WHAT IS TIME? The fifth century Christian theologian Augustine wrote: What then is time? I know well enough what it is, provided that nobody asks me; but if I am asked what it is and try to explain, I am baffled. The passage of fifteen hundred years has, in fact, done little to clarify the real meaning of time. Throughout history, human beings have always puzzled over time's profound but seemingly vague nature. The subject of time has fascinated poets, writers, and philosophers of every generation. Modern science has given us a way of incorporating the concept of time into measurements of physical quantities. We know that time is a measure of change. We know that change involves the expenditure of energy. By the second law of thermodynamics, energy expenditure produces unwanted entropy. Thus time gets directly linked to the tendency of physical systems to disorder. Time involves irreversibility. SATURN DEVOURING HIS CHILDREN (1824) by the Spanish artist Francisco de Goya. http://cosmology.uwinnipeg.ca/cosmology/time.htm (1 of 9) [03/25/2002 1:43:05 PM]
Saturn was the roman name for Cronus, the god associated in late Greek mythology with time. He swallowed his children after being told that one of them would dethrone him. Only Zeus, who later did dethrone him, escaped. The painting symbolizes how time ravages and destroys all humans. Unlike Zeus, mere mortals cannot evade his clutches. The irreversibility of time does not hold on small scales. None of the major theories in physics seem to preferentially pick out a certain direction in time. On small scales processes are genuinely reversible. On large macroscopic scales, processes are unquestionably irreversible. How this switch over, from irreversibility to reversibility, occurs is a fundamental mystery. The irreversible direction of time was not always accepted by ancient civilizations. The cyclic character of time was a common feature in Greek cosmological thought. Aristotle wrote that 'there is a circle in all other things that have a natural movement and coming into being and passing away. This is because all other things are discriminated by time and end and begin as though conforming to a cycle; for even time itself is thought to be a circle.' The Stoics believed that when the planets returned to the same relative positions, as at the beginning of time, the cosmos would be renewed again and again. http://cosmology.uwinnipeg.ca/cosmology/time.htm (2 of 9) [03/25/2002 1:43:05 PM]
The reason for the acceptance of cyclic-time ideas was because time's arrow, a unique direction of time, aroused deep fear, even terror, amongst the thoughtful of ancient civilizations. A time direction implied instability, flux, and change. It was associated with the end of the world and not associated with rebirth and renewal. It was the Judeo-Christian tradition that imposed a 'linear' irreversible time in Western culture. Through the Christian belief in the birth and death of Christ and the Crucifixion as unique events, a direction of time was accepted. These events were assumed to be unrepeatable. Western civilization came to regard time as a linear path that stretches between past and future. Before the Christians, only the Hebrews and the Zoroastrian Persians had adopted this progressive view of time. Scientists became involved with the concept of time when Newton (1687) felt compelled to define time as being necessarily absolute. This meant that all events could be regarded as having a distinct and definite position in space and occur at a particular moment of time. This moment of time was taken to be the same for observers everywhere in the universe. Time was an unstretchable quantity, in terms of which, change in the whole universe could be uniquely described. Newton constructed a deterministic set of mathematical relations that allowed prediction of the future behavior of moving objects and allowed deduction of the past behavior of the moving objects. All that one needed in order to do this was data in the present regarding these moving objects. Einstein changed these ideas totally. In his special theory of relativity and his general relativity theory time became a relative concept. This meant that time varied from place to place. Time was stretchable. In special relativity time became stretched when you moved with any speed. The effect is biggest when you move at speeds close to one billion kilometers per hour. In general relativity time became stretched when you placed yourself into a gravitational field. This effect is biggest when the gravitational field is strong. In relativity we can draw diagrams, called spacetime diagrams, of the paths people take as they go forward in time. The path is called the world-line of the individual. World-line of a person walking to the right, stopping, and then walking back to the left. http://cosmology.uwinnipeg.ca/cosmology/time.htm (3 of 9) [03/25/2002 1:43:05 PM]
Note that the vertical displacement of the individual is the time elapsed since she started the journey. The individual cannot move in any way such that the vertical direction is traversed in the negative (down) direction. That direction would correspond to going backwards in time. On spacetime diagrams such as the one above, light rays will travel along paths always at 45 degree angles to the horizontal and vertical lines. Nothing can travel faster than these light paths. All objects having mass must move slower that the light rays. Consequently the motion of everyday objects falls within the 45 degree angled lines. These 45 degree lines form what is called the light cone. Paths outside the light cone are forbidden. They represent object paths corresponding to speeds greater that the speed of light. http://cosmology.uwinnipeg.ca/cosmology/time.htm (4 of 9) [03/25/2002 1:43:05 PM]
This is a diagram for the world-line of a particle that travels at less than the speed of light. The place where the two forbidden triangles meet is called the present. The future is the triangular shaped region at the top of the diagram. The past is the triangular shaped diagram at the bottom of the diagram. At no time can the particle divert into the forbidden regions (red triangles) without going faster than the speed of light. Every point along the world-line of the particle must never be greater than 45 degrees. To go backwards in time however this is exactly what the observer would have to do. Hence, special relativity predicts that you can only go backwards in time if you travel faster than the speed of light. http://cosmology.uwinnipeg.ca/cosmology/time.htm (5 of 9) [03/25/2002 1:43:05 PM]
The effect of gravity on light cones is to tip them in the direction of the gravitational curvature. Black holes tip the light cones of nearby particles so much that the future part of the light cones points directly into the black hole. In this case, no matter how the particle moves, its future lies trapped inside the black hole. Both the general and the special theory of relativity give a method for time travel into the future. In special relativity all one has to do is to leave the Earth behind and travel in a super fast space ship. Your time intervals will dilate, making your clocks run very slow compared with the clocks that remain fixed to the Earth's surface. When you return to the Earth, you will have aged a small fraction of the time that people who remained on the Earth have aged. In effect you have traveled to the future of the Earth in your fast 'time-machine' spaceship. In general relativity you leave the Earth and encircle a dense object such as a neutron star for a small amount of time. To increase the amount you travel into the future, you orbit closer to the neutron star. The gravitational field dilates http://cosmology.uwinnipeg.ca/cosmology/time.htm (6 of 9) [03/25/2002 1:43:05 PM]
your time intervals. When you finally leave the vicinity of the neutron star and return to the Earth, all inhabitants of the Earth will have aged much more than you have. You have effectively traveled to the future using a strong gravitational field to carry out the time travel. For short mpeg movies simulating trips to neutron stars and black holes click here. The laws of physics do not preclude time travel. They seem to work irrespective of the direction of time. There are, however, two logical objections to travel into the past in the same universe. They are as follows: 'Grandfather Paradox'- You cannot travel to the past and disrupt some event in such a way that you never existed in the first place. This is called the grandfather paradox because you can think of going backwards in time and killing your own grandfather. This is problematic, since if you really did kill your own grandfather, you shouldn't exist at all. 'No Free Lunch' Paradox'- Within the same universe time travelers should not be able to bring information and energy to the past that could be used to create new ideas and products. This would involve no creative energy on the part of the "inventor." Example: Imagine that the famous artist, Pablo Picasso, had traveled back in time to meet his younger self. He could give his younger self his portfolio containing copies of his art. If the young version of Picasso meticulously copied the reproductions, he could have affected the future of 20th century art at a very early age. His reproductions would exist because they were copied from the originals, and the originals would exist because they were copied from the reproductions. The time-traveling artist would have created masterpieces without ever having expended any energy in their creation. It is possible to construct time machines in general relativity that take you back into the past. These machines usually involve rapidly rotating dense objects which are dragging spacetime around with them. An example is the rotating massive cylinder shown below. You travel forwards in time if you go around the massive rotating cylinder in its direction of rotation. You travel backwards in time if you go around the cylinder in the opposite direction that it is rotating. http://cosmology.uwinnipeg.ca/cosmology/time.htm (7 of 9) [03/25/2002 1:43:05 PM]
Alternatively you can time travel using wormholes that connect our universe with itself at a earlier time (or some other universe that is a copy of our universe at a earlier time). To avoid the two logical problems given above, it is mandatory to allow for the existence of multiple worlds. It turns out that the theory of physics, called quantum mechanics, that governs all physical phenomena on the smallest scales can be interpreted literally as a theory of parallel universes. Hence, quantum theory is quite consistent with the idea that time travel can exist without logical difficulties. How is time viewed in a parallel universe scenario? Each moment of time is seen as a distinct universe recreated from the previous universe such that the rules of existence (the laws of thermodynamics) are obeyed. Below is a diagram of a stack of 2-D universes. Time proceeds from the bottom of the stack to the top of the stack. Time is then seen as just a universe label: Time 1 = Universe #1, Time 2 = Universe #2, Time 3 = Universe #3, etc. http://cosmology.uwinnipeg.ca/cosmology/time.htm (8 of 9) [03/25/2002 1:43:05 PM]
The recreation of the universe at any instant is done so rapidly that no experiment can pick up any part of the universe's recreation at that instant. If you have two or more adjacent stacks of universes then it is possible for time travel to be self-consistent. The time-traveler has to hop from a certain time in the left stack of universes to a different time in the right stack of copy universes. Click here for some resources on time travel: NOVA Time Travel http://cosmology.uwinnipeg.ca/cosmology/time.htm (9 of 9) [03/25/2002 1:43:05 PM]