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Old 11-21-2017, 02:14 PM
Jim B. Jim B. is offline
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Temporal Paradoxes and the Possibility of Time Travel.

I guess like many people, my knowledge of physics doesn't go much beyond high school. And then there is a lot I've learned from PBS and the Science Channel. I'm serious.

Anyway, my question is simply this: Do temporal paradoxes prove time travel is impossible?

Because I know of no other example of this in physics. There are no motion paradoxes. There are no gravity paradoxes. There are no mass paradoxes. You see what I am getting at.

But then again as I have said before on these boards, I for one am open to any possibility. Flight certainly seemed impossible, and not too long ago, relatively speaking.

So why or why not, then?

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Old 11-21-2017, 02:23 PM
CalMeacham CalMeacham is offline
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Physicist and Science Fiction Author Robert L. Forward was wary of temporal paradoxes, and said that he wanted to see the concept rigorously proven (or disproven) -- something much more formal than, say, my sig line below.

I observe that in his time-travel novel Time Master he takes the view that anything you do in the past becomes an irrevocable and necessary part of that past -- no "alternate realities", "branch points", "Many Worlds" stuff. the situation in the book, in fact, resembles an example he used in one of his papers.

Of course, that's unsatisfying to most of us. Larry Niven wrote an essay "The Theory and Practice of Time Travel" back in the 1960s (It's in his collection All the Myriad ways) in which he looks at different methods of time travel and at the paradoxes involved, without committing to any of them. (If you went back in time to the Crucifiction with a machine gun, he observes, tongue in cheek, your gun would positively jam.)
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Old 11-21-2017, 02:39 PM
Blue Blistering Barnacle Blue Blistering Barnacle is offline
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I have heard of physicist setting up mathematical systems wherein the time travel doesn't lead to paradox.

For example, the cue ball deflects off a ball, knocking the 3-ball into a (wormhole) pocket. The 3-ball emerges from another pocket in the past with just the right position and momentum to knock the cue ball, into its past self.

I am no expert, but I call baloney on that. Just because you could set up a situation in which a paradox resolves itself (whether in arts or in maths) doesn't mean that many (probably most) "real" situations wouldn't induce paradoxes.

I don't think the universe is smart enough to "censor itself"*. I just don't think it can happen.

*Reminds me of a cute story where someone noticed that any civilization which attempted to build a time machine would experience a civilization ending crisis prior to completion. He proposed cosmic censoring of time travel as explanation. He proposed to his emporer to leak the existence of a partially completed time travel device and plans for completion to their rivals, hoping to doom them. The universe nipped the plan in the bud by having their sun go nova while they were discussing the feasibility of the plan. Fine for stories, but...
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Old 11-21-2017, 02:45 PM
John Mace John Mace is offline
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You can hypothesis an infinite number of universes where all possibilities have happened, and if you change something in the timeline, it ends up in another universe, not affecting the universe you (if there even is a you) are traveling thru.
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Old 11-21-2017, 02:45 PM
Little Nemo Little Nemo is online now
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I think the key issue with temporal paradoxes is causality. With time travel you can easily create a situation where two events are causing each other to occur. (The most basic example is I am given the plans for a time machine by my future self. I use the plans to build a time machine. I then travel back in time to give my past self the plans.)
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Old 11-21-2017, 02:53 PM
Exapno Mapcase Exapno Mapcase is online now
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Check out Hawking's Chronology protection conjecture.
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Old 11-21-2017, 03:00 PM
gnoitall gnoitall is offline
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Originally Posted by CalMeacham View Post
Of course, that's unsatisfying to most of us. Larry Niven wrote an essay "The Theory and Practice of Time Travel" back in the 1960s (It's in his collection All the Myriad ways) in which he looks at different methods of time travel and at the paradoxes involved, without committing to any of them. (If you went back in time to the Crucifiction with a machine gun, he observes, tongue in cheek, your gun would positively jam.)
AFAIK, Niven's final take on time travel from that essay is crystallized as Niven's Law: "If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe." In other words, time travelers will keep meddling with the universe until they fall into a universe in which time travel is impossible. At which point time travel doesn't exist. In other fewer words, time travel erases itself. The only point of temporal stability is the one in which time travel cannot exist.

See also Novikov self-consistency, a more formal physics principle that winds up looking eerily like Niven's Law to an amateur.

Last edited by gnoitall; 11-21-2017 at 03:02 PM.
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Old 11-21-2017, 03:25 PM
Chronos Chronos is offline
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Niven's essay on the subject isn't particularly well thought-out, and misses a lot of possibilities. What it amounts to is that Larry Niven can't imagine how time travel could work, and therefore it couldn't work. In particular,
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See also Novikov self-consistency, a more formal physics principle that winds up looking eerily like Niven's Law to an amateur.
Not really, since Niven's Law is based on the (unstated) assumption that Novikov's principle is false.
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Old 11-21-2017, 03:31 PM
Mangetout Mangetout is online now
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Because I know of no other example of this in physics. There are no motion paradoxes. There are no gravity paradoxes. There are no mass paradoxes. You see what I am getting at.
There are paradoxes in physics though - here's one:
https://www.youtube.com/watch?v=zcqZHYo7ONs
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Old 11-21-2017, 03:43 PM
StrTrkr777 StrTrkr777 is online now
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I definitely do not understand the physics or science or math or any of that of time travel.

While it is a fun concept and allows for some potential interesting writing and movies/tv, I do not really see that it is achievable.

My reasoning is thus, if it were possible, then the inventors of such would likely have "fixed" the worst parts of the past. i.e. killed Hitler or other murderous political leaders prior to their killing so many. They could not likely do much about natural disasters, but they could affect things brought about by humans. Yes, it is possible that the creators of a time travel machine could be bad, but then it is just as likely that they would have good intentions.

I agree that the universe would not have a mechanism by which it would stop such a change, so the fact that our past is littered with evil humans, leads me to believe that no one will ever invent time travel.
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Old 11-21-2017, 03:56 PM
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I definitely do not understand the physics or science or math or any of that of time travel.

While it is a fun concept and allows for some potential interesting writing and movies/tv, I do not really see that it is achievable.

My reasoning is thus, if it were possible, then the inventors of such would likely have "fixed" the worst parts of the past. i.e. killed Hitler or other murderous political leaders prior to their killing so many. They could not likely do much about natural disasters, but they could affect things brought about by humans. Yes, it is possible that the creators of a time travel machine could be bad, but then it is just as likely that they would have good intentions.

I agree that the universe would not have a mechanism by which it would stop such a change, so the fact that our past is littered with evil humans, leads me to believe that no one will ever invent time travel.
Maybe we're just looking at the big picture. To us, Hitler is still part of our living history. There are people whose parents and grandparents were killed by the Nazis.

But will Hitler still be remembered in a thousand years? Even if he is, will people feel a personal connection to his crimes? How many modern people would feel the need to go back in time and kill somebody like Tamerlane?
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Old 11-21-2017, 04:12 PM
md2000 md2000 is offline
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Maybe we're just looking at the big picture. To us, Hitler is still part of our living history. There are people whose parents and grandparents were killed by the Nazis.

But will Hitler still be remembered in a thousand years? Even if he is, will people feel a personal connection to his crimes? How many modern people would feel the need to go back in time and kill somebody like Tamerlane?
Who did that cartoon where a time traveler materializes and is immediately shot by an SS guard. "Another one, " he says to his companion. "Our Fueher is a great leader, but why do so many time travelers try to kill him?"

* * *
There are the several possibilities.
-The immutable past - what happened, happened. Nobody took a machine gun to the crucifixion (or sent a giant bomb to Hitler) because it never happened/happens. Circular paradoxes are possible, but they're no paradoxes because they always happen that way.
-The past changes; like Back to the Future - if you go back and change things, they change. But then the movie tried to introduce an extension that produced a paradox, you the time traveler will slowly fade in the here and now as the probability of your future changes... but will still return to the new future with memories of the lost alternate future... Which makes sense, because those brain cells existed with those memories in the unchanged past.
-If there are multiple futures, like alternate dimensions, then how does the mass of the universe double every millisecond as random choices produce multiple versions of the future?

maybe we need Heisenberg's future - we can know one or the other thing, but not both. Or Schrodinger's future - we won't know what the future we return to will be until we open the box.
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Old 11-21-2017, 04:12 PM
Jim B. Jim B. is offline
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You can hypothesis an infinite number of universes where all possibilities have happened, and if you change something in the timeline, it ends up in another universe, not affecting the universe you (if there even is a you) are traveling thru.
I have heard that argument before. I think it is called the multiverse theory, of alternate quantum realities or something. I don't know (IANAPhysicist).

But that actually brings up another question, in my mind at least. Then where are all the time tourists, from other realities?

No time tourists is often the argument used against time travel. But wouldn't the same argument apply to alternate time realities?
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Old 11-21-2017, 04:19 PM
Mangetout Mangetout is online now
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When time machines were invented, people travelled back in time extensively and accidentally altered the flow of history in such a way that eventually resulted in time machines never being invented, ever. We happen to inhabit the universe that is the result of that process.
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Old 11-21-2017, 04:25 PM
Little Nemo Little Nemo is online now
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Who did that cartoon where a time traveler materializes and is immediately shot by an SS guard. "Another one, " he says to his companion. "Our Fueher is a great leader, but why do so many time travelers try to kill him?"
Winston Rowntree
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Old 11-21-2017, 04:30 PM
Czarcasm Czarcasm is online now
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When time machines were invented, people travelled back in time extensively and accidentally altered the flow of history in such a way that eventually resulted in time machines never being invented, ever. We happen to inhabit the universe that is the result of that process.
And we should be thankful for that, because in all the other scenarios but one they destroyed the universe.
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Old 11-21-2017, 04:38 PM
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My reasoning is thus, if it were possible, then the inventors of such would likely have "fixed" the worst parts of the past. i.e. killed Hitler or other murderous political leaders prior to their killing so many. They could not likely do much about natural disasters, but they could affect things brought about by humans. Yes, it is possible that the creators of a time travel machine could be bad, but then it is just as likely that they would have good intentions.
I don't see why they would do that - if they go back and make a major change to the past, it will probably result in the inventor and his entire society vanishing into never-existence on the spot. The second world war had such huge effects on politics, society, people living or dying, people meeting, and technological development that majorly altering it will almost certainly majorly alter what happens after in large and hard-to-predict ways. I also doubt that future people would feel some strong moral imperative to save people in the past who are already dead, much like there's no big push for the US to go in and sort out Africa.

Also if you kill Hitler as a one-off, you may end up with a much worse result. The timing of the atomic bomb is pretty perfect for it to be used in anger (so that people are scared of it) before there were enough of them to really damage the world. Imagine that killing Hitler results in a more rational Germany that still wants revenge against France, but waits a few years to build up gradually, and develops an atomic bomb via the scientists it doesn't persecute. It's quite possible that WW2 then ends with a large scale nuclear exchange. Or maybe killing Hitler makes WW2 end early, so the bomb is never used, so couple of decades later the US and USSR launch a doomsday war since no one has that visceral dread of atomic bombs.
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Old 11-21-2017, 05:08 PM
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Anyway, my question is simply this: Do temporal paradoxes prove time travel is impossible?

Because I know of no other example of this in physics. There are no motion paradoxes. There are no gravity paradoxes. There are no mass paradoxes. You see what I am getting at.
There are lots of paradoxes in physics. For instance, the paradox of matter falling into a black hole. We take the phenomenon of infalling matter to be routine and even posit black holes colliding and merging. The paradox is, how can this be when time dilation at the event horizon should cause time to freeze from the standpoint of an external observer? There was some discussion of that here and elsewhere in that thread.
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You can hypothesis an infinite number of universes where all possibilities have happened, and if you change something in the timeline, it ends up in another universe, not affecting the universe you (if there even is a you) are traveling thru.
I've always liked that hypothesis. In fact, Hugh Everett's "many worlds" is a subset of it to explain the quantum paradox of superposition and quantum collapse.
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I have heard that argument before. I think it is called the multiverse theory, of alternate quantum realities or something. I don't know (IANAPhysicist).

But that actually brings up another question, in my mind at least. Then where are all the time tourists, from other realities?

No time tourists is often the argument used against time travel. But wouldn't the same argument apply to alternate time realities?
Not necessarily. What if there are an infinite number of those universes? We don't know how to work out those probabilities. As an analogy, we know that there probably aren't aliens among us and never have been, yet this is not inconsistent with the view that intelligent alien life is both very likely to exist and very unlikely to ever contact us. It may be a lot like that.
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Old 11-21-2017, 05:09 PM
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But will Hitler still be remembered in a thousand years? Even if he is, will people feel a personal connection to his crimes? How many modern people would feel the need to go back in time and kill somebody like Tamerlane?
Going back in time from 2017 to kill Hitler before he can take control of Germany would change the world of 2017 a lot. I am 49 years old I have a very hard time imagining that my parents would have met without world war 2. That is probably the case of for at least 1/2 or more of people born to people who met after WW2. In Europe it is probably much much higher. If Hitler was not around would the world powers have done more to keep Japan from taking Manchuria? If Chiang Kai-shek was able to not be so week against the Japanese would communism have taken hold in China? To go back in time more than a few generations to fix big problems is to basically kill off every body you know.
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Old 11-21-2017, 05:12 PM
Andy L Andy L is offline
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*Reminds me of a cute story where someone noticed that any civilization which attempted to build a time machine would experience a civilization ending crisis prior to completion. He proposed cosmic censoring of time travel as explanation. He proposed to his emporer to leak the existence of a partially completed time travel device and plans for completion to their rivals, hoping to doom them. The universe nipped the plan in the bud by having their sun go nova while they were discussing the feasibility of the plan. Fine for stories, but...
"Rotating Cylinders and the Possibility of Global Causality Violation" by Larry Niven, in fact.
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Old 11-21-2017, 05:16 PM
TimeWinder TimeWinder is offline
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But that actually brings up another question, in my mind at least. Then where are all the time tourists, from other realities?
In the many-branches model, though, we're usually talking about effectively infinite universes. Given finite time tourists, you'd expect few or none to appear in any given universe.
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Old 11-21-2017, 05:32 PM
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Obligatory SMBC references:

https://www.smbc-comics.com/comic/killing-hitler

https://www.smbc-comics.com/?id=3266

https://www.smbc-comics.com/?id=2920
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Old 11-21-2017, 05:51 PM
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The least impossible method of timetravel is probably the relativistically-displaced wormhole concept. Here's a quick explanation.
https://en.wikipedia.org/wiki/Wormhole#Time_travel

But you don't start to get 'time travel tourists' until after the first time-travel wormhole is invented. In fact the universe which allows time-travel-wormholes exists in two very different states; the normal state, before time-travel-wormholes are invented, and causality is conserved; and the abnormal state, in which time-travel tourists are everywhere, and causality is an endangered concept.
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Old 11-22-2017, 03:24 AM
SamuelA SamuelA is offline
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The least impossible method of timetravel is probably the relativistically-displaced wormhole concept. Here's a quick explanation.
https://en.wikipedia.org/wiki/Wormhole#Time_travel

But you don't start to get 'time travel tourists' until after the first time-travel wormhole is invented. In fact the universe which allows time-travel-wormholes exists in two very different states; the normal state, before time-travel-wormholes are invented, and causality is conserved; and the abnormal state, in which time-travel tourists are everywhere, and causality is an endangered concept.
One of the ways to resolve this is that if you build such a wormhole, since an infinite number of possible time travelers could come out (not travelers in the sense of full human beings in practice, just subatomic particles), you do start to get an infinite flow of travelers. The catch is, every particle the wormhole emits costs it mass-energy, so what would actually happen is that the instant you put the wormhole into a configuration that permits time travel, a flood of light comes out of it and it basically just explodes basically instantly.
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Old 11-22-2017, 03:55 AM
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-If there are multiple futures, like alternate dimensions, then how does the mass of the universe double every millisecond as random choices produce multiple versions of the future?
I've often wondered that (actually it would not just be doubling, because not every choice is binary, and not every choice is independent of others - it would be increasing infinitely at every clock tick.

Or, there's one universe that contains stuff - our perception of that stuff being arranged into time, space and the precise possibilities we have experienced, is just a viewpoint - an interpretation of the 'stuff'. Had we made different choices, we would be experiencing the same stuff, in a different arrangement - or maybe those different arrangements of the same stuff are being experienced by different versions of us, simultaneously* (well that's the wrong word, since the path of time is part of the stuff)
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Old 11-22-2017, 06:51 AM
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-If there are multiple futures, like alternate dimensions, then how does the mass of the universe double every millisecond as random choices produce multiple versions of the future?
Simple really; the alternate arrangements of mass/energy manifest as what we currently call 'Dark Matter' and 'Dark Energy' - which will only be properly accessible to us when we invent time-travel.
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Old 11-22-2017, 09:20 AM
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The many-worlds interpretation of quantum mechanics only works if there's no possibility of interaction at all, not even gravitational, between the "worlds".
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Old 11-22-2017, 09:32 AM
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How about the practical approach: even if theoretically possible, time travel would be very hard to achieve. Hard enough to where it will never did happen. Sort of like me running a sub 2 hr marathon. Will never happen, regardless of theoretical possibility. (Frankly, neither will a 3 hr marathon)

I have a problem with the paradox being the reason for impossibility, though. The paradox resolved itself in that we wouldnít know of the change. I invent a time machine, kill my granddad, and cease to exist. Now thereís just some people excised from reality, but the time machine makes the concept of before and after useless. At one point I existed, and then I didnít. So what? For all we know we are missing a billion people who were around at one point and then never were.
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Old 11-22-2017, 09:35 AM
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If it were as simple as some people falling out of existence, there would be no paradox. But now ask, if you never existed, who was it who killed your father, and where did that killer come from? That's where the paradox comes in.
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Old 11-22-2017, 10:04 AM
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Originally Posted by StrTrkr777 View Post
...
My reasoning is thus, if it were possible, then the inventors of such would likely have "fixed" the worst parts of the past. i.e. killed Hitler or other murderous political leaders prior to their killing so many.
...
I agree that the universe would not have a mechanism by which it would stop such a change, so the fact that our past is littered with evil humans, leads me to believe that no one will ever invent time travel.
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Maybe we're just looking at the big picture. To us, Hitler is still part of our living history. There are people whose parents and grandparents were killed by the Nazis.

But will Hitler still be remembered in a thousand years? Even if he is, will people feel a personal connection to his crimes? How many modern people would feel the need to go back in time and kill somebody like Tamerlane?
How about this:

What we think of as the past is the sanitized version. There were vastly greater Evils perpetrated than we know about. Only small potato pipsqueaks like Hitler remain in our record. The time travelers have already fixed all the biggies.

Now there's a sobering thought. Also makes me wonder who's next to get the 'ol temporal eraser used on them. Is that somebody in our past or our future?


FTR I'm not making a serious proposal here. Just tossing out a wild conjecture for entertainment. Might make a good short story plot.

Last edited by LSLGuy; 11-22-2017 at 10:05 AM.
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Old 11-22-2017, 11:31 AM
Just Asking Questions Just Asking Questions is online now
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I came up with this theory from TNG Tapestry. I wondered, in Q's little torture experiment on Picard, what happened to all the people whose lives were totally different in the time between Picard getting stabbed/not getting stabbed and the show's present? Or the "time" (as it were) between the loss of the Enterprise C and the events of Yesterday's Enterprise? Million, billions, of people live or die who did not before. So do their lives count for nothing? Does the universe not care about their hopes, dreams, adventures? Do they just disappear, poof!, never having been? It seems so unfair.

My theory is, there are not parallel universes, there is only one, and it is a the result of probability. There is no "correct" version of the universe. Every time a time travel change is made, the universe just shifts to this new setting.

There are no "souls" and no free will in this iterpretation. The universe, as it were, doesn't care. We're all just collapsed quantum waveforms, and the version where you get killed at birth, or become Hitler, are equally valid and equally possible and equally neutral.

The universe is a giant complex equation, and it is so complex that it rearranges itself and whole civilizations appear and disappear with no great impact. In this universe, there is no "time travel" as we understand it. The universe's equation just bounces around a little.

We are not in the universe, we are the universe.

It's a very depressing interpretation of the universe.
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Old 11-22-2017, 11:44 AM
Johnny L.A. Johnny L.A. is online now
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I haven't read the whole thread, but I'm posting my thoughts anyway.

Assume space-time is a continuum. Since the Past has already happened, it can't be changed. Since we're in a continuum, the Future 'already exists' -- like the number 1,276,598 exists in the continuum even though we're living on 12. You can travel into the Future by slowing down your personal time (approaching the speed of light), but that's about it.

Assume a temporal wormhole. If you're in a single continuum, then the wormhole may deposit you in the Past or the Future, but nothing you do in the Past will change anything in the Present because whatever you did in the Past, you've already done. If you go forward, the Future is your new Present. If you hop forward a week and get the winning lottery numbers and then come back to the Present and play them, then of course you'll win because you've already won in the Future.

But if there is a multiverse with an infinite number of possibilities, then you can 'change the Past'. But in the universe you are now in, your changes already exist in that multiverse's Future.
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Old 11-22-2017, 11:53 AM
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This will sound flippant, but not meant to be: If time travel becomes possible in the future, we would have heard of it by now.
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Old 11-22-2017, 12:00 PM
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Frankly, after decades of hearing various versions of this debate, pro and con, immutable vs. multiple-world, on and on, etc. etc, I've adopted a mantra given to me by an expert in the field:

"Wibbly wobbly. Timey wimey."
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Old 11-22-2017, 12:02 PM
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This will sound flippant, but not meant to be: If time travel becomes possible in the future, we would have heard of it by now.
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Old 11-22-2017, 12:24 PM
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I came up with this theory from TNG Tapestry. I wondered, in Q's little torture experiment on Picard, what happened to all the people whose lives were totally different in the time between Picard getting stabbed/not getting stabbed and the show's present? Or the "time" (as it were) between the loss of the Enterprise C and the events of Yesterday's Enterprise? Million, billions, of people live or die who did not before. So do their lives count for nothing? Does the universe not care about their hopes, dreams, adventures? Do they just disappear, poof!, never having been? It seems so unfair.

My theory is, there are not parallel universes, there is only one, and it is a the result of probability. There is no "correct" version of the universe. Every time a time travel change is made, the universe just shifts to this new setting.

There are no "souls" and no free will in this iterpretation. The universe, as it were, doesn't care. We're all just collapsed quantum waveforms, and the version where you get killed at birth, or become Hitler, are equally valid and equally possible and equally neutral.

The universe is a giant complex equation, and it is so complex that it rearranges itself and whole civilizations appear and disappear with no great impact. In this universe, there is no "time travel" as we understand it. The universe's equation just bounces around a little.

We are not in the universe, we are the universe.

It's a very depressing interpretation of the universe.
Like I said - Heisenberg's universe... or worse yet, Schrodinger's Universe. All possibilities exist until you find the dead cat.
  #37  
Old 11-22-2017, 12:38 PM
jz78817 jz78817 is online now
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Originally Posted by CalMeacham View Post
Physicist and Science Fiction Author Robert L. Forward was wary of temporal paradoxes, and said that he wanted to see the concept rigorously proven (or disproven) -- something much more formal than, say, my sig line below.

I observe that in his time-travel novel Time Master he takes the view that anything you do in the past becomes an irrevocable and necessary part of that past -- no "alternate realities", "branch points", "Many Worlds" stuff. the situation in the book, in fact, resembles an example he used in one of his papers.

Of course, that's unsatisfying to most of us. Larry Niven wrote an essay "The Theory and Practice of Time Travel" back in the 1960s (It's in his collection All the Myriad ways) in which he looks at different methods of time travel and at the paradoxes involved, without committing to any of them. (If you went back in time to the Crucifiction with a machine gun, he observes, tongue in cheek, your gun would positively jam.)

the 2002 film version of The Time Machine kind of addressed it that way:

SPOILER:
Alexander invents the time machine after his fiancee was shot and killed by a mugger. He goes back and prevents the murder, but then she's struck and killed by a carriage shortly thereafter. realizing no matter what he does she will still die at the same point in time, he travels around time to see if he can find a solution. accidentally traveling thousands of years into the future. the "Uber-Morlock" explains why there's nothing he can do to save Emma:

"You built your time machine because of Emma's death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you."
  #38  
Old 11-22-2017, 01:15 PM
B-Rad B-Rad is online now
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Originally Posted by Johnny L.A. View Post
Assume space-time is a continuum. Since the Past has already happened, it can't be changed. Since we're in a continuum, the Future 'already exists' -- like the number 1,276,598 exists in the continuum even though we're living on 12. ...
Basically, Vonnegut's depiction of time in Slaughterhouse 5. Free will is a human construct. Time/history are immutable, everything that will happen in the future is already set. So, even if you invent a time machine and go backward or forward, you will change nothing because everything will always be as it has always been, including your travels and any butterflies you crush. Want to take a machine gun to Calvary? Fine, but you won't do anything with it, because a machine gun is never used at Calvary.

I like this because it is simple, and it comports to predictable mathematics. Once something is set in motion, it will not alter its course until it is acted upon by something else. Given the human mind is the product of the arrangement of physical things (neurons, chemicals, etc.), even human behavior ought to be predictable IF we know all the connections, what they mean, and what stimulus is coming--there will be no free will, the "mind" will respond according to its construction. The concept of entropy is just intellectual shorthand for, "I didn't know all the variables and/or couldn't work out the math." There is no reason to suggest anything random happens in the universe, simply because there are too many moving parts for our simian noodles to keep track of. With no random actions, there are no multiple possibilities on the timeline.
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Old 11-22-2017, 02:42 PM
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Originally Posted by Mangetout View Post
When time machines were invented, people travelled back in time extensively and accidentally altered the flow of history in such a way that eventually resulted in time machines never being invented, ever. We happen to inhabit the universe that is the result of that process.

Or deliberately, as Asimov did 62 years ago.
  #40  
Old 11-22-2017, 02:45 PM
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Originally Posted by TimeWinder View Post
In the many-branches model, though, we're usually talking about effectively infinite universes. Given finite time tourists, you'd expect few or none to appear in any given universe.
But since the time tourists would be definition be returning to a different universe than the one they came from, I doubt it would be too popular a trip.
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Old 11-22-2017, 02:48 PM
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Originally Posted by Chronos View Post
If it were as simple as some people falling out of existence, there would be no paradox. But now ask, if you never existed, who was it who killed your father, and where did that killer come from? That's where the paradox comes in.
Simple. It’s the me from before I didn’t exist. It is only a paradox if you try to make time, which is now obviously looping or at least bending, linear again by imposing linear cause-effect rules on it. (Also, I killed my grandfather. Killing my dad - you think I’m some sort of monster?)

Last edited by Isosleepy; 11-22-2017 at 02:49 PM.
  #42  
Old 11-22-2017, 02:49 PM
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Originally Posted by gazpacho View Post
Going back in time from 2017 to kill Hitler before he can take control of Germany would change the world of 2017 a lot. I am 49 years old I have a very hard time imagining that my parents would have met without world war 2. That is probably the case of for at least 1/2 or more of people born to people who met after WW2. In Europe it is probably much much higher. If Hitler was not around would the world powers have done more to keep Japan from taking Manchuria? If Chiang Kai-shek was able to not be so week against the Japanese would communism have taken hold in China? To go back in time more than a few generations to fix big problems is to basically kill off every body you know.
More than that. My parents were married before WW II, but if my father wasn't in Europe for a few years they might have had a child before me, and, trust me, that changes lots of things. None of us would have been born if the phone had rang at an inopportune time. (Someone else with different genes would have been born instead.)
Just about any time travel would have immense ripple effects, even if it does not start or stop wars.
  #43  
Old 11-22-2017, 03:00 PM
Trinopus Trinopus is offline
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Originally Posted by B-Rad View Post
. . . I like this because it is simple, and it comports to predictable mathematics. Once something is set in motion, it will not alter its course until it is acted upon by something else. . . .
I don't want to highjack the thread...but to me, the problem with absolute Newtonian determinism is: where does the information come from? Shakespeare's sonnets must have been encoded, in some form, at the Big Bang, and only "shook out" as they were destined to. But where did that encoding come from?

I think you need some fluidity in the time-stream, in order for evolutionary processes to "create" the information we see all about us.

lissener does bring up an excellent point: if Time Travel were possible, big historical events should be just rotten with tourists. Maybe the travellers have developed really good stealth tech, and we don't see them. But, as always, the temptation to interfere is just too great for darn near anyone to resist.

(And who knows what the chaos threshholds are. Just observing a big historical event might introduce small changes -- oops, I bumped into a guy who jostled another guy, who cussed out a third guy, who misses meeting the next lady over, who never gives birth to the Emperor Hadrian's great-grandfather...)
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Old 11-22-2017, 03:05 PM
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Originally Posted by Isosleepy View Post
Simple. Itís the me from before I didnít exist. It is only a paradox if you try to make time, which is now obviously looping or at least bending, linear again by imposing linear cause-effect rules on it. (Also, I killed my grandfather. Killing my dad - you think Iím some sort of monster?)
???

Could someone please provide or link to at least a semi-rigorous description of a quantum particle in the presence of a closed time-like curve, so we can see whether such a thing is at first glance OK or violates some obvious physical law.
  #45  
Old 11-22-2017, 03:06 PM
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The ripple effects are greater than that, even. Think of all of the millions of sperm in a single ejaculation, and how small the distances are between them. If your parents' positions were even that much different at the time of the act, then it probably would have been a different sperm that won the race. And that's even assuming that we had the same set of sperm and the same egg to begin with, but that's also the result of very small random processes.
Quote:
Quoth Isosleepy:

Simple. It’s the me from before I didn’t exist. It is only a paradox if you try to make time, which is now obviously looping or at least bending, linear again by imposing linear cause-effect rules on it. (Also, I killed my grandfather. Killing my dad - you think I’m some sort of monster?)
It's you who is trying to impose linearity, with language like "the me from before I didn't exist". When is that "before"? It can't mean "at an earlier time". So what does it mean? Is there some other meta-time in which one timeline existed "first", and then some other timeline exists "after" that one, and so on? In that case, is it possible to travel through that meta-time? You're so used to assuming a linear time that, when asked to consider a nonlinear time, you invent a whole new linear time to embed it in, without even realizing that that's what you're doing.
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Old 11-22-2017, 03:22 PM
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@Isosleepy - how does your "simple" approach to paradox resolution explain a situation like this: a time traveller takes a copy of the Shakespeare's plays back in time and gives it to a young William Shakespeare. Shakespeare copies them and those copies become the "original" plays. Now they seem to have been created without anyone actually writing them.

Or even simpler, you throw a ball into a time machine. The ball emerges a few seconds in the past and knocks aside the earlier version of the ball so that it never enters the time machine. There seems to be no consistent way to describe this event. If the ball didn't enter the time machine, where did the second ball come from that prevented it from doing so? And if it did enter the time machine, then it prevented itself from entering the time machine, so it didn't.
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Old 11-22-2017, 03:49 PM
Asympotically fat Asympotically fat is offline
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Einstein's theory of general relativity apparently admits solutions with "time travel". More specifically it allow closed timelike curves (CTCs) in which a test particle travels around in a closed loop of time where its past is its future and its future is its past such that there is no distinction you can make between its past and future. CTCs even occur in what otherwise seem like physically reasonable solutions for example the Kerr solution which describes a rotating black hole contains CTCs.

However it is questionable whether CTCs really constitute time travel, at least the sci-fi notion of time travel. By the nature of CTCs there can be no global distinction between past and future in any region of spacetime that contains one. Also CTCs describe the trajectories of theoretical test particles lacking any mass-energy rather than real particles through spacetime. CTCs though clearly strike a blow against causality and indeed any spacetime containing a CTC can be described as acausal. CTCs do not introduce any degree of stochastic behaviour in spacetime, but they always mean the spacetime is inherently unpredictable in that there is no set of initial data that will allow the exact prediction of the future state. and in fact to achieve predictability even stronger conditions than "no CTCs" must be applied.

Perhaps more fatal to any potential idea of time travel physics seems to conspire to prevent them from existing or at least hides them from view. A few reasonable assumptions on the mass-energy contents tends to suppress CTCs or make the solutions which contain them unstable or hide them behind event horizon. For example the CTCs in the Kerr solution are hidden in the black holes event horizon in a region of the Kerr solution that is unstable.

CTCs do not inherently create paradoxes, at least within the theoretical confines of general relativity, more they are seen as problematic as, though opinions differ, CTCs are usually seen to some degree or another as hallmarks of unphysicality rather than truly offering the chance of time travel.
  #48  
Old 11-22-2017, 03:58 PM
Isosleepy Isosleepy is offline
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Originally Posted by markn+ View Post
@Isosleepy - how does your "simple" approach to paradox resolution explain a situation like this: a time traveller takes a copy of the Shakespeare's plays back in time and gives it to a young William Shakespeare. Shakespeare copies them and those copies become the "original" plays. Now they seem to have been created without anyone actually writing them.

Or even simpler, you throw a ball into a time machine. The ball emerges a few seconds in the past and knocks aside the earlier version of the ball so that it never enters the time machine. There seems to be no consistent way to describe this event. If the ball didn't enter the time machine, where did the second ball come from that prevented it from doing so? And if it did enter the time machine, then it prevented itself from entering the time machine, so it didn't.
The second ball came from before the ball was knocked out of the time machine. Shakespeare got to copy plays from Shakespeare who had to figure them out (or steal them from Contemporaries). It is a paradox to an observer who knows of both states, and who canít reconcile them, cannot construct a timeline in which all of it happens. If there ever was a moment where there was no universe, I posit there was an occurrence without cause to make the universe into being. Or some sort of loopy thing as above. If either is true, then the whole linear cause-> effect doesnít always have to occur.
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Old 11-22-2017, 04:39 PM
Chronos Chronos is offline
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Again, you're describing the possibility of time travel by inventing some other "time" through which travel is impossible. What is this "before" you speak of?
  #50  
Old 11-22-2017, 04:56 PM
Just Asking Questions Just Asking Questions is online now
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Originally Posted by markn+ View Post

Or even simpler, you throw a ball into a time machine. The ball emerges a few seconds in the past and knocks aside the earlier version of the ball so that it never enters the time machine. There seems to be no consistent way to describe this event. If the ball didn't enter the time machine, where did the second ball come from that prevented it from doing so? And if it did enter the time machine, then it prevented itself from entering the time machine, so it didn't.
That's an easy one. Each time travel creates a new reality.

Initial reality: time machine sits on table, nothing comes out. One ball exists in the universe. You throw ball into time machine.

First change: ball coming out of time machine creates new time track. Initial ball does not go into time machine. Universe now has extra mass of one ball. Initial time track "no longer exists" or "never existed" (for lack of better terminology to describe it).

but you say - this violates conservation of mass! And it would, except the laws of physics allow fudging. If the universe ended with the extra mass, then there would be a violation, and the entire universe would disappear. But since it never ends, the extra mass is tolerated. The books don't have to balance until the end.

Each time something travels through time, it creates a new reality.

Did you ever wonder what happened to the Marty McFly who owned the truck and had cool parents? He actually created a new reality when he traveled, and he's still out there, somewhere (some when?). Perhaps he actually followed Doc's advice, and made no significant changes to the timeline, so when he returned to 1985 he couldn't tell he wasn't in his original universe. But he is no longer in the universe of the movie. Marty Prime has no connection to the universe of the end of the film. None of the atoms in his body came from that reality. He has no connection to it at all. Good thing, because if Marty 2.0 came back to the same reality he left, he'd want his life back. And poor Marty Prime would be forced to live with Doc.
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