Over On The Poul Anderson Appreciation Blog

Recent discussion of time travel has been on the Poul Anderson Appreciation blog, e.g., here. Please look there for posts that would have been published here if they had not been mainly about works by Anderson.

See also here.

Dear Tomorrow

I mentioned this anthology here.

In "Dear Tomorrow" by Simon Clark, the presenters of a regular live TV show plan to broadcast from the top of a mountain, asking any time travelers who find the archived show to travel back to this date and join them on the mountain. Heavy rain prevents the broadcast. The main event in the story is that a widower and a widow who had participated in the show because of their respective bereavements start to get together and move on, leaving the past behind, into tomorrow. The story hints that one person present is in fact a time traveler but I think that this was unnecessary.

If any time travelers see the archived show, then they will also see whether they appear on the show. If they do not (see themselves) appear, then they will not (travel back to) appear.

Check Out Poul Anderson

I have recently discussed time travel a lot on the Poul Anderson Appreciation blog. See here.

And here.

Temporal Vehicles

(i) A "time machine" (antiquated terminology from HG Wells) moves fictitious characters pastwards or futurewards along a single timeline. A variation is a space-time machine, like Poul Anderson's Time Patrol timecycles or Doctor Who's Tardis, which can also move the characters to a different place.

(ii) A different kind of vehicle, with no agreed terminology, would move characters sideways into a different timeline, assuming that other timelines exist.

So how can a character with a mere time machine get into a different timeline? He might travel pastwards and (a) initiate a divergent timeline or (b) return futurewards, passing through the moment at which another time traveler had initiated a divergent timeline.

It follows that a time traveler cannot enter another timeline merely by traveling pastwards.

Stepsons Of Terra by Robert Silverberg

I had two bad experiences with time travel fiction recently, a film and a novel.

While on holiday, I borrowed, but did not finish reading, Stepsons Of Terra by Robert Silverberg.

(i) The characters refer to the "Absolute Past." Every moment is past to some moments but present to itself and future to others so there is no Absolute Past. The relativity of simultaneity means that event A can be simultaneous with event B to one observer but before or after B to another.

(ii) We are told that a building is safely invisible and undetectable because it is three microseconds in the past. It does not make sense to say that anything is now in the past. Even if it were somehow valid to say at 1.00 pm, "The building is now at 1.00 pm minus three microseconds," then, at 1.00 pm plus three microseconds, it would be necessary to say, "The building is now at 1.00 pm," so the building should be detectable at 1.00 pm.

(iii) Ewing is being tortured and is going to be killed but is rescued by an armed, masked man who returns him to his hotel room where he sleeps and recovers. On waking, he meets a group who have time travel. With their help, he travels a few days into the past to become his own armed, masked rescuer. After leaving his younger self asleep in the hotel room, Ewing continues to exist so he should still be in existence when his younger self has traveled into the past. This entails, on the simplest hypothesis, that there is a single timeline containing a single Ewing whose single world-line has a single loop.

However, Ewing concocts a much more elaborate scenario. He thinks that the Ewing whom he rescued is not only younger but also other and that this other Ewing will rescue yet another Ewing. The implications, not spelled out at least in the part of the novel that I read, are:

there is a succession of timelines;
pastward time travel is to the past not of the current timeline but of a succeeding timeline;
each Ewing is born, grows up and is eventually tortured in one timeline but is rescued by the Ewing from the preceding timeline and rescues the Ewing of the succeeding timeline.

Ewing feels obliged to end this regression. To do this, he leaves a written message beside his sleeping self advising that self not to travel into the past, then kills himself! This will ensure that the current timeline will continue to be inhabited by the Ewing who was born in it. It also ensures that, if there is a succeeding timeline, then it will be one in which Ewing is tortured and killed. Why the Ewing who kills himself regards this as a desirable outcome is beyond me.

Before dying, the Ewing who commits suicide reflects that the single factor for which he has made no provision is his own rescuer. That rescuer should have lived out the rest of his life in the preceding timeline. Either there has been an infinity of previous timelines or there has not. If not, then the Ewing of the first timeline could not have been rescued by a Ewing from any previous timeline so the progression could not have got started.

Dimensions

I am going back to basics with dimensions. Many page viewers will not need to read this post.

On the flat surface of a blank sheet of paper, there are two dimensions: horizontal and vertical. Each dimension has two directions. Horizontal: left and right. Vertical: up and down. Each dimension is at right angles to the other. Thus, it is possible to move up or down without moving left or right and vice versa.

We can let the horizontal dimension represent the temporal dimension. In that case, the vertical can represent either one of the three spatial dimensions or a second temporal dimension. In this post, it will represent the latter. A temporal dimension has a direction or "arrow" defined by causality, memory and entropy whereas a spatial dimension does not.

If the horizontal represents our familiar temporal dimension, then a horizontal straight line represents a history or "timeline" from beginning to end. Each point on the line represents a moment of time. To an observer located in one of these points, every point to the left is earlier or past and every point to the right is later or future whereas, to an external observer looking down on the sheet, the points coexist with each other simultaneously.

A second straight line drawn above and parallel to the first line can represent a second timeline existing later then the first timeline in the second temporal dimension which extends up the page. If a time traveler, reversing his arrow of time, "travels" leftwards, then he moves pastward in the first temporal dimension (T1). If he travels upwards, then he moves futureward in the second temporal dimension (T2).

The hero of Ward Moore's Bring The Jubilee leaves a timeline in which the South won the American Civil War and enters a timeline in which the North won. He moves left/pastward in T1 to a decisive battle but also up/futureward in T2 to the second timeline. Thus, he can say, from the point that he has reached in T2, that the first timeline no longer exists. However, it is still visible to an external observer of the sheet of paper. Further, it is of no concern to the inhabitants of the first timeline that they will no longer exist according to an observer in the second timeline.

Contradictions only occur when we try to cram all of these diverse events into a single temporal dimension.

Revisiting Successive Timelines

Rereading Poul Anderson's Time Patrol series reopens the issues of the two "Logic of Time Travel" articles at the beginning of this blog.

In the standard science fiction causality violation scenario, a time traveler originates in timeline 1 and travels to the past of that timeline but then "changes the past," thus generating timeline 2. One way to picture this is to represent timeline 1 by a horizontal line and timeline 2 by a second line emerging at an angle from the point representing the moment of the causality violation but this entails that, at that moment, the time traveler disappears from timeline 1 and creates around himself an entire new universe for timeline 2! I do not think that either the functioning of a time machine or the actions of a time traveler would be able to create all that organized matter and energy.

I think that it makes more sense to model temporal change on experienced change. Thus, in experienced change, a single temporal dimension connects states changed from to states changed to. Each of these states is a configuration of the entire three dimensional universe. Similarly, in temporal change, a second temporal dimension connects changing states. Each of these states is an entire four dimensional continuum with its own internal temporal dimension. It is these temporal dimensions that we call timelines 1, 2 etc. 

A time traveler originates in timeline 1 but either transforms timeline 1 into timeline 2 or causes timeline 2 to succeed timeline 1 along the second temporal dimension - these are alternative descriptions of a single process. In The Shield Of Time, Poul Anderson presents another scenario: a quantum change in space-time-energy transforms timeline 1 into timeline 2.

If a story were set in the timeline 2 of the quantum change scenario but without time travelers, then readers would recognize an alternative history or parallel universe story. However, "parallel" implies simultaneity or co-existence whereas I argue that timeline 1 does not coexist with timeline 2 but preexists and causes it along the second temporal dimension. In that dimension, timeline 1 is not contemporary with but earlier than timeline 2 and therefore is inaccessible to a time traveler who can either remain in timeline 2 or advance to timeline 3 but not return to timeline 1. That is how Anderson describes the relationship between the current and deleted timelines in the Time Patrol series.