I’ve always loved space opera, but when I was growing up, as I learned more about science, I discovered that a lot of the tropes in space opera are problematic. Space operas, to tell adventure stories among the stars, often have to make compromises. One of the earliest and most pervasive is FTL (faster than light) travel.
Interestingly, the earliest interstellar space opera stories in the late 1920s largely ignored relativity. E.E. “Doc” Smith and Edmond Hamilton simply had their adventurers accelerate away at thousands of times the speed of light. If relativity was mentioned, it was just as a superseded or wrong theory.
But by the early 1930s, authors found a way to seemingly avoid outright ignoring Einstein by simply hand waving technologies that bypassed the laws of physics. One of the earliest and most enduring was hyperspace, a separate realm that a spaceship could enter to either travel faster than light, or where distances were compressed. Over the decades, hyperspace came in a wide variety of fashions and with a lot of different names: subspace, u-space, slipstream, etc.
One variant, popularized by Isaac Asimov in his Robot and Foundation series, has hyperspace as a realm where ships jump through it to instantly move light years away. (I’ll be using this version in an example below.)
There are a wide variety of other FTL technologies that often show up in science fiction. An interesting example is the ansible, a device that allows instant communication across interstellar distances. Often the ansible shows up in stories where actual FTL travel is impossible, but an interstellar community is enabled by the instant communications.
I’ve written before that there are lots of problems with all of these ideas. Generally they’re not based on actual science. They’re just plot gimmicks to enable the type of stories authors want to tell. And the few that are somewhat based on science, such as wormholes or Alcubierre drives, involve speculative concepts that haven’t been observed in nature.
But FTL has another issue, one that I only started appreciating a few years ago. FTL, no matter how you accomplish it, opens the door to time travel. Most FTL concepts are conceptualized within a Newtonian understanding of the universe. In that universe, there is an absolute now which exists throughout all of space. If we imagine a two dimensional diagram with space as the horizontal axis and time as the vertical, then now, or the absolute plane of simultaneity, exists as a flat line throughout the universe.
But that’s not the universe we live in. We live in a universe governed by special and general relativity (or at least one where those theories are much more predictive than Newton’s laws). In our universe, there is no single plane of simultaneity, no universal version of now. In this universe, talking about what is happening “right now” for cosmically distant locations is a meaningless exercise.
Most people are aware that, under special relativity, time flows slower for a traveler at speeds approaching the speed of light. But not everyone is aware that, from the traveler’s perspective, it’s the rest of the universe that is traveling near the speed of light and experiencing slower time. How can both see the other as having slower time than themselves? Because simultaneity is relative.
As this image animation shows (which I grabbed from the Wikipedia article on the relativity of simultaneity), under relativity, whether certain events occur simultaneously is no longer an absolute thing, but a relative one. If B is stationary, then events A, B, and C all happen simultaneously. However, if B is moving toward C, B’s plane of simultaneity slopes upward, leaving C in its past. On the other hand, if B is moving toward A, C is now in its future. (Note: this never allows information to influence the past because, in normal physics, such information can only travel at the speed of light.)
An important point here is that these effects do not only happen at speeds approaching the speed of light. They happen with any motion. However, in normal everyday life, the effect is too small to notice, which is why Newton’s laws work effectively for relatively slow speeds and short distances.
Crucially, the upward or downward slope of simultaneity still happens at slow speeds, but the angle of difference is small, and again we don’t notice. However, while a small angle of deviation may not be noticeable for everyday distances (say between New York and Sydney), or even for distances within the solar system, when the distances start expanding to thousands, millions, or even billions of light years, then even minute angle deviations grow to significant variances.
So imagine we have a spaceship heading out of the solar system at 1% of c (the speed of light). Using the Asmovian version of hyperspace, the spaceship jumps to a destination 1000 light years away.
Which plane of simultaneity, which version of now, does the ship’s instant jump happen in? The plane associated with stationary observers back on Earth? Or the plane associated with the ship traveling at 1% c? If it’s the ship’s plane, then when the ship exits hyperspace 1000 light years away, it will do so 18 days in the future of the stationary Earth observers.
That is true if the spaceship’s hyperspace jump is in the direction of its 1% c velocity. But if the 1000 light year jump is in the direction opposite the one of it’s velocity, it will arrive 18 days in the stationary observer’s past.
It doesn’t take a whole lot of imagination to see how this technology could be used to travel to arbitrary points in the past or future. All a ship would need to do is jump in circles either in the direction of their rate of travel or opposite it to travel forward or backward in time.
We encounter exactly the same issue with other versions of FTL, such as warp drives or versions of hyperspace that take time to travel through, it’s just more of gradual than sharp jump in time.
In the case of ansibles, which version of simultaneity are the communications happening over? The chances that the two correspondents happen to be traveling at the same speeds are nil. The variances in the speeds of their star’s movement around the galaxy, the orbits of the planets, etc, will all conspire to ensure that their various planes of simultaneity are out of sync with and constantly changing in relation to each other. An ansible accelerated to relativistic speeds could be used to communicate with the past or future.
Even wormholes would be an issue. The wormholes in fiction always connect distant points together in the same now, but wormholes are connections between two points in spacetime. There’s no particular reason it would be limited to some arbitrary version of now. Indeed, a natural wormhole, like the one in Star Trek Deep Space Nine, would be more likely to open to some distant point in future, long after the heat death of the universe, than somewhere along the Bajoran plane of simultaneity.
We might imagine that if the FTL technology allowed us to choose which plane of simultaneity we moved under, maybe everyone would just agree on some standard, albeit an arbitrary one. But that only makes the time travel capability more pronounced. Orson Scott Card made the point years ago that if you’re going to introduce a technology into your fictional universe, you should account for all the ways that technology might be used, or abused.
It’s often said that the absence of tourists from the future probably indicates that time travel is impossible. Even if future societies have strict taboos against interfering with the past, the idea that such taboos would hold for all societies until the end of time seems unsustainable. Since FTL is also time travel, the same observation would seem to rule out most forms of it. (Star gates or wormholes where a destination version has to be built might be the only ones that avoid this issue.)
Unless of course there’s something I’m missing about this?