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Time Travel

There are new cosmology theories with 4 or more dimensions and with the 4th dimension as time.
Some theoretical physicists will say something sensational like  'imagine moving forward or backward in time.' That statement of imagination about moving in time might not be possible in reality.

In some of these theories time is assumed to be directly connected with motion.

In a simple practical sense:

Time is an increasing count of a defined event, with the standard being Caesium 133 (or an equivalent, like in a computer; USNO uses several elements). A physical motion can be started, stopped, resumed with no direct connection or relationship to the current time.

Chemical reactions also require time, defined as the reaction rate, and this time has nothing to do with linear motion.

The reaction rate calculation is complicated by many factors. From wikipedia: 'Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. '

Time is involved in these rates because one of the critical factors in the reaction rate is the molecular collision frequency.

In new theories involving time as the 4th dimension the consequences of possible time reversal must be addressed.
excerpts from online articles:
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according to the Theory of Relativity, the reality of the universe can be described by four-dimensional space-time, so that time does not actually "flow", it just "is".
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A test of time dilation (implied in Einstein's relativity) was conducted in 1971: 'From the actual flight paths of each trip, the theory predicted that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40±23 nanoseconds during the eastward trip and should have gained 275±21 nanoseconds during the westward trip. Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59±10 nanoseconds during the eastward trip and gained 273±7 nanoseconds during the westward trip (where the error bars represent standard deviation)'
Time dilation is not really time travel ( with a difference in elapsed times between two observer's clocks in nanoseconds).

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Thermodynamics, then, appears to be one of the only physical processes that is NOT time-symmetric, and so fundamental and ubiquitous is it in our universe that it may be single-handedly responsible for our perception of time as having a direction. '

about quantum mechanics:

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 the wave function only collapses when the particle is actually observed. At that point, the particle can finally be said to be in one particular position, and all the information from the wave function is then lost and cannot be recreated. In this respect, the process is time-irreversible, and an arrow of time is created.
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It is not clear how the quantum arrow of time, if indeed it exists at all, is related to the other arrows, but it is possible that it is linked to the thermodynamic arrow, in that nature shows a bias for collapsing wave functions into higher entropy states versus lower ones.
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Thus, [Stephen] Hawking argues, a strong thermodynamic arrow of time is a necessary condition for intelligent life as we know it to develop. For example, beings like us need to consume food (a relatively ordered form of energy) and convert it into heat (a relatively disordered form of energy), for which a thermodynamic arrow like the one we see around us is necessary. If the universe were any other way, we would not be here to observe it.
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It seems time travel, especially backward, is just a fantasy.

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