Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 21/11/2010 18:07:44
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J. Richard Gott (Time Travel in Einstein's Universe) has this to say on the subject of time travel to the past:
“A time traveller who visits the past is just someone whose world line somehow loops back in time, where it could even intersect with itself. This would allow the time traveller to shake hands with himself. The older man could meet up with his younger self and say, “Hi! I’m your future self! I’ve travelled back in time to say hello!”…..The surprised younger man would reply, “Really?” He would then continue his life, becoming old and eventually looping back to that same event - where he would recognize his younger self, shake hands, and say, “Hi! I’m your future self! I’ve travelled back in time to say hello!”
One slightly worrying thought that springs to mind regarding the above scenario is that, once the first time traveller has succeeded in looping back in time, and has completed the loop by meeting himself, he seems to have set in motion what looks very much like a potentially indefinite succession of loops, in which he continually returns to his past and meets himself, after which his younger self continues with his life; but once the loop has been formed, can he ever progress beyond the point from which he first went back?
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That is probably why a complex structure like a person is very unlikely ever to survive going through this process. A one subatomic particle is totally indistinguishable from another subatomic particle of the same type and this sort of paradox cannot occur because no change can happen.
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In all the Sci Fi stories I have seen they manage to get out of the loop.
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What if he chooses not to go back in time after the first time travel... The first man never met with himself when he was young...???
(i just don't believe in the possibility of going back in time, too many paradoxes. I am a downer on this one.. sorry...)
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In all the Sci Fi stories I have seen they manage to get out of the loop.
Does this mean that you think Gott's book should have been published as Sci Fi, rather than Pop Sci? You could have a point. [;D]
I have done quite a lot of reading, and some thinking, about time travel, and I am yet to be convinced that the past-directed variety is possible, at least, not within our 4D confines.
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There's some interesting stuff about past-directed time travel in the current New Scientist. I would be interested to know what people think.
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It's like bifurcations in chaos theory. If you do a 'closed time loop' you are creating a paradox. The only way out of such a one is to assume that the guy 'forces' his younger 'self', and that universe, to develop in another 'continuum' enabling that guy to have his, and the universes 'arrow of time' unbroken. Or possibly disallowing the ones universe that first goes 'back'. any which way the 'universes' will have to 'split'. And that's a 'bifurcation.
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I shall have to think about this bifurcations for a while; presumably it requires a multiverse.
In the meantime here are some more thoughts inspired by the fact that serious scientists can make the kind of statement in my OP.
Gott’s story, perhaps conveniently, makes no mention of what happens to the older “self” after the meeting. There seem to be two possibilities: In the first of these he returns to the point in time from which he went back, (let us call this point “X”). In which case, presumably, he continues to age, and eventually dies. At first glance, this may seem to answer the question as to whether the original self can progress beyond point X, once the time loop has been formed. However, we must remember that point X, like every other point in spacetime, is a unique and immutable event; so when the younger self reaches point X, he must, again meet his older self, (because point X cannot occur with and without the older self) thus raising the question as to whether the original self has progressed beyond this point, or not, bearing in mind that this is not a repeat visit to point X, it is the one and only time that point X will occur. If we were right in thinking that the formation of the initial loop set a succession of loops into action it is difficult to see how we can avoid a scenario in which progressively more “selfs” meet at point X and progress together, presumably, all to die at the same time, only to “reappear” at point X to meet the next self as he reaches the time travel event. At first glance, it may seem as though we have a contradiction here, in that I talk of the older self as having progressed from point X; yet I have the younger self meeting the older self again at point X when he arrives there, surely the older self would have moved on by then. What we have to keep in mind, though, is that, as we saw above, point X is a unique spacetime event. It cannot occur with and without the older self, or any other character who is ever at that event. Obviously, this must mean that if the build up of multiple “selfs” at point X does occur, then all those selfs must always have been at point X. In fact, because there seems to be no end to the number of selfs who will eventually arrive at point X, it might be argued that this gathering of selfs could be limitless, some might say “infinite”. It begins to look as though, if this scenario ever became a reality, the Universe would be stuck, “eternally”, in a time loop beyond which it could never progress. Can such a cosmic misfortune be brought about by the action of one person who succeeds in travelling back through time and meeting her or his younger self? One more point that has to be considered is that if point X is to play host to an unlimited number of selfs, then all these selfs must have been there when the original self took his trip back in time. Bearing in mind the fact that, at this juncture, the time loop had not been formed, where did all these other selfs come from and who decided how many there should be? There is still another complication. Not only must the original self meet all the other selfs at point X before going on the journey through time, all these selfs approached point X from the past, along a “continuous succession” of unique spacetime events, none of which can have occurred with and without a single one of the multitude of selfs.
The second possibility is the bifurcation one, which needs some thought.
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Paradoxes of activity need to be duplicated; the vast window of opportunity is too wide to comprehend.
Every event that has transpired leading towards the construction of the time travel apparatus or a facsimile must occur, in order for it to exist. Participants may change, but events remain the same.
This loop is needed to be maintained, for main event to occur, which we are referring to as time travel.
Time travel within the relative view point of the traveler is not instantaneous, it takes a second to go back a second it takes a year to go back a year, and so on. Everything he carries back with him also is affected to rot.
The traveler’s metabolic rate of aging will also be affected; they also will age as the journey does. The time traveler is limited to the duration of time in which they have been in existence. Maybe a Hostess Twinkie may survive.
So as the time traveler travels back there is only but a moment, that all his present knowledge, of the latest date of his previous existence can be comprehended by his younger self.
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Participants may change, but events remain the same.
Are you suggesting that a single spacetime event can occur with changed participants? Surely, that would change the event; which we are assured is not possible.
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Participants may change, but events remain the same.
Are you suggesting that a single spacetime event can occur with changed participants? Surely, that would change the event; which we are assured is not possible.
Take notice in the event of a race if the number one winner were somehow removed from existance at an earlier point in time prior to the race, there will be a winner. If the race were removed also from earlier existance we would never know.
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A 'event' can only happen once, each one is unique. To believe that we have 'events' that are indistinguishable you will have to isolate your system into absurdity. That we say that a experiment is 'repeatable' doesn't imply it is the same exact 'experiment'. It's like going in for a drink, 'I'll have the same' is just not possible :)
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Take notice in the event of a race if the number one winner were somehow removed from existance at an earlier point in time prior to the race, there will be a winner. If the race were removed also from earlier existance we would never know.
This is an impossible situation. If the winner were removed prior to the race, he would not be the winner, and would not be there to remove. You cannot have a situation in which he was/was not the winner in the same spacetime event.