatever happens in nature is a matter of chance like the game of dice. **Einstein** strongly opposed this interpretation of quantum mechanics. **Einstein** believed that **Quantum Mechanics** was an incomplete theory. He said, "**God Does Not Play Dice**". Then, **Bohr** replies to **Einstein**, "**Stop Telling God What to Do**". This debate went on until **Einstein**'s death in 1955. Even today, there is no clear picture of **Quantum Mechanics**. That is why the prominent physicist **Richard Feynman** said, "***If you think that you understand Quantum Mechanics, you actually don't understand Quantum Mechanics***".

**Quantum Mechanics** is the most successful thing that humans have done in the last century that has blessed the present day world. Even though the Quantum phenomena seem counter-intuitive, it has given us many revolutionary technologies. 

<center>https://www.robomart.com/image/cache/catalog/Transistor/bc547-npn-general-purpose-transistor-pack-of-5-bc-series-rm0384-by-robomart-445-500x500.jpg</center>
<center>**Transistor**: [**Source**](https://www.robomart.com/bc-557)</center>

**Transistor**, which is the fundamental building block of a computer processor was invented from the laws of **Quantum Mechanics**. the scanning tunneling microscope, diodes, electron microscope and many other technologies of today are born of **Quantum Mechanics**. **Quantum Tunneling** has been recently used for teleporting fundamental particles. **Quantum Mechanics** is the basis of the future generation computers known as [**Quantum Computing**](https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiTzoDG69nYAhXEmJQKHcFoCjgQFggoMAA&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FQuantum_computing&usg=AOvVaw2W9mwRWxb-VEN2GUnWd03Z). Though strange, **Quantum Mechanics** is really a blessing to us.

#### References
1. **NOVA: Quantum Physics** [[**Link**](https://www.youtube.com/watch?v=UVxKrcFwTtI)]
2. **Jay Shenoy: Quantum Tunneling Explained**  [[**Link**](https://www.youtube.com/watch?v=c6bn_tPDb1M)]
3. **LiveScience: Spooky Action at a Distance: Quantum Entanglement** [[**Link**](https://www.livescience.com/28550-how-quantum-entanglement-works-infographic.html)]
4. **Science News: What is Quantum Entanglement** [[**Link**](https://www.youtube.com/watch?v=6yfWdb-JOA8)]

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