Author: Stephen Hawking
_Stephen Hawking_
Reading time: 26 minutes
Synopsis
A Brief History of Time explains how scientific ideas and theories have changed our understanding of the universe. From the Big Bang to black holes and the smallest particles, Hawking gives a clear overview of the universe’s history. He also talks about the most important scientific discoveries. He uses simple language, so anyone can understand these complex topics, even if they have no scientific background.
What’s in it for you: Find answers to some questions about the universe.
You have probably thought about the big mysteries of the universe. When we look up at the sky and see thousands of stars, we naturally ask questions like: “What is happening up there?” and “How did it all begin?”
A Brief History of Time gives you answers. It teaches you about the rules that control the universe. The book is easy to understand. You don’t need any science knowledge to learn why the universe exists, how it started, and what its future will be.
You will learn a lot about strange things like black holes. These suck in everything, or almost everything. The book also talks about the mysteries of time: How fast does time move? How can we be sure it only goes forward? The following summaries will answer these big questions.
Blink 1 – A look into the future: What makes scientific theories special.
When scientists try to explain something, they create a theory. Many people know famous theories, like Newton’s law of gravity or the theory of evolution.
A theory is like a model. It gives a good explanation for certain things we observe. Scientists collect information from what they see. From this, they form a general idea. This idea explains how and why things happen. For example, Isaac Newton created his theory of gravity. First, he watched apples fall from a tree. Then, he watched how planets moved. Based on this information, he built a theory that described all these things.
Theories have two main benefits: They help scientists guess what will happen in the future. Also, they can be proven wrong. This means they can be changed if new evidence appears.
So, a theory lets us make predictions about the future. For example, we can use Newton’s law of gravity. This helps us predict how objects in space, like Mars, will move in the future.
Theories can be proven wrong by new observations. If evidence goes against a theory, it can be changed or replaced. For example, people once thought everything in the universe moved around the Earth. Galileo showed this was wrong. He saw moons moving around Jupiter. This was proof that not everything moves around the Earth.
A theory can never be proven right forever. Future observations might prove it wrong. So, scientific knowledge is always changing and improving.
Blink 2 – Everything is relative and nothing stays still.
Before Isaac Newton, people thought that the natural state of an object was to be completely still. They believed that objects would not move if no force pushed them.
In the 17th century, Newton showed this old idea was wrong. He said that all objects in the universe are always moving.
Newton learned this by seeing that planets and stars constantly move in relation to each other. For example, the Earth always moves around the Sun. And our whole solar system always moves around the galaxy. Nothing stays still.
Newton created three laws. They explained how all objects in the universe move:
Newton’s first law says that objects move in a straight line if no other force acts on them. Galileo Galilei showed this idea in 1638 with a simple test. He rolled two balls down a slope. Only gravity pulled on the balls. So, they rolled down in a straight line.
Newton’s second law says that how fast an object speeds up depends on the force pushing it. For example, a car with a stronger engine can speed up faster than a car with a weaker engine. This law also says that if an object is heavier, a force will move it less. If two cars have engines with the same power, the heavier car will take longer to speed up.
Newton’s third law talks about gravity. It says that all objects in the universe pull on other objects. This pull depends on how heavy each object is. This means if you double an object’s weight, its pull will also double. If you double one object’s weight and triple another’s, the pull between them will become 6 times stronger.
Blink 3 – The big problem in Newton’s theory: Why is the speed of light always the same?
Newton’s theory showed that objects are not completely still. Instead, he said their movement depends on the movement of other objects. And their acceleration also depends on other things.
Imagine you are reading a book on a train going 100 km/h. How fast are you moving? Someone outside, watching the train pass, would say you are moving 100 km/h. But compared to the book you are reading, you are moving zero kilometers per hour. So, your movement can only be measured compared to other objects.
This sounds very logical at first. But if we think more about this, we find a big problem in Newton’s theory: The speed of light is always the same. It does not depend on other movement. It is always 299,792,458 meters per second. That’s about one billion kilometers per hour. It doesn’t matter how fast anything else is moving – the speed of light stays the same.
If we are on a train going 100 km/h and move towards a light beam, the light’s speed is still 299,792,458 m/s. If the train stops at a red light, the speed of light is still 299,792,458 m/s. It doesn’t matter how fast the person watching the light is moving, the speed of light is always the same.
This was a problem for Newton’s theory: How can something’s speed not depend on the person watching it?
Albert Einstein answered this question in the early 1900s. He created his theory of relativity.
Blink 4 – Einstein provided the answer: Time is relative.
It was hard to match Newton’s theory with the fact that light speed is always constant. This showed that speed is not always relative. So, science needed a better model that included the constant speed of light.
Albert Einstein developed such a theory: the theory of relativity.
The theory of relativity says that the laws of physics are the same for all observers who are moving freely. This means that everyone measures the same speed for light, no matter how fast they are moving.
This idea seems simple at first. But one of its main points is very hard for many people to understand: the idea that time is relative.
This means: If observers traveling at different speeds measure the same speed for light, then observers moving relative to each other must measure different times for the same event.
For example, imagine two different people see a flash of light. One person moves towards the light source, while the other moves away from it at a higher speed. But the speed of light is the same for both people, even though they are moving at different speeds in opposite directions.
It seems strange at first to think that the travelers would see the same event, the flash, at two different times. This is because time is measured like this: You divide the distance traveled by the speed you are moving. The speed of light is the same for both observers. But because the distance is different, the time of the event is also relative to each observer.
If both observers had clocks to measure the flash, they would measure two different times for the event. Which person would measure the correct time? Nobody, because time is relative and depends on the observer. There is no “one correct time” that applies to everyone.
Blink 5 – The exact position of particles cannot be measured – but quantum mechanics helps.
All matter is made of tiny particles, like electrons and photons. To learn more about the universe, scientists want to measure these particles and find out their speed.
But particles behave strangely: The more exactly we can find a particle’s position, the less exactly we can find its speed. And if we measure its speed very exactly, we become less sure about its position! This was first discovered in the 1920s. It is called the uncertainty principle.
Because of this uncertainty, scientists had to watch particles in a different way. So, they started looking at their quantum mechanical state. This is made up of all the many possible positions and speeds a particle could have.
Since scientists cannot find the exact position or speed of a particle, they look at the many likely positions and speeds the particle could have. As the particle moves, scientists track all its possible places. They then figure out which one is most likely.
To find a particle, it helps to think of particles like waves. The many different positions a particle can take form a series of continuous, moving waves.
Imagine a vibrating string. As it vibrates, the string moves up and down, forming peaks and valleys. A particle behaves in the same way. But its path is a series of many such waves happening at the same time and overlapping.
So, the quantum mechanical view helps scientists find where a particle is most likely to be at a certain time. The most likely position is where the peaks and valleys of many waves cross each other. The least likely position is where they do not match. In science, this crossing, which shows us the most likely speed and position for a particle, is called interference.
Blink 6 – Gravity happens because heavy objects bend the universe.
When you look around you, you see three dimensions: You can describe an object by its height, width, and depth. But there is a fourth dimension we cannot see: time. Einstein showed that time is relative. So, it must always be included when calculating an event. Together with the other three dimensions, this creates a four-dimensional system. It is called the space-time continuum.
Scientists use this four-dimensional model to describe events in the universe. This has surprising results for how we think about gravity:
Gravity is caused by very heavy objects that make curves in the space-time continuum. A heavy object, like our Sun, bends and changes the structure of space-time. Imagine the space-time continuum as a bedsheet stretched out in the air. If you put an object in the middle of it, the sheet will stretch and form a dip where the object sinks. Heavy objects cause the space-time continuum to curve in the same way.
Other objects follow these curves. To understand this better, think again about the bedsheet example. If you place a large object, like an orange, on the sheet, and then roll a lighter object next to it – for example, a marble – the marble will roll in a path around the orange. Gravity works in the same way.
Blink 7 – A very special rebirth: When a massive star dies, it becomes a black hole.
During their lives, stars use huge amounts of energy to make heat and light. But this energy does not last forever. When it runs out, the star dies.
What happens when a star dies depends on its size. If a very large star runs out of energy, something amazing happens: a black hole forms.
A black hole forms because the gravity of a massive star is very strong. During its life, the star uses its energy to stop itself from collapsing. But when the star loses its energy, it can no longer fight its own gravity. Its dying body falls inward. Everything pulls into a super-dense, round point. This is called a singularity, which we more commonly call a black hole.
When a black hole forms, the space-time continuum bends so much that even light curves around it. The black hole swallows everything near it. Once an object gets too close, it cannot escape. This point of no return is called the event horizon. Not even light, which is faster than anything else in the universe, can escape from this point.
This brings up a difficult question: If black holes take in everything that crosses their event horizon, even light, how can we know they exist?
Scientists look for black holes by watching for the effects of their gravity on the universe. For example, stars that orbit dark, heavy objects can be a sign of a black hole. Also, X-rays are produced when matter is sucked into a black hole and broken apart. At the center of our galaxy, there is a source of radio and infrared waves. This could be a very massive black hole.
Blink 8 – Black holes give off radiation and then disappear.
If a black hole’s gravity is so strong that not even light can escape it, you might think that nothing else can escape either.
But that is wrong. In reality, black holes must release something. If not, they would go against the second law of thermodynamics.
The second law of thermodynamics is true everywhere. It says that entropy, which is the amount of disorder in a physical state, always increases over time. As entropy increases in a closed system, its temperature also goes up. You can see this clearly, for example, when fireplace tools glow red after being put in a fire and release heat as radiation.
According to the second law of thermodynamics, the entropy of a black hole must always increase. This happens as it absorbs disordered energy from the universe. This increase, in turn, must cause the black hole to release heat.
Virtual particle-antiparticle pairs cause this release of energy. They make sure the second law of thermodynamics is followed. Virtual particles are particles that we cannot see. But their effect can be measured. One particle in each pair is always positively charged, and the other is negatively charged.
In a black hole, gravity is so strong that it absorbs the negatively charged particle. This gives the positive particle of the pair enough energy to escape into the universe as heat. So, this radiation escapes the black hole, and the second law of thermodynamics is followed here too.
The amount of positive radiation that is released is balanced by the negative particles that disappear into the black hole. The flow of negative particles can reduce the black hole’s mass until it starts to shrink and dies. When its mass becomes small enough, there will likely be a massive explosion. This explosion would be as powerful as millions of hydrogen bombs.
Blink 9 – Upside down world? Why time can only move forward for us.
Imagine the universe started to shrink. And imagine time started to go backward. What would that be like? Maybe clocks would run counter-clockwise. And history would rewind to its beginning.
But don’t worry: Scientists don’t completely rule out time going backward. However, there are three strong reasons that suggest time can only move forward.
The first sign that time moves forward, from past to future, is the thermodynamic arrow of time. According to the second law of thermodynamics, entropy – the disorder in a closed system – tends to increase over time. This means we can measure time by seeing how disorder increases.
For example, if a cup rolls off a table and breaks, its physical state becomes less ordered. Its entropy increases. We know from experience that a broken cup never spontaneously puts itself back together and becomes ordered again. This shows us that time can only move forward.
The broken cup and the thermodynamic arrow of time also show the second sign: the psychological arrow of time, which is made of our memories. After the cup breaks, we can remember that it was on the table before. But when it was whole on the table, we could not remember its “future” broken state on the floor.
The third sign is the cosmological arrow of time. This means the expansion of the universe. This also fits with how we see the thermodynamic arrow of time. This is because entropy increases as the universe expands.
If the chaos in the universe reached its maximum, then the universe would start to shrink. This would reverse the cosmological arrow of time. However, we would never know this. This is because increasing entropy is necessary for us to exist. Without it, our food would not turn into energy. So, as long as we exist as we do now, we will only see the cosmological arrow of time moving forward.
Blink 10 – Besides gravity, there are three basic forces in the universe.
What types of forces work in the universe?
Most people have only heard of one: gravity. This force makes objects pull each other. In our daily lives, we feel it as the force that pulls us to Earth.
Most people do not know that three other forces also act on particles:
We can see the electromagnetic force in daily life. For example, when a magnet sticks to a fridge. Or when you charge your phone. It affects particles with an electric charge, like electrons.
Electromagnetic forces, like the North and South poles of a magnet, can either pull or push. Positively charged particles attract negatively charged ones. They push away other positively charged particles, and vice versa. This force is much stronger than gravity. It works inside an atom. It makes electrons move around the atom’s center.
The second force is the weak nuclear force. It acts on all particles that make up matter. It is responsible for radioactivity. This force is called “weak” because it can only work over very small distances. At higher energy levels, the weak nuclear force gets stronger. It can become as strong as the electromagnetic force.
The third basic force in physics is the strong nuclear force. It holds protons and neutrons together inside the atom’s center. Unlike electromagnetism and the weak nuclear force, its strength decreases as energy levels get higher.
At very high energy levels, which scientists call grand unification energy, electromagnetism and the weak nuclear force become stronger. At the same time, the strong nuclear force shrinks.
At this point, the three forces become equally strong. They become different parts of a single force. This single force might have helped create the universe.
Blink 11 – Scientists are still searching for exact explanations of the Big Bang as the universe’s birth.
Most scientists believe that the Big Bang was the start of time. It was the moment when the universe changed from a super-dense point into a quickly expanding area. And it is still growing today.
However, science has not yet found an exact answer to how the Big Bang happened. Even though there are a number of theories that try to explain it.
The most widely accepted theory for how the universe began is the “Hot Big Bang” model.
According to this theory, the universe started as an infinitely hot and dense point. As it expanded after the Big Bang, it cooled down. In the first few hours after the Big Bang, most of the elements we find today were created.
As the universe continued to expand, gravity caused dense areas of expanding matter to form. This led to the creation of spinning, growing galaxies. Inside these new galaxies, hydrogen and helium gases collapsed. Their atoms hit each other, starting nuclear fusion. This created stars.
The death and collapse of these stars then caused huge, star-like explosions. These explosions sent more elements into the universe. This gave the material from which new stars and planets could be born.
Even though this model is a commonly accepted version of the Big Bang and the birth of time, it is not the only one.
Another model is the “inflationary model“. This theory says that the energy in the early universe was so huge that the strength of the strong and weak nuclear forces and magnetism were all the same. As the universe expanded, the three forces quickly became different in strength. When the forces separated, a massive amount of energy was released. This had an anti-gravity effect, so the universe began to expand at a decreasing rate.
Blink 12 – General relativity and quantum physics cannot be combined.
Scientists want to understand and describe the universe. This led to the creation of two main theories:
The first is the theory of general relativity. It deals with the big idea of gravity. The second is quantum theory. It describes some of the smallest known objects in the universe: particles smaller than atoms.
Both theories give very important insights. But there is a big difference in what scientists can see and predict using quantum physics compared to general relativity. This difference is so huge that right now, there is no way to combine the two theories into one big theory of everything.
For example, many equations used in quantum physics give endless results. But we know from observations that these values are limited. According to these equations, for instance, the curve of space-time is endless. But observations have shown this is wrong.
To fix these wrongly endless values, scientists add other endless numbers into their equations. But this stops scientists from making accurate predictions. Instead of using quantum physics equations to predict events, scientists have to add events and change the equations to fit them.
Another similar problem is that quantum theory believes the empty space in the universe is made of virtual pairs of particles and antiparticles.
But the existence of these virtual particles is hard to fit with the theory of general relativity.
Since the amount of empty space in the universe is infinitely large, the energy of these particle pairs should add up to an infinite energy.
This would be a problem. Einstein’s famous equation E = mc² says that mass and energy are the same. The infinite energy of these particles would mean they also have an infinite mass. If such an infinite mass had gravity, the universe would collapse and become a single black hole.
Summary
The main message of this book is:
There are many rules and laws that help us understand the mysteries of the universe, at least a little. Scientists have already learned a lot about the universe. But some things, like how the Big Bang started, still need more research.
To think about:
Theories help you test ideas and make predictions.
Theories let us guess what will happen in the future. You can make a theory by creating some simple rules that can be checked. Then, you test if they work in an experiment. If the experiment does not prove them wrong, you can assume they will be true in the future under the same conditions. So, you can use them to make predictions.
Time is relative. Even if it’s barely noticeable, this has big effects on your daily life!
Time is relative. This fact has very interesting results. For example, if we could move at the speed of light, we would age slower. If we could travel through space at that speed, we would return to Earth thousands of years in the future, without having aged much ourselves.
But even in our daily lives, time’s relativity affects some things. For instance, navigation systems must consider this relativity. Otherwise, they would guide users kilometers away from their real destination. Also interesting: A twin living on a mountain would age a little faster than their twin living at sea level, because of their position.
You can only describe your position in relation to something else.
Where are you? At home? On Earth? Orbiting the Sun? On one of the Milky Way’s spiral arms? All of these are true. As bodies like you move in the universe, they can only be located in relation to other bodies. Think about where you really are, as you drift through the universe with other planets, stars, and galaxies!
Source: https://www.blinkist.com/https://www.blinkist.com/de/books/eine-kurze-geschichte-der-zeit-de