I watched an excellent discussion with Carlo Rovelli.
Here are some of my thoughts on the discussion:
Entropy and direction of time
At: https://youtu.be/r_fUPbBNmBw?t=5394 Carlo says that the increasing entropy is the reason why time has direction. There is a notion of past present and future. Other scientists like, Sean Carroll say similar things elsewhere. IMO that is not correct reasoning.
Let me explain...
IMO we remember the past NOT because entropy increases but because there are effects (traces) of the events in the past on the environment, which can be retrieved/measured/observed at present moment. For example, the markings on the Rosetta stone which we can see today tell us that someone wrote the same information in 3 languages/scripts sometime in the past. However, if the entropy keeps increasing and the markings on the Rosetta stone wear off, we will not know that someone wrote that information. Similarly, if someone walks on a wet beach we can know that someone walked on the beach for a few more hours, but after some time when tides wash away the footprints, we will not be able to tell if someone walked on the beach earlier. Please note, in both cases, the entropy was increasing. IMO this confusion of tying an increase in entropy to the direction of time is the cause of confusion about Janus point (Ref: Julian Barbour). According to the idea of Janus point, what does it mean to say that there is a future on both sides of the Janus point as the entropy increases in both directions away from the Janus point. I can understand that before the Janus point time was still moving forward (like on this side of the Janus point) but by randomness, the entropy decreased to reach the minima of Janus point and then started increasing again. Also if the entropy at Janus point was minimum and not zero it is possible that the last Janus point (i.e. the big bang) was a local minimum. The point I am making is that time is a name given to changing configuration of constituents of the universe. I think Julien Barbour has views similar to this. Tim Maudlin also thinks that time flows "forward" meaning that one configuration of the universe follows the previous configuration and will be followed by the next configuration. As soon as there is order in the configurations of the parts of the universe, as indicated by previous and next, there is that thing called time. And when we locally measure the changes in configurations of the local parts of the universe, that gives us the familiar clock time. In other words, the concept of time (without any quantitative units) is nothing but ordered changes to configurations of parts of the universe. Once we build clocks then the "rate" of the flow of time comes into existence.
Difficulty in traveling backward in Time
In SCIFY and in some science discussions there is talk about traveling back in time. Can subpart of the universe travel backward in time? Now why I think this is not possible: If a subpart of the universe goes backward in time and it does so at the same spatial location, won't it run into STUFF that was there at that same spatial location in the past. Just like when cars go across a signal intersection at different times they do not collide, but if they try to cross the intersection at the same time there is a collision. Based on this I think that only the whole universe can travel backward in time (rewind) so that room is made such that things keep out of each other's location.
Another point, if a subpart of the universe, like a human, can travel backward in time, does its own time continues to flow in forward direction? Our thoughts and experiences are based on forward-going time processes inside our brain and will/do remain meaningful and coherent only if our brains run on forward-going time. Thus even if a human goes back in time their own brain processes need to run forward in time, right? For example, when Marty goes back in time in the movie Back to the future, the implicit assumption is that his spatial coordinates also change to the location of earth billions of miles away where earth was at that moment in the past, right? Also what if the location where Marty goes to in the past there was a mountain rock at that exact spatial location. What would have happened?
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