Human Extraterrestrials


Even though science fiction is often focused on the future, its assumptions are tied to the present.

Aldrin descends from Apollo 11In some respects this is obvious.  A story about the near future can become dated by history itself.  Every SF story prior to 1969 that describes the first moon landing in detail (happy 51st anniversary, last week!) is obsolete.  And every story that predicted a smooth reach out into colonizing the solar system directly after that first landing, unfortunately, is also defunct.  Stories can also be rendered unbelievable by scientific advance:  all the delightful tales based on a habitable Venus or Mars are gone with the, er, vacuum.

But there’s also a subtler way.  Even though F&SF specialize in examining our assumptions about the universe, the assumptions that seem plausible shift over time.  Fashions change.  To take a heartening example:  SF stories from the late 1940s and the 1950s tended to take it for granted that there would shortly be a nuclear world war.  (Hence it’s spot-on characterization when the 1955 version of Doc Brown in “Back to the Future” accepts Marty’s recorded appearance in a hazmat suit as logical because of the “fallout from the atomic wars.”)  But for over seventy years, we’ve managed to avoid that particular catastrophe.

One assumption that’s always intrigued me is whether we are likely to meet people like ourselves—and I mean, exactly like ourselves—on another planet.  If we discovered an Earthlike planet of another sun, might we climb down the ladder from our spaceship to shake hands with a biologically human alien?

Not Really Alien

I’m talking about a “convergent evolution” hypothesis—the notion that the human species might have developed independently more than once.  And, incidentally, the standard biological definition of “species” as “interfertile” (a more precise definition can be found on Wikipedia) is what I’m using here; because, obviously, one of the potential uses of the assumption in a story is to make possible a romance between two characters from different worlds, and romance is not unrelated to sex and reproduction.

The Cometeers coverSo we want to set aside, to begin with, a class of stories in which people from different planets are all human because they have a common ancestry.  For example, in Jack Williamson’s classic space opera The Cometeers (1936), Bob Star finds his true love Kay Nymidee among the human subjects of the decidedly nonhuman masters of an immense assemblage of space-traveling planets, the “comet.”  But the reason there are human beings present is that a research ship from Earth was captured by the Cometeers long ago, and these are the descendants of the crew.

It’s not uncommon for the inheritance to work the other way around.  David Weber’s “Mutineers’ Moon” (1991) starts with the eye-opening assumption that our Moon is actually a long-inert giant spaceship—and reveals that the humanity of Earth is descended from the original crew members of that spaceship.  Thus, it’s perfectly plausible when hero Colin MacIntyre falls for a preserved member of the original crew; they’re from the same stock.  Similarly, in at least the original 1978 version of Battlestar Galactica, the human survivors of the “rag-tag fugitive fleet” are human because Earth itself was one of their original colonies, which apparently fell out of touch.

The Era of Planetary Romance

In the early days of modern SF—say, from about 1912 through the 1930s—it was commonly assumed that the answer was yes:  human beings (with minor variations) might be found independently on other planets.  Arguably, this may have been because the early planetary romances—melodramas set on exotic worlds, heavy on adventure and love stories—were less interested in science than in plot devices.  But biology was less advanced in those days; recall that DNA was not identified as the basis of genetic inheritance until 1952.  It’s easy to forget how little we knew about things we take for granted today, even in relatively recent periods.

A classic early case is that of Edgar Rice BurroughsBarsoom.  In A Princess of Mars (1912), Earthman John Carter is transported by obscure means to Mars, called by its inhabitants “Barsoom.”  Those inhabitants include the nonhuman “Green Martians,” but also people identical to humans in several colors, particularly the “Red Martians” among whom Carter finds his lady-love, Dejah Thoris.  As a Red Martian, Dejah is human enough for Carter to mate with, and they have a son, Carthoris, thus meeting the “interfertile” criterion.

Lynn Collins as Dejah Thoris in John Carter of Mars

Lynn Collins as Dejah Thoris

To be sure, the biology here is a little mysterious.  Dejah looks entirely human, and even, to borrow a Heinlein phrase, “adequately mammalian” (see, for example, Lynn Collins’ portrayal in the loosely adapted movie John Carter (2012)).  But Martians don’t bear their young as Earth-humans do; they lay eggs, which then develop for ten years before hatching.  It’s not easy to imagine the genetics that could produce viable offspring from an individual whose genes direct live birth and one whose genes result in egg-laying.  But that didn’t stop Burroughs.

E.E. Smith, whose initial SF writing goes back just about as far as that of Burroughs, was willing to accept this trope as well.  In The Skylark of Space (published 1928, but written between 1915 and 1921), our intrepid heroes travel to a planet inhabited by two nations of essentially human people—although the double wedding in the story does not involve any interplanetary romances, but is between two pairs of characters from Earth.  Smith’s later Lensman series (1948-1954), which features one of the most diverse arrays of intelligent creatures in SF, also allows for apparently interfertile humans from a variety of planets.  My impression is that this sort of duplication was also true of some of the nonhuman species in the Lensman unverse—there might be, say, Velantian-types native to planets other than Velantia.

This approach wasn’t universal in old-time SF.  The more scientifically-minded John W. Campbell’s extraterrestrial character Torlos in Islands of Space (1930) was generally humanoid in form, but quite different in makeup:  his iron bones, for instance.  It’s been argued that a roughly humanoid form has some advantages for an intelligent species, and hence that we might find vaguely humanoid aliens on different planets—though this is pure speculation.  But “humanoid” is a far cry from biologically human.

Darkover Landfall coverWe see some persistence of this tradition into the second half of the twentieth century.   Marion Zimmer Bradley’s iconic planet Darkover, for instance (first novel published 1958), is populated by the descendants of Terran humans from a colony ship and also by the elf-like indigenous Chieri, who, despite minor differences like six fingers and golden eyes, not to mention the ability to change sex at will, have interbred with the Terran immigrants.

An interesting variation can be seen in Julian May’s Saga of Pliocene Exile (first story published in 1981).  When modern humans are sent on a one-way trip into the distant past, they are enslaved by the Tanu, aliens from another galaxy who have settled on Earth.  The story indicates that the Tanu were specifically searching for a place where the local gene pool was similar to theirs—which might also account for why they came all the way from another galaxy (also a somewhat antique trope) to get here.

It’s slightly odd that, even where basically identical human beings turn up on other planets, other animals never seem to be similarly duplicated.  On Burroughs’ Barsoom, one doesn’t ride horses, but thoats; is menaced not by tigers, but by banths; and keeps a calot, not a dog, as a pet.  In a planetary romance or science fantasy setting, one is less likely to see Terran-equivalent fauna than parallel creatures with exotic names and slight differences—whence the SF-writing gaffe “Call a Rabbit a Smeerp” (see TV Tropes and the Turkey City Lexicon).

At the Movies

The all-too-human trope is carried on into the present day in video media—movies and TV.  Again, this may be partly because the science is often subordinated to the plot; but the cost and difficulty of putting convincing nonhuman characters on-screen is surely another factor.  Filmmakers’ ability to depict exotic creatures, however, has changed immensely in the last forty years, to a point where almost any imaginable creature can be created if the budget is sufficient.  Thus, the original Star Trek series of the 1960s stuck largely to slightly disguised humanoid aliens, perhaps relying on the ‘universal humanoid’ hypothesis mentioned above, while later series were able to branch out a bit.  Similarly, the Star Wars movies could readily give us nonhuman characters like Jabba the Hutt, Chewbacca, and C3PO; they, too, grew in variety as the capabilities of CGI and other techniques expanded.

Jupiter Ascending movie posterStill, it may be harder for us to adjust to interactions among characters where we can see their nonhumanity, rather than just reading about it.  So we still tend to see extraterrestrial humans on-screen.  The Kree in Captain Marvel (2019), for example, are indistinguishable from humans—an actual plot point, since this makes it possible for Yon-Rogg to tell Carol that she’s an enhanced Kree rather than a kidnapped human.  The Kree do have blue blood, in the movie; it’s not clear what kind of biological difference (hemocyanin?) might result in that feature.  We also see a number of alien humans in Jupiter Ascending (2015), though I think of that tale as a deliberate throwback to pulpish science fantasy or planetary romance.

A Match Made in Space, fictional coverI keep wanting to cite the fictional novel written by George McFly as shown in the closing scenes of Back to the Future, “A Match Made in Space,” since the cover seems to suggest an interplanetary romance (and one thinks of George as a nerdy romantic); but it isn’t actually clear whether that’s the case.  All we have to go on is the title and the cover, and that could just as easily depict a match between two humans, fostered by an alien matchmaker (or vice versa).

The Modern Era

We don’t see nearly as many extraterrestrial humans in modern SF, and for good reason.

The more we understand about genetics, the less likely it seems that another human species, so closely similar as to be interfertile, could evolve independently.  What we know about evolution suggests that there are just too many random chances along the way—cases where the prevailing mutations might have turned out differently.  Even if we assume that humanoid form is probable, why not have six fingers, or hemocyanin rather than hemoglobin?  While I’m not well enough educated in biology to venture any actual probabilities, I think our growing sense of the complexity of the human body and its workings, over the last seventy years or so, has simply made it seem vanishingly unlikely that an independently evolved intelligence would come out that close to the human genotype.

For example, the scientifically-minded Arthur C. Clarke depicted a galaxy in which each intelligent species, including humans, was unique:  The City and the Stars (1956, developed from an earlier story published in 1948).  In one of the unused story fragments he wrote while working on 2001:  A Space Odyssey (1968), his hero, well along on his journey into mystery, thinks:

He did not hesitate to call them people, though by the standards of Earth they would have seemed incredibly alien.  But already, his standards were not those of Earth; he had seen too much, and realized by now that only a few times in the whole history of the Universe could the fall of the genetic dice have produced a duplicate of Man.  The suspicion was rapidly growing in his mind—or had something put it there?—that he had been sent to this place because these creatures were as close an approximation as could readily be found to Homo sapiens, both in appearance and in culture.  (Clarke, The Lost Worlds of 2001, ch. 39, p. 220)

Contemporary SF writers who are really adept at building interesting and coherent aliens—David Brin and Becky Chambers, to name two of the best—give us a wide range of wildly exotic creatures from other planets, but not humans.

The Uplift War, coverIf we are still fond of the idea of interplanetary romance, we might find a possible work-around in the shapeshifter.  The Tymbrimi female Athaclena in Brin’s The Uplift War (1987) uses her species’ unusual abilities to adjust her appearance closer to that of a human female—but of course she has an entirely different genetic heritage, as that ability itself demonstrates.  The result wouldn’t meet our criterion of interfertility, no matter how close the similarity in physical structure.  To adjust one’s genes in the same way would be another order of change altogether.

Starman movie posterThe 1984 movie Starman, in a way, plays off this idea.  The alien in this case is apparently an entity made of pure energy, without a physical structure of its own.  Using hair from the female lead’s deceased husband, it creates a new body with a human genetic structure.  The two do, eventually, prove to be interfertile.  If we’re willing to accept the notion of an energy being in the first place, this approach is actually more plausible than, say, mating with the oviparous Dejah Thoris.

If one were writing a SF story today, it would be rash to assume that Earthborn characters could run across independently evolved humans elsewhere.  The idea may not be entirely inconceivable.  But it’s out of fashion for good reasons.  Attractive as the notion of interplanetary romance may be, at this point we’d best confine it to the kind of case noted above, where some common ancestry—no matter how far-fetched—can account for the common humanity.

Changing the Past – Or Avenging It


Avengers Endgame posterI set out to do an analytical essay on Three Theories of Time Travel—until I realized that Larry Niven’s astute and entertaining brief article “The Theory and Practice of Time Travel” (1971) had already covered those theories pretty well.  (You can find that article in Niven’s All the Myriad Ways, and a couple other locations.)  So I decided instead to comment on how they’re used in Avengers:  Endgame, which seems to invoke at least two and possibly three different theories.

Maybe I’d have been better off sticking with the original plan; this post has turned out to be considerably longer than I’d planned.

Endgame came out on April 26, 2019, and was released on disc August 13, so it’s still new enough at this writing that I should issue a

Spoiler Alert!

I’m not going to address the mechanics of how one might travel into the past—whether via Tipler machines, or wormholes, or simply thinking oneself into the past à la Jack Finney.  (Endgame manages it via what the movies refer to as the “Quantum Realm,” which is completely incoherent in one way but rather fascinating in another—a side issue I won’t go into here.)  I’m interested in what happens if you let causality turn back on itself.  I can think of three main ways of handling the question of changing the past.  Each has its pros and cons, from a storytelling point of view.

“Make It Didn’t Happen”

First, let’s suppose we can change the past (and, by extension, the present and future).  The idea arises because we often wish we could go back and undo something—either our own actions, or the broader course of history.  Niven observes, “When a child prays, ‘Please, God, make it didn’t happen,’ he is inventing time travel in its essence.”  He goes on to note, “The prime purpose of time travel is to change the past; and the prime danger is that the Traveler might change the past.”  These twin aspects of the idea generate plot tensions and conflicts immediately, on both a personal and a historical scale, so it’s not surprising they’re so popular.

Back to the Future posterThe most familiar example, of course, is Back to the Future (1985-1990).  In the three movies, Zemeckis played several variations on the idea of making history come out differently.  The cultural reference is so well-known that Marvel was able to riff off it for a comic moment in Endgame.  Scott Lang, the young and relatively naïve Ant-Man, says they’ll be okay if they obey the ‘rules of time travel’ (at about 0:35).  Tony Stark, the all-round genius of the Marvel movies, derides Scott for having gotten his “rules” from BTTF, and proceeds to shoot the notion down as hopelessly unscientific.

And Tony’s right, in the sense that building a theory of time travel purely on the assumptions made in fictional stories is silly.  We don’t know what would happen if it were possible to change the past; we haven’t done it.  That would make time travel really dangerous if it could be attempted in real life.  On the other hand, that same lack of knowledge leaves a wide field open for the fiction writer.  We can make whatever assumptions we like, as long as they’re consistent.  We can imagine that you can only go back in time a certain distance, at a certain geographical location, as in Julian May’s Saga of Pliocene Exile (1981-84).  We can imagine that the transition requires vast energies, as in Arthur C. Clarke’s story “Technical Error” (1950).  Or we can invoke the imaginary “Pym particles” of Ant-Man lore and time-travel at will.

This first theory of time travel generates the paradoxes we know and love.  We have the “grandfather paradox,” in which an effect removes its own cause.  (I go back in time and kill my grandfather.)  We have what Wikipedia calls the “ontological paradox,” in which an effect becomes its own cause.  (I go back but my grandfather fails to show up, so I marry my grandmother instead and name my son after my dad…)  I talked about these a bit in a 2016 post on the TV series Timeless.

One thing that’s not always obvious is that the idea of changing the past requires a second time dimension.  There’s the familiar one that’s typically represented by a “timeline,” a one-dimensional line ordering events from past to future.  But if someone changes the past, then the old line has to be replaced by a new one:  imagine a second timeline lying next to the first.  Every time a change is made, another timeline gets added.  The set of lines forms a plane, extending through a second dimension, in which each new timeline happens after (in some Pickwickian sense) the last.  Otherwise, it wouldn’t make any sense to say that we’d changed history.  Marty can’t rejoice in having “fixed” his family unless the new timeline succeeds the first, just as events along the timeline succeed each other.  Hence, a second time dimension, to accommodate the sequence of timelines.  (This may, or may not, be related to what TV Tropes calls “San Dimas Time,” a reference from Bill and Ted’s Excellent Adventure (1989).

As a narrative device, the chance to change the past creates suspense.  But it only works if you don’t look too closely.  The author has to stage-manage things carefully so that changes of all sorts don’t start happening in all directions, and this means that time travel must be rare.  If we imagine a period of hundreds or thousands of years, during which people invent time machines every so often and start changing the past, it would become impossible to make sense of what was happening.  Different changes, each with their rippling “butterfly effects,” would take place, one after another—or even at the same, er, time.  (I tried playing around with that idea in an as-yet-unpublished story called Getting to Gettysburg.)  So I’m skeptical about stories based on letting time travel become routine, as in “Time Patrol” scenarios or Asimov’s The End of Eternity.

Avengers Disassemble

Does Endgame, after all Tony’s disclaimers, involve changing the past?  Maybe not; but it’s hard to see how the story can avoid it.

Thanos with Infinity GauntletThe screenwriters chose to set themselves an interesting dilemma that makes the simple time-travel solution (go back and kill Thanos) unusable.  When the time-travel possibility arises, five years have passed since the Snap, in which Thanos killed off half the people in the universe.  Life has gone on.  Tony and Pepper, for example, have an adorable little girl.  But eliminating the Snap would also eliminate Tony’s little daughter Morgan, along with everything else that’s happened since.  That’s unacceptable (at least to Tony).  So the Avengers are not trying to avert the Snap; instead, they want to bring back, in the present time, all those who disintegrated.

The reason they have to go into the past is to retrieve the six Infinity Stones, which Thanos destroyed after the Snap.  The Avengers will need to use the Stones for a Snap of their own to bring back all the people Thanos destroyed.  But in order to avoid changing the past, they will have to put the Stones back in their earlier times after they’ve been used.  This is a clever idea, but it’s going to be really tricky to execute in practice, as we’ll discuss below.

It’s Already Happened

Meanwhile, the business of a second time dimension may make us start to wonder about the whole idea of changing the past.  Maybe we’ve forgotten to take into account the integrity of the original time dimension.  After all, if something happened in the past, it has already happened.  The effects of past events should be baked into the present that follows from them.  If I go back to 1800 and leave a hidden time capsule, let’s say, I should be able to dig it up in 2019.  You might say that the change I wish to make has already taken place.

Kate and Leopold posterBut it follows that if I can find the evidence in the present, then I know the event occurred in the past.  (That’s what “evidence” means.)  If I find the time capsule, I know that it was buried.  This may allow me to predict or “retrodict” my future changes to the past on the basis of what’s known now. If I find the time capsule, I know I’m going to bury it—or someone else will.  A key scene in Kate and Leopold (2001) relies on just such a discovery about a future event that changes the past.  (Have we mixed up the tenses enough yet?)  Bill and Ted makes even more comically inventive use of this aspect.

But on this theory, the event in the past isn’t really a change.  It was always that way.  The time capsule persisted through all the intervening time.  You can’t change the past, because your change is already included in the past we know and thus embedded in the present.  As Niven puts it, “any attempt on the part of a time traveler to change the past has already been made, and is a part of the past.”

This approach deprives us of the fun of changing history, but I rather like it.  It ensures the timeline remains consistent with itself.  In fact, one version of this postulate is referred to as the “Novikov self-consistency principle,” named for Russian physicist Igor Dmitriyevich Novikov.  We avoid grandfather paradoxes:  we already know I didn’t succeed in traveling into the past and killing my grandfather, because here I am.  If I try, something will go wrong.  On the other hand, ontological paradoxes are still allowed, as in Heinlein’s classic novella By His Bootstraps (1941).  In fact, I tend to think of this as ‘Heinlein’s theory of time travel,’ because he used it extensively—not only in Bootstraps and the even more baffling  “—All You Zombies—” (1959), but also in the delightful The Door Into Summer (1957).  Of course, Heinlein’s by no means the only writer using a Novikov-type theory.

One reason I like this type of time travel story is that everything fits neatly together, like a puzzle.  The fun of the story is in seeing how they’ll fit.  In that sense, the enjoyment of you-already-changed-the-past stories resembles that of the Greek tragedies, in which an oracular pronouncement tells what’s going to happen, and the story shows how it happens.  No matter how Oedipus tries to avoid the awful future foretold, he can’t.  The efforts to avoid the predicted outcome may themselves produce it.

In such a tragedy, where time travel isn’t involved (except to the extent the oracle itself is future information acting on the past), the Greek tragedy tends to suggest that the outcome is determined by some kind of Fate, whether we like it or not.  (Niven puts this view under the heading of “determinism.”)  But the Novikov-type theory can also be seen as compatible with free will.  Even actions freely taken, once they are complete, become part of the fabric of history, not subject to further change afterwards—except to the extent that backward causation via time travel is possible, which alters the whole meaning of “afterwards.”

The Door Into Summer, coverA subclass of these stories assumes that the time continuum somehow defends itself against change.  It may automatically “self-heal” to swallow up minor changes, or all changes:  Edison doesn’t invent the light bulb, but someone else does.  Or the time stream may simply be designed so that with “fail-safes” that prevent catastrophic causality failures.  At the end of The Door Into Summer, the engineer hero seems to be speculating in this direction:  if time travel could be used commercially, he thinks,

it will be because the Builder designed the universe that way.  He gave us eyes, two hands, a brain; anything we do with them can’t be a paradox.  He doesn’t need busybodies to “enforce” His laws; they enforce themselves.  (p. 158)

To Say Nothing of the Dog coverIn a modern context, God seems to take over the role of Fate—not by predetermining everything, but by designing the system (i.e., the universe) so nothing can go fatally wrong with causality.  Something similar, I think, lies behind the way the time travel “net” portal functions in Connie Willis’s time travel stories.  If allowing something through the net would create a paradox, the net simply won’t open—which leads to some tortuous reasoning by the characters as to what is keeping the net from openingaat  a particular moment.  Something like Providence seems to be at work.  The only causal loops allowed are what we might call ‘virtuous loops’—those that work out right.

What makes this confusing is that we’re used to analyzing causality by looking at the conditions preceding the effect.  Here, we don’t see the ‘virtuous loop’ conditions being set at any particular point in time.  The conditions have to apply to the continuum as a whole—from outside it, in effect.

You Can’t Avenge the Future

When Tony initially declares Scott’s proposed “time heist” impossible, the remaining Avengers bring in Bruce Banner as a substitute scientific resource.  Banner (who now combines his own brain with the Hulk’s body) does make a nod to the fact that his scientific expertise is primarily in biology, not physics, but the story remains basically true to the comic-book idea that a scientific genius is a genius in every science.  At about 0:59, Banner says something that sounds rather like the Novikov principle we’ve been discussing:  if you kill someone in the past, that doesn’t erase their later selves.  Apparently causality doesn’t propagate down the world lines of already-existing characters to wipe them out when their original causes go away.  On this theory, Marty wouldn’t have had to worry about disappearing even if he couldn’t get his parents back together.

On the other hand, Bruce doesn’t seem to be saying you can’t kill the person in the past; he seems to be saying that if you did kill them, it wouldn’t make any difference.  This may have more to do with what TV Tropes calls “ontological inertia” (see here, but also here).  Bruce’s approach seems to allow for wild inconsistency in the timeline, because I can be alive in 2019 even after being killed in 1971.

The simplest answer may be to conclude that Bruce wasn’t a very good physicist; maybe Tony silently corrected Bruce’s theory when Tony finally did agree to join the party.

Branching Timelines

At some point in SF history, people realized that the whole paradox thing could be avoided by introducing a third theory, the notion of multiple branching timelines.  Niven’s phrase is “multiple time tracks.”  If you change the past, the original future going forward from that point remains unchanged, but a new future comes into existence, branching off to take into account the change.  (The character making the change always seem to end up in the new branch, not the old.)  We can have our cake and eat it too:  one version of me devours the cake, but another, equally real, version of me prudently saves the cake for later.

The multiple-timeline approach gains some headway from the general popularity of alternate-history stories, and some plausibility from the fact that physicists take seriously the suggested “many-worlds” interpretation of quantum mechanics.  It appears to solve the problem of time paradoxes.  However, it runs very close to an assumption that would make it impossible to tell a good story at all.

Stories are about action and choice.  A mere recounting of a series of experiences that happen to someone wouldn’t be much of a story (which is one reason the ending of 2001:  A Space Odyssey is so weak).  James Michener’s introduction to the novel Hawaii (1959), which describes the geological formation of the islands, is only part of a story because it lays the groundwork for what the characters later say and do.

All the Myriad Ways coverIf every possible alternative branched off a new timeline whenever there were options, there would be no point in making a choice, because whichever choice I made, another version of me would make the opposite choice.  Niven captures the problem exactly:

. . . did you ever sweat over a decision?  Think about one that really gave you trouble, because you knew that what you did would affect you for the rest of your life.  Now imagine that for every way you could have jumped, one of you in one universe did jump that way.

Now don’t you feel silly?  Sweating over something so trivial, when you were going to take all the choices anyway.  And if you think that’s silly, consider that one of you still can’t decide . . .  (p. 117)

The title story in All the Myriad Ways explores exactly that issue—what would happen if people really started to believe that all alternatives were equally real.

But suppose we assume that every choice doesn’t spawn alternate universes—just the changes caused by time travel, by backward causality.  That doesn’t destroy all narrative in the way just described.  It just ruins the story you’re trying to tell.  The main characters move heaven and earth to get into the past and make the necessary change.  They succeed!  Whew.  Victory.  —Except that in another universe, the original one, they didn’t succeed.  Somewhere, the sad failures who are Marty McFly’s parents still languish by the TV.  That’s not a really satisfying conclusion.

Alternating Avengers

The multiple-timeline approach certainly comes up in Endgame.  What I can’t make out is whether it prevails in the end, or is averted.

Ancient One and Banner with timeline simulationAt about 1:24 in the movie, Bruce Banner is having a tense conversation with the Ancient One (Dr. Strange’s mentor) about the plan to return the stones to their original places in time.  The idea is that if he takes the Time Stone from the Ancient One at (let’s say) 1:03:12 p.m. on January 31, 2010, and eventually Steve Rogers returns it to her at 1:03:13 p.m. on January 31, 2010, there won’t be a need for a branch to form.  History continues on as it had always been.  (Steve describes his mission concisely at 2:43 in the movie:  “I know.  Clip all the branches.”)  Thus, the timeline of the movie, in which Thanos Snapped half the universe away, and five years later the assembled Avengers brought them back and did away with Thanos, remains the one-and-only timeline.  There’s a helpful description of this procedure in an article from July 2019 (which is also full of spoilers, by the way).

If we leave aside how hard it would have been to put things back exactly as they were, given the butterfly effect—not all the Stone retrievals were as simple as Bruce’s—does this work?  Did the screenwriters (Christopher Markus and Stephen McFeely) come up with a way to manage the dizzying time loops and still save the story?

I’m still not quite sure.  One glaring plot hole, as various people have pointed out, is that we have to account for Thanos himself.  In order to give us a great battle at the end (and what a battle it is!), the movie has Thanos in pre-Snap 2014 discover what’s going to happen and time-travel forward to 2019, where he’s ultimately disintegrated by the Avengers.  He never returns to 2014.  That seems to mean that the disappearance of Thanos did create a branch, since if he vanished from 2014 and never came back, the Snap would never have occurred.

At least that reduces us to two timelines, the one we see in the movie and another where Thanos does not continue to exist after 2014.  And, interestingly enough, the Avengers’ actions saved both of those timelines from the Snap.  The people who lived through the movie timeline experienced the Snap, but the lost people were eventually returned.  Meanwhile, in the new alternate timeline, Thanos never came back, he never got the Infinity Stones, and the Snap never occurred.  That’s not such a bad (dual) ending.

I don’t know.  All these causal loops produce a kind of shell game in which I’m not quite sure how things came out.  Nonetheless, it’s a great movie, if you like the Marvel characters at all.  If you haven’t seen it, you shouldn’t have been reading this (but maybe the circuitous account above will be helpful).  If you have—see it again!  Just don’t try to go back to April to catch the premiere a second time; who knows what that would do to the space-time continuum.

The Large Spaceship Competition


We space opera fans like big things:  sweeping plots and intrigues, larger-than-life characters, huge explosions.  And big spaceships.  The bigger, the better.

You remember the opening of the original Star Wars.  At the top of the frame, the Imperial Star Destroyer looms into view.  And keeps coming, for what seems like forever.  With this visual cue, Lucas communicates immediately the sheer scale of his story.

In a long story, this can lead to a Lensman Arms Race in terms of spaceship size—each new construct eclipsing the last.  One might wonder:  How far can we go with this?  Who holds the record for Largest Spaceship of All?

Traditional spacecraft

Saturn V-Apollo on transporterOrdinary, garden-variety depictions of spaceships are long since out of the running—not just real vehicles like the Saturn V-Apollo, but fictional ones that are meant to be big.  Kate Wilhelm’s title “The Mile-Long Spaceship” (1957) doesn’t sound even slightly impressive any more.  According to Wikipedia, those Star Destroyers range up to 2,915 meters, almost two miles.  The famous size comparison chart by Dirk Loechel displays these, along with the Independence Day mothership, the Dune Spacing Guild Heighliner, the Borg cube, and many more—but Loechel set his cutoff for the image, arbitrarily, at 24,000 meters:  “I had to draw the line somewhere.”

That’s just getting started.

“That’s no moon”

The alien probe in Clarke’s Rendezvous with Rama, which I’ve mentioned a couple of times before, is 50 kilometers long and 16 wide (31 by 10 miles).  That’s bigger than a lot of Earthly cities.

Death StarStar Wars’ Death Star, famously misidentified as a moon, has been estimated at between 100 and 160 kilometers in diameter (62 to 100 miles), at least three times larger.  The second Death Star, from Return of the Jedi, spanned somewhere between 200 and 400 kilometers (up to 250 miles), according to the same Wikipedia article.

Skylark of Valeron coverFor a long time I idolized E.E. Smith’s Skylark of Valeron as the epitome of spaceship scale.  The ship (not the novel of the same name), “almost of planetary dimensions,” is a sphere one thousand kilometers in diameter—about 621 miles (ch. 20).  That’s over twice the size of the large economy-size Death Star.  (A blogger going under the title Omnivorenz has done a detailed analysis of the various Skylark series spaceships.)

However, we’re still in the moon category, not yet up to planet-sized.  (Of course, if astronomers keep changing the rules on what counts as a planet, the two size ranges can overlap.)

Planets in motion

At this stage, we need to think about what counts as a “spaceship.”  Surely one requirement is that it be able to move in a directed fashion—otherwise the International Space Station would count.  But does it have to be a construct, a made thing?  Or can you take an existing mass and equip it with motive power?

Quite a few science fiction writers have depicted mobile planets (not simply in the sense of moving in an orbit, but of guided, directed motion).  In Blish’s Cities in Flight series, the characters twice use spindizzy drives to send whole worlds careening through the heavens.  E.E. Smith’s Lensmen, after a point, deploy “dirigible” planets almost routinely—not to mention antimatter objects (“negaspheres”) of comparable mass.  In The Wanderer, Fritz Leiber has not one, but two, traveling planets approach the Earth.  (The original source of the word “planet” is a Greek word meaning “wanderer.”)

There’s a series of stories by Robert Reed about something called “the Greatship,” but I haven’t managed to read them yet.  The blurbs describe the Greatship as “larger than worlds” and “apparently built from a re-engineered gas giant” – which would put us into a distinctly different size class from even an Earth-type planet.

And beyond

Here’s a question:  Does a “spaceship” have to be a single contiguous object?  Or can it be an array of objects that travel together, but are not physically connected?

In Larry Niven’s Known Space universe, the species known as Puppeteers flees a coming galactic disaster, not by leaving their homeworld, but by taking their planet with them—along with four others, arranged in a “Klemperer rosette” around their common center of gravity.  That’s traveling in style!  This “Fleet of Worlds” moves slower than light, but not much—and it’s five times larger, altogether, than a Blish or Smith flying planet.

The Cometeers coverBut Niven was writing hard science fiction, with some attempt at scientific plausibility.  The great space operas of earlier eras weren’t constrained by such trivial considerations.  For a long time, my list of giant spacecraft was topped by the so-called “comet” piloted by the adversaries in Jack Williamson’s 1936 classic The Cometeers.  It’s not a comet at all, but a green force-field shell enclosing an entire system of worlds:  the heroes count 143 planets, plus an artificial sun (ch. 13).  The eponymous alien Cometeers sail this congeries of worlds about the universe like space pirates on a grand scale.  It’s the biggest guided space-traveling object I’m run across that’s described with any precision.

The real prize, however, belongs to Arthur C. Clarke.  In the unthinkably far future of The City and the Stars (1956), a reconstruction of history shows the leading intelligences of a galactic civilization leaving the Milky Way on a voyage of exploration in what is apparently an entire star cluster, set into motion by energies on a galactic scale:

They had assembled a fleet before which imagination quailed.  Its flagships were suns, its smallest vessels, planets.  An entire globular cluster, with all its solar systems and all their teeming worlds, was about to be launched across infinity.

The long line of fire smashed through the heart of the Universe [i.e., the Galaxy], leaping from star to star.  In a moment of time a thousand suns had died, feeding their energies to the monstrous shape that had torn along the axis of the Galaxy, and was now receding into the abyss. . . .  (ch. 24)

That’s about as far as my imagination will go, to be sure.

The pursuit of wonder

TV Tropes collects a number of these references, and more, under the heading Planet Spaceship.

As we contemplate these logarithmic jumps in scale, we may note that after a point, the whole exercise can become meaningless.  If your “ship” includes whole planets and solar systems, why go anywhere?  It’s rather mysterious why Williamson’s Cometeers would need to roam the universe preying on other star systems (other than to provide a challenge for our dauntless heroes).  What are they getting that they haven’t already got ‘on board’?

The one likely answer may be, to see what’s out there (assuming you need this large a vessel to take you).  This is the primary reason Clarke’s far-future intelligences set out on their journey:

All we know is that the Empire made contact with—something—very strange and very great, far away around the curve of the Cosmos, at the other extremity of space itself.  What it was we can only guess, but its call must have been of immense urgency, and immense promise.  (ch. 24)

In a way, the exploration motive—“new life and new civilizations”—brings us back to where we started this exercise in scope.  Simply contemplating larger and larger objects brings us some degree of awe—a touch of that “sense of wonder” for which science fiction and fantasy are famous.  But a much greater wonder springs from the idea of meeting wholly new experiences—“very strange and very great.”

The grandest justification for leaving our world may be sheer discovery—even if we end up, like the Puppeteers, taking our world with us when we go.


One of the specialties of science fiction—and to some extent fantasy—is to evoke a sense of strangeness.  In dealing with the alien, the cosmic, that which is far away in space or time, SF can make us feel we are encountering something that passes the limits of our knowledge or understanding.

This isn’t as easy as it looks.

The Used and the Unusual

Since at least the original Star Wars (1977), it’s been good practice to portray a “Used Future.”  Star Wars gave us a world full of beaten-up, grimy equipment that looked as if it had been duct-taped together.  This is generally a good technique.  It adds realism.  We feel at home in a world where everything is not perfectly cleaned and aligned; it’s like where we actually live.  There’s a sense of familiarity.

One opposite to the “used future,” of course, is the kind of earlier SF movie that was full of shiny, spotless spaceships and immaculate gizmos.  But the sense of familiarity also has its own opposite:  the thrill of unfamiliarity.

One way the challenge arises is with extraterrestrials.  Suppose a story has us meeting intelligent aliens.  If they seem just like us—“rubber-forehead aliens”—they won’t be convincing.  We expect something from another world to be different.  The writer or director has to show creatures, technologies, behaviors that are unlike anything we’ve seen on Earth.

Escher: Wallpaper CaveYet these things must also be believable.  Something that simply looks random or arbitrary, like an abstract swirl of colors, won’t convince us we’re seeing a real thing at all.  How do we thread the needle between the too-familiar and the unintelligible?

Just Alien Enough

Natural laws do enforce certain constraints on physical objects.  But other characteristics are a matter of custom, design choices, or aesthetics.  To show something convincingly alien, we need to know the difference.

Alien ship from movie ArrivalSometimes a single feature can be odd enough to alert us that we’re “not in Kansas any more.”  The alien ship that appears in the movie Arrival looks strange at once, because it’s smaller at the bottom than at the top.  It looks as if it’s upside-down or sideways. Not the way we’d build, yes.  But is it physically impossible?  Nope.  The ship isn’t on the ground, balanced implausibly on a narrow end.  It’s floating in the air.  This not only frees the ship from the usual need for wheels or other supports; it also introduces a second, subtler strangeness.  When we humans land somewhere, we expect to land, to set ourselves down securely on a surface.  These folks seem quite comfortable floating just above the ground.

A classic example is Arthur C. Clarke’s Rendezvous with Rama.  A massive spacecraft—a spinning O’Neill cylinder—enters the solar system, apparently inert.  A human crew matches course to explore it before its hyperbolic orbit takes it out into interstellar space again.  The ship begins to “come alive” around them—but there’s no sign of intelligent life aboard.  The explorers find one strange and amazing feature after another.  The purpose of some becomes clear:  the long, shallow rectangular valleys turn out to be immense lights that illuminate the interior.  But they never find out the reasons for many other objects.  In the end they have to cut loose from the vessel, letting it go on its mysterious way.

Rendezvous with Rama interior illustrationClarke’s mastery of clear detail—how the airlock doors open, for instance—gives us the necessary sense of realism.  But leaving many things mysterious evokes the sense of mystery and wonder that is among the most distinctive experiences in science fiction.  The unfamiliar is clearly and concretely depicted, but the purpose remains obscure.

(Parenthetically, I advise paying no attention at all to the dreadful sequels Gentry Lee wrote to Rama under Clarke’s direction.  They make the classic mistake of erasing the mystery without replacing it with anything at all interesting.  As with certain other sequels, the only thing for a conscientious reader to do is declare them non-canonical and pretend they never happened.)

For another Clarke treatment, remember 2001:  A Space Odyssey.  The mundane and even humdrum character of the long space voyage makes the psychedelic sequence at the end feel even weirder than it is in itself.

Sufficiently Advanced Technology

Extraterrestrials need not be involved.  Distance in time or space, and the concurrent advances in technology, can also provide a good foundation for the sense of strangeness.  (It was, after all, Clarke’s Third Law that “any sufficiently advanced technology is indistinguishable from magic.”)

Among the numerous virtues of David Brin’s Hugo-winning novel Startide Rising is that sense of entering a new and unaccountable world.  His Earthly spaceship crew of “uplifted” dolphins, with their small group of human companions, use advanced techniques that are still recognizable to us.  But they’re dealing with galactic cultures that draw on hundreds of millions of years of accumulated science.  The results can be mind-boggling.  One species, for example, travels by using a captive creature that creates portals “by the adamant power of its ego—by its refusal to concede anything at all to Reality.”  This isn’t your grandmother’s hyperdrive.

Toy stack of ringsThe body of another species, the Jophur, consists of a stack of distinct rings, like a child’s toy.  The Brothers of the Ebony Shadows employ a probability weapon that sends out “waves of uncertainty.”  The fact that these species are nonhuman is incidental to the fact that their immense background of far-advanced science lets them use techniques that seem to surpass our understanding.

For a purely human example, let’s look at Arthur C. Clarke’s The City and the Stars.  (Clarke really had the knack for this sort of thing.)  The main character, who bears the pedestrian name of Alvin, lives in Diaspar, the last city on Earth, billions of years in our future.  The city’s structure does not erode or decay; it’s maintained by “eternity circuits” according to the model held in its master computers.  The people do not die in a conventional sense.  After living for a thousand years, each individual walks back into the Hall of Creation and is dissolved—but is also retained in the memory circuits, to be rematerialized eons later.  Thus the population of the city is always changing, but the individuals continue.  And that’s only the beginning . . .

The City and the Stars, illustration

Exotic Ways of Life

Technology is one thing; behavior is another.  The City and the Stars does a terrific job of imagining how the society of Diaspar is shaped by the extraordinary conditions under which its people live.

When I read Yoon Ha Lee’s Ninefox Gambit, it was billed as ‘military science fiction’—but it’s nothing like the general run of military SF.  The six factions in the story make use of technologies that create real-world effects based on “formations” of people and their consensus beliefs.  Much of the plot revolves around a revolt based on “calendrical heresy”—which is just what it sounds like:  deviation from the standard calendars.  In Lee’s world, calendrical uniformity isn’t just a matter of convenience, but of crucial importance.  The resulting society is correspondingly peculiar.  Reading the story makes you feel as if you’re constantly being knocked sideways.

Greg Bear’s City at the End of Time combines present-day characters with those living in a city one hundred trillion years in the future.  The far-future people consist of “noötic” or virtual mass, are defended by “reality generators,” and are trying to fight a cosmic entity that’s trying to destroy the universe by disintegrating its history, acting backward through time.  The present-day people in mundane Seattle keep us grounded, but trying to understand the end-of-time characters and what they are doing requires a constant stretching of the imagination.

Strangeness and Wonder

The sense of strangeness or mystery is one form of the “sense of wonder” often used to characterize science fiction.  It takes us out of the mundane, makes us strain to conceive the inconceivable.  We’re often told that world travel expands our horizons by exposing us to different places and cultures.  Science fiction goes further:  it exposes us to ideas and places and people that don’t exist in the world at all.  At its limits, SF seeks to show us more than we can even comprehend.  The lack of reality is compensated by the greater impetus to go beyond our mental limitations.

To achieve that experience, we seem to need the right combination of the familiar and the exotic.  The weird stuff at the end of 2001 isn’t entirely successful, in my view:  it’s too strange.  Not only do we not understand what’s happening; we don’t quite feel there is anything to understand.  You have to read the book to figure out what’s going on.

But when we have enough groundedness to effect the “willing suspension of disbelief,” yet enough mystery to defeat (in part) our attempt to understand, the combination is uniquely fascinating.  As I noted at the beginning, this isn’t an easy balance to strike.  But the payoff makes it worth attempting.