Feb 26, 2020

## Quantum Mechanics is the branch of physics that studies very small objects, such as atoms, molecules and nuclei, which in turn are composed of elementary particles.

In the past, the physical world was explained according to the principles of Classical Mechanics, or Newtonian Mechanics.

However, at the end of the 19th century, it started to become insufficient to answer some questions.

Therefore, the Theories of Relativity and Quantum Mechanics were developed.

### Relativity vs. Quantum Mechanics

Relativity is the theory that describes the physics of massive, high-speed objects, while Quantum Mechanics, or Quantum Physics, studies the physics of very small objects. Many attempts have been made to unify both theories (String Theory), but this is a subject for another time.

Many of the Classical Mechanics equations, which describe how things move in sizes and speeds in our daily lives, are no longer useful on the scale of atoms and electrons, which can now be explained by the principles of Quantum Mechanics.

### 1. Particles are waves, and vice versa

On the macroscopic scale, we are used to two types of occurrences: particles and waves.

• Particles take up space, transporting mass and energy as they move.
• Waves propagate throughout space, transporting energy as they move, but without mass.

When particles collide, their trajectory is predictable and can be calculated using Newton’s laws of motion.

When waves pass through slits, they generate new waves, and when these new waves collide they can reinforce or cancel each other out.

In quantum mechanics, however, this distinction between waves and particles no longer exists.

Objects that we normally see as particles, like electrons, can behave like waves in certain situations, while objects that we normally think of as waves, like light, can behave like particles.

In this way, electrons can create wave diffraction patterns as they pass through narrow slits, just as waves emerge in a lake when we throw a rock into the water. On the other hand, the photoelectric effect (that is, the absorption of light by electrons in solid objects) can only be explained if light behaves like a particle.

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Such ideas led De Broglie to conclude that all entities had wave and particle aspects, and that different aspects were manifested according to the type of process they were submitted to.

This became known as the wave-particle duality principle.

### 2. All we can know are probabilities

When physicists use quantum mechanics to predict the results of an experiment, the only thing they can predict is the probability of detecting one of the possible results.

For example, if we do an experiment where an electron will stop at place A or B, in the end, we can say that there is a 17% probability of finding it at point A and an 83% probability of finding it at point B.

However, we can never say with certainty that the electron will end up in A or B.

No matter how careful the preparation of each electron is, we cannot definitively know what the result of the experiment will be.

Each electron is a completely new experiment, and the end result is random.

### 3. Measurement determines reality

Until the exact state of a quantum particle is measured, that state is undetermined.

Only after the measurement is made will the state of the particle be determined, and all subsequent measurements on that particle will produce exactly the same result.

This is the problem that inspired the experiment of Erwin Schrödinger, the cat in the box that can be dead and alive at the same time.

The double-slit experiment confirms this indeterminacy. Until the position of the electron is measured on the opposite side of the slit, it can exist in all possible ways.

A quantum particle will occupy several states until the moment it is measured, and after its measurement, it will only exist in one state.

### 4. Quantum correlations are not local

One of the strangest and most important consequences of this understanding is the idea of “quantum entanglement“.

When two quantum particles interact, their states will depend on each other, regardless of how far apart they are.

You can “hold” one particle in the United States and send the other to China, and then measure them simultaneously. The measurement results in the United States will determine the results in China and vice versa.

The correlation between these states cannot be described by any local theory, in which particles have defined states.

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This has been confirmed experimentally dozens of times over the past thirty years, and each new experiment has reinforced this theory.

Although the measurement in China determines the state of a particle in the United States, the result of each measurement will be completely random.

There is no way to manipulate the Chinese particle to produce a specific result in the United States. It’s random.

The correlation between the measurements will only be evident after the action, when the two results are compared, and this process must occur at speeds slower than the speed of light.

### 5. Quantum physics is real

Although quantum mechanics has many concepts that defy our classical intuition, such as indeterminate states, probabilistic measures and non-local effects, it is still subject to rules.

As strange as its predictions may be, quantum mechanics does not contradict the fundamental principles of physics.

You cannot exploit quantum effects to build a ship that travels at the speed of light, or invent telepathy.

Quantum mechanics is a rigorous and precise mathematical science, and every effect you hear about it is real and confirmed by experience.

### And where can I find quantum physics in my daily life?

Quantum physics is all around us and determines everything about the world we live in.

The red glow in a heated metal and the color of the light from a neon lamp are due to the quantum nature of light and atoms.

The Sun itself is powered by quantum physics! If it weren’t for the quantum effect known as “tunneling“, the Sun would not be able to fuse hydrogen into helium, producing the light that makes life on Earth possible.

Additionally, modern computers that we have are built on silicon chips, which contain millions of small transistors. Without understanding the quantum physics of how atoms and electrons act, it would be impossible to build a single transistor, let alone millions of them.

Modern telecommunications networks, such as the Internet, also depend on Quantum Mechanics. In these networks the information is transmitted through pulses of light that travel through fiber optic cables.

These pulses of light are produced by diode lasers, which use small semiconductor chips to generate intense beams of light.

The construction of such lasers would be impossible without understanding the quantum physics of semiconductors and the quantum nature of light.

In other words, much of the technology we have today exists thanks to Quantum Mechanics.