In this quantum physics introduction for beginners, we will explain quantum physics, also called quantum mechanics, in simple terms. Quantum physics is possibly the most fascinating part of physics that exists. It is the amazing physics that becomes relevant for small particles, where the so-called classical physics is no longer valid. Where classical mechanics describes the movement of sufficiently big particles, and everything is deterministic, we can only determine probabilities for the movement of very small particles, and we call the corresponding theory quantum mechanics.

You may have heard Einsteins saying, “Der Alte würfelt nicht” which roughly means “God does not roll dice”. Well, even geniuses can be wrong. Again, quantum mechanics is not deterministic, but we can in general only determine probabilities. Since we are used to reasonably big objects in our everyday life, quantum mechanics and its laws may initially seem strange and quantum theory is often considered complex. But for example, electrons and photons are sufficiently small that quantum physics is needed, and on this website, we will show you that understanding the basics of quantum physics is easy and fun.

In the following paragraph, we will describe a thought experiment that we perform at two different length scales: With bullets as known from pistols (the large scale) and with electrons (the very small scale). While the experiment is essentially the same but for the size, we will show you how the result is very different. This will be your first lecture in quantum mechanics.

## Classical Bullets vs. Electrons in a Two-Slit Experiment

### a) Classical bullets

Consider first a machine gun that fires bullets to a wall. Between the wall and the machine gun, another wall has two parallel slits that are big enough to easily allow a bullet to pass through them. To make the experiment interesting, we take a “bad” machine gun that has a lot of spread. This means it sometimes shoots through the first slit and sometimes through the second, and sometimes it hits the intermediate wall.

If we block the second slit, all bullets that reach the outer wall will have come through the first slit. If we count the number of bullets as a function of the distance from the center of the outer wall, we will find a curve distribution that could be similar to a Gaussian curve. We can call this probability curve P1.

If we block the first slit, all bullets that reach the outer wall will have come through the second slit. The probability curve will be mirrored around the center, and we call it P2.

If we open both slits, all bullets at the outer wall will have come through either slit 1 or 2. Typical for classical mechanics in this situation is that the total probability distribution P can be determined as the sum of the previously-mentioned probability distributions, P = P1 + P2.

### b) Electrons – Quantum Mechanics

Now consider the same experiment on a much smaller scale. Instead of bullets from a machine gun we consider electrons that for example can stem from a heated wire parallel to the two slits in an intermediate wall. The electron direction will have a natural spread. The slits are also much smaller than before but much broader than a single electron.

#### The electron experiment results

Consider again the case that the second slit is blocked. For proper sizes of the slits and distance between the wire and the walls, the probability distribution P1 will be similar to before. Similarly, if we block the slit 1, we will for proper distances find a probability distribution P2 similar to before.

What do you expect will happen if we do not block any slit? Will we find a probability distribution P = P1 + P2 as before? Well, after all we said you may guess that this is not the case. Indeed, we will instead find a probability distribution that has various minima and maxima. That is, for x = 0 there would be the strongest peak of electrons, for a certain +-Delta x there wouldn’t be any electrons at all, but for +-2 Delta x there would be another peak of electrons, and so on.

#### Explanation of the electron experiment results

How can we explain these results? Well, the explanation is rather straight forward if we assume that electrons in this specific case do not behave as particles, but as waves. “Waves?” you may ask. Well, consider a plain of water, and the same wall as before and the same intermediate wall with a double slit as before. At the place where the machine gun or the wire where, consider a pencil punching periodically downwards into the water. If you do this, you will get concentric waves around the point where you punch the water, until the intermediate plain with the two slits.

Behind each slit, there will be a half circle of concentric waves, up to the point where the new waves from the two slits cross each other. There, the waves from the two slits can add up or eliminate each other. As a function of the periodic punching you will find points where the height of the wave is always the same. There will be other places where the wave is sometimes very high and sometimes very low. At the outer wall, these two phases will be repeatedly following one another. The places where there is a lot of variation correspond to the places where there are the most electrons. The places with no variation correspond to the places where there are no electrons on the wall at all.

So, why do electrons in this case behave like waves and not like particles? Well, this is the thing where you will not find a satisfying answer. You just need to accept it.

### c) Photons (light particles)

What if you do not believe this? Well, the thought experiment with the electrons is rather difficult to perform with the proper scale of all elements of the experiment. But there is another very similar experiment that you can do at home. Instead of the electrons you use the photons (light particles) from a laser which you can buy for a few bucks. You let the laser shine through a double slit, darken the room, and look at the outer wall. And boom! What you see is not just two light lines on the outer wall, but a pattern of light line, dark line, light line, dark line, and so on. The intensity of the lighter region becomes less far away from the center. It corresponds exactly to the result of our thought experiments with electrons.

Why does the laser experiment give the same result as the thought experiment with electrons? It is quite easy: Light particles, called photons, are also very small and therefore behave quantum mechanically. And like electrons, they behave like waves in this specific situation. As a side remark, research has shown that light behaves like particles in another respect: If one reduces the intensity a lot, one will find single light spots from single photons on the wall. This means the light behaves like particles as well. One therefore talks about the**particle-wave duality**of photons or electrons.

What do you wait for? Do the experiment, and you will become a believer of quantum mechanics, or more generally phrased, of quantum physics.

## Advanced Remarks

### Don’t watch!

The pattern with maxima and minima is called an interference pattern, since it comes about by the interference of the waves through slit 1 and slit 2. It has been found that you only get this interference pattern if you do not by other means (some additional measurement instrument) watch through which of the two slits the electrons or photons pass. If you do measure which of the two ways the particles pass by any other means, the interference pattern goes away. You will then find the sum distribution P = P1 + P2 as in the classical experiment.

### Uncertainty principle

A measurement device for electrons would typically disturb the electrons. More precisely, their momentum p would typically change due to a measurement device, while the place x of its path would become known more precisely. In general, there will be some uncertainty left in the momentum and in the place of the electron. Heisenberg postulated that the product of these uncertainties can never be lower than a specific constant h: Delta x times Delta p >= h. No one ever managed to disproof this relation, which is at the heart of quantum mechanics. Essentially it says, we cannot measure both momentum and place with arbitrary precision at the same time.

### Single Slit Experiments

We said that for proper distributions, you will find a similar result P1 and P2 as in the classical case. However, for other sizes one can achieve an interference pattern even for the single slits. This is the case when the slit is so broad that one can achieve an interference of the wave stemming from one side of the slit with the wave stemming from the other side of the slit.

### How Small Is Small?

We said above that quantum physics becomes relevant for small particles — whereby we mean that*naturally*, quantum effects are only seen for small particles. However,the theory itself is thought to provide correct results for large particles as well. Why is it then, that quantum effects (which cannot be explained with classical theory) become increasingly difficult to observe for larger particles? Larger compound particles in general experience more interaction both within themselves and with their surroundings. These interactions typically lead to an effect physicists call “decoherence” — which simply put means that quantum effects get lost. In this case (for sufficiently large matter), quantum physics and classical physics yield the same result.

Now you may wonder: “At which size does this happen?”.While one doesn’t naturally observe quantum effects in large particles, ingenious people have managed to specifically prepare test environments that showed quantum effects for an ever-growing size of particles. Already 1999 an experiment showed a quantum superposition in particles as large as C_{60}molecules. A2013articlealready claims to observe quantum superpositions in molecules that weighmore than 10000 atomic mass units. The question of where the achievable limit lies, and whether one can be sure that experiments really demonstrate quantum behavior, is still of interest. That these questions are not finally concluded is also reflected in a more recentarticle on the American Physical Society site. In principle, if one would be able to somehow get rid of decoherence effects in specifically prepared systems, the theory itself imposes no upper size limits on where quantum effects could be shown.

### Quantum Effects To a Satellite And Back

The aspect of the length scale for quantum physics that we just discussed was the particle size – which typically is on the microscopic scale. A completely different matter is the length scale of how far you can move or separate such particles afteran initial interaction, without losing quantum effects. You can view the two-slit experiment as showingan interaction between particles at the slit. If you tried out the experiment yourself, you probably realized, that the distance between the slit and the wall were you observe interference patterns can easily be some meters – not microscopic at all!

Other experiments prepare two particles in a special quantum superposition called entanglement — which, by the way, lies at the heart ofquantum computation— and then separate these particles. In someexperiments, it was possible to show interactions between these particles despite a separation over many miles. Essentially, if one measures the state of one such particle, one can thereafter predict the state of the other particle (within errors), despite the large separation between the particles. Arecent experimentdemonstrated this entanglement effect over extreme distances. Particles were sent to a satellite and back to earth – a fairly large scale distance compared to the size of a human.

## Summary of this Quantum Physics Introduction

In this quantum physics introduction, we told you that both photons and electrons behave as both particles and waves. This particle-wave duality is not understandable with classical mechanics. It results in us only being able to predict probabilities, while one classically can make deterministic predictions. You can easily test these results at home by performing the two-slits experiment with a laser pointer. Have fun! We hope you enjoyed this quantum physics introduction for beginners. If you haven’t read it yet, you should continue with our articleWhat Everyone Should Know About Quantum Physics. And if you want to learn even more, why not have a look at our articleBest Quantum Physics Books for Beginners?

## FAQs

### What is quantum physics short answer? ›

Quantum physics is **the study of matter and energy at the most fundamental level**. It aims to uncover the properties and behaviors of the very building blocks of nature. While many quantum experiments examine very small objects, such as electrons and photons, quantum phenomena are all around us, acting on every scale.

**How do you explain quantum physics to a child? ›**

Quantum physics is **the study of things that are very, very small**. This branch of science investigates the behavior of matter and the activities happening inside of atoms in order to make sense of the smallest things in nature.

**Is quantum physics for Dummies a good book? ›**

**It is a great supplement to the many textbooks on Quantum Physics**. I've bought many other good reference books on Quantum Physics, all of them that included extensive math, but I actually got through this entire book, and understood what was covered. For that alone, I have to give this book five stars.

**What is a quantum state for dummies? ›**

In quantum physics, a quantum state is **a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system**. Knowledge of the quantum state together with the rules for the system's evolution in time exhausts all that can be predicted about the system's behavior.

**What is the actual meaning of quantum? ›**

In physics, a quantum (plural quanta) is **the minimum amount of any physical particle (physical property) that has entropy**. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantization".

**What are the basic laws of quantum physics? ›**

**Six Things Everyone Should Know About Quantum Physics**

- Everything Is Made Of Waves; Also, Particles.
- Quantum Physics Is Discrete.
- Quantum Physics Is Probabilistic.
- Quantum Physics Is Non-Local.
- Quantum Physics Is (Mostly) Very Small.
- Quantum Physics Is Not Magic.

**What type of math is used in quantum physics? ›**

The mathematical prerequisites are **multi-variable calculus (as in Calculus IV), and Linear Algebra**. This course is open to both undergraduate and graduate students. It can be taken independently and in addition to any of the Physics department courses on quantum mechanics.

**What does quantum mechanics mean to you explain in your own words with examples? ›**

quantum mechanics, **science dealing with the behaviour of matter and light on the atomic and subatomic scale**. It attempts to describe and account for the properties of molecules and atoms and their constituents—electrons, protons, neutrons, and other more esoteric particles such as quarks and gluons.

**What is the biggest problem in quantum physics? ›**

The biggest challenge with quantum gravity, from a scientific point of view, is that **we cannot do the experiments required**. For example, a particle accelerator based on present technology would have to be larger than our whole galaxy in order to directly test the effects.

**What is quantum energy in simple terms? ›**

Quantum refers to **a particular packet of substance or energy in chemistry and physics**. The energy is not transferred continuously but as discrete packets of energy. It corresponds to the minimal amount of energy needed for a transition.

### Why is quantum physics so hard? ›

Quantum mechanics is deemed the hardest part of physics. Systems with quantum behavior don't follow the rules that we are used to, they are hard to see and hard to “feel”, can have controversial features, exist in several different states at the same time - and even change depending on whether they are observed or not.

**Is there a lot of math in quantum physics? ›**

**A lot of complex mathematics underlies the principles of quantum mechanics**. Physics students are introduced to this field largely through a mathematical lens, which is great — they need that perspective.

**What are the 4 quantum states? ›**

In atoms, there are a total of four quantum numbers: **the principal quantum number (n), the orbital angular momentum quantum number (l), the magnetic quantum number (m _{l}), and the electron spin quantum number (m_{s})**.

**What are the four types of quantum? ›**

There are four quantum numbers, namely, **principal, azimuthal, magnetic and spin quantum numbers**.

**Is the universe a quantum state? ›**

**The quantum state of the universe is determined by the specification of the class of metrics and matter field configurations that are summed over in the path integral**. The only natural choice of this class seems to be compact euclidean (i.e. positive definite) metrics and matter fields that are regular on them.

**What is another name for quantum? ›**

**Synonyms of quantum**

- amount.
- quantity.
- volume.
- measure.
- portion.
- degree.
- coefficient.
- number.

**How does quantum physics affect our lives? ›**

As they govern the behaviour of atoms, the effects of quantum physics underpin everything from **the ability of plants to turn sunlight into chemical energy to the behaviour of semiconductors in microchips**. Their influence is, however, usually subtle and hard to see directly.

**What did Einstein say about quantum? ›**

**Einstein saw Quantum Theory as a means to describe Nature on an atomic level, but he doubted that it upheld "a useful basis for the whole of physics."** He thought that describing reality required firm predictions followed by direct observations.

**What are the two golden rules of quantum mechanics? ›**

The Two Golden Rules of Quantum Mechanics will focus on the quantum concepts of **superposition and measurement uncertainty**. These concepts are absolutely essential for students to understand when considering future technologies based on quantum physics.

**What are the 4 principles of physics? ›**

The four fundamental forces are **gravity, electromagnetism, weak nuclear force, and strong nuclear force**.

### What are the three pillars of quantum mechanics? ›

The framework of quantum mechanics rests on three pillars: **the Hilbert space of quantum states; the Hermitian operators, also called observables; and the unitary evolution operators**.

**What is the first law of quantum physics? ›**

An analogous quantum "first law" would be: **Every arrow-like body continues in its state of rest, or of uniform spinning motion, unless it is compelled to change that state by forces impressed upon it**.

**What should I learn before quantum physics? ›**

To be a working quantum physicist, you will need a working knowledge of all of **calculus; PDE's(partial differential equations) and ODE's(ordinary differential equations); and linear algebra**.

**Can anyone learn quantum physics? ›**

**Anyone can learn quantum mechanics, but only with the proper motivation**. The extent of this knowledge then depends on the mathematical background.

**How long does it take to learn quantum physics? ›**

If you seek to become a quantum physicist, you have to complete **four years of undergraduate training**. You also can complete an additional two years of schooling to earn a master's degree followed by five years of doctoral degree training.

**What are some examples of quantum mechanics in everyday life? ›**

Many modern electronic devices are designed using quantum mechanics. Examples include **lasers, electron microscopes, magnetic resonance imaging (MRI) devices and the components used in computing hardware**.

**What does quantum theory successfully explain? ›**

It successfully explained phenomena such as **radioactivity and antimatter**, and no other theory can match its description of how light and particles behave on small scales.

**What are the 7 biggest unanswered questions in physics? ›**

- Quantum Gravity. The biggest unsolved problem in fundamental physics is how gravity and the quantum will be made to coexist within the same theory. ...
- Particle Masses. ...
- The “Measurement” Problem. ...
- Turbulence. ...
- Dark Energy. ...
- Dark Matter. ...
- Complexity. ...
- The Matter-Antimatter.

**How does quantum affect the brain? ›**

The quantum mind or quantum consciousness is a group of hypotheses proposing that classical mechanics alone cannot explain consciousness, positing instead that **quantum-mechanical phenomena, such as entanglement and superposition, may play an important part in the brain's function and could explain critical aspects of** ...

**How many dimensions exist? ›**

The world as we know it has three dimensions of space—length, width and depth—and one dimension of time. But there's the mind-bending possibility that many more dimensions exist out there. According to string theory, one of the leading physics model of the last half century, the universe operates with **10 dimensions**.

### What is quantum technology in simple words? ›

Quantum technology is **a class of technology that works by using the principles of quantum mechanics (the physics of sub-atomic particles), including quantum entanglement and quantum superposition**.

**Why is it called quantum physics? ›**

Albert Einstein (opens in new tab) won a Nobel Prize for proving that energy is quantized. Just as you can only buy shoes in multiples of half a size, so **energy only comes in multiples of the same "quanta"** — hence the name quantum physics.