Paradox-free time travel is theoretically possible

https://www.npr.org/2020/09/27/917556254/paradox-free-time-travel-is-theoretically-possible-researchers-say

84 206 6 years ago

npmaile• 6 years ago

When you take this theory to it's logical conclusion, you end up with some strange outcomes. The strangest is the idea of going back and killing baby Hitler. When you attempt to do that, the universe, in it's self righting ways makes the assassination attempt not work. Since Hitler is a pretty big enemy for a lot of people, many people over time try to go back and kill Hitler.

Hitler, on the other hand is just living his life as an Austrian child and young adult suffering from constant attempts on his life from people who he has no way of knowing. He gets the idea that people are always out to get him (because they are) and becomes evil, and uses the assassination attempts to justify creating the Holocaust.

gear54rus• 6 years ago

Self-fulfilling prophecy, yes.

But the article reads like some sci-fi bs tbh. What's considered the 'same outcome'. If I kill baby Hitler, would the universe create another just like him? Or someone else will do same things? If someone else does them, surely it won't (for example) kill as much people. So someone dead would now be alive in the future (or vice versa). And now the outcome isn't the same yeah?

Sorry but this is bs.

suby• 6 years ago

I find the concept absurd as well. If you do something like go back in time and murder Hitler, you're going to be impacting the actions of every single human being alive. Within a few generations there's going to be an entirely different set of people alive than the set of people alive in the universe in which Hitler was not murdered. This idea that you would still have to go back in time because the universe corrects itself makes no sense because there's just no way you would still exist. I'd be willing to entertain the idea that someone else would go back in time for the same reason, but it's all just wild speculation.

MetaWhirledPeas2• 6 years ago

This is all just a bunch of nonsense, but I enjoy discussing it regardless.

Why bother going back so far? Let's think of what would happen if you went back 30 seconds. As soon as you arrive in the past, take a baseball bat to the time machine. What then?

And then let's take the article's scenario at face value. If you went back 5 minutes and somehow events always led back to you successfully using the time machine, your consciousness would be stuck in a permanent 5-minute loop. Maybe you could spice up those 5 minutes but you'd be right back where you started soon enough. Wouldn't the same be true if you went back 5 seconds? That would be hell.

But that's not the end of it. If you went back in time, why would we assume you'd displace to a new location? One of two things would need to happen: A) you'd need to teleport AND erase a blank area of space perfectly sized for your arrival, or B) you'd rewind like a tape playing backwards.

I like to think of the B scenario. If it were true we would never know it. The effects of the time machine would never be observable. We could have working time machines all around us right now for all we know. As I sit in this chair I could be at the destination of a trip through a time machine which I will one day invent. And when I finally enter the machine and flip the button, nothing will seem to happen. A pointless device.

captn3m0• 6 years ago

Somewhat portrayed in a "Love, Death & Robots" episode (Alternate Histories).

https://www.imdb.com/title/tt9788516/

Want to see Hitler die in a variety of comically fantastic ways? Now you can. Welcome to Multiversity!

MetaWhirledPeas2• 6 years ago

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

I'm sure these researchers are pretty smart, but nothing in this article goes beyond the level of a grade-school debate. Maybe there's something enlightening in the paper. But if there was, you'd think it would make its way into the article.

paxys2• 6 years ago

Exactly. The article mentions the grandfather paradox, but doesn't really address it at all. Can't "self correct" shooting your past self in the head.

mpalmer• 6 years ago

Think what they're getting at is that it won't happen that way. Gun will jam, you die of a brain aneurysm. We don't exist in a universe where impossible things happen, so we can't bring them about. But yes, the article doesn't really sink its teeth into it.

gremlinsinc• 6 years ago

It also pre-supposes that you aimed to kill your grandfather. Perhaps you accidently did this. The reason for you going back in time still exists.

However, I believe the worst that could come of this would be to split the timeline in the case that string theory and multiverses exist where the timeline we return to is one that is different than the one we came from.

What if we really are living in a simulation?

Could it be possible that any moment we arrive in the past would possibly have a random outcome on the future? What if every choice from the moment you arrive to the moment you left could have a random effect, and thus the world you arrive back at if it is a closed loop could be different as night and day, simply because all events that happened after your arrival were erased and all choices were given a second chance to arrange themselves like coin tosses mixed with end-user free agency.

If that were the case, how would you prove that it's a randomized closed loop vs string theory since to you the time-traveler, everything has changed which the easiest reason would be you're in a different timeline. It never dawned on you that events along the timeline could be shuffled at anytime and are inconsequential to how the universe works because we're actually in a simulation.

markus_zhang• 6 years ago

I think it's still a lot easier to treat each going-back-to-the-past as a fork().

forgingahead• 6 years ago

Given that this is NPR, a reference to Gell-Mann amnesia[0] seems appropriate:

“Briefly stated, the Gell-Mann Amnesia effect is as follows. You open the newspaper to an article on some subject you know well. In Murray’s case, physics. In mine, show business. You read the article and see the journalist has absolutely no understanding of either the facts or the issues. Often, the article is so wrong it actually presents the story backward—reversing cause and effect. I call these the “wet streets cause rain” stories. Paper’s full of them.

In any case, you read with exasperation or amusement the multiple errors in a story, and then turn the page to national or international affairs, and read as if the rest of the newspaper was somehow more accurate about Palestine than the baloney you just read. You turn the page, and forget what you know.”

[0] https://www.epsilontheory.com/gell-mann-amnesia/

nimbleal• 6 years ago

I didn’t know this had a name, but it’s long been a cause of amusement (or despair!) for me. I too have noticed that any time I read an article on a subject on which I have even a passable level of expertise, I find it chock-full of errors.

pontifier2• 6 years ago

I take it with a grain of quantum physics. When traveling back in time you are entangling the state of the moment of time travel with a past state of the universe, and the universe must collapse into a steady state with a single history... The history in which the time travel did nothing.

omarhaneef• 6 years ago

Just give it to me in popular movies: Back to the future style time travel can’t happen but Terminator style time travel can.

It doesn’t say what constitutes a “major” change and what is a minor one though. The only guidance we are given is that the original motivation for going back must be preserved but otherwise very large changes can happen (planet of the apes marky wahlberg version)

TimTheTinker• 6 years ago

Pasting the abstract from the paper:

The theory of general relativity predicts the existence of closed time-like curves (CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a framework for deterministic, reversible, dynamics compatible with non-trivial time travel, where observers in distinct regions of spacetime can perform arbitrary local operations with no contradiction arising. However, only scenarios with up to three regions have been fully characterised, revealing only one type of process where the observers can verify to both be in the past and future of each other. Here we extend this characterisation to an arbitrary number of regions and find that there exist several inequivalent processes that can only arise due to non-trivial time travel. This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.

FWIW, the last sentence of the paper itself (prior to the conclusion) reads: It is an open question how generic is this situation and what are the spacetimes and physical systems for which nontrivial, self-consistent time travel is possible.

As usual, non-scientific reporting overplays the ramifications.

Link to paper: https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc

baking• 6 years ago

"Further studies will be necessary to find genuine physical scenarios realising the acausal processes we have discovered."

To bad no one went back in time to tell the authors whether they actually discovered anything or not.

rhn_mk1• 6 years ago

Maybe we just need to wait until someone builds the womhole or the time machine from Primer.

mmaunder• 6 years ago

This suggests concepts like fate and the universe having a will and intent. It seems absurd. Wouldn’t a branching creating a new universe with its own timeline make more sense? Each unit of influence you exert as a time traveler branches a new universe in which you continue to exist and which has a completely new and separate timeline without paradoxes.

spijdar• 6 years ago

Neither possibility really makes more "sense" than the other. And it's effectively impossible to prove experimentally -- in a "many branching worlds" universe, you could never conclusively prove or disprove its existence, even if you experimentally demonstrated this kind of "self correcting" time travel, as you may have simply created a branching universe where it appears to you that you did so.

It's metaphysical philosophizing as far as I'm concerned -- interesting, but not terribly applicable, and heavily influenced by one's own beliefs and preexisting notions about the world.

ex3ndr• 6 years ago

No, because time and universe is not discrete, there are no branching.

v642• 6 years ago

Sounds like they're describing the Novikov self-consistency principle [1] in a roundabout way.

[1] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

dghf• 6 years ago

Are they, though? I thought the Novikov conjecture postulated that time-travel would change nothing at all: if you travelled back to the past, you had "always" travelled back to the past, and anything you did there was already part of history before you started on your journey (aargh, time-travel tenses are hard). Michael Moorcock's Behold the Man[0] would be a good fictional example.

This, on the other hand, suggests you can change the fine detail of history (e.g. the identity of patient zero) but not its broad strokes (the occurrence of a global pandemic).

At least, that's what I gather from the article: I haven't read the paper, and suspect it would be a waste of time as my maths and physics are both rudimentary.

[0] https://en.wikipedia.org/wiki/Behold_the_Man_(novel)

musingsole• 6 years ago

If there are such physical laws to allow details to change while broad strokes must happen, I'd be curious what exactly differentiates a detail from a broad stroke. But all the same, this idea imagines a more timey-wimey view of history that doesn't have just one state. This is an idea explored heavily in Neal Stephenson's "The Rise and Fall of D.O.D.O"

pontifier2• 6 years ago

Im reading "The Time Patrol" by Paul Anderson. It describes that even if an individual is killed, it's unlikely to matter much because a species is more like a collective DNA wise. They even go hunting in the past, have possible children there, etc. It's only in very specific cases that the time patrol has to step in to make sure that "X" specific significant event happens the way it's "Supposed to"

I haven't finished the book yet though :)

v642• 6 years ago

I thought the Novikov conjecture postulated that time-travel would change nothing at all

Not exactly, from the wiki article:

The simplest way to impose the principle of self-consistency in quantum mechanics (in a classical space-time) is by a sum-over-histories formulation in which one includes all those, and only those, histories that are self-consistent.

So it is possible to change the past, but only in ways that are self-consistent and don't give rise to paradoxes.

Regarding the ramifications for free will:

Novikov supports this point of view with another argument: physics already restricts your free will every day. You may will yourself to fly or to walk through a concrete wall, but gravity and condensed-matter physics dictate that you cannot. Why, Novikov asks, is the consistency restriction placed on a time traveler any different?

hawktheslayer• 6 years ago

In Isaac Asimov's "End of Eternity" he posed a similar concept where contrary to the butterfly effect where minor changes ripple through time, time has a mechanism of self-preservation which is resistant to change. Asimov never ceases to amaze me with how prescient he was for his time.

interfixus• 6 years ago

Such is also the underlying premise in Poul Anderson's Time Patrol universe.

djmips• 6 years ago

I don't think you can travel back in time so it's moot as far as I'm concerned.

newsbinator• 6 years ago

Why would it be impossible to travel back in time?

elil172• 6 years ago

The article makes it seem like "the timeline corrects itself." What the scientific paper actually says (as best I can understand it) is that, if the universe is entirely deterministic, the conditions necessary for time travel can only occur when the time traveling wouldn't cause a paradox. As the authors note, this isn't super relevant to our universe because:

- the conditions necessary for time travel probably don't exist

- the universe probably isn't totally deterministic

pdonis• 6 years ago

What the scientific paper actually says (as best I can understand it) is that, if the universe is entirely deterministic, the conditions necessary for time travel can only occur when the time traveling wouldn't cause a paradox.

It's actually even more general than that: under non-deterministic laws (such as the way most people currently understand quantum mechanics), if time travel is possible at all, the probability of it will only be nonzero for time travel scenarios that do not cause a paradox. (I don't think this is discussed in the paper referenced in the article, but it is discussed in some of the other papers that that paper references--IIRC the Novikov paper is one of them.)

elil172• 6 years ago

Thank you! That explanation actually makes the paper make a lot more sense.

moonchild• 6 years ago

Relevant webcomic: https://www.smbc-comics.com/comic/2013-02-09

xwdv2• 6 years ago

It’s impossible to create a paradox, because the events that led to you being able to go back in time were created by whatever you did in the past in an attempt to change the future.

If you did successfully change the past so that you never ended up going back, you would not live the life where you go back in time to make that change. But if you did find yourself even going back in time it means your efforts already failed anyway, so might as well not even bother.

maxerickson• 6 years ago

It's possible that reality is defined by different rules than you are reasoning with.

(I don't think arbitrary travel backwards in time is possible in our universe; there could be very rare circumstances where it ends up occurring, but not where you can build a device that takes you anywhen)

paxys2• 6 years ago

This is the "there's no such thing as free will" argument, which is even more implausible than time travel.

xwdv2• 6 years ago

If cause comes before effect, then there is free will but there is a limited possibility of effects. If cause comes after effect, there isn’t free will but there is an unlimited possibility of effects.

ffhhj• 6 years ago

Why time travelers in sci-fi also move to the position and rotation of the Earth at the given moment? They should end floating in space.

nirui• 6 years ago

Oh actually, I've found the way to convenience myself about that: The "reality" is, when somebody travels through time, most of them (god knows how many) got killed in the process, but the story will just pick another "instance" who has survived the journey from the pool of "Multiverse" and continues.

pmontra• 6 years ago

I watched this short movie yesterday night "One-Minute Time Machine" https://www.youtube.com/watch?v=vBkBS4O3yvY

Only 5 minutes and it's kind of along this line of thought. I won't spoil the details.

jennyyang• 6 years ago

Time travel violates the law of conservation of energy. By travelling back in time, you yourself are injecting more mass/energy into a system. So it's great as a science fiction trope, but if you're going to talk physics, that needs to be addressed.

dnautics3• 6 years ago

Iirc there is no requirement for mass/energy to be conserved, and some models of expansion require conservation to be violated.

nautilus123• 6 years ago

Depends if the system is open or closed and if mass is converting to energy or visa versa. Also conditions near big bang are not representative.

loblollyboy• 6 years ago

I don’t have the details but I’m pretty sure passage of time is due to a tendency for entropy to increase, and that if I go back in time a second by, say, walking backwards, it is no more a violation of the cons of energy as if I walk forward.

nautilus123• 6 years ago

I think the point about conservation of mass still applies. There are literally 2x yourself at a certain point in time, mass is not conserved.

akvadrako2• 6 years ago

How do you know you aren't walking backwards back in time? To be complete you'll also need to be losing your new memories and becoming younger - it's experimentally undistinguishable.

deviation2• 6 years ago

I suppose it would be possible under certain assumptions- The main one being that both the past and present would need to be parts of the same system.

Take this with a grain of salt, as I'm vastly underqualified to even form an opinion on this subject.

rk06• 6 years ago

it is possible to explain it assuming there are multiple timelines. overall multiverse has finite energy/mass.

when you move to a different timeline, you are moving mass from a timeline to another timeline, so total mass in multi verse remains the same

amelius• 6 years ago

Just consider the entire thing (spacetime) as one system.

noxer• 6 years ago

If you could just send information back in time that would not violate conservation of energy. But otherwise would have a similar effect as traveling back yourself. You simply tell your past what to do rather than going back and to do it "with your present yourself". This also avoids a lot of other paradoxes and we already know that the reverse (sending information to the future) is super simple.

shmageggy• 6 years ago

One of the short stories in Ted Chiang’s recent book Exhalation deals with this theme. It’s a pretty interesting and entertaining read.

whoisburbansky• 6 years ago

I believe you're referring to The Merchant and The Alchemist's Gate. All of Exhalation is great, but I particularly enjoyed that one!

MH15• 6 years ago

Yes, the chapter titled "The Merchant and the Alchemists Gate". In my opinion one of the more powerful shorts in the collection.

krapp• 6 years ago

I find every solution except "time travel always results in a parallel universe" to be too messy and weird. I also prefer that solution for faster than light travel, since that's also basically time travel.

Of course, this is entirely an aesthetic opinion and not scientific on my part.

jand• 6 years ago

Are there ideas out there which "explain away" the problems arising from planetary/stellar movement?

My naive understanding of time travelling back a single day would put me in a spot where earth will be a day later.

psyc• 6 years ago

And why stop this line of reasoning with our local star system? There’s no absolute reference frame.

CogitoCogito• 6 years ago

The time-travel in the third Harry Potter book seemed consistent and paradox-free. Seems like a good reference for this sort of discussion.

greentimer• 6 years ago

Worth noting that you can’t conclude time travel is impossible or irrelevant just because other people have. There could exist time travelers who secretly have control either of your life or the world with advanced technology (potentially up to the level of mind control chips) who are keeping the evidence from getting out.

mikemoka• 6 years ago

what if he goes back in time to the moment before leaving and disintegrates the time machine?

if selfconsistency is right it makes me wonder if a time traveler can actually be interacting with the past around him

8bitsrule2• 6 years ago

I've never seen the past or future. Only physical things still or moving, combining with other things or not ...

What we call 'the past' is a mental picture of where physical things used to be positioned and their condition. But everything in 'that now' has changed. Just as when we 'travel' in physical space, when we get 'back home' it has changed.

If I were to 'time travel' I'd be gone from now. But everyone else would go on living in now. So where was the 'instant of time' I travelled to stored so I could go there? And the infinite number of instants between it and now?

Fanciful idea, but no evidence ... none zero zip nada

drdeca3• 6 years ago

How do you reconcile this idea with special relativity?

Daniel_sk2• 6 years ago

By the speed of how these things move, interact or change state relative to each other.

npmisdown• 6 years ago

Every single oscillating particle needs to travel more absolute distance when speed increases, so it will oscillate with lower frequency for the external observer, thus causing time dilation.

GoblinSlayer2• 6 years ago

In special theory of relativity time is local, so objects interact only with immediate moment - present, and not with past and future. You need to first somehow overcome locality to postulate global time, which you would be hard pressed to do.

8bitsrule2• 6 years ago

The Earth is not the center of the universe. Similarly, time-travel seems to be an idea still stuck in the Newtonian idea of absolute time. As SR demonstrated: There is no such thing.

“Einstein said, ‘Time has no independent existence apart from the order of events by which we measure it,’”

https://phys.org/news/2011-04-scientists-spacetime-dimension...

guerrilla• 6 years ago

So where was the 'instant of time' I travelled to stored so I could go there?

Not where, when. [1] The block universe ontology is pretty commonly offered as an interpretation of general relativity. People consider GR as pretty strong evidence for such an ontology. Personally I don't but I'm just saying your criticism isn't very good.

1. https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time...

GoblinSlayer2• 6 years ago

That article doesn't mention general theory or relativity, quoted discussions indicate that eternalism mostly relies on geometric expression of (plain) Minkowsky spacetime, which is interpreted literally as indication of existence of global time dimension. General theory or relativity is bad news for eternalism, because curved space is more obviously local and thus less compatible with eternalism which favors globality.

deviation2• 6 years ago

I'm so agnostic concerning the concept of time itself at the end of the day. I feel that we're so busy struggling to find a way to make time travel work, that we forget that time itself isn't a mechanism of the universe, and that it's just a word we use to describe some sort of effect or bi-product that we're observing.

Here's a can of worms... Compare the concept of time travel to something like 'smell travel'. Is it really that different? We are conditioned to know what smell is because our body has a mechanism to "interpret" particles and send signals to our brain which we 'remember'... How do we know that Time is any different? Perhaps travelling back in time is a flawed concept in itself, in the same way that "travel back in hearing" or "travel back in smell" makes little sense at all because we're just describing an effect or mechanism of our biology, rather than an effect or mechanism of the universe.

drdeca3• 6 years ago

So do you think that special relativity is misleading, or, if not, how do you reconcile this point with special relativity?

Daniel_sk2• 6 years ago

I think you partly misunderstood his comment. Basically instead of time we can look at it as a change of states, just a linear process. There is no time. If you place something on a table (in a perfect environment with no decay) you can't tell how much time it sat there. It could have sat there for 1 hour or 1 million years. It just didn't move and no one tracked the "time". Time is just a concept in our minds to track changes in space (or state of space). Special relativity I think doesn't contradict this. It just says that the "speed" of the state changes (e.g. you go from A to B) is relative. But this speed can't go negative. There is no trace of previous events where you could go back to.

thaumasiotes• 6 years ago

Compare the concept of time travel to something like 'smell travel'. Is it really that different?

What do you mean by moving from one smell to another smell?

Time is a kind of position; smell isn't.

zaroth2• 6 years ago

Why can’t the linear flow of time and a singular instance of reality just be the narrow lense through which we view the universe?

If reality is infinitely branching, then time travel is merely a way for the traveler to perceive and move between those infinite branches.

If you kill your grandfather in an alternate timeline your grandfather is still alive in the timeline you came from, hence no paradox. You (an alien in the new timeline) can then watch a world develop in that alternate reality where your grandfather died.

If time travel were in fact possible, this would be the only way my puny brain could conceive of it actually working.

If there’s only strictly one path that reality takes even from the perspective of an outside observer (God) then I can’t see how time travel could be possible in such a construct. If it is possible it seems you must implicitly admit that there is not in fact a singular thread of reality/time.

Just because we define some equations where t can flow in reverse and fit them into the standard model doesn’t seem to me to be persuasive evidence one way or another.

BlueGh0st• 6 years ago

Is there some sort of highly entropic and massive hash string being constantly printed, recorded, and widely distributed so that one could see if any changes happened in the past? Or rather, ensure, with a comparison, that the observer is still in the same timeline?

Is there any similar data that can be used? Perhaps global weather records?

technick• 6 years ago

I won't find my young self and give him all of the lotto numbers for every major drawing. Instead I would go claim it myself and enjoy life at that point. Younger me will eventually go in the same loop.

mr_mitm• 6 years ago

In case anyone is interested, Sean Carroll did an amazing podcast episode on the mechanics of time travel in a scientifically accurate way [1]. He sometimes works as a scientific advisor on Hollywood movies and claims to be responsible (in part) for the line "You mean Back to the Future is bullshit?" in 'The Avengers: Endgame'. He also touches on the topic of killing Hitler as well as different tropes in time travel movies. He also points out one particular movie which he thinks did time travel logic best: Bill and Ted's excellent adventures.

[1] https://www.preposterousuniverse.com/podcast/2020/11/23/124-...

offtop52• 6 years ago

Time travel stories always seem to be centered around what if fantasy.

I think one of the movie adaptations of the time machine has the protagonist trying to save his wife.

I would like to explore what would happen if one went years into the future to party ? Would certain time periods become tourist traps

avereveard2• 6 years ago

You need to chart the future first, for even in moderately advanced countries you risk showing up during a riot or other agitations that could get you arrested and stranded, especially as our surveillance capabilities expand it will be hard to evade detection.

And, if we assume we live in a one timeline hypothesis, all that work is still at risk from other time travelers altering a midpoint between now and where you mapped.

offtop52• 6 years ago

Assuming I have enough gas in my time machine, I could just jump back. Eventually in this dream, it would be known , say Paris 2500 , is a cool spot . So people would travel there and spend money ( barter) .

Would love to see this explored in a novel

Cyber_squad• 6 years ago

Sometimes I came here to read good news and interesting article... most of the time just clickbait...

littlestymaar5• 6 years ago

The time-travel paradox arises only if you presuppose the existence of free will: if we don't, and just consider human beings as physical objects acting deterministically, then there can be no paradox: the time traveler will simply do what was already had been done in his past.

rightbyte3• 6 years ago

Third law of the thermodynamics has to be false for that to be possible. It is still a paradox.

littlestymaar5• 6 years ago

Would you mind expanding on that?

akvadrako2• 6 years ago

Not if he has more information because he has seen the future - then he'll likely act different.

mjburgess• 6 years ago

Indeed.

What people mean by "time travel" isnt time-reversal. It's taking information from the future and injecting it into the past by moving the atoms of your now-body into the past.

This isn't "time travel" in any sense that physics uses the term. And is certainly impossible on very many grounds.

Physics has nothing to say on "time travel" in the popular imagination.

schwartzworld3• 6 years ago

Ah yes, can you change the past or was there a time traveler version of you running around failing to change things the whole time? See season 5 of Lost for endless discussion on this point. You also can avoid paradoxes if it turns out that time travel just puts you on an alternate timeline.

There's a different paradox with this version of time travel though. Suppose I'm a kid and future-Me comes back and tells me a story. "you will write down this story and publish it and it will be famous." I do as he asks and he's absolutely right, leading me to eventually return the favor to my past self in a time loop.

Where did the story come from?

littlestymaar5• 6 years ago

Your story allows a “closed timelike curves” to exist, then it must be considered as a physical constant.

Where do c (the speed of light), G (Gravitational constant) or π comes from?

dnh44• 6 years ago

The film Predestination also deals with this paradox.

https://www.imdb.com/title/tt2397535/

Digit-Al• 6 years ago

Have you ever read "All you zombies" by Heinlein? https://gist.github.com/defunkt/759182/ad44c6135d168ae54503a...

diggernet4• 5 years ago

See also "As Never Was", by P. Schuyler Miller.

simiones• 5 years ago

It may be that, just as it's impossible for you to be killed by yourself from the future in this type of universe, it's also impossible to create information out of nothing, so the specific scenario you described would just be impossible.

This could be a kind of conservation law - imagine that instead of a story you had a golden coin, that you receive from your future self, keep, and give back to your past self after you travel, you've added some mass to the universe, which would be impossible because of conservation of energy.

DebtDeflation• 6 years ago

So yet another paradox (like so many in quantum mechanics) that's easily solved with Superdeterminism.

https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...

yrimaxi10• 6 years ago

This explains absolutely nothing. “Will simply do what was already done in his past” is the non-sequitur: if they shoot themselves in the past then that was what was done in the past. Sounds paradoxical? Because it is, and determinism can’t stop a human from shooting a gun any more than free will can because this has got nothing to do with volition.

skc• 6 years ago

This really got me thinking.

Does anyone know of any great sci-fi stories centered around the lengths the universe goes to to prevent a curious time traveler from testing the grandfather paradox.

schwartzworld3• 6 years ago

Probably not what you're asking for, but there's a classic Futurama episode that fits that description https://en.m.wikipedia.org/wiki/Roswell_That_Ends_Well

gitgud• 6 years ago

[1] Predestination is a good Time Travelling Film, similar to the plot you describe.

[1] https://www.imdb.com/title/tt2397535/

JNRowe• 6 years ago

While also not quite what you asked for, Ted Chiang's short story "What's Expected of Us" deals with some of the emotional issues of determinism.

Oh, and his "The Merchant and the Alchemist's Gate" is a stylistically beautiful exploration of people intertwined through time travel.

bArray2• 6 years ago

I would recommend Steins;Gate as a fun exploration of time travel [1].

[1] https://en.wikipedia.org/wiki/Steins;Gate_(TV_series)

bArray2• 6 years ago

Just some thoughts:

Would you be aware of some force stopping you from performing an action that would change your motivation to time travel in the first place? For example, if you tried to walk up to yourself in the past, would it feel like some force field stopping you? Or will it be false sense of agency, where you suddenly make the decision not to do something? Or will some event happen to simply kill you? Or will you suddenly vanish from this timeline to live out a life you over-wise would have based on the consequences of your actions?

If we are a result of a series of tiny influences from our environment, will the entire past refuse to be changed? Or will you be able to live out a version of a timeline where a different set of tiny influences add up to the same future where you make the same actions? As a result, does that also limit what you are actually able to do in the past without changing so much that one of these alternative states is impossible?

If the past doesn't want to be changed, how about the future? Just because we perceive the clock running in one way, doesn't mean that it actually does. I would think that if the past must happen in some specific way, then the present will always occur and so will a specific future. Whilst we claim to experience free-will, we have so far not measured it. I guess this is really the idea that the quantum realm simply consists of hidden variables that are only perceived as 'random', rather than actually having any form of random element.

throwaway9d0291• 6 years ago

I watch a lot of both American-made movies and TV shows as well as a lot of Japanese anime.

One thing I find curious is how different the models for time travel usually are.

In American media, it seems more often than not, time travel is modelled as if the past and the future are simultaneously existing parallel worlds, where the future is affected by the past in "real-time", e.g. Back to the Future's Marty gradually fading as the chances of his birth diminish, or Timeless's people in the future "watching" people arrive in the past and making sure their own team leaves "in time" to catch them.

These models don't really offer a solution to the grandfather paradox.

In anime however, time travel is near universally modelled with timelines, where time travel essentially creates a new parallel world each time. If you travel back in time and kill your grandfather, you simply create a timeline in which you were not born, but can continue to exist, because you are from a timeline where you were born.

I'm by no means suggesting that either model is unique to America/Japan (Rick and Morty for example uses the branching timeline model), I just find it interesting how they differ.

caractacus• 6 years ago

I think it depends on the show in anime. SPOILERS FOLLOW. Erased is not a parallel world, but someone trying to change their past. Madoka finds Homura repeatedly doing whatever is possible to save Madoka. Steins:Gate is timelines and parallel worlds (think of how many times Okabe again and again tries to save Mayuri). Girl who Leapt Through Time is the Groundhog Day story, same day over and again; bit like Tatami Galaxy. No idea where something like Haruhi Suzumiya might fall where time is played with in many different ways.

Meanwhile Evangelion (the later films) and Attack on Titan both seem to be veering towards some kind of inability to stop the same grand narrative cycle repeating again and again...

Izkata• 6 years ago

It also depends on the show in American media. Star Trek, for example, has nearly every version of time travel at some point or another - except the "simultaneous worlds" version described in GP with Back to the Future and Timeless.

surfsvammel• 6 years ago

I’m not an expert, but why would the paradox they are talking about be a problem for the concept of time travel?

To me, a believer in the multiverse, it seems that the obvious answer is that in our particular instance of the universe no one in our future has figured out time travel, or have not gone back in such a way to let us know about it. For whatever reason.

We pay too much attention to what we perceive to be the “arrow of time”, the direction. Forward, backward? What’s the difference really? Is it our illusion of “free will”, that somehow there is more to the atoms of our body than can be explained by the same laws of nature that governs the rest of the universe, that simply won’t accept that we might just be like billiard balls moving along predetermined paths (admittedly unbelievably complex ones) and if we accept that, then the direction of time doesn’t really matter at all.

kstenerud2• 6 years ago

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.
"No matter what you did, the salient events would just recalibrate around you," Tobar said.

I'd love to see how this works when you go back in time and kill your toddler self.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being,"

But WHICH events? If things change around you when you change the past, then by definition some events will have happened, and others not (otherwise NOTHING would change, including your arrival in the past, and therefore time travel cannot occur). After all, a single breath you took at a specific point in time is an event. Waking up 2 seconds later is an event. Using your left foot first as you walked out of the house that day instead of the right is an event. So who decides which events are "important" enough that "forces bring Past Event X into being"? You'd need some kind of omniscient, omnipotent intelligence in order to accomplish that.

The very logic behind this idea is a contradiction.

darkcha0s• 6 years ago

I'd love to see how this works when you go back in time and kill your toddler self.

Logically, this would be consistent, as you are no longer present in the future, because you have travelled to the past. What if killing the toddler means you can't travel back to your understood future?

Your 'gotcha' doesn't really disprove anything, beyond raising another question which we can't conceivably answer.

kstenerud2• 6 years ago

What if killing the toddler means you can't travel back to your understood future?

How would killing a toddler prevent me from time travelling back? Killing someone will not affect the functionality of my time machine.

mjsir911• 6 years ago

Nothing changes in the past, including your arrival, because your arrival would have always happened. The past stays consistent because it was already determined to have happened.

How this would work going back in time to kill your toddler self is that, well, you don't. Gun jams, you have an aneurysm before you can, or maybe you killed the wrong toddler and there was just always a toddler killed in the past.

The fact that you are alive to go back in time means you were not killed in toddler form.

wired_devil• 6 years ago

It leads me to think that no one that would fuck up the future will be able to do time travel in the first place. So it is not possible because anyone who dreams about going back in time wants to change something.

alex_young• 6 years ago

Doesn’t conservation of energy preclude backwards time travel? If not, would a hypothetical machine which sends an atom back in time be able to fill some previous universe with infinite numbers of the same atom?

mathattack• 6 years ago

This article claims to refute the butterfly effect with no support. I get that strange things happen in Physics. I would have liked more support.

dcanelhas• 6 years ago

I like William Gibsons version of interactions with the past, as featured in The Peripheral.

Since no-one altered our past we can't be in a timeline where people from the future had influence on the outcome. Therefore any timelines after the point of contact run parallel to ours and we cannot change our own present by traveling back in time.

Interesting to imagine the different "solutions" to the paradox though.

gpsx• 6 years ago

I don't understand how people view changing the past with time travel. If I go back in time I might experience a given point in time twice, but that point in time still only happens once. So if I go back to 1940 and kill my grandfather, then the first time 1940 happened he got shot. The second time...there is not second time. (Fortunately my parents were born in 1930s.)

JoeAltmaier5• 6 years ago

I always imagined that time travel into the past would involve some statistical uncertainty. If you got close to creating a paradox, you'd simply transition back. Disappear essentially.

Make a nice story - ghosts are time travelers, appearing and disappearing without making any permanent changes.

ohiovr• 6 years ago

There might not be a paradox but there could be unintended consequences to time travel. A physical object can be degraded in space. Why not also degrade the time it exists in also.

risk000• 5 years ago

Andrew Basiago claims to have time-travelled:

https://www.youtube.com/watch?v=Gr_qHl9dWRU

Pretty amazing, if its true.

Paradox-free time travel is theoretically possible

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