The speed of light was first measured in 1676 by the Danish astronomer Ole Christensen Roemer. The speed of light was found to be very fast, but at a finite speed. However, scientists found a problem when they tried to say that light always travelled at the same speed. The scientists created a new idea, called the "ether", which tried to explain light's speed.
Albert Einstein said the idea of the ether was not needed if another idea, the idea of "absolute time" (or time that is always the same) was dropped. Einstein's idea was also the same as Henry Poincare's idea. Einstein's idea is called the theory of relativity.
Hawking also talks about light. He says that events can be described by "light cones". The top of the light cone tells where the light from the event will travel. The bottom tells where the light was in the past. The center of the light cone is the event. Besides light cones, Hawking also talks about how light can bend. When light goes past a big mass, like a star, the light changes direction slightly towards the mass.
After talking about light, Hawking talks about time in Einstein's theory of relativity. One prediction that Einstein's theory makes is that time will go by slower when something is near huge masses. However, when something is farther away from the mass, time will go by faster. Hawking used the idea of two twins living at different places to describe his idea. If one of the twins went to live on a mountain, and another twin went to live near the sea, the twin who went to live on the mountain would be a little bit older than the twin who went to live at the sea.
Hawking talks about the expanding universe. The universe is getting bigger over time. One of the things he uses to explain his idea is the "Doppler shift". The Doppler shift happens when something moves toward or away from another object. There are two types of things that happen in Doppler shift - "red shifting" and "blue shifting". Red shifting happens when something is moving away from us. This is caused by the wavelength of the visible light reaching us increasing, and the frequency decreasing, which shifts the visible light towards the red/infra-red end of the electromagnetic spectrum. Red-shift is linked to the belief that the universe is expanding as the wavelength of the light is increasing, almost as if stretched as planets and galaxies move away from us, which shares similarities to that of the Doppler effect, involving sound waves. Blue shifting happens when something is moving toward us, the opposite process of red-shift, in which the wavelength decreases and frequency increases, shifting the light towards the blue end of the spectrum. A scientist named Edwin Hubble found that many stars are red shifted and are moving away from us. Hawking uses the Doppler shift to explain that the universe is getting bigger. The beginning of the universe is thought to have happened through something called the "Big Bang". The Big Bang was a very big explosion that created the universe.
The uncertainty principle says that the speed and the position of a particle cannot be found at the same time. To find where a particle is, scientists shine light at the particle. If a high frequency light is used, the light can find the position more accurately but the particle's speed will be unknown (because the light will change the speed of the particle). If a lower frequency light is used, the light can find the speed more accurately but the particle's position will be unknown. The uncertainty principle disproved the idea of a theory that was deterministic, or something that would predict everything in the future.
How light behaves is also talked more about in this chapter. Some theories say that light acts like particles even though it really is made of waves; one theory that says this is Planck's quantum hypothesis. A different theory also says that light waves also act like particles; a theory that says this is Heisenberg's uncertainty principle.
Light waves have "crests" and "troughs". The highest point of a wave is the crest, and the lowest part of the wave is a trough. Sometimes more than one of these waves can interfere with each other - the crests and the troughs line up. This is called "light interference". When light waves interfere with each other, this can make many colors. An example of this is the colors in soap bubbles.
Quarks are very small things that make up everything we see (matter). There are six different "flavors" of quarks: the up quark, down quark, strange quark, charmed quark, bottom quark, and top quark. Quarks also have three "colors": red, green, and blue. There are also anti-quarks, which are the opposite of the regular quarks. In total, there are 18 different types of regular quarks, and 18 different types of anti quarks. Quarks are known as the "building blocks of matter" because they are the smallest thing that make up all the matter in the universe.
All elementary particles (for example, the quarks) have something called "spin". The "spin" of a particle shows us what a particle looks like from different directions. For example, a particle of spin 0 looks the same from every direction. A particle of spin 1 looks different in every direction, unless the particle is spun completely around (360 degrees). Hawking's example of a particle of spin 1 is an arrow. A particle of spin two needs to be turned around halfway (or 180 degrees) to look the same. The example given in the book is of a double-headed arrow. There are two groups of particles in the universe: particles with a spin of 1/2, and particles with a spin of 0, 1, or 2. All of these particles follow "Pauli's exclusion principle". Pauli's exclusion principle says that particles cannot be in the same place or have the same speed. If Pauli's exclusion principle did not exist, then everything in the universe would look the same, like a roughly uniform and dense "soup".
Particles with a spin of 0, 1, or 2 move force from one particle to another. Some examples of these particles are "virtual gravitons" and "virtual photons". Virtual gravitons have a spin of 2 and they represent the force of gravity. This means that when gravity affects two things, gravitons move to and from the two things. Virtual photons have a spin of 1 and represent electromagnetic forces (or the force that holds atoms together).
Besides the force of gravity and the electromagnetic forces, there are weak and strong nuclear forces. "Weak nuclear forces" are the forces that cause radioactivity, or when matter emits energy. Weak nuclear force works on particles with a spin of 1/2. "Strong nuclear forces" are the forces that keep the quarks in a neutron and a proton together, and keeps the protons and neutrons together in an atom. The particle that carries the strong nuclear force is thought to be a gluon. The gluon is a particle with a spin of 1. The gluon holds together quarks to form protons and neutrons. However, the gluon only holds together quarks that are three different colors. This makes the end product have no color. This is called "confinement".
Some scientists have tried to make a theory that combines the electromagnetic force, the weak nuclear force, and the strong nuclear force. This theory is called a "grand unified theory" (or a GUT). This theory tries to explain these forces in one big unified way or theory.