Temporal Paradoxes and the Possibility of Time Travel.

Factual Questions
81

One way to resolve the paradoxes is that, when you alter the past, you aren’t altering your own past but rather you are altering the past of a parallel universe. Every time you go to the past, you create an duplicate universe which is identical to the past of the one you left. Marty A starts out in Universe A, takes the DeLorean from the Twin Pine Mall back to 1955 and finds himself at 1955 in Universe B, which is exactly the same as Universe A was in 1955 but now it’s a duplicate. He is now Marty AB. He meets Doc B, who is an exact duplicate of Doc A. They make all sorts of changes in Universe B and that doesn’t affect Universe A in the slightest.* All the changes in Universe B only affect the future of Universe B. When Marty AB goes forward to 1985, he’s still in Universe B. And now there are two Martys because George B and Elaine B had a child Marty B. Marty B grew up in Universe B and looks a lot like Marty AB, which makes George B suspect that Elaine B cheated on him with her old high school crush who called himself Calvin Klein. Marty AB goes to the Lone Pine Mall and finds Doc B is there with Marty B. Marty AB watches Marty B disappear. Marty B is never seen again because he ends up in Universe C, a duplicate of Universe B. Doc B goes to 2015 (still in Universe B) and finds that Marty AB has married Jennifer B and had a son, who is about to go to jail. So Doc B goes back to 1985 and thereby creates another duplicate universe, Universe D. Now he is Doc BD. In that universe, there’s a Marty D who remembers growing up in Universe A, traveling to 1955 in Universe B, and seeing Doc go to 2015. Marty D is an exact duplicate of Marty AB. Marty D sees Doc B come back from 2015 with a Mister Fusion. But there’s also a Marty AB in Universe B who simply watches Doc B leave and never come back. However, there’s no Doc D because Universe D is a copy of Universe B, where Doc B left and never came back. Then Doc BD, Marty D, and Jennifer D (an exact duplicate of Jennifer B) all go forward to 2015 (in Universe D), where Biff D steals the DeLorean and goes back to 1955, creating Universe E. All the changes he makes in Universe E have no effect on D. Doc BD, Marty D, and Jennifer D, are now stranded in 2015-D without a DeLorean. They find that Marty Jr. does not exist in universe D because Marty D and Jennifer D disappeared in 1985 without having any kids. They have all been missing and presumed dead for 30 years. After Biff D goes back to 1955-E, he is now Biff DE. He can interact with Biff E, who is an exact duplicate of what Biff D was at that moment in Universe D’s history. He leaves the sports Almanac which came from Universe D, but it won’t be 100% accurate in Universe E because the two universe begin to diverge as soon as Biff DE arrives. Biff DE goes forward to 2015-E. Doc B isn’t there. He’s over in Universe D wondering what happened to his DeLorean. But there’s a Doc E who is a duplicate of the Doc B who met Marty AB. This Doc E can still build a DeLorean in 1985-E. And there will be a Marty E, son of George E and Elaine E and stepson of Biff E after he kills George E and marries Elaine. Do Doc E and Marty E go somewhere in their DeLorean?

Remember Universe E is a copy of D, which was a copy of B, which was a copy of A. Universe B split from A when Marty AB arrived. If Biff DE arrived before that point, then Marty AB isn’t there and there’s no reason for him to show up at all. But if Biff DE arrives after the point in time where Mary AB arrived in A, then there’s a copy of Marty AB in Universe E. So Biff DE and Marty ABe fight but Marty ABe knows nothing about the Almanac. We, the audience, got to see two Martys and two Biffs, and two Docs, which means we weren’t watching Universe E. We must have been watching Universe F. That’s the one created when Marty E confronted Biff and found out about the Almanac, then went back to 1955 with Doc E. Marty E steals the almanac from Biff F right after Biff DEF gives it to him. Biff F never becomes rich. But this doesn’t change the fact that Biff E did become rich and did murder George E. George F and Elaine F have a son Marty F who marries Jennifer F and they have a son Marty Jr F.

There are multiple copies of 1955 and 1985 and 2015. In some of them, Marty never existed. In others, Marty disappeared and never came back. In some, there is more than one Marty. It’s complicated, and events sometimes seem to happen for no reason, but there are no paradoxes. Every event has a cause, either a cause which took place at a prior point in time or a cause that took place in a different Universe.
*One might argue that this still violates the law of conservation of mass/energy, but that law could be reimagined to say “If mass/energy is added to one universe, the same amount must be subtracted from another universe”.

82

Well, look, you say there’s no need, and maybe you’re right; but I tell you, I’m perfectly happy to go back in time and pass along that same warning anyway.

83

sbunny8 if you invent a machine that can create and destroy universes wholesale, time travel is small beer, don’t you think? :slight_smile:

You can use it to cut down on your winter heating bill, too.

84

Sure, if you want to fracture the time line into millions of pieces, then go ahead!

85

Eh, it’s all just a big ball of timey-wimey… stuff, anyway.

86

Does it make a difference if we put constraints on where you can time travel to?

I ask because IIRC one plausible* means of time travel suggests you can only travel between the point the time machine was created and some arbitrary time in the future (or past?..confuses me which way it works).

The idea being (super, super brief version) you get two plates entangled on a quantum level then put one plate on a spaceship and fly it around at relativistic speeds for a while. When done the two plates will be in different time frames and, in theory, you could walk through the “portal” between the two and travel in time.

But you can only go to when it was created. Dunno if that changes much but figured I would throw it out there.

*[sub]“Plausible” in a theoretical sense only. As a practical matter almost certainly impossible.[/sub]

87

Whack-a-Mole: That limitation applies to the rotating cylinder model (Tipler cylinder.) Trajectories near such a thing can involve motion through time, even (in theory) backward. But the trajectory has to be around the cylinder, so you can’t go back to before the thing was put in place and spun up.

88

shrug According to the Many Worlds Interpretation of quantum physics, a new universe is created every time a wave function collapses, trillions of them in a fraction of a nanosecond. I don’t see the big deal about creating just one more on the day that a time machine goes backward.

Another model which also eliminates paradoxes is to imagine that a huge number of parallel universes already exist and time travel just jumps from one to another. Universe B is 30 years behind Universe A and Universe C is 10 minutes behind A. When Marty A goes from 1985 to 1955, he’s just jumping sideways from 1985-A to 1955-B. Any changes in Universe B have zero effect on Universe A. When Marty A goes back to 1985 minus ten minutes, then he’s in Universe C, which is also completely unaffected by what he did in 1955-B. He should see Doc C get shot at Twin Pines Mall (not Lone Pine Mall). This model doesn’t require creating universes, just jumping between them. It solves the grandfather paradox quite easily. You can go back in time and kill your own grandfather as many times as you want and it never threatens your existence because those grandfathers are in different universes. They aren’t the same grandfather which previously existed in the universe where you were born. Once again, this model only works if you permit effects which apparently have no cause (but actually have a cause which took place in a parallel universe).

89

I do not think it is a Tipler Cylinder I was talking about.

IIRC it has to do with getting two plates really, really close together and charging them in some fashion so they become linked ala quantum entanglement. Once entangled you can separate them spatially but they remain “linked”. Put one on a spaceship and move it near light speed and the two plates will be in different time frames since one moved through time more slowly than the other.

Since they are entangled in some fashion you could, in theory, move between them and thus travel in time.

And before anyone starts I realize how hocus pocus the above seems. I also realize I am throwing out vague sciency sounding things with zero backup. It is all a vague memory of one “possible” (big grain of salt there) method of time travel. I only put it out there to explore if it changes the discussion in any way.

Also, in case it has not been mentioned in this thread (I have not read it all) time travel to the future is 100% possible. Obviously we do it now at 1 second per second but hop on a spaceship that moves at relativistic speeds and you can move from now to the far future in a short time (as you perceive it). Or if you could hover close to the event horizon of a black hole and not die that would do it too.

90

Hmm… isn’t the many-worlds interpretation merely one way of “interpreting” the fact that a single particle can be in a linear combination of pure states? Either way, the point is that there is no interaction among the “worlds”, so I am not getting it even as a thought experiment for space or time travel since by definition nothing could travel from one “world” into another.

Also, in basic quantum mechanics as exemplified by Schrödinger’s equation, isn’t time a simple variable, in which case how would we deduce anything about time travel from philosophical interpretations of it?

91

Re. quantum teleportation via entanglement, you cannot send any information faster than light/back in time that way. At least not if you are thinking about the same experiment I am thinking of.

92

Coming back around to this: isn’t this just “Shakespeare’s Plays” again?

Imagine that a clean-shaven Future-Me with a cane visits me tomorrow morning, and explains how to keep my wife from dying when a car slams into the mailbox at noon; he also hands me some papers, saying they accurately detail the experiments he’d spent the last year performing due to her death. “It’s been a year since I even heard her voice,” he says sadly, as if on the brink of tears.

So, okay, I relay the warning to my wife, who stays inside at lunchtime and survives. And I then start thumbing through those papers – and I see that they’re dated, and that the older ones closer to the front look a little more yellowed – and, when I get to the last page, I see the entry for Experiment #361, which reveals that the secret to time travel involves running an electrical current through a container of orange juice with some aspirin floating in it.

Anyhow, I then flip back to the start; and, as each day goes by for a year, I copy out the relevant day’s notes from the original on to fresh white paper. And maybe I don’t actually bother doing any of the first couple hundred experiments – I don’t need to run an electrical current through a 50/50 blend of champagne and blood, I just need to write down that (a) I did, and that (b) it didn’t work.

So that takes a minute each day, and I otherwise spend the year growing out a beard as a happy guy with a loving wife who talks a lot; and after a year, I burn the papers I’d been handed, and I shave off my beard, and I grab a cane, and I travel back in time to visit my young self: I tell him how to save his wife, and I hand him papers I say accurately detail experiments I spent the last year performing, and I pretend to be on the brink of tears when I say I haven’t heard her voice in a year.

And then I leave, wondering what the deal was with the cane.

Anyhow, figure my wife and I then spend the rest of our days helping mankind and getting rich and otherwise making good use of my great invention, and – is there a paradox, there? Is there some reason that couldn’t work?

93

Yes that would work. Because when yourself from the future where your wife dies comes back, a time-stream fracture starts. You continue along in a new time-stream, one where yourself from another time-stream came and gave you time-machine instructions.

94

And where did that guy learn how to make a time machine? Did he learn it from a time-traveler from yet another time-stream? Is it time-streams all the way up?

95

It’s time-streams all the way across.

96

That’s the version of time travel where everything already happened, so changing the past doesn’t actually change the past because how could it?

This version of time travel has no paradoxes, and any seeming paradoxes are only because you didn’t have complete information. You thought that guy you killed was your grandfather, but he never was, never will be, never would have been, and never is. Who is/was/will be your real grandfather? I don’t know for sure, all I know is that it wasn’t that guy whose body lies accidentally crushed under the chassis of your time machine.

The only question then is, could you have not warned your past self about an event that didn’t happen? Yeah, you didn’t, but could you have? If in every multiversal instance you use your free will to freely choose to follow predestination, then no.

97

I guess it’s this: figure the original guy learned how to do it by actually performing hundreds of experiments and documenting the results first-hand; and figure he then goes back in time and gives his records to a guy who (a) whips up strikingly similar documentation, but without ever learning a damn thing by actually performing those experiments – because he just shrugged and skipped ahead to the final entry before copying out each page in longhand as if he’d made those attempts and learned from those failures before, finally, having an experiment unexpectedly succeed.

And then figure that second guy (b) goes back in time, and hands off the records he’d written as if with first-hand knowledge. You know, to a guy who’ll shrug, and flip to the final entry, and then copy the whole thing out in longhand before going back in time and handing it off as if he’d learned it by performing the experiments.

Would that be a paradox, or is it potentially okay?

98

The kind of scenario where someone goes back and time and gives themselves the plan to the time machine which allows them to travel back in time is non-paradoxical as it is self-consistent and indeed arguably when you have time travel such kind of scenario can be seen as an inevitable outcome of the causal structure that must result whenever there is a time machine.

99

To explain a little more physicists prefer to work with spacetimes that are classified as “globally hyperbolic”. A globally hyperbolic spacetime is one where you can take a spacelike slice (i.e. space at an instant of time) and then determine the whole spacetime just from that slice. Such a ‘predictable’ slice of space is called a Cauchy surface and it is intersected by every timelike curve once and once only, where each timelike curve can be thought of as representing a real or theoretical observer. Each Cauchy surface is a set of initial conditions which we can develop in to the past or the future to completely determine the state at any other time. Physicists like globally hyperbolic spacetimes as the conditions at one time completely determine the conditions at any other time, just as in Newtonian physics.

However once you have a timelike loop in the spacetime no Cauchy surface can exist as a spacelike surface intersected by every timelike curve, must be intersected by a timelike loop an even number of times. In other words, when there is time travel the current conditions cannot determine all future and/or past conditions.
To reduce this a bit, time travel necessitates that you can never predict the future and/or determine the past, and an illustration of this is just such a scenario where someone goes back in time and gives themselves the plans for the time machine. It is self-consistent, but you could not predict this happening at a time before the time traveller first arrives from the future.

Even worse given a set of initial conditions which allow a globally hyperbolic development, though as you would expect there is only one globally hyperbolic development, there are always an infinite number of non-globally hyperbolic developments. That is to say, if time travel is allowed or not constrained in some way you can never discount or predict that a time traveller might suddenly appear from the future.