“Why FTL Will End the Universe—and Six Ways to Avoid It in an SF Story” by John Lambshead

Every schoolboy with any interest in space opera knows two things: the first is that it is absolutely de rigueur to have your heroes buzz around in FTL ships of some sort, and the second is that it takes an infinite amount of energy to accelerate a mass to light speed because said object increases in mass with additional velocity until that mass itself becomes infinite.

This has led space opera authors to devise all sorts of clever side-steps to travel faster than light without accelerating mass to light speed. An early solution was E.E. “Doc” Smith’s inertialess drives, an idea later reused by well-known writers such as Robert A. Heinlein, Larry Niven, and Alastair Reynolds as well as in Rick Priestley’s 40K miniature game. It’s an idea not without issues. A photon may have a theoretical mass of zero at rest but it surely has momentum when it gets underway—all of which means that an inertialess space ship would bounce back like a super-ball if it collided with a photon. As there are quite a few photons knocking around in space, a trip in an inertialess starship would be a bare-knuckle ride to beat everything else in the cosmic fairground.

Other science fiction universes, such that found in the Star Wars movies, have a gizmo called a “hyperdrive,” or something similar, and leave it at that. Press the big red button and off you go. A variant is the hyper-jump where the spacecraft instantly jumps from point A to point B without actually moving. Battlestar Galactica employs something along these lines. Arguably the cleverest, and certainly the most amusing, hyper jump variant is the Infinite Improbability Drive in Douglas Adam’s The Hitchhiker’s Guide to the Galaxy.

Star Trek used warp drives, where a ship compresses space in front and expands it behind the vessel. Not too far from this concept are starships which “bend” space to “reduce” the distance between two points. People can even do it with their minds by “tesseracting” in Madeleine L’Engle’s A Wrinkle in Time. And then there are our old friends the worm holes, whether natural or synthetic. Down the rabbit hole you go, and where you end up is . . . a long way away. A variant on this are star gates as used as plot devices as in, well, the Stargate series and movies. Arthur C. Clarke probably invented the term “star gate” but A. E. van Vogt employed a similar plot device, the cyclopex, in the early fifties.

The last major group of FTL travel methods involve stepping outside the universe into somewhere else where the laws of physics are subtly different. David Drake’s RCN series books provide a highly imaginative example. In his stories normal Newtonian spaceships drop out of space to sail their way through bubble universes like Napoleonic men o’ war.

However, ingenious those these ideas be, they fail to address the key problem that makes FTL impossible, in so far as we currently understand the universe: an FTL drive is also a time machine.

We experience a Newtonian universe in our daily lives. When we travel across a fixed distance our watches inflexibly tick away the seconds at one second per second. If we want to get somewhere quicker we have to move faster: the rate of time is fixed, we can’t alter it, but speed is variable. In like manner, the same seconds are ticking away at the same rate on the watch faces of the people around us, even if they are travelling in different directions. But our experience is illusory or, to be more accurate, a special case of large objects moving slowly.

In the universe described by relativity, the speed of light is always constant irrespective of whether the object from which the light derives is stationary or moving. The laws of physics are constant throughout the universe, i.e. the speed of light is the same everywhere. It is time that is variable. Hence, the rate of time slows down as velocity climbs towards light speed.

One other key difference between the relativistic universe and our common perception of it has serious implications. We tend to see the Earth around us as stationary and to judge velocity according to that frame of reference. If I ride a bike at ten miles an hour relative to the Earth and throw a ball in front of me at ten miles an hour, the ball has an initial speed of twenty miles an hour, not ten. Fighter pilots trying to attack a distant target will accelerate their plane as much as possible before firing a missile to increase its range.

Fig 1. A confusion of starships.

When we start considering space opera warships, or any object, moving fast and firing laser cannons or similar at each other, then completely different rules apply. Take three warships moving in convoy equidistant apart in a line and let’s see the universe from Ship 2’s frame of reference. From its viewpoint, it is stationary and, because Ships 1 and 3 have the same velocity they are also stationary in 2’s frame of reference. Now let’s assume the ships have faster than light “sub-space” radios that broadcast a constant signal.

There is a mutiny on Ship 2, but the mutineers can’t just take off on a new heading because they will be seen by Ships 1 and 3 who, when they fail to get a satisfactory answer on the subspace radio, will open fire. Fortunately, one of the mutineers understands relativity and comes up with a cunning plan. He advises the mutineers to open fire simultaneously on Ships 1 and 3. Because Ship 2 is stationary in its frame of reference, both targets are at the same fixed range, and as the speed of light is also fixed, then both 1 and 3 will be destroyed simultaneously. Neither will be able to get off a warning on their FTL transmitters so that the other can take evasive action.

Now let’s introduce Ship 4 scudding along at velocity Y. In 4’s frame of reference the ship is stationary, so the convoy is moving at Y on a reciprocal heading. Therefore 2’s laser shot will have farther to go to hit Ship 1 than Ship 3 because 3 is moving towards the laser burst but 1 is moving away. The fact that 2 was moving when it fired is irrelevant. Both laser bursts cross space at the speed of light.

Ship 3 will therefore be destroyed first in Ship 4’s frame of reference. Cessation of its FTL transmission warns Ship 1 that something is wrong, so it takes evasive action. The laser misses and Ship 1 survives.

Now introduce another ship moving in the same direction as the convoy but faster so it is overtaking at speed X. Ship 5 is also stationary in its frame of reference so the convoy is moving backward, at speed X on a reciprocal course to Ship 5. The laser shot now hits Ship 1 before Ship 3 so Ship 3 can take evasive action and survive.

Frames of Reference

Ship 2: Ships 1 & 3 are destroyed but it can get on its FTL transmitter and talk to Ships 4 & 5.

Ship 4: Can talk to Ships 4, 2 & 1 but not 3 because it’s destroyed.

Ship 5: Can talk to Ships 2, 3 & 4 but not 1 because it’s destroyed.

Should be an interesting conversation!

Now this is a simplified and rather contrived tongue-in-cheek example and much better explanations written by physical scientists (I’m a biologist) can be found on the web.

For example:


And a more detailed explanation here:


The example employed here used FTL radios. Once you introduce FTL drives, then it is possible to come up with examples where a starship can return home from a journey before it sets off.

Just think, a crewman could meet himself before he left, and warn himself not to go because the captain will turn out to be a tyrant and the food terrible. But if he didn’t go then he wouldn’t warn himself not to travel so he would sign aboard.

The fact that everything can have a different frame of reference doesn’t matter so much if nothing moves faster than light. Once anything does, then the universe hits the buffers of: FTL = Time Travel = Potential Causality Paradox.

As Matthew Buckley points out, it’s as if the universe is saying to us that when it comes down to Relativity, Causality and FTL, pick two—because you can’t have all three.

Note that it is the act of sending information (including large objects as information) FTL that causes the problem, not the method by which the information is sent.

So could relativity be wrong? Well, I’m a biologist not a physicist so I have no idea ultimately, but the physical scientists seem struck on the idea and it has been repeatedly tested and not found wanting. Then what about causality? Is our idea of causality erroneous?

Causality is one of those primary building blocks of science (and reason). Although there can be some subtle effects when considering quantum mechanics, it is difficult to imagine a universe where events precede causes. And that’s before we get into the classic time paradoxes of shooting one’s grandfather.

In the final analysis, if we are not prepared to give up on causality, and relativity also remains as one of our scientific core theories, is there any way we can twist the universe to allow us to have FTL?

The key point here is to prevent a time paradox occurring. One suggestion is that the universe might act in some blind fashion to ensure causality always “works,” and so prevent paradox or, to put it another way, an immutable law ensures that the past can’t be changed. What has happened has happened and can’t be altered. So you can’t go back in time and shoot your grandfather. You may try to go back in time with that intention, but the time machine or gun will malfunction, or you will shoot your grandfather only to find out that your grandmother was “no better than she should be,” and you came from different stock.

Another related possibility is that the universe might allow a time paradox to occur but then realign itself to wipe the paradox from history as if it never occurred. If you shoot your grandfather you cease to exist, which may mean he ceases to exist to tidy things up, or your family ceases to exist or your nation, species, planet, solar system, right down to the universe itself, ceases to exist.


The third logical solution is that a paradox splits the universe into two, so each event can happen in some way without breaking causality.

Doctor Who has taken the first option: horrible monsters fly out of the Time Vortex and seal off the paradox, trying to destroy everything involved to repair the fabric of time (“Father’s Day” episode). Of course, Doctor Who is one of the few popular SF franchises that has tried to seriously tackle issues of time paradox.

A TARDIS is a highly complex sentient device and one can bet that not many were made. Also a “Time Lord” is not a normal Gallifreyan. They are superhuman (supergallifreyan?) in many ways, physically different having “looked into the abyss” with “non-linear perception of time.” So presumably Time Lords can be trusted to travel in space and time FTL without carelessly causing time paradoxes.

There are “fixed points” in the Doctor Who universe which can’t be altered. These are events that have happened to the Doctor in his past so they can’t be changed without risking a causality paradox. For example, he can’t bring a dead assistant back to life when he witnessed the death because “it’s already happened”—already happened in his frame of reference, that is.

But doesn’t the Doctor often change time? Well, not in his time line. He may have been the cause of that event that happened in our past all along and it occurs in his future (interesting ramifications from this last point—see below). There is no change in our frame of reference because, say, what the Doctor did in 55 A.D. was what had always happened. We just didn’t know that until the episode was broadcast.

In the early stories, when the Doctor returns to Gallifrey, which is in our past, he arrives a set time after he left. It’s as if the Time Lord/TARDIS combination can maintain its own frame of reference in synchronicity with Gallifrey—and hence all other time travelling Time Lords— irrespective of how he hops in and out of our time line. The Doctor also tends to drop assistants back on Earth further down their own time line from where they took off, thus preventing paradox. The one time The Doctor breaks this rule with Rose Tyler, she prevents her father’s death creating a causality break—Father’s Day—hence the appearance of the Time Vortex monsters acting as a sort of Causality Police.

It’s true that you can find all sorts of anomalies in Doctor Who, given that hundreds of script writers must have worked on the plot lines since 1966. Nevertheless, you have to give the series points for at least trying.

This brings me to think how space opera writers can handle the FTL = Time Travel conundrum. Here are some suggestions.

#1: Ignore the problem

No, really, you are writing fiction in a fantastic setting. Who cares about frames of reference? Not your readers. Not if you offer them a cracking good story populated with fascinating characters. For example, the giant sentient spacecraft in Iain M. Banks’ Culture universe “push against the grid” to go FTL. How this avoids time paradox is never touched upon and no one notices. It just does, okay! Banks and Drake, The Culture and RCN are my all-time favorite space opera book series, and that they never mention causality bothers me not a jot.

#2: Choose Causality and FTL

Abandon the Theory of Relativity as a flawed concept. In your universe the speed of light is not fixed, so unwanted time travel ceases to be an issue. Essentially, this is the approach that Dave Drake and I took in the Citizen series, although we never say so explicitly.

#3: Invent a “Plot Voucher”

Plot voucher is a term created by Nick Lowe for an object used by the protagonist to move the plot along. Q’s gadgets in James Bond films are classic examples. In this case the plot voucher is some way of negating time travel impacts. It could be a Time Standardisation Rectification Medium, better known as TIMSRECTUM, which controls the frame of reference so your heroes can’t land back at base before they take off. Just don’t go into the physics of your magic gubbins too deeply.

#4: Embrace the Issue

Tackle time paradoxes and causality maintenance as a component of your plot. Use the difficulties that can be created as problems for your protagonists to overcome. Make your space opera universe a strange and terrifying environment where the normally accepted rules of life hold no sway.

#5: Don’t Go FTL

It is possible to write space opera with STL (slower than light) spaceships using (i) the time-slowing property of fast moving objects, (ii) “stasis fields” or (iii) multi-generation colony ships. Personally speaking, I don’t find such stories as exciting as FTL based novels, but maybe that’s just me. Throw in immortality if you want the characters in your novel to meet up in the future after riding on different STL ships

#6: Sidestep the Issue

Have your protagonists travel through gates that come out in different universes so causality maintenance is then irrelevant. I really like Philip Farmer’s World of Tiers setting, which has many of the features of space opera of the star gate type but doesn’t involve FTL.

I am sure that there are many other solutions that I haven’t thought of.

So which way should an author jump? Which approach should be selected? Well, a good writer can make something out of anything. They can spin fantastic and wonderful stories out of the most unpromising material. By that token, any of these will work if handled right.

If I am prodded into choosing, I would favor approach number 1—ignore the problem. If it’s good enough for Drake, Weber, and Banks it might be good enough for you. Option 3 is fascinating and, with the right author and the right touch, could make for amazing stories. However, it is probably the most difficult of all the options to plot.

Don’t say I didn’t warn you.

Mind how you go, people, especially if you’re going FTL!

Copyright © 2018 John Lambshead

Dr. John Lambshead is a retired senior research scientist in marine biodiversity at the Natural History Museum, London. He was also the Visiting Chair at Southampton University, Oceanography, and Regent’s Lecturer, University of California. He writes military history and designs computer and fantasy games. Lambshead is the author of swashbuckling fantasy Lucy’s Blade, contemporary urban fantasy Wolf in Shadow, and coauthor, with nationally best-selling author David Drake, of science fiction adventures, Into the Hinterlands and Into the Maelstrom.