I have reservations about participating in this discussion. This is more about philosophy than electronics, but it is a discussion that promotes understanding and interest of science and technology. I will share some thoughts. This is a mind twister to write,
I hope it is as interesting to read as it is to write.
1. Space Time vs Space and TimeMcFry looks at time travel as rather like spatial travel. You can go back to 1955 and then return to 1985 as if you can go to Boston for dinner then head back to New York. They treat the past as if it is a place and that it still exists.
Now let say you can indeed travel in an enclosed machine in your basement and you go back exactly one day. You will not find yourself in your basement one day back - Earth is not in the same place as it was one day ago. You will find yourself in that machine alone, in space, with Earth at a distance about 1.6 million miles away as Earth orbits the Sun. If you travel further back in time, you will find yourself closer to other stars than to our Sun.
You can receive information from the past - if you travel faster than light away from earth to about 50 light years, you could receive TV broadcast of the Apollo 11 launch and the first moon-landing. But the world of 1969 no longer exists today. World of 1969 is not a place you can go to, not any more. It doesn't exist any more. The world of 2069 is yet to be, so there is no there to go.
I will however attempt to contradict myself later.2. Rewinding the clock or fast-forward the clockHow would one control every single particles in the universe to modify their time vector?
3. Wormholes, the most promisingThis is
one theory yet to be debunked by Science in-so-far as I know. Reading published scientific paper is a rare event for me so I could be wrong here. You can in theory create a wormhole with one end anchored to the north pole of the Moon and the other end at the south pole of the moon of different time. In theory, you can also find wormholes elsewhere connecting some other spatial locations and some other times,
Current theory is, there must be something to prevent the "Grand Father Paradox". But who knows if that is indeed the case, or not.
Note that current theory is, you can't go back father than the creation time of the wormhole -- that is if you can indeed travel through a wormhole without it collapsing. That said, wormholes is the most viable way of time travel (viable as in not yet debunked). Again, I could be out of date with my knowledge here.
4. Trying to contradict myself.Let say you have a chess board with three chess pieces only, one Bishop, one Knight, and one Queen, and you randomly put the three chess pieces onto your chess board squares. Lets also say everyone on Earth will do the same thing. Will you find your placement position of Bishop, Knight, and the Queen on one of the other 8 billion chess boards done by others? Possibly so!
If you limit the chess board's size to 3 squares by 3 squares (9 spots) and again 8 billion people on earth each doing the same random placement of 3 pieces on 9 spots on their chess boards.
Statistically, it is a near-certainty that you will find the exact same placement of the 3 chess pieces as yours on one of those 8 billion chess board done by others.
As space expands, star that was visible to us reduce because those stars are so far away. With so much space between us and those stars, they are receding from us faster than the speed of light - they are beyond our cosmic horizon. We will never see them, ever.
Let's call what is within our cosmic horizon our local universe for short. What happen within our local universe can not affect elsewhere, and what happen elsewhere cannot affect our local universe.
There is a limited number of particles within our local universe. A very big number but limited none the less. I'll call that P for now. Light and Forces are particles as well, so they are included in P. For that matter, you can add the particles property and count each property into P so P is all inclusive of particles and its properties. So our local universe has P particles and
P is a very big number but not infinite.
If the entire universe is
sufficiently large, it can have many local universes, lets call that N. N is the number of local universes out there.
Now you know where I am going. While the permutation of P is huge, if N is
sufficiently large, one will statistically find every permutation of P within one of the N's. So if the universe is sufficiently large, every single arrangement of a local universe should exist. So, you may well find a local universe arranged exactly as 1955 Earth would have it!
So, indeed, if the universe is sufficiently large, we could have every moment of our local universe represented on some other local universe!Well, even if it is out there on another local universe, how do you find it and get there is another matter.
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EDIT: Per
Eugene's suggestion/request, added one that omitted at first, click link below.
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3a. Time travel by time dilation
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