Topic : Time Travel

Topic : Time Travel Welcome To Our Physics Workshop Presented By TESSERACT Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically with the use of a hypothetical device known as a time machine. Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction. The idea of a time machine was popularized by H. G. Wells' 1895 novel The Time Machine. It is uncertain if time travel to the past is physically possible. Forward time travel, outside the usual sense of the perception of time, is an extensively observed phenomenon and well-understood within the framework of special relativity and general relativity. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology. As for backward time travel, it is possible to find solutions in general relativity that allow for it, such as a rotating black hole. Traveling to an arbitrary point in spacetime has very limited support in theoretical physics, and is usually connected only with quantum mechanics or wormholes. HISTORY OF TIME TRAVEL Some ancient myths depict a character skipping forward in time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is surprised to learn when he returns to Earth that many ages have passed. The Buddhist Pali Canon mentions the relativity of time. The Payasi Sutta tells of one of the Buddha's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that time in the Heavens passes differently than on Earth. The Japanese tale of "Urashima Taro", first described in the Manyoshu tells of a young fisherman named Urashima-no-ko who visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is in ruins, and his family has died. In Jewish tradition, the 1st-century BC scholar Honi ha-M’agel is said to have fallen asleep and slept for seventy years. When waking up he returned home but found none of the people he knew, and no one believed his claims of who he was. Quantum mechanics of time travel Until recently, most studies on time travel are based upon classical general relativity. Coming up with a quantum version of time travel requires physicists to figure out the time evolution equations for density states in the presence of closed time like curves (CTC). Novikov had conjectured that once quantum mechanics is taken into account, self-consistent solutions always exist for all time machine configurations, and initial conditions. However, it has been noted such solutions are not unique in general, in violation of determinism, unitarity and linearity. The application of self-consistency to quantum mechanical time machines has taken two main routes. Novikov's rule applied to the density matrix itself gives the Deutsch prescription. Applied instead to the state vector, the same rule gives nonunitary physics with a dual description in terms of post-selection. TIME MACHINE The Time Machine is a science fiction novella by H. G. Wells, published in 1895 and written as a frame narrative. The work is generally credited with the popularization of the concept of time travel by using a vehicle or device to travel purposely and selectively forward or backward through time. The term "time machine", coined by Wells, is now almost universally used to refer to such a vehicle or device. The Time Machine has been adapted into two feature films of the same name, as well as two television versions and many comic book adaptations. It has also indirectly inspired many more works of fiction in many media productions. The Time Traveller had finally finished work on his time machine, and it rocketed him into the future. When the machine stops, in the year 802,701 AD, he finds himself in a paradisiacal world of small humanoid creatures called Eloi. They are frail and peaceful, and give him fruit to eat. He explores the area, but when he returns he finds that his time machine is gone. He decides that it has been put inside the pedestal of a nearby statue. He tries to pry it open but cannot. In the night, he begins to catch glimpses of strange white ape-like creatures the Eloi call Morlocks. He decides that the Morlocks live below ground, down the wells that dot the landscape. Meanwhile, he saves one of the Eloi from drowning, and she befriends him.. Time dilation In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them (special relativistic "kinetic" time dilation) or to a difference in gravitational potential between their locations (general relativistic gravitational time dilation). When unspecified, "time dilation" usually refers to the effect due to velocity After compensating for varying signal delays due to the changing distance between an observer and a moving clock (i.e. Doppler effect), the observer will measure the moving clock as ticking slower than a clock that is at rest in the observer's own reference frame. In addition, a clock that is close to a massive body (and which therefore is at lower gravitational potential) will record less elapsed time than a clock situated further from the said massive body (and which is at a higher gravitational potential). These predictions of the theory of relativity have been repeatedly confirmed by experiment, and they are of practical concern, for instance in the operation of satellite navigation systems such as GPS and Galileo. Time dilation has also been the subject of science fiction works. Time dilation caused by a relative velocity Special relativity indicates that, for an observer in an inertial frame of reference, a clock that is moving relative to them will be measured to tick slower than a clock that is at rest in their frame of reference. This case is sometimes called special relativistic time dilation. The faster the relative velocity, the greater the time dilation between one another, with time slowing to a stop as one approaches the speed of light (299,792,458 m/s). Theoretically, time dilation would make it possible for passengers in a fast- moving vehicle to advance further into the future in a short period of their own time. For sufficiently high speeds, the effect is dramatic. For example, one year of travel might correspond to ten years on Earth. Indeed, a constant 1 g acceleration would permit humans to travel through the entire known Universe in one human lifetime. With current technology severely limiting the velocity of space travel, however, the differences experienced in practice are minuscule: after 6 months on the International Space Station (ISS), orbiting Earth at a speed of about 7,700 m/s, an astronaut would have aged about 0.005 seconds less than those on Earth. The cosmonauts Sergel Krikalev and Sergei Avdeyev both experienced time dilation of about 20 milliseconds compared to time that passed on Earth. Twin Paradox In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, as a consequence of an incorrect and naive application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey. Another way of looking at it is by realising that the travelling twin is undergoing acceleration, which makes him a non-inertial observer. In both views there is no symmetry between the spacetime paths of the twins. Therefore, the twin paradox is not a paradox in the sense of a logical contradiction. Grandfather Paradox The grandfather paradox is a paradox of time travel in which inconsistencies emerge through changing the past . The name comes from the paradox's description: a person travels to the past and kills their own grandfather before the conception of their father or mother, which prevents the time traveller's existence. Despite its title, the grandfather paradox does not exclusively regard the contradiction of killing one's own grandfather to prevent one's birth. Rather, the paradox regards any action that alters the past, since there is a contradiction whenever the past becomes different from the way it was. The grandfather paradox was mentioned in written stories as early as 1929. In 1931, it was described as "the age-old argument of preventing your birth by killing your grandparents" in a letter to American science fiction magazine Amazing Stories. Early science- fiction stories dealing with the paradox are the short story Ancestral Voices by Nathaniel Schachner, published in 1933, and the 1944 book Future Times Three by Rene Barjavel, although a number of other works from the 1930s and 1940s touched upon the topic in various degrees of detail. Einstein's Theory of Special Relativity The theory of special relativity explains how space and time are linked for objects that are moving at a consistent speed in a straight line. One of its most famous aspects concerns objects moving at the speed of light.

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