Quote:
Originally Posted by cosmicfish
But what is the mechanism of this? If there is only one "past" at a time, then when he goes backwards in time he creates a new past, so there are multiple, realized "pasts" even if only one appears accessible at a time.
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The way to think of this is from BEM's perspective. Let's first make some definitions. Let's let A(0) represent a point on a worldliness where time travel begins and A(
t) represent a point on a worldline at time
t from A(0).
Quote:
Originally Posted by cosmicfish
Let's consider time travel between the years 1960, 1980, and 2000. If our intrepid traveler starts in 1980, the years 1960-1980 are that single, fixed past. If they then travel to 2000, all of 1960-2000 are now that single, fixed past.
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So if we let A(0) = the year 1980, then A(
t<0) is indeed a single, deterministic path. But when our time traveler decides to go to time
t, there is no single path. So they choose one from the available worldlines and choose a point, A'(
t). The choice is limited by the light cone that extends from any given point A(
t). Any worldlines outside the light cone are not accessible. So our traveler is not creating the worldline that contains A'(
t), but simply goes to A'(
t) which is already on an accessible worldline. Once the traveller arrives at A'(
t), it becomes A(0) and again there is just a single deterministic worldline for all A(
t<0). So we agree here.
Quote:
Originally Posted by cosmicfish
But when they travel back to 1960, any change means replacing all of that 1960-2000 past with some new past. The old past is not probability or theory, it actually existed and must now be replaced by something else that is, again, neither probability nor theory.
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So now our traveller is going to A(-40). As this is a simple deterministic route, no problem. But now when we reset this to be A(0), the additional information that the time traveller beings back, will change the probability of the worldline and normal time may follow a different path. So we have three worldlines that all exist. The first would be the one that started in 1980 and would have progressed along a worldline, on which "future" events would be on, that we will call A
1980(
t). The second is the worldline that contains choice the traveler made and all "future" events from that point we can call A'
1980(
t). The third worldline is the one that will be followed after bringing back the information, A'
1960(
t).
So we have three worldlines that co-exist, but the people in the worldlines only know about the one they have always been on. Therefore, only BEM knows there have been differences. So quite literally, BEM perception is reality. But because his available choices of worldlines is limited, there is only so much change he can effect. Also, this is what caused him to lose his family. The change in worldline that was introduced when he stopped the 1989 war moved him to a point where his family only exists outside of his lightcone. He has no way to get back to them, unless he goes and stops himself from preventing the 1989 war, but that will put him on yet another worldline and still may not be able to reach them. Who knows, maybe he has tried it many times and just can't get back to them. It's kind of sad. But if we look at this from his family's perspective, he disappeared one day and just never came back.
Most of this heavily draws on work by David Deutsch and presumes a multiverse. Though Deutsch's work has been criticized because even though his models do eliminate things like the Grandfather paradox, it suffers from a knowledge paradox that Deutsch's model cannot solve. But since this is just a game, close counts.
If you are interested, here are some published papers:
http://thelifeofpsi.com/wp-content/u...utsch-1991.pdf
http://www.academia.edu/6059479/The_...of_Time_Travel
https://conjecturesandrefutations.co...f-time-travel/