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.
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.)
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…
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 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.)
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.
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,
Not really, since Niven’s Law is based on the (unstated) assumption that Novikov’s principle is false.
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?
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.
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:confused:?
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.
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.
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.
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.
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.
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.
