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?)
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.
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…)
???
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.
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.
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.
@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.
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.
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 can’t 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 doesn’t always have to occur.
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?
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.
To simplify the Shakespeare writings, let’s say you are standing around, and a box mysteriously pops into existence in front of you. You open it, and there is a note in your handwriting, from your future self, warning you to do, or not do, something. You put the note in your pocket. (Amazed that time travel exists, and you are still alive in The Future.) You later do (or don’t do) whatever the note warned.
Later, after whatever crisis your future self was warning you about passes, you are walking around the University campus, and you walk into a lab with crazy scientists that say they invented a time machine. They want to test it. You have an idea - you take a box off the table, put the note in it, and have them send it back in time, to where you were. They do.
Who wrote the note?
Did anyone ever write that note? Was there a time where many-iterations removed version of you wrote the note? Or is it an example of a completely closed time loop? If so, where did the note come from? Did it spontaneous generate out of the fabric of space? Why did it have your handwriting?
(borrowed from Harry Harrison, twice. The Stainless Steel Rat Saves the World, and The Technicolor Time Machine.)
The note exists and originates in the timelike loop. It’s demonstration of the unpredictability caused by CTCs, as information can originate from the CTC.
So to the scientists who invented the time machine, it looks like what they invented is something completely different, a machine that destroys balls. Is that right?
The real use of time machines. We can send our trash to alternate timelines.
This was what Fritz Leiber invented in his novel “The Big Time.”
It’s sort of like you’re playing a time-travel board game. On Turn 1, I move my agent to 400 B.C. On Turn 2, you move your agent to 800 B.C. Even though your agent is in my agent’s past, my turn is in your turn’s past.
(Leiber also invented “The Law of Conservation of Reality,” in which the past really, really does not want to be changed. Go back in time and murder your grandfather? You’ll find yourself having sex with your grandmother…just to make things end up “the same.”)
And again, if there’s a “meta-time”, then is it possible to travel through that “meta-time”? If so, then how is meta-time any different than regular time? And if not, then you’re really just chickening out on the whole time travel thing.
That’s fun handwaving but it can’t work. First, your DNA is not the same as your grandfather’s. Second, the combination of sperm and egg couldn’t possibly be the same. Third, the epigenetics of your development would differ greatly. Fourth, the DNA of offspring would also change so your father or mother would be a different person.
The only rejoinder to that is that you’ve always been your grandfather, but that creates a closed time loop, creating a myriad of questions of its own.
You can’t fanwank time travel paradoxes. Dozens of ingenious people have tried. There’s always problems.
A potential answer to that was already advanced – not an issue if the number of alternate universes is infinite. Anyway, an excellent treatment of this idea is the great sci-fi short story Vintage Season, by Henry Kuttner and his wife C. L. Moore, I believe originally published under one of their pseudonyms, Lawrence O’Donnell.
Rudy Rucker has an interesting discussion of a model of the universe with a “para-time” in addition to real time Rudy Rucker, The Lifebox, the Seashell, and the Soul
My idea is to combine a Wolfram-style view of reality with Cramer’s transactional interpretation. Suppose with Cramer that causality runs both forward and backward in time, and also suppose that our world is deterministic in both these temporal directions. This means that spacetime is a coherent whole, with both past and future fully determined by the world’s state at any single instant. If you fix upon some arbitrary moment in time—say, the instant when you read this sentence, then the question becomes: How was the world’s structure at this particular instant determined? If you can explain the now, you get the entire past and future for free—for the past and future follow deterministically from the now.
Now I add in the Wolframite element. Think like a universal automatist and suppose that the great structure of quantum-mechanically patterened spacetime arises from a higher-dimensional deterministic computation. Since our time-bound human nature makes it easier to imagine a deterministic computation as being embedded in some kind of time, let’s invoke a (possibly imaginary) second time dimension in which to compute our world—call this extra time dimension paratime. Paratime is perpendicular to our ordinary dimensions of space and time, and we want the entire universe to be the result of a computation that’s taken place in the direction of paratime, as illustrated in Figure 45.
Note that the paratime notion reintroduces the theme of parallel worlds. Presumably the people in each of the spacetimes feel themselves to be in a unique reality with time flowing forward as usual. Note also that, if we take the paratime view seriously, it’s possible or even likely that the spacetime in which we find ourselves isn’t the last one in the series. Reality evolves further along the paratime axis. In terms of an analogy to a novel, our world is very well plotted, but it may not be the final draft.