It is said that microphysics is time reversible. In other words if the time runs in reverse the microphysics stays the same. A crude example of this is - if we see a short film of two billiard balls hitting each other in forward and backward, we will not be able to tell any difference. OK. But when we think of it in details, I do not really understand what that means. Of course in the example of billiard balls above, if we measure the temperature of each ball before and after the collision we will be able to tell the difference. In forward running scenario, the two billiard balls will be slightly hotter after the collision compared to that before the collision. So the example breaks down.
The short term memories in our brains are continuously recorded in the connections of neurons. Thus we remember more and more as we live our lives forward in time. If the time is revered will we not start forgetting things? Won't our memories start getting erased?
If a protagonist decides to go back in time, enters a time machine to go backwards in time and turns on the time machine, wont s/he be ejected out as the time will start flowing backward and s/he will end up outside the time machine where s/he came from and eventually even the decision to go into the time machine will get erased? Or is the rest of the universe is supposed to go backward in time but the protagonist continues to go forward in time so that the thinking and memories continue to build in her/his brain in the normal way.
May be for all these reasons going backward in time is forbidden. May be time only has forward direction and can never go in reverse direction. It may even be a non-sensical thing to think of.
May be it is more correct to say that microphysics is velocity reversible. Remember velocity is a vector and equal but opposite velocity may provide the negative sign which represents the reverse microphysics. Thus the time does not have to be reversed to imagine a reverse microphysics. All the particles involved still move forward in time, only in opposite direction.
In the billiard balls example above the film motor is run forward (clockwise) and backward (anti-clockwise) but it still rotates forward in time.
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