When I think of Asimov I imagine the greatest sci-fi writer in history: Maybe this isn’t true, but I was very impressed by some of his ideas and I expect always to find great ones when I read his books.

But when I start to read The End of Eternity I feel a bit annoyed by his time-changing structure. The thing that I don’t understand is the “two times” evolution of the universe: It seems to me that the time the eternals can travel in is more another space dimension that only they can access and there is a another time, a universal one, that flows in the eternity and in different first time’s point.

Indeed when eternals make a history-change they have to wait for the effect to propagate in time, but if they could travel in time and see the future in what time are propagating these effects? I think that there is what I’ve called the second one.

I’ve nothing against a sci-fi universe with five dimensions, in which two of these are time-like, but it seems to me that Asimov would want to set his novel in a four-dimensional universe, so my question is: is there any way to have a consistent view of The End of Eternity events in a four-dimensional universe?

  • Hello and welcome to SFF! This is a well written and thought out question. Just to clarify are you asking for how the events could play out if there was only a four dimensional universe or if that universe is possible? I'm also not sure if SFF is the best place to ask your question.
    – TheLethalCarrot
    Commented Apr 7, 2018 at 10:31
  • The first one (how the events could play out if there was only a 4d universe), I’d want the users opinion on two points: if it is possible to imagine book’s event, or something similar, happen with only one time and, if there is someone creative, if is impossible what story is possible to tell with only one time that has someting in common with Asimov’s one.
    – Annibale
    Commented Apr 7, 2018 at 11:03
  • That seems to be a bit opinion based for our site, there might be a subreddit or forum where this would be better suited.
    – TheLethalCarrot
    Commented Apr 7, 2018 at 11:09
  • I think that you're right, but this time I'd like an opinion based answer (even if well-supported is also better, but not necessary for me)
    – Annibale
    Commented Apr 7, 2018 at 11:14
  • Eternity is outside of normal time and space. This is mentioned several times in the book. Therefore your proposed "second time" already exists.
    – JRE
    Commented Apr 7, 2018 at 11:14

3 Answers 3


The idea of changes to the past propagating forward necessarily involves a second time dimension -- there's no avoiding it. To be perceived, "change" necessarily involves a time dimension against which the change occurs. When what is changing is events in time, then you need a second time dimension for it to change in. You can't observably change the past in a 4D universe!

If we confine ourselves to our usual four dimensions and try to imagine a change to the past (and its consequent effect of its future), without a way to step outside of those four dimensions, we can't speak of change to the past at all. We remember how Lincoln's second term in office was spent touring the country, North and South, to heal the wounds of war and effect a lasting reconciliation. We remember building a time machine to observe his great series of speeches and how careful we were to avoid doing anything which might contaminate the past. And ?suddenly? we remember his assassination and the botched Reconstruction that followed it. How can that be? If Lincoln died of an assassin's bullet 150 years ago, then our memories from earliest childhood are of that death and its consequences. And if he lived to an honored old age, then our memories from earliest childhood are of the Grand Old Man himself.

A few writers have appreciated that -- a particularly good example of one who did was William Tenn in "The Brooklyn Project". In that story, the Brooklyn Project was to prove that the present does not change even though the past is changed.

After an introduction, the chief scientists is ranting about people who think that changes to the past change the present:

“As you know, one of the fears entertained about travel to the past was that the most innocent-seeming acts would cause cataclysmic changes in the present. You are probably familiar with the fantasy in its most currently popular form—if Hitler had been killed in 1930, he would not have forced scientists in Germany and later occupied countries to emigrate, this nation might not have had the atomic bomb, thus no third atomic war, and Venezuela would still be part of the South American continent."

He goes on a bit more and then presses the button to send back a a series of probes. After each probe, things change more -- more radically each time -- and finally, when the experiment is over:

“See,” cried the thing that had been the acting secretary to the executive assistant on press relations. “See, no matter how subtly! Those who billow were wrong: we haven’t changed.” He extended fifteen purple blobs triumphantly. “Nothing has changed!”

Bottom line: Asimov had to do something like what he did if he wanted to have the people in the story perceive changes to history.

BTW, I agree with your criticism of how he did it: He wasn't being terribly consistent but was focusing on the story. (Which IMO made him a greater writer, not a lesser one.) So to answer your question, In a 4D universe, I don't see any way you could have The End of Eternity.

For an example of someone who did a better (IMO) job of handling the perception of changes to history, look at Poul Anderson's Time Patrol series. In that series there is only one 4-D space-time, and when one changes the past everything futureward of the change changes. The only way to retain your memories of the "former" timeline is to be to the past of the change. Nicely done. (He's not the only person to use that idea and I doubt he's the first. But I do believe he's one of the best.)

  • 1
    Thank you very much, I agree with you and I accept your reading suggestions, and also I liked very much your judgment on Asimov policy of focusing on the story. Moreover I’d like to know if you or someone else has an alternative 4D background in which set “The End of the Eternity”’s events. Maybe would be better if I assign the best answer and ask another question on the topic... (is there someone who knows what is the SFF policy inthis cases? is there any metapost on this?)
    – Annibale
    Commented Apr 7, 2018 at 16:04
  • For the record, I concur that you need some way to (in some sense) step "outside" of the four dimensions of space-time, but I don't agree that this way needs to constitute another time dimension except perhaps in a very metaphorical sense. Eternity looks a bit like another time dimension if you squint, but that interpretation doesn't really hold up IMO. Commented Apr 7, 2018 at 23:16
  • @HarryJohnston I agree with you that there is no neef to put a time2 for setting Eternity in it. The real point is not Eternity, indeed, but the world outside Eternity, in which changes happens at "finite velocity" in time, but the finite velocity cannot be a velocity1, because changes are propagating from a point in time1 to another point in time1: it has to be a velocity2 dt1/dt2 (in particular its time1 component)
    – Annibale
    Commented Apr 8, 2018 at 9:24
  • As already mentioned, the novel makes it clear that your dt2 is what in-universe is referred to as physiotime. From a theoretical physics standpoint I don't think there's any compelling reason to think of it as a fifth dimension, so you could interpet "The End of Eternity" as set in a 4D universe if you wished. Of course if you prefer to think of it as 5D that's your prerogative, but I doubt that's what Asimov intended. :-) Commented Apr 9, 2018 at 5:44

It seems to me that [...] there is another time

Yes, in-universe the Eternals called this physiotime.

It seems to me that Asimov would want to set his novel in a four-dimensional universe

Physiotime really doesn't seem to be a fifth dimension; it is more complicated than that.

According to Noÿs the people of the Hidden Centuries can view all the alternate timelines, not just the one that constitutes Reality. So in some sense, all possible histories exist simultaneously. This applies to Eternity as well as to Time - Noÿs specifically talks about them studying multiple alternative Eternities.

(This is actually rather reminiscent of what we would now call a multiverse or a many-world theory, which is impressive considering that Asimov was writing several years before Everett first proposed the original relative state formulation of quantum mechanics.)

One might therefore sensibly interpret Reality as following a path through the multiverse, as different histories become briefly Real and are then replaced. Even when the Eternals aren't messing with time, there are still microchanges. Physiotime could be considered to be a measure of how far along that path Reality had gotten.

... whether you could really make all this mathematically sound is a bigger question. A point in physiotime would have to in some way represent a particular history of the universe, including the history of Eternity, but ordinary physiotime as experienced by the Eternals involves changes in Time but not in Eternity.

I think that's OK, though, as it just means that the path Reality takes is ordinarily constrained to keep Eternity unchanged; Twissel spoke of infinite loops around an infinite physiotime circle, and IIRC specifically talked about microchanges in Eternity, so that isn't an absolute rule.

Is [the story] consistent?

Maybe, maybe not. But it isn't obviously inconsistent, and that's more than most writers manage. At any rate, you certainly don't need to postulate a fifth space-time dimension to make the story work.

  • 1
    Two additional notes: firstly no matter how consistent they may be, I don't think time travel stories qualify as hard science fiction. :-) Secondly, I wanted to emphasize how difficult it is (IMO) to write changing-the-past stories that aren't obviousy inconsistent - the only other one I'm aware of is Poul Anderson's "Time Patrol" series, and to be honest he kind of cheated. Commented Apr 7, 2018 at 12:30
  • Essentially, what you have is a place "outside" of time where the people making the changes can study the impact of those changes without getting changed themselves.in the process. Commented Apr 7, 2018 at 15:31
  • @HarryJohnston thanks very much for your answer, but I think it’s not really correct: multiverse is a bit different, and could be similar, at least in structure, in a 5-dimensional space-time only if the “alternatives” that the universe could choose are a continuum of dimension 1 and interlaces among them, in the sense that I could reach a different point also from two different paths, and it is the truly the same inside the multiverse structure (I’m sorry if a misuse a tag, I’ve read the description and it seemed to me that fits my question)
    – Annibale
    Commented Apr 7, 2018 at 15:40
  • @HarryJohnston I stress my hypothesis of a 2nd time dimension in the base of propagating changes. I suggest to think of that as to propagating fields: if you change your source position fields propagate this information throughout the space and this is done with a certain velocity, i.e. it last a finite time-interval to arrive in a distant space point. In this way the novel’s changes propagate through time: it last a finite time2-interval to reach a distant time point, i.e. another century.
    – Annibale
    Commented Apr 7, 2018 at 15:49
  • 2
    The novel definitely makes clear that the delay in the propagation of changes in Time is measured by physiotime, so I think your time2 really is the same as physiotime. Physiotime isn't just one person's proper time, because all of Eternity experiences physiotime at the same rate. When Andrew spent a month in Time, everybody in Eternity was exactly one month older when he got back. Commented Apr 7, 2018 at 23:02

When I thought at Asimov I imagine the greatest sci-fi writer of the history:

You can have that opinion for sure; he objectively was one of the three great ones at the time he was active in the SciFi business (which is basically the 1950s). "The greatest" is surely a big shoe to fill. Jules Verne certainly was The Greatest SciFi Author when he lived as well...

But do keep in mind that he also was a child of his times, they only just had invented early rocketry back then, and it was certainly much too early to expect any physically sane or "realistic" idea about concepts like time (or even time travel) from a book of that era.

are there any way to have a consistent view of The End of Eternity events in a four-dimensional universe?

No. There is no consistent view of time travel with our current knowledge about physics and reality, period. If this were Philosopy.SE, then we could have a long talk about what "time" is anyway (spoiler: nobody knows, and people are arguing about it to this very day...). The very best we can come up with are theories (or rather conjectures) that just turn out to feel like magic to prevent time travel (to prevent anomalies).

I admit that I have not read that particular book of Asimov, just gave the Wikipedia page a quick glance. Obviously it has been received very well, but I'd daresay that's mostly because of his intense plot and not of a revolutionary, "real" implementation of time travel.

  • Just to know: who are the other biggest two according you?
    – Annibale
    Commented Apr 7, 2018 at 22:14
  • 1
    @Annibale: not according to me... I've added a link, and if you check out the pages of those three authors (Asimov, Heinlein, Clarke), you find links to original papers/books which look at them and clearly show them to stick out from the rest (if only in amount of books, influentality etc.; not talking about taste here). I do hope my first paragraph is "tongue in cheek" enough to let it stand as is. ;) And no disrespect to Philip K Dick or other great ones of that time....
    – AnoE
    Commented Apr 7, 2018 at 22:22
  • @AnoE Asimov himself talks about the ABCs of SF in prefaces of some of his books (Asimov, "Bob" Heinlein, Clarke)
    – muru
    Commented Apr 8, 2018 at 1:55

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