The Universe in your hand | A journey through space, time and beyond

The Universe in your hand - Christophe Galfard

Summary of “The Universe in your Hand”: This novel will take you on a journey through time and space, from the infinitely small to the infinitely large, to help you better understand our Universe and everything that surrounds us.

By Christophe Galfard, 2016, 603 pages.

Note: This is a guest chronicle written by Laura and Marion from the blog AstronomiePratique.

Book review and summary of “The Universe in your Hand”:

In The Universe in your Hand, Christophe Galfard takes us on a discovery of the universe and all of its mysteries.

Who is the author of The Universe in your Hand?

Christophe Galfard is a French physicist and writer. As a student, he was a pupil of the renowned Stephen Hawking.

Among the questions which he answers in his book The Universe in your Hand are the following:

  • What is the Universe made of?
  • What is its history?
  • And what is the origin of space and time?
  • What happens in the vicinity of a black hole?
  • What is outside the Universe?

You will learn a lot, especially about the immensely small and the immensely large. You will also learn about black holes, light, gravitation, time, and lots of other exciting topics! To convince you that The Universe in your Hand is within everyone’s reach, in the foreword the author tells us about his deep conviction that scientific knowledge is accessible to everyone.

This is his ambition: “…in this book I will not leave any readers behind.”

There are no equations or complex scientific language here. At least not without explaining them first. No, this is simply a story to better understand the world in which we live and everything that surrounds us.

Without further ado, let’s dive in together.

Part 1: The end of humanity

From the first pages, the reader is sucked into the body, and above all the mind of a person on an island holiday with friends. We will tell you a bit about this trip in this article.

It all starts on a beach. You are waiting for your friends who are supposed to join you for the evening. As you wait, you look up and contemplate the heavens. This is where the journey begins. You are beset by questions about the stars when suddenly you see a shooting star go by. You decide to make a wish. Instantly, you are projected 5 billion years ahead into the space void.

Lost in this silent immensity, you witness a terrifying spectacle – the death of the sun. This violent demise leads to the eradication of our own dear planet.

You returning to the beach in shock. You become aware that in order to prevent the Sun from dragging the Earth, and therefore Humanity down with it, you need to develop your knowledge.

This is the point of departure of a long journey, over the course of which Christophe Galfard will introduce us to the wonders of the Universe.

Part 2: The Cosmos

The first part takes us gently into space, beginning by introducing us to the most basic elements of astronomy.

We set off to discover the Moon, the Sun, and the planets that orbit it, and then our entire galaxy.

The Moon

After your vertiginous leap 5 billion years into the future, you are back on your island once again. But not for long.

As if by magic, you find yourself thrown onto the dark side of the Moon. Here you will discover the major difference between our satellite and our planet: there is no atmosphere on the Moon. Therefore, there is no blue sky. It is night-time all the time. Another difference is that the stars do not shine from the Moon. Once again, this is because of the non-existent atmosphere of our satellite.

All of a sudden, in a flash, you see the birth of the Moon.

Four billion years ago, while the Earth was still in formation, another planet around the size of Mars collided violently with the Earth.

The collision was incredibly violent.

Part of the Earth broke off and flew into space. Over the course of the next few thousand years, all this debris came together to form the Moon.

Suddenly, you notice a blue dot in space. It is the Earth. You now become aware of the beauty of our planet, and the minuscule place that it occupies in this seemingly infinite space.

This vision puts your life into perspective. You feel minuscule and insignificant faced with such immensity. At the same time, you become aware of how lucky you are to live on a magnificent planet that is so well suited to the formation of life.

The moon is nice enough, but now it is time to move on in order to pay a visit to the Sun.

The Sun

The Sun

Image credit: NASA/SDO

The Sun is big. Very big.

In order for you to get an idea of just how big, the author compares it to a watermelon. If you put the Earth next to it, you would need a magnifying glass in order to see it!

Impressive, right?

To understand the Sun a little, you need to know that it is a star and how it functions.

Unfortunately, it would take too long to explain this here. However, there are important basic principles.

What makes a star, is what is known as stellar nucleosynthesis. This phenomenon is what makes it shine and differentiates it from a planet, for example.

At its heart, a star creates new atoms by merging with smaller ones.

Without going into detail, this process releases tremendous energy, in particular in the form of particles called photons that constitute light. The author now takes us away to discover these photons.

To finish on the subject of stars, all of the matter that surrounds us is made from stardust.

When a star dies, it releases all of the atoms that it has created throughout its life into the Universe. They are what assembles to form matter.

Let’s now visit the other planets in our solar system.

The planets


As you stroll around the solar system, you will discover two different types of planet.

The four that are closest to the Sun (Mercury, Venus, Earth, and Mars) are rocky planets. In other words, they have a solid core and a rocky outer layer.

Then there are four other planets (Jupiter, Saturn, Uranus, and Neptune) known are gassy giants. You probably guessed that this is because they are much bigger than the others and they are essentially made of gas.

After a quick tour of these planets, you continue your journey through the Kuiper belt. On the way, you say a quick hello to Pluto (which lost its status as a planet a few years ago), then cross the Oort Cloud.

Now you are outside the Solar system and a long, long way from our blue planet. Your next stop is the star that is closest to our Sun: Proxima Centauri.

Proxima Centauri

You have traveled a very long way. How far do you think?

Well, Proxima, which is the star that is closest to the Sun, is located at around 4 light-years from us. This means that if you travel at the speed of light, or 300,000 km/second, it will take you 4 years to get there! Yes, it is very, very far away… But at the same time, in the scale of the Universe, this distance is tiny.

Proxima Centauri is what scientists call a red dwarf. It is a star that is smaller than the Sun (Proxima is around 7 times smaller), and it is obviously red in color.

According to astrophysicists, red dwarves constitute the majority of the stars we know.

A planet discovered in an observatory in Chile in 2016 gravitates around it. It is what is known as an exoplanet, in other words, a planet that is outside our Solar system.

As Proxima is smaller, its life expectancy is much greater than that of the Sun. The smaller a star, the longer it lives.

We continue our journey to discover what is going on in the center of our galaxy. And you are not going to be disappointed…

Supermassive black hole

We suspect that you are familiar with the term “black hole”. It describes a star-eating monster. But do you know what it really is?

To keep it simple, it is a minuscule object with a gigantic mass that exercises enormous gravitational pull.

The direct consequence of this particularity is that, over a certain limit (called the event horizon), nothing, including light, can escape the black hole.

Light can longer get out. It is a prisoner, and that is why we call it a black hole.

black hole

Researchers in astrophysics think that one exists at the center of every galaxy. Our galaxy is no exception, and the one at the center of the Milky Way is named Sagittarius A* (pronounced “Astar”).

Its mass is around 4 million times that of the Sun!

Just imagine what that can represent… Actually no… You can try to imagine it for a long time, but in reality, it is unfortunately impossible for us to grasp such enormous values.

Black holes are the most mysterious celestial objects we know. Even though in reality we know very little about them. Many astronomers and dreamers find them fascinating.

Let me take you very quickly up close to one to see what is happening there. Among the strange phenomena that take place in the vicinity of a black hole, here are two of them:

  • Time slows down, to the point that you would be present for the death of the Sun, and much further still.
  • You would stretch and turn into a giant spaghetti.

Yes, we are talking science here, not science fiction. But throughout this article, you will see that science sometimes seems stranger than science fiction.

We are going to travel upwards, to place ourselves above our galaxy in order to better study it.

The Milky Way

But what in fact is a galaxy?

Before we go on, let’s take a look at what a galaxy actually is.

Here is one simple definition: a galaxy is a set of stars, of dust (there’s not a lot of cleaning going on) and of gas. In most cases, as we just learned, there is a black hole at the center.

Now let’s study our galaxy in more detail – the Milky Way.

Our galaxy

The Milky Way contains approximately 300 billion stars. The gravity of the black hole that we just explored unites them.

It is composed of a center and four arms that form a spiral.

Perhaps you are wondering: where do we fit into all this?

Well, we are on one of the arms. Closer to the periphery than to the center.


Picture credit: NASA/Adler/U. Chicago/Wesleyan/JPL-Caltech

Don’t bother looking, you won’t find us!

What place do you think the Milky Way has in our Universe?

Perhaps you think that 300 billion stars is already huge and that our galaxy is the only one. Well, you are wrong! The Milky Way is just one galaxy among billions

We will let you think about how many stars that represents. Or even more numerous, the planets…

It makes you dizzy, doesn’t it?

Let’s get back down to Earth. In your opinion, how far do all of the stars that we can see at night stretch? Do we see ones from other galaxies?

The answer is no. All the stars that you can see when you raise your eyes to heaven are only the stars that are closest to us. So, just a very small part of our galaxy.

However, you can observe other galaxies through a telescope, even an amateur one. In reality, you can even see our closest neighbor (in the same category as the Milky Way) with the naked eye! Its name is the Galaxy of Andromeda.

It is now time to start the next part of The Universe in your Hand, and it is just as fascinating. This part is devoted to the laws that govern our Universe.

Part 3: The laws of our Universe


We suppose that you are already familiar with the law of gravitation. We feel it all the time. It is what keeps you sitting on your chair, or wherever you currently find yourself.

This part looks into how two men “discovered” it and revolutionized our way of seeing the world.

Firstly, Newton in 1687. What Newton did was precisely describe the effects of gravitation. He produced three fundamental laws that can describe the movement of an object. However, while the laws are correct and have served us for more than two centuries without any problem, in fact, they still serve us today, they do not explain why they function.

Gravitation remains a mysterious force and we do not know its origin. Thanks to Newton, we know how it works and the consequences that it has on matter, but we don’t know what causes it.

We would have to wait 18 centuries before another very famous physicist explained what gravitational attraction really is.

In 1915, Albert Einstein wrote an article entitled The Foundation of the Generalised Theory of Relativity published in 1916.

What Einstein tells us is that, contrary to what we thought since Newton, gravity is not a force but rather a deformation of space-time.

To better understand, imagine stretching a sheet between two poles. Do not stretch it too much, you need to see it move in the wind.

Now choose an object and place it on the sheet. A dent will form in the place where you place the object. If you take a small marble and place it sufficiently close to this object, you will see it slide into the dent formed by the object.

Well, what we learn from Einstein is that the Universe is made up of what is known as space-time, and it plays the same role as the sheet.

Now, if we place asteroids, stars, planets, etc. into this Universe, then the stars each distort the lines of the space-time a little. They form hollows, curves that may be big or small depending on their size and their mass.

Imagine placing a tennis ball on your sheets and a bowling ball a little further. You will surely notice that the tennis ball distorts the sheet much less than the bowling ball.

It is exactly the same thing with the Universe. The Sun deforms space-time more than the Earth. Accordingly, it attracts objects that are further away towards it. At a certain distance from the sun, the objects find themselves at the top of a slope and they tend to “fall”.

This is what causes the gravitational attraction described by Newton 228 years earlier.

You probably suspect it is maybe a little more complicated than that, but you get the essential idea.

Except we cannot see the sheet. It is more like a 3D or even 4D cosmic fabric that can be deformed.


The speed of light is around 300,000 km/second.

For example, as the author notes, by the time you finish reading this sentence, light will have already traveled around the Earth 26 times…

This must seem very fast to you. And that is an understatement… At the same time, when you think about the distances in the Universe, it is in reality very slow.

Look into the past

To illustrate this, the light of the Sun takes about 8.5 minutes to reach us. So when we “look” at the Sun, we see it as it was 8.5 minutes ago.

The more distant the object you look at, the more time its light will take to reach you.

Yes, that means that when you look at the stars, you travel backward in time.

“Looking out is looking back.”

But this is not the only prodigious thing that light offers us.

Determining the composition of celestial objects

Light allows us to determine the composition of distant objects like stars, clouds of gas, or the atmosphere of the planets.

It would take too long to explain how this miracle is possible but the author explains the process very well.

We will, however, demonstrate the concept a little.

Light is composed of several wavelengths that each correspond to a color. We can only see one part that goes from red to violet, but infra-red and ultraviolet light also exist, for example.

We can break down white light, emitted by a star, into a spectrum like this:

spectrum 1

When it passes through an element, it absorbs a certain part of this light. We get absorption rays.

These rays are characteristics of each element. By observing the absorption spectrum of an object, you can determine its composition.

Here the absorption spectrum of hydrogen presents four absorption rays. This spectrum is characteristic of this type of atom.


We can therefore determine the composition of the stars.

Gravitational waves

You may have already heard of these. There was a lot of talk about them a few years ago.

More precisely in 2015. On 14 September 2015, gravitational waves were detected for the first time.

Before we explain how they were detected, let’s take a quick look at what these waves are.

A gravitational wave is a periodic fluctuation of the curvature of space-time. Do you remember the fabric that is at the origin of gravity?

This fluctuation spreads over long distances starting from its formation point. You can picture it like a wave.

Massive stars produce it when their movement accelerates. These waves can, for example, come from a collision between two black holes.

They were detected thanks to LIGO (Laser Interferometer Gravitational-Wave Observatory), consisting of two sensors located in two different American states. There was one in Louisiana and another in the state of Washington.

laser The Universe space and time

To make it simple, let us just say that these are extremely precise sensors that can measure a very, very small variation in distance that can only be caused by gravitational waves.

The LIGO was designed and is used specifically to detect gravitational waves. It was created based on projections by Einstein. But he thought that we could never observe them ourselves because it was too complex.

Despite this, researchers in astrophysics took on the project, and years later they managed a feat that even Einstein thought impossible.

Remember that if people succeeded in a project that even Einstein thought impossible, then your project can be successful too, whatever it may be…!

This famous date of 14 September 2015 is therefore a very important date for astrophysics. It was the first time that we perceived a signal other than light.

Hubble’s law

The study of the composition of the stars from light also allowed us to make another important astrophysical discovery. Do you remember the spectra obtained with the characteristic rays of certain elements?

When they studied them, scientists realized that the characteristic absorption rays of the elements all shifted to red. What was even odder was that the more distant the objects being observed, the more their spectrum shifted towards red.

How could this be possible? What is behind this shift?

Well, prepare to be amazed, because it is simply due to the fact that our universe is expanding.

It’s a pity that the author does not talk about this, but it comes from what is known as the Doppler effect. Doppler tells us that when an object moves away from us, the light that reaches us has a higher wavelength than if it was stationary. In contrast, if an object is closer to us, the wavelength decreases. In terms of colors, this translates into a shift towards red when it is distant and towards blue when it is closer.

This leads to the conclusion that our universe is expanding because the galaxies are moving away from each other.

You can also note that because of this discovery we can also know how fast a star is traveling by studying the shift that appears on its spectrum.

In 1929 Edwin Hubble set out a law, which bears his name, which connects the velocity at which galaxies recede from the Earth to their distance.

This leads us to a fairly simple conclusion: if the Universe expands over time, it means that it was smaller before… You will agree with this.

Well, this leads us straight to the famous Big Bang theory.

Big Bang

According to this theory, the entire Universe was included in a single point in the beginning, 13.8 billion years ago. This tiny point contained both space and time.

It’s true, imagining our entire Universe compressed into a tiny point is a difficult concept to grasp. But let’s accept it.

How do we know that the Big Bang took place 13.8 billion years ago? Do we have evidence of this phenomenon or is it simply a theory?

Cosmic microwave background

To answer the second question, yes, we have evidence. One piece of evidence is called the cosmic microwave background. Its discovery was something of an accident in 1965.

Two American physicists were in charge of installing an antenna designed to receive radio waves. But they found themselves facing a problem: an unknown noise was crackling on the antenna and jamming the signal. They couldn’t find the source of the problem. They blamed it on birds and began to clean and re-clean their device. But nothing worked and the noise was still there. They called on other scientists to help them. What they discovered amazed them…

The noise that they heard was not a problem with the device. In reality, it came from space.

The noise had been there for nearly 13.8 billion years (which answers the first question). It was in fact one of the consequences foreseen in Einstein’s equations.

The noise, named Cosmic Microwave Background, corresponds to the “picture” of the Big Bang, below.

The Universe space and time

Image credit: ESA and the Planck Collaboration

You should probably know that prior to this discovery, the theory of the  Big Bang had seemed completely absurd to some astrophysicists. The phenomenon is now widely accepted by the scientific community.

After this plunge into the infinitely large, it is time to take a look at what it is made of – the infinitely small.

Part 4: The quantum world

Before we take a trip to the infinitely small, the author suggests that we prepare psychologically.

He explains that our senses are limited, adapted to our scale, to life in our world.

That is why it is impossible for us to understand concepts like the immensely big and the immensely small.

“Our sense are our windows on the world, but they are only tiny portholes overlooking a huge sea which is unknown to us.”

You get the picture – we cannot rely on our senses alone to understand our Universe. Do not try to grasp the concepts we are about to examine with your senses. In fact, do not try to grasp them using anything you know about the world on our scale.

What is the Quantum world?

As we have just said, it is the world of the infinitely small. It is the one in which the elementary particles are born and exist, such as light for example.

It is absolutely everywhere. In the soil, in the water, in the air, in space, in you and in me… everywhere.

But it is well hidden.

It took us a long time to discover it. Today, even if it still holds many mysteries, we know that it exists.

If you don’t understand everything (or even anything) of what comes next, that is completely normal!

Einstein himself would have said the same to his students in quantum physics (the science that attempts to explain the world of the infinitely small):

“The more success the quantum theory has, the sillier it looks”.

Or Richard Feynman, the winner of the Nobel prize for physics:

“If you think you understand quantum mechanics, you don’t understand quantum mechanics.”

One last one from Niels Bohr, another winner of the Nobel prize for physics:

“If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.”

So you have to take the following without any prior judgment, even if it sounds completely mad. We can reassure you that it’s the same for everyone, even the scientists working in this field.

Now that you are prepared, let’s go down the rabbit hole from Alice in Wonderland together.

The quantum field

As we have seen, atoms make up matter. What are these atoms made of?

That is what we are going to look at next.

We are now in the quantum world, a world in which our reality has no meaning and in which the laws of physics as we have seen them have no place.

An atom is composed of two charged parts: the nucleus and a sort of cloud that revolves around it.

  • The nucleus has a positive charge. It consists of particles called protons (as well as neutrons, which are neutral).
  • Around this nucleus are electrons, which form a sort of cloud. The electrons have a negative charge.

They form the electromagnetic field, the first of the quantum fields.

The Universe space and time

There are three quantum fields: the electromagnetic field, the field of strong interaction and the weak nuclear field.

The field of strong interaction is what stabilizes all the nuclei of atoms in the Universe. We talk about strong interaction because it is the most powerful natural quantum field we know of.

There is much to say about this field but it would take too long here. We will let you discover it when you read “The Universe in your hand”.

When it comes to the weak nuclear field, unlike the first two fields, it is a destructive force. As its name indicates, it is a nuclear force. It only occurs on the constituents of atomic nuclei.

You will have no doubt realized that this force is much less powerful than its cousin – strong interaction. But it does have the power to destroy what the latter produces. This interaction has the power to lead to the disintegration of an atomic nucleus. It is responsible for what is called radioactivity, which, as you know is extremely dangerous.

So, these three fields surround us, they are everywhere. What appears empty to us is in fact composed of billions of particles such as those that we are about to see.

You have just taken the first steps in a world made of elementary particles, the quantum world. It is time to move a little deeper into our discovery of this world.

The particles

We have just seen that what appears empty to us is in fact composed of different kinds of particles.

Let’s look at these particles in more detail. For this, we are going to take the example of an electron. (What we are going to see applies to all elementary particles and not only to electrons.)

We advise giving this your full attention – it may well astound you!

Electrons have several types of behavior that are very strange.

Everything we know on our scale or on a greater scale is either a wave or a particle. Electrons are simultaneously wave and particle.

This principle is wave-corpuscle dualism.

You can see that we do not exactly know how to represent an electron. But this is not the strangest thing you are going to see…

According to mathematical formulas, when we know the speed of an object, we also systematically known its position. And vice versa.

Well, this principle does not work for electrons. This is what the physicist Werner Heisenberg discovered in 1927. He called it the uncertainty principle.

But that’s not all!


Here we come to a concept that is one of the most disturbing and most difficult to imagine: electrons can teleport…!

For a particle, the phenomenon is in reality called tunnel effect or quantum leap.

There remains one final notion, an extraordinarily overwhelming and also fundamental one in quantum mechanics, presented in “The Universe in your Hand”. Let’s take a closer look.

“Particles that no one looks at travel through all possible paths spacetime can offer”.

What exactly does that mean?

Let us imagine a particle, which is in position A. It could go to position B, C, D, E, etc. Where do you think it will you? The answer is everywhere at once. At least as long as nobody is watching.

This behavior is impossible to reproduce in our everyday life. But a particle that is not observed can be found everywhere at once.

This raises an interesting question…

You know that, like everything else, you and I are made of particles. So why can’t we live every possible and imaginable life at the same time?

Imagine. You would be simultaneously happy and sad, rich and poor, married and unmarried…

But you know that this is not what happens. So why not?

Beware – the answer from the scientists may surprise you…

The answer is that they don’t know. It is one of the great mysteries of modern science. We have no way of knowing why the reality that we can see does not correspond to the quantum world.

Let’s go back to “as long as nobody is watching”, which is perhaps the most disturbing thing.

The role of the observer

We will put it differently so that it is clear. Once an observer is present, the particle changes behavior. It is no longer everywhere, but in a specific place, the place in which it is detected.

Picture it like a game of statues. When the child turns her back and does not look, all the other children move. As soon as the child turns around, everyone freezes.

So what does it mean to be an observer? According to the experiments carried out, observers can be machines, detectors, cameras… or the human eye.

Well, a particle that can be located in several places at once already seems incredible. But that its behavior changes as soon as it is observed sounds even more absurd, right?!

It reminds us of the principle of the Law of Attraction.

We could also put it this way: reality changes when you look at it.

Moreover, some scientists are looking at the role our consciousness plays in all this.

“Some others believe that our consciousness has something to do with it all, that it is the very act of experimenting or even thinking that freezes reality in one state, thereby creating it.”

The sentence “our thoughts create our reality” suddenly takes on its full meaning.

Of course, this is pure assumption and the personal opinions of people who are passionate about science and personal development.

We will close the parenthesis on the role of our consciousness in this.

If you have your doubts about what we have just seen in this chapter, you should know that several experiments have been performed. They all arrived at this conclusion.

Part 5: To the origin of space and time

Schrödinger’s cat

The experiment of Schrödinger’s cat is a very well-known experiment that attempts to reproduce the phenomenon of the quantum world on the human scale.

Let’s reproduce the experiment together.

To start with, pick up a cat and put it in a closed box with:

  • A radioactive substance that has a 50% chance of disintegrating
  • A radiation detector
  • A hammer
  • A vial of deadly poison

Nobody should be able to guess what happens inside the box.

Now you may be thinking that things don’t look good for the cat. You would be right. But let’s go on.

The hammer will only fall on the vial and the poison will only be released if it detects the radiation emitted by the radioactive substance. You have therefore understood that the poison has a 50% chance of spreading and the cat has a 50% chance of dying.

Let’s get to the question that we are interested in.

Is the cat dead or alive?

You will agree that it is impossible to know the answer as long as you do not open the box.

In real life, you open the box and the cat is either dead or alive. But in this specific case, both answers are wrong.

Oh yes, quantum effects come into play in radioactive disintegration. And that is the point.

In the quantum world, anything that can happen, happens. Both disintegration and non-disintegration take place.

Finally, and you will remember, this is only as long as nobody is watching. Since the box is closed and nobody can see what is happening inside, we can in fact say that nobody is watching.

The answer is, therefore: the cat is dead and alive, at the same time.

Of course, this seems absurd.

You are suddenly seized with panic and you open the box to check. The cat leaps out, perfectly unharmed, and therefore very much alive.

But was it really dead and alive before you opened the box?

To begin with, when you opened the box, you observed, and as the laws of quantum physics say, from this time, there is only one possibility and no longer an infinite number of possibilities.

Schrödinger devised this experiment in 1935. We should specify that it is a thought experiment. Fortunately, no cat was the victim of this experiment.

For years, this has been an unresolved enigma for scientists. Until two physicists, the Frenchman Serge Haroche and the American David J. Wineland, performed the experiment for real. We can reassure you right away that they did not use a cat, but atoms and light. They could confirm that the particles really do exist simultaneously. The laws of the quantum world are therefore not just theories put forward by physicists.

To answer the previous question: as long as the box is closed, the cat is both dead and alive.

This brings us to another question. Where does the dead cat go when you open the box?

You may think that everything we have been telling you is completely absurd?

Well, hold on tight, because it’s going to get worse…

According to one theory, when you open the box, the entire Universe splits in two, and a parallel universe appears.

We, therefore, have our Universe in which the cat is alive, and another in which it is dead.

Up to now, you probably only heard about parallel universes in science-fiction movies. And even then you probably thought the director had an overactive imagination…

Let’s look at this business about parallel universes in more detail.

Parallel universes

So, how did we get to theories about multiple universes?

Remember, according to the laws of the quantum world, particles are everywhere at once.

Well, according to the physicist Hugh Everett, there should be a multitude of parallel universes in which all possibilities are real.

Imagine you cannot decide where to go for your next holiday, Marbella or Cornwall?

Once you make your decision, a parallel universe is created in which a replica of yourself made the other choice. There are therefore as many parallel universes as there are choices, so that all possibilities can be realized.

After presenting his hypothesis, Everett gave up physics because it no longer had any meaning for him. And we can understand… But the idea of parallel universes was not abandoned, far from it. We will see this a little later.

What is the point of quantum physics?

These experiments are all well and good, but what is the point in paying scientists to conduct experiments like this?!

Well, discoveries about the quantum world have practical applications. Many of the objects around you only exist because of these discoveries.

It is the case of the computer or the telephone on which you are reading this article, and many medical devices.

The Universe space and time

Let’s take a quick look at new technologies. Scientists are working on the computer of the future: a quantum computer.

Its capacity for calculation will far, far exceed the most powerful computers today.

This will be possible by using the quantum phenomenon of particles that are found everywhere at the same time. It will be a little bit as if the computer was located in billions of parallel universes at the same time.

Now that you are reassured about the usefulness of the research carried out in the quantum world, let’s continue our journey.

Dark matter and dark energy

Dark matter and dark energy are two very interesting and very strange elements that we cannot really understand as yet.

Let’s look at them in more detail.

Dark matter

The astronomer Jan Oort was the first to suggest the hypothesis of dark matter. This is matter which is everywhere in the Universe, and which does not interact with light (which is why we cannot see it).

“Dark matter is not matter. Nor is it antimatter. It is something else. But nobody knows what. ”

Scientists have performed many tests since the 1930s, and they all confirm the presence of dark matter.

But to this day, nobody knows what this matter is composed of. We know that it is there, and know where it is located, but that is all.

We also know that it is heavy: for 1 kg of ordinary matter, there is 5 kg of dark matter!

Therefore, there is more that is invisible than visible in our Universe. And that is an understatement, as you will see…

Dark energy

The principle is more or less the same for dark energy. It is something that we cannot see but we know that it exists.

Dark energy is behind the accelerated expansion of our Universe. It is an anti-gravitational force that took over all of the other forces in the universe approximately 5 billion years ago.

This energy is present today, very much so. And there is much more of it than dark matter.

To give you an idea, here is an estimate of the breakdown in our Universe, according to NASA:

  • Dark energy: 72 %.
  • Dark matter: 23 %.
  • Matter that we know (including light): 4.6 %.

Yes indeed, everything you know, everything that we have seen up to now, represents only 4.6% of the total content of our universe.

The rest is completely unknown…


Have you seen the film Interstellar?

If you haven’t, it is a science-fiction film. In fact, it’s our favorite film. Anyway. The reason we mention it is because the term “singularity” is used in the film. Perhaps you remember?

A singularity occurs when there is too much mass or energy in a volume that is too small. The infinitely large and the infinitely small join together.

So, in a singularity, should we use general relativity or quantum mechanics? That is the problem…

Yes, singularities really do exist in our Universe. Do you any idea where they might be hiding?

Yes, inside black holes!

Inside a black hole, the “fabric” of the Universe that we talked about earlier gets torn. This also means that the notions of space and time, as we know them, no longer apply.

In other words, Einstein’s equations and his theory of relativity no longer work.

Our Universe is also born from a singularity. It was the Big Bang.

This phenomenon of singularity has not yet been explained. This poses a problem that researchers in astrophysics are currently trying to solve (it is also the object of their interstellar research).

The theory of everything

Up to now, we have seen two models, two theories. That of the infinitely large (general relativity), which describes the structure of our universe, and that of the infinitely small (quantum theory), which explains everything that it contains.

But as you have already seen, there are problems. In this case, the singularities that we have just seen and cannot explain.

Astrophysicists set off on a quest for a theory of everything. This is a theory that can explain everything in a single formula, bringing together gravitation and the quantum world.

One theory to rule them all…

Einstein himself looked for a formula to solve this problem, but he died without finding it.

In recent years, several theories have been put forward. One of them has become the preferred candidate.

Let’s take a look at what it contains. But beware, it may surprise you and you may find yourself wondering whether all the physicists have not gone definitively mad…

String theory

Like we said, it is one theory among others. But we have heard a lot about it in recent times.

We all agree that it is simply a theory that is not confirmed. We won’t go into it in detail here, but as you may suspect, if it is the preferred theory and lots of physicians are looking into it, then there must be something behind it.

String theory questions many things, and here are a few examples:

As its name implies, the theory applies to a principle of strings. Quantum strings to be precise. Tiny, tiny strings that are invisible on our scale.

You can imagine these strings like guitar strings. Instead of playing a musical note, they emit light.

The Universe space and time

Therefore, all the light that reaches our eyes is in fact the result of the vibrations of quantum strings.

There are closed strings for gravitation and open strings that emit light.

So far, you are probably thinking: ok, why not?

We’re not finished. For this model to work, something needs to change.

Other dimensions

These strings cannot exist in our 4-dimensional space-time. No, they need more, many more…

What? 5 dimensions? 6 maybe…?

You’re still way off. No, these little strings need 10 dimensions… That’s all!

But how can we imagine these extra dimensions?!

You need to imagine directions other than up, down, left, right, forward, backward…

It’s not easy, is it? That’s understandable, we don’t have any words for it.

Extra dimensions sounds crazy, right? But that’s not all…

We end on a high note with another prediction from string theory.


Our Universe is nothing more than one “brane” (from the word “membrane”, except that it is not flat), among many others. We are back to the concept of parallel universes.

All these Universes (or multiverses really) may be nothing more than a tiny part of an even more vast reality…

It makes you dizzy, doesn’t it?

Black holes could then be distorted space-time tubes that link one universe to another.

Finally, this theory also explains the Big Bang as a collision between two branes.

This is where our journey through the cosmos ends.

To conclude, here is a lovely thought which is a good summary of the magic of the cosmos: “We are nothing but stardust”.

Please note that we have only introduced you to some of the phenomena explained in the book. Among the ones we have not mentioned are:

  • Comets
  • The limit of the visible universe
  • Time dilation
  • Special relativity
  • Particle accelerator
  • Higgs boson
  • Anti-matter

And we have only scraped the surface of the topics which we have mentioned. The Universe in your Hand goes into far greater depth.

Book critique of “The Universe in your Hand”

This book is extremely well written and very easy to understand. It is the basis for a beginner who wants to understand our Universe and its various laws a little better.

The level of complexity increases as you read The Universe in your Hand. You may need to read some parts, especially towards the end, a few times in order to understand them.

We both loved this book and we strongly recommend it to anyone interested in astronomy and astrophysics.

It gave us a better understanding of the world and the way the laws that govern our Universe work. Faced with such nature that is powerful and mysterious makes us feel very humble.

Finally, it is important to remember that science, as powerful as it may be, does not explain everything. We are a long, long way from understanding everything. Remember, we know less than 5% of what makes up the Universe.

Keep your head in the stars and be curious about the world around you!

Laura and Marion from the blog AstronomiePratique.

Strong Points of The Universe in Your Hand:
  • Very comprehensive
  • Accessible to novices
  • No mathematical formulas
  • Novel format
Weak Points of The Universe in Your Hand:
  • Quite long
  • Some notions are complex to understand if you are reading them for the first time

My rating : The Universe space and time The Universe space and time The Universe space and timeThe Universe space and timeThe Universe space and timeThe Universe space and timeThe Universe space and timeThe Universe space and timeThe Universe space and time

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