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Physicists prove time travel is theoretically ‘possible,’ but changing the past is not
https://themindunleashed.com/2020/09...st-is-not.html
based on https://iopscience.iop.org/article/1...61-6382/aba4bc
Is the report an accurate summary of the article?
I don't think this is going to appear in Nature. :doh:Quote:
Originally Posted by Tom Mazanec
I think this is more complex than just a statement that " time travel is theoretically ‘possible,’ but changing the past is not."
First of all, they haven't proved that time travel to the past is possible, which would require something more substantial than a theoretical paper.
Secondly, they seem to be saying that you actually 'can' change the past in certain inconsequential ways, if you can travel or signal back into the past; but the consequences of such a change would be so insignificant that they would have no discernable effect on the future. This has a certain significance for the concept of free will, since you do have a certain amount of freedom of action in this model; but the end result is just the same. If History shows that X events have happened, then that will also be true after the formation of a closed timelike curve, even if the exact sequence and nature of those events may have changed slightly. So long as the same X events have occurred (as seen from and recorded by the future), then history is consistent. So if you do somehow kill your grandfather before your father is conceived, you would need to be descended from his identical twin, or from previously frozen sperm.
Thirdly this paper assumes that alternate timelines are not significant. If causality can be significantly reversed and the past can be changed, then this seems to imply the creation of new timelines. Instead of constraining the changes that can be made to insignificant tweaks, the creation of alternate timelines allows the past to be changed significantly. This paper does not consider that possibility at all, as far as I can see.
And fourth, not testable. :whistle:Quote:
Originally Posted by eburacum45
Until and unless physical time travel actually happens, this is an inapplicable abstraction at best.
Maybe there's a related effect or spin-off of this hypothesis that has some useful application in models predicting the future, but as far as the past goes? It's already over. Let it go.
Maybe it happens all the time and we are living in the changed history, but because we are part of the changed universe, we don't know it has changed (we changed with it). To paraphrase THHGTTG "There is a theory which states that if anyone ever goes back in time and changes history, the universe will instantly disappear and be replaced by something even more bizarre and inexplicable.Quote:
Originally Posted by Noclevername
There is another theory which states that this has already happened.”
I don't get what the paper says about changes being inconsequential. If you kill Hitler, someone's going to notice.
The only way you could kill Hitler in this model is if he is immediately replaced by a look-alike who behaves exactly the same.
I don't think the the article is that great of a summary of the paper itself. Looking at just the paper, it appears largely concerned with showing that it's possible to have closed timelike curves and still have consistent, stable spacetime. But one of the authors is quoted as suggesting that this means that "Try as you might to create a paradox, the events will always adjust themselves to avoid any inconsistency". I'm not sure the mathematics of the paper particularly support that, though. I'd think a stable CTC in general relativity would be more like the sort of time travel where you go back in time to try to change something, but your actions end up making everything turn out the way you experienced it in the first place (though perhaps you end up with a more complete understanding than you did before). Something like Heinlein's "By His Bootstraps".
Kill who Who's that? :doh:Quote:
Originally Posted by Noclevername
Well that would fit. Occasionally some people on the internet claim things about the world that are so radically different from the world I see that I suspect they actually live on something like the Star Trek mirror world, but have been connected to my Earth’s internet through some oddity of physics. For instance, some will make statements about certain public figures that indicate they are radically different from my world’s. It’s like Kirk vs mirror Kirk.
The interesting thing to me is that this would seem to suggest FTL travel or communication could be made to be compatible with relativity theory, but wouldn’t completely destroy causality. It would just make it more complex. Of course, a method would be needed.Quote:
Originally Posted by Grey
two things
one the current location of the time traveler is not the past location one wishes to be at
the earth moves as does the sun and the galaxy a LOT over long periods so one would need a space ship with a time machine
IF THERE EVER WAS TO BE TIME TRAVEL IN THE FUTURE
THE WOULD BE TIME TRAVELERS NOW
unless the UFO's are them and not from somewhere else ?
If you can choose you point in time you can choose your point in space. "Time travel" is also effectively teleportation. Location, location, location!Quote:
Originally Posted by nota
Many concepts of time travel involve travelling through a wormhole to a different location in time and space. The exit point of the wormhole dictates exactly where, and when, you will emerge. If this is not the right location in space you would need to travel from there to your chosen location.
magic thinkingQuote:
Originally Posted by eburacum45
if your science project need an unproven trick thing [worm holes]
to make another thing ''work'' [time travel]
you are not doing science
just casting magic spells
btw I would love to see a worm hole or a white hole
wouldn't a white hole be very very hard to miss ?
but anything that needs a black hole as one end point
is not likely user friendly
This is something I've never considered before, and makes sense. Adams's Heart of Gold seems so much more effective now than Wells's time machine.Quote:
Originally Posted by nota
But I'm curious, what about momentum? Is there some chance the machine could be "carried along" on it's trajectory, and land, in time, about where it should?
Makes me wonder how many backup clones the world had in stock in case someone actually did come along and kill Hitler.Quote:
Originally Posted by eburacum45
Was the original Hitler named Untermeer?
This seems like a good way to run into counting problems. If somebody has to stand in for somebody else, who is going to stand in for them?
Won't we always run one person short?
Maybe it's that when you make changes, someone always reverts your edits: Wikihistory.
Just before a time traveler pulls the trigger on Hitler, another time traveler knocks the gun aside because the original timeline is better in the long run.
All methods of travelling into the past require some exotic 'trick' to work. The mathematics and physics of wormholes have been extensively worked out by various theoreticians, including the Nobel laureate Kip Thorne.Quote:
Originally Posted by nota
But there are other methods to do the same 'trick'; simply travelling faster than light in certain directions could take you into the absolute past.
Both of these methods of time travel (wormholes and FLT) have well-defined characteristics, so should allow a careful chrononaut the opportunity to arrive at the correct coordinates in time and space, assuming they work at all. It's not a question of just pressing a button and hoping for the best.
If the past can't be changed, could anything I do while visiting the past be considered to be unethical? If someone was shot dead at noon yesterday and I decide to time travel back and become the shooter, I didn't cut his life short. I'm just discovering that I pulled the trigger. If I decide not to time travel at all it just means that someone else did it. If the past can't be changed, no decision that I make now can have any effect on it and would be irrelevant, not unethical.
Let's say I want to sell some vintage comic books for a fortune but don't happen to have any. I can just time travel back a few decades, sell a calculator or something, and use the money to buy a bunch. I can't just bring them back with me because they won't be properly aged, so I stash them somewhere and go get them when I return to the present. Did I change the past? If not, why don't I just go get the stash without bothering to time travel? But it won't be there if I don't go back in time, so something is different, right?
The calculator you took back in time would need batteries. So take a few with you.
But it it was too advanced, the boffins at DARPA (or its predecessors) would love to get hold of it (and reverse engineer it), and you'd potentially change history anyway. Even the batteries might change the history of technology significantly, and we'd all be driving electric cars by now.
I could take a solar powered calculator if I wanted to be a nice guy. But dead batteries wouldn't be my problem since I'd be gone before the calculator became worthless.
Ah, so you want time travel with return to your own timeline? Gonna have to charge you extra for the premium package.Quote:
Originally Posted by Chuck
My comic book stash will be in the new timeline so that's where I'd want to go, if there is a new timeline. But if there's only one timeline that can't be changed then the comic book stash existed before I decided to time travel, so I should just go get it and not bother time traveling. But if I'm not going to time travel then the stash wasn't there after all.
I think that there are a variety of hypothetical versions of time travel, including versions where the past can be changed (resulting in changes to the present), as well as versions where any travel to the past just produces a stable time loop (that's the way it always was, but maybe you just didn't know it). Any of those can work in fiction, depending on what kind of story you want to tell. From a scientific perspective, obviously, we'd have to experiment with time travel to see which of these options is the way it "really" works. And of course it's entirely possible (and actually seems very likely at this point) that time travel to the past is just not actually possible.
I'm pretty sure that collectors don't pay extra for aging: they want the condition to be perfect, or as close to it as possible.Quote:
Originally Posted by Chuck
I guess that perfect books that are 60-odd years old would be suspected as counterfeit, but yours would pass every test for authenticity, of course. Well, they might register too young upon carbon-dating. I don't know if such is economically feasible for comic book people.
I do know that books from the 1940s in excellent condition have been recovered. These were effectively sealed nicely in a dry climate under exceptional conditions.