A Brief History of Time summary

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.