There was a young lady named
bright
Who traveled much
faster than light
She started one day in a relative way
and returned on the previous
night.
- A. H. Reginald Buller
and its possibility have been an intriguing concept for many. It has been a theme in several science fiction stories and films. Below is a brief excerpt explaining the relationship of space/time based upon Einstein's Theory of Relativity. (a good teacher helper)
QUANTUM STRANGENESS
AND SPACETIME
by Sherrill Roberts
The scientific premises suggesting a theoretical time travel mechanism are Einstein's Theory of Relativity and its successor, quantum mechanics. Einstein's inclusion of time as simply another basic dimension of physical reality, like width and height, and his mathematical equations using the speed of light as a cosmic "speed limit," paved the way for quantum mechanics' description of the physical universe in terms of black holes, singularities, and "cosmic strings," concepts which at times defy "rationality." MIT Professor Alan Guth has given us a concise summary of the Theory of Relativity: "Space tells matter how to move. Matter tells space how to curve."
If we envision the concept of spacetime as a bedsheet held at the four corners, we can immediately see these implications of Relativity if we place a tennis ball in the center of the sheet; the flat sheet of spacetime is distorted into a curve with the ball at the center, matter telling space how to curve. If we place a second ball on the surface, the new ball rolls toward the indentation made by the first, curved space telling matter how to move. If we place a bowling ball in the center of our flat spacetime, the indentation will be very deep, possibly tearing a hole in the fabric of our spacetime, a black hole. If we view spacetime from beneath the flat sheet, we will see the bowling ball as a protruding shape, the black hole has emerged on the "other side of time" as a white hole or possibly a wormhole.
Keeping this scenario in mind, it becomes clear that what is needed for time travel is an object which is massive enough to create a significant distortion of spacetime, something larger and heavier than a ping-pong ball on the surface of our bedsheet.
Paradoxes inherent in time travel have provided inspiration for numerous science fiction tales. Assuming that a civilization has the technical capability to orbit black holes and move wormhole mouths, there is still the question of the time traveler's journey into the past and his possible influence on his own present existence. This issue has been called the "granny paradox,"so named because a time traveler in the past could cause the demise of his/her own grandmother and would cease to exist in the present. One attempt to resolve the granny paradox is Hugh Everett's "many-worlds" interpretation of quantum mechanics. Everett's hypothesis is that, at the quantum level, all possible states potentially exist and that a universe confronted with a choice brings both realities into being. Everett's theory is consistent with certain experimental findings that photons (light particles) exist simultaneously as particles and as waves, so the possibility of an infinite number of parallel universes is not as far-fetched as it may seem. Other speculations on the time-travel paradox hold that a person traveling back from the future would not be "allowed" by circumstances, to do anything which would jeopardize his or her future existence. Given the massive distortions of spacetime involved in time travel, a person would need to think very carefully about the possibility of returning to find all his friends long dead, his apartment rented, and his job nonexistent. Perhaps Stephen Hawking is correct in assuming that the laws of quantum mechanics preclude time travel, as, "we have not been invaded by hordes of tourists from the future."
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