Book Summary: A Brief History of Time (Part 1)

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Unlock the secrets of the cosmos.

It’s hard to imagine a more arresting and thought-provoking sight than a starry night sky. Something about the twinkle of the cosmos compels us to pause and ponder the deepest mysteries of the universe.

A Brief History of Time will help illuminate these secrets by unlocking the laws which govern the universe. Written in accessible language, it will help even the non-scientifically minded to understand why the universe exists, how it started and what it will look like in the future.

You will also find out about strange phenomena; like black holes which suck everything (well, almost everything) toward them. What’s more, you’ll also discover the secrets of time itself; as these blinks provide the answers to questions like “how fast is time going?” and “how do we know it’s going forwards?”

It’s safe to say that after these blinks, you’ll never view the night sky in quite the same way again.


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.

Newton determined this through his discovery that the planets and stars in the universe were constantly moving in relation to each other. For example, the Earth is constantly orbiting the Sun and the entire solar system is rotating around the galaxy. Therefore, nothing is ever still.

To describe how all objects in the universe move, Newton developed three laws:

The first of Newton’s laws states that all objects will continue moving in a straight line if not acted on by another force. This was demonstrated in an experiment by Galileo in which he rolled balls down a slope. As gravity was the only force acting on the balls, they rolled in a straight line.

Newton’s second law states that an object will speed up at a rate proportional to the force acting on it. For example, a car with a more powerful engine will accelerate faster than one with a less powerful engine. This law also states that the greater the body’s mass, the less a force affects its motion. For example, if there are two cars with the same engine, the heavier car will take longer to accelerate.

Newton’s third law describes gravity. It states that all bodies in the universe attract other bodies with a force proportional to the mass of each object. This means that if you double the mass of one object, the force will be twice as great. If you double one object’s mass and triple the other, the force will be six times as great.


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.

Yet, one major hole developed in Newton’s theory: the speed of light. The speed of light is constant, not relative. It is always 186,000 miles per second. It doesn’t matter how fast something else is going, the speed of light remains the same.

For example, if that train were speeding towards a beam of light at 100 mph, the speed of light would be 186,000 miles per second. Yet if that train stopped at a red signal, the beam of light would still be 186,000 miles per second. It doesn’t matter who is viewing the light or how quickly they are traveling, its speed will always be the same.

This fact causes problems for Newton’s theory. How can the speed of something be constant regardless of the state of the observer? The answer was discovered in the early twentieth century when Albert Einstein postulated his theory of relativity.


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. The theory of relativity states that the laws of science are the same for all freely moving observers. This means that no matter what someone’s speed might be, they would observe the same speed of light. This might seem quite straightforward at first glance, but one of its central suggestions is actually very difficult for many to comprehend; it states that time is relative.

What this means is that because the speed of light doesn’t change for observers moving at different speeds, observers traveling relative to one another would actually measure different times for the same event. For example, say a flash of light is sent out to two observers: one is travelling toward the light while the other is traveling at a quicker speed in the opposite direction. For both observers, the speed of the light would be the same, even though they are traveling at relatively different speeds and going in different directions.

Unbelievably, this would mean that they each experience the flash event as if it happened at two different times. This is because time is determined by the distance something has traveled divided by its speed. The speed of light is the same for both observers, but as the distance is different, time is relative to each observer.

If both observers carried clocks to record when the pulse of light was emitted, these would confirm two different times for the same event. So who’s right? Neither observer; time is relative and unique to both observers’ perspectives!


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.

Because of the uncertainty principle, scientists had to use other ways of looking at particles, so they began to look at a particle’s quantum state instead. Quantum state combines many likely possible positions and speeds of a particle.

Since scientists cannot pinpoint a particle’s definite position or velocity, they look at the many likely positions particles might occupy and velocities they might have. As a particle moves about, scientists track all the likely places it could be and determine which of these is the most likely.

To help them determine this, scientists treat particles as if they are waves. The multitude of different positions that a particle can be in means that they appear like a series of continuous, oscillating waves. Imagine a piece of vibrating string. When it vibrates, the string will arc and dip through peaks and troughs. A particle also behaves like this, although its possible path is a series of such overlapping waves, all happening at once.

Looking at particles like this helps scientists figure out where a particle is most likely to be. The likeliest positions of the particle occur where the arcs and dips on the many waves correspond with each other, and the least likely positions are where they don’t. This is called interference, and it shows which positions and speeds are most probable for the particle wave’s path.


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.

Scientists have to take time into consideration when determining the position of an event because the theory of relativity states that time is relative. It is therefore an important factor in describing the nature of an event.

An amazing consequence of the combination of space and time is how it changed our conception of gravity.

Gravity is the result of massive objects curving space-time. A huge mass, like that of our sun, curves and actually alters space-time. Think of it like this: Imagine space-time to be a blanket stretched out and held in the air. If you place an object in the middle of the blanket, the blanket will curve and the object will sink a little. This is what massive objects do to space-time.

Other objects then follow these curves in space-time. This is because an object always takes the shortest journey between two points, which is a circular orbit around a larger object. You can see this if you look at that blanket again. If you put a large object like an orange on the blanket and then try to roll a smaller one – say, a marble – past it, the marble will follow the indentation made by the orange. Gravity works in the same way!



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