This is mark Joseph “young” blog entry #201, on the subject of The Grandfather Paradox Solution.

I sent birthday greetings to a time travel fan on Facebook–one who deserves special mention, as he has provided copies of several time travel movies analyzed on our Temporal Anomalies site–and in response received a discussion of a time travel issue.  I would have said that this is addressed already on the site, but I recognize that the site has become unwieldy in some ways and it’s difficult to find, let alone absorb, it all.  I have edited his comments for space, and added links to references on the site for those who are uncertain of the terminology.

I’ve been thinking about Niven’s Law (ie the popular “if you change it in the past it stays changed even if you undo the time travel” version).

Here’s the thing–without it, it seems to me that things work by magic.  Let’s use the old example of going back and killing my grandfather as a child.

Fixed time this is just impossible….

Parallel universes, no problem….

Replacement theory is where it gets interesting (of course).  Let’s first postulate that I’m not going back to kill Granddad.  Let’s say instead that I’d discovered in talking to other people that there was some sort of childhood toy in my granddad’s house…that was extremely rare, and if I went back and got it I could sell it for a fortune in the future….Unfortunately while I’m back in the past I interrupt a burglar, he shoots at me and misses but kills my granddad who was hiding behind the couch watching this armed burglar tussle with me….

So…I haven’t erased my motivation for going back.  However, obviously if I never exist, I can’t go back, which means that I won’t interrupt the burglar, which means he won’t shoot….

But what exactly happens?  What does the burglar see?  Does he just see me vanish into thin air?  That’s what I mean–there’s no real known phenomena that would cause that.  And in fact he wouldn’t see it anyway, because the whole idea is that I could never have been there in the 1st place.

I think in reality, if time travel is possible at all…either Niven’s Law must exist or else something like Hawking’s Conjecture must be true (the one where he says that you will be physically unable to successfully perform any actions that would create a paradox…).  I find the Conjecture even less likely (it pretty much falls under your “God won’t let it happen” thing).

Mind you that doesn’t get off the hook with “uncaused causes“.  There’s no perfect answers.  It just always seemed weird to me that things could magically change just because I remove the reason for the change.

This happens to be exactly the problem that is resolved by the standard concept of the infinity loop, two histories each of which causes the other.  My reader has missed this, falling into the notion presented by other time travel stories, perhaps most notoriously the ending of The Philadelphia Experiment II, in which the death of the childless father causes the son, a moment later, to dissolve into non-existence.  The reality postulated by the theory is much less complicated.

The postulated problem suggests that when I travel to the past I accidentally cause the death of my own grandfather.  The questioner then wonders whether I flicker out of existence, but recognizes that the problem is more complicated, that in fact if I never existed I never made the trip to the past and the burglar never shot at me.  That, though, means he never killed my grandfather, and I am able to make the trip to the past.  This much the question recognizes; it then gets caught in trying to make both versions of time real simultaneously, as if the death of my grandfather means that I must immediately vanish.  This fails to grasp the significance of causal chains, which we will here review.

In all of our science, we have causal chains:  A causes B, B causes C.  If B does not happen, C does not happen, because C only happens if caused by B; similarly, B only happens if caused by A, so if we prevent A, we prevent B, and in so doing we also prevent C.  This is simple for us in most situations, because of two “rules” that have always applied to everything we have observed.  One is that causes and effects have always happened in temporal sequence, that is, A happens before B and B before C even if only infinitessimally (the hammer strikes the firing pin which compresses and ignites the gunpowder which drives the bullet out of the shell, all in a fraction of a second but that fraction divided into sequential fractions).  The other is that once a cause has brought about an effect we are unable to remove the cause.

Time travel erases both of those rules, and therein lies our confusions.

In the present circumstance, the original history has Burglar invading Grandfather’s house, observed perhaps by grandfather but otherwise unmolested.  Decades pass and Traveler learns of the valuable toy in Grandfather’s attic.  Having access to a time machine, he travels to a time when he believes he can obtain the toy without changing anything significant in history.

• There is an issue here which is not addressed in the problem:  we do not know how Traveler became aware of the presence of the toy in the attic, but if he removes it too soon he might well break the chain of information such that he does not know about the toy.  For example, if his information about the toy comes from the estate sale records, the toy will not be listed there once he has removed it.  However, our theorist having been careful on all other points, we will assume that Traveler got the information through a source that predates his effort to steal the toy.

He arrives in the past, and interrupts Burglar, who in attempting to kill him accidentally kills Grandfather.  There are scores of steps in this causal chain, but simplifying it we have A: Traveler travels to the past; B: Traveler interferes with Burglar; and C: Burglar kills Grandfather.

However, there was a causal chain in the original history in which Grandfather sired Father who sired Traveler, who eventually left for the past.  Our logic problem recognizes that because Grandfather is now prematurely dead, Father will never be born, and Traveler in turn will never be born.  It is precisely because the original causal chain has been disrupted that Traveler is never born–there is nothing magical about that, and no one imagines that it is.  We understand completely that if you remove the cause of an effect, the effect never happens; if you kill someone’s grandfather before he has children, the grandchild is never born.

Yet exactly the same rule applies at the other end.  If Traveler is never born, he never makes the trip to the past, which means A: Traveler travels to the past never happens.  Since A is the cause of B: Traveler interferes with Burglar, B never happens, and since B never happens, C: Burglar kills grandfather, also never happens.  If it applies to the A-B-C sequence that is Grandfather sires Father, Father sires Traveler, then it also applies equally to the A-B-C sequence Traveler travels to the past, Traveler interferes with Burglar, Burglar kills Grandfather.  The removal of the cause A undoes the effects B and C.

We balk at this because what we perceive as inaction in the future is becoming a cause of a change in the past, and we feel as if whether or not the past can be changed it can only be changed by someone traveling to the past.  However, if we look at it a different way, it might become clearer.  If I know that Gary traveled to the past, leaving tomorrow, and that what he changed altered history in a disastrous way, in theory I might attempt to travel to the past and prevent him from making that mistake, but could I not just as easily act to prevent him from making the disastrous trip?  (I admit that this would cause an infinity loop, but the point is only that preventing the trip to the past will prevent the changes to the past just as surely as traveling to the past to do so would.)  At the same time, we are mistaken to think of “not traveling to the past” as inaction.  It is much more properly different action, and different action becomes a different cause that has a different effect.  Further, since the effect B which is the cause of the effect C is itself the effect of A, if A is undone–if Traveler does not go to the past–then B is also undone–Traveler does not interfere with Burglar–and C is in turn undone–Grandfather is not killed.

But we return to what it is that Burglar experiences when his stray bullet kills Grandfather, theoretically undoing the existence of Traveler.

I admit that it is plausible that this event will cause time to unravel entirely, and the universe will cease to exist.  I think, though, that this is a bit extreme, and further it seems to require that the universe “knows” that history has changed in an irreconcilable way.  I don’t think the universe can know anything of the sort–for the universe, despite the fact that someone arrived from the future and became a new cause, this is the first time through these events, and as far as the universe “knows” (if it can be said to “know” anything in any sense), this is the history that exists.  It does not “know” that the man who just died is the grandfather, and thus the necessary cause of the life, of the Traveler who incidentally caused his death.  It has to “discover” that by playing through the events which follow.

There is thus an interweaving of two histories, in a sense.  Traveler comes from a universe in which Grandfather had a child.  The history of the universe is being rewritten, event by event, cause by cause, moment by moment, but it has not been rewritten yet.  Since under replacement theory there is ultimately only one history of the universe, each moment that is created erases and replaces the moment that was the same time in the other history.  That means the cause of Traveler’s presence in the past, cause A, has not yet been erased, and so Traveler still exists in the past even while his history is being erased and rewritten.

Ultimately the moment comes when cause A needs to happen in order for effect B, in the past, to be supported.  If we had an N-jump, that would happen.  To use our example modified, there was no Burglar, Traveler successfully collected the toy and stored it in a place where he could recover it in the future, and returned to the future without significantly altering the past.  Thus as the moment of his departure approaches he is the same person planning the same trip, and at the right moment he does so, cause A creating effect B, his arrival in the past.  This creates a stable history, and we have a sort of diverging hiccough:  because traveler leaves for the past on schedule, time continues into the future based on the history Traveler created and now confirmed.

However, with Burglar in the mix, we know that Grandfather died and Traveler was never born.  That means cause A never happens, and effect B never happens–we already know what happens if no time traveler arrives from the future, because that was the original history.  Burglar passes through the house unmolested, Grandfather survives to sire Father who sires Traveler.  That results in Traveler making the trip, creating the other history.

In no history does anyone simply disappear.  In no history does something inexplicably change without cause.  The difference between the original history and the altered history is that in the altered history someone arrives from the future and introduces causes that create a different set of events leading to its own undoing, while in the original history no one arrives from the future and so events follow the undisturbed path of events to the moment when someone decides to change them.

I should note that in all of this we experience the changes at the speed of time.  There is a sense in which at the instant Grandfather dies, Traveler ceases ever to have existed–but that only happens because of the intervening causes and effects which fail to bring him to life.  We experience those events at the speed of time; using time travel we presumably could skip ahead to the outcomes in the future.  That, though, means that in some sense all of those events happen instantaneously–and as I have suggested in The Spreadsheet Illustration, it can be understood as all happening simultaneously–it is Einstein who said that time exists so that everything would not happen at once, but if the nature of time is such that time travel is possible, the reality is that everything does happen “at once”, and time exists so that we can experience the causal chains in the order in which events cause each other.  So in that sense the moment Burglar kills Grandfather, Traveler ceases to exist, but his non-existence can only be discovered by following the causal chain to the moment when he fails to arrive in the past.

I hope this clarifies the problem and the solution.  I should mention that we previously addressed the matter in relation to a supposed “multiverse” solution in web log post #81:  The Grandfather Paradox Problem just over a year ago.

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