Time and Cosmology
The illimitable, silent, never-resting thing called Time, rolling, rushing on, swift, silent, like an all embracing ocean-tide, on which we and all the Universe swim like exhalations. [Thomas Carlyle]
Throughout history, Man has tried to explain the universe: why things occur as they do, how it all began, and how it will end. Ancient people had myths to explain their observations of the universe. Some beliefs were that the sun moved across the sky because Apollo pulled it along behind his chariot, the Earth was created by God in six days, and the whole world rested on the back of a very large turtle. In modern times, science is used to explain the mysteries of the universe. One of the more recent theories is that there are four dimensions: up-down, back-forth, in-out, and time. These four dimensions are known as spacetime.
The phenomenon of gravity is described as a curvature of spacetime. A massive object, such as the sun, would cause spacetime to be curved near it. It has been shown by experiments that light rays passing close to the surface of the sun are curved. When gravity is explained in terms of spacetime, it is no longer a mysterious force, but rather a natural property of four-dimensional geometry.
Time was believed in the past to be constant, but now it is seen that it is not. Time is relative to the inertial frame of reference it is being observed from. Although two events may occur simultaneously from one perspective, from a frame of reference moving relative to the first, the events may not be simultaneous.
Time dilation was a concept introduced by Albert Einstein. Time dilation occurs when an object is moving at a high velocity. Time passes more slowly for that object than it does from another frame of reference which is "at rest." Muons moving at high velocities have their internal clocks slowed down, and they have a longer lifetime than what would be expected at slower velocities. To understand the concept of time dilation, one could picture two scientists, one who is at rest relative to the surface of the earth, and one who is on a train moving relative to the surface of the earth. Each scientist has in their possession a light-clock. It consists of two parallel mirrors one meter apart. A photon is reflected back and forth between the mirrors. Since the velocity of light is known to be constant, the light-clocks are accurate. The units of time for these clocks are light-meters: the amount of time it takes a photon to travel from one mirror to the other. Each scientist observes that the photon in their clock moves straight up and down. However, the scientist at rest relative to the earth sees the scientist on the train pass by, and observes that that scientist's photon moves in a zig-zag pattern, not straight up and down. They observe the photon on the train to be travelling a longer distance in a unit of time than their own photon. Using the simple physics equation v = d/t, the scientist on the ground calculates that the clock on the train is running slower than their own clock, although the scientist on the train experiences time passing by the same way in their inertial frame of reference as the scientist on the ground experiences.
Time dilation leads to the concept known as the "twin paradox." In theory, if one twin remained on earth while the other twin travelled through space on a very fast rocket ship, the travelling twin would return to find they were younger than their sibling. This has been tested by placing cesium beam clocks on commercial airplane flights. A cesium beam clock was left at the Naval Observatory for reference, while other cesium beam clocks were sent travelling east and west. It was found that the eastward clock was nanoseconds slow, while the westward clock was nanoseconds fast.
One cannot help but wonder what would happen if an object were to move faster than the speed of light. However, this is not possible, as explained by a thought experiment. An astronaut accelerates away from the Earth in a spaceship with an infinite fuel supply. Once the astronaut reaches a velocity of seventy-five percent of the speed of light, scientists observing from the Earth notice that the clocks on the spaceship are running slower than the ones on Earth. As the astronaut gets even closer to the speed of light, the scientists observe the clocks running slower and slower. When the astronaut reaches the speed of light, the scientists observe that the clocks, and therefore time on the spaceship, have stopped completely. If the engines of the spaceship are considered to be clocks, then the rate that they burn fuel stops at the speed of light. Since the engines stop at the speed of light, they can never consume the last little bit of fuel that they would require to go beyond the speed of light. Therefore, nothing can travel faster than the speed of light.
Gravitational redshift occurs because clocks close to a massive object run slower than clocks far from a massive object. If atoms that radiate light are thought of as clocks, the frequency of their light in the presence of gravity will be lowered. Their spectrum will be redshifted.
The arrows of time are: the Thermodynamic Arrow of Time, where entropy, the level of disorder, is increased; the Cosmological Arrow of Time, which has to do with the expansion of the universe; and a proposed Information Arrow of Time, which has to do with increasing order in a system.
Scientists are still considering the role that time plays in the universe. Although it is hard for the human mind to picture four dimensions, the concept of time helps to explain phenomena such as gravity. As time progresses, scientists will increase their knowledge of time, thus increasing their knowledge of the universe.
Carlyle, Thomas in Dictionary of Quotations. London: Bloomsbury Books, 1994.
Charon, Jean. Cosmology. New York: McGraw-Hill Book Company, 1970.
"Cosmology (astronomy)," Groiler Multimedia Encyclopedia (CD ROM, 1993 ed.)
"Einstein, Albert," Groiler Multimedia Encyclopedia (CD ROM, 1993 ed.)
"Experimental Proof of Relativity," Eyewitness Encyclopedia of Science (CD ROM, 1994 ed.)
Kaufmann, William J. III. Relativity and Cosmology. New York: Harper & Row, 1977.
Kopczynski, W., and A. Trautman. Spacetime and Gravitation. United Kingdom: John Wiley & Sons Ltd., 1992.
Martin, J. L. General Relativity: A Guide to its Consequences for Gravity and Cosmology. England: Ellis Horwood Limited, 1988.
Questions and Answers (From "Ask the Astronomer," Internet.)
"Relativity," Groiler Multimedia Encyclopedia (CD ROM, 1993 ed.)
Salmon, Wesley C. Space, Time & Motion: A Philosophical Introduction. California: Dickenson Publishing Co., Inc., 1975.
"Spacetime," Dictionary of Astronomy (from Redshift CD ROM, 1994 ed.)
"Special Theory," Eyewitness Encyclopedia of Science (CD ROM, 1994 ed.)
"Time (concept)," Groiler Multimedia Encyclopedia (CD ROM, 1993 ed.)
"Time Reversal Invariance," Groiler Multimedia Encyclopedia (CD ROM, 1993 ed.)
Zeilik, Michael, Stephen A. Gregory, and Elske V. P. Smith. Introductory Astronomy & Astrophysics. Third edition. USA: Saunders College Publishing, 1992.
Related books for sale on Amazon.com:
Dictionary of Quotations
Cosmology by Jean E. Charon
Grolier Multimedia Encyclopedia (CD-ROM)
Eyewitness Encyclopedia of Science (CD-ROM)
Relativity and Cosmology by William J. Kaufman III
Spacetime and Gravitation by Wojciech Kopczynski and Andrzej Trautman
General Relativity - A Guide to its Consequences for Gravity and Cosmology by J. L. Martin
Space, Time, and Motion: A Philosophical Introduction by Wesley C. Salmon
Introductory Astronomy and Astrophysics by Stephen A. Gregory and Michael Zeilik
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