The time machine........????
Time travel is the concept of movement (such as by a human) between certain points in time, analogous to movement between different points in space, typically using a hypothetical device known as a time machine, in the form of a vehicle or of a portal connecting distant points in time. Time travel is a recognized concept in philosophy and fiction, but traveling to an arbitrary point in time has a very limited support in theoretical physics, and usually only connected with quantum mechanics or wormholes, also known as Einstein-Rosen bridges. In a more narrow sense, one-way time travel into the future via time dilation is a well-understood phenomenon within the frameworks of special relativity and general relativity, but advancing a large amount of time is not feasible with current technology.[1] The concept was touched upon in various earlier works of fiction, but was popularized by H. G. Wells' 1895 novel The Time Machine, which moved the concept of time travel into the public imagination, and it remains a popular subject in science fiction.
General relativity Edit
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, transversable wormholes, and Alcubierre drive.[22][23]:33–130 The theory of general relativity does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed.[24] These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[25] but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.[26][27]:150
Special spacetime geometries Edit
The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves", and hence time travel into the past. The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have.[19]:499 Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.
General relativity Edit
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, transversable wormholes, and Alcubierre drive.[22][23]:33–130 The theory of general relativity does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed.[24] These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[25] but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.[26][27]:150
Special spacetime geometries Edit
The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves", and hence time travel into the past. The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have.[19]:499 Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.
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