Time: a phenomenon that is involved in all observable changes.

Changes appear to us as a movement in the present from the past into the future. The past is the realm of facts, fixed and immutable. The future, however, is the realm of possibilities. The movement in the present turns future into past, possibilities into facts. The direction in which the present moves is called the axis of time. Things not affected by the progression of the present, that is, things that remain unchanged along the axis of time are called ►eternal.

To us, time appears to flow from the future into the past because our consciousness is firmly attached to the present. Since the present moves along the axis of time (represented as an upward movement in the diagram), time in a sense flows toward us, thus enabling us to perceive changes. Thus, the wind of change does not at all blow into the face of time; rather, the wind of time blows into the the face of the present.

We can conceive of creatures whose consciousness is not as firmly rooted in the present, or even not rooted in it at all. Such creatures would perceive all changes in an object simultaneously, thus perceiving the object in a temporally extended manner. A tree, for example, would present itself to such creatures simultaneously in all the states of its existence from the seed to the dead stump. Though creatures with such time-independent consciousness would also be immortal, they would not profit very much from this, as time would not mean anything to them.

The flow of time as we know it is also a product of our manner of perception. ►Relativity theory considers time as a ►dimension that is equal to the three standard spatial dimensions except in one respect. With that, relativity theory is based on a four-dimensional space-time. Events are described by four coordinates in space-time: their three spatial positions and their temporal position. Since time and space are relative — each object having its own temporal and spatial coordinates — the concept of simultaneity is meaningless in physics. From the standpoint of one observer, one of two spatially separate events may take place earlier, while from the standpoint of another observer the other event is earlier. From the standpoint of a static observer, time would proceed more slowly in moving objects.

Since actually nothing in the universe is static, a movement in time is normally also a movement in space. In a single second,

the Earth's surface moves 460 meters (at the equator),
the Earth moves 30 kilometers (on its orbit around the sun),
the solar system moves 250 kilometers (on its orbit around the center of the Milky Way),
the Milky Way moves 600 kilometers (toward the ►Great Attractor).

Special relativity theory limits the speed of objects to the speed of light. In the above diagram the cones (see also ►event horizon) correspond to the possible orbits of objects moving at the speed of light, that is, of light particles. Material objects can move only on orbits lying within these cones. An orbit that transgresses the limits of a cone means a surpassing of the speed of light. Again, an orbit that looks like a closed loop represents a ►journey through time into our past and back to our present location.

Such time travel strikes us as impossible because it creates paradoxes such as the grandfather paradox (someone travels into the past and murders his own grandfather when the latter is still an infant). One reason for this paradox is that the laws of nature allow for complex interactions in our world such as the interactions between the molecules of a gas. This imposes on ►entropy a direction in time. In the 1940s the mathematician Kurt Gödel developed the model of an infinitely large, rotating world evenly filled with matter as a consistent solution to the field equations of general relativity theory. In this world the cones in the above diagram would be so distorted that objects could indeed move along closed loops in space-time. Time travel would thus be possible. However, such time travel would always involve a change of location, making it impossible to go back and kill one's grandfather.

In our own world, time travel would be theoretically conceivable in the form of journeys to ►parallel worlds or in the vicinity of ►singularities. For example, along an orbit around a ►black hole that rotates sufficiently fast, time might move backwards. But such fast-rotating black holes probably do not exist in the universe. In 1974, the notoriously theatrical physicist Frank Tipler proposed the construction of a time machine made of a cylinder-shaped arrangement of neutron stars orbiting one another at a high speed. A spiral-shaped orbit through the center of the cylinder would carry objects back in time. As it is not so easy to construct an object out of neutron stars, however, many physicists do not believe that a time machine could be created in this manner.

To measure time, we use objects that change along the axis of time in a specified way: clocks. Clocks can be human-made, such as the ►ten-thousand-year clock. But in physics we frequently also use natural effects such as the decay of ►atoms or the oscillation frequency of light waves to measure very short time periods. Psychology teaches that measured and perceived time often clearly differ. An eventful period is perceived as short, while uneventful periods seem to us long or even ►eternal. Paradoxically, in our memory the respective time periods will appear to us in precisely the opposite way.

Perceived time is not infinitely divisible. Whether two events are perceived as separate depends on the sense organ: Optical impressions must be 20 to 30 milliseconds apart in order to be perceived as separate events. Acoustic signals require only three milliseconds, and tactile stimuli require up to 40 milliseconds. Present perceptions extend over a period of approximately three seconds.

Furthermore, almost all living creatures (including some monads) possess some kind of biological inner clock, which usually adjusts to the day-night rhythm. Shifting this rhythm, for example due to a plane trip, may confuse this inner clock, leading to jet lag. If there no external signs of time whatsoever, as when people stay in bunkers or submarines, after a while a sleeping-waking rhythm of around 25 hours sets in. At present we do not understand why this natural rhythm of ours is slightly longer than the terrestrial day.

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