what is time?
There are many definitions for times, some of the short definitions follows
- what clocks measure (attr. to physicists Albert Einstein, Donald Ivey, and others)
- what prevents everything from happening at once (physicist John Wheeler and others)
- a linear continuum of instants (philosopher Adolf Grünbaum)
- a certain period during which something is done (Medical Dictionary)
- a continuum that lacks spatial dimensions (Encyclopaedia Britannica)
Definition of time in dictionaries.
- The indefinite continued progress of existence and events in the past, present and future regarded as a whole (Oxford Dictionary)
- The measured or measurable period during which an action, process or condition exists or continues (Webster’s Collegiate Dictionary)
- the continuous passage of existence in which events pass from a state of potentiality in the future, through the present, to a state of finality in the past (World English Dictionary)
- a continuous, measurable quantity in which events occur in a sequence proceeding from the past through the present to the future (Science Dictionary)
Arrow of time.
When you walk forward two steps, you can turn around and walk backward two steps, too, and get back where you started. Time is different. If you wait twenty-four hours to travel to tomorrow, there’s no way to turn around and travel back a day to where you started. This irreversibility is called the arrow of time.
Einstein's explanation of time.
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| Albert Einstein |
What is time? While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative, it can vary for different observers depending on your speed through space. To Einstein, time is the "fourth dimension." Space is described as a three-dimensional arena, which provides a traveler with coordinates such as length, width, and height —showing location. Time provides another coordinate, direction, although conventionally, it only moves forward.
Einstein's theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home.
Time travel.
Time travel moving between different points in time has been a popular topic for science fiction for decades.
Not all scientists believe that time travel is possible. Some even say that an attempt would be fatal to any human who chooses to undertake it.
Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel. Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the correct way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date
Physicists have been thinking about tunnels in time too, but we come at it from a different angle. We wonder if portals to the past or the future could ever be possible within the laws of nature. As it turns out, we think they are. What's more, we've even given them a name: wormholes. The truth is that wormholes are all around us, only they're too small to see. Wormholes are very tiny. They occur in nooks and crannies in space and time.
A wormhole is a theoretical 'tunnel' or shortcut, predicted by Einstein's theory of relativity, that links two places in space-time - visualized above as the contours of a 3-D map, where negative energy pulls space and time into the mouth of a tunnel, emerging in another universe. They remain only hypothetical, as obviously nobody has ever seen one, but have been used in films as conduits for time travel - in Stargate (1994), for example, involving gated tunnels between universes, and in Time Bandits (1981), where their locations are shown on a celestial map.
Nothing is flat or solid. If you look closely enough at anything you'll find holes and wrinkles in it. It's a basic physical principle, and it even applies to time. Even something as smooth as a pool ball has tiny crevices, wrinkles and voids. Now it's easy to show that this is true in the first three dimensions. But trust me, it's also true of the fourth dimension. There are tiny crevices, wrinkles and voids in time. Down at the smallest of scales, smaller even than molecules, smaller than atoms, we get to a place called the quantum foam. This is where wormholes exist. Tiny tunnels or shortcuts through space and time constantly form, disappear, and reform within this quantum world. And they actually link two separate places and two different times.
Infinite cylinder theory
Black holes theory
Cosmic string theory
Time machines
Travelling at light speed
Infinite cylinder theory
A Tipler Cylinder uses a massive and long cylinder spinning around its longitudinal axis. The rotation creates a frame-dragging effect and fields of closed time-like curves traversable in a way to achieve subluminal time travel to the past.
Civilizations with the technology to harness black holes might be better advised to leave wormholes alone and try the time-warp method suggested by U.S. astronomer Frank Tipler. He has a simple recipe for a time machine: First take a piece of material 10 time the mass of the Sun, squeeze it together and roll it into a long, thin, super-dense cylinder – a bit like a black hole that has passed through a spaghetti factory. Then spin the cylinder up to a few billion revolutions per minute and see what happens.
Tipler predicts that a ship following a carefully plotted spiral course around the cylinder would immediately find itself on a "closed, time-like curve." It would emerge thousands, even billions, of years from its starting point and possibly several galaxies away. There are problems, though. For the mathematics to work properly, Tipler’s cylinder has to be infinitely long. Also, odd things happen near the ends and you need to steer well clear of them in your timeship. However, if you make the device as long as you can, and stick to paths close to the middle of the cylinder
Physicists have been thinking about tunnels in time too, but we come at it from a different angle. We wonder if portals to the past or the future could ever be possible within the laws of nature. As it turns out, we think they are. What's more, we've even given them a name: wormholes. The truth is that wormholes are all around us, only they're too small to see. Wormholes are very tiny. They occur in nooks and crannies in space and time.
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| Travel through wormhole |
Nothing is flat or solid. If you look closely enough at anything you'll find holes and wrinkles in it. It's a basic physical principle, and it even applies to time. Even something as smooth as a pool ball has tiny crevices, wrinkles and voids. Now it's easy to show that this is true in the first three dimensions. But trust me, it's also true of the fourth dimension. There are tiny crevices, wrinkles and voids in time. Down at the smallest of scales, smaller even than molecules, smaller than atoms, we get to a place called the quantum foam. This is where wormholes exist. Tiny tunnels or shortcuts through space and time constantly form, disappear, and reform within this quantum world. And they actually link two separate places and two different times.
Time tarvel theories.
Black holes theory
Cosmic string theory
Time machines
Travelling at light speed
Infinite cylinder theory
A Tipler Cylinder uses a massive and long cylinder spinning around its longitudinal axis. The rotation creates a frame-dragging effect and fields of closed time-like curves traversable in a way to achieve subluminal time travel to the past.
Civilizations with the technology to harness black holes might be better advised to leave wormholes alone and try the time-warp method suggested by U.S. astronomer Frank Tipler. He has a simple recipe for a time machine: First take a piece of material 10 time the mass of the Sun, squeeze it together and roll it into a long, thin, super-dense cylinder – a bit like a black hole that has passed through a spaghetti factory. Then spin the cylinder up to a few billion revolutions per minute and see what happens.
Tipler predicts that a ship following a carefully plotted spiral course around the cylinder would immediately find itself on a "closed, time-like curve." It would emerge thousands, even billions, of years from its starting point and possibly several galaxies away. There are problems, though. For the mathematics to work properly, Tipler’s cylinder has to be infinitely long. Also, odd things happen near the ends and you need to steer well clear of them in your timeship. However, if you make the device as long as you can, and stick to paths close to the middle of the cylinder
Black hole theory
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.
possibility would be to move a ship rapidly around a black hole, or to artificially create that condition with a huge, rotating structure.
"Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time," physicist Stephen Hawking wrote in the Daily Mail in 2010.
"Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had."
However, he added, the crew would need to travel around the speed of light for this to work. Physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel pointed out another limitation if one used a machine: it might fall apartbefore being able to rotate that quickly.
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