A Brief History of Time Book Summary - A Brief History of Time Book explained in key points

A Brief History of Time summary

Name: A Brief History of Time
Rating: 4.5 (408 reviews)
Author: Stephen Hawking

Stephen Hawking

From the Big Bang to Black Holes

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4.5 (408 ratings)

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What is A Brief History of Time about?

A Brief History of Time (1988) takes a look at both the history of scientific theory and the ideas that form our understanding of the universe today. From big bangs and black holes to the smallest particles in the universe, Hawking offers a clear overview of both the history of the universe and the complex science behind it, all presented in a way that even readers who are being introduced to these ideas for the first time will understand.

About the Author

Stephen Hawking, PhD, (1942-2018) was a theoretical physicist, cosmologist and author best known for his work exploring Hawking radiation and Penrose-Hawking theorems. Serving as the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009, Hawking was the recipient of the Presidential Medal of Freedom, an Honorary Fellow at the Royal Society of Arts, and a lifetime member of the Pontifical Academy of Sciences.

Table of Contents

Key idea 1 of 13

Theories based on what you’ve seen in the past can help predict the future.

You’ve probably heard of the theory of gravity or the theory of relativity? But have you ever paused to think what we really mean when we talk about theories?

A theory, in its most basic terms, is a model that accurately explains large groups of observations. Scientists collect data from observations they see in, for example, experiments, and use it to develop explanations of how and why phenomena happen.

For example, Isaac Newton developed the theory of gravity after observing many phenomena, from apples falling from trees to the movements of planets. Using the data he collected he was able to describe gravity in a theory.

Theories have two great benefits:

First, they allow scientists to make definite predictions about future events.

For example, Newton’s theory of gravity allowed scientists to predict the future movements of objects like planets. If you want to know, say, where Mars will be six months from now, it’s possible to predict this precisely using the theory of gravity.

Second, theories are always disprovable, meaning they’re open to reform if new evidence that doesn’t fit the theory is found.

For example, people once believed in the theory that everything in the universe revolved around the Earth. Galileo disproved this theory when he noticed moons orbit Jupiter; he could therefore show that actually not everything orbit the Earth.

So in effect, a single future observation can always invalidate a theory, no matter how reliable it seems at the moment. This means theories can never be proven correct, and this makes science a constantly evolving process.

In the 1600s, Isaac Newton revolutionized the way we think about how objects move.

Before Isaac Newton, people thought an object’s natural state was at absolute rest. This means that if no force was acting on it, then the object would remain completely still.

In the 1600s, Newton thoroughly disproved this long-held belief. In its place, he introduced a theory which stated that all objects in the universe, instead of being still, were in fact in constant motion.

The fact that the speed of light is constant shows that you can’t always measure something’s speed relative to something else’s.

We have seen how Newton’s theory did away with absolute rest and replaced it with the idea that the movement of an object is relative to the movement of something else. Yet, the theory also suggested the speed of an object is relative.

For example, imagine you are reading a book while sitting on a train travelling at 100 mph. How fast are you travelling? Well, to a bystander watching the train speed past, you are travelling at 100 mph. But relative to the book you are reading, your speed is zero mph. So your speed is relative to another object.

The theory of relativity states that time itself is not fixed.

The speed of light being constant was problematic for Newton’s theory, because it proved that speed wasn’t always relative. Therefore, scientists needed an updated model that took the speed of light into account.

Albert Einstein developed such a theory, the theory of relativity.

Since one can’t make exact measurements of particles, scientists use something called quantum state to make predictions.

All matter is made up of particles such as electrons or photons. In order to learn more about the universe, scientists want to measure them and study their speed.

However, particles do something very strange when you try to study them. Bizarrely, the more precisely you try to measure the position of a particle, the more uncertain its speed becomes; and the more exactly its speed is measured, the less certain its position becomes! This phenomenon, first discovered in the 1920s, is called the uncertainty principle.

Gravity is the result of massive objects curving the universe.

When you view the world around you, you are seeing it in three dimensions, i.e., you can describe any object by its height, width and depth. Yet there is also a fourth dimension, although we ourselves cannot see it: it is time, and it combines with the other three dimensions to form something called space-time.

Scientists use this four-dimensional model of space-time to describe events in the universe. An event is something that occurs at a particular position in space and time. So when calculating an event’s position along with the three-dimensional coordinates, scientists add a fourth coordinate to indicate time.

When a star with a very high mass dies, it collapses into a singularity called a black hole.

During their lifetimes, stars need enormous amounts of energy to produce heat and light. Yet, this energy doesn’t last forever; eventually it runs out, leaving the star to die.

What happens to a star when it dies depends on its size. When a very large star runs out of energy, something spectacular is created: a black hole.

Black holes emit radiation, which can lead to their demise through evaporation.

If the gravitational pull of a black hole is so strong that not even light can escape it, then you’d think nothing else could escape either.

But you’d be wrong. In fact, black holes must release something; otherwise they’d break the second law of thermodynamics.

Although we can’t be sure, there are strong indicators that suggest that time can only move forwards.

Imagine a scenario where the universe began to contract and time started running backward. What would that be like? Perhaps clocks would run backward and the course of history would reverse. Scientists haven’t completely ruled it out, but there are three strong indicators that suggest time only moves forward.

The first indicator showing that the passage of time goes from past to future is the thermodynamic arrow of time. According to the second law of thermodynamics, entropy – the disorder of a closed system – tends to increase with time. This means that time can be measured by the tendency of disorder to increase.

In addition to gravity, there are three fundamental forces in the universe.

What kinds of forces are at work in the universe?

Most people will have heard about only one: gravity, the force that attracts objects to one another and which is experienced in the way that Earth’s gravity pulls us to its surface.

Although scientists believe that the universe started with the big bang, they are unsure of exactly how this happened.

Most scientists believe that time began with the big bang – the moment when the universe went from an infinitely dense state to a rapidly expanding entity which is still growing today.

Scientists, however, don’t exactly know how this big bang occurred, although a number of theories have been proposed to explain how this huge expansion might have happened.

Physicists haven’t been able to unify general relativity and quantum physics.

In their desire to understand and describe the universe, scientists have developed two major theories. The first is general relativity, which concentrates on a very large phenomenon in the universe: gravity. The second is quantum physics, which describes some of the smallest known objects in the universe: particles smaller than atoms.

While both theories provide great insights, there are big differences in what is predicted with the equations of quantum physics, and what is predicted and observed with general relativity. This means that currently there is no way of combining them together to make one complete unified theory of everything.

Final Summary

The main message in these blinks:

Many people are put off physics because they see it as an impenetrable world of lengthy equations and complex theories. And, to a certain extent, this is true. But the complexity of physics shouldn’t stop us non-experts from learning how and why the universe works.

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Who should read A Brief History of Time

Anyone who wonders how the universe began
Anyone who wonders what quantum mechanics is
Anyone interested how black holes work

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A Brief History of Time summary

Listen to the first key idea

What is A Brief History of Time about?

About the Author

Theories based on what you’ve seen in the past can help predict the future.

In the 1600s, Isaac Newton revolutionized the way we think about how objects move.

The fact that the speed of light is constant shows that you can’t always measure something’s speed relative to something else’s.

The theory of relativity states that time itself is not fixed.

Since one can’t make exact measurements of particles, scientists use something called quantum state to make predictions.

Gravity is the result of massive objects curving the universe.

When a star with a very high mass dies, it collapses into a singularity called a black hole.

Black holes emit radiation, which can lead to their demise through evaporation.

Although we can’t be sure, there are strong indicators that suggest that time can only move forwards.

In addition to gravity, there are three fundamental forces in the universe.

Although scientists believe that the universe started with the big bang, they are unsure of exactly how this happened.

Physicists haven’t been able to unify general relativity and quantum physics.

Final Summary

Key ideas in A Brief History of Time

Theories based on what you’ve seen in the past can help predict the future.

In the 1600s, Isaac Newton revolutionized the way we think about how objects move.

The fact that the speed of light is constant shows that you can’t always measure something’s speed relative to something else’s.

The theory of relativity states that time itself is not fixed.

Since one can’t make exact measurements of particles, scientists use something called quantum state to make predictions.

Gravity is the result of massive objects curving the universe.

When a star with a very high mass dies, it collapses into a singularity called a black hole.

Black holes emit radiation, which can lead to their demise through evaporation.

Although we can’t be sure, there are strong indicators that suggest that time can only move forwards.

In addition to gravity, there are three fundamental forces in the universe.

Although scientists believe that the universe started with the big bang, they are unsure of exactly how this happened.

Physicists haven’t been able to unify general relativity and quantum physics.

Final Summary

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