We begin with a reader signing himself "Waggs", who raised some issues in response to the theory articles on parallel and divergent dimension theories, which required a longer response than could be addressed in the comments sections.

He began,

Before you start running around saying "because there is an infinite number, it must be like this" consider that although the number is really really really big, it is not infinite.

From this, he observes correctly,

Conservation of Energy is not as absolute as we [...] thought....the lost energy is always taken from somewhere else.

There is a confusion here, in which two distinct concepts, parallel dimensions and divergent dimensions, are being conflated.

That "the lost energy is always taken from somewhere else" is conservation of energy:  the same amount always exists, but we move it from place to place.  In parallel universe theory, the total matter/energy of the entire multiverse has always existed.  When we suggest that matter and energy moved from one universe to another, there is no need to compensate:  the total still exists, just in a different location.

In divergent theory, though, the time travel event creates a new universe which takes the history of the old universe to the point of the time travel arrival but then diverges from it.  A new universe is created at that instant, of the same size and in the same state as the old universe, which also still exists.  Thus there is a matter/energy creation problem here, because what is needed to create the new universe must come from somewhere, and must be equal to the total matter/energy of the original universe.  That is where the conservation problem arises.

It is true that the number of divergent universes would never be infinite; but under certain forms of parallel dimension theory time travel only works if the number of identical parallel universes is infinite, because for any given universe there must be at least one other unique universe to which a traveler to the past could go from which no traveler from the future could come.

Waggs, however, cites a different concept of the multiverse, in which there are not many discrete universes but a single universe in which all possibilities occur but each individual experiences only one set.  Citing the famous thought experiment Schrödinger's cat, he asserts,

Each entangled operation that makes up our world, then, may or may not exist in the others....Future and past, then, would be illusions fixated on cause and effect.  Altering your own past would only produce a new configuration and would not alter your existence.

That Schrödinger created his thought experiment specifically to demonstrate that this conception of the multiverse was absurd does not matter to those who cite him for the explanation of this view; this is what is believed, that all possible states are actual.  Waggs takes the notion one step further, asserting that all are actual within a single cosmos, coexisting but known to individuals only in coherent causal chains.  The causal chains themselves are thus illusory; the fire both lights and fails to light, the forest burns and does not burn, and the events are not causally connected but in our perceptions.  It is a conception of reality which denies reality, in which time travel cannot really change the past because all possible versions of the past already exist within the one universe; it only changes which version of the past the time traveler knows, without eliminating the version from which he came, such that he now knows two distinct realities, both of them real and co-existing.  It is fixed time overlaying parallel dimensions in a single universe that has no specific history.

As interesting as that is as a thought experiment, few could embrace such a subjective conception of reality.  It means that those who claim the Holocaust never happened are as correct as those who assert that it did; that Obama and McCain and many other men each won the 2008 American presidential election and are each currently serving as President of the United States.  Reality ultimately is whatever you believe it to be, while being entirely different for someone else.  Rewriting history is not inventing a different world, but recounting a different true version.  Nothing is false; everything that might have happened did.

This runs afoul of objections previously raised in discussing a divergent dimension theory multiverse, the version of parallel dimension theory in which every possible universe exists.  Additionally, in invalidating the necessity of causality it also invalidates the conservation of matter and energy--the point from which this began.  It is, finally, the scientific declaration that nothing is true, a claim which Socrates observed must apply even to itself.

Next, a reader signing himself "Jeff" raised some issues in response to the theory articles on infinity loops concerning variants of replacement theory.

He begins,

What do you call the 'self-correcting theory'?

He provides this example:

...if I kill Hitler, someone else will still take over in his place and essentially commit the same crimes killing millions of people.

In some cases, this is just the inexorable nature of history:  some events are ripe to occur, such as scientific discoveries and political tides; individuals are flashpoints for these events, but the crushing war debt Germany faced after World War I would ultimately have led to another war absent some relief.  In some ways, though, this smacks of divine intervention, that certain things were ordained to happen and a power beyond humanity ensures that they do.  It is difficult to distinguish the inevitable from the preventable.

It is also important to consider the scope of the correction.  If you mean that the same soldiers would die on the same days on the same battlefields, then you are looking at something more than the tides of history; if you mean less than that, then the world will have changed drastically, such that very few of the people who are alive today would have been born.

He then suggests

...the 'made worse theory' where if I kill Hitler, events turn out much worse like Germany rules the world....

This is an even odder view of providence:  it suggests that the world we have is the best of all possible, and thus that any tampering will upset the good that has happened.  Certainly it is possible to make things worse by tampering, but if they are already bad odds are that they will wind up better rather than worse.  If not, then God must have brought about the world that is, declaring it the best possible world, but at the same time must be powerless to prevent us from changing it.

Finally,

...the 'made better theory' where the changes all lead to some sickly sweet, everything is perfect theory, like back1.html">McFly and Lake House, Kate and Leopold....

These films are ultimately rather varied in their approaches to time, but they do all have Hollywood endings.  It is certainly possible to make things better accidentally; we can forgive a good story for its happy ending as long as it does not suggest that happy endings are inevitable.  After all, people do like stories with happy endings.  Not all happy endings are plausible, of course, but that's an individual question:  could it have happened that way?  So the happy ending in and of itself is not at issue, but how it is achieved.

In all these cases, the issues remain the same to this degree:  if what the time traveler did would prevent the same time traveler from making the same trip for the same reason, then history fails (barring reliance on some form of Niven's Law).  Thus if killing Hitler changed the world, for better, for worse, or simply by putting someone else in command, it is almost impossible for a time traveler to decide to kill Hitler (who now never mattered to history) and thus he will not be killed.  That is the classic form of the infinity loop and of the classic grandfather paradox:  the time traveler undoes his own actions, creating a causal loop that cycles between two histories, one which leads him to take action in the other, the other which prevents him from taking action in the one.

A reader signing himself "Hubert" wrote in challenge to our description of replacement theory that he was "dismayed and alarmed" anyone might believe it.  His reasons were entirely theological, that as a Christian he thought the theory "flies in the face of loving God".

As I mentioned briefly there, I am a Christian, and while it is inappropriate to claim to be a "good" Christian, I am a credentialed one, with degrees in the field and ministry experience.  I have reason to believe and reason to hope that time travel will never be a reality--but it is hazardous to assert that there is any particular thing which God will not allow.  (It is undoubtedly so that there are things He will not permit, but He has neglected to give us the list.)  Therefore I have approached time travel from the perspective of wishing to determine what would happen if it were possible.  That Hubert is offended by replacement theory (which I embrace) is of some concern to me; it is the theory I prefer, both metaphysically and theologically.

Fixed time theory, though, is easiest to address.  It is the determinism of this theory which seems so offensive.  It is immediately evident with this theory that one cannot change the past; what's done is done, as they say, and you cannot fix it even if you could go back to it.  Not as immediately evident, though, is that what is not yet done is also done.  There is no point of speaking of repentance or contrition, because you cannot change the future, either.  You will remain the scoundrel that you are if that is your destiny.  Even "destiny" is too weak a word.  It is not that you are fated to do what you have not yet done; it is that you have already done all that you do not know you have done.  Thus, as with extreme forms of Calvinism, there is no point to preaching repentance.  There is equally no moral basis for guilt:  we cannot do otherwise than we will do, and we cannot repent if we have not repented.  Choice is non-existent in this view of the universe.

Parallel dimension theory is more difficult to discuss because of the variant approaches to it.  If all parallel universes are the same, then what is the connection between the various duplicates who are me in each of them?  If I am "saved" in all of them, will I be many in heaven, or only one?  On the other hand, if all those universes are different (not useful for time travel) or if time travel changes events in some such that I am "saved" in some but not in others, are some of me then saved, or all of me or none of me?  Can I as a believer be denied heaven because I in another dimension am an unbeliever?

Divergent dimension theory is similarly complicated.  If we take the view that for every choice we make there is a universe in which we chose otherwise, the value of our moral choices is negated.  Is there a universe where you are a serial killer, a serial rapist, a notorious thief, and also those in which you are a televangelist, an aid worker in Asia, a country pastor?  It seems then that the world we have is improbable and ought to change drastically more frequently (where is the morning on which everyone awoke and decided to mend his ways, or on which everyone chose to kill someone--both of which ought to happen every morning in some universe yet never happens in ours?).  If it is only time travel that creates new universes, there are still complications concerning our alternate selves, made the more complicated by virtue of the fact that they are us, many born in the same universe but carried on a different path through no fault of their own.

Hubert did not explain his problem with replacement theory, but it would seem to be in the fact that it is possible to change the past, and perhaps to destroy time itself.  To the first, as C. S. Lewis observed on another topic, it is not remarkable that God permits us to change the world by prayer but that He permits us to change it at all--and it seems we do change the world by choice.  To add that we could change the world in the past by our choice in the future does not seriously alter that fact theologically; it only gives more options.  As to the consequences, all choices have consequences.  The choice to build nuclear weapons meant the destruction of thousands of human lives and a reign of fear that is not entirely ended.  It cannot be said whether the Cold War never could have gone hot because God prevented it, or whether we chose wisdom instead of folly.  It may be that God would not allow us to destroy time--but whether that will be by intervening in events such that they do not result in disaster or by preventing us from having that power at all is more than we can predict.  I would not dare say it would be impossible to destroy time.  Jesus may have timed his return based on the expectation that we would do so (and the exact details of how that event will occur are hidden even from the best theologians, despite the pretensions of some otherwise).  If the only problem with this theory is that it does not fit with our prophetic expectations, it is worth remembering that the Crucifixion did not fit with anyone's prophetic expectations at the time, either, and nowhere are we told that we have been given a completely clear and complete picture of the future beyond the fact that God is in control.

If there are other theological objections, I would be particularly interested in hearing them.

Perennially readers write to ask whether this or that scheme would work to change the past.  It usually involves telling your younger self to do what you have done.  He is not you, though.  Even apart from the fact that you did not have the experience of being told what to do, he lived in a world that you made, not the world you experienced.  He does not have the same motivation, which will show even when he in turn tries to tell his younger self what to do.  The best hope is that the sawtooth snap this creates ultimately resolves to a stable history and an N-jump; the probabilities are against it.

Yet there might be a way to control the variables and hopefully create a successful outcome.  If you have seen the show 7 Days, they had part of it, but not near enough.  We will use them as the model, though, to finish the system.

To do this you need four teams and a time machine capable of short hops of a precise and unvarying length.  Seven days is the length we will use.  Our teams are the investigators, time travelers, responders, and coordinators.  Each has an essential part in the plan.

When something happens that needs to be prevented after the fact, the investigators learn everything they can about it.  The quicker they can compile their information, the more time will be available at the other end to prevent it.  Everything they learn they save in something akin to a digital form, and give to the coordinators.

The coordinators work with all three of the other teams; otherwise those teams have no contact with each other.  At this point, the coordinators receive the information from the investigators and store it on a durable permanent medium such as a digital video disk.  They have a stock of these, and they use the first one in the stock.  That matters.  They have a delivery system that will transfer that disk to the time travel team without any other contact.  The delivery time is encoded on the disk, and the delivery system delivers it at the designated time.

The time travel team is completely isolated from the outside world.  All information coming into their base is on a seven-day delay.  They do not know what happened today, or yesterday, or any day back seven days.  That means they do not know whether the events on the disk have actually happened or have already been prevented.  As far as they know, this is something that really did happen in the seven days which are blacked out in their news feeds.  Any member of the team who leaves the enclave is quarantined for seven days upon his return so as not to contaminate the information base inside the enclave.  When they receive the disk, they prepare to send it back in time seven days.  If they send someone with it, it is always the same person, and he remains in quarantine when he returns so as not to contaminate their news feed.  Note that he cannot re-enter the enclave until after his doppelganger has departed.

The time traveler himself delivers the data disk to the coordinating team, and then goes into quarantine.  The coordinating team now does two things.  First, they take the top disk from their supply--which is the temporal duplicate of the disk they just received, the same disk seven days younger--and they transfer the data from the disk they received to the new disk.  That disk will include the encoding for when it is to be delivered, so they prepare to deliver it to the time travel team.  The data they received is also given to the response team, which uses the received information to attempt to prevent the disaster.  If they succeed, the coordinators inform the time travel team by means of their seven-day delay.

Note that the time travel team does not know when they receive the disk whether they have already succeeded in preventing the disaster recorded; they can only assume that it needs to be prevented.  Likewise, the response team does not know whether they have already prevented this in a previous timeline or how they did it, and can only do what they think would be best.  Since all the information is the same, it is for everyone the first time through every time, and so history stabilizes into the corrected form, and we have changed history.

We cannot stretch the time too long.  Quite apart from the fact that it would be difficult to isolate anyone for a significantly longer time, we also risk overlapping disasters.  But for something like the Backstep Program, this would be a workable system, and nearly foolproof, depending on the competence of your local fools.

Somewhere in a comment I can no longer find, in response to a previous article, Charles "Chuck" Endicott posited that you

can't travel back [in time]; all the energy used to create [the] past has dissipated so [it is] no longer there.  Energy for the future has yet to be created so where are you really going?

At the time I suggested that an article would be needed to address the issue; this is that article.

He objects to time travel in principle, on the grounds that those "times" do not exist as substantial destinations.  The matter and energy which was (or will be) "there" "then" is "now" occupied "here", and thus that it is not and cannot be in the configuration it had or will have at a different moment in time.

As to whether the past and future exist in any sense of the word "now", quite a few scientists believe the "fixed time" theory, holding that all events past, present, and future, exist in a static continuum and we only experience them in temporal order; Stephen Hawking has considered that possibility and not rejected it.  Thus the issue for these physicists is not whether the past and future exist, but whether they can be altered by time travel.  We plan to publish a second series on temporal theory (Temporal theory 102) which will discuss the nature of time in this regard, but it is for the moment sufficient that the concept of the future and past existing in some form "now" is not incompatible with modern physics.

For literary purposes, though, in approaching a story it is necessary to accept those assumptions the book makes which are necessary to its story and neither patently absurd nor known to be false (excepting, of course, those books that are intended to be patently absurd).  The effort of analyzing time travel films has been to determine which ones do and do not work under such rules for time travel as seem rationally likely, given that time travel is possible and persons have free choice to act to guide events based on their knowledge, with due consideration given to whether other plausible theories of time might explain them.  For example, Source Code is clearly a multiple dimension theory story.

If the objection is fundamentally theological (that the kind of time God created exists only as a present moment), somewhere in the Psalms (and I wish I had noted the spot) the author comments that to God his life is like a book (or scroll?), such that God can turn to the final words and read how it ends--and that can only be so if the future already exists even as we are living in the past.  This certainly does not prove that we can go to that future (or return to the past), but it does suggest that however it is we experience time, the future already exists and the past still exists, at least in the sense that it can be observed by God.

Ultimately the objection appears to apply to all time travel stories that do not involve multiple dimensions in some way, and thus that discussing what "would" happen "if" someone could travel through time is moot.  Yet people write time travel stories, and try to make them rationally consistent with our expectations.  I do not regard parallel or divergent dimension theories to be time travel, because the traveler is in someone else's past; fixed time theory requires a rigorous determinism that eliminates not only free choice but often the integrity of causal chains.  Thus I work with a form of replacement theory as rationally the most probable theory of the effects of time travel.

These issues will be further elucidated in the upcoming theory series.

Schrödinger's Cat walks into a bar.  And doesn't.

One theory of time travel involves travel to parallel worlds, which as we saw [in a Theory 102 article yet to be republished] rapidly become "unparalleled" as soon as a time traveler moves to one.  Yet the more popular theory of parallel universes suggests that no two are the same, that all are divergent based on possibilities, that every universe which might conceivably exist in fact does.  I am asked questions about this notion, perhaps because of the notion of "sideways time travel", or perhaps because I am co-creator of Multiverser:  The Game, which is based on that concept.

The core of the concept is that at any time that something happens, whether someone chooses to do something or an event occurs randomly, the possibility exists that it could have not happened, that something else could have happened instead.  Under the theory, all possibilities occur, dividing the universe into a vast multiverse approaching an infinite number of worlds.  Thus there is a universe in which my game took the role playing community by storm, others where my band is bigger than The Beatles, where I am a successful radio announcer or television talk show host, a Federal Court judge being considered for the Supreme Court, a bestselling author, or any of a thousand other grand things of which I have not dreamed.  John Cross (who analyzed The Final Countdown) insists that in some world I am a molecular biologist, or something of the sort.

The basis for this lies in quantum theory, and the fact that some physicist at some point proposed that the reason we did not know whether any particular unstable atom had decayed is that at any given moment it existed in both the decayed and the undecayed state in different universes, and when we checked we would determine in which universe we were by establishing whether it was decayed or undecayed; until that moment, it existed in both states in our universe.  Another scientist named Schrödinger thought this absurd, and so created a "thought experiment" to demonstrate the absurdity:  place a single unstable atom in a box with a geiger counter which on detection of atomic decay will release cyanide gas into the box, in which there is a cat who will die instantly.  Since the molecule has both decayed and not decayed until we check it, the cat in the box is both alive and dead until we check.  Obviously, Schrödinger argued, the cat is either alive or dead; it is not both.  Therefore the multiverse theory about atomic decay is incorrect.

However, theorists rallied to the side of the original proposal:  indeed, the cat is both alive and dead.  Then, from this very limited and controlled starting point, there developed a notion that all creatures are both alive and dead, that everything that might happen is happening in some universe, and so the number of universes is increasing exponentially.  Every imaginable universe and most unimaginable ones are real.

I side with Schrödinger:  I do not believe it.  Quite apart from the fact that the cat notion was a very specific scenario in which the radioactive decay of a single atom controlled the fate of the nominal creature, and such supposedly random radioactive decay does not normally alter the lives of anything, quite apart from the fact that Schrödinger's point was supposed to be that it was not true and this was supposed to demonstrate it, the notion is not rational.  It requires me to believe that there are other versions of myself who are successful at those things which in my own life amounted to very little.  It is on some level an inspiring thought to believe that I am rich and famous in many universes for things I accomplished, and at the same time depressing to recognize that in this universe, the one which I am experiencing, I am not rich (although some might argue I am famous).  Certainly it also means that there are universes in which I am worse off than I am, but that is small consolation next to the other image.  Those universes are pipe dreams.

Of course, that is something of an emotional response.  There are people in this world who are rich and famous, and the fact that I don't happen to be one of them does not mean that there is not a universe in which I am.  However, when I look deeper at it, there is still reason to challenge this view of the multiverse--a challenge that I shall raise next.

We introduced the notion of a multiverse in which every possible universe exists--the universe in which I am a famous rock star, the one in which I am a bestselling author, and many others.  I said that I did not believe those universes exist--but admitted that that was an emotional reaction.  I have a more logical reason, though, for my disbelief.

One of the things that sometimes happens to people is we "go off the deep end".  Someone who has been seemingly normal his entire life without any advance warning shoots up an elementary school, or rapes a co-worker, or robs a bank.  We would all like to believe that it could not happen to us, but anyone who has seen the film Falling Down should recognize that quite a bit of our self-composure is simply that we have not been pushed past our limits.  Any one of us could "snap" at any moment.  It could be this moment; it could be tomorrow morning.  There are probably a thousand things we could do when that happened, all of them bad, most of them newsworthy.

If we assume that all possible universes exist, then there is a universe in which you snap right now, and another universe in which you snap right now, and another in which--you get the picture.  Not only that, there is a universe in which you and I both go at the same time, another in which you go first and then I go, and another the other way around; and this is true for everyone in the world.  There is, in fact, a universe in which all of us "lose it" right now, for every "right now".  It certainly means that of billions of people on Earth, if only thousands are doing so on any given second, or even on any given day, that is a very small fraction of the total.  There are worlds that are very bad, because everyone is going crazy right now.  By this theory they must exist.

In fact, by this theory, we cannot be certain that our world is not going to descend into such a nightmare any second now.  Yet despite the possibility that anyone might go crazy at any moment, and that by simple random chance there ought to be universes in which vast numbers of people do so simultaneously with no connection to each other, it never happens.  We have random acts of violence that are unexpected, but we do not have them as frequently or at the saturation level that must be true in many universes in order for all possible universes to exist.  We have wars and riots, but these events occur because of the connections between the people involved.

That suggests that the universe in which we find ourselves is really extremely improbable.  It might have happened a moment ago; it might have happened this morning, or yesterday, or last week.  It has never happened.  Yet if this theory of the multiverse is true, it might happen today, tonight, tomorrow, next week, at any moment when we do not expect it.  The entire human population could go crazy all at once; even if it were limited to one tenth of one percent of individuals randomly scattered throughout the world, it would be a devastating event.

We might propose all kinds of reasons why it does not happen, but the fact remains that those universes must be realities if the theory is true, and it must be the case that at this moment the universe has once again splintered into decillions of diverging histories, in quadrillions of which vast numbers of people have gone over the edge and begun some type of violent spree.  Why have you and I never been in any of those universes?  There must be versions of us who are; we never seem to be those versions.

I would like to believe that this would not happen to me, that because of my character and the work of God in my life I would not become one of those violent statistics.  You probably believe that of yourself as well.  If we are correct, though, then not all possible universes are real, and not every choice you make creates a universe in which you chose otherwise.

There might be parallel dimensions, but there are not divergent histories of the sort suggested by the advocates of Schrödinger's Cat.  The demonstration that there is some kind of multiverse is not a demonstration of histories parallel to our own, but merely of facets of the universe not spatially connected to us in a way we can perceive.  That is more than I know, really; but I know that there is not a version of me in another universe that just blew up the mall.