# A Briefer History of Time Report

At the end of the first chapter, Hawking ends with a thought provoking statement; “Someday these answers may seem as obvious to us as the Earth orbiting the sun – or perhaps as ridiculous as a tower of

turtles. Only time (whatever that may be) will tell.” He illustrates the elusive ambiguity of science – what we are determined to prove true today, we may laugh at tomorrow.

In the first two chapters, Stephen Hawking describes what we live in to be a “strange and wonderful universe,” introducing his topic with contrast to the old fashioned theory of the flat world resting upon a tower of turtles, quoted earlier. What can we really prove, and how can we determine if the theories we have today are legitimate or if they are as ridiculous as a pile of turtles? He brings up and then disproves various past ideas of the world and the galaxy, like the world being flat; if the world was flat, the Greeks had reasoned, you would see a ship to appear first as a dot and then, as it got closer, you would gradually be able to make out more detail. However, the first thing you see are the ship’s sails, and then later you see it’s hull, and “the fact that a ship’s masts, rising high above the hull, are the first part if the ship to poke up horizon is evidence that the Earth is a ball.” He also introduces Newton’s laws, his ideas of gravity, and the elliptical orbits of the planets.

In the third chapter, Hawking defines the nature of a scientific theory, and the process of abandoning or modifying these theories. He gives us a brief understanding of the general theory of relativity and quantum mechanics, while explaining that the two theories are inconsistent with each other; they cannot both be correct. The fourth chapter, entitled “Newton’s Universe,” talks about how gravity is proportional to mass. Although an object with twice the weight will have twice as much gravity pulling it down, it will also have twice the mass, thus it will only have half the acceleration per unit force. These two effects cancel each other out; therefore, every object has the same acceleration when falling. Another idea introduced in this chapter was the ambiguity of time and space. Hawking uses the example that if a person were on a train, bouncing a ping-pong ball, it would appear to them as if it was going straight up and down, whereas someone standing beside the track would see the two bounces as forty meters apart, because the train would have raveled that far down the track between the bounces.

In the fifth chapter, the fact that light travels at a very high yet finite speed is explained. Danish astronomer Ole Christensen Roemer observed that the eclipses of Jupiter’s moons were not evenly spaced. If Jupiter remained the same distance from the Earth at all times, the delay would be uniform for every eclipse. Because light has a farther distance to travel when Jupiter moves away from the earth, the light we would normally see is late. In addition, when Jupiter is closer to the earth, and the “signal” from each eclipse has les distance to travel, it arrives earlier. Ether is defined as a substance present everywhere, even in the vacuum of “empty” space. It is believed that light waves go through ether as sound waves do through air, and that, although different observers could see light coming toward them at different speeds, the light’s speed relative to the ether remains fixed. Hawking explains that “we must accept that time is not completely separate from and independent of space but is combined with it to form an object called space-time.” It is understood that position is relative; it is nothing unless compared to something else.

In chapter six, a geodesic is explained as the shortest (or longest) distance between two points. The shortest distance between two points on the globe is along a great circle, or a circle around the globe (one of the largest circles you can draw on the globe) whose center coincides with the center of the earth. Therefore, although a straight line would be considered the shortest distance between two points, a curved geodesic is actually the shortest distance between two points on the globe.

Some interesting topics in this chapter are the predictions of general relativity. General relativity predicts that gravitational fields should bend light. This means that the light from a distant star that passes near the sun would be deflected, appearing in a different position to someone on the Earth. Another prediction of general relativity is that time should appear to run slower near a massive body, such as the Earth. This prediction was tested in 1962 with a pair of very accurate clocks, mounted to the top and bottom of a water tower. The clock at the bottom was found to run more slowly, in exact agreement with general relativity. Our biological clocks are equally affected by this change in the flow of time. Throughout this chapter, the idea of absolute time was disproved.

Chapter seven contains another discovery about the universe, that it is the same in every direction. It also introduces the method of parallax, using the change in relative position to plot locations. It was not possible to use this method for Edwin Hubble because the distances he was trying to find appeared fixed, because they were too far away. Instead, he catalogued the brightness of each star, dependant on the luminosity. The same types of stars have the same luminosity, and the types and luminosities of nearby stars could be determined, so Hubble was able to calculate the distance to that galaxy, eventually working out nine different galaxies. After this, he spent his time tracking where the galaxies were moving to, as most people expected them to be moving around quite randomly. He was surprised to find that every galaxy was moving away from us. This means that the universe could not be static or unchanging in size, rather it is expanding. This can be described by the expanding balloon model:
“The situation is rather like a balloon with a number of spots painted on it being steadily blown up. As the balloon expands, the distance between any two spots increases, but there is no spot that can be said to be the center of the expansion.

Moreover, as the radius of the balloon steadily increases, the farther apart the spots on the balloon, the faster they will be moving apart. For example, suppose the radius of the balloon doubles in one second. Two spots that were previously one centimeter apart will now e two centimeters apart (as measured along the surface of the balloon.)”
In chapter eight, the big bang is explained, using reference to extreme temperatures, like one second after the big bang, when “the universe would have expanded enough to bring its temperature down to about ten billion degrees Celsius. This is about a thousand times the temperature at the center of the sun.” In addition, a black hole is explained as a collapsing star with a gravitational field so strong that light cannot escape. Therefore, these “black holes” are just black voids in space, stars massive enough where the escape velocity is higher than the speed of light. According to the theory of relativity, nothing can travel faster than light. Therefore, if light is not able to escape, how could anything else be able to? Black holes are very common – one satellite discovered fifteen hundred black holes in just one small area of the sky. If an astronaut was on the surface of a collapsing star, the change in gravity between his feet and the one or two meters up to his head would literally “stretch him out like spaghetti or tear him apart before the star had contracted to the critical radius at which the event horizon formed.” Sometimes, when a very massive star collapses, parts of the star may be blown off in an explosion called a supernova. A supernova explosion is so gigantic that it can radiate more light than all the other stars in its galaxy combined.

In chapter nine, quantum mechanics are used to describe the unavoidable element of unpredictability or randomness into science. The particles in this uncertainty behavior behave in some respects like waves. They do not have a definite position but are “smeared out” with a probability distribution. A nice way of visualizing this wave/particle duality is the “sum over histories” introduced by Feynman. Instead of a particle moving in one single path, it is supposed to go from point A to point B by every possible path. With each path between point A and point B, Feynman associated a couple of numbers – one represents the size of a wave, the other represents the position in the cycle. Quantum theory has been an outstandingly successful theory and underlies almost all of modern science and technology.

Chapter ten, titled “Wormholes and Time Travel,” touches on the science-fiction aspects of true science. It is stated that traveling to the future is possible, if one accelerates to the speed of light. The first indication that traveling to the past might be possible was when Gödel discovered a new solution to Einstein’s equations; that is, “a new space-time allowed by the theory of general relativity.” His space-time had the curious property that the universe was rotating. There is a problem, however, with breaking the speed-of-light barrier. The theory of relativity states that the rocket power needed to accelerate a spaceship gets greater and greater the nearer it gets to the speed of light. Particles so far can be accelerated up to 99.99% of the speed of light, but they cannot get them beyond the speed-of-light barrier. There is a possible way out of this predicament – a wormhole, which is a think tube of space-time that connects two nearly flat regions far apart.

In chapter eleven, force-carrying particles are grouped into four different categories, the first being the gravitational force. This force is universal, and every particle feels the force of gravity according to its mass or energy. The second is electromagnetic force, which interacts with electrically charged particles, and is much stronger than the gravitational force. The third category is called the weak nuclear force. We do not come in direct contact with this force, however it is responsible for radioactivity – the decay of atomic nuclei. The last category, and the strongest of all forces, is the strong nuclear force. This is another force with which we don’t have direct contact, yet it is responsible for holding most of our everyday world together. Without the strong force, the electric repulsion between the positively charged protons would blow apart every atomic nucleus in the universe (except those of hydrogen gas.) This chapter also touches on the string theory. In string theories, the basic objects are things that have length but no other dimension, and they either have ends (open strings) or they may be joined up with themselves in closed loops (closed strings.) String theories lead to infinities, but it is thought that in the right version they will all cancel out (though this is not known for certain.) Also, these theories seem only to be consistent if space-time has either ten or twenty-six dimensions instead of the usual four. There are many problems with more than three space dimensions. In four dimensions, the gravitational force would drop to 1/8th, in five to 1/16th, and so on. The orbits of planets around the sun would be unstable. People have searched for the underlying theory, but so far, it has been without success. In Newton’s time, it was possible for an educated person to have a grasp of the whole human knowledge; today that clearly is not true. The goal of science is to get a complete understanding of the events around us, and of our own existence.