A Brief History of Time

Trae Pina
A Brief History of Time
Stephen Hawking



The name of this book is A Brief History of Time by Stephen Hawking. This book explains many different concepts about space, time, laws of physics, and even many laws by Einstein including his general theory of relativity. In the first chapter, Hawking starts by telling the story of a professor who gave a lecture, in which he said that the earth orbits the sun. A woman said he was lying and that the earth is a flat plate supported on the backs of giant turtles. Now we all now this to be false, and that the earth truly a sphere and does orbit the sun. He then ends the chapter by saying that what we are trying to prove true today may seem as obvious as the earth orbiting the sun or as ridiculous as a tower of turtles. He finishes by saying that only time, whatever that may be, will tell.

In the second chapter: Our Evolving Picture of the Universe, Hawking talks about the past ideas of the world and space. One example from the book is how the Greeks that that the earth was flat and when a boat sailed closer, you would first see it as a small dot, then as it got closer you would be able to make out more detail. This is not the case though. You first see the sails, then the hull. This is proof that the earth is a sphere. He then goes on to talk about the orbit of the planets and how they are elliptical, not a perfect circle.

In the third chapter: The Nature of a Scientific Theory, Hawking explains a scientific theory and the abandoning or modifying of this theory. He then goes on to explain the general theory of relativity and quantum mechanics. He then says that these theories contradict each other and that they can’t both be true. In chapter 4: Newton’s Universe, Hawking explains how gravity has to do with mass. He says that an object with twice the weight will have twice the gravitational force pulling down on it, but that it will also have twice the mass, which means it will have half the acceleration. These two effects cancel each other out though, and that is why two objects, no matter what the size, will always have the same acceleration. Another idea introduced is space and time. One example is if a ping pong ball is bounce on a train it will have looked as if the ball had bounced up and down, but to someone on the side of the tracks, it will have looked as if the ball had bounced forty meters, because that’s how far the train would have traveled in-between bounces.

In chapter 5: Relativity, The fact that light travels at a finite speed and not infinite speed is explained. Danish astronomer Christensen Roemer observed that eclipses of Jupiter’s moons were not evenly spaced. This is due to the earth moving further away from Jupiter as the earth orbits the sun. Since light then has a farther distance to travel we would see it later than we normally would if we were closer. He goes on to talk about an ether which is a substance present every where even in “empty” space. It was believed that this is what light waves went through. He then goes on to say that time is not separate from space, but that we must think of them as one, space-time.

In chapter six: Curved Space, a geodesic is explained as the shortest or longest distance between two places. The shortest distance between two places on a globe is called a great circle, one of the biggest circles you can draw on a globe. Another topic in this chapter is the prediction of general relativity. This theory states that gravitational fields should bend light. This means that light passing the sun from a distant star could be bent to make it look like the star is in a different position than it truly is. This law also states that a clock should appear to run slower near a massive body such as the earth. An experiment was done with two clocks, one on a water tower and the other on the ground, and the experiment found the belief true. This all disapproved of absolute time.

In chapter 7: The Expanding Universe, it is said that the universe is the same in every direction. It also introduces the parallax using a change in relative position to plot a location. This method was helpful to Edwin Hubble because the distances he was trying to measure were to far away and appeared fixed. He instead kept the brightness of each star. The same types of stars had the same luminosity. After his plotting and mapping, he was surprised to find out that all other galaxies are moving away from us, and that the universe is expanding. In chapter eight: The Big Bang, Black Holes, and the Evolution of Physics, the big bang is explained ling how one second after the big bang it would have been ten billion degrees Celsius. A black whole is also explained as a collapsed star whose gravity is so strong that not even light can escape it. If an astronaut were on the surface of a collapsing star, the gravitational difference from his head to his toes would be so great he would be torn apart. It also talks about how supernovas are when stars blow away material.

In chapter 9: Quantum Gravity, quantum mechanics are used to describe the unavoidable element of unpredictability or randomness into science. A nice way of thinking about this is instead of a particle moving in one single path; it is supposed to go from point A to point B by every possible path. Quantum theory has been a great success, and underlies almost all of modern science and technology. Chapter ten: Wormhole and time travel, talks about how traveling to the future is possible if one reaches the speed of light. Travel to the past is also possible but there is a problem. The theory of relativity says that the rocket power needed to accelerate a spaceship gets greater and greater the nearer it gets to the speed of light. We have been able to get single particles to 99.9% the speed of light but no faster. One alternative though is a wormhole which connects two regions.

In chapter eleven: The Forces of Nature and the Unification of Physics, force carrying particles are grouped into four groups. The first being gravitational force this force is felt by every particle and is universal. The second is electromagnetic force which only interacts with electrically charged particles and is much stronger than gravitational force. The third is the weak nuclear force. We don’t come in direct contact with this force, but it is responsible for radioactivity. The forth power is the strongest of all the forces, and is called the strong nuclear force. We don’t come in direct contact with this force, but it holds most of the world together. Without this force, every atomic nucleus would be blown apart except for hydrogen. This chapter also briefly talks about the string theory.

One theory that is brought up throughout the whole book is Einstein’s general theory of relativity, which plays a major role in the development of other laws and theories. One other concept is space time and how they are not two separate things, but one thing known as space-time. Another is the string theory which is mentioned later in the book. In the string theory a basic object is something with length but no other dimensions. They also either have no ends or meet up with themselves.

I believe this book changed science by gathering up most if not all of what we believe or did believe about the universe in one book. It explains past beliefs, disapproves of them, and talks about our current beliefs. It also talks about how many theories contradict each other. It further explains why modern theories must be true, gives descriptive examples that mostly anyone could understand. This book sums up what is modern in physic, astronomy, and the worlds current beliefs. It also shows you how some theories are unbelievably accurate. This book also shows how far our understanding of the universe has come and gone in the past few centuries, and how we continue to think into the future.