Timetravel inconsistenciesAll possibilities mentioned in this page are part of the Sea of Possibilities, are part of the superposition. The Sea itself is still larger than that. Physical existability is the only demand.
Suppose, you want to go back to a place and moment in the past from which you know with certainty YOU WERE NOT THERE THEN, by memory, witness declaration, painting, photograph, film or anything. If you succeed to go there, it cannot be the past you meant to return to, because YOU WERE NOT THERE THEN.
In your home at the wall hangs a photograph of the specific place and time you want to go back to. The photograph shows a small low hill with grass and a tree, but without you. At the background there is a church tower that shows you
You step out of the timemachine into your home, back again. The photograph has changed now, you see yourself on it, standing next to the tree. The tower shows you 12.03 at the third of July in the year 1012. Onto your local environment - the timemachine - has clicked on a different surrounding and a different past too. Your past has been changed.
Compare Changing the links between local environments.
Or just read the whole storyline starting with page 3, Pasts in superposition.
And your memory? In a number of possibilities the memory of the photograph of the tree without you fleets away as a dream, out of existence. People can have dreams, this is an allowed development.
In other possibilities the rest of the brain has clicked on to a different memory in which you stand next to the tree. But clicking-on between brain parts is more difficult since the brainparts are so connected, leaving little different possibilities to fit too.
The arrival at the past moment will make the past to split in two possible pasts, at least. In one past you did arrive, in the other past you didn't. Both possibilities are in superposition for the people in the future time you originate from.
You step out of the timemachine. The photograph hasn't changed. You have no remembrance to an arrival in the past. Disappointed you think “How could I have ever thought a time travel would be possible!” While a second you is adjusting the controls on its way to the past.
And when you might remember - all ways that yield this memory, do superpose.
Electron A wants to know about another electron B. A knows nothing about B yet and A experiences B in superposition of all its possible states. When A starts receiving photons from B, the number of states B can possibly be in, soon reduces to the limits given by the Heisenberg Uncertainty Relations (HUR, or UR). From then on, as experienced by A, more photons from B will not reduce any further the number of states B can be in.
In another past you might exist twice IF the laws of physics provide a way how this came to existence, at least in this specific possibility. It doesn’t have to be time-travel. ANY possible way would do.
Is it necessary to replace a suiting amount of matter from the past by the matter you’re consisting of, to obey conservation laws?
Often the timetravel itself takes time. Not much, but nevertheless. When you see the timetraveler being busy during the timetravel, then time is elapsing. But since the timetraveler goes backwards in time, the time in the timemachine goes backward. I mean, IF the traveler didn’t enter a new dimension, THEN it used the old timeline and so evolves backwards in time. So as observed by us, the Outside Observers, the entropy in the timemachine decreases. But we will not come to see this. The timetravel machine will be dark, see item 11 in page 1 of THE DIRECTION OF TIME. And, most inconvenient, the timemachine will have to consist of antimatter, is the conclusion further on that page.
Quit a number of time-travel stories are about this, the main character going back in time and then is changing a lot of things that are necessary to yield his or her future. In the course of the story then things are turned such that everything fits again at the end, leaving the timeline contradiction-free. A closed time loop.
A timetraveler named A, goes back in time and prevents his or her parents meeting. So A never came to birth. So A never traveled backwards in time. Thus A’s parents did meet. So A came to birth. And when A has grown up, A goes back in time and prevents his or her parents meeting. A closed time loop, going around twice before neatly connecting to its starting point.
(With thanks to J. Richard Gott, Time travel in Einstein's universe, page 13.)
Key is, IF the past is changed by A entering there by timetravel, THEN the past is restored by A not leaving to the past by the same timetravel.
Suppose, moment 1 of arrival of A in the past is a superposition of A arriving there and A not arriving there. The world there splits in two worlds with different course of events from then on.
The moment 2 of A departing to the past then is a superposition of A leaving and A not leaving. The world there splits in one world where the timemachine doesn't work and one where the timemachine disappears, on its way as it is to the past.
The world of A-not-leaving clicks on to a past of moment 2 where A's parents did meet to give birth to A. The world rejoins there with the “normal” world where A never arrived there in the past.
The world of A arriving at moment 1 in the past clicks on to a different past of moment 1 that explains A's existence there - there's just no other way. Therefore the worlds of successful timetravel and without timetravel only share moment 1. They cross there.
In fact they do not share even moment 1, because in one world moment 1 is WITH A while in the other world moment 1 is WITHOUT A. So in fact the timelines decouple.
The world of A successfully leaving to the past at moment 2 clicks on to a lot of pasts. One in which A prevents his or her parents meting, one in which A missed the event and died unknown; and so on.
The different pasts may cause different developments of time from then on. Worlds may cross, or turn out to be decoupled.
The Sea of Possibilities.
A bizarre story
with a questionable sense of justice.
Stories that are meant to show the inconsistencies in timetravel often are quite rude. You have to go back and shoot your parents and that sort of things. So is this one. I timetraveled to the place where my parents had just met and shot them both. When I returned to my own time, I made a little mistake, so I landed just before I departed to the past and about 4 meters aside. I saw myself on the back, just entering the timemachine going to kill my parents. I shot myself, what should you do? Not my own self of course, but my other self. So I was left WITH my parents healthy alive, 2 timemachines and a dead corpse of myself.
Thirty years later. After a good and fulfilled life, my parents had died. I once again timetraveled to the place my parents had just met and shot them both, again. When I returned, I made the same mistake as the previous time but this time I didn't shoot myself. I thought there was no evil in killing two people who already had died. I tapped him on the shoulder, he shook scared, recognized me, laughed and said, “Hey, how are you?” We shook hands. He never left and we had a good time together, twice as strong as we used to be now. In the garage were 3 timemachines now.
We sat at the kitchen table. I discussed with myself that the first time I was there in the past, if I would have had withhold myself from killing my parents then, everything else could have remained the same. I mean, no other me withholding me, just me withholding myself, I said. At the return I should have killed myself still (my other self, I mean), I thought by myself, just to be sure. “And when I wouldn't have killed them the second time - ” and here my other self interrupted me and said “I didn't kill them the second time; you withhold me from entering the timemachine!”.
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