For a rather hard-science based sci-fi setting, I am checking for faster-than-light (FTL) possibilities. One option that I consider is the following:

Interstellar travel is achieved through portals. They can be imagined during their construction as two giant rings initially superposed. Once finished, they are separated but have a remarkable property : they behave as a "tunnel" in space. Something watching or entering in the space circled by Ring A will see or exit through Ring B. No special effects or anything fancy : both parts of spaces are "simply" connected and behave accordingly. After that, you can move Ring B to anywhere in space, at slower-than light speed. Even if takes a few hundred years to achieve, you can end-up connecting stars or regions of space, building a web of connected portals.

This system makes the portals key elements in my setting (for obvious strategy, communication, and trade/economic advantages) and avoid some FTL. I'd like to have Worldbuilding reader's feedback on this :

  • Is this something that is commonly used in existing settings ?
  • Is this plausible ? While not really hard-science, it avoids effective FTL by playing with the space shortcut, without any hyperspace involved.

Any other remarks on this "pseudo-FTL" scenario are welcome.

For clarification : this is a 2D visual representation of the resulting space-time : space deformation So there is no fancy glowing portal here : simply a permanent connection that is created between two regions of space. This is indeed nearly identical to a wormhole, with the specific characteristic that both endpoints must start in one single point. The initial "pinch/hole" can after that be moved.

Hereunder is the initial unfortunate attempt to connect Earth and Mars, resulting in Earth's atmosphere leakage to Mars. By regulation, next gates were enforced to be located in outer space and limited gravitation/magnetic fields.

Gate to Mars

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    $\begingroup$ See also: Stargate. $\endgroup$
    – Frostfyre
    May 7, 2016 at 16:27
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    $\begingroup$ Possibly relevant: physics.stackexchange.com/questions/144427/… $\endgroup$ May 7, 2016 at 20:41
  • $\begingroup$ M-Theories postulate up to 11 dimensions in the universe, but what if there are an infinite number of dimensions, with most having an extention of exactly zero? If you could unfurl yourself a set of dimensions, you might be able to travel along them at any rate you care to, since they wouldn't necessarily be bounded by normal laws of physics. $\endgroup$ May 7, 2016 at 22:22
  • $\begingroup$ Don't worry about plausible. This is basically magic. Trying to justify it must come off as lame. Just declare that it is, and get on with the story. David Weber, in the Honor Harrington series, has such things for starships. What's interesting is that his version can only move so much mass over a certain amount of time. So if a fleet passes through, another can't use it for several days. $\endgroup$
    – Tony Ennis
    May 8, 2016 at 1:25
  • $\begingroup$ An interesting website which discusses various aspects of wormhole physics and construction is here: aleph.se/Trans/Tech/Space-Time/wormholes.html. The Orions Arm website also has interesting discussions on the nature and implications of wormholes. $\endgroup$
    – Thucydides
    May 8, 2016 at 1:36

6 Answers 6


What you're talking about is known as a wormhole, and variations on it are actually pretty common in science fiction (partial list here). The only franchise I can think of off the top of my head that uses the particular version you mentioned is the Galactic Civilizations series of games, but I'm sure that there are many others.

As far as we can tell, it's at least plausible, although there's a lot of argument about whether or not it would allow true FTL (your movement appearing faster than light to an outside observer) or apparent FTL (your movement appearing slower than light to an outside observer but instantaneous to you).

For more information you can look at wikipedia.org and projectrho.com (the latter website being an excellent resource for space travel in general).

  • $\begingroup$ Wormholes (or gates of the same kind) are indeed common, but I am more focusing on the way they are build so that they bend space definitely to "connect" regions of space. I already reviewed projectrho on the topic, but did not find similar mechanisms. $\endgroup$
    – Uriel
    May 7, 2016 at 18:54
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    $\begingroup$ @Uriel I would think string theory and general relativity would be the ideal direction to go to understand how to connect regions of space. $\endgroup$ May 8, 2016 at 1:11
  • $\begingroup$ Schlock Mercenary's pre-Big Invention society is set up around this premise. $\endgroup$ May 8, 2016 at 5:37
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    $\begingroup$ @chrylis: Could be. It's been a while since I read Schlock Mercenary, so I don't remember whether or not they towed the wormgates at STL speed before using them, which is why I didn't mention them. Mass Effect is another one I didn't mention because I have no idea whether or not the mass relays were built in place or towed. $\endgroup$
    – emo bob
    May 8, 2016 at 17:28

This setup has a very slight problem - it forms a perpetual motion machine.

Let's take two portals, A and B. Place A on the ground. Suspend B (oriented horizontally) 10 meters directly above it. Now take a mass M, place it 1 meter above A and let go. Oh yes, and let's put the whole thing a vacuum chamber, shall we?

The mass will fall into A and appear at B. Since its velocity vector is vertically downwards, it will fall into A, reappear at B, ad infinitum. Each drop from B to A will increase its kinetic energy by Mgh. In the early stages it will gain about 14 m/sec in downward velocity for each pass through the portals. A 1 kg mass will gain 98 joules of energy for each pass.

In effect, the object will experience a constant acceleration of 1 g, and you know that won't end well.

After 1 hour the object's velocity will be about 35 km/sec, far greater than escape velocity, and its energy will be about 600 MJ.

After a day, the energy of the object is about 358 GJ, which gives a velocity of about 846 km/sec.

After a month, if you don't accept relativity the velocity of the object will be 248,000,000 m/sec, which is just a bit shy of the speed of light.

There are some complications with this, most notably the rotation of the earth, which will tend to move the path of the falling body out of the portals, but this is clearly just a matter of engineering. Or, to a first approximation, you just put the portals at either the North or South Poles.

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    $\begingroup$ Sucking energy as heat from the environment still violates thermodynamics. The hot environment contains more entropy than the cold one. Either you have to push an object to make it go through to get its gravitational potential up, or wormholes take energy to make and such use would shrink them. $\endgroup$ May 7, 2016 at 20:00
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    $\begingroup$ @DonaldHobson: This is true for sucking energy out of a passive environment, but my laptop does a great job sucking energy out of the environment of the local power grid. Maybe the wormhole has solar cells on it? $\endgroup$
    – Kevin
    May 8, 2016 at 2:30
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    $\begingroup$ There are two solutions to this. First would be that the portals must share a vector in space. Thus having portals facing each other is not possible. The proposed scenario would instead have mass M fall into the Portal A, and fly up out of Portal B. Second is that you're not actually creating a perpetual motion machine. It only appears like it, because of the frame of reference. After all, the "machine" gains its energy from the Earth, and with each pass, the Earth's mass would also be effected. Moving a pebble into near light speed is less energy than moving Earth a few inches. $\endgroup$
    – TUSF
    May 8, 2016 at 5:36
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    $\begingroup$ Please give credit to Larry Niven - he wrote about this decades ago: obooksbooks.com/2015/3975_4.html (see section VIII on page 4 of the linked). $\endgroup$
    – DoxyLover
    May 8, 2016 at 5:55
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    $\begingroup$ Traversable wormholes are a real theoretical possibility in general relativity, and this theory does respect energy conservation in asymptotically flat spacetimes which include ones with wormholes. So, one should expect that conservation of energy must work out somehow in your scenario. Note pervect's post #6 on this thread which points out that when a mass exits a wormhole mouth, the mass of the wormhole mouth decreases to compensate, and can actually $\endgroup$
    – Hypnosifl
    May 8, 2016 at 13:09

Your suggested method is not an alternative to FTL travel, it is a means of FTL travel. However you do it, if you reach one part of the universe from another faster than light could make the trip, you are indulging in Faster Than Light travel, and you will encounter problems with both Causality and Relativity.

One of the best and most comprehensive explanations of FTL travel and the problems it raises with Causality and Relativity is hosted on this page of Winchell Chung's excellent Atomic Rocket site.

This page also includes a comprehensive catalogue of FTL methods used in science fiction, known as The Canonical List of StarDrives or the Landis list, compiled by author Geoffrey A. Landis. Your postulated method is included on the list as Portal-to-portal.

  • $\begingroup$ The described setting never allows to go faster than light. It creates another alternate space that is shorter (it may be quite stretched). What is specific is that you have two ways to reach another point in space that have different distance. It does not violate causality as you have to move endpoints at conventional speed. If you have an example in mind, I am very interested. $\endgroup$
    – Uriel
    May 8, 2016 at 12:51
  • $\begingroup$ Hi Uriel, I understand that the setting does not include a drive that makes ships travel through real space at faster than light speed. I was trying to say that the method proposed was one of only two categories of getting around the light speed barrier - that is making the distance shorter. This still runs into problems with physics, around the energy or matter required to open and maintain a wormhole and around causality. remember space and time are not separate, they are one entity called Space-Time, and tunnelling through one tunnels through the other. Wormholes are time machines. $\endgroup$
    – John
    May 8, 2016 at 13:03
  • $\begingroup$ Indeed, my option is to mess with distance rather than pure FTL speed. I am pretty sure such gate/hole involve exotic or impossible physics, but hopefully less apparent than blatant FTL scenarios. My question (and my knowledge) is too light for physics.stackexchange.com. I am unsure about causality breaking here, but the stretched spacetime has for sure some drawbacks, like unfortunate spaghettification when going through such gate/wormhole... $\endgroup$
    – Uriel
    May 8, 2016 at 13:18

It's been used in science fiction a number of times, though I don't know off-hand of any rpg settings that use them.

Some quick examples:

Constellation Games by Leonard Richardson has an multi-species alien civilization visit Earth via just such a wormhole. They send a probe carrying one end of the wormhole at slower-than-light speeds, then send a group of explorers through. They have both one-way and two-way wormholes, and use one-way wormholes for exploration, because sending a million people to visit a primitive society is less disruptive than letting a trillion people have casual access. Edit: I almost forgot to mention it, but an interesting detail is that wormholes have a limited lifespan, measured in the amount of mass they can transfer. This means that a thriving multi-stellar civilization would need continuous production of new wormholes, and a continuous stream of ships carrying endpoints to where they are needed, in order to maintain consistent connectivity. That's the type of detail that would make for a lot of fun gameplay in an RPG.

Glasshouse by Charles Stross has a human society living on the fringes of the solar system and in nearby stellar systems in immense habitats connected by wormholes. They're cheap enough that most people have apartments spread out over all of human space, with individual rooms connected by bi-directional wormholes. Although now that I think about it, Accelerando is really where the wormholes are mentioned more. Glasshouse is more about a group that splits itself off from the rest of the wormhole network after a war; the domestic uses are more clearly seen in Accelerando.

A much earlier book that I recall, though I cannot recall the name, was about a society that used wormholes on Earth to connect up their homes and businesses. I believe the plot was mostly about a child who goes wandering around outside; the child's mother thinks he has a mental problem. Not a very interesting story, really.

The Light of Other Days by Arthur C. Clarke (perhaps in collaboration with someone?) is quite interesting. Here the wormholes start out simply as a means of transmitting information between distant parts of the Earth without any time lag, making everyone's internet connections faster. Eventually they discover that the endpoints can be positioned outside of the equipment that generates it, and so the real story begins. They start out by placing the microscopic endpoint right in front of the eyes of a famous quarterback, to capture exactly what he was seeing when he made the big play, etc. At the end of the book they start using wormholes to scan people's minds just prior to their deaths, with the goal of bringing literally everyone who every lived back to life in reverse the order that they actually died.

I recall another where the inventor of the wormhole was blackmailed, and sent his blackmailer to live for a time on another planet. I thought it was called Gone Fishing, but a quick search doesn't turn it up.

Diaspora by Greg Egan has the descendants of humanity try to build a wormhole and fail. They spent 800 years setting up the experiment to create a wormhole (think of a particle accelerator like the LHC, but the size of the solar system), only to find out that although wormholes exist, they aren't actually shortcuts. All wormholes are as long on the inside as they are on the outside. This is a great book, superb on several levels, so I don't want to spoil it. They do ultimately discover a use for these long wormholes, and it's even more fantastic than short wormholes would have been.

As others have mentioned, if your wormhole endpoints can dial up any other wormhole endpoint then you have Stargate, which is actually quite a fun show. You could have a lot of fun games in that kind of setting.

As for plausibility, no, they're not plausible at all. The mathematics can "work" if you introduce some strange matter with negative mass, but that's precisely the sort of thing that the real laws of physics won't let you have. On the other hand, it would be hard to argue that wormholes are less plausible than FTL, so don't let that stop you. Your readers (or players) will suspend their disbelief of at least one thing in the setting.

Oh yea, and Schlock Mercenary, mentioned in the comments above, is excellent. Very good world-building there, and good stories well told.

  • $\begingroup$ @db48x The Light of Other Days was written by Arthur C. Clarke and Stephen Baxter (his collaborator who probably did all the heavy lifting of actually writing most of the text). Baxter is one of the better current hard Sf writers. Glad you liked Greg Egan's Diaspora too. $\endgroup$
    – a4android
    Jul 17, 2016 at 13:38

I’m not sure I fully understand your concept — I totally do not understand your comments on WhatRoughBeast’s answer — but it sounds like you are talking about a technique that is also known as “matter transmission,” a.k.a. teleportation — like the “teleporter” in Star Trek, but with a terminal at each end.  Like wormholes, this is also common in science fiction.  In particular, I am reminded of The Space Eater, by David Langford, in which humanity develops interstellar matter transmission (but not faster-than-light travel, per se), and then uses it to travel to another solar system, where they pretend that they got there via FTL (see review/plot summary).

  • $\begingroup$ It is not "matter transmission" or any kind of teleportation. It is more a deformation of space-time. Imagine that the door of your room is such a portal : you could take the door "A" with you, put it in the Mojave desert, and you would still see your room through it, and walk in your room if you cross through. $\endgroup$
    – Uriel
    May 7, 2016 at 20:40
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    $\begingroup$ It feels like you're splitting hairs and waving your hands.  If I can walk from my room to the Mojave desert in one step, that is teleportation. $\endgroup$ May 7, 2016 at 20:58
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    $\begingroup$ Well, then you have teleportation on every door :) $\endgroup$
    – Uriel
    May 7, 2016 at 22:49

If you could somehow create a spacecraft capable of producing 1g acceleration constantly for multiple decades, you could travel to any place in the galaxy within one lifetime without ever exceeding the speed of light. And even some other galaxies are within reach.

What saves you is time dilation. At 1g acceleration you will reach relativistic speeds after one year. As you keep accelerating at 1g the craft will appear to move closer and closer to the speed of light as seen by an outside observer, and time dilation will slow down the aging of the passengers. For the passengers on the craft the entire universe will appear to shrink in their direction of travel, which shortens the journey to an achievable distance.

Once you are halfway there you'll have to start slowing down again.

As a byproduct of accelerating at 1g the entire way, you get artificial gravity of 1g on your craft.

There is a few caveats to this approach:

  • You better be sure you don't hit anything on the way to your destination. Even the tiniest object you may hit will hit you quite hard at close to the speed of light.
  • The rocket equation is problematic. You will need an extremely high exhaust velocity.
  • You could travel to the other side of the Milky Way and back again in a lifetime. But though the trip took you only a lifetime, the time on Earth will not have slowed down, and once you got back millennia would have passed.
  • $\begingroup$ "time dilation will slow down the aging of the passengers" is incorrect + "time on Earth will not have slowed down, and once you got back, millennia would have passed", but the clock in your time reference frame will keep on ticking through those millennia and you'll die of old age. Maybe solveable by hibernating your biological body/processes $\endgroup$ May 8, 2016 at 10:19
  • $\begingroup$ @GuntherStruyf Somebody have already done the calculations. You could travel all the way to Andromeda and only be 28 years older when you arrived there. $\endgroup$
    – kasperd
    May 8, 2016 at 10:33
  • $\begingroup$ This is an option i also use before gates were created. It also requires some handwavium because the 1g acceleration requires a lot if energy (the paper mention this point, btw). These first traveller now continue to travel at high speed and form a cast that influence very long-term outcomes, for unknown purposes... $\endgroup$
    – Uriel
    May 8, 2016 at 13:26

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