In Anderson's world, people travel to the past all the time--but most of them do so because they work for an organization dedicated to preventing changes to the past.  It seems that the day people, somewhere in our future, discovered time travel, they were visited by people from a yet much more distant future, an incomprehensibly distant future, who had a vested interest in preventing change to the past, and so informed those earliest inventors of the technology that they were now drafted into a temporal police force to prevent anyone else from using their technology to change the world.  They also recruited at least a few people from earlier times, including several from the twentieth century, to work as researchers, historians, and enforcers.  These were provided with the equipment needed to travel through time, and given life extension therapy so that others from their own age would not wonder either why they aged so quickly or where they were all the time.

Even a casual reader of this site should recognize that such a scenario is not possible under any of the familiar models of time.  Bear with a brief exposition of the flaws.

Under fixed time, perhaps the most popular model of time travel, the people in the future are wasting their efforts trying to preserve the past, because the past cannot be changed--all effects of all time travel events are already part of history, and those who will at some point in the future travel to the past in some sense have already done so, have already arrived in the past.  The time patrol itself is nonsense, as it is enforcing rules that cannot be broken by attempting to break them.  One might was well organize a police force to enforce gravity, and say that in their efforts to enforce the law of gravity they are permitted to break it.  Of course, Anderson's stories would be rather boring in that case.  That's not to say that you can't tell an interesting story in fixed time--only that a story about a police force that attempts to correct changes made to history before they become serious problems is not such a story.

Parallel and divergent dimension theory is vexed by the problems outlined in The Two Brothers:  Why Parallel Dimension Theory Is Not Time Travel.  Notably, our future society that is attempting to preserve itself is simply creating other universes.  As with fixed time, the society of the future cannot be changed--in this case, because its history is fixed, and the time traveler is tampering with someone else's history.  Further, if we assume that someone from the future has traveled to the past and created a new universe in which that future society does not exist, then someone else from that future "fixes" the universe such that that society is restored, we have gone from having one universe in which that future society exists to having three universes--one in which the future society existed and was never endangered, one in which the future society never existed and never would have come into existence, and one in which the future society came into existence because after a traveler from the future tampered with history, someone else tampered with history again.

Even more problematic for telling such tales under the parallel/divergent dimensions theory is the problem created by the linear nature of each such dimension.  Although Back to the Future Part II is a disaster of a time travel story, it gets this part right:  once Doc and Marty are in a divergent dimension, they cannot go forward to the point from which the time traveler departed to correct the problem, but only backward to the root where the change occurred.  Once a change has been made in the past that would destroy the society of the future, there is no society in that future who can detect and correct this.  In that dimension, that society never came into existence.

The replacement theory--the theory supported by this site--also rejects such a concept.  For there to be any future universe, all anomalies must already have resolved. Let us suppose that the future society lives in twenty thousand A.D.; let us suppose that time travel is discovered in three thousand A.D.  If our time travelers in three thousand travel to two thousand and completely alter history between two thousand and three thousand, maybe they will destroy time, and maybe they will be very lucky and preserve time--but by twenty thousand, that's all ancient history, more ancient to them than the earliest Egyptian inscriptions are to us.  Whatever that distant future society is, it became that because of whatever changes were made to history by all time travelers before then.  Their intervention cannot prevent settled history from changing, because for them the changed version is and has always been the settled version.  They exist because of every change that was made, not in spite of these.  There is, again, nothing to protect.

Yet the stories seem plausible.  This suggests that there is a way of understanding time that does not fit any of these models.

The solution that seems to be in view is to perceive time in two dimensions.

On one level, this is similar to the conception of time outlined in The Spreadsheet Illustration of Temporal Anomalies--similar enough that the reader should understand that concept before attempting to grasp this one.  In short, there is a sense in which all of history occurs metaphysically simultaneously.  The future is not unformed, merely undiscovered; the past is not unalterable, merely known.  If there is a change in 2000, there is a consequent change in 3000, an immediate change because the events in 3000 are in direct causal dependency upon the events of 2000.  It does not take a thousand years for the change to occur; it takes a thousand years for us to reach the place in time where the change occurred.

Each of the popular theories of time travel treats this problem differently, but in each case it can be comprehended by visualizing time as if it were space.  In the fixed time theory, that space is unidimensional:  time exists in a continuous line from the past to the future, and cannot be altered.  With the parallel and divergent dimension theories, there are multiple timelines lying alongside each other, and travelers leaving one arrive in another.  This is the concept of sideways time, suggested in a John Pertwee Dr. Who episode and exploited in Sliders.  In this conception, these parallel or divergent universes exist temporally "alongside" each other, but are disconnected save by the acts of time travelers, who are really dimension hoppers.  Also, the past is immutable, but the future is being created.

The replacement theory gives the impression of two-dimensional time, but it does not support the conception.  Rather, as with fixed time, it maintains that there is only one history of the world--but that it might be altered, and if it is then there is still only one history of the world, the other having existed only in some metaphysical past, something like the program on your video recorder that you erased to record another.  Still, the description of history in this case involves tracing causal lines to determine whether the past still supports the future which supports the past.  The past can be changed, but once it is, nothing from the original past remains.

For Time Patrol to work, there needs to be two-dimensional time.  Time moves from the past to the future, as is familiar to all of us; but it also moves laterally, from one version of history to another.  Yet lateral versions of history still exist, and anything which originated in one can continue to exist in another.

What ought to make this work is the interaction between chronological time and lateral time.  Having accepted that chronological history all exists simultaneously, and that time is merely the way we experience it, we can understand that a change in 2000 could cause an immediate change in twenty thousand.  We would have to wait until we reached twenty thousand to see that change, but there are people in twenty thousand whose lives would change immediately.

The problem is, no one in twenty thousand would know that history had changed.  It would always have been thus--unless there is another aspect to time.

That other aspect would be the lateral.  This rests on the notion that change requires a medium within which to occur.  In history, change occurs through chronological time:  I drop my pen now, and it hits the floor now.  We are accepting that this is perception, that the falling and landing of the pen are accomplished instantaneously in various points in time.  Yet if we involve time travel, and we eliminate at least some of the conception of time as the medium in which changes occur (because we are now looking at global changes, changes which occur instantly across all of time), it might be asked how that change can happen.  One possible answer is lateral time.  With each tick of lateral time, the history of the universe is established; if a time traveler from a previous tick makes a change to the past of this one, the entire history of this tick forms to accommodate that change, but still accommodates causes which have moved from the previous tick into this one.

That means that time is not really moving "forward" toward the end of history, but it is really moving "sideways" from iteration to iteration of all of history.  Anyone who moves backward in time also moves sideways, into a subsequent iteration of the entire history of the world.  He might undo his own birth, but this paradox is inconsequential, since his birth still exists in the iteration of history from which he came, a spatio-temporal location as real yet inaccessible to him as yesterday is to us.

Unfortunately, this does not solve the anomalies portrayed in Anderson's work.

As noted in connection with parallel and divergent dimension theory, the problem still remains that in any iteration of history in which the desired future society is destroyed, it does not exist.  Thus it cannot send enforcers back to correct a problem that it cannot detect.

Anderson's answer to this seems plausible on its face.  Time Patrol training facilities exist in an era in the distant past--prehistoric past, dinosaur past.  This facility includes among its resources everything that is known about the future history of the world, and specifically what events are thought to be important in establishing the existence of the future society which sponsors their work.  Since this facility exists at a point prior to any known ventures to the past, it continues to exist despite changes made in its future.  Since it can still possess information and personnel and equipment which were delivered from the future of one iteration of time, the shifts in the existence of the future society will not impact its ability to perform its mission.

The problem is finding the relationship between historic time and lateral time.  Think of it this way:  those who are stationed in the past are charged with preserving the future whose records they have in their files.  The moment some change in that future is detected, one of their own must undertake to identify the change, determine how to restore what would be as near as possible to the original history, and travel to whatever point in time this repair can be implemented.  But it is still that aspect of knowing when the future has been changed that creates the problem.

From the perspective of historic time, the answer seems to be either that it will be changed or it will not be changed at a particular moment.  Our theory of two-dimensional time, though, suggests that there is an original history of the universe in which no time travel events ever occurred, and then "when" someone traveled to the past, we moved laterally across time to the altered history.  That lateral shift, though, is not something we can perceive or experience; it also is not something that happens at a specific moment in the historic timeline:  it happens to all of history simultaneously.  Thus at time L1, the original history of the world, no one can detect any change in history.  Then at time L2, following the "first" time travel event, everyone who can detect such events detects that one simultaneously.

To clarify, let us suppose that the time patrol base in the past exists for a thousand years.  In the L1 moment, no one ever detected any change in the history of the world.  Then as we moved to the L2 moment, a change occurred.  That change became detectable--but it was detectable simultaneously through every second of those thousand years.  Then the L3 moment occurs, due to another trip through time, and both changes are detectable for that entire thousand years.  More significantly, those who live in the time patrol base when it reaches L3 cannot know which of the two detected anomalies occurred first.  Further, those who live at year 1 have no means of knowing whether they are actually detecting a change made by their own people at year 5.  In fact, every time someone is sent back to the base, it creates a detectable anomaly, a change in the history of the world, a next tick on the lateral timeline.  Without a complete roster of everyone who was recruited and sent for training, the people at year 1 cannot sort out who is what.

You might think it a simple enough matter for the people at year 1000 to send a that complete roster to the people at year 1, so that they can use that to compare against whatever they detect.  The problem is, as time moves laterally the information at year 1000 will change but the information already sent to year 1 will not--otherwise, the record of the history of the future society these people are trying to preserve would also change, and they could not know whether history had changed or not, nor what it was they were trying to preserve.  In order for the people at year 1 to have current information, the people at year 1000 must send back an update with ever click of lateral time.  That, though, is impossible.  Even were we to suppose that our time travelers have a way of detecting clicks of lateral time, they cannot act in lateral time, only in linear time.  They can send a roster back from the earliest lateral moment in which they have that roster, but in order to send an updated roster they would have to be able to send both rosters simultaneously.  Again, if we assume that having sent the one roster at L1 they automatically will have sent the most currently updated roster at L1000, we must also assume that whatever history of the world was sent back from the distant future is that version that exists at L1000, having been updated by virtue of the fact that the people in that future will have sent the most current version.

Too, this ignores the detail that the very sending of the current roster is itself an event, a change to history which needs to be recorded on the roster of those events which are to be ignored.

The concept of lateral time does not solve the problems created by the Time Patrol stories, nor does it appear to resolve any other problems.  My impression is that the replacement theory is still the best theory.  At the same time, the concept of lateral time might help resolve some of the uncertainties of two-dimensional time, particularly in relation to what is the future for a time traveler who travels to a point prior to a previous anomaly.  In any case, I offer this as a starting point for any readers who think there might be something of value here, and invite you to make your suggestions for how this might be brought to a functional theory of time.  I am available by e-mail, but would prefer to have such discussions publicly, and so invite anyone interested in posting to do so at the official Multiverser forum at Gaming Outpost, where there are at least a few persons interested in such discussions.